EP2068699A1 - Système à double électrode pour un capteur de substance à analyser en continu - Google Patents

Système à double électrode pour un capteur de substance à analyser en continu

Info

Publication number
EP2068699A1
EP2068699A1 EP06816228A EP06816228A EP2068699A1 EP 2068699 A1 EP2068699 A1 EP 2068699A1 EP 06816228 A EP06816228 A EP 06816228A EP 06816228 A EP06816228 A EP 06816228A EP 2068699 A1 EP2068699 A1 EP 2068699A1
Authority
EP
European Patent Office
Prior art keywords
sensor
working electrode
glucose
electrode
analyte
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP06816228A
Other languages
German (de)
English (en)
Other versions
EP2068699A4 (fr
Inventor
Mark Brister
Sean Saint
James R. Petisce
Curtis Hanson
Peter Simpson
Jack Pryor
James H. Brauker
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Dexcom Inc
Original Assignee
Dexcom Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Dexcom Inc filed Critical Dexcom Inc
Publication of EP2068699A1 publication Critical patent/EP2068699A1/fr
Publication of EP2068699A4 publication Critical patent/EP2068699A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
    • A61B5/1486—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using enzyme electrodes, e.g. with immobilised oxidase
    • A61B5/14865—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue using enzyme electrodes, e.g. with immobilised oxidase invasive, e.g. introduced into the body by a catheter or needle or using implanted sensors
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/145—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue
    • A61B5/14532—Measuring characteristics of blood in vivo, e.g. gas concentration, pH value; Measuring characteristics of body fluids or tissues, e.g. interstitial fluid, cerebral tissue for measuring glucose, e.g. by tissue impedance measurement

Definitions

  • the present invention relates generally to systems and methods for measuring an analyte concentration in a host.
  • Diabetes mellitus is a disorder in which the pancreas cannot create sufficient insulin (Type I or insulin dependent) and/or in which insulin is not effective (Type 2 or non-insulin dependent).
  • Type I or insulin dependent in which the pancreas cannot create sufficient insulin
  • Type 2 or non-insulin dependent in which insulin is not effective
  • a hypoglycemic reaction low blood sugar may be induced by an inadvertent overdose of insulin, or after a normal dose of insulin or glucose-lowering agent accompanied by extraordinary exercise or insufficient food intake.
  • SMBG self-monitoring blood glucose
  • a variety of continuous glucose sensors have been developed for detecting and/or quantifying glucose concentration in a host. These sensors have typically required one or more blood glucose measurements, or the like, from which to calibrate the continuous glucose sensor to calculate the relationship between the current output of the sensor and blood glucose measurements, to provide meaningful values to a patient or doctor.
  • continuous glucose sensors are conventionally also sensitive to non-glucose related changes in the baseline current and sensitivity over time, for example, due to changes in a host's metabolism, maturation of the tissue at the biointerface of the sensor, interfering species which cause a measurable increase or decrease in the signal, or the like.
  • continuous glucose sensors should be responsive to baseline and/or sensitivity changes over time, which requires recalibration of the sensor. Consequently, users of continuous glucose sensors have typically been required to obtain numerous blood glucose measurements daily and/or weekly in order to maintain calibration of the sensor over time.
  • the preferred embodiments provide improved calibration techniques that utilize electrode systems and signal processing that provides measurements useful in simplifying and updating calibration that allows the patient increased convenience (for example, by requiring fewer reference glucose values) and confidence (for example, by increasing accuracy of the device).
  • One aspect of the preferred embodiments is a method for measuring a sensitivity change of a glucose sensor implanted in a host over a time period comprising: 1) measuring a first signal in the host by obtaining at least one glucose-related sensor data point, wherein the first signal is measured at a glucose-measuring electrode disposed beneath an enzymatic portion of a membrane system on the sensor; 2) measuring a second signal in the host by obtaining at least one non-glucose constant data point, wherein the second signal is measured beneath the inactive or non-enzymatic portion of the membrane system on the sensor; and 3) monitoring the second signal over a time period, whereby a sensitivity change associated with solute transport through the membrane system is measured.
  • the second signal is indicative of a presence or absence of a water-soluble analyte.
  • the water-soluble analyte may comprise urea.
  • the second signal is measured at an oxygen-measuring electrode disposed beneath a non-enzymatic portion of the membrane system.
  • the glucose-measuring electrode incrementally measures oxygen, whereby the second signal is measured.
  • the second signal is measured at an oxygen sensor disposed beneath the membrane system.
  • the sensitivity change is calculated as a glucose-to-oxygen ratio, whereby an oxygen threshold is determined that is indicative of a stability of the glucose sensor.
  • One embodiment further comprises filtering the first signal responsive to the stability of the glucose sensor.
  • One embodiment further comprises displaying a glucose value derived from the first signal, wherein the display is suspended depending on the stability of the glucose sensor.
  • One embodiment further comprises calibrating the first signal, wherein the calibrating step is suspended when the glucose sensor is determined to be stable.
  • One embodiment further comprises calibrating the glucose sensor when the sensitivity change exceeds a preselected value.
  • the step of calibrating may comprise receiving a reference signal from a reference analyte monitor, the reference signal comprising at least one reference data point.
  • the step of calibrating may comprise using the sensitivity change to calibrate the glucose sensor.
  • the step of calibrating may be performed repeatedly at a frequency responsive to the sensitivity change.
  • One embodiment further comprises determining a stability of glucose transport through the membrane system, wherein the stability of glucose transport is determined by measuring the sensitivity change over a time period.
  • One embodiment further comprises a step of prohibiting calibration of the glucose sensor when glucose transport is determined to be unstable.
  • One embodiment further comprises a step of filtering at least one glucose-related sensor data point when glucose transport is determined to be unstable.
  • Another aspect of the preferred embodiments is a system for measuring glucose in a host, comprising a glucose-measuring electrode configured to generate a first signal comprising at least one glucose-related sensor data point, wherein the glucose- measuring electrode is disposed beneath an enzymatic portion of a membrane system on a glucose sensor and a transport-measuring electrode configured to generate a second signal comprising at least one non-glucose constant analyte data point, wherein the transport- measuring electrode is situated beneath the membrane system on the glucose sensor.
  • One embodiment further comprises a processor module configured to monitor the second signal whereby a sensitivity change associated with transport of the non-glucose constant analyte through the membrane system over a time period is measured.
  • the transport-measuring electrode is configured to measure oxygen.
  • the processor module is configured to determine whether a glucose-to-oxygen ratio exceeds a threshold level, wherein a value is calculated from the first signal and the second signal, wherein the value is indicative of the glucose-to-oxygen ratio.
  • the processor module is configured to calibrate the glucose-related sensor data point in response to the sensitivity change.
  • the processor module is configured to receive reference data from a reference analyte monitor, the reference data comprising at least one reference data point, wherein the processor module is configured to use the reference data point for calibrating the glucose-related sensor data point.
  • the processor module is configured to use the sensitivity change for calibrating the glucose-related sensor data point.
  • the processor module is configured to calibrate the glucose- related sensor data point repeatedly at a frequency, wherein the frequency is selected based on the sensitivity change.
  • One embodiment further comprises a stability module configured to determine a stability of glucose transport through the membrane system, wherein the stability of glucose transport is correlated with the sensitivity change.
  • the processor module is configured to prohibit calibration of the glucose-related sensor data point when the stability of glucose transport falls below a threshold.
  • the processor module is configured to initiate filtering of the glucose-related sensor data point when the stability of glucose transport falls below a threshold.
  • Another aspect of the preferred embodiments is a method for processing data from a glucose sensor in a host, comprising: 1) measuring a first signal associated with glucose and non-glucose related electroactive compounds, wherein the first signal is measured at a first electrode disposed beneath an active enzymatic portion of a membrane system; 2) measuring a second signal associated with a non-glucose related electroactive compound, wherein the second signal is measured at a second electrode that is disposed beneath a non-enzymatic portion of the membrane system; and 3) monitoring the second signal over a time period, whereby a change in the non-glucose related electroactive compound in the host is measured.
  • One embodiment further comprises a step of subtracting the second signal from the first signal, whereby a differential signal comprising at least one glucose sensor data point is determined.
  • the step of subtracting may be performed electronically in the sensor. Alternatively, the step of subtracting may be performed digitally in the sensor or an associated receiver.
  • One embodiment further comprises calibrating the glucose sensor, wherein the step of calibrating comprises: 1) receiving reference data from a reference analyte monitor, the reference data comprising at least two reference data points; 2) providing at least two matched data pairs by matching the reference data to substantially time corresponding sensor data; and 3) calibrating the glucose sensor using the two or more matched data pairs and the differential signal.
  • One embodiment further comprises a step of calibrating the glucose sensor in response to a change in the non-glucose related electroactive compound over the time period.
  • the step of calibrating may comprise receiving reference data from a reference analyte monitor, the reference data comprising at least one reference data point.
  • the step of calibrating may comprise using the change in the non-glucose related electroactive compound over the time period to calibrate the glucose sensor.
  • the step of calibrating may be performed repeatedly at a frequency, wherein the frequency is selected based on the change in the non-glucose related electroactive compound over the time period.
  • One embodiment further comprises prohibiting calibration of the glucose sensor when the change in the non-glucose related electroactive compound rises above a threshold during the time period.
  • One embodiment further comprises filtering the glucose sensor data point when the change in the non-glucose related electroactive compound rises above a threshold during the time period.
  • One embodiment further comprises measuring a third signal in the host by obtaining at least one non-glucose constant data point, wherein the third signal is measured beneath the membrane system.
  • One embodiment further comprises monitoring the third signal over a time period, whereby a sensitivity change associated with solute transport through the membrane system is measured.
  • an oxygen-measuring electrode disposed beneath the non-enzymatic portion of the membrane system measures the third signal.
  • the first electrode measures the third signal by incrementally measuring oxygen.
  • an oxygen sensor disposed beneath the membrane system measures the third signal.
  • One embodiment further comprises determining whether a glucose-to-oxygen ratio exceeds a threshold level by calculating a value from the first signal and the second signal, wherein the value is indicative of the glucose-to-oxygen ratio.
  • One embodiment further comprises calibrating the glucose sensor in response to the sensitivity change measured over a time period.
  • the step of calibrating may comprise receiving reference data from a reference analyte monitor, the reference data comprising at least one reference data point.
  • the step of calibrating may comprise using the sensitivity change.
  • the step of calibrating may be performed repeatedly at a frequency, wherein the frequency is selected based on the sensitivity change.
  • One embodiment further comprises determining a glucose transport stability through the membrane system, wherein the glucose transport stability corresponds to the sensitivity change over a period of time.
  • One embodiment further comprises prohibiting calibration of the glucose sensor when the glucose transport stability falls below a threshold.
  • One embodiment further comprises filtering the glucose-related sensor data point when the glucose transport stability falls below a threshold.
  • Still another aspect of the preferred embodiments is a system for measuring glucose in a host, comprising a first working electrode configured to generate a first signal associated with a glucose related electroactive compound and a non-glucose related electroactive compound, wherein the first electrode is disposed beneath an active enzymatic portion of a membrane system on a glucose sensor; a second working electrode configured to generate a second signal associated with the non-glucose related electroactive compound, wherein the second electrode is disposed beneath a non-enzymatic portion of the membrane system on the glucose sensor; and a processor module configured to monitor the second signal over a time period, whereby a change in the non-glucose related electroactive compound is measured.
  • One embodiment further comprises a subtraction module configured to subtract the second signal from the first signal, whereby a differential signal comprising at least one glucose sensor data point is determined.
  • the subtraction module may comprise a differential amplifier configured to electronically subtract the second signal from the first signal.
  • the subtraction module may comprise at least one of hardware and software configured to digitally subtract the second signal from the first signal.
  • One embodiment further comprises a reference electrode, wherein the first working electrode and the second working electrode are operatively associated with the reference electrode.
  • One embodiment further comprises a counter electrode, wherein the first working electrode and the second working electrode are operatively associated with the counter electrode.
  • One embodiment further comprises a first reference electrode and a second reference electrode, wherein the first reference electrode is operatively associated with the first working electrode, and wherein the second reference electrode is operatively associated with the second working electrode.
  • One embodiment further comprises a first counter electrode and a second counter electrode, wherein the first counter electrode is operatively associated with the first working electrode, and wherein the second counter electrode is operatively associated with the second working electrode.
  • One embodiment further comprises a reference input module adapted to obtain reference data from a reference analyte monitor, the reference data comprising at least one reference data point, wherein the processor module is configured to format at least one matched data pair by matching the reference data to substantially time corresponding glucose sensor data and subsequently calibrating the system using at least two matched data pairs and the differential signal.
  • the processor module is configured to calibrate the system in response to the change in the non-glucose related electroactive compound in the host over the time period.
  • the processor module is configured to request reference data from a reference analyte monitor, the reference data comprising at least one reference data point, wherein the processor module is configured to recalibrate the system using the reference data.
  • the processor module is configured to recalibrate the system using the change in the non-glucose related electroactive compound measured over the time period.
  • the processor module is configured to repeatedly recalibrate at a frequency, wherein the frequency is selected based on the change in the non-glucose related electroactive compound over the time period.
  • the processor module is configured to prohibit calibration of the system when a change in the non-glucose related electroactive compound rises above a threshold during the time period. In one embodiment, the processor module is configured to filter the glucose sensor data point when the change in the non-glucose related electroactive compound rises above a threshold during the time period.
  • One embodiment further comprises a third electrode configured to generate a third signal, the third signal comprising at least one non-glucose constant analyte data point, wherein the third electrode is disposed beneath the membrane system on the sensor. The third electrode may be configured to measure oxygen.
  • the processor module is configured to determine whether a glucose-to-oxygen ratio exceeds a threshold level, wherein a value indicative of the glucose-to-oxygen ratio is calculated from the first signal and the second signal. In one embodiment, the processor module is configured to monitor the third signal over a time period, whereby a sensitivity change associated with solute transport through the membrane system is measured. In one embodiment, the processor module is configured to calibrate the glucose-related sensor data point in response to the sensitivity change. In one embodiment, the processor module is configured to receive reference data from a reference analyte monitor, the reference data comprising at least' one reference data point, wherein the processor module is configured to calibrate the glucose sensor data point using the reference data point.
  • the processor module is configured to calibrate the glucose-related sensor data point repeatedly at a frequency, wherein the frequency is selected based on the sensitivity change.
  • One embodiment further comprises a stability module configured to determine a stability of glucose transport through the membrane system, wherein the stability of glucose transport is correlated with the sensitivity change.
  • the processor module is configured to prohibit calibration of the glucose-related sensor data point when the stability of glucose transport falls below a threshold.
  • the processor module is configured to filter the glucose-related sensor data point when the stability of glucose transport falls below a threshold.
  • an analyte sensor configured for insertion into a host for measuring an analyte in the host
  • the sensor comprising a first working electrode disposed beneath an active enzymatic portion of a sensor membrane; and a second working electrode disposed beneath an inactive-enzymatic or a non-enzymatic portion of a sensor membrane, wherein the first working electrode and the second working electrode each integrally form at least a portion of the sensor.
  • the first working electrode and the second working electrode are coaxial.
  • At least one of the first working electrode and the second working electrode is twisted or helically wound to integrally form at least a portion of the sensor.
  • the first working electrode and the second working electrode are twisted together to integrally form an in vivo portion of the sensor.
  • one of the first working electrode and the second working electrode is deposited or plated over the other of the first working electrode and the second working electrode.
  • the first working electrode and the second working electrode each comprise a first end and a second end, wherein the first ends are configured for insertion in the host, and wherein the second ends are configured for electrical connection to sensor electronics.
  • the second ends are coaxial.
  • the second ends are stepped.
  • the senor further comprises at least one additional electrode selected from the group consisting of a reference electrode and a counter electrode.
  • the additional electrode together with the first working electrode and the second working electrode, integrally form at least a portion of the sensor.
  • the additional electrode is located at a position remote from the first and second working electrodes.
  • a surface area of the additional electrode is at least six times a surface area of at least one of the first working electrode and the second working electrode.
  • the senor is configured for implantation into the host.
  • the senor is configured for subcutaneous implantation in a tissue of a host.
  • the senor is configured for indwelling in a blood stream of a host.
  • the senor substantially continuously measures an analyte concentration in a host.
  • the senor comprises a glucose sensor, and wherein the first working electrode is configured to generate a first signal associated with glucose and non-glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential.
  • the second working electrode is configured to generate a second signal associated with noise of the glucose sensor, the noise comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential.
  • the non-glucose related electroactive species comprises at least one species selected from the group consisting of interfering species, non-reaction-related hydrogen peroxide, and other electroactive species.
  • the senor further comprises electronics operably connected to the first working , electrode and the second working electrode, and configured to provide the first signal and the second signal to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the senor further comprises a non- conductive material positioned between the first working electrode and the second working electrode.
  • each of the first working electrode, the second working electrode, and the non-conductive material are configured to provide at least two functions selected from the group consisting of electrical conductance, insulative property, structural support, and diffusion barrier.
  • the senor comprises a diffusion barrier configured to substantially block diffusion of at least one of an analyte and a co- analyte between the first working electrode and the second working electrode.
  • a glucose sensor configured for insertion into a host for measuring a glucose concentration in the host, the sensor comprising a first working electrode configured to generate a first signal associated with glucose and non- glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential; and a second working electrode configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential, wherein the first working electrode and the second working electrode each integrally form at least a portion of the sensor.
  • the first working electrode and the second working electrode integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises a reference electrode, wherein the first working electrode, the second working electrode, and the reference electrode each integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises an insulator, wherein the first working electrode, the second working electrode, and the insulator each integrally form a substantial portion of the sensor configured for insertion in the host.
  • a system configured for measuring a glucose concentration in a host
  • the system comprising a processor module configured to receive or process a first signal associated with glucose and non-glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential, and to receive or process a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential, wherein the first working electrode and the second working electrode each integrally form at least a portion of the sensor, and wherein the processor module is further configured to process the first signal and the second signal to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the first working electrode and the second working electrode are coaxial.
  • At least one of the first working electrode and the second working electrode is twisted or helically wound to form at least a portion of the sensor.
  • the first working electrode and the second working electrode are twisted together to form an in vivo portion of the sensor.
  • one of the first working electrode and the second working electrode is deposited or plated over the other of the first working electrode and the second working electrode.
  • the first working electrode and the second working electrode each comprise a first end and a second end, wherein the first ends are configured for insertion in the host, and wherein the second ends are configured for electrical connection to sensor electronics.
  • the second ends are coaxial.
  • the second ends are stepped.
  • an analyte sensor configured for insertion into a host for measuring an analyte in the host, the sensor comprising a first working electrode disposed beneath an active enzymatic portion of a membrane; a second working electrode disposed beneath an inactive-enzymatic or non-enzymatic portion of a membrane; and a non- conductive material located between the first working electrode and the second working electrode, wherein each of the first working electrode, the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of electrical conductance, insulative property, structural support, and diffusion barrier.
  • each of the first working electrode and the second working electrode are configured to provide electrical conductance and structural support.
  • the sensor further comprises a reference electrode, wherein the reference electrode is configured to provide electrical conductance and structural support.
  • the senor further comprises a reference electrode, wherein the reference electrode is configured to provide electrical conductance and a diffusion barrier.
  • the non-conductive material is configured to provide an insulative property and structural support.
  • the non-conductive material is configured to provide an insulative property and a diffusion barrier.
  • the senor further comprises a reference electrode, wherein the reference electrode is configured to provide a diffusion barrier and structural support
  • the non-conductive material is configured to provide a diffusion barrier and structural support.
  • the sensor further comprises at least one of a reference electrode and a counter electrode.
  • At least one of the reference electrode and the counter electrode is located at a position remote from the first working electrode and the second working electrode.
  • a surface area of at least one of the reference electrode and the counter electrode is at least six times a surface area of at least one of the first working electrode and the second working electrode.
  • the senor is configured for implantation into the host.
  • the senor is configured for subcutaneous implantation in a tissue of the host.
  • the senor is configured for indwelling in a blood stream of the host. [0061] In an embodiment of the fourth aspect, the sensor substantially continuously measures an analyte concentration in the host.
  • the senor comprises a glucose sensor, and wherein the first working electrode is configured to generate a first signal associated with glucose and non-glucose related electroactive compounds, the glucose and non-glucose related compounds having a first oxidation potential.
  • the second working electrode is configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential.
  • the non-glucose related electroactive species comprises at least one species selected from the group consisting of interfering species, non-reaction-related hydrogen peroxide, and other electroactive species.
  • the senor further comprises electronics operably connected to the first working electrode and the second working electrode, and configured to provide the first signal and the second signal to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the senor further comprises a non- conductive material positioned between the first working electrode and the second working electrode.
  • the first working electrode, the second working electrode, and the non-conductive material integrally form at least a portion of the sensor.
  • the first working electrode and the second working electrode each integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises a reference electrode, wherein the first working electrode, the second working electrode, and the reference electrode each integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises an insulator, wherein the first working electrode, the second working electrode, and the insulator each integrally form a substantial portion of the sensor configured for insertion in the host.
  • the sensor comprises a diffusion barrier configured to substantially block diffusion of an analyte or a co-analyte between the first working electrode and the second working electrode.
  • a glucose sensor configured for insertion into a host for measuring a glucose concentration in the host, the sensor comprising a first working electrode configured to generate a first signal associated with glucose and non- glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential; a second working electrode configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential; and a non-conductive material located between the first working electrode and the second working electrode, wherein each of the first working electrode, the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of electrical conductance, insulative property, structural support, and diffusion barrier.
  • each of the first working electrode and the second working electrode are configured to provide electrical conductance and structural support.
  • the senor further comprises a reference electrode, wherein the reference electrode is configured to provide electrical conductance and structural support.
  • the senor further comprises a reference electrode, wherein the reference electrode is configured to provide electrical conductance and a diffusion barrier.
  • the senor further comprises a reference electrode, wherein the reference electrode is configured to provide a diffusion barrier and structural support.
  • the non-conductive material is configured to provide an insulative property and structural support.
  • the non-conductive material is configured to provide an insulative property and a diffusion barrier.
  • the non-conductive material is configured to provide a diffusion barrier and structural support.
  • an analyte sensor configured for insertion into a host for measuring an analyte in the host, the sensor comprising a first working electrode disposed beneath an active enzymatic portion of a membrane; a second working electrode disposed beneath an inactive-enzymatic or non-enzymatic portion of a membrane; and an insulator located between the first working electrode and the second working electrode, wherein the sensor comprises a diffusion barrier configured to substantially block diffusion of at least one of an analyte and a co-analyte between the first working electrode and the second working electrode.
  • the diffusion barrier comprises a physical diffusion barrier configured to physically block or spatially block a substantial amount of diffusion of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the physical diffusion barrier comprises the insulator.
  • the physical diffusion barrier comprises the reference electrode.
  • a dimension of the first working electrode and a dimension of the second working electrode relative to an in vivo portion of the sensor provide the physical diffusion barrier.
  • the physical diffusion barrier comprises a membrane.
  • the membrane is configured to block diffusion of a substantial amount of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the diffusion barrier comprises a temporal diffusion barrier configured to block or avoid a substantial amount of diffusion or reaction of at least one of the analyte and the co-analyte between the first and second working electrodes.
  • the senor further comprises a potentiostat configured to bias the first working electrode and the second working electrode at substantially overlapping oxidation potentials, and wherein the temporal diffusion barrier comprises pulsed potentials of the first working electrode and the second working electrode to block or avoid a substantial amount of diffusion or reaction of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the senor further comprises a potentiostat configured to bias the first working electrode and the second working electrode at substantially overlapping oxidation potentials, and wherein the temporal diffusion barrier comprises oscillating bias potentials of the first working electrode and the second working electrode to block or avoid a substantial amount of diffusion or reaction of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the analyte sensor is configured to indwell in a blood stream of the host, and wherein the diffusion barrier comprises a configuration of the first working electrode and the second working electrode that provides a flow path diffusion barrier configured to block or avoid a substantial amount of diffusion of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the flow path diffusion barrier comprises a location of the first working electrode configured to be upstream from the second working electrode when inserted into the blood stream.
  • the flow path diffusion barrier comprises a location of the first working electrode configured to be downstream from the second working electrode when inserted into the blood stream.
  • the flow path diffusion barrier comprises an offset of the first working electrode relative to the second working electrode when inserted into the blood stream.
  • the flow path diffusion barrier is configured to utilize a shear of a blood flow of the host between the first working electrode and the second working electrode when inserted into the blood stream.
  • the senor is a glucose sensor, and wherein the diffusion barrier is configured to substantially block diffusion of at least one of glucose and hydrogen peroxide between the first working electrode and the second working electrode.
  • the senor further comprises at least one of a reference electrode and a counter electrode.
  • the reference electrode or the counter electrode together with the first working electrode, the second working electrode and the insulator, integrally form at least a portion of the sensor.
  • the reference electrode or the counter electrode is located at a position remote from the first working electrode and the second working electrode.
  • a surface area of at least one of the reference electrode and the counter electrode is at least six times a surface area of at least one of the first working electrode and the second working electrode.
  • the senor is configured for implantation into the host.
  • the senor is configured for subcutaneous implantation in a tissue of the host.
  • the senor is configured for indwelling in a blood stream of the host.
  • sensor substantially continuously measures an analyte concentration in the host.
  • the analyte sensor comprises a glucose sensor and wherein the first working electrode is configured to generate a first signal associated with glucose and non-glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential.
  • the second working electrode is configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential.
  • the non-glucose related electroactive species comprise at least one species selected from the group consisting of interfering species, non-reaction-related hydrogen peroxide, and other electroactive species.
  • the senor further comprises electronics operably connected to the first working electrode and the second working electrode, and configured to provide the first signal and the second signal to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the first working electrode, the second working electrode, and the insulator integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises a reference electrode, wherein the first working electrode, the second working electrode, and the reference electrode integrally form a substantial portion of the sensor configured for insertion in the host.
  • each of the first working electrode,the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of electrical conductance, insulative property, structural support, and diffusion barrier.
  • a glucose sensor configured for insertion into a host for measuring a glucose concentration in the host, the sensor comprising a first working electrode configured to generate a first signal associated with glucose and non- glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential; a second working electrode configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential; and a non-conductive material located between the first working electrode and the second working electrode, wherein the sensor comprises a diffusion barrier configured to substantially block diffusion of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the diffusion barrier comprises a physical diffusion barrier configured to physically or spatially block a substantial amount of diffusion of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the diffusion barrier comprises a temporal diffusion barrier configured to block or avoid a substantial amount of diffusion or reaction of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the analyte sensor is configured to indwell in a blood stream of the host, and wherein the diffusion barrier comprises a configuration of the first working electrode and the second working electrode that provides a flow path diffusion barrier configured to block or avoid a substantial amount of diffusion of at least one of the analyte and the co-analyte between the first working electrode and the second working electrode.
  • the senor further comprises at least one of a reference electrode and a counter electrode.
  • the senor is configured for implantation into the host.
  • the senor substantially continuously measures an analyte concentration in the host.
  • the sensor further comprises electronics operably connected to the first working electrode and the second working electrode, and configured to provide the first signal and the second signal to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the first working electrode, the second working electrode, and the insulator integrally form a substantial portion of the sensor configured for insertion in the host.
  • each of the first working electrode, the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of: electrical conductance, insulative property, structural support, and diffusion barrier.
  • a glucose sensor system configured for insertion into a host for measuring a glucose concentration in the host, the sensor comprising a first working electrode configured to generate a first signal associated with glucose and non- glucose related electroactive compounds, the glucose and non-glucose related electroactive compounds having a first oxidation potential; a second working electrode configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential; and electronics operably connected to the first working electrode and the second working electrode and configured to process the first signal and the second signal to generate a glucose concentration substantially without signal contribution due to non-glucose related noise.
  • the non-glucose related noise is substantially non-constant.
  • the electronics are configured to substantially remove noise caused by mechanical factors.
  • the mechanical factors are selected from the group consisting of macro-motion of the sensor, micro-motion of the sensor, pressure on the sensor, and stress on the sensor.
  • the first working electrode and the second working electrode are configured to substantially equally measure noise due to mechanical factors, whereby noise caused by mechanical factors is substantially removed.
  • the electronics are configured to substantially remove noise caused by at least one of biochemical factors and chemical factors.
  • the at least one of the biochemical factors and the chemical factors are substantially non-constant and are selected from the group consisting of compounds with electroactive acidic groups, compounds with electroactive amine groups, compounds with electroactive sulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides, amino acids, amino acid precursors, amino acid break-down products, nitric oxide, nitric oxide-donors, nitric oxide-precursors, electroactive species produced during cell metabolism, electroactive species produced during wound healing, and electroactive species that arise during body pH changes.
  • the first working electrode and the second working electrode are configured to substantially equally measure noise due to at least one of the biochemical factors and the chemical factors whereby noise caused by at least one of the biochemical factors and the chemical factors can be substantially removed.
  • the electronics are configured to subtract the second signal from the first signal, whereby a differential signal comprising at least one glucose sensor data point is determined.
  • the electronics comprise a differential amplifier configured to electronically subtract the second signal from the first signal.
  • the electronics comprise at least one of hardware and software configured to digitally subtract the second signal from the first signal.
  • the first working electrode and the second working electrode are configured to be impacted by mechanical factors and biochemical factors to substantially the same extent.
  • the first working electrode and the second working electrode have a configuration selected from the group consisting of coaxial, helically twisted, bundled, symmetrical, and combinations thereof.
  • the senor further comprises a non- conductive material positioned between the first working electrode and the second working electrode.
  • each of the first working electrode, the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of electrical conductance, insulative property, structural support, and diffusion barrier.
  • the senor comprises a diffusion barrier configured to substantially block diffusion of at least one of the analyte and the co- analyte between the first working electrode and the second working electrode.
  • the first working electrode, the second working electrode, and the insulator integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises a reference electrode, wherein the first working electrode, the second working electrode, and the reference electrode integrally form a substantial portion of the sensor configured for insertion in the host.
  • an analyte sensor configured for insertion into a host for measuring an analyte in the host, the sensor comprising a first working electrode disposed beneath an active enzymatic portion of a membrane; a second working electrode disposed beneath an inactive-enzymatic or non-enzymatic portion of a membrane, wherein the first working electrode and the second working electrode are configured to substantially equally measure non-analyte related noise, whereby the noise is substantially removed; and electronics operably connected to the first working electrode and the second working electrode, and configured to process the first signal and the second signal to generate sensor analyte data substantially without signal contribution due to non-analyte related noise .
  • the non-glucose related noise is substantially non-constant.
  • the non-analyte related noise is due to a factor selected from the group consisting of mechanical factors, biochemical factors, chemical factors, and combinations thereof.
  • the electronics are configured to substantially remove noise caused by mechanical factors.
  • the mechanical factors are selected from the group consisting of macro-motion of the sensor, micro-motion of the sensor, pressure on the sensor, and stress on the sensor.
  • the first working electrode and the second working electrode are configured to substantially equally measure noise due to mechanical factors, whereby noise caused by mechanical factors can be substantially removed.
  • the electronics are configured to substantially remove noise caused by at least one of biochemical factors and chemical factors.
  • At least one of the biochemical factors and the chemical factors are substantially non-constant and are selected from the group consisting of compounds with electroactive acidic groups, compounds with electroactive amine groups, compounds with electroactive sulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides, amino acids, amino acid precursors, amino acid break-down products, nitric oxide, nitric oxide-donors, nitric oxide-precursors, electroactive species produced during cell metabolism, electroactive species produced during wound healing, and electroactive species that arise during body pH changes.
  • the first working electrode and the second working electrode are configured to substantially equally measure noise due to at least one of biochemical factors and chemical factors, whereby noise caused by at least one of the biochemical factors and the chemical factors is substantially removed.
  • the senor further comprises at least one of a reference electrode and a counter electrode.
  • a surface area of at least one of the reference electrode and the counter electrode is at least six times a surface area of at least one of the first working electrode and the second working electrode.
  • the senor is configured for implantation into the host.
  • the senor is configured for subcutaneous implantation in a tissue of the host.
  • the senor is configured for indwelling in a blood stream of the host.
  • the senor substantially continuously measures an analyte concentration of the host.
  • the analyte sensor comprises a glucose sensor, and wherein the first working electrode is configured to generate a first signal associated with glucose and non-glucose related electroactive compounds, the glucose and the non-glucose related electroactive compounds having a first oxidation potential.
  • the second working electrode is configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds with an oxidation potential that substantially overlaps with the first oxidation potential.
  • the non-glucose related electroactive species comprises at least one species selected from the group consisting of interfering species, non-reaction-related hydrogen peroxide, and other electroactive species.
  • the senor further comprises a non- conductive material positioned between the first working electrode and the second working electrode.
  • each of the first working electrode, the second working electrode, and the non-conductive material are configured provide at least two functions selected from the group consisting of: electrical conductance, insulative property, structural support, and diffusion barrier.
  • the sensor comprises a diffusion barrier configured to substantially block diffusion of at least one of an analyte and a co- analyte between the first working electrode and the second working electrode.
  • the first working electrode, the second working electrode, and the insulator integrally form a substantial portion of the sensor configured for insertion in the host.
  • the senor further comprises a reference electrode, wherein the first working electrode, the second working electrode, and the reference electrode integrally form a substantial portion of the sensor configured for insertion in the host.
  • the first working electrode and the second working electrode are configured to be impacted by mechanical factors and biochemical factors to substantially the same extent.
  • the first working electrode and the second working electrode have a configuration selected from the group consisting of coaxial, helically twisted, bundled, symmetrical, and combination thereof.
  • Fig. IA is a perspective view of a continuous analyte sensor, including an implantable body with a membrane system disposed thereon
  • Fig. IB is an expanded view of an alternative embodiment of a continuous analyte sensor, illustrating the in vivo portion of the sensor.
  • Fig. 2A is a schematic view of a membrane system in one embodiment, configured for deposition over the electroactive surfaces of the analyte sensor of Fig. IA.
  • Fig. 2B is a schematic view of a membrane system in an alternative embodiment, configured for deposition over the electroactive surfaces of the analyte sensor of Fig. IB.
  • FIG. 3A which is a cross-sectional exploded schematic view of a sensing region of a continuous glucose sensor in one embodiment wherein an active enzyme of an enzyme domain is positioned only over the glucose-measuring working electrode.
  • Fig. 3B is a cross-sectional exploded schematic view of a sensing region of a continuous glucose sensor in another embodiment, wherein an active portion of the enzyme within the enzyme domain positioned over the auxiliary working electrode has been deactivated.
  • Fig. 4 is a block diagram that illustrates continuous glucose sensor electronics in one embodiment.
  • Fig. 5 is a drawing of a receiver for the continuous glucose sensor in one embodiment.
  • Fig. 6 is a block diagram of the receiver electronics in one embodiment.
  • Fig. 7Al is a schematic of one embodiment of a coaxial sensor having axis A-A.
  • Fig. 7A2 is a cross-section of the sensor shown in Fig. 7Al.
  • Fig. 7B is a schematic of another embodiment of a coaxial sensor.
  • Fig. 7C is a schematic of one embodiment of a sensor having three electrodes.
  • Fig. 7D is a schematic of one embodiment of a sensor having seven electrodes.
  • Fig. 7E is a schematic of one embodiment of a sensor having two pairs of electrodes and insulating material.
  • Fig. 7F is a schematic of one embodiment of a sensor having two electrodes separated by a reference electrode or insulating material.
  • Fig. 7G is a schematic of another embodiment of a sensor having two electrodes separated by a reference electrode or insulating material.
  • Fig. 7H is a schematic of another embodiment of a sensor having two electrodes separated by a reference electrode or insulating material.
  • Fig. 71 is a schematic of another embodiment of a sensor having two electrodes separated by reference electrodes or insulating material.
  • Fig. 7J is a schematic of one embodiment of a sensor having two electrodes separated by a substantially X-shaped reference electrode or insulating material.
  • Fig 7K is a schematic of one embodiment of a sensor having two electrodes coated with insulating material, wherein one electrode has a space for enzyme, the electrodes are separated by a distance D and covered by a membrane system.
  • Fig. 7L is a schematic of one embodiment of a sensor having two electrodes embedded in an insulating material.
  • Fig. 7M is a schematic of one embodiment of a sensor having multiple working electrodes and multiple reference electrodes.
  • Fig. 7N is a schematic of one step of the manufacture of one embodiment of a sensor having, embedded in insulating material, two working electrodes separated by a reference electrode, wherein the sensor is trimmed to a final size and/or shape.
  • Fig. 8A is a schematic on one embodiment of a sensor having two working electrodes coated with insulating material, and separated by a reference electrode.
  • Fig. 8B is a schematic of the second end (e.g., ex vivo terminus) of the sensor of Fig 8 A having a stepped connection to the sensor electronics.
  • Fig. 9A is a schematic of one embodiment of a sensor having two working electrodes and a substantially cylindrical reference electrode there around, wherein the second end (the end connected to the sensor electronics) of the sensor is stepped.
  • Fig. 9B is a schematic of one embodiment of a sensor having two working electrodes and an electrode coiled there around, wherein the second end (the end connected to the sensor electronics) of the sensor is stepped.
  • Fig. 10 is a schematic illustrating metabolism of glucose by Glucose Oxidase (GOx) and one embodiment of a diffusion barrier D that substantially prevents the diffusion Of H 2 O 2 produced on a first side of the sensor (e.g., from a first electrode that has active GOx) to a second side of the sensor (e.g., to the second electrode that lacks active GOx).
  • a diffusion barrier D that substantially prevents the diffusion Of H 2 O 2 produced on a first side of the sensor (e.g., from a first electrode that has active GOx) to a second side of the sensor (e.g., to the second electrode that lacks active GOx).
  • Fig. 11 is a schematic illustrating one embodiment of a triple helical coaxial sensor having a stepped second terminus for engaging the sensor electronics.
  • Fig. 14 is a graph that illustrates an in vitro signal (counts) detected from a dual-electrode sensor with a bundled configuration similar to that shown in Fig. 7C (two platinum working electrodes and one silver/silver chloride reference electrode, not twisted).
  • Fig. 15 is a graph that illustrates an in vivo signal (counts) detected from a dual-electrode sensor with a bundled configuration similar to that shown in Fig. 7C (two platinum working electrodes, not twisted, and one remotely disposed silver/silver chloride reference electrode).
  • analyte as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a substance or chemical constituent in a biological fluid (for example, blood, interstitial fluid, cerebral spinal fluid, lymph fluid or urine) that can be analyzed. Analytes may include naturally occurring substances, artificial substances, metabolites, and/or reaction products. In some embodiments, the analyte for measurement by the sensor heads, devices, and methods disclosed herein is glucose.
  • analytes include but not limited to acarboxyprothrombin; acylcarnitine; adenine phosphoribosyl transferase; adenosine deaminase; albumin; alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle), histidine/urocanic acid, homocysteine, phenylalanine/tyrosine, tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers; arginase; benzoylecgonine (cocaine); biotinidase; biopterin; c-reactive protein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholic acid; chloroquine; cholesterol; cholinesterase; conjugated 1- ⁇ hydroxy- cholic acid; Cortisol; creatine kinase; creatine kinase
  • Salts, sugar, protein, fat, vitamins, and hormones naturally occurring in blood or interstitial fluids may also constitute analytes in certain embodiments.
  • the analyte may be naturally present in the biological fluid, for example, a metabolic product, a hormone, an antigen, an antibody, and the like.
  • the analyte may be introduced into the body, for example, a contrast agent for imaging, a radioisotope, a chemical agent, a fluorocarbon-based synthetic blood, or a drug or pharmaceutical composition, including but not limited to insulin; ethanol; cannabis (marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide, amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine (crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin, Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine); depressants (barbituates, methaqualone, tranquilizers such as Valium, Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens (phencyclidine, lysergic acid, mescaline, peyote, p
  • Analytes such as neurochemicals and other chemicals generated within the body may also be analyzed, such as, for example, ascorbic acid, uric acid, dopamine, noradrenaline, 3-methoxytyramine (3MT), 3,4-Dihydroxy ⁇ henylacetic acid (DOPAC), Homovanillic acid (HVA), 5- Hydroxytryptamine (5HT), and 5-Hydroxyindoleacetic acid (FHIAA).
  • continuous glucose sensor is a broad term, and is to be given/ its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a device that continuously or continually measures glucose concentration, for example, at time intervals ranging from fractions of a second up to, for example, 1 , 2, or 5 minutes, or longer. It should be understood that continuous glucose sensors can continually measure glucose concentration without requiring user initiation and/or interaction for each measurement, such as described with reference to U.S. Patent 6,001,067, for example.
  • continuous glucose sensing is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the period in which monitoring of plasma glucose concentration is continuously or continually performed, for example, at time intervals ranging from fractions of a second up to, for example, 1 , 2, or 5 minutes, or longer.
  • biological sample as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a sample of a host body, for example, blood, interstitial fluid, spinal fluid, saliva, urine, tears, sweat, tissue, and the like.
  • host as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to plants or animals, for example humans.
  • biointerface membrane is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a permeable or semi-permeable membrane that can include one or more domains and is typically constructed of materials of a few microns thickness or more, which can be placed over the sensing region to keep host cells (for example, macrophages) from gaining proximity to, and thereby damaging the membrane system or forming a barrier cell layer and interfering with the transport of glucose across the tissue-device interface.
  • host cells for example, macrophages
  • membrane system as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a permeable or semi-permeable membrane that can be comprised of one or more domains and is typically constructed of materials of a few microns thickness or more, which may be permeable to oxygen and are optionally permeable to glucose.
  • the membrane system comprises an immobilized glucose oxidase enzyme, which enables an electrochemical reaction to occur to measure a concentration of glucose.
  • domain is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to regions of a membrane that can be layers, uniform or non-uniform gradients (for example, anisotropic), functional aspects of a material, or provided as portions of the membrane.
  • copolymer as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to polymers having two or more different repeat units and includes copolymers, terpolymers, tetrapolymers, and the like.
  • the term "sensing region" as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the region of a monitoring device responsible for the detection of a particular analyte.
  • the sensing region generally comprises a non-conductive body, at least one electrode, a reference electrode and a optionally a counter electrode passing through and secured within the body forming an electrochemically reactive surface at one location on the body and an electronic connection at another location on the body, and a membrane system affixed to the body and covering the electrochemically reactive surface.
  • the sensing region generally comprises a non-conductive body, a working electrode (anode), a reference electrode (optionally can be remote from the sensing region), an insulator disposed therebetween, and a multi-domain membrane affixed to the body and covering the electrochemically reactive surfaces of the working and optionally reference electrodes.
  • electrochemically reactive surface is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the surface of an electrode where an electrochemical reaction takes place.
  • a working electrode measures hydrogen peroxide creating a measurable electronic current.
  • electrochemical cell is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a device in which chemical energy is converted to electrical energy.
  • a cell typically consists of two or more electrodes held apart from each other and in contact with an electrolyte solution. Connection of the electrodes to a source of direct electric current renders one of them negatively charged and the other positively charged.
  • Positive ions in the electrolyte migrate to the negative electrode (cathode) and there combine with one or more electrons, losing part or all of their charge and becoming new ions having lower charge or neutral atoms or molecules; at the same time, negative ions migrate to the positive electrode (anode) and transfer one or more electrons to it, also becoming new ions or neutral particles.
  • the overall effect of the two processes is the transfer of electrons from the negative ions to the positive ions, a chemical reaction.
  • Electrode as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a conductor through which electricity enters or leaves something such as a battery or a piece of electrical equipment.
  • the electrodes are the metallic portions of a sensor (e.g., electrochemically reactive surfaces) that are exposed to the extracellular milieu, for detecting the analyte.
  • the term electrode includes the conductive wires or traces that electrically connect the electrochemically reactive surface to connectors (for connecting the sensor to electronics) or to the electronics.
  • enzyme as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a protein or protein-based molecule that speeds up a chemical reaction occurring in a living thing. Enzymes may act as catalysts for a single reaction, converting a reactant (also called an analyte herein) into a specific product.
  • a reactant also called an analyte herein
  • an enzyme glucose oxidase (GOX) is provided to react with glucose (the analyte) and oxygen to form hydrogen peroxide.
  • co-analyte is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a molecule required in an enzymatic reaction to react with the analyte and the enzyme to form the specific product being measured.
  • an enzyme glucose oxidase (GOX) is provided to react with glucose and oxygen (the co-analyte) to form hydrogen peroxide.
  • constant analyte is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to an analyte that remains relatively constant over a time period, for example over an hour to a day as compared to other variable analytes.
  • oxygen and urea may be relatively constant analytes in particular tissue compartments relative to glucose, which is known to oscillate between about 40 and 400 mg/dL during a 24-hour cycle.
  • analytes such as oxygen and urea are known to oscillate to a lesser degree, for example due to physiological processes in a host, they are substantially constant, relative to glucose, and can be digitally filtered, for example low pass filtered, to minimize or eliminate any relatively low amplitude oscillations. Constant analytes other than oxygen and urea are also contemplated.
  • proximal is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to near to a point of reference such as an origin or a point of attachment.
  • the electrolyte domain is located more proximal to the electrochemically reactive surface than the resistance domain.
  • distal is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to spaced relatively far from a point of reference, such as an origin or a point of attachment.
  • a resistance domain is located more distal to the electrochemically reactive surfaces than the electrolyte domain.
  • the term "substantially” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a sufficient amount that provides a desired function.
  • the interference domain of the preferred embodiments is configured to resist a sufficient amount of interfering species such that tracking of glucose levels can be achieved, which may include an amount greater than 50 percent, an amount greater than 60 percent, an amount greater than 70 percent, an amount greater than 80 percent, or an amount greater than 90 percent of interfering species.
  • modem is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to an electronic device for converting between serial data from a computer and an audio signal suitable for transmission over a telecommunications connection to another modem.
  • processor module and “microprocessor” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to a computer system, state machine, processor, or the like designed to perform arithmetic and logic operations using logic circuitry that responds to and processes the basic instructions that drive a computer.
  • ROM read-only memory
  • EEPROM electrically erasable programmable read-only memory
  • RAM refers without limitation to a data storage device for which the order of access to different locations does not affect the speed of access.
  • RAM is broad enough to include SRAM, for example, which is static random access memory that retains data bits in its memory as long as power is being supplied.
  • A/D Converter as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to hardware and/or software that converts analog electrical signals into corresponding digital signals.
  • RF transceiver as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a radio frequency transmitter and/or receiver for transmitting and/or receiving signals.
  • raw data stream and “data stream” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to an analog or digital signal directly related to the analyte concentration measured by the analyte sensor.
  • the raw data stream is digital data in "counts" converted by an A/D converter from an analog signal (for example, voltage or amps) representative of an analyte concentration.
  • raw data includes one or more values (e.g., digital value) representative of the current flow integrated over time (e.g., integrated value), for example, using a charge counting device, or the like.
  • counts is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a unit of measurement of a digital signal.
  • a raw data stream measured in counts is directly related to a voltage (for example, converted by an A/D converter), which is directly related to current from a working electrode.
  • the term "electronic circuitry” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the components (for example, hardware and/or software) of a device configured to process data.
  • the data includes biological information obtained by a sensor regarding the concentration of the analyte in a biological fluid.
  • potentiostat as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to an electrical system that applies a potential between the working and reference electrodes of a two- or three-electrode cell at a preset value and measures the current flow through the working electrode.
  • the potentiostat forces whatever current is necessary to flow between the working and reference or counter electrodes to keep the desired potential, as long as the needed cell voltage and current do not exceed the compliance limits of the potentiostat.
  • operably connected and “operably linked” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to one or more components being linked to another component(s) in a manner that allows transmission of signals between the components.
  • one or more electrodes can be used to detect the amount of glucose in a sample and convert that information into a signal; the signal can then be transmitted to an electronic circuit. In this case, the electrode is "operably linked” to the electronic circuit.
  • These terms are broad enough to include wired and wireless connectivity.
  • smoothing and filtering are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to modification of a set of data to make it smoother and more continuous and remove or diminish outlying points, for example, by performing a moving average of the raw data stream.
  • algorithm is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the computational processes (for example, programs) involved in transforming information from one state to another, for example using computer processing.
  • regression is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to finding a line in which a set of data has a minimal measurement (for example, deviation) from that line.
  • Regression can be linear, non-linear, first order, second order, and so forth.
  • One example of regression is least squares regression.
  • pulsed amperometric detection is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to an electrochemical flow cell and a controller, which applies the potentials and monitors current generated by the electrochemical reactions.
  • the cell can include one or multiple working electrodes at different applied potentials. Multiple electrodes can be arranged so that they face the chromatographic flow independently (parallel configuration), or sequentially (series configuration).
  • calibration is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the relationship and/or the process of determining the relationship between the sensor data and corresponding reference data, which may be used to convert sensor data into meaningful values substantially equivalent to the reference.
  • calibration may be updated or recalibrated over time if changes in the relationship between the sensor and reference data occur, for example due to changes in sensitivity, baseline, transport, metabolism, or the like.
  • sensor analyte values and “sensor data” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to data received from a continuous analyte sensor, including one or more time-spaced sensor data points.
  • reference analyte values and “reference data” as used herein are broad te ⁇ ns, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to data from a reference analyte monitor, such as a blood glucose meter, or the like, including one or more reference data points.
  • the reference glucose values are obtained from a self-monitored blood glucose (SMBG) test (for example, from a finger or forearm blood test) or an YSI (Yellow Springs Instruments) test, for example.
  • SMBG self-monitored blood glucose
  • YSI Yellow Springs Instruments
  • matched data pairs is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to reference data (for example, one or more reference analyte data points) matched with substantially time corresponding sensor data (for example, one or more sensor data points).
  • interferants and “interfering species” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and they are not to be limited to a special or customized meaning), and refer without limitation to effects and/or species that interfere with the measurement of an analyte of interest in a sensor to produce a signal that does not accurately represent the analyte measurement.
  • interfering species ' are compounds with an oxidation potential that overlaps with the analyte to be measured, producing a false positive signal.
  • interfering species are substantially non-constant compounds (e.g., the concentration of an interfering species fluctuates over time).
  • Interfering species include but are not limited to compounds with electroactive acidic, amine or sulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides, amino acids, amino acid precursors or break-down products, nitric oxide (NO), NO-donors, NO-precursors, acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine, dopamine, ephedrine, ibuprofen, L-dopa, methyl dopa, salicylate, tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid electroactive species produced during cell metabolism and/or wound healing, electroactive species that arise during body pH changes and the like.
  • NO nitric oxide
  • NO-donors NO-precursors
  • acetaminophen ascorbic acid
  • bilirubin cholesterol
  • creatinine dopamine
  • bifunctional as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to having or serving two functions.
  • a metal wire is bifunctional because it provides structural support and acts as an electrical conductor.
  • electrical conductor is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning) and refers without limitation to materials that contain movable charges of electricity.
  • an electric potential difference is impressed across separate points on a conductor, the mobile charges within the conductor are forced to move, and an electric current between those points appears in accordance with Ohm's law.
  • electrical conductance is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning) and refers without limitation to the propensity of a material to behave as an electrical conductor. In some embodiments, the term refers to a sufficient amount of electrical conductance (e.g., material property) to provide a necessary function (electrical conduction).
  • insulative properties are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning) and refers without limitation to the tendency of materials that lack mobile charges to prevent movement of electrical charges between two points.
  • an electrically insulative material may be placed between two electrically conductive materials, to prevent movement of electricity between the two electrically conductive materials.
  • the terms refer to a sufficient amount of insulative property (e.g., of a material) to provide a necessary function (electrical insulation).
  • insulator and “non-conductive material” can be used interchangeably herein.
  • structural support is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning) and refers without limitation to the tendency of a material to keep the sensor's structure stable or in place.
  • structural support can include "weight bearing” as well as the tendency to hold the parts or components of a whole structure together.
  • a variety of materials can provide "structural support" to the sensor.
  • a diffusion barrier is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning) and refers without limitation to something that obstructs the random movement of compounds, species, atoms, molecules, or ions from one site in a medium to another.
  • a diffusion barrier is structural, such as a wall that separates two working electrodes and substantially prevents diffusion of a species from one electrode to the other.
  • a diffusion barrier is spatial, such as separating working electrodes by a distance sufficiently large enough to substantially prevent a species at a first electrode from affecting a second electrode.
  • a diffusion barrier can be temporal, such as by turning the first and second working electrodes on and off, such that a reaction at a first electrode will not substantially affect the function of the second electrode.
  • At least a portion (e.g., the in vivo portion) of the sensor is formed from at least one platinum wire at least partially covered with an insulative coating, which is at least partially helically wound with at least one additional wire, the exposed electroactive portions of which are covered by a membrane system (see description of Fig. IB or 9B); in this exemplary embodiment, each element of the sensor is formed as an integral part of the sensor (e.g., both functionally and structurally). . :
  • coaxial is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to having a common axis, having coincident axes or mounted on concentric shafts.
  • twisted is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to united by having one part or end turned in the opposite direction to the other, such as, but not limited to the twisted strands of fiber in a string, yarn, or cable.
  • helix as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a spiral or coil, or something in the form of a spiral or coil (e.g. a corkscrew or a coiled spring).
  • a helix is a mathematical curve that lies on a cylinder or cone and makes a constant angle with the straight lines lying in the cylinder or cone.
  • a "double helix” is a pair of parallel helices intertwined about a common axis, such as but not limited to that in the structure of DNA.
  • in vivo portion is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a portion of a device that is to be implanted or inserted into the host.
  • an in vivo portion of a transcutaneous sensor is a portion of the sensor that is inserted through the host's skin and resides within the host.
  • background is composed substantially of signal contribution due to factors other than glucose (for example, interfering species, non- reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that overlaps with hydrogen peroxide).
  • the background comprises components related to constant and non- constant factors.
  • the term "constant noise” and “constant background” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refer without limitation to the component of the background signal that remains relatively constant over time.
  • constant background noise can slowly drift over time (e.g., increases or decreases), however this drift need not adversely affect the accuracy of a sensor, for example, because a sensor can be calibrated and re-calibrated and/or the drift measured and compensated for.
  • non-constant noise or non-constant background
  • non-constant background refers without limitation to a component of the background signal that is relatively non- constant, for example, transient and/or intermittent.
  • certain electroactive compounds are relatively non-constant (e.g., intermittent interferents due to the host's ingestion, metabolism, wound healing, and other mechanical, chemical and/or biochemical factors), which create intermittent (e.g., non-constant) "noise" on the sensor signal that can be difficult to "calibrate out” using a standard calibration equations (e.g., because the background of the signal does not remain constant).
  • intermittent interferents due to the host's ingestion, metabolism, wound healing, and other mechanical, chemical and/or biochemical factors
  • inactive enzyme or “inactivated enzyme” as used herein are broad terms, and are to be given their ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refer without limitation to an enzyme (e.g., glucose oxidase, GOx) that has been rendered inactive (e.g., "killed” or “dead”) and has no enzymatic activity.
  • Enzymes can be inactivated using a variety of techniques known in the art, such as but not limited to heating, freeze-thaw, denaturing in organic solvent, acids or bases, cross-linking, genetically changing enzymatically critical amino acids, and the like.
  • a solution containing active enzyme can be applied to the sensor, and the applied enzyme subsequently inactivated by heating or treatment with an inactivating solvent.
  • non-enzymatic as used herein is a broad term, and is to be given their ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a lack of enzyme activity.
  • a "non-enzymatic" membrane portion contains no enzyme; while in other embodiments, the "non-enzymatic" membrane portion contains inactive enzyme.
  • an enzyme solution containing inactive enzyme or no enzyme is applied.
  • GOx is a broad term, and is to be given their ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to the enzyme Glucose Oxidase (e.g., GOx is an abbreviation).
  • the preferred embodiments provide a continuous analyte sensor that measures a concentration of the analyte of interest or a substance indicative of the concentration or presence of the analyte.
  • the analyte sensor is an invasive, minimally invasive, or non-invasive device, for example a subcutaneous, transdermal, or intravascular device.
  • the analyte sensor may analyze a plurality of intermittent biological samples.
  • the analyte sensor may use any method of analyte-measurement, including enzymatic, chemical, physical, electrochemical, spectrophotometric, polarimetric, calorimetric, radiometric, or the like.
  • analyte sensors provide at least one working electrode and at least one reference electrode, which are configured to measure a signal associated with a concentration of the analyte in the host, such as described in more detail below, and as appreciated by one skilled in the art.
  • the output signal is typically a raw data stream that is used to provide a useful value of the measured analyte concentration in a host to the patient or doctor, for example.
  • the analyte sensors of the preferred embodiments may further measure at least one additional signal.
  • the additional signal is associated with the baseline and/or sensitivity of the analyte sensor, thereby enabling monitoring of baseline and/or sensitivity changes that may occur in a continuous analyte sensor over time.
  • continuous analyte sensors define a relationship between sensor-generated measurements (for example, current in nA or digital counts after AfD conversion) and a reference measurement (for example, mg/dL or mmol/L) that are meaningful to a user (for example, patient or doctor).
  • a reference measurement for example, mg/dL or mmol/L
  • the sensing mechanism generally depends on phenomena that are linear with glucose concentration, for example: (1) diffusion of glucose through a membrane system (for example, biointerface membrane and membrane system) situated between implantation site and the electrode surface, (2) an enzymatic reaction within the membrane system (for example, membrane system), and (3) diffusion of the H 2 O 2 to the sensor.
  • y mx + b
  • y the sensor signal (counts)
  • x the estimated glucose concentration (mg/dL)
  • m the sensor sensitivity to glucose (counts/mg/dL)
  • b the baseline signal (counts). Because both sensitivity m and baseline (background) h change over time in vivo, calibration has conventionally required at least two independent, matched data pairs (xj, yu % 2 , yi) to solve for m and b and thus allow glucose estimation when only the sensor signal, y is available.
  • Matched data pairs can be created by matching reference data (for example, one or more reference glucose data points from a blood glucose meter, or the like) with substantially time corresponding sensor data (for example, one or more glucose sensor data points) to provide one or more matched data pairs, such as described in co- pending U.S. Publication No. US-2005-0027463-A1.
  • reference data for example, one or more reference glucose data points from a blood glucose meter, or the like
  • sensor data for example, one or more glucose sensor data points
  • the sensing region is configured to measure changes in sensitivity of the analyte sensor over time, which can be used to trigger calibration, update calibration, avoid inaccurate calibration (for example, calibration during unstable periods), and/or trigger filtering of the sensor data.
  • the analyte sensor is configured to measure a signal associated with a non-analyte constant in the host.
  • the non-analyte constant signal is measured beneath the membrane system on the sensor.
  • a non-glucose constant that can be measured is oxygen, wherein a measured change in oxygen transport is indicative of a change in the sensitivity of the glucose signal, which can be measured by switching the bias potential of the working electrode, an auxiliary oxygen-measuring electrode, an oxygen sensor, or the like, as described in more detail elsewhere herein.
  • the sensing region is configured to measure changes in the amount of background noise (e.g., baseline) in the signal, which can be used to trigger calibration, update calibration, avoid inaccurate calibration (for example, calibration during unstable periods), and/or trigger filtering of the sensor data.
  • the baseline is composed substantially of signal contribution due to factors other than glucose (for example, interfering species, non- reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that overlaps with hydrogen peroxide).
  • the glucose sensor is configured to measure a signal associated with the baseline (all non-glucose related current generated) measured by sensor in the host.
  • an auxiliary electrode located beneath a non- enzymatic portion of the membrane system is used to measure the baseline signal.
  • the baseline signal is subtracted from the glucose signal (which includes the baseline) to obtain the signal contribution substantially only due to glucose. Subtraction may be accomplished electronically in the sensor using a differential amplifier, digitally in the receiver, and/or otherwise in the hardware or software of the sensor or receiver as is appreciated by one skilled in the art, and as described in more detail elsewhere herein.
  • sensors of the preferred embodiments describe a variety of sensor configurations, wherein each sensor generally comprises two or more working electrodes, a reference and/or counter electrode, an insulator, and a membrane system.
  • the sensors can be configured to continuously measure an analyte in a biological sample, for example, in subcutaneous tissue, in a host's blood flow, and the like.
  • each exemplary sensor design (e.g., Figs. IA, 2A, 7A through 9B, and 11) includes a first working electrode, wherein the working electrode is formed from known materials.
  • each electrode is formed from a fine wire with a diameter of from about 0.001 or less to about 0.010 inches or more, for example, and is formed from, e.g., a plated insulator, a plated wire, or bulk electrically conductive material.
  • the working electrode comprises a wire formed from a conductive material, such as platinum, platinum-iridium, palladium, graphite, gold, carbon, conductive polymer, alloys, or the like.
  • the electrodes can by formed by a variety of manufacturing techniques (bulk metal processing, deposition of metal onto a substrate, and the like), it can be advantageous to form the electrodes from plated wire (e.g., platinum on steel wire) or bulk metal (e.g., platinum wire). It is believed that electrodes formed from bulk metal wire provide superior performance (e.g., in contrast to deposited electrodes), including increased stability of assay, simplified manufacturability, resistance to contamination (e.g., which can be introduced in deposition processes), and improved surface reaction (e.g., due to purity of material) without peeling or delamination.
  • plated wire e.g., platinum on steel wire
  • bulk metal e.g., platinum wire
  • the working electrode is configured to measure the concentration of an analyte.
  • the working electrode measures the hydrogen peroxide produced by an enzyme catalyzed reaction of the analyte being detected and creates a measurable electronic current.
  • hydrogen peroxide H 2 O 2
  • the working electrode reacts with the surface of the working electrode producing two protons (2H + ), two electrons (2e " ) and one molecule of oxygen (O 2 ), which produces the electronic current being detected.
  • each exemplary sensor design includes at least one additional working electrode configured to measure a baseline (e.g., background noise) signal, to measure another analyte (e.g., oxygen), to generate oxygen, and/or as a transport-measuring electrode, all of which are described in more detail elsewhere herein.
  • a baseline e.g., background noise
  • another analyte e.g., oxygen
  • the additional working electrode(s) can be formed as described with reference to the first working electrode.
  • the auxiliary (additional) working electrode is configured to measure a background signal, including constant and non-constant analyte signal components.
  • each exemplary sensor design includes a reference and/or counter electrode.
  • the reference electrode has a configuration similar to that described elsewhere herein with reference to the first working electrode, however may be formed from materials, such as silver, silver/silver chloride, calomel, and the like.
  • the reference electrode is integrally formed with the one or more working electrodes, however other configurations are also possible (e.g., remotely located on the host's skin, or otherwise in bodily fluid contact).
  • the reference electrode is helically wound around other component(s) of the sensor system.
  • the reference electrode is disposed remotely from the sensor, such as but not limited to on the host's skin, as described herein.
  • each exemplary sensor design (e.g., Figs. IA, 2A, 7A through 9B, and 11) includes an insulator (e.g., non-conductive material) or similarly functional component.
  • an insulator e.g., non-conductive material
  • one or more electrodes are covered with an insulating material, for example, a non-conductive polymer. Dip-coating, spray-coating, vapor- deposition, or other coating or deposition techniques can be used to deposit the insulating material on the electrode(s).
  • the insulator is a separate component of the system (e.g., see Fig. 7E) and can be formed as is appreciated by one skilled in the art.
  • the insulating material comprises parylene, which can be an advantageous polymer coating for its strength, lubricity, and electrical insulation properties.
  • parylene is produced by vapor deposition and polymerization of para-xylylene (or its substituted derivatives).
  • any suitable insulating material can be used, for example, fluorinated polymers, polyethyleneterephthalate, polyurethane, polyimide, other nonconducting polymers, or the like. Glass or ceramic materials can also be employed.
  • Other materials suitable for use include surface energy modified coating systems such as are marketed under the trade names AMC 18, AMC 148, AMC 141, and AMC321 by Advanced Materials Components Express of Bellafonte, PA.
  • each exemplary sensor design (e.g., Figs. IA, 2A, 7A through 9B, and 11) includes exposed electroactive area(s).
  • a portion of the coated electrode(s) can be stripped or otherwise removed, for example, by hand, excimer lasing, chemical etching, laser ablation, grit-blasting (e.g., with sodium bicarbonate or other suitable grit), and the like, to expose the electroactive surfaces.
  • grit-blasting e.g., with sodium bicarbonate or other suitable grit
  • a portion of the electrode can be masked prior to depositing the insulator in order to maintain an exposed electroactive surface area.
  • grit blasting is implemented to expose the electroactive surfaces, preferably utilizing a grit material that is sufficiently hard to ablate the polymer material, while being sufficiently soft so as to minimize or avoid damage to the underlying metal electrode (e.g., a platinum electrode).
  • a grit material that is sufficiently hard to ablate the polymer material, while being sufficiently soft so as to minimize or avoid damage to the underlying metal electrode (e.g., a platinum electrode).
  • grit a variety of "grit” materials can be used (e.g., sand, talc, walnut shell, ground plastic, sea salt, and the like)
  • sodium bicarbonate is an advantageous grit-material because it is sufficiently hard to ablate, a coating (e.g., parylene) without damaging, an underlying conductor (e.g., platinum).
  • One additional advantage of sodium bicarbonate blasting includes its polishing action on the metal as it strips the polymer layer, thereby eliminating a cleaning step that might otherwise be necessary.
  • the tip (e.g., end) of the sensor is cut to expose electroactive surface areas, without a need for removing insulator material from sides of insulated electrodes. In general, a variety of surfaces and surface areas can be exposed.
  • each exemplary sensor design includes a membrane system.
  • a membrane system is deposited over at least a portion of the electroactive surfaces of the sensor (working electrode(s) and optionally reference electrode) and provides protection of the exposed electrode surface from the biological environment, diffusion resistance (limitation) of the analyte if needed, a catalyst for enabling an enzymatic reaction, limitation or blocking of interferents, and/or hydrophilicity at the electrochemically reactive surfaces of the sensor interface.
  • the membrane system includes a plurality of domains, for example, one or more of an electrode domain 24, an optional interference domain 26, an enzyme domain 28 (for example, including glucose oxidase), and a resistance domain 30, as shown in Figs. 2A and 2B, and can include a high oxygen solubility domain, and/or a bioprotective domain (not shown), such as is described in more detail in U.S. Publication No. US-2005-0245799-A1, and such as is described in more detail below.
  • the membrane system can be deposited on the exposed electroactive surfaces using known thin film techniques (for example, vapor deposition, spraying, electro-depositing, dipping, or the like).
  • vapor deposition processes e.g., physical and/or chemical vapor deposition processes
  • other vapor deposition processes can be useful for providing one or more of the insulating and/or membrane layers, including ultrasonic vapor deposition, electrostatic deposition, evaporative deposition, deposition by sputtering, pulsed laser deposition, high velocity oxygen fuel deposition, thermal evaporator deposition, electron beam evaporator deposition, deposition by reactive sputtering molecular beam epitaxy, atmospheric pressure chemical vapor deposition (CVD), atomic layer CVD, hot wire CVD, low-pressure CVD, microwave plasma-assisted CVD, plasma-enhanced CVD, rapid thermal CVD, remote plasma-enhanced CVD, and ultrahigh vacuum CVD, for example.
  • the membrane system can be disposed over (or deposited on) the electroactive surfaces using any known method, as will be appreciated by one skilled in the art.
  • one or more domains of the membrane systems are formed from materials such as silicone, polytetrafluoroethylene, polyethylene-co- tetrafluoroethyiene, polyolefm, polyester, polycarbonate, biostable polytetrafluoroethylene, homopolymers, copolymers, terpolymers of polyurethanes, polypropylene (PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutylene terephthalate (PBT), polymethylmethacrylate (PMMA), polyether ether ketone (PEEK), polyurethanes, cellulosic polymers, polysulfones and block copolymers thereof including, for example, di-block, tri- block, alternating, random and graft copolymers.
  • PP polypropylene
  • PVC polyvinylchloride
  • PVDF polyvinylidene fluoride
  • PBT polybutylene terephthalate
  • the membrane system comprises an electrode domain 24 (Figs. 2A-2B).
  • the electrode domain is provided to ensure that an electrochemical reaction occurs between the electroactive surfaces of the working electrode and the reference electrode, and thus the electrode domain is preferably situated more proximal to the electroactive surfaces than the interference and/or enzyme domain.
  • the electrode domain includes a coating that maintains a layer of water at the electrochemically reactive surfaces of the sensor.
  • the electrode domain is present to provide an environment between the surfaces of the working electrode and the reference electrode, which facilitates an electrochemical reaction between the electrodes.
  • a humectant in a binder material can be employed as an electrode domain; this allows for the full transport of ions in the aqueous environment.
  • the electrode domain can also assist in stabilizing the operation of the sensor by accelerating electrode start-up and drifting problems caused by inadequate electrolyte.
  • the material that forms the electrode domain can also provide an environment that protects against pH-mediated damage that can result from the formation of a large pH gradient due to the electrochemical activity of the electrodes.
  • the electrode domain includes a flexible, water- swellable, hydrogel film having a "dry film” thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns.
  • “Dry film” thickness refers to the thickness of a cured film cast from a coating formulation by standard coating techniques.
  • the electrode domain is formed of a curable mixture of a urethane polymer and a hydrophilic polymer.
  • Particularly preferred coatings are formed of a polyurethane polymer having carboxylate or hydroxyl functional groups and non- ionic hydrophilic polyether segments, wherein the polyurethane polymer is crosslinked with a water-soluble carbodiimide ⁇ e.g., l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the presence of polyvinylpyrrolidone and cured at a moderate temperature of about 5O 0 C.
  • a water-soluble carbodiimide ⁇ e.g., l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)
  • the electrode domain is formed from a hydrophilic polymer such as polyvinylpyrrolidone (PVP).
  • PVP polyvinylpyrrolidone
  • An electrode domain formed from PVP has been shown to reduce break-in time of analyte sensors; for example, a glucose sensor utilizing a cellulosic-based interference domain such as described in more detail below.
  • the electrode domain is deposited by vapor deposition, spray coating, dip coating, or other thin film techniques on the electroactive surfaces of the sensor.
  • the electrode domain is formed by dip-coating the electroactive surfaces in an electrode layer solution and curing the domain for a time of from about 15 minutes to about 30 minutes at a temperature of from about 40°C to about 55°C (and can be accomplished under vacuum (e.g., 20 to 30 mmHg)).
  • a preferred insertion rate of from about 1 to about 3 inches per minute into the electrode layer solution, with a preferred dwell time of from about 0.5 to about 2 minutes in the electrode layer solution, and a preferred withdrawal rate of from about 0.25 to about 2 inches per minute from the electrode layer solution provide a functional coating.
  • values outside of those set forth above can be acceptable or even desirable in certain embodiments, for example, depending upon solution viscosity and solution surface tension, as is appreciated by one skilled in the art.
  • the electroactive surfaces of the electrode system are dip-coated one time (one layer) and cured at 50°C under vacuum for 20 minutes.
  • an independent electrode domain is described herein, in some embodiments sufficient hydrophilicity can be provided in the interference domain and/or enzyme domain (the domain adjacent to the electroactive surfaces) so as to provide for the full transport of ions in the aqueous environment (e.g. without a distinct electrode domain). In these embodiments, an electrode domain is not necessary.
  • Interferents are molecules or other species that are reduced or oxidized at the electrochemically reactive surfaces of the sensor, either directly or via an electron transfer agent, to produce a false positive analyte signal.
  • an optional interference domain 26 is provided that substantially restricts, resists, or blocks the flow of one or more interfering species (Figs. 2A-2B).
  • Some known interfering species for a glucose sensor include acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine, dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate, tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid.
  • the interference domain of the preferred embodiments is less permeable to one or more of the interfering species than to the analyte, e.g., glucose.
  • the interference domain is formed from one or more cellulosic derivatives.
  • cellulosic derivatives include polymers such as cellulose acetate, cellulose acetate butyrate, 2-hydroxyethyl cellulose, cellulose acetate phthalate, cellulose acetate propionate, cellulose acetate trimellitate, and the like.
  • the interference domain is formed from cellulose acetate butyrate.
  • Cellulose acetate butyrate with a molecular weight of about 10,000 daltons to about 75,000 daltons, preferably from about 15,000, 20,000, or 25,000 daltons to about 50,000, 55,000, 60,000, 65,000, or 70,000 daltons, and more preferably about 20,000 daltons is employed. In certain embodiments, however, higher or lower molecular weights can be preferred.
  • a casting solution or dispersion of cellulose acetate butyrate at a weight percent of about 15% to about 25%, preferably from about 15%, 16%, 17%, 18%, 19% to about 20%, 21%, 22%, 23%, 24% or 25%, and more preferably about 18% is preferred.
  • the casting solution includes a solvent or solvent system, for example an acetone:ethanol solvent system. Higher or lower concentrations can be preferred in certain embodiments.
  • a plurality of layers of cellulose acetate butyrate can be advantageously combined to form the interference domain in some embodiments, for example, three layers can be employed.
  • cellulose acetate butyrate components with different molecular weights in a single solution, or to deposit multiple layers of cellulose acetate butyrate from different solutions comprising cellulose acetate butyrate of different molecular weights, different concentrations, and/or different chemistries (e.g., functional groups). It can also be desirable to include additional substances in the casting solutions or dispersions, e.g., functionalizing agents, crosslinking agents, other polymeric substances, substances capable of modifying the hydrophilicity/hydrophobicity of the resulting layer, and the like.
  • additional substances in the casting solutions or dispersions e.g., functionalizing agents, crosslinking agents, other polymeric substances, substances capable of modifying the hydrophilicity/hydrophobicity of the resulting layer, and the like.
  • the interference domain is formed from cellulose acetate.
  • Cellulose acetate with a molecular weight of about 30,000 daltons or less to about 100,000 daltons or more, preferably from about 35,000, 40,000, or 45,000 daltons to about 55,000, 60,000, 65,000, 70,000, 75,000, 80,000, 85,000, 90,000, or 95,000 daltons, and more preferably about 50,000 daltons is preferred.
  • a casting solution or dispersion of cellulose acetate at a weight percent of about 3% to about 10%, preferably from about 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, or 6.5% to about 7.5%, 8.0%, 8.5%, 9.0%, or 9.5%, and more preferably about 8% is preferred.
  • higher or lower molecular weights and/or cellulose acetate weight percentages can be preferred. It can be desirable to employ a mixture of cellulose acetates with molecular weights in a single solution, or to deposit multiple layers of cellulose acetate from different solutions comprising cellulose acetates of different molecular weights, different concentrations, or different chemistries (e.g., functional groups). It can also be desirable to include additional substances in the casting solutions or dispersions such as described in more detail above.
  • Layer(s) prepared from combinations of cellulose acetate and cellulose acetate butyrate, or combinations of layer(s) of cellulose acetate and layer(s) of cellulose acetate butyrate can also be employed to form the interference domain.
  • additional polymers such as Nafion®
  • cellulosic derivatives can be used in combination with cellulosic derivatives to provide equivalent and/or enhanced function of the interference domain.
  • a 5 wt % Nafion® casting solution or dispersion can be used in combination with a 8 wt % cellulose acetate casting solution or dispersion, e.g., by dip coating at least one layer of cellulose acetate and subsequently dip coating at least one layer Nafion® onto a needle-type sensor such as described with reference to the preferred embodiments. Any number of coatings or layers formed in any order may be suitable for forming the interference domain of the preferred embodiments.
  • more than one cellulosic derivative can be used to form the interference domain of the preferred embodiments.
  • the formation of the interference domain on a surface utilizes a solvent or solvent system in order to solvate the cellulosic derivative (or other polymer) prior to film formation thereon.
  • acetone and ethanol are used as solvents for cellulose acetate; however one skilled in the art appreciates the numerous solvents that are suitable for use with cellulosic derivatives (and other polymers).
  • the preferred relative amounts of solvent can be dependent upon the cellulosic derivative (or other polymer) used, its molecular weight, its method of deposition, its desired thickness, and the like.
  • a percent solute of from about 1% to about 25% is preferably used to form the interference domain solution so as to yield an interference domain having the desired properties.
  • the cellulosic derivative (or other polymer) used, its molecular weight, method of deposition, and desired thickness can be adjusted, depending upon one or more other of the parameters, and can be varied accordingly as is appreciated by one skilled in the art.
  • the interference domain includes a thin, hydrophobic membrane that is non-swellable and restricts diffusion of low molecular weight species.
  • the interference domain is permeable to relatively low molecular weight substances, such as hydrogen peroxide, but restricts the passage of higher molecular weight substances, including glucose and ascorbic acid.
  • Other systems and methods for reducing or eliminating interference species that can be applied to the membrane system of the preferred embodiments are described in U.S. Publication No. US-2005-0115832-A1, U.S. Publication No. US-2005- 0176136-A1, U.S. Publication No. US-2005-0161346-A1, and U.S. Publication No. US- 2005-0143635-A1.
  • a distinct interference domain is not included.
  • the interference domain is deposited directly onto the electroactive surfaces of the sensor for a domain thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 1, 1.5 or 2 microns to about 2.5 or 3 microns.
  • Thicker membranes can also be desirable in certain embodiments, but thinner membranes are generally preferred because they have a lower impact on the rate of diffusion of hydrogen peroxide from the enzyme membrane to the electrodes.
  • the membrane systems of the preferred embodiments can be formed and/or deposited on the exposed electroactive surfaces (e.g., one or more of the working and reference electrodes) using known thin film techniques (for example, casting, spray coating, drawing down, electro-depositing, dip coating, and the like), however casting or other known application techniques can also be utilized.
  • the interference domain is deposited by vapor deposition, spray coating, or dip coating.
  • the interference domain is formed by dip coating the sensor into an interference domain solution using an insertion rate of from about 20 inches/min to about 60 inches/min, preferably 40 inches/min, a dwell time of from about 0 minute to about 5 seconds, preferably 0 seconds, and a withdrawal rate of from about 20 inches/minute to about 60 inches/minute, preferably about 40 inches/minute, and curing (drying) the domain from about 1 minute to about 30 minutes, preferably from about 3 minutes to about 15 minutes (and can be accomplished at room temperature or under vacuum (e.g., 20 to 30 mmHg)).
  • a 3-minute cure (i.e., dry) time is preferred between each layer applied.
  • a 15 minute cure (i.e., dry) time is preferred between each layer applied.
  • the dip process can be repeated at least one time and up to 10 times or more.
  • the preferred number of repeated dip processes depends upon the cellulosic derivative(s) used, their concentration, conditions during deposition (e.g., dipping) and the desired thickness (e.g., sufficient thickness to provide functional blocking of (or resistance to) certain interferents), and the like.
  • 1 to 3 microns may be preferred for the interference domain thickness; however, values outside of these can be acceptable or even desirable in certain embodiments, for example, depending upon viscosity and surface tension, as is appreciated by one skilled in the art.
  • an interference domain is formed from three layers of cellulose acetate butyrate.
  • an interference domain is formed from 10 layers of cellulose acetate.
  • an interference domain is formed of one relatively thicker layer of cellulose acetate butyrate.
  • an interference domain is formed of four relatively thinner layers of cellulose acetate butyrate.
  • the interference domain can be formed using any known method and combination of cellulose acetate and cellulose acetate butyrate, as will be appreciated by one skilled in the art.
  • the electroactive surface can be cleaned prior to application of the interference domain.
  • the interference domain of the preferred embodiments can be useful as a bioprotective or biocompatible domain, namely, a domain that interfaces with host tissue when implanted in an animal (e.g., a human) due to its stability and biocompatibility.
  • Enzyme Domain a domain that interfaces with host tissue when implanted in an animal (e.g., a human) due to its stability and biocompatibility.
  • the membrane system further includes an enzyme domain 28 disposed more distally from the electroactive surfaces than the interference domain; however other configurations can be desirable (Figs. 2A-2B).
  • the enzyme domain provides an enzyme to catalyze the reaction of the analyte and its co-reactant, as described in more detail below.
  • the enzyme domain includes glucose oxidase; however other oxidases, for example, galactose oxidase or uricase oxidase, can also be used.
  • the sensor's response is preferably limited by neither enzyme activity nor co-reactant concentration.
  • enzymes including glucose oxidase (GOx)
  • GOx glucose oxidase
  • the enzyme domain is constructed of aqueous dispersions of colloidal polyurethane polymers including the enzyme.
  • the enzyme domain is constructed from an oxygen enhancing material, for example, silicone, or fluorocarbon, in order to provide a supply of excess oxygen during transient ischemia.
  • the enzyme is immobilized within the domain. See, e.g., U.S. Publication No. US-2005-0054909-A1.
  • the enzyme domain is deposited onto the interference domain for a domain thickness of from about 0.05 micron or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns.
  • the enzyme domain can be deposited directly onto the electroactive surfaces.
  • the enzyme domain is deposited by spray or dip coating.
  • the enzyme domain is formed by dip coating the interference domain coated sensor into an enzyme domain solution and curing the domain for from about 15 to about 30 minutes at a temperature of from about 4O 0 C to about 55°C (and can be accomplished under vacuum (e.g., 20 to 30 mmHg)).
  • a preferred insertion rate of from about 0.25 inch per minute to about 3 inches per minute, with a preferred dwell time of from about 0.5 minutes to about 2 minutes, and a preferred withdrawal rate of from about 0.25 inch per minute to about 2 inches per minute provides a functional coating.
  • the enzyme domain is formed by dip coating two times (namely, forming two layers) in an enzyme domain solution and curing at 5O 0 C under vacuum for 20 minutes.
  • the enzyme domain can be formed by dip coating and/or spray coating one or more layers at a predetermined concentration of the coating solution, insertion rate, dwell time, withdrawal rate, and/or desired thickness.
  • the membrane system includes a resistance domain 30 disposed more distal from the electroactive surfaces than the enzyme domain (Figs. 2A-2B).
  • a resistance domain 30 disposed more distal from the electroactive surfaces than the enzyme domain (Figs. 2A-2B).
  • an immobilized enzyme-based glucose sensor employing oxygen as co-reactant is preferably supplied with oxygen in non-rate-limiting excess in order for the sensor to respond linearly to changes in glucose concentration, while not responding to changes in oxygen concentration. Specifically, when a glucose-monitoring reaction is oxygen limited, linearity is not achieved above minimal concentrations of glucose.
  • a linear response to glucose levels can be obtained only for glucose concentrations of up to about 40 mg/dL. However, in a clinical setting, a linear response to glucose levels is desirable up to at least about 400 mg/dL.
  • the resistance domain includes a semipermeable membrane that controls the flux of oxygen and glucose to the underlying enzyme domain, preferably rendering oxygen in a non-rate-limiting excess.
  • the resistance domain exhibits an oxygen to glucose permeability ratio of from about 50:1 or less to about 400:1 or more, preferably about 200:1.
  • one-dimensional reactant diffusion is adequate to provide excess oxygen at all reasonable glucose and oxygen concentrations found in the subcutaneous matrix (See Rhodes et al, Anal. Chem., 66:1520-1529 (1994)).
  • a lower ratio of oxygen-to-glucose can be sufficient to provide excess oxygen by using a high oxygen solubility domain (for example, a silicone or fluorocarbon-based material or domain) to enhance the supply/transport of oxygen to the enzyme domain. If more oxygen is supplied to the enzyme, then more glucose can also be supplied to the enzyme without creating an oxygen rate-limiting excess.
  • a high oxygen solubility domain for example, a silicone or fluorocarbon-based material or domain
  • the resistance domain is formed from a silicone composition, such as is described in U.S. Publication No. US-2005-0090607-A1.
  • the resistance domain includes a polyurethane membrane with both hydrophilic and hydrophobic regions to control the diffusion of glucose and oxygen to an analyte sensor, the membrane being fabricated easily and reproducibly from commercially available materials.
  • a suitable hydrophobic polymer component is a polyurethane, or polyetherurethaneurea.
  • Polyurethane is a polymer produced by the condensation reaction of a diisocyanate and a difunctional hydroxyl-containing material.
  • a polyurethaneurea is a polymer produced by the condensation reaction of a diisocyanate and a difunctional amine-containing material.
  • Preferred diisocyanates include aliphatic diisocyanates containing from about 4 to about 8 methylene units.
  • Diisocyanates containing cycloaliphatic moieties can also be useful in the preparation of the polymer and copolymer components of the membranes of preferred embodiments.
  • the material that forms the basis of the hydrophobic matrix of the resistance domain can be any of those known in the art as appropriate for use as membranes in sensor devices and as having sufficient permeability to allow relevant compounds to pass through it, for example, to allow an oxygen molecule to pass through the membrane from the sample under examination in order to reach the active enzyme or electrochemical electrodes.
  • non-polyurethane type membranes examples include vinyl polymers, polyethers, polyesters, polyamides, inorganic polymers such as polysiloxanes and polycarbosiloxanes, natural polymers such as cellulosic and protein based materials, and mixtures or combinations thereof.
  • the hydrophilic polymer component is polyethylene oxide.
  • one useful hydrophobic-hydrophilic copolymer component is a polyurethane polymer that includes about 20% hydrophilic polyethylene oxide.
  • the polyethylene oxide portions of the copolymer are thermodynamically driven to separate from the hydrophobic portions of the copolymer and the hydrophobic polymer component.
  • the 20% polyethylene oxide-based soft segment portion of the copolymer used to form the final blend affects the water pick-up and subsequent glucose permeability of the membrane.
  • the resistance domain is formed from a silicone polymer modified to allow analyte (e.g., glucose) transport.
  • analyte e.g., glucose
  • the resistance domain is formed from a silicone polymer/hydrophobic-hydrophilic polymer blend.
  • the hydrophobic- hydrophilic polymer for use in the blend may be any suitable hydrophobic-hydrophilic polymer, including but not limited to components such as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid, polyethers such as polyethylene glycol or polypropylene oxide, and copolymers thereof, including, for example, di-block, tri-block, alternating, random, comb, star, dendritic, and graft copolymers (block copolymers are discussed in U.S. Patent Nos.
  • the hydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitable such polymers include, but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblock copolymers, alternating block copolymers of PEO-PPO, random copolymers of ethylene oxide and propylene oxide, and blends thereof. In some embodiments, the copolymers may be optionally substituted with hydroxy substituents.
  • PEO and PPO copolymers include the PLURONIC® brand of polymers available from BASF®. In one embodiment, PLURONIC® F- 127 is used. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Other suitable commercial polymers include, but are not limited to, SYNPERONICS® products available from UNIQEMA®.
  • the resistance domain is deposited onto the enzyme domain to yield a domain thickness of from about 0.05 microns or less to about 20 microns or more, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns.
  • the resistance domain is deposited onto the enzyme domain by vapor deposition, spray coating, or dip coating.
  • spray coating is the preferred deposition technique. The spraying process atomizes and mists the solution, and therefore most or all of the solvent is evaporated prior to the coating material settling on the underlying domain, thereby minimizing contact of the solvent with the enzyme.
  • the resistance domain is deposited on the enzyme domain by spray coating a solution of from about 1 wt. % to about 5 wt. % polymer and from about 95 wt. % to about 99 wt. % solvent.
  • a solution of resistance domain material including a solvent
  • Tetrahydrofuran (THF) is one solvent that minimally or negligibly affects the enzyme of the enzyme domain upon spraying.
  • Other solvents can also be suitable for use, as is appreciated by one skilled in the art.
  • each exemplary sensor design (e.g., Figs. IA, 2A, and 7A through 9B) includes electronic connections, for example, one or more electrical contacts configured to provide secure electrical contact between the sensor and associated electronics.
  • the electrodes and membrane systems of the preferred embodiments are coaxially formed, namely, the electrodes and/or membrane system all share the same central axis. While not wishing to be bound by theory, it is believed that a coaxial design of the sensor enables a symmetrical design without a preferred bend radius.
  • the coaxial design of the preferred embodiments do not have a preferred bend radius and therefore are not subject to regular bending about a particular plane (which can cause fatigue failures and the like).
  • non-coaxial sensors can be implemented with the sensor system of the preferred embodiments.
  • the coaxial sensor design of the preferred embodiments enables the diameter of the connecting end of the sensor (proximal portion) to be substantially the same as that of the sensing end (distal portion) such that a needle is able to insert the sensor into the host and subsequently slide back over the sensor and release the sensor from the needle, without slots or other complex multi-component designs, as described in detail in U.S. Publication No. US-2006-0063142-A1 and U.S. Application No. 11/360,250 filed February 22, 2006 and entitled "ANALYTE SENSOR,” which are incorporated in their entirety herein by reference.
  • the senor is an enzyme-based electrochemical sensor, wherein the glucose-measuring working electrode 16 (e.g., Figs. 1A-1B) measures the hydrogen peroxide produced by the enzyme catalyzed reaction of glucose being detected and creates a measurable electronic current (for example, detection of glucose utilizing glucose oxidase produces hydrogen peroxide (H 2 O 2 ) as a by product, H 2 O 2 reacts with the surface of the working electrode producing two protons (2H + ), two electrons (2e " ) and one molecule of oxygen (O 2 ) which produces the electronic current being detected, see Fig. 10), such as described in more detail elsewhere herein and as is appreciated by one skilled in the art.
  • the glucose-measuring working electrode 16 e.g., Figs. 1A-1B
  • H 2 O 2 reacts with the surface of the working electrode producing two protons (2H + ), two electrons (2e " ) and one molecule of oxygen (O 2 ) which produces the electronic current being detected
  • one or more potentiostat is employed to monitor the electrochemical reaction at the electroactive surface of the working electrode(s).
  • the potentiostat applies a constant potential to the working electrode and its associated reference electrode to determine the current produced at the working electrode.
  • the current that is produced at the working electrode (and flows through the circuitry to the counter electrode) is substantially proportional to the amount OfH 2 O 2 that diffuses to the working electrodes.
  • the output signal is typically a raw data stream that is used to provide a useful value of the measured analyte concentration in a host to the patient or doctor, for example.
  • Some alternative analyte sensors that can benefit from the systems and methods of the preferred embodiments include U.S. Patent No. 5,711,861 to Ward et al., U.S. Patent No. 6,642,015 to Vachon et al., U.S. Patent No. 6,654,625 to Say et al., U.S. Patent 6,565,509 to Say et al. , U.S. Patent No. 6,514,718 to Heller, U.S. Patent No. 6,465,066 to Essenfeld et al., U.S. Patent No. 6,214,185 to Offenbacher et al., U.S. Patent No. 5,310,469 to Cunningham et al., and U.S.
  • glucose sensor configurations are described in detail below, it should be understood that the systems and methods described herein can be applied to any device capable of continually or continuously detecting a concentration of analyte of interest and providing an output signal that represents the concentration of that analyte, for example oxygen, lactose, hormones, cholesterol, medicaments, viruses, or the like.
  • Fig. IA is a perspective view of an analyte sensor, including an implantable body with a sensing region including a membrane system disposed thereon.
  • the analyte sensor 10a includes a body 12 and a sensing region 14 including membrane and electrode systems configured to measure the analyte.
  • the sensor 10a is preferably wholly implanted into the subcutaneous tissue of a host, such as described in U.S. Publication No. US-2006-0015020-A1; U.S. Publication No. US-2005-0245799-A1; U.S. Publication No. US-2005-0192557-A1; U.S. Publication No.
  • the body 12 of the sensor 10a can be formed from a variety of materials, including metals, ceramics, plastics, or composites thereof.
  • the sensor is formed from thermoset molded around the sensor electronics.
  • U.S. Publication No. US-2004- 0199059-A1 discloses suitable configurations for the body, and is incorporated by reference in its entirety.
  • the sensing region 14 includes a glucose-measuring working electrode 16, an optional auxiliary working electrode 18, a reference electrode 20, and a counter electrode 24.
  • the sensing region 14 includes means to measure two different signals, 1) a first signal associated with glucose and non-glucose related electroactive compounds having a first oxidation potential, wherein the first signal is measured at the glucose-measuring working electrode disposed beneath an active enzymatic portion of a membrane system, and 2) a second signal associated with the baseline and/or sensitivity of the glucose sensor.
  • the second signal measures sensitivity
  • the signal is associated with at least one non-glucose constant data point, for example, wherein the auxiliary working electrode 18 is configured to measure oxygen.
  • the signal is associated with non-glucose related electroactive compounds having the first oxidation potential, wherein the second signal is measured at an auxiliary working electrode 18 and is disposed beneath a non-enzymatic portion of the membrane system, such as described in more detail elsewhere herein.
  • a membrane system (see Fig. 2A) is deposited over the electroactive surfaces of the sensor 10a and includes a plurality of domains or layers, such as described in more detail below, with reference to Figs. 2 A and 2B.
  • the membrane system may be disposed over (deposited on) the electroactive surfaces using methods appreciated by one skilled in the art. See U.S. Publication No. US-2006-0015020-A1.
  • the sensing region 14 comprises electroactive surfaces, which are in contact with an electrolyte phase (not shown), which is a free-flowing fluid phase disposed between the membrane system 22 and the electroactive surfaces.
  • the counter electrode is provided to balance the current generated by the species being measured at the working electrode.
  • the species being measured at the working electrode is H 2 O 2 .
  • Glucose oxidase catalyzes the conversion of oxygen and glucose to hydrogen peroxide and gluconate according to the following reaction:
  • H 2 O 2 Glucose + O2 -> Gluconate + H2O2
  • the change in H 2 O 2 can be monitored to determine glucose concentration because for each glucose molecule metabolized, there is a proportional change in the product H 2 O 2 (see Fig. 10).
  • Oxidation Of H 2 O 2 by the working electrode is balanced by reduction of ambient oxygen, enzyme generated H 2 O 2 , or other reducible species at the counter electrode.
  • the H 2 O 2 produced from the glucose oxidase reaction further reacts at the surface of the working electrode and produces two protons (2H + ), two electrons (2e ⁇ ), and one oxygen molecule (O 2 ).
  • one or more potentiostats are employed to monitor the electrochemical reaction at the electroactive surface of the working electrode(s).
  • the potentiostat applies a constant potential to the working electrode and its associated reference electrode to determine the current produced at the working electrode.
  • the current that is produced at the working electrode (and flows through the circuitry to the counter electrode) is substantially proportional to the amount OfH 2 O 2 that diffuses to the working electrodes.
  • the output signal is typically a raw data stream that is used to provide a useful value of the measured analyte concentration in a host to the patient or doctor, for example.
  • Fig. IB is a schematic view of an alternative exemplary embodiment of a continuous analyte sensor 10b, also referred to as an in-dwelling or transcutaneous analyte sensor in some circumstances, particularly illustrating the in vivo portion of the sensor.
  • the in vivo portion of the sensor 10b is the portion adapted for insertion under the host's skin, in a host's blood stream, or other biological sample, while an ex vivo portion of the sensor (not shown) is the portion that remains above the host's skin after sensor insertion and operably connects to an electronics unit.
  • the analyte sensor 10b is coaxial and includes three electrodes: a glucose-measuring working electrode 16, an optional auxiliary working electrode 18, and at least one additional electrode 20, which may function as a counter and/or reference electrode, hereinafter referred to as the reference electrode 20.
  • the sensor 10b may include the ability to measure two different signals, 1) a first signal associated with glucose and non-glucose related electroactive compounds having a first oxidation potential, wherein the first signal is measured at the glucose-measuring working electrode disposed beneath an active enzymatic portion of a membrane system, and 2) a second signal associated with the baseline and/or sensitivity of the glucose sensor, such as described in more detail above with reference to Fig. IA.
  • the analyte sensor of Fig. IB can have a variety of configurations.
  • the sensor is generally configured of a first working electrode, a second working electrode, and a reference electrode.
  • the first working electrode 16 is a central metal wire or plated non-conductive rod/filament/fiber and the second working and reference electrodes (20 and 18, respectively OR 18 and 20, respectively) are coiled around the first working electrode 16.
  • the first working electrode is a central wire, as depicted in Fig. IB
  • the second working electrode is coiled around the first working electrode
  • the reference electrode is disposed remotely from the sensor, as described herein.
  • first and second working electrodes (20 and 18) are coiled around a supporting rod 16 of insulating material.
  • the reference electrode (not shown) can be disposed remotely from the sensor, as described herein, or disposed on the non- conductive supporting rod 16.
  • the first and second working electrodes (20 and 18) are coiled around a reference electrode 16 (not to scale).
  • each electrode is formed from a fine wire, with a diameter in the range of 0.001 to 0.010 inches, for example, and may be formed from plated wire or bulk material, however the electrodes may be deposited on a substrate or other known configurations as is appreciated by one skilled in the art.
  • the glucose-measuring working electrode 16 comprises a wire formed from a conductive material, such as platinum, palladium, graphite, gold, carbon, conductive polymer, or the like.
  • the glucose-measuring working electrode 16 can be formed of a non-conductive fiber or rod that is plated with a conductive material.
  • the glucose-measuring working electrode 16 is configured and arranged to measure the concentration of glucose.
  • the glucose-measuring working electrode 16 is covered with an insulating material, for example a non-conductive polymer. Dip-coating, spray-coating, or other coating or deposition techniques can be used to deposit the insulating material on the working electrode, for example.
  • the insulating material comprises Parylene, which can be an advantageous conformal coating for its strength, lubricity, and electrical insulation properties, however, a variety of other insulating materials can be used, for " example, fluorinated polymers, polyethyleneterephthalate, polyurethane, polyimide, or the like.
  • the auxiliary working electrode 18 comprises a wire formed from a conductive material, such as described with reference to the glucose- measuring working electrode 16 above.
  • the reference electrode 20, which may function as a reference electrode alone, or as a dual reference and counter electrode, is formed from silver, Silver/Silver chloride, or the like.
  • the electrodes are juxtapositioned and/or twisted with or around each other; however other configurations are also possible.
  • the auxiliary working electrode 18 and reference electrode 20 may be helically wound around the glucose- measuring working electrode 16 as illustrated in Fig. IB.
  • the auxiliary working electrode 18 and reference electrode 20 may be formed as a double helix around a length of the glucose-measuring working electrode 16.
  • the working electrode, auxiliary working electrode and reference electrodes may be formed as a triple helix.
  • the assembly of wires may then be optionally coated together with an insulating material, similar to that described above, in order to provide an insulating attachment.
  • Electrodes may be included within the assembly, for example, a three-electrode system (including separate reference and counter electrodes) as is appreciated by one skilled in the art.
  • Figs. 2A and 2B are schematic views membrane systems in some embodiments that may be disposed over the electroactive surfaces of an analyte sensors of Fig. IA and IB, respectively, wherein the membrane system includes one or more of the following domains: a resistance domain 30, an enzyme domain 28, an optional interference domain 26, and an electrolyte domain 24, such as described in more detail below.
  • the membrane system 22 can be modified for use in other sensors, by including only one or more of the domains, additional domains not recited above, or for other sensor configurations.
  • the interference domain can be removed when other methods for removing interferants are utilized, such as an auxiliary electrode for measuring and subtracting out signal due to interferants.
  • an "oxygen antenna domain" composed of a material that has higher oxygen solubility than aqueous media so that it concentrates oxygen from the biological fluid surrounding the biointerface membrane can be added.
  • the oxygen antenna domain can then act as an oxygen source during times of minimal oxygen availability and has the capacity to provide on demand a higher rate of oxygen delivery to facilitate oxygen transport to the membrane. This enhances function in the enzyme reaction domain and at the counter electrode surface when glucose conversion to hydrogen peroxide in the enzyme domain consumes oxygen from the surrounding domains.
  • this ability of the oxygen antenna domain to apply a higher flux of oxygen to critical domains when needed improves overall sensor function.
  • the membrane system generally provides one or more of the following functions: 1) protection of the exposed electrode surface from the biological environment, 2) diffusion resistance (limitation) of the analyte, 3) a catalyst for enabling an enzymatic reaction, 4) optionally limitation or blocking of interfering species, and 5) hydrophilicity at the electrochemically reactive surfaces of the sensor interface, such as described in U.S. Publication No. US-2005-0245799-A1.
  • the membrane system additionally includes a cell disruptive domain, a cell impermeable domain, and/or an oxygen domain (not shown), such as described in more detail in U.S. Publication No. US-2005-0245799-A1.
  • a membrane system modified for other sensors for example, by including fewer or additional domains is within the scope of the preferred embodiments.
  • a sensor for transcutaneous, wholly implantable, or intravascular short-term or long-term use
  • integrally formed parts such as but not limited to a plurality of electrodes, a membrane system and an enzyme.
  • the parts may be coaxial, juxtapositioned, helical, bundled and/or twisted, plated and/or deposited thereon, extruded, molded, held together by another component, and the like.
  • the components of the electrode system are integrally formed, (e.g., without additional support, such as a supporting substrate), such that substantially all parts of the system provide essential functions of the sensor (e.g., the sensing mechanism or "in vivo" portion).
  • a first electrode can be integrally formed directly on a second electrode ⁇ e.g., electrically isolated by an insulator), such as by vapor deposition of a conductive electrode material, screen printing a conductive electrode ink or twisting two electrode wires together in a coiled structure.
  • Some embodiments provide an analyte sensor that is configured for insertion into a host and for measuring an analyte in the host, wherein the sensor includes a first working electrode disposed beneath an active enzymatic portion of a membrane (e.g., membrane system) on the sensor and a " second working electrode disposed beneath an inactive- or non-enzymatic portion of the membrane on the sensor.
  • the first and second working electrodes integrally form at least a portion of the sensor.
  • Fig. IB is a schematic view of a sensor in one embodiment.
  • the sensor is configured to be integrally formed and coaxial.
  • one or more electrodes are helically wound around a central core, all of which share axis A-A.
  • the central core 16 can be an electrode (e.g., a wire or metal-plated insulator) or a support made of insulating material.
  • the coiled electrodes 18, 20 are made of conductive material (e.g., plated wire, metal-plated polymer filaments, bulk metal wires, etc.) that is helically wound or twisted about the core 16.
  • at least the working electrodes are coated with an insulator I of non-conductive or dielectric material.
  • the core 16 is a first working electrode and can be substantially straight.
  • One of the coiled electrodes (18 or 20) is a second working electrode and the remaining coiled electrode is a reference or counter electrode.
  • the reference electrode can be disposed remotely from the sensor, such as on the host's skin or on the exterior of the sensor, for example.
  • this exemplary embodiment illustrates an integrally formed coaxial sensor, one skilled in the art appreciates a variety of alternative configurations.
  • the arrangement of electrodes is reversed, wherein the first working electrode is helically wound around the second working electrode core 16.
  • the reference electrode can form the central core 16 with the first and second working electrodes coiled there around.
  • the sensor can have additional working, reference and/or counter electrodes, depending upon the sensor's purpose.
  • one or more of the electrode wires are coated with an insulating material, to prevent direct contact between the electrodes.
  • a portion of the insulating material can be removed (e.g., etched, scraped or grit-blasted away) to expose an electroactive surface of the electrode.
  • An enzyme solution can be applied to the exposed electroactive surface, as described herein.
  • the electrodes each have first and second ends.
  • the electrodes can be of any geometric solid shape, such as but not limited to a cylinder having a circular or oval cross-section, a rectangle (e.g., extruded rectangle), a triangle (e.g., extruded triangle), an X- cross section, a Y-cross section, flower petal-cross sections, star-cross sections, melt-blown fibers loaded with conductive material (e.g., conductive polymers) and the like.
  • the first ends (e.g., an in vivo portion, "front end") of the electrodes are configured for insertion in the host and the second ends (e.g., an ex vivo portion, "back end") are configured for electrical connection to sensor electronics.
  • the sensor includes sensor electronics that collect data from the sensor and provide the data to the host in various ways. Sensor electronics are discussed in detail elsewhere herein.
  • Figs. 7Al and 7A2 are schematics of an analyte sensor in another embodiment.
  • Fig. 7Al is a side view and Fig. 7A2 is a side-cutaway view.
  • the sensor is configured to be integrally formed and coaxial, with an optional stepped end.
  • the sensor includes a plurality of electrodes El, E2, E3 to En, wherein n equals any number of electrode layers.
  • Layers of insulating material I e.g., non-conductive material
  • All of the electrode and insulating material layers share axis A-A.
  • the layers can be applied by any technique known in the art, such as but not limited to spraying, dipping, spraying, etc.
  • a bulk metal wire electrode El can be dipped into a solution of insulating polymer that is vulcanized to form a layer of non-conductive, electrically insulating material I.
  • a second electrode E2 can be plated (e.g., by electroplating or other plating technique used in the art) on the first insulating layer, followed by application of a second insulating layer I applied in the same manner as the first layer.
  • Additional electrode layers e.g., E3 to En
  • insulating layers can be added to the construct, to create the desired number of electrodes and insulating layers.
  • multiple sensors can be formed from a long wire (with insulating and electrode layers applied) that can be cut to yield a plurality of sensors of the desired length.
  • the various electrode and/or insulator layers can be applied by dipping, spraying, printing, vapor deposition, plating, spin coating or any other method known in the art.
  • this exemplary embodiment illustrates an integrally formed coaxial sensor, one skilled in the art appreciates a variety of alternative configurations.
  • the sensor can have two, three, four or more electrodes separated by insulating material I.
  • the analyte sensor has two or more electrodes, such as but not limited to a first working electrode, an auxiliary working electrode, a reference electrode and/or counter electrode. Fig.
  • a coiled first electrode El is manufactured from an electrically conductive tube or cylinder, such as but not limited to a silver Hypotube. A portion of the Hypotube is trimmed or carved into a helix or coil 702. A second electrode E2 that is sized to fit (e.g., with minimal tolerance) within the first electrode El mates (e.g., slides into) with the first electrode El, to form the sensor.
  • the surfaces of the electrodes are coated with an insulator, to prevent direct contact between the electrodes. As described herein, portion of the insulator can be stripped away to expose the electroactive surfaces.
  • the first electrode El is a reference or auxiliary electrode
  • the second electrode E2 is a working electrode
  • the first electrode El can be a working electrode
  • the second electrode E2 can be a reference or auxiliary electrode.
  • additional electrodes are applied to the construct (e.g., after E2 is inserted into El).
  • the silver Hypotube can be cut to increase or decrease the flexibility of the sensor.
  • the spiral cut can space the coils farther apart to increase the sensor's flexibility.
  • Another example of this configuration is that it is easier to construct the sensor in this manner, rather than winding one electrode around another (e.g., as is done for the embodiment shown in Fig. IB).
  • Figs. 7C to 7E are schematics of three embodiments of bundled analyte sensors.
  • the sensors are configured to be integrally formed sensors, wherein a plurality (El, E2, E3, to Ew) of electrodes are bundled, coiled or twisted to form a portion of the sensor.
  • the electrodes can be twisted or helically coiled to form a coaxial portion of the sensor, which share the same axis.
  • the first and second working electrodes are twisted or helically wound together, to form at least a portion of the sensor (e.g., a glucose sensor).
  • the electrodes can be twisted in a double helix.
  • additional electrodes are provided and twisted, coiled or wound with the first and second electrodes to form a larger super helix, such as a triple helix, a quadruple helix, or the like.
  • a larger super helix such as a triple helix, a quadruple helix, or the like.
  • three wires (El, E2, and E3) can be twisted to form a triple helix.
  • at least one reference electrode can be disposed remotely from the working electrodes, as described elsewhere herein.
  • the tip of the sensor can be cut at an angle (90° or other angle) to expose the electrode tips to varying extents, as described herein.
  • Fig. 7C is a schematic of an exemplary embodiment of a sensor having three bundled electrodes El, E2, and E3.
  • the electrodes can be non-identical.
  • the sensor can have a glucose-sensing electrode, an oxygen-sensing electrode and a reference electrode.
  • this exemplary embodiment illustrates a bundled sensor, one skilled in the art appreciates a variety of alternative sensor configurations. For example, only two electrodes can be used or more than three electrodes can be used.
  • holding one end of the bundled wires in a clamp and twisting the other end of the wires, to form a cable-like structure can coil the electrodes together.
  • Such a coiled structure can hold the electrodes together without additional structure (e.g., bound by a wire or coating).
  • non-coiled electrodes can be bundled and held together with a wire or fiber coiled there around, or by applying a coating of insulating material to the electrode bundle.
  • the reference electrode can be disposed remotely from the working electrodes, as described elsewhere herein.
  • Fig. 7D is a schematic view of a sensor in one embodiment.
  • the sensor is designed to be integrally formed and bundled and/or coaxial.
  • the sensor includes seven electrodes, wherein three electrodes of a first type (e.g., 3 x El) and three electrodes of a second type (e.g., 3 x E2) are bundled around one electrode of a third type (e.g., E3).
  • a first type e.g., 3 x El
  • 3 x E2 three electrodes of a second type
  • E3 e.g., E3
  • the different types of electrodes can be alternated or not alternated.
  • the two types of electrodes are alternately disposed around E3.
  • the two types of electrodes can be grouped around the central structure. As described herein, some or all of the electrodes can be coated with a layer of insulating material, to prevent direct contact between the electrodes.
  • the electrodes can be coiled together, as in a cable, or held together by a wire or fiber wrapping or a coating of insulating material.
  • the sensor can be cut, to expose the electroactive surfaces of the electrodes, or portions of the insulating material coating can be stripped away, as described elsewhere herein.
  • the sensor can include additional (or fewer) electrodes.
  • the El and E2 electrodes are bundled around a non-conductive core (e.g., instead of electrode E3), such as an insulated fiber.
  • El, E2, and E3 electrodes can be used (e.g., two El electrodes, two E2 electrodes, and three E3 electrodes).
  • additional electrode type can be included in the sensor (e.g., an electrode of type E4, E5 or E6, etc.).
  • three glucose-detecting electrodes (e.g., El) and three reference electrodes (e.g., E2) are bundled and (optionally) coiled around a central auxiliary working electrode (e.g., E3).
  • Fig. 7E is a schematic of a sensor in another embodiment.
  • two pairs of electrodes e.g., 2 x El and 2 x E2
  • Fibers or strands of insulating material I also separate the electrodes from each other.
  • the pair of El electrodes can be working electrodes and the pair of E2 electrodes can be reference and/or auxiliary electrodes.
  • the El electrodes are both glucose-detecting electrodes, a first E2 electrode is a reference electrode and a second E2 electrode is an auxiliary electrode.
  • one El electrode includes active GOx and measures a glucose-related signal; the other El electrode lacks active GOx and measures a non-glucose-related signal, and the E2 electrodes are reference electrodes.
  • one El electrode detects glucose and the other El electrode detects urea, and both E2 electrodes are reference electrodes.
  • Electrode size and insulating material size/shape are not constrained by their depiction of relative size in the Figures, which are schematic schematics intended for only illustrative purposes.
  • Fig. 7F is a schematic view of a cross-section of an integrally formed sensor in another embodiment.
  • the sensor is configured to be bifunctional.
  • the sensor includes two working electrodes E1/E2 separated by either a reference electrode R or an insulating material I.
  • the electrodes El, E2 and optionally the reference electrode R are conductive and support the sensor's shape.
  • the reference electrode R (or the insulating material I) can act as a diffusion barrier (D, described herein) between the working electrodes El, E2 and support the sensor's structure.
  • the working electrodes El, E2 can be relatively larger or smaller in scale, with regard to the reference electrode/insulator R/I separating them.
  • the working electrodes El, E2 are separated by a reference electrode that has at least 6-times the surface area of the working electrodes, combined. While the working electrodes El, E2 and reference electrode/insulator R/I are shown and semi-circles and a rectangle, respectively, one skilled in the art recognizes that these components can take on any geometry know in the art, such as but not limited to rectangles, cubes, cylinders, cones, and the like.
  • Fig. 7G is a schematic view of a sensor in yet another embodiment.
  • the sensor is configured to be integrally formed with a diffusion barrier D, as described herein.
  • the working electrodes El, E2 (or one working electrode and one counter electrode) are integrally formed on a substantially larger reference electrode R or an insulator I that substantially prevents diffusion of analyte or other species from one working electrode to another working electrode (e.g., from the enzymatic electrode (e.g., coated with active enzyme) to the non-enzymatic electrode (e.g., no enzyme or inactive enzyme)).
  • the reference electrode is designed to include an exposed electroactive surface area that is at least equal to, greater than, or more than about 2, 3, 4, 5, 6, 7, 8, 9, 10 or more times greater than the surface area of the working electrodes (e.g., combined).
  • the surface of the reference electrode is about 6 (e.g., about 6 to 20) or more times greater than the working electrodes.
  • each working electrode detects a separate analyte (e.g., glucose, oxygen, uric acid, nitrogen, pH, and the like).
  • one of the working electrodes is a counter electrode.
  • an enzyme solution containing active GOx is applied to the El electroactive surface, while an enzyme solution containing inactive GOx (or no GOx at all) is applied to the E2 electroactive surface.
  • this configuration allows the measurement of two signals. Electrode El measures both a signal related to glucose concentration and a signal that is not related to glucose concentration. Electrode E2 measures a signal that is not related to glucose concentration.
  • the sensor electronics as described herein, can use these data to calculate glucose concentration without signal due to non-glucose-related contributions.
  • Fig. 7H is a schematic view of a sensor in another embodiment.
  • the sensor is configured of a geometric solid (e.g., cylindrical) reference electrode R having two or more working electrodes El, E2 to En disposed within two or more grooves or channels carved in the sides of the reference electrode R (parallel to the axis of the reference electrode R).
  • the grooves are sized such that the electrodes El, E2 can snuggly fit therein.
  • the depth of the grooves can be configured that the electrode placed therein is externally exposed to a greater or lesser degree.
  • the opening to the groove may be wider or narrower.
  • a portion of an electrode protrudes from the groove in which the electrode has been disposed.
  • an insulator (e.g., I) takes the place of a reference electrode (which can be disposed elsewhere, such remotely as described in more detail elsewhere herein).
  • the reference electrode/insulator R/I can take any geometric structure known in the art, such as but not limited to cylinders, rectangles, cones, and the like.
  • the relative sizes of the working electrodes El, E2 and the reference electrode/insulator R/I can be varied to achieve a desired signal level, to enable the use of the desired voltage (e.g., to bias the sensor), and the like, as described herein.
  • a diffusion barrier D separates the working electrodes.
  • the diffusion barrier can be spatial, physical, or temporal.
  • the distance around the reference electrode e.g., from the first working electrode El to the second working electrode E2, around a portion of the circumference of the reference electrode R
  • the working electrodes are coated with a layer of insulating material I (e.g., non-conductive material or dielectric) to prevent direct contact between the working electrodes El, E2 and the reference electrode R.
  • a portion of the insulator I on an exterior surface of each working electrode is etched away, to expose the electrode's electroactive surface.
  • an enzyme solution e.g., containing active GOx
  • an enzyme solution is applied to the electroactive surfaces of both electrodes, and dried. Thereafter, the enzyme applied to one of the electroactive surfaces is inactivated.
  • enzymes can be inactivated by a variety of means, such as heat, treatment with inactivating (e.g., denaturing) solvents, proteolysis, laser irradiation or UV irradiation (e.g., at 254-320 nm).
  • the enzyme coating one of the electroactive surfaces can be inactivated by masking one of the electroactive surfaces/electrodes (e.g., El, temporarily covered with a UV-blocking material); irradiating the sensor with UV light (e.g., 254-320 nm; a wavelength that inactivates the enzyme, such as by cross-linking amino acid residues) and removing the mask.
  • UV light e.g., 254-320 nm; a wavelength that inactivates the enzyme, such as by cross-linking amino acid residues
  • an enzyme solution containing active enzyme is applied to a first electroactive surface (e.g., El) and an enzyme solution containing either inactivated enzyme or no enzyme is applied to the second electroactive surface (e.g., E2).
  • the enzyme-coated first electroactive surface e.g., El
  • the second electroactive surface e.g., E2
  • the sensor electronics can use the data collected from the two working electrodes to calculate the analyte-only signal.
  • the reference electrode is formed of at least two adjacent pieces shaped such that the working electrodes fill at least some space between them.
  • the at least two pieces can be any shape known in the art, as described herein.
  • the at least two pieces are symmetrical and/or mirror images of each other, but one skilled in the art will recognize that this is not a requirement.
  • an insulating material can be coated on the working electrodes and/or the reference electrode(s) to prevent contact there between.
  • the working electrodes can detect the same analyte or separate analytes, or one of the working electrodes may act as a counter electrode (e.g., auxiliary electrode).
  • a counter electrode e.g., auxiliary electrode
  • this exemplary embodiment illustrates one example of a sensor having a reference electrode R that is formed of at least two pieces shaped such that the working electrodes fill at least some space between the pieces, one skilled in the art appreciates that a variety of sensor configurations are possible.
  • the reference electrode can be formed of three or more pieces.
  • the sensor can be configured with more than two working electrodes (e.g., 3, 4, or 5 working electrodes, or more).
  • Fig. 7J is a schematic view of an integrally formed sensor in yet another embodiment.
  • the reference electrode R is formed in any desired extruded geometry, such as an approximate X-shape.
  • Two or more working electrodes El, E2 are disposed on substantially opposing sides of the reference electrode, with a diffusion barrier D between them.
  • the diffusion barrier is a physical diffusion barrier, namely the distance between the two working electrodes (e.g., around the reference electrode).
  • the electrodes are bundled and held together by a wrapping of wire or fiber.
  • the electrodes are twisted around the lengthwise axis of the extruded X-shaped reference electrode, to form a coaxial sensor.
  • the reference electrode can be Y-shapes, star-shaped, flower-shaped, scalloped, or any other convenient shape with multiple substantially isolated sides.
  • an insulating material I takes the place of the reference electrode of Fig. 7 J, which is remotely located.
  • a working electrode is replaced with a counter electrode.
  • the sensor components are bifunctional.
  • the electrodes and reference electrode provide electrical conduction and the sensor's structure.
  • the reference electrode (or insulating material) provides a physical diffusion barrier D.
  • the insulating material acts as insulator by preventing direct electrical contact between the electrodes.
  • the materials selected to construct the sensor determine the sensor's flexibility.
  • active enzyme is applied to the electroactive surface of at least one working electrode (e.g., El). In some embodiments, no enzyme (or inactivated enzyme) is applied to the electroactive surface of a second working electrode (e.g., E2).
  • a second enzyme is applied to the second working electrode (e.g., E2) such that the sensor can measure the signals of two different analytes (e.g., glucose and aureate or oxygen).
  • Fig. 7K is a schematic of a sensor in another embodiment.
  • the sensor is configured to be integrally formed of two working electrodes.
  • the sensor includes two electrodes El, E2 (e.g., metal wires), wherein each electrode is coated with a non-conductive material I (e.g., and insulator).
  • a non-conductive material I e.g., and insulator
  • an enzyme solution 702 (e.g., GOx for detecting glucose) is disposed within the space 701.
  • the second working electrode E2 extends substantially flush with the insulator I.
  • a membrane system 703 coats the electrodes.
  • a diffusion barrier D separates the working electrodes.
  • the first and second electrodes are separated by a distance D that substantially prevents diffusion of H 2 O 2 from the first electrode (e.g., with active enzyme) to the second electrode (e.g., without active enzyme).
  • Fig. 7L is a schematic of a sensor in one embodiment.
  • the sensor is designed to be integrally formed.
  • two electrodes El, E2 are embedded within an insulator I.
  • the sensor can be formed by embedding conductive wires within a dielectric, curing the dielectric and then cutting sensors of the desired length. The cut end provides the exposed electroactive electrode surfaces and can be polished or otherwise treated.
  • this exemplary embodiment illustrates one integrally formed sensor, one skilled in the art appreciates a variety of alternative configurations. For example, additional electrode wires can be embedded in the dielectric material.
  • a reference electrode e.g., wire or cylinder
  • the sensor can be coiled or wrapped around the sensor (e.g., on the surface of the insulator).
  • the reference electrode can be disposed remotely from the working electrodes El, E2, such as on the host's skin or on another portion of the sensor.
  • Fig. 7M is a schematic cross-sectional view of a sensor having multiple working and reference electrodes, in one embodiment. In some preferred embodiments, the sensor is integrally formed.
  • the senor includes a plurality of working electrodes (e.g., El, E2, E3) that are layered with a plurality of reference electrodes (e.g., Rl, R2, Rn).
  • the working electrodes are coated with an insulating material to prevent direct contact with adjacent reference electrodes.
  • the reference electrodes are also coated with insulative material.
  • layers of insulating material separate the layers.
  • at least one of the working electrodes is a counter electrode.
  • electroactive surfaces are exposed on one or more electrodes, such as by stripping away a portion of an insulating coating, such as on the sides of the sensor.
  • an extended electrode structure e.g., a long sandwich of electrode layers
  • the cut end includes the exposed electroactive surfaces of the electrodes.
  • An enzyme layer can be applied to one or more of the electroactive surfaces, as described herein.
  • the working electrodes can be configured to detect the same analyte (e.g., all electroactive surfaces coated with GOx glucose) or different analytes (e.g., one working electrode detects glucose, another detects oxygen and the third detects ureate), as described herein.
  • this exemplary embodiment illustrates a sensor having a plurality of working and reference electrodes, one skilled in the art appreciates a variety of alternative configurations.
  • the electrodes can be of various sizes, depending upon their purpose. For example, in one sensor, it may be preferred to use a 3 mm oxygen electrode, a 10 mm glucose electrode and a 4 mm counter electrode, all separated by reference electrodes.
  • each reference electrode can be functionally paired with a working electrode.
  • the electrodes can be pulsed on and off, such that a first reference electrode Rl is active only when the first working electrode El is active, and a second reference electrode R2 is active only when the second working electrode E2 is active.
  • a flat sensor e.g., disk-shaped
  • a flat sensor can be manufactured by sandwiching reference electrodes between working electrodes, cutting the sandwich into a cylinder, and the cutting the cylinder cross-wise (perpendicularly or at an angle) into disks.
  • Fig. 7N is a schematic cross-sectional view of the manufacture of an integrally formed sensor, in one embodiment.
  • at least two working electrodes (El, E2) and optionally a reference electrode R are embedded in a quantity 704 of insulating material I.
  • the working electrodes are separated by a diffusion barrier D.
  • the structure is shaped (e.g., carved, scraped or cut etc.) to the final sensor shape 705, such that excess insulation material is removed.
  • multiple sensors can be formed as an extended structure of electrode wires embedded in insulator, which is subsequently cut to the desired length, wherein the exposed electrode ends (e.g., at the cut surface) become the electroactive surfaces of the electrodes.
  • portions of the insulator adjacent to the electrodes e.g., windows
  • an enzyme solution can be applied to one or more of the electroactive surfaces, as described elsewhere herein.
  • a diffusion barrier D can comprise both the reference electrode R and the insulating material I, or only the reference electrode.
  • windows exposing the electroactive surfaces can be formed adjacent to each other (e.g., on the same side of the reference electrode) or on opposite sides of the reference electrode.
  • more working or reference electrodes can be included, and the working and reference electrodes can be of relatively larger or smaller size, depending upon the sensor's configuration and operating requirements (e.g., voltage and/or current requirements).
  • Figs. 8A and 8B are schematic views of a sensor in yet another embodiment.
  • Fig. 8A is a view of the cross-section and side of an in vivo portion of the sensor.
  • Fig. 8B is a side view of the ex vivo portion of the sensor (e.g., the portion that is connected to the sensor electronics, as described elsewhere herein).
  • a reference electrode R such as a silver or silver/silver chloride electrode
  • the working electrodes are separated by a diffusion barrier D that can include a physical barrier (provided by the reference electrode and/or the insulating material coatings), a spatial barrier (provided by staggering the electroactive surfaces of the working electrodes), or a temporal barrier (provided by oscillating the potentials between the electrodes).
  • the reference electrode R has a surface area at least 6-times the surface area of the working electrodes. Additionally, the reference electrode substantially can act as a spatial diffusion barrier between the working electrodes due to its larger size (e.g., the distance across the reference electrode, from one working electrode to another).
  • the electrodes can be held in position by wrapping with wire or a non- conductive fiber, a non-conductive sheath, a biointerface membrane coating, or the like.
  • the electroactive surfaces of the working electrodes are exposed.
  • the end of the sensor is cut off, to expose the ends of the wires.
  • the ends of the wires are coated with insulating material; and the electroactive surfaces are exposed by removing a portion of the insulating material (e.g., a window 802 cut into the side of the insulation coating the electrode).
  • the windows exposing the electroactive surfaces of the electrodes can be staggered (e.g., spaced such that one or more electrodes extends beyond the other one or more electrodes), symmetrically arranged or rotated to any degree; for example, to substantially prevent diffusion of electroactive species from one working electrode (e.g., 802a) to the other working electrode (e.g., 802b), as will be discussed in greater detail elsewhere herein.
  • the reference electrode is not coated with a nonconductive material.
  • the ex vivo end of the sensor is connected to the sensor electronics (not shown) by electrical connectors 804a, 804b, 804c.
  • the ex vivo end of the sensor is stepped.
  • the ex vivo end of the reference electrode R terminates within electrical connector 804a.
  • the ex vivo end of the first working electrode El is exposed (e.g., nonconductive material removed therefrom) and terminates a small distance past the reference electrode R, within electrical connector 804b.
  • the ex vivo end of the second working electrode E2 is exposed (e.g., nonconductive material removed therefrom) and terminates a small distance past the termination of the first working electrode El, within electrical connector 804c.
  • this exemplary embodiment illustrates one configuration of an integrally formed sensor
  • a portion of the in vivo portion of the sensor can be twisted and/or stepped.
  • More working, reference, and/or counter electrodes, as well as insulators, can be included.
  • the electrodes can be of relatively larger or smaller size, depending upon the sensor's intended function.
  • the electroactive surfaces can be staggered.
  • the reference electrode can be disposed remotely from the sensor, as described elsewhere herein.
  • the reference electrode shown in Fig. 8A can be replaced with a non-conductive support and the reference electrode disposed on the host's skin.
  • a portion of the ex vivo portion of the sensor can be twisted or coiled.
  • the working and reference electrodes can be of various lengths and configurations not shown in Fig. 8B.
  • the reference electrode R can be the longest (e.g., connect to electrical contact 804c) and the first second working electrode E2 can be the shortest (e.g., connect to electrical contact 804a).
  • the first working electrode El may be either the longest electrode (e.g., connect to electrical contact 804c) or the shortest electrode (e.g., connect to electrical contact 804a).
  • Fig. 9A is a schematic view that illustrates yet another exemplary embodiment of an integrally formed analyte sensor.
  • two working electrodes El, E2 are bundled together and substantially encircled with a cylindrical silver or silver/silver chloride reference electrode R (or the like).
  • the reference electrode can be crimped at a location 902, to prevent movement of the working electrodes El, E2 within the reference electrode R cylinder.
  • a reference electrode can be rolled or coiled around the working electrodes El, E2, to form the reference electrode R.
  • the working electrodes are at least partially insulated as described in more detail elsewhere herein; such as by coating with a non-conductive material, such as but not limited to Parylene.
  • a non-conductive material such as but not limited to Parylene.
  • Fig. 9B illustrates another embodiment of an integrally formed analyte sensor. Namely, two working electrodes El, E2 are bundled together with a silver or silver/silver chloride wire reference electrode R coiled there around. The reference electrode can be coiled tightly, to prevent movement of the working electrodes El, E2 within the reference electrode R coil.
  • windows 904a and 904b are formed on the working electrodes El, E2. Portions of the non-conductive material (e.g., insulator) coating each electrode is removed to form windows 904a and 904b. The electroactive surfaces of the electrodes are exposed via windows 904a and 904b. As described elsewhere herein, the electrode electroactive surfaces exposed through windows 904a and 904b are coated with a membrane system.
  • An active enzyme e.g., GOx is used if glucose is the analyte
  • the membrane covering the other window can include inactivated enzyme (e.g., GOx inactivated by heat, solvent, UV or laser irradiation, etc., as described herein) or no enzyme.
  • the electrode having active enzyme detects a signal related to the analyte concentration and non-analyte related signal (e.g., due to background, etc.).
  • the electrode having inactive enzyme or no enzyme detects substantially only the non-analyte related signal.
  • the windows 904a and 904b are separated or staggered by a distance D, which is selected to be sufficiently large that electroactive species (e.g., H 2 O 2 ) do not substantially diffuse from one window to the other (e.g., from 904a to 904b).
  • electroactive species e.g., H 2 O 2
  • active enzyme is included in the membrane covering window 904a and inactive enzyme is included in the membrane covering window 904b.
  • Distance D is configured to be large enough that H 2 O 2 cannot diffuse from window 904a to window 904b, which lacks active enzyme (as discussed elsewhere herein). In some embodiments, the distance D is at least about 0.020 inches or less to about 0.120 inches or more.
  • D is at least about 0.030 to about 0.050 inches. In other embodiments, D is at least about 0.090 to about 0.095 inches.
  • the diffusion barrier D can be spatial (discussed herein with relation to Figs. 9A and 9B), physical or temporal (see discussion of Diffusion Barriers herein and Fig. 10).
  • a physical diffusion barrier D such as but not limited to an extended non-conductive structure placed between the working electrodes (e.g., Fig. 8A), substantially prevents diffusion Of H 2 O 2 from one working electrode (having active enzyme) to another working electrode (having no active enzyme).
  • one of the windows 904a or 904b comprises an enzyme system configured to detect the analyte of interest (e.g., glucose or oxygen).
  • the other window comprises no active enzyme system (e.g., wherein the enzyme system lacks enzyme or wherein the enzyme has been de-activated).
  • the "enzyme system lacks enzyme” a layer may be applied, similar to an active enzyme layer, but without the actual enzyme included therein.
  • the enzyme can be inactivated (e.g., by heat or solvent) prior to addition to the enzyme system solution or the enzyme can be inactivated after application to the window.
  • an enzyme is applied to both windows 904a and 904b followed by deactivation of the enzyme in one window.
  • one window can be masked (e.g., to protect the enzyme under the mask) and the sensor then irradiated (to deactivate the enzyme in the unmasked window).
  • one of the enzyme-coated windows e.g., the first window but not the second window
  • an enzyme-deactivating solvent e.g., treated with a protic acid solution such a hydrochloric acid or sulfuric acid.
  • a window coated with GOx can be dipped in dimethyl acetamide (DMAC), ethanol, or tetrahydrofuran (THF) to deactivate the GOx.
  • DMAC dimethyl acetamide
  • THF tetrahydrofuran
  • the enzyme-coated window can be dipped into a hot liquid (e.g., water or saline) to deactivate the enzyme with heat.
  • a hot liquid e.g., water or saline
  • the design of the active and inactive enzyme window is at least partially dependent upon the sensor's intended use. In some embodiments, it is preferred to deactivate the enzyme coated on window 904a. In other embodiments, it is preferred to deactivate the enzyme coated on window 904b. For example, in the case of a sensor to be used in a host's blood stream, the choice depends upon whether the sensor will be inserted pointing upstream (e.g., against the blood flow) or pointing downstream (e.g., with the blood flow).
  • an intravascular sensor is inserted into the host's vein pointing upstream (against the blood flow), an enzyme coating on electrode El (window 904a) is inactivated (e.g., by dipping in THF and rinsing) and an enzyme coating on electrode E2 (in window 904b) is not inactivated (e.g., by not dipping in THF). Because the enzyme on the first electrode El (e.g., in window 904a) is inactive, electroactive species (e.g., H 2 O 2 ) will not be substantially generated at window 904a (e.g., the first electrode El generates substantially no H 2 O 2 to effect the second electrode E2).
  • electroactive species e.g., H 2 O 2
  • the active enzyme on the second electrode E2 (in window 904b) generates H 2 O 2 which at least partially diffuses down stream (away from the windows) and thus has no effect on the first electrode El, other features and advantages of spatial diffusion barriers are described in more detail elsewhere herein.
  • an intravascular sensor is inserted into the host's vein pointing downstream (with the blood flow), the enzyme coating on electrode El (window 904a) is active and the enzyme coating on electrode E2 (in window 904b) is inactive. Because window 904a is located farther downstream than window 904b, the H 2 O 2 produced by the enzyme in 904a diffuses downstream (away from window 904b), and therefore does not affect substantially electrode E2.
  • the enzyme is GOx
  • the sensor is configured to detect glucose. Accordingly, H 2 O 2 produced by the GOx in window 904a does not affect electrode E2, because the sensor is pointing downstream and the blood flow carries away the H 2 O 2 produced on electrode El.
  • Figs. 9A and 9B illustrate two embodiments of a sensor having a stepped second end (e.g., the back end, distal end or ex vivo end, described with reference to Fig. 8B) that connects the sensor to the sensor electronics.
  • each electrode terminates within an electrical connector 804 such as but not limited to an elastomeric electrical connector.
  • each electrode is of a different length, such that each electrode terminates within one of a plurality of sequential electrical connectors.
  • the reference electrode R is the shortest in length and terminates within the first electrical connector 804.
  • the first working electrode El is longer than the reference electrode R, and terminates within the second electrical connector 804.
  • the second working electrode E2 is the longest electrode and terminates within the third electrical connector 804.
  • the first working electrode El can be longer than the second working electrode E2. Accordingly, the second working electrode E2 would terminate within the second (e.g., middle) electrical connector 804 and the first working electrode El would terminate within the third (e.g., last) electrical connector 804.
  • the second ends of the sensor may be separated from each other to connect to non-parallel, non-sequential electrical connectors.
  • Fig. 11 is a schematic view of a sensor in yet another embodiment.
  • the sensor is integrally formed, coaxial, and has a stepped ex vivo end (e.g., back or second end).
  • Electrodes El, E2 and E3 are twisted to form a helix, such as a triple helix.
  • the electrodes are stepped and each electrode is individually connected to the sensor electronics by an electrical connector 804.
  • the electrode engages an electrical connector 804 that joins the electrode to the sensor electronics.
  • the second end of electrode El electrically connects electrical connector 1106.
  • each sensor component is difunctional, and provides electrical conductance, structural support, a diffusion barrier, or insulation (see description elsewhere herein).
  • this exemplary embodiment illustrates an integrally formed, coaxial sensor having a stepped back end, one skilled in the art appreciates a variety of alternative configurations.
  • one of the electrodes El, E2 or E3 can be a reference electrode, or the reference electrode can be disposed remotely from the sensor, such as but not limited to on the host's skin.
  • the sensor can have only two electrodes or more than three electrodes.
  • the reference electrode (and optionally a counter electrode) can be disposed remotely from the working electrodes.
  • the reference electrode R can be replaced with a non-conductive material, such as an insulator I.
  • the reference electrode R can then be inserted into the host in a location near to the sensor, applied to the host's skin, be disposed within a fluid connector, be disposed on the ex-vivo portion of the sensor or even disposed on the exterior of the sensor electronics.
  • Fig. 7L illustrates an embodiment in which the reference and/or counter electrode is located remotely from the first and second working electrodes El and E2, respectively.
  • the sensor is a needle-type sensor such as described with reference to Fig. IB, and the working electrodes El, E2 are integrally formed together with a substantially X-shaped insulator I and the reference electrode (and/or counter electrode) is placed on the host's skin (e.g., a button, plate, foil or wire, such as under the housing) or implanted transcutaneously in a location separate from the working electrodes.
  • the host's skin e.g., a button, plate, foil or wire, such as under the housing
  • one or more working electrodes can be inserted into the host's blood via a catheter and the reference and/or counter electrode can be placed within the a fluid connector (on the sensor) configured to be in fluid communication with the catheter; in such an example, the reference and/or counter electrode is in contact with fluid flowing through the fluid connector but not in direct contact with the host's blood.
  • the reference and/or counter electrodes can be placed exterior to the sensor, in bodily contact for example.
  • the surface area of the electroactive portion of the reference (and/or counter) electrode is at least six times the surface area of one or more working electrodes.
  • the reference (and/or counter) electrode surface is 1, 2, 3, 4, 5, 7, 8, 9 or 10 times the surface area of the working electrodes.
  • the reference (and/or counter) electrode surface area is 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 times the surface area of the working electrodes.
  • the surface area of the reference electrode e.g., 18 or 20
  • the exposed surface of the reference electrode such as but not limited to the electrode surface facing away from the working electrode 16.
  • the electrodes can be stacked or grouped similar to that of a leaf spring configuration, wherein layers of electrode and insulator (or individual insulated electrodes) are stacked in offset layers.
  • the offset layers can be held together with bindings of non-conductive material, foil, or wire.
  • the strength, flexibility, and/or other material property of the leaf spring-configured or stacked sensor can be either modified (e.g., increased or decreased), by varying the amount of offset, the amount of binding, thickness of the layers, and/or materials selected and their thicknesses, for example.
  • the senor e.g., a glucose sensor
  • the sensor is configured for implantation into the host.
  • the sensor may be wholly implanted into the host, such as but not limited to in the host's subcutaneous tissue (e.g., the embodiment shown in Fig. IA).
  • the sensor is configured for transcutaneous implantation in the host's tissue.
  • the sensor can have a portion that is inserted through the host's skin and into the underlying tissue, and another portion that remains outside the host's body (e.g., such as described in more detail with reference to Fig. IB).
  • the sensor is configured for indwelling in the host's blood stream.
  • a needle-type sensor can be configured for insertion into a catheter dwelling in a host's vein or artery.
  • the sensor can be integrally formed on the exterior surface of the catheter, which is configured to dwell within a host's vein or artery. Examples of indwelling sensors can be found in co-pending U.S. patent application / filed on even date herewith and entitled "ANALYTE SENSOR.”
  • the in vivo portion of the sensor can take alternative configurations, such as but not limited to those described in more detail with reference to Figs. 7A-9B and 11.
  • the analyte sensor substantially continuously measures the host's analyte concentration.
  • the sensor can measure the analyte concentration every fraction of a second, about every fraction of a minute or every minute.
  • the sensor measures the analyte concentration about every 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes.
  • the sensor measures the analyte concentration every fraction of an hour, such as but not limited to every 15, 30 or 45 minutes.
  • the sensor measures the analyte concentration about every hour or longer.
  • the sensor measures the analyte concentration intermittently or periodically.
  • the analyte sensor is a glucose sensor and measures the host's glucose concentration about every 4-6 minutes. In a further embodiment, the sensor measures the host's glucose concentration every 5 minutes.
  • the analyte sensor is a glucose sensor having a first working electrode configured to generate a first signal associated with both glucose and non-glucose related electroactive compounds that have a first oxidation potential.
  • Non-glucose related electroactive compounds can be any compound, in the sensor's local environment that has an oxidation potential substantially overlapping with the oxidation potential of H 2 O 2 , for example. While not wishing to be bound by theory, it is believed that the glucose-measuring electrode can measure both the signal directly related to the reaction of glucose with GOx (produces H 2 O 2 that is oxidized at the working electrode) and signals from unknown compounds that are in the extracellular milieu surrounding the sensor.
  • the glucose sensor includes a second (e.g., auxiliary) working electrode that is configured to generate a second signal associated with non-glucose related electroactive compounds that have the same oxidation potential as the above-described first working electrode (e.g., para supra).
  • the non- glucose related electroactive species includes at least one of interfering species, non-reaction- related H 2 O 2 , and other electroactive species.
  • interfering species includes any compound that is not directly related to the electrochemical signal generated by the glucose- GOx reaction, such as but not limited to electroactive species in the local environment produces by other bodily processes (e.g., cellular metabolism, wound healing, a disease process, and the like).
  • Non-reaction-related H 2 O 2 includes H 2 O 2 from sources other than the glucose-GOx reaction, such as but not limited to H 2 O 2 released by nearby cells during the course of the cells' metabolism, H 2 O 2 produced by other enzymatic reactions (e.g., extracellular enzymes around the sensor or such as can be released during the death of nearby cells or such as can be released by activated macrophages), and the like.
  • Other electroactive species includes any compound that has an oxidation potential similar to or overlapping that
  • the non-analyte signal produced by compounds other than the analyte obscured the signal related to the analyte, contributes to sensor inaccuracy, and is considered background noise.
  • background noise includes both constant and non-constant components and must be removed to accurately calculate the analyte concentration.
  • the senor of the preferred embodiments are designed (e.g., with symmetry, coaxial design and/or integral formation) such that the first and second electrodes are influenced by substantially the same external/environmental factors, which enables substantially equivalent measurement of both the constant and non- constant species/noise.
  • This advantageously allows the substantial elimination of noise (including transient biologically related noise that has been previously seen to affect accuracy of sensor signal due to it's transient and unpredictable behavior) on the sensor signal (using electronics described elsewhere herein) to substantially reduce or eliminate signal effects due to noise, including non-constant noise (e.g., unpredictable biological, biochemical species or the like) known to effect the accuracy of conventional continuous sensor signals.
  • the senor includes electronics operably connected to the first and second working electrodes.
  • the electronics are configured to provide the first and second signals that are used to generate glucose concentration data substantially without signal contribution due to non-glucose- related noise.
  • the electronics include at least a potentiostat that provides a bias to the electrodes.
  • sensor electronics are configured to measure the current (or voltage) to provide the first and second signals. The first and second signals are used to determine the glucose concentration substantially without signal contribution due to non-glucose-related noise such as by but not limited to subtraction of the second signal from the first signal or alternative data analysis techniques.
  • the sensor electronics include a transmitter that transmits the first and second signals to a receiver, where additional data analysis and/or calibration of glucose concentration can be processed.
  • U.S. Publication Nos. US-2005-0027463-A1, US-2005-0203360-A1 and US-2006-0036142-A1 describe systems and methods for processing sensor analyte " data and " are incorporated herein by reference in their entirety.
  • the sensor electronics are operably connected to the first and second working electrodes.
  • the electronics are configured to calculate at least one analyte sensor data point.
  • the electronics can include a potentiostat, A/D converter, RAM, ROM, transmitter, and the like.
  • the potentiostat converts the raw data (e.g., raw counts) collected from the sensor to a value familiar to the host and/or medical personnel.
  • the raw counts from a glucose sensor can be converted to milligrams of glucose per deciliter of glucose (e.g., mg/dl).
  • the electronics are operably connected to the first and second working electrodes and are configured to process the first and second signals to generate a glucose concentration substantially without signal contribution due to non- glucose noise artifacts.
  • the sensor electronics determine the signals from glucose and non- glucose related signal with an overlapping measuring potential (e.g., from a first working electrode) and then non-glucose related signal with an overlapping measuring potential (e.g., from a second electrode).
  • the sensor electronics then use these data to determine a substantially glucose-only concentration, such as but not limited to subtracting the second electrode's signal from the first electrode's signal, to give a signal (e.g., data) representative of substantially glucose-only concentration, for example.
  • the sensor electronics may perform additional operations, such as but not limited to data smoothing and noise analysis. Bifunctionality
  • the components of at least a portion (e.g., the in vivo portion or the sensing portion) of the sensor possess bifunctional properties (e.g., provide at least two functions to the sensor). These properties can include electrical conductance, insulative properties, structural support, and diffusion barrier properties.
  • the analyte sensor is designed with two working electrodes, a membrane system and an insulating material disposed between the working electrodes.
  • An active enzymatic membrane is disposed above the first working electrode, while an inactive- or non-enzymatic membrane is disposed above the second working electrode.
  • the working electrodes and the insulating material are configured provide at least two functions to the sensor, including but not limited to electrical conductance, insulative properties, structural support, and diffusion barrier.
  • the two working electrodes support the sensor's structure and provide electrical conductance; the insulating material provides insulation between the two electrodes and provides additional structural support and/or a diffusional barrier.
  • a component of the sensor is configured to provide both electrical conductance and structural support.
  • the working electrode(s) and reference electrode are generally manufactured of electrically conductive materials, such as but not limited silver or silver/silver chloride, copper, gold, platinum, iridium, platinum-iridium, palladium, graphite, carbon, conductive polymers, alloys, and the like. Accordingly, the electrodes are both conductive and they give the sensor its shape (e.g., are supportive).
  • all three electrodes 16, 18, and 20 are manufactured from plated insulator, a plated wire, or electrically conductive material, such as but not limited to a metal wire. Accordingly, the three electrodes provide both electrical conductance (to measure glucose concentration) and structural support. Due to the configuration of the electrodes (e.g., the wires are about 0.001 inches in diameter or less, to about 0.01 inches or more), the sensor is needle-like and only about 0.003 inches or less to about 0.015 inches or more.
  • the electrodes of Fig. 7A through Fig. 9 provide electrical conductance, to detect the analyte of interest, as well as structural support for the sensor.
  • the sensors depicted in Figs. 7A through 7L embodiments that are substantially needle-like. Additionally, these sensors are substantially resilient, and therefore able to flex in response to mechanical pressure and then to regain their original shapes.
  • Fig. 7M depicts a cross-section of another sensor embodiment, which can be a composite (e.g., built up of layers of working and reference electrode materials) needle-like sensor or the composite "wire" can be cut to produce pancake-shaped sensors [describe its bifunctionality without unnecessary characterizations (e.g., not "pancake-shaped").
  • Fig. 7N through Fig. 9 illustrate additional sensor embodiments, wherein the electrodes provide electrical conductance and support the sensor's needle-like shape.
  • the first and second working electrodes are configured to provide both electrical conductance and structural support.
  • the working electrodes are often manufactured of bulk metal wires (e.g., copper, gold, platinum, iridium, platinum-iridium, palladium, graphite, carbon, conductive polymers, alloys, and the like).
  • the reference electrode which can function as a reference electrode alone, or as a dual reference and counter electrode, are formed from silver or silver/silver chloride, or the like.
  • the metal wires are conductive (e.g., can conduct electricity) and give the sensor its shape and/or structural support.
  • one electrode metal wire may be coiled around the other electrode metal wire (e.g., Fig. IB or Fig. 7B).
  • the sensor includes a reference electrode that is also configured to provide electrical conductance and structural support (e.g., Fig. IB, Figs. 7C to 7E).
  • reference electrodes are made of metal, such as bulk silver or silver/silver chloride wires. Like the two working electrodes, the reference electrode both conducts electricity and supports the structure of the sensor.
  • the first and second working electrode and the insulating material are configured provide at least two functions, such as but not limited to electrical conductance, insulative properties, structural support, and diffusion barrier.
  • the working electrodes are electrical conductors and also provide support for the sensor.
  • the insulating material e.g., I
  • the insulating material also provides structural support or substantially prevents diffusion of electroactive species from one working electrode to the other, which is discussed in greater detail elsewhere herein.
  • the senor has a diffusion barrier disposed between the first and second working electrodes.
  • the diffusion barrier is configured to substantially block diffusion of the analyte or a co-analyte (e.g., H 2 O 2 ) between the first and second working electrodes.
  • a sheet of a polymer through which H 2 O 2 cannot diffuse can be interposed between the two working electrodes. Diffusion barriers are discussed in greater detail elsewhere herein.
  • the analyte sensor includes a reference electrode that is configured to provide electrical conductance and a diffusion barrier. Electrical conductance is an inherent property of the metal used to manufacture the reference electrode.
  • the reference electrode can be configured to prevent species (e.g., H 2 O 2 ) from diffusing from the first working electrode to the second working electrode.
  • species e.g., H 2 O 2
  • a sufficiently large reference electrode can be placed between the two working electrodes.
  • the reference electrode projects farther than the two working electrodes.
  • the reference electrode is so broad that a substantial portion of the H 2 O 2 produced at the first working electrode cannot diffuse to the second working electrode, and thereby significantly affect the second working electrode's function.
  • the reference electrode is configured to provide a diffusion barrier and structural support.
  • the reference electrode can be constructed of a sufficient size and/or shape that a substantial portion of the H 2 O 2 produced at a first working electrode cannot diffuse to the second working electrode and affect the second working electrode's function.
  • metal wires are generally resilient and hold their shape, the reference electrode can also provide structural support to the sensor (e.g., help the sensor to hold its shape).
  • the insulating material is configured to provide both electrical insulative properties and structural support.
  • portions of the electrodes are coated with a non- conductive polymer.
  • the non-conductive polymer electrically insulates the coated electrodes from each other, and thus substantially prevents passage of electricity from one coated wire to another coated wire.
  • the non-conductive material e.g., a non- conductive polymer or insulating material
  • a sensor component is configured to provide electrical insulative properties and a diffusion barrier.
  • the electrodes are coated with the non-conductive material that substantially prevents direct contact between the electrodes, such that electricity cannot be conducted directly from one electrode to another. Due to the non-conductive coatings on the electrodes, electrical current must travel from one electrode to another through the surrounding aqueous medium (e.g., extracellular fluid, blood, wound fluid, or the like). Any non-conductive material (e.g., insulator) known in the art can be used to insulate the electrodes from each other.
  • the electrodes can be coated with non-conductive polymer materials (e.g., parylene, PTFE, ETFE, polyurethane, polyethylene, polyimide, silicone and the like) by dipping, painting, spraying, spin coating, or the like.
  • non-conductive polymer materials e.g., parylene, PTFE, ETFE, polyurethane, polyethylene, polyimide, silicone and the like
  • Non-conductive material applied to or separating the electrodes can be configured to prevent diffusion of electroactive species (e.g., H 2 O 2 ) from one working electrode to another working electrode. Diffusion of electroactive species from one working electrode to another can cause a false analyte signal.
  • electroactive species e.g., H 2 O 2
  • a first working electrode having active enzyme e.g., GOx
  • active enzyme e.g., GOx
  • the second electrode registers a signal (e.g., as if the second working electrode comprised active GOx).
  • the signal registered at the second working electrode due to the diffusion of the H 2 O 2 is aberrant and can cause improper data processing in the sensor electronics.
  • the second electrode is configured to measure a substantially non-analyte related signal (e.g., background) the sensor will record a higher non-analyte related signal than is appropriate, possibly resulting in the sensor reporting a lower analyte concentration than actually is present in the host. This is discussed in greater detail elsewhere herein.
  • the non-conductive material is configured to provide a diffusion barrier and structural support to the sensor. Diffusion barriers are described elsewhere herein.
  • Non-conductive materials can be configured to support the sensor's structure.
  • non-conductive materials with relatively more or less rigidity can be selected. For example, if the electrodes themselves are relatively flexible, it may be preferred to select a relatively rigid non-conductive material, to make the sensor stiffer (e.g., less flexible or bendable).
  • a very flexible non-conductive material may be coated on the electrodes to bind the electrodes together (e.g., keep the electrodes together and thereby hold the sensor's shape).
  • the non-conductive material can be coated on or wrapped around the grouped or bundled electrodes, to prevent the electrodes from separating and also to prevent the electrodes from directly touching each other.
  • each electrode can be individually coated by a first non- conductive material and then bundled together. Then the bundle of individually insulated electrodes can be coated with a second layer of the first non-conductive material or with a layer or a second non-conductive material.
  • a sensor having the structure shown in Fig.
  • each electrode El, E2 is coated with a non-conductive material/insulator I, and then coated with a second non-conductive material 703 (e.g., instead of a biointerface membrane).
  • a non-conductive material/insulator I e.g., instead of a biointerface membrane.
  • the non-conductive material I prevents electrodes El and E2 from making direct contact with each other as well as giving the needle-like sensor its overall dimensions and shape.
  • Fig. 7N illustrates one method of configuring a sensor having a non- conductive material I that both provides electrical insulation between the electrodes El, E2, R and provides structural support to the sensor.
  • a component of the sensor is configured to provide both insulative properties and a diffusion barrier. Diffusion barriers are discussed elsewhere herein.
  • the working electrodes are separated by a non- conductive material/insulator that is configured such that electroactive species (e.g., H 2 O 2 ) cannot diffuse around it (e.g., from a first electrode to a second electrode).
  • electroactive species e.g., H 2 O 2
  • the electrodes El, E2 are placed in the groves carved into a cylinder of non-conductive material I. The distance D from El to E2 (e.g., around I) is sufficiently great that H 2 O 2 produced at El cannot diffuse to E2 and thereby cause an aberrant signal at E2.
  • the senor in addition to two working electrodes and a non-conductive material/insulator, includes at least a reference or a counter electrode.
  • the reference and/or counter electrode together with the first and second working electrodes, integrally form at least a portion of the sensor.
  • the reference and/or counter electrode is located remote from the first and second working electrodes.
  • the reference and/or counter electrodes can be located on the ex vivo portion of the sensor or reside on the host's skin, such as a portion of an adhesive patch.
  • the reference and/or counter electrode can be located on the host's skin, within or on the fluid connector (e.g., coiled within the ex vivo portion of the device and in contact with fluid within the device, such as but not limited to saline) or on the exterior of the ex vivo portion of the device.
  • the surface area of the reference and/or counter electrode is as least six times the surface area of at least one of the first and second working electrodes. In a further embodiment, the surface area of the reference and/or counter electrode is at least ten times the surface area of at least one of the first and second electrodes.
  • the senor is configured for implantation into the host.
  • the sensor can be configured for subcutaneous implantation in the host's tissue (e.g., transcutaneous or wholly implantable).
  • the sensor can be configured for indwelling in the host's blood stream (e.g., inserted through an intravascular catheter or integrally formed on the exterior surface of an intravascular catheter that is inserted into the host's blood stream).
  • the senor is a glucose sensor that has a first working electrode configured to generate a first signal associated with glucose (e.g., the analyte) and non-glucose related electroactive compounds (e.g., physiological baseline, interferents, and non-constant noise) having a first oxidation potential.
  • glucose e.g., the analyte
  • non-glucose related electroactive compounds e.g., physiological baseline, interferents, and non-constant noise
  • glucose has a first oxidation potential.
  • the interferents have an oxidation potential that is substantially the same as the glucose oxidation potential (e.g., the first oxidation potential).
  • the glucose sensor has a second working electrode that is configured to generate a second signal associated with noise of the glucose sensor.
  • the noise of the glucose sensor is signal contribution due to non-glucose related electroactive compounds (e.g., interferents) that have an oxidation potential that substantially overlaps with the first oxidation potential (e.g., the oxidation potential of glucose, the analyte).
  • the non-glucose related electroactive species include an interfering species, non-reaction-related hydrogen peroxide, and/or other electroactive species.
  • the glucose sensor has electronics that are operably connected to the first and second working electrodes and are configured to provide the first and second signals to generate glucose concentration data substantially without signal contribution due to non-glucose-related noise.
  • the sensor electronics analyze the signals from the first and second working electrodes and calculate the portion of the first electrode signal that is due to glucose concentration only.
  • the portion of the first electrode signal that is not due to the glucose concentration can be considered to be background, such as but not limited to noise.
  • the glucose sensor has a non-conductive material (e.g., insulative material) positioned between the first and second working electrodes.
  • the non-conductive material substantially prevents cross talk between the first and second working electrodes.
  • the electrical signal cannot pass directly from a first insulated electrode to a second insulated electrode. Accordingly, the second insulated electrode cannot aberrantly record an electrical signal due to electrical signal transfer from the first insulated electrode.
  • the first and second working electrodes and the non-conductive material integrally form at least a portion of the sensor (e.g., a glucose sensor).
  • the first and second working electrodes integrally form a substantial portion of the sensor configured for insertion in the host (e.g., the in vivo portion of the sensor).
  • the sensor e.g., a glucose sensor
  • the sensor includes a reference electrode that, in addition to the first and second working electrodes, integrally forms a substantial portion of the sensor configured for insertion in the host (e.g., the in vivo portion of the sensor).
  • the senor e.g., a glucose sensor
  • the insulator e.g., non-conductive material
  • the first and second working electrodes and the insulator integrally form a substantial portion of the sensor configured for insertion in the host (e.g., the in vivo portion of the sensor).
  • the senor e.g., a glucose sensor
  • the sensor includes a diffusion barrier configured to substantially block diffusion of the analyte (e.g., glucose) or a co-analyte (e.g., H 2 O 2 ) between the first and second working electrodes.
  • a diffusion barrier D e.g., spatial, physical and/or temporal blocks diffusion of a species (e.g., glucose and/or H 2 O 2 ) from the first working electrode El to the second working electrode E2.
  • the diffusion barrier D is a physical diffusion barrier, such as a structure between the working electrodes that blocks glucose and H 2 O 2 from diffusing from the first working electrode El to the second working electrode E2.
  • the diffusion barrier D is a spatial diffusion barrier, such as a distance between the working electrodes that blocks glucose and H 2 O 2 from diffusing from the first working electrode El to the second working electrode E2.
  • the diffusion barrier D is a temporal diffusion barrier, such as a period of time between the activity of the working electrodes such that if glucose or H 2 O 2 diffuses from the first working electrode El to the second working electrode E2, the second working electrode E2 will not substantially be influenced by the H 2 O 2 from the first working electrode El..
  • the diffusion barrier is spatial, a distance D separates the working electrodes, such that the analyte or co-analyte substantially cannot diffuse from a first electrode El to a second electrode E2.
  • the diffusion barrier is physical and configured from a material that substantially prevents diffusion of the analyte or co-analyte there through.
  • the insulator I and/or reference electrode R is configured from a material that the analyte or co-analyte cannot substantially pass through. For example, H 2 O 2 cannot substantially pass through a silver/silver chloride reference electrode.
  • a parylene insulator can prevent H 2 O 2 diffusion between electrodes.
  • the two electrodes are activated at separate, non-overlapping times (e.g., pulsed).
  • the first electrode El can be activated for a period of one second, , followed by activating the second electrode E2 three seconds later (e.g., after El has been inactivated) for a period of one second.
  • a component of the sensor is configured to provide both a diffusional barrier and a structural support, as discussed elsewhere herein.
  • the diffusion barrier can be configured of a material that is sufficiently rigid to support the sensor's shape.
  • the diffusion barrier is an electrode, such as but not limited to the reference and counter electrodes (e.g., Fig. 7G to 7J and Fig. 8A).
  • the diffusion barrier is an insulating coating (e.g., parylene) on an electrode (e.g., Fig. 7K to 7L) or an insulating structure separating the electrodes (e.g., Fig. 8A and Fig. 10).
  • One preferred embodiment provides a glucose sensor configured for insertion into a host for measuring a glucose concentration in the host.
  • the sensor includes a first working electrode configured to generate a first signal associated with glucose and non- glucose related electroactive compounds having a first oxidation potential.
  • the sensor also includes a second working electrode configured to generate a second signal associated with noise of the glucose sensor comprising signal contribution due to non-glucose related electroactive compounds that have an oxidation potential that substantially overlaps with the first oxidation potential (e.g., the oxidation potential of H 2 O 2 ).
  • the glucose sensor includes a non-conductive material located between the first and second working electrodes. Each of the first working electrode, the second working electrode, and the non- conductive material are configured to provide at least two functions selected from the group consisting of: electrical conductance, insulative properties, structural support, and diffusion barrier.
  • each of the first working electrode and the second working electrode are configured to provide electrical conductance and structural support.
  • the metal plated wire of electrodes conducts electricity and helps maintain the sensor's shape.
  • the glucose sensor includes a reference electrode that is configured to provide electrical conductance and structural support.
  • the silver/silver chloride reference electrode is both electrically conductive and supports the sensor's shape.
  • the glucose sensor includes a reference electrode that is configured to provide electrical conductance and a diffusion barrier.
  • the silver/silver chloride reference electrode can be configured as a large structure or protruding structure, which separates the working electrodes by the distance D (e.g., Fig. 7G).
  • the glucose sensor includes a reference electrode that is configured to provide a diffusion barrier and structural support.
  • the non- conductive material is configured to provide electrical insulative properties and structural support.
  • non-conductive dielectric materials can insulate an electrode and can be sufficiently rigid to stiffen the sensor.
  • the non-conductive material is configured to provide electrical insulative properties and a diffusion barrier.
  • a substantially rigid, non-conductive dielectric can coat the electrodes and provide support, as shown in Fig. 7L.
  • the non-conductive material is configured to provide diffusion barrier and structural support.
  • a dielectric material can protrude between the electrodes, to act as a diffusion barrier and provide support to the sensor's shape, as shown in Fig. 10. Noise Reduction
  • the senor is configured to reduce noise, including non- constant non-analyte related noise with an overlapping measuring potential with the analyte.
  • noise can occur when a sensor has been implanted in a host.
  • implantable sensors measure a signal (e.g., counts) that generally comprises at least two components, the background signal (e.g., background noise) and the analyte signal.
  • the background signal is composed substantially of signal contribution due to factors other than glucose (e.g., interfering species, non-reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that overlaps with the analyte or co-analyte).
  • the analyte signal (e.g., glucose) is composed substantially of signal contribution due to the analyte. Consequently, because the signal includes these two components, a calibration is performed in order to determine the analyte (e.g., glucose) concentration by solving for the equation y-mx+b, where the value of b represents the background of the signal.
  • the background is comprised of both constant (e.g., baseline) and non-constant (e.g., noise) factors.
  • constant e.g., baseline
  • non-constant e.g., noise
  • noise as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to a substantially intermittent signal caused by relatively non-constant factors (e.g., the presence of intermittent noise-causing compounds that have an oxidation potential that substantially overlaps the oxidation potential of the analyte or co-analyte and arise due to the host's ingestion, metabolism, wound healing, and other mechanical, chemical and/or biochemical factors, also non-analyte related).
  • relatively non-constant factors e.g., the presence of intermittent noise-causing compounds that have an oxidation potential that substantially overlaps the oxidation potential of the analyte or co-analyte and arise due to the host's ingestion, metabolism, wound healing, and other mechanical, chemical and/or biochemical factors, also non-analyte related.
  • Noise can be difficult to remove from the sensor signal by calibration using standard calibration equations (e.g., because the background of the signal does not remain constant). Noise can significantly adversely affect the accuracy of the calibration of the analyte signal. Additionally noise, as described herein, can occur in the signal of conventional sensors with electrode configurations that are not particularly ' designed to measure noise substantially equally at both active and in-active electrodes (e.g., wherein the electrodes are spaced and/or non symmetrical, noise may not be equally measured and therefore not easily removed using conventional dual electrode designs).
  • noise can be recognized and/or analyzed.
  • the sensor data stream is monitored, signal artifacts are detected, and data processing is based at least in part on whether or not a signal artifact has been detected, such as described in U.S. Publication No. US-2005-0043598-A1 and co-pending U.S. Application No. 11/503,367 filed August 10, 2006 and entitled "ANALYTE SENSOR,” herein incorporated by reference in its entirety.
  • an analyte sensor e.g., glucose sensor
  • the sensor includes a first working electrode disposed beneath an active enzymatic portion of a membrane on the sensor; a second working electrode disposed beneath an inactive- or non- enzymatic portion of the membrane on the sensor; and electronics operably connected to the first and second working electrode and configured to process the first and second signals to generate an analyte (e.g., glucose) concentration substantially without signal contribution due to non-glucose related noise artifacts.
  • the senor has a first working electrode El and a second working electrode E2.
  • the sensor includes a membrane system (not shown) covering the electrodes, as described elsewhere herein.
  • a portion of the membrane system on the first electrode contains active enzyme, which is depicted schematically as oval 904a (e.g., active GOx).
  • a portion of the membrane system on the second electrode is non-enzymatic or contains inactivated enzyme, which is depicted schematically as oval 904b (e.g., heat- or chemically-inactivated GOx or optionally no GOx).
  • a portion of the sensor includes electrical connectors 804. In some embodiments, the connectors 804 are located on an ex vivo portion of the sensor.
  • Each electrode (e.g., El, E2, etc.) is connected to sensor electronics (not shown) by a connector 804. Since the first electrode El includes active GOx, it produces a first signal that is related to the concentration of the analyte (in this case glucose) in the host as well as other species that have an oxidation potential that overlaps with the oxidation potential of the analyte or co-analyte (e.g., non- glucose related noise artifacts, noise-causing compounds, background). Since the second electrode E2 includes inactive GOx, it produces a second signal that is not substantially related to the analyte or co-analyte. Instead, the second signal is substantially related to noise- causing compounds and other background noise.
  • the analyte in this case glucose
  • the second electrode E2 includes inactive GOx, it produces a second signal that is not substantially related to the analyte or co-analyte. Instead, the second signal is substantially related to noise- causing compounds
  • the sensor electronics process the first and second signals to generate an analyte concentration that is substantially free of the non-analyte related noise artifacts. Elimination or reduction of noise (e.g., non-constant background) is attributed at least in part to the configuration of the electrodes in the preferred embodiments, e.g., the locality of first and second working electrode, the symmetrical or opposing design of the first and second working electrodes, and/or the overall sizing and configuration of the exposed electroactive portions. Accordingly, the host is provided with improved analyte concentration data, upon which he can make medical treatment decisions (e.g., if he should eat, if he should take medication or the amount of medication he should take).
  • medical treatment decisions e.g., if he should eat, if he should take medication or the amount of medication he should take.
  • the host can be maintained under tighter glucose control (e.g., about 80 mg/dl to about 120 mg/dl) with a reduced risk of hypoglycemia and hypoglycemia's immediate complications (e.g., coma or death).
  • tight glucose control e.g., about 80 mg/dl to about 120 mg/dl
  • the reduced risk of hypoglycemia makes it possible to avoid the long-term complications of hyperglycemia (e.g., kidney and heart disease, neuropathy, poor healing, loss of eye sight) by consistently maintaining tight glucose control (e.g., about 80 mg/dl to about 120 mg/dl).
  • the senor is configured to substantially eliminate (e.g., subtract out) noise due to mechanical factors.
  • Mechanical factors include macro-motion of the sensor, micro-motion of the sensor, pressure on the sensor, local tissue stress, and the like. Since both working electrodes are constructed substantially symmetrically and identically, and due to the sensor's small size, the working electrodes are substantially equally affected by mechanical factors impinging upon the sensor.
  • both working electrodes will measure the resulting noise to substantially the same extend, while only one working electrode (the first working electrode, for example) will also measure signal due to the analyte concentration in the host's body.
  • the sensor calculates the analyte signal (e.g., glucose-only signal) by removing the noise that was measured by the second working electrode from the total signal that was measured by the first working electrode.
  • Non-analyte related noise can also be caused by biochemical and/or chemical factors (e.g., compounds with electroactive acidic, amine or sulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides, amino acids (e.g., L-arginine), amino acid precursors or break-down products, nitric oxide (NO), NO-donors, NO-precursors or other electroactive species or metabolites produced during cell metabolism and/or wound healing).
  • biochemical and/or chemical factors e.g., compounds with electroactive acidic, amine or sulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides, amino acids (e.g., L-arginine), amino acid precursors or break-down products, nitric oxide (NO), NO-donors, NO-precursors or other electroactive species or metabolites produced during cell metabolism and/or wound healing).
  • the sensor electronics can use these data to calculate the glucose-only signal, as described elsewhere herein.
  • the analyte sensor is a glucose sensor that measures a first signal associated with both glucose and non-glucose related electroactive compounds having a first oxidation potential.
  • the oxidation potential of the non-glucose related electroactive compounds substantially overlaps with the oxidation potential of H 2 O 2 , which is produced according to the reaction of glucose with GOx and subsequently transfers electrons to the first working electrode (e.g., El; Fig. 10).
  • the glucose sensor also measures a second signal, which is associated with background noise of the glucose sensor.
  • the background noise is composed of signal contribution due to noise- causing compounds (e.g., interferents), non-reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that substantially overlaps with the oxidation potential Of H 2 O 2 (the co-analyte).
  • the first and second working electrodes integrally form at least a portion of the sensor, such as but not limited to the in vivo portion of the sensor, as discussed elsewhere herein. Additionally, each of the first working electrode, the second working electrode, and a non-conductive material/insulator are configured provide at least two functions (to the sensor), such as but not limited to electrical conductance, insulative properties, structural support, and diffusion barrier (described elsewhere herein). Furthermore, the sensor has a diffusion barrier that substantially blocks diffusion of glucose or H 2 O 2 between the first and second working electrodes. Diffusion Barrier
  • the sensor is a diffusion barrier, to prevent an undesired species, such as H 2 O 2 or the analyte, from diffusing between active (with active enzyme) and inactive (without active enzyme) electrodes.
  • the sensor includes a diffusion barrier configured to be physical, spatial, and/or temporal.
  • Fig. 10 is a schematic illustrating one embodiment of a sensor (e.g., a portion of the in vivo portion of the sensor, such as but not limited to the sensor electroactive surfaces) having one or more components that act as a diffusion barrier (e.g., prevent diffusion of electroactive species from one electrode to another).
  • the first working electrode El is coated with an enzyme layer 1000 comprising active enzyme.
  • the first working electrode El is coated with glucose oxidase enzyme (GOx).
  • a second working electrode E2 is separated from the first working electrode El by a diffusion barrier D, such as but not limited to a physical diffusion barrier (e.g., either a reference electrode or a layer of non-conductive material/insulator).
  • the diffusion barrier can also be spatial or temporal, as discussed elsewhere herein.
  • Glucose and oxygen diffuse into the enzyme layer 1000, where they react with GOx, to produce gluconate and H 2 O 2 .
  • At least a portion of the H 2 O 2 diffuses to the first working electrode El, where it is electrochemically oxidized to oxygen and transfers two electrons (e.g., 2e " ) to the first working electrode El, which results in a glucose signal that is recorded by the sensor electronics (not shown).
  • the remaining H 2 O 2 can diffuse to other locations in the enzyme layer or out of the enzyme layer (illustrated by the wavy arrows).
  • a portion of the H 2 O 2 can diffuse to the second working electrode E2, which results in an aberrant signal that can be recorded by the sensor electronics as a non-glucose related signal (e.g., background).
  • Preferred embodiments provide for a substantial diffusion barrier D between the first and second working electrodes (El, E2) such that the H 2 O 2 cannot substantially diffuse from the first working electrode El to the second working electrode E2. Accordingly, the possibility of an aberrant signal produced by H 2 O 2 from the first working electrode El (at the second working electrode E2) is reduced or avoided.
  • the senor is provided with a spatial diffusion barrier between electrodes (e.g., the working electrodes).
  • a spatial diffusion barrier can be created by separating the first and second working electrodes by a distance that is too great for the H 2 O 2 to substantially diffuse between the working electrodes.
  • the spatial diffusion barrier is about 0.010 inches to about 0.120 inches.
  • the spatial diffusion barrier is about 0.020 inches to about 0.050 inches.
  • the spatial diffusion barrier is about 0.055 inches to about 0.095 inches.
  • a reference electrode R e.g., a silver or silver/silver chloride electrode
  • a non-conductive material I e.g., a polymer structure or coating such as Parylene
  • Figs. 9A and 9B illustrate two exemplary embodiments of sensors with spatial diffusion barriers.
  • the sensor has two working electrodes El and E2.
  • Each working electrode includes an electroactive surface, represented schematically as windows 904a and 904b, respectively.
  • the sensor includes a membrane system (not shown). Over one electroactive surface (e.g., 904a) the membrane includes active enzyme (e.g., GOx). Over the second electroactive surface (e.g., 904b) the membrane does not include active enzyme.
  • the portion of the membrane covering the second electroactive surface contains inactivated enzyme (e.g., heat- or chemically-inactivated GOx) while in other embodiments, this portion of the membrane does not contain any enzyme (e.g., non-enzymatic).
  • the electroactive surfaces 904a and 904b are separated by a spatial diffusion barrier that is substantially wide such that H 2 O 2 produced at the first electroactive surface 904a cannot substantially affect the second electroactive surface 904b.
  • the diffusion barrier can be physical (e.g., a structure separating the electroactive surfaces) or temporal (e.g., oscillating activity between the electroactive surfaces).
  • the senor is an indwelling sensor, such as configured for insertion into the host's circulatory system via a vein or an artery.
  • an indwelling sensor includes at least two working electrodes that are inserted into the host's blood stream through a catheter.
  • the sensor includes at least a reference electrode that can be disposed either with the working electrodes or remotely from the working electrodes.
  • the sensor includes a spatial, a physical, or a temporal diffusion barrier.
  • a spatial diffusion barrier can be configured as described elsewhere herein, with reference to Fig. 7A through Fig. 8 A.
  • Fig. 9B provides one exemplary embodiment of an indwelling analyte sensor, such as but not limited to an intravascular glucose sensor to be used from a few hours to ten days or longer.
  • the sensor includes two working electrodes.
  • One working electrode detects the glucose-related signal (due to active GOx applied to the electroactive surface) as well as non-glucose related signal.
  • the other working electrode detects only the non-glucose related signal (because no active GOx is applied to its electroactive surface).
  • H 2 O 2 is produced on the working electrode with active GOx. If the H 2 O 2 diffuses to the other working electrode (the no GOx electrode) an aberrant signal will be detected at this electrode, resulting in reduced sensor activity. Accordingly, it is desirable to separate the electroactive surfaces with a diffusion barrier, such as but not limited to a spatial diffusion barrier. Indwelling sensors are described in more detail in copending U.S. patent application
  • the location of the active enzyme is dependent upon the orientation of the sensor after insertion into the host's artery or vein.
  • active GOx e.g., GOx
  • inactive GOX or no GOx
  • electroactive surface 904a e.g., upstream from 904b, relative to the direction of blood flow. Due to this configuration, H 2 O 2 produced at electroactive surface 904b would be carrier down stream (e.g., away from electroactive surface 904a) and thus not affect electrode El.
  • the indwelling electrode can also be configured for insertion of the sensor into the host's vein or artery in the direction of the blood flow (e.g., pointing downstream).
  • the active GOx can be advantageously applied to electroactive surface 904a on the first working electrode El.
  • the electroactive surface 904b on the second working electrode E2 has no active GOx. Accordingly, H 2 O 2 produced at electroactive surface 904a is carried away by the blood flow, and has no substantial effect on the second working electrode E2.
  • the reference electrode which is generally configured of silver/silver chloride, can extend beyond the working electrodes, to provide a physical barrier around which the H 2 O 2 generated at the electrode comprising active GOx cannot pass the other working electrode (that has active GOx).
  • the reference electrode has a surface area that is at least six times larger than the surface area of the working electrodes.
  • a 2- working electrode analyte sensor includes a counter electrode in addition to the reference electrode. As is generally know in the art, the inclusion of the counter electrode allows for a reduction in the reference electrode's surface area, and thereby allows for further miniaturization of the sensor (e.g., reduction in the sensor's diameter and/or length, etc.).
  • Fig. 7H provides one exemplary embodiment of a spatial diffusion barrier, wherein the reference electrode/non-conductive insulating material R/I is sized and shaped such that H 2 O 2 produced at the first working electrode El (e.g., with enzyme) does not substantially diffuse around the reference electrode/non-conductive material R/I to the second working electrode E2 (e.g., without enzyme).
  • the X- shaped the reference electrode/non-conductive material R/I substantially prevents diffusion of electroactive species from the first working electrode El (e.g., with enzyme) to the second working electrode E2 (e.g., without enzyme).
  • the layer of non-conductive material I (between the electrodes) is of a sufficient length that the H 2 O 2 produced at one electrode cannot substantially diffuse to another electrode, (e.g., from El to either E2 or E3; or from E2 to either El or E3, etc.).
  • a physical diffusion barrier is provided by a physical structure, such as an electrode, insulator, and/or membrane.
  • a physical structure such as an electrode, insulator, and/or membrane.
  • the insulator (I) or reference electrode (R) act as a diffusion barrier.
  • the diffusion barrier can be a bioprotective membrane (e.g., a membrane that substantially resists or blocks the transport of a species (e.g., hydrogen peroxide), such as CHRONOTHANE®-H (a polyetherurethaneurea based on polytetramethylene glycol, polyethylene glycol, methylene diisocyanate, and organic amines).
  • a species e.g., hydrogen peroxide
  • CHRONOTHANE®-H a polyetherurethaneurea based on polytetramethylene glycol, polyethylene glycol, methylene diisocyanate, and organic amines.
  • the diffusion barrier can be a resistance domain, as described in more detail elsewhere herein; namely, a semipermeable membrane that controls the flux of oxygen and an analyte (e.g., glucose) to the underlying enzyme domain.
  • analyte e.g., glucose
  • Numerous other structures and membranes can function as a physical diffusion barrier as is appreciated by one skilled in the art.
  • a temporal diffusion barrier is provided (e.g., between the working electrodes).
  • temporal diffusion barrier is meant a period of time that substantially prevents an electroactive species (e.g., H 2 O 2 ) from diffusing from a first working electrode to a second working electrode.
  • the differential measurement can be obtained by switching the bias potential of each electrode between the measurement potential and a non-measurement potential.
  • the bias potentials can be held at each respective setting (e.g., high and low bias settings) for as short as milliseconds to as long as minutes or hours.
  • Pulsed amperometric detection is one method of quickly switching voltages, such as described in Bisenberger, M.; Brauchle, C; Hampp, N. A triple-step potential waveform at enzyme multisensors with thick-film gold electrodes for detection of glucose and sucrose. Sensors and Actuators 1995, B, 181-189, which is incorporated herein by reference in its entirety. In some embodiments, bias potential settings are held long enough to allow equilibration.
  • One preferred embodiment provides a glucose sensor configured for insertion into a host for measuring glucose in the host.
  • the sensor includes first and second working electrodes and an insulator located between the first and second working electrodes.
  • the first working electrode is disposed beneath an active enzymatic portion of a membrane on the sensor and the second working electrode is disposed beneath an inactive- or non- enzymatic portion of the membrane on the sensor.
  • the sensor also includes a diffusion barrier configured to substantially block diffusion of glucose or hydrogen peroxide between the first and second working electrodes.
  • the glucose sensor includes a reference electrode configured integrally with the first and second working electrodes.
  • the reference electrode can be located remotely from the sensor, as described elsewhere herein.
  • the surface area of the reference electrode is at least six times the surface area of the working electrodes.
  • the sensor includes a counter electrode that is integral to the sensor or is located remote from the sensor, as described elsewhere herein.
  • the glucose sensor detects a first signal associated with glucose and non-glucose related electroactive compounds having a first oxidation potential (e.g., the oxidation potential of H 2 O 2 ).
  • the glucose sensor also detects a second signal is associated with background noise of the glucose sensor comprising signal contribution due to interfering species, non-reaction-related hydrogen peroxide, or other electroactive species with an oxidation potential that substantially overlaps with the oxidation potential of hydrogen peroxide; the first and second working electrodes integrally form at least a portion of the sensor; and each of the first working electrode, the second working electrode and the non-conductive material/insulator are configured provide at least two functions such as but not limited to electrical conductance, insulation, structural support, and a diffusion barrier [0430]
  • the glucose sensor includes electronics operably connected to the first and second working electrodes.
  • the electronics are configured to calculate at least one analyte sensor data point using the first and second signals described above.
  • the electronics are operably connected to the first and second working electrode and are configured to process the first and second signals to generate a glucose concentration substantially without signal contribution due to non-glucose noise artifacts.
  • Figs. 3A to 3B are cross-sectional exploded schematic views of the sensing region of a glucose sensor 10, which show architectures of the membrane system 22 disposed over electroactive surfaces of glucose sensors in some embodiments.
  • the membrane system 22 is positioned at least over the glucose-measuring working electrode 16 and the optional auxiliary working electrode 18; however the membrane system may be positioned over the reference and/or counter electrodes 20, 22 in some embodiments.
  • Fig. 3A is a cross-sectional exploded schematic view of the sensing region in one embodiment wherein an active enzyme 32 of the enzyme domain is positioned only over the glucose-measuring working electrode 16.
  • the membrane system is formed such that the glucose oxidase 32 only exists above the glucose-measuring working electrode 16.
  • the enzyme domain coating solution can be applied as a circular region similar to the diameter of the glucose-measuring working electrode 16. This fabrication can be accomplished in a variety of ways such as screen-printing or pad printing.
  • the enzyme domain is pad printed during the enzyme domain fabrication with equipment as available from Pad Print Machinery of Vermont (Manchester, VT).
  • This embodiment provides the active enzyme 32 above the glucose-measuring working electrode 16 only, so that the glucose-measuring working electrode 16 (and not the auxiliary working electrode 18) measures glucose concentration. Additionally, this embodiment provides an added advantage of eliminating the consumption of O 2 above the counter electrode (if applicable) by the oxidation of glucose with glucose oxidase.
  • Fig. 3B is a cross-sectional exploded schematic view of a sensing region of the preferred embodiments, and wherein the portion of the active enzyme within the membrane system 22 positioned over the auxiliary working electrode 18 has been deactivated 34.
  • the enzyme of the membrane system 22 may be deactivated 34 everywhere except for the area covering the glucose-measuring working electrode 16 or may be selectively deactivated only over certain areas (for example, auxiliary working electrode 18, counter electrode 22, and/or reference electrode 20) by irradiation, heat, proteolysis, solvent, or the like.
  • a mask for example, such as those used for photolithography
  • exposure of the masked membrane to ultraviolet light deactivates the glucose oxidase in all regions except that covered by the mask.
  • the membrane system is disposed on the surface of the electrode(s) using known deposition techniques.
  • the electrode-exposed surfaces can be inset within the sensor body, planar with the sensor body, or extending from the sensor body.
  • the sensing region may include reference and/or electrodes associated with the glucose-measuring working electrode and separate reference and/or counter electrodes associated with the optional auxiliary working electrode(s).
  • the sensing region may include a glucose-measuring working electrode, an auxiliary working electrode, two counter electrodes (one for each working electrode), and one shared reference electrode.
  • the sensing region may include a glucose-measuring working electrode, an auxiliary working electrode, two reference electrodes, and one shared counter electrode.
  • a variety of electrode materials and configurations can be used with the implantable analyte sensor of the preferred embodiments.
  • the working electrodes are interdigitated.
  • the working electrodes each comprise multiple exposed electrode surfaces; one advantage of these architectures is to distribute the measurements across a greater surface area to overcome localized problems that may occur in vivo, for example, with the host's immune response at the biointerface.
  • the glucose-measuring and auxiliary working electrodes are provided within the same local environment, such as described in more detail elsewhere herein.
  • Fig. 4 is a block diagram that illustrates the continuous glucose sensor electronics in one embodiment.
  • a first potentiostat 36 is provided that is operatively associated with the glucose-measuring working electrode 16.
  • the first potentiostat 36 measures a current value at the glucose-measuring working electrode and preferably includes a resistor (not shown) that translates the current into voltage.
  • An optional second potentiostat 37 is provided that is operatively associated with the optional auxiliary working electrode 18.
  • the second potentiostat 37 measures a current value at the auxiliary working electrode 18 and preferably includes a resistor (not shown) that translates the current into voltage.
  • the optional auxiliary electrode can be configured to share the first potentiostat with the glucose-measuring working electrode.
  • An A/D converter 38 digitizes the analog signals from the potentiostats 36, 37 into counts for processing. Accordingly, resulting raw data streams (in counts) can be provided that are directly related to the current measured by each of the potentiostats 36 and 37.
  • a microprocessor 40 also referred to as the processor module, is the central control unit that houses EEPROM 42 and SRAM 44, and controls the processing of the sensor electronics. It is noted that certain alternative embodiments can utilize a computer system other than a microprocessor to process data as described herein. In other alternative embodiments, an application-specific integrated circuit (ASIC) can be used for some or all the sensor's central processing.
  • the EEPROM 42 provides semi-permanent storage of data, for example, storing data such as sensor identifier (ID) and programming to process data streams (for example, such as described in U.S. Publication No. US-2005-0027463-A1, which is incorporated by reference herein in its entirety.
  • the SRAM 44 can be used for the system's cache memory, for example for temporarily storing recent sensor data.
  • memory storage components comparable to EEPROM and SRAM may be used instead of or in addition to the preferred hardware, such as dynamic RAM, non-static RAM, rewritable ROMs, flash memory, or the like.
  • a battery 46 is operably connected to the microprocessor 40 and provides the necessary power for the sensor 10a.
  • the battery is a Lithium Manganese Dioxide battery, however any appropriately sized and powered battery can be used (for example, AAA, Nickel-cadmium, Zinc-carbon, Alkaline, Lithium, Nickel-metal hydride, Lithium-ion, Zinc-air, Zinc-mercury oxide, Silver-zinc, and/or hermetically-sealed).
  • the battery is rechargeable.
  • a plurality of batteries can be used to. power the system.
  • one or more capacitors can be used to power the system.
  • a Quartz Crystal 48 may be operably connected to the microprocessor 40 to maintain system time for the computer system as a whole.
  • An RF Transceiver 50 may be operably connected to the microprocessor 40 to transmit the sensor data from the sensor 10 to a receiver (see Figs. 4 and 5) within a wireless transmission 52 via antenna 54.
  • a receiver see Figs. 4 and 5
  • the receiver can be transcutaneously powered via an inductive coupling, for example.
  • a second quartz crystal 56 can provide the system time for synchronizing the data transmissions from the RF transceiver. It is noted that the transceiver 50 can be substituted with a transmitter in other embodiments. In some alternative embodiments other mechanisms such as optical, infrared radiation (IR), ultrasonic, or the like may be used to transmit and/or receive data.
  • Receiver such as optical, infrared radiation (IR), ultrasonic, or the like may be used to transmit and/or receive data.
  • Fig. 5 is a schematic drawing of a receiver for the continuous glucose sensor in one embodiment.
  • the receiver 58 comprises systems necessary to receive, process, and display sensor data from the analyte sensor, such as described in more detail elsewhere herein.
  • the receiver 58 may be a pager-sized device, for example, and house a user interface that has a plurality of buttons and/or keypad and a liquid crystal display (LCD) screen, and which may include a backlight.
  • the user interface may also include a speaker, and a vibrator such as described with reference to Fig. 6.
  • Fig. 6 is a block diagram of the receiver electronics in one embodiment.
  • the receiver comprises a configuration such as described with reference to Fig. 5, above.
  • the receiver may comprise any reasonable configuration, including a desktop computer, laptop computer, a personal digital assistant (PDA), a server (local or remote to the receiver), or the like.
  • a receiver may be adapted to connect (via wired or wireless connection) to a desktop computer, laptop computer, a PDA, a server (local or remote to the receiver), or the like in order to download data from the receiver.
  • the receiver may be housed within or directly connected to the sensor in a manner that allows sensor and receiver electronics to work directly together and/or share data processing resources. Accordingly, the receiver, including its electronics, may be generally described as a "computer system.”
  • a quartz crystal 60 may be operably connected to an RF transceiver 62 that together function to receive and synchronize data streams via an antenna 64 (for example, transmission 52 from the RF transceiver 50 shown in Fig. 4). Once received, a microprocessor 66 can process the signals, such as described below.
  • the microprocessor 66 also referred to as the processor module, is the central control unit that provides the processing, such as storing data, calibrating sensor data, downloading data, controlling the user interface by providing prompts, messages, warnings and alarms, or the like.
  • the EEPROM 68 may be operably connected to the microprocessor 66 and provides semi-permanent storage of data, storing data such as receiver ID and programming to process data streams (for example, programming for performing calibration and other algorithms described elsewhere herein).
  • SRAM 70 may be used for the system's cache memory and is helpful in data processing.
  • the SRAM stores information from the continuous glucose sensor for later recall by the patient or a doctor; a patient or doctor can transcribe the stored information at a later time to determine compliance with the medical regimen or a comparison of glucose concentration to medication administration (for example, this can be accomplished by downloading the information through the pc com port 76).
  • the SRAM 70 can also store updated program instructions and/or patient specific information.
  • memory storage components comparable to EEPROM and SRAM can be used instead of or in addition to the preferred hardware, such as dynamic RAM, non-static RAM, rewritable ROMs, flash memory, or the like.
  • a battery 72 may be operably connected to the microprocessor 66 and provides power for the receiver.
  • the battery is a standard AAA alkaline battery, however any appropriately sized and powered battery can be used.
  • a plurality of batteries can be used to power the system.
  • a power port (not shown) is provided permit recharging of rechargeable batteries.
  • a quartz crystal 84 may be operably connected to the microprocessor 66 and maintains system time for the system as a whole.
  • a PC communication (com) port 76 can be provided to enable communication with systems, for example, a serial communications port, allows for communicating with another computer system (for example, PC, PDA, server, or the like). In one exemplary embodiment, the receiver is able to download historical data to a physician's PC for retrospective analysis by the physician.
  • the PC communication port 76 can also be used to interface with other medical devices, for example pacemakers, implanted analyte sensor patches, infusion devices, telemetry devices, or the like.
  • a user interface 78 comprises a keypad 80, speaker 82, vibrator 84, backlight 86, liquid crystal display (LCD) 88, and one or more buttons 90. The components that comprise the user interface 78 provide controls to interact with the user.
  • the keypad 80 can allow, for example, input of user information about himself/herself, such as mealtime, exercise, insulin administration, and reference glucose values.
  • the speaker 82 can provide, for example, audible signals or alerts for conditions such as present and/or predicted hyper- and hypoglycemic conditions.
  • the vibrator 84 can provide, for example, tactile signals or alerts for reasons such as described with reference to the speaker, above.
  • the backlight 94 can be provided, for example, to aid the user in reading the LCD in low light conditions.
  • the LCD 88 can be provided, for example, to provide the user with visual data output. In some embodiments, the LCD is a touch-activated screen.
  • the buttons 90 can provide for toggle, menu selection, option selection, mode selection, and reset, for example.
  • a microphone can be provided to allow for voice-activated control.
  • the user interface 78 which is operably connected to the microprocessor 70, serves to provide data input and output for the continuous analyte sensor.
  • prompts can be displayed to inform the user about necessary maintenance procedures, such as "Calibrate Sensor” or "Replace Battery.”
  • prompts or messages can be displayed on the user interface to convey information to the user, such as malfunction, outlier values, missed data transmissions, or the like. Additionally, prompts can be displayed to guide the user through calibration of the continuous glucose sensor, for example when to obtain a reference glucose value.
  • Keypad, buttons, touch-screen, and microphone are all examples of mechanisms by which a user can input data directly into the receiver.
  • a server, personal computer, personal digital assistant, insulin pump, and insulin pen are examples of external devices that can be connected to the receiver via PC com port 76 to provide useful information to the receiver.
  • Other devices internal or external to the sensor that measure other aspects of a patient's body for example, temperature sensor, accelerometer, heart rate monitor, oxygen monitor, or the like
  • the user interface can prompt the patient to select an activity most closely related to their present activity, which can be helpful in linking to an individual's physiological patterns, or other data processing.
  • a temperature sensor and/or heart rate monitor can provide information helpful in linking activity, metabolism, and glucose excursions of an individual. While a few examples of data input have been provided here, a variety of information can be input and can be helpful in data processing as will be understood by one skilled in the art. Calibration Systems and Methods
  • continuous analyte sensors define a relationship between sensor-generated measurements and a reference measurement that is meaningful to a user (for example, blood glucose in mg/dL). This defined relationship must be monitored to ensure that the continuous analyte sensor maintains a substantially accurate calibration and thereby continually provides meaningful values to a user.
  • sensitivity m and baseline b of the calibration are subject to changes that occur in vivo over time (for example, hours to months), requiring updates to the calibration.
  • any physical property that influences diffusion or transport of molecules through the membrane can alter the sensitivity (and/or baseline) of the calibration.
  • Physical properties that can alter the transport of molecules include, but are not limited to, blockage of surface area due to foreign body giant cells and other barrier cells at the biointerface, distance of capillaries from the membrane, foreign body response/capsule, disease, tissue ingrowth, thickness of membrane system, or the like.
  • an implantable glucose sensor is implanted in the subcutaneous space of a human, which is at least partially covered with a biointerface membrane, such as described in U.S. Publication No. US-2005- 0112169-Al, which is incorporated by reference herein in its entirety.
  • a biointerface membrane such as described in U.S. Publication No. US-2005- 0112169-Al, which is incorporated by reference herein in its entirety.
  • ingrowth of vascularized tissue matures (changes) over time, beginning with a short period of high solute transport during the first few days after implantation, continuing through a time period of significant tissue ingrowth a few days to a week or more after implantation during which low solute transport to the membrane has been observed, and into a mature state of vascularized tissue during which the bed of vascularized tissue provides moderate to high solute transport, which can last for months and even longer after implantation.
  • this maturation process accounts for a substantial portion of the change in sensitivity and/or baseline of the calibration over time due to changes in solute transport to the membrane.
  • the sensitivity measurement is a signal obtained by measuring a constant analyte other than the analyte being measured by the analyte sensor.
  • a constant analyte other than the analyte being measured by the analyte sensor.
  • a non-glucose constant analyte is measured, wherein the signal is measured beneath the membrane system 22 on the glucose sensor 10.
  • a biointerface monitor is provided, which is capable of monitoring changes in the biointerface surrounding an implantable device, thereby enabling the measurement of sensitivity changes of an analyte sensor over time.
  • the analyte sensor 10 is provided with an auxiliary electrode 18 configured as a transport-measuring electrode disposed beneath the membrane system 22.
  • the transport-measuring electrode can be configured to measure any of a number of substantially constant analytes or factors, such that a change measured by the transport- measuring electrode can be used to indicate a change in solute (for example, glucose) transport to the membrane system 22.
  • solute for example, glucose
  • substantially constant analytes or factors that can be measured include, but are not limited to, oxygen, carboxylic acids (such as urea), amino acids, hydrogen, pH, chloride, baseline, or the like.
  • the transport- measuring electrode provides an independent measure of changes in solute transport to the membrane, and thus sensitivity changes over time.
  • the transport-measuring electrode measures analytes similar to the analyte being measured by the analyte sensor.
  • analytes similar to the analyte being measured by the analyte sensor.
  • water soluble analytes are believed to better represent the changes in sensitivity to glucose over time than non-water soluble analytes (due to the water- solubility of glucose), however relevant information may be ascertained from a variety of molecules.
  • sensitivity measurements that can be used as to qualify or quantify solute transport through the biointerface of the analyte sensor.
  • the transport-measuring electrode is configured to measure urea, which is a water-soluble constant analyte that is known to react directly or indirectly at a hydrogen peroxide sensing electrode (similar to the working electrode of the glucose sensor example described in more detail above).
  • urea is directly measured by the transport-measuring electrode
  • the glucose sensor comprises a membrane system as described in more detail above, however, does not include an active interference domain or active enzyme directly above the transport- measuring electrode, thereby allowing the urea to pass through the membrane system to the electroactive surface for measurement thereon.
  • the glucose sensor comprises a membrane system as described in more detail above, and further includes an active uricase oxidase domain located directly above the transport-measuring electrode, thereby allowing the urea to react at the enzyme and produce hydrogen peroxide, which can be measured at the electroactive surface thereon.
  • the change in sensitivity is measured by measuring a change in oxygen concentration, which can be used to provide an independent measurement of the maturation of the biointerface, and to indicate when recalibration of the system may be advantageous.
  • oxygen is measured using pulsed amperometric detection on the glucose-measuring working electrode 16 (eliminating the need for a separate auxiliary electrode).
  • the auxiliary electrode is configured as an oxygen-measurir ⁇ g electrode.
  • an oxygen sensor (not shown) is added to the glucose sensor, as is appreciated by one skilled in the art, eliminating the need for an auxiliary electrode.
  • a stability module wherein the sensitivity measurement changes can be quantified such that a co-analyte concentration threshold is determined.
  • a co-analyte threshold is generally defined as a minimum amount of co-analyte required to fully react with the analyte in an enzyme-based analyte sensor in a non- limiting manner.
  • the minimum co-analyte threshold is preferably expressed as a ratio (for example, a glucose-to-oxygen ratio) that defines a concentration of co-analyte required based on a concentration of analyte available to ensure that the enzyme reaction is limited only by the analyte.
  • the processor module can be configured to compensate for instabilities in the glucose sensor accordingly, for example by filtering the unstable data, suspending calibration or display, or the like.
  • a data stream from an analyte signal is monitored and a co-analyte threshold set, whereby the co-analyte threshold is determined based on a signal-to-noise ratio exceeding a predetermined threshold.
  • the signal-to- noise threshold is based on measurements of variability and the sensor signal over a time period, however one skilled in the art appreciates the variety of systems and methods available for measuring signal-to-noise ratios. Accordingly, the stability module can be configured to set determine the stability of the analyte sensor based on the co-analyte threshold, or the like.
  • the stability module is configured to prohibit calibration of the sensor responsive to the stability (or instability) of the sensor. In some embodiments, the stability module can be configured to trigger filtering of the glucose signal responsive to a stability (or instability) of the sensor.
  • sensitivity changes can be used to trigger a request for one or more new reference glucose values from the host, which can be used to recalibrate the sensor.
  • the sensor is re-calibrated responsive to a sensitivity change exceeding a preselected threshold value.
  • the sensor is calibrated repeatedly at a frequency responsive to the measured sensitivity change.
  • sensitivity changes can be used to update calibration.
  • the measured change in transport can be used to update the sensitivity m in the calibration equation.
  • the sensitivity m of the calibration of the glucose sensor is substantially proportional to the change in solute transport measured by the transport- measuring electrode.
  • the implementation of sensitivity measurements of the preferred embodiments typically necessitate an addition to, or modification of, the existing electronics (for example, potentiostat configuration or settings) of the glucose sensor and/or receiver.
  • the signal from the oxygen measuring electrode may be digitally low-pass filtered (for example, with a passband of 0-10 "5 Hz, dc-24 hour cycle lengths) to remove transient fluctuations in oxygen, due to local ischemia, postural effects, periods of apnea, or the like. Since oxygen delivery to tissues is held in tight homeostatic control, this filtered oxygen signal should oscillate about a relatively constant. In the interstitial fluid, it is thought that the levels are about equivalent with venous blood (40 rhmHg). Once implanted, changes in the mean of the oxygen signal (for example, > 5%) may be indicative of change in transport through the biointerface (change in sensor sensitivity and/or baseline due to changes in solute transport) and the need for system recalibration.
  • a passband for example, with a passband of 0-10 "5 Hz, dc-24 hour cycle lengths
  • the oxygen signal may also be used in its unfiltered or a minimally filtered form to detect or predict oxygen deprivation-induced artifact in the glucose signal, and to control display of data to the user, or the method of smoothing, digital filtering, or otherwise replacement of glucose signal artifact.
  • the oxygen sensor may be implemented in conjunction with any signal artifact detection or prediction that may be performed on the counter electrode or working electrode voltage signals of the electrode system.
  • U.S. Publication No. US-2005-0043598-A1 which is incorporated by reference in its entirety herein, describes some methods of signal artifact detection and replacement that may be useful such as described herein.
  • the transport-measuring electrode is located within the same local environment as the electrode system associated with the measurement of glucose, such that the transport properties at the transport-measuring electrode are substantially similar to the transport properties at the glucose-measuring electrode.
  • auxiliary working electrode is configured to measure the baseline of the analyte sensor over time.
  • the glucose-measuring working electrode 16 is a hydrogen peroxide sensor coupled to a membrane system 22 containing an active enzyme 32 located above the electrode (such as described in more detail with reference to Figs. 1 to 4, above).
  • the auxiliary working electrode 18 is another hydrogen peroxide sensor that is configured similar to the glucose-measuring working electrode however a portion 34 of the membrane system 22 above the base- measuring electrode does not have active enzyme therein, such as described in more detail with reference to Figs. 3 A and 3B.
  • the auxiliary working electrode 18 provides a signal substantially comprising the baseline signal, b, which can be (for example, electronically or digitally) subtracted from the glucose signal obtained from the glucose-measuring working electrode to obtain the signal contribution due to glucose only according to the following equation:
  • Olg ⁇ l£ll glucose only Slg ⁇ .3.1 glucose-measuring working electrode - olgH.il baseline-measuring working electrode
  • electronic subtraction of the baseline signal from the glucose signal can be performed in the hardware of the sensor, for example using a differential amplifier.
  • digital subtraction of the baseline signal from the glucose signal can be performed in the software or hardware of the sensor or an associated receiver, for example in the microprocessor.
  • the senor is made less dependent on the range of values of the matched data pairs, which can be sensitive to human error in manual blood glucose measurements, for example. Additionally, by subtracting the baseline at the sensor (rather than solving for the baseline b as in conventional calibration schemes), accuracy of the sensor may increase by altering control of this variable (baseline b) from the user to the sensor. It is additionally believed that variability introduced by sensor calibration may be reduced.
  • the system can trigger a request (for example, from the patient or caregiver) for a new reference glucose value (for example, SMBG), which can be used to recalibrate the sensor.
  • a new reference glucose value for example, SMBG
  • the auxiliary working electrode signal as an indicator of baseline shifts, recalibration requiring user interaction (namely, new reference glucose values) can be minimized due to timeliness and appropriateness of the requests.
  • the sensor is re-calibrated responsive to a baseline shifts exceeding a preselected threshold value.
  • the sensor is calibrated repeatedly at a frequency responsive to the rate-of- change of the baseline.
  • the electrode system of the preferred embodiments is employed as described above, including determining the differential signal of glucose less baseline current in order to calibrate using the simplified equation (y - mx ), and the auxiliary working electrode 18 is further utilized as an indicator of baseline shifts in the sensor signal. While not wishing to be bound by theory, it is believed that shifts in baseline may also correlate and/or be related to changes in the sensitivity m of the glucose signal. Consequently, a shift in baseline may be indicative of a change in sensitivity m. Therefore, the auxiliary working electrode 18 is monitored for changes above a certain threshold.
  • the system can trigger a request (for example, from the patient or caregiver) for a new reference glucose value (for example, SMBG), which can be used to recalibrate the sensor.
  • a new reference glucose value for example, SMBG
  • recalibration requiring user interaction can be minimized due to timeliness and appropriateness of the requests.
  • infrequent new matching data pairs may be useful over time to recalibrate the sensor because the sensitivity m of the sensor may change over time (for example, due to maturation of the biointerface that may increase or decrease the glucose and/or oxygen availability to the sensor).
  • the baseline shifts that have conventionally required numerous and/or regular blood glucose reference measurements for updating calibration can be consistently and accurately eliminated using the systems and methods of the preferred embodiments, allowing reduced interaction from the patient (for example, requesting less frequent reference glucose values such as daily or even as infrequently as monthly).
  • An additional advantage of the sensor of the preferred embodiments includes providing a method of eliminating signal effects of interfering species, which have conventionally been problematic in electrochemical glucose sensors.
  • electrochemical sensors are subject to electrochemical reaction not only with the hydrogen peroxide (or other analyte to be measured), but additionally may react with other electroactive species that are not intentionally being measured (for example, interfering species), which cause an increase in signal strength due to this interference.
  • interfering species are compounds with an oxidation potential that overlap with the analyte being measured.
  • Interfering species such as acetaminophen, ascorbate, and urate, are notorious in the art of glucose sensors for producing inaccurate signal strength when they are not properly controlled.
  • Some glucose sensors utilize a membrane system that blocks at least some interfering species, such as ascorbate and urate.
  • interfering species such as ascorbate and urate.
  • it is difficult to find membranes that are satisfactory or reliable in use, especially in vivo, which effectively block all interferants and/or interfering species for example, see U.S. Patent No. 4,776,944, U.S. Patent No. 5,356,786, U.S. Patent No. 5,593,852, U.S Patent No. 5776324B1, and U.S. Patent No. 6,356,776).
  • the preferred embodiments are particularly advantageous in their inherent ability to eliminate the erroneous transient and non-transient signal effects normally caused by interfering species. For example, if an interferant such as acetaminophen is ingested by a host implanted with a conventional implantable electrochemical glucose sensor (namely, one without means for eliminating acetaminophen), a transient non-glucose related increase in signal output would occur. However, by utilizing the electrode system of the preferred embodiments, both working electrodes respond with substantially equivalent increased current generation due to oxidation of the acetaminophen, which would be eliminated by subtraction of the auxiliary electrode signal from the glucose-measuring electrode signal.
  • the system and methods of the preferred embodiments simplify the computation processes of calibration, decreases the susceptibility introduced by user error in calibration, and eliminates the effects of interfering species. Accordingly, the sensor requires less interaction by the patient (for example, less frequent calibration), increases patient convenience (for example, few reference glucose values), and improves accuracy (via simple and reliable calibration).
  • the analyte sensor is configured to measure any combination of changes in baseline and/or in sensitivity, simultaneously and/or iteratively, using any of the above-described systems and methods. While not wishing to be bound by theory, the preferred embodiments provide for improved calibration of the sensor, increased patient convenience through less frequent patient interaction with the sensor, less dependence on the values/range of the paired measurements, less sensitivity to error normally found in manual reference glucose measurements, adaptation to the maturation of the biointerface over time, elimination of erroneous signal due to non- constant analyte-related signal so interfering species, and/or self-diagnosis of the calibration for more intelligent recalibration of the sensor.
  • Example 1 Dual- Electrode Sensor with Coiled Reference Electrode
  • Dual-electrode sensors (having a configuration similar to the embodiment shown in Fig. 9B) were constructed from two platinum wires, each coated with non- conductive material/insulator. Exposed electroactive windows were cut into the wires by removing a portion thereof. The platinum wires were laid next to each other such that the windows are offset (e.g., separated by a diffusion barrier). The bundle was then placed into a winding machine & silver wire was wrapped around the platinum electrodes. The silver wire was then chloridized to produce a silver/silver chloride reference electrode. The sensor was trimmed to length, and a glucose oxidase enzyme solution applied to both windows (e.g., enzyme applied to both sensors).
  • a glucose oxidase enzyme solution applied to both windows (e.g., enzyme applied to both sensors).
  • the window was dipped into dimethylacetamide (DMAC) and rinsed. After the sensor was dried, a resistance layer was sprayed onto the sensor and dried.
  • DMAC dimethylacetamide
  • Fig. 12 shows the results from one experiment, comparing the signals from the two electrodes of the dual-electrode sensor having a coiled silver/silver chloride wire reference electrode described above.
  • the "Plus GOx” electrode included active GOx in its window.
  • the "No GOx” electrode included DMAC-inactivated GOx in its window.
  • the sensor was incubated in room temperature phosphate buffered saline (PBS) for 30 minutes. During this time, the signals from the two electrodes were substantially equivalent. Then the sensor was moved to a 40-mg/dl solution of glucose in PBS.
  • PBS room temperature phosphate buffered saline
  • This sensor was constructed similarly to the sensor of Example 1, except that the configuration was similar to the embodiment shown in Fig. 13. Two platinum electrode wires were dipped into non-conductive material and then electroactive windows formed by removing portions of the nonconductive material. The two wires were then bundled with an X-shaped silver reference electrode therebetween. An additional layer of non-conductive material held the bundle together.
  • Fig. 13 shows the results from one experiment, comparing the signals from the two electrodes of a dual-electrode sensor having an X-shaped reference electrode.
  • the "Plus GOx” electrode has active GOx in its window.
  • the "No GOx” electrode has DMAC- inactivated GOx in its window.
  • the sensor was tested as was described for Experiment 1 , above. Signal from the two electrodes were substantially equivalent until the sensor was transferred to the 40-mg/dl glucose solution. As this point, the "Plus GOx" electrode signal increased but the "No GOx” electrode signal did not.
  • a dual-electrode sensor was assembled similarly to the sensor of Example 1, with a bundled configuration similar to that shown in Fig. 7C (two platinum working electrodes and one silver/silver chloride reference electrode, not twisted).
  • the electroactive windows were staggered by 0.085 inches, to create a diffusion barrier.
  • Fig.- 14 shows the experimental results.
  • the Y-axis shows the glucose signal (volts) and the X-axis shows time.
  • the "Enzyme” electrode included active GOx.
  • the “No Enzyme” electrode did not include active GOx.
  • the “Enzyme minus No Enzyme” represents a simple subtraction of the “Enzyme” minus the “NO Enzyme.”
  • the “Enzyme” electrode measures the glucose-related signal and the non-glucose-related signal.
  • the “No Enzyme” electrode measures only the non-glucose-related signal.
  • the “Enzyme minus No Enzyme” graph illustrates the portion of the "Enzyme” signal related to only the glucose- related signal.
  • the sensor was challenged with increasing concentrations of glucose (-20 mg/dl, 200 mg/dl, 400 mg/dl) in PBS. As glucose concentration increased, the "Enzyme” electrode registered a corresponding increase in signal. In contrast, the "No Enzyme” electrode did not record an increase in signal. Subtracting the “No Enzyme” signal from the “Enzyme” signal shows a step-wise increase in signal related to only glucose concentration.
  • the sensor was challenged with the addition of acetaminophen (-.22 mM) to the highest glucose concentration.
  • Acetaminophen is known to be an interferent (e.g., produces non-constant noise) of the sensors built as described above, e.g., due to a lack of acetaminophen-blocking membrane and/or mechanism formed thereon or provided therewith.
  • Both the "Enzyme” and “No Enzyme” electrodes showed a substantial increase in signal.
  • the "Enzyme minus No Enzyme” graph substantially shows the portion of the signal that was related to glucose concentration.
  • ANALYTE SENSOR Namely, the sensor was built by providing two platinum wires (e.g., dual working electrodes) and vapor-depositing the platinum wires with Parylene to form an insulating coating. A portion of the insulation on each wire was removed to expose the electroactive surfaces (e.g., 904a and 904b). The wires were bundled such that the windows were offset to provide a diffusion barrier, as described herein, cut to the desired length, to form an "assembly.” A silver/silver chloride reference electrode was disposed remotely from the working electrodes (e.g., coiled inside the sensor's fluid connector). [0488] An electrode domain was formed over the electroactive surface areas of the working electrodes by dip coating the assembly in an electrode solution (comprising BAYHYDROL® 123 with PVP and added EDC)) and drying.
  • an electrode solution comprising BAYHYDROL® 123 with PVP and added EDC
  • An enzyme domain was formed over the electrode domain by subsequently dip coating the assembly in an enzyme domain solution (BAYHYDROL 140AQ mixed with glucose oxidase and glutaraldehyde) and drying. This dip coating process was repeated once more to form an enzyme domain having two layers and subsequently drying. Next an enzyme solution containing active GOx was applied to one window; and an enzyme solution without enzyme (e.g., No GOx) was applied to the other window.
  • an enzyme domain solution BAYHYDROL 140AQ mixed with glucose oxidase and glutaraldehyde
  • a resistance domain was formed over the enzyme domain by subsequently spray coating the assembly with a resistance domain solution (Chronothane H and Chronothane 1020) and drying.
  • the sensors Prior to use, the sensors were sterilized using electron beam radiation.
  • the forelimb of an anesthetized dog (2 years old, ⁇ 40 pounds) was cut down to the femoral artery and vein.
  • An arterio-venous shunt was placed from the femoral artery to the femoral vein using 14 gauge catheters and 1/8-inch IV tubing.
  • a pressurized arterial fluid line was connected to the sensor systems at all times.
  • the test sensor system included a 20 gauge x 1.25-inch catheter and took measurements every 30 seconds.
  • the catheter was aseptically inserted into the shunt, followed by insertion of the sensor into the catheter.
  • the dog's glucose was checked with an SMBG, as well as removing blood samples and measuring the glucose concentration with a Hemocue.
  • Fig. 15 shows the experimental results. Glucose test data (counts) is shown on the left-hand Y-axis, glucose concentration for the controls (SMBG and Hemocue) are shown on the right-hand y-axis and time is shown on the X-axis. Each time interval on the X-axis represents 29-minutes (e.g., 12:11 to 12:40 equals 29 minutes). An acetaminophen challenge is shown as a vertical line on the graph.
  • the term "Plus GOx” refers to the signal from the electrode coated with active GOx., which represents signal due to both the glucose concentration and non-glucose- related electroactive compounds as described elsewhere herein (e.g., glucose signal and background signal, which includes both constant and non-constant noise).
  • "No GOx” is signal from the electrode lacking GOx, which represents non-glucose related signal (e.g., background signal, which includes both constant and non-constant noise).
  • the “Glucose Only” signal (e.g., related only to glucose concentration) is determined during data analysis (e.g., by sensor electronics). In this experiment, the "Glucose Only” signal was determined by a subtraction of the "No GOx” signal from the "Plus GOx” signal.
  • Acetaminophen is known to be an interferent (e.g., produces non-constant noise) of the sensors built as described above, e.g., due to a lack of acetaminophen-blocking membrane and/or mechanism formed thereon or provided therewith.
  • the SMBG or Hemocue devices utilized in this experiment do include mechanisms that substantially block acetaminophen from the signal (see Fig. 15).
  • the signals from both working electrodes ("Plus GOx" and "No GOx"
  • the "Glucose Only" signal was determined, it substantially paralleled the signals of the control devices and was of a substantially similar magnitude.
  • an indwelling, dual-electrode glucose sensor system in contact with the circulatory system can provide substantially continuous glucose data that can be used to calculate a glucose concentration that is free from background components (e.g., constant and non- constant noise), in a clinical setting.
  • background components e.g., constant and non- constant noise

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Abstract

La présente invention concerne des systèmes et procédés pour un capteur de substances à analyser en continu, comme un capteur continu de glucose. Un tel système utilise des première et seconde électrodes actives pour mesurer un signal additionnel relié à une substance à analyser ou à ne pas analyser. De telles mesures permettent de réaliser des mesures de fond et/ou de sensibilité utilisées dans le traitement de données de capteur et peuvent être utilisées pour déclencher des événements comme le filtrage numérique de données ou la suspension d'affichage de données. Le capteur peut être conçu de façon avantageuse pour être réalisé intégralement, y compris les deux électrodes E1, E2 incorporées dans un isolant I.
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US9743871B2 (en) * 2012-09-24 2017-08-29 Dexcom, Inc. Multiple electrode system for a continuous analyte sensor, and related methods
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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6175752B1 (en) * 1998-04-30 2001-01-16 Therasense, Inc. Analyte monitoring device and methods of use
US20050143635A1 (en) * 2003-12-05 2005-06-30 Kamath Apurv U. Calibration techniques for a continuous analyte sensor

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB9120144D0 (en) * 1991-09-20 1991-11-06 Imperial College A dialysis electrode device
US6119028A (en) * 1997-10-20 2000-09-12 Alfred E. Mann Foundation Implantable enzyme-based monitoring systems having improved longevity due to improved exterior surfaces
US6088608A (en) * 1997-10-20 2000-07-11 Alfred E. Mann Foundation Electrochemical sensor and integrity tests therefor
US6081736A (en) * 1997-10-20 2000-06-27 Alfred E. Mann Foundation Implantable enzyme-based monitoring systems adapted for long term use
US20030032874A1 (en) * 2001-07-27 2003-02-13 Dexcom, Inc. Sensor head for use with implantable devices
US10022078B2 (en) * 2004-07-13 2018-07-17 Dexcom, Inc. Analyte sensor

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6175752B1 (en) * 1998-04-30 2001-01-16 Therasense, Inc. Analyte monitoring device and methods of use
US20050143635A1 (en) * 2003-12-05 2005-06-30 Kamath Apurv U. Calibration techniques for a continuous analyte sensor

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
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