WO2009055865A1

WO2009055865A1 - Implantable prosthesis with sensor

Info

Publication number: WO2009055865A1
Application number: PCT/AU2008/001619
Authority: WO
Inventors: John Chambers; Peter Van Gerwen
Original assignee: Cochlear Limited
Priority date: 2007-10-31
Filing date: 2008-10-31
Publication date: 2009-05-07

Abstract

A system and method are disclosed for sensing temperature in an implanted medical device with a primary operative purpose. The temperature sensor (20) is provided so as to allow an alarm to be generated if predetermined criteria are met, indicated e.g. that the patient has a post operative fever.

Description

IMPLANTABLE PROSTHESIS WITH SENSOR

FIELD OF THE INVENTION

The present invention relates to implantable medical devices with enhanced functionality.

BACKGROUND TO THE INVENTION

Devices are implanted in the body for a variety of purposes including monitoring, heart defibrillation, drug delivery and as neural and organ prostheses. Such devices assist the body to carry out a function that it can no longer perform, monitor organs for regular function, or provide a combination of functions such as providing an electrical impulse should monitoring reveal irregular function. Such implanted devices generally include electronic components and other elements for a variety of purposes, including delivery of electrical stimulation or drugs, monitoring of parameters, communication with or control of other devices to store information, and to communicate via RF or other means.

A cochlear implant prosthesis is one such surgically implantable device that provides hearing sensation to individuals with severe-to-profound hearing loss. Other implantable auditory devices include any acoustic or electrical auditory devices, middle ear implants, intra-cochlear implants, brain stem implants, implanted acoustic devices or any combination of these, for example devices providing combined electrical and acoustic stimulation.

A conventional cochlear implant prosthesis includes an electrode array. The electrode array is inserted into the inner ear region during an operation that usually takes between 2-3 hours depending on the device to be implanted. Complications can arise due to the surgery. For inner ear surgery, for example, there is an increased likelihood of the implant recipient contracting meningitis. Any infection is of concern, but particularly so in areas of the body like the inner ear which have limited immune response due to the lack of blood flow to the area. Meningitis is an infection of the meninges that can be more readily treated when diagnosed early. Current postoperative practice to monitor implant recipients for infection is for nurses to wake the implant recipient at regular intervals and monitor the patient's temperature, with an increase indicating a potential infection. In some cases, this can mean hourly waking for the patient, which interrupts restorative sleep.

It is therefore desirable to provide an improved method for monitoring for infection after implantation of a medical device. SUMMARY OF THE INVENTION

Broadly, the present invention provides an implantable device including a temperature sensor, arranged so that if predefined criteria are exceeded, an indication of an alarm condition is generated.

According to one aspect the present invention provides an implantable device system including: an implantable device adapted to be implanted in a user, said device having a primary operative purpose and being adapted to transmit data after implantation; a temperature sensor associated with said device for monitoring the temperature of the user; wherein if the monitored temperature meets predefined criteria, said system provides an alarm or other indication that a specific physiological condition has been detected.

The primary operative purpose may be any purpose for which the device is implanted, for example neural stimulation, augmentation of function, monitoring, or any other purpose, other than acting purely as a temperature sensor.

According to another aspect, the present invention provides a method of monitoring body temperature using an implantable device, the method including at least the steps of: providing a temperature sensor associated with said device; comparing the measured temperature data to predetermined criteria; and if said predetermined criteria are met, providing an alarm or other indication that a specific physiological condition has been detected.

An embodiment of the present invention accordingly provides a mechanism by which the device itself can measure temperature and detect changes which may be indicative of infection, illness, or metabolic status and communicate this to its user and or to one or more external devices. This invention may be applied more generally to the measurement, recording, analysis, and alerting of human body temperature values and trends. A rapid overnight rise in body temperature might for example allow the invention to alert a user as to the onset of influenza or certain other types of illness such as an acute malarial attack. Conversely the invention could be used to monitor long- term metabolism or hormone induced temperature changes to allow a female user to track the status of their monthly menstrual cycle. BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention will now be described with reference to the accompanying figures, in which:

Figure 1 is a schematic diagram of a typical cochlear implant hearing prosthesis, in which the present invention can be implemented;

Figure 2 illustrates the implanted location of the cochlear implant of Figure 1 ; Figure 3 is a schematic diagram of an embodiment of the present invention implemented in the intra-cochlear electrode array of a cochlear implant prosthesis;

Figure 4 shows an alternative implementation showing in particular a possible configuration where an existing electrode wire forms a part of the temperature sensor;

Figure 5 shows a schematic view of another alternative embodiment of the invention whereby much of the temperature sensor is disposed at the distal tip of an intra-cochlear stimulating electrode array;

Figure 6 is an electrical schematic diagram of an embodiment of the invention showing how the temperature sensor incorporated in the stimulating electrode is electrically connected to other circuit elements.

Figure 7 is an electrical schematic diagram showing an alternative configuration to that depicted in Figure 6;

Figure 8 is a functional block diagram of the implanted part of a cochlear implant hearing prosthesis, which incorporates an embodiment of the invention; and

Figure 9 is a functional block diagram of the external part of a cochlear implant hearing prosthesis, which incorporates an embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENT

Embodiments of the present invention are described herein primarily in connection with one type of implantable device, an implantable hearing prosthesis. Hearing prostheses in this sense include but are not limited to any acoustic or electrical auditory device, such as a cochlear implant, middle ear implants, intra-cochlear array implants, brain stem implants, implanted mechanical stimulators, implanted acoustic device or any combination, for example combined electrical and acoustic stimulation. However, it will be understood that this invention could well apply to other implantable prosthetic devices including but not limited to pacemakers, implantable defibrillators, muscle stimulators, neurological stimulators, monitoring devices, drug pumps, and so on. It will be understood that the embodiments described are intended to be illustrative of the present invention, but not limitative of its scope.

Figure 1 illustrates the main implanted parts of a typical cochlear implant hearing prosthesis. A hermetically sealed housing 1 encloses and protects the electronic circuitry contained therein. An induction coil 2 is used for the reception of power and bi-directional conveyance of data signals while wiring cables 3 connect the electronic circuitry to the stimulating electrodes 4. The location of such an implant 5 is shown on a recipient in Figure 2. In order to monitor for infection or other illness, a temperature sensor 20 is provided as part of the cochlear implant hearing prosthesis. The sensor 20 shown is constructed entirely of biocompatible materials, allowing the temperature measurement means to be incorporated in the stimulation electrode system 21. The temperature measurement may be of use either to monitor immediate postoperative issues such as infection, or of an ongoing nature to monitor temperature rises in body tissue to assist in avoiding body tissue or cellular damage.

The following example utilises a platinum wire 6 and a semiconductor junction incorporated either separately or jointly with other electronics of an implanted prosthesis, thereby creating a temperature sensor 20 within a neural stimulating electrode array 21 by the use of one additional platinum metal conductor 6. The use of thin, electrically insulated biocompatible platinum alloy wire in existing cochlear implant neural stimulating electrodes readily facilitates the incorporation of such a temperature sensor 20. Accordingly, the temperature sensor may be provided by the simple addition of one extra wire 6 in parallel with existing wires and the connection of this wire to an electrode wire near the distal tip of the array, so as to form a loop that may also incorporate a coiled up section. Since the current and voltage characteristics of silicon semi-conducting gates and diode junctions are known and are strongly temperature dependent, the electrical parameters of a diode, for example can be used to calculate the temperature of the die (the term die refers to the chip inside the implant).

Further, since the temperature co-efficients α and β of platinum are well known, calibration is achieved using these co-efficients, and measuring or calculating the applied current and voltage across the sensor. While a current of several milliamperes may be necessary to achieve sufficient accuracy to resolve temperature trends below 0.1 °C, this measurement current need only be applied momentarily for a millisecond or so, every few minutes. At such a low duty cycle the average current is so low that its effect on battery life and sensor self-heating is almost negligible.

When an extra platinum wire 6 is added to a platinum conductor neural stimulating electrode array 21 , temperature measurements can be based on the temperature dependence of the average electrical resistance of the platinum wires, as will now be described.

At a reference temperature T₀, the electrical resistivity of a given material is denoted as po. The electrical resistivity p at any temperature T is then given by

where a is the linear temperature coefficient and β is the 2nd order temperature coefficient. Both parameters may be determined empirically.

R, the resistance of a wire with fixed cross-sectional area A, a constant uniform resistivity and total length L being exposed to a temperature gradient, can then be calculated by taking the integral along its length, indicated by the coordinate x.

(₂₎

By combining equations (1) and (2) together, the new equation can be simplified to

Introducing the average temperature T given by,

L and the reference resistance R₀,

equation (3) can further be simplified to

By defining the average temperature difference

_i given by

Equation (6) becomes

where is the temperature variance.

By using equation (8), if β=0 (linear temperature dependence) or στ=0 (no temperature variance, hence a uniform temperature along the temperature sensor), then the average temperature can be derived from the total resistance.

However, even in the non-linear case with a non-uniform temperature, the resistance will always be a good indication of the real temperature.

For example, a 200Ω platinum metal resistor at 25°C will increase in resistance to 200.38Ω at 25.5°C,

( )

In the simplest case, one assumes a linear temperature rise from the chip down to the electrode. In that case, the average temperature is just the mean between the two tips:

where Tdie is the temperature of the die and T_eiectrode is the temperature of the electrode. Therefore, the average temperature T can be measured by measuring the total resistance of the extra wire 6 and one of the standard electrode wires 32, the die temperature T_die can be measured with the integrated resistor on the chip in the implant case.

While the temperature profile between the die and the tip of the electrode may be more complex than is previously expressed, it should be appreciated that, given a fixed geometry and material parameters, the electrode temperature will always be

As such, an approximating formula can be derived from calibration measurements and stored in a simple look-up table.

If it is only the temperature increase of the electrode that is of interest, an isothermal calibration of the diode on the die and of the resistance is then sufficient. That is, the platinum resistance and the diode characteristics are measured at a given temperature. Then, absolute temperature is not known, but changing temperature will result in a deviation of the calibrated value, and the change in temperature, ΔT, can then be calculated. It is expected that measurements of temperature changes of 0.5°C are achievable.

Temperature measurements based on the voltage developed across a thermocouple junction can also be calculated using a type R or S thermocouple. In this case the single extra wire 6, shown in Figure 3, comprises PtRd wire such that it is a different material to the thermocouple material of the existing electrode wire 32 to which it connects. The measured voltage across these two wires is proportional to the temperature difference of both its connections, being close to the electrode and close to the die.

Apart from electronic pre-amplification and offset-compensation no calibration is needed. Such a thermocouple will typically give 6μV/K at temperatures between 20°C and 40°C; accordingly, for a 0.5°C increase, a voltage of 3μV has to be measured. Cold junction compensation for the cold side of the thermocouple can be derived from a diode junction within the body of the implant where temperatures matching that of the platinum to copper wire connection within the implant are similar.

Figure 3 shows the additional platinum metal conductor 6 and its return path, that can be added to an intra-cochlear electrode array 3 for the purpose of functioning as a temperature sensor. This embodiment is relevant for both the temperature information obtained based on measurement of the resistance and that based on thermoelectric measurement.

Figure 4 depicts an enlarged view of an intra-cochlear electrode array whereby a platinum metal conductor 6 has been added so as to electrically connect with one of the electrodes 4, and/or the electrical conductor 32, that is connected to this electrode. In addition to conveying electrical current to and from the electrode, conductor 32, in conjunction with conductor 6, completes an electrical circuit, the electrical resistance and hence temperature of which is determinable from Ohm's law and the temperature co-efficient of resistance for platinum.

Figure 5 is an embodiment of the measurement based on resistance, it illustrates a configuration of an intra-cochlear stimulating electrode array 3, whereby much of the length of the additional platinum wire 6 and hence temperature dependant resistance, is configured or coiled 20, to be disposed in close proximity to the stimulating electrodes 21 at the distal tip. In this case the additional platinum wire 6, connects with the conductor associated with the electrode labelled as 4. Body temperature measurements for medical diagnosis should ideally reflect the temperature of the blood supplying the subject's brain (ie core body temperature). Therefore having the temperature sensor disposed in very close proximity to the intra-cochlear neural stimulating electrodes, deep within a users head inherently provide good quality temperature information.

Furthermore, the subsequent increase in the electrical resistance that may result from doing so can be used to increase the amplitude of temperature measurement signals when a fixed current is applied.

An electrical schematic diagram showing how additional platinum wire 6 is . connected to existing electrode wire 32 and electrically connected to other circuit elements is shown in Figure 6. The components located within the hermetic housing 24 of the implant are outlined by a dotted rectangle. For the sake of simplicity, the temperature sensor formed by the combined resistance of wires 6 and 32 is depicted as a discrete element 20 located outside of this hermetic housing 24 with the neural stimulating electrodes 21. The associated interconnecting wires 26, in which existing electrode wire 32 forms a part thereof, are also external to the implant housing.

A pair of constant current sources and an electronically controlled switch

25 are used to periodically supply a known current through the resistive path of the temperature sensor 20. An electronic signal amplifier 22 amplifies the voltage signal measured across the temperature sensor 20, and conveys an output signal

23 to other circuitry.

An alternative electronic circuit configuration is shown in Figure 7 in which three additional conductors have been added so as to allow what is commonly known as a four wire resistance measurement to be undertaken. This configuration is advantageous when used in conjunction with the embodiment of

Figure 5, since the measurement excludes the voltage drop across the length of wires connecting the temperature sensor 20 to the implant, such that a more direct measurement of the sensor's resistance and subsequent temperature is possible.

The operation of the sensor as part of a cochlear implant hearing prosthesis system is shown in Figures 8 and 9. A current, known within measurement limits, is applied periodically by current source 25 to a resistive path 6 made up of platinum wire and incorporating a temperature sensor 20. Programmed electronics within the implant system momentarily measure the voltage across this loop while the known pulsatile current is applied. The resulting voltage signal is conditioned and converted to digital form for processing to extract temperature information, either locally by the implanted prosthesis, or remotely after having been conveyed to external parts of the prosthesis outside the body.

Routine processing and statistical analysis of the stored data is performed by the invention so as to determine the minimum temperature, maximum temperature, mean temperature and trending and cyclic temperature characteristics of the user. The results of this analysis are regularly compared with data representing the range and variation that is safely allowable before a user or carer alert is issued by the prosthesis. The invention or parts associated with the invention can be arranged to emit a flashing light or alarm sound, and to convey an alarm to a remote electronic wireless receiver device such as mobile phone, a commercial broadcast receiver or a baby monitor like device from where a message can be delivered to a carer or guardian. A dedicated alarm or alerting system could flash a light and emit a beeping sound followed by the sound of an electronic synthesised voice describing the nature of the alert in plain language. Such examples might include, "WARNING Your baby is developing a fever that may be related to recent surgery, please consult a healthcare professional as soon as possible." A temperature sensor and associated monitoring system as described is particularly suitable for incorporation within cochlea implants as these hearing prostheses often remain switched off during the first weeks after implantation, while the recipient's neurological sensitivity to electrical stimulation stabilizes and when the risk of post operative infection or tissue rejection is greatest. As the electronic data gathering, storage, processing and conveyance capabilities of the implant system are not required for hearing during this period, these capabilities can be fully dedicated to execute the aims of this invention at a time when post operative diagnostics and the detection of infection are most likely to benefit the user. By the time the hearing functionality of the prosthesis is required, the acute risk of infection is likely to be lower, such that the rate at which the invention acquires body temperature data for the purpose of detecting infection can be lowered so as not to impact hearing functionality. In this arrangement, a specific purpose external charging and/or communication arrangement may be used for the first period of time, to take best advantage of the temperature alarm feature.

However, although the present invention has been principally described with reference to a cochlear implant prosthesis, it will be appreciated that this construction can readily be applied to other implant prosthesis. It will be understood that the present invention is capable of being used for any active implantable medical device, that is, where it is necessary to provide an electrical connection from the exterior of the device to electrical or electronic components inside the device. Any implantable medical prosthesis such as those incorporating, for example, programmable electronics, telemetry data links and electrode systems could be adapted to use this invention to measure the body temperature of the implanted user.

Further, other noble metals, ailoys, or composite material with a higher resistive or thermocouple temperature to voltage co-efficient that can be made suitable for implantation can be used to increase sensitivity. Alternative designs of temperature sensor and measurement system calibration reference sources for such parameters as resistance, voltage, current or time could also be used to implement the present invention.

It will be appreciated that the present invention may be applied to a range of implanted devices, and that it is not limited to any particular device or application. Further, additional features may be used in conjunction with the present invention.

Claims

CLAIMS:

1. An implantable device system including: an implantable device adapted to be implanted in a user, said device having a primary operative purpose and being adapted to transmit data after implantation; a temperature sensor associated with said device for monitoring the temperature of the user; wherein if the monitored temperature meets predetermined criteria stored or programmed in the device, said system provides an alarm or other indication that a specific physiological condition has been detected.

2. The system according to claim 1 , wherein the device is adapted to provide electrical stimulation to the user.

3. The system according to claim 1 or claim 2, wherein the temperature sensor includes thermoelectric components.

4. The system according to claim 1 , wherein the temperature sensor uses components of the implanted device having other purposes.

5. The system according to claim 4, wherein the temperature sensor uses an electrode wire of the device.

6. The system according to claim 1 or claim 2, wherein the temperature of the user is monitored by supplying a substantially known current through a resistive path, said resistive path including a temperature sensor, measuring the voltage signal across the temperature sensor; and processing a corresponding measured voltage signal to extract temperature information.

7. The system according to claim 6, wherein the supply of the substantially known current is selected from periodic, intermittent, or continuous.

8. The system according to any one of the preceding claims, wherein further data is utilised by the device to determine whether the predetermined criteria are met, said including one or more of the minimum temperature, maximum temperature, mean temperature, trending and cyclic temperature characteristics of the user.

9. The system according to any one of the preceding claims, wherein the alarm is selected from a flashing light or alarm sound on a remote receiver device or mobile phone, or a commercial broadcast receiver.

10. The system according to any one of the preceding claims, wherein the implantable device is a cochlear implant.

11. The system according to claim 10 when dependent on claim 2, wherein the cochlear implant has an electrode array and the temperature sensor is located adjacent to, or coiled around, the electrode array.

12. The system according to claim 11 , wherein voltage is additionally measured across an electrode wire of the electrode array, and temperature information is obtained on the basis of the voltages measured across each respective wire being proportional to the temperature difference between the respective wires.

13. A method of monitoring body temperature using an implantable device, the method including at least the steps of: providing a temperature sensor associated with said device; using the temperature sensor to obtain temperature data of a user; and comparing the temperature data to predetermined criteria; wherein if said predetermined criteria are met, providing an alarm or other indication that a specific physiological condition has been detected.

14. A method according to claim 13, wherein further data is utilised by the device to determine whether the predetermined criteria are met, said further data including one or more of the minimum temperature, maximum temperature, mean temperature, trending and cyclic temperature characteristics.

15. An implantable medical device, including an electrode array, said electrode array including a conductor arranged to at least periodically carry a current, such that said conductor is operatively arranged to function as a temperature sensor.

16. A device according to claim 15, wherein the device is a cochlear implant and the conductor is an additional wire associated with the electrode array.

17. A device according to claim 16, wherein the sensor detects differential changes in resistance of the conductor.