WO2008135930A1

WO2008135930A1 - Sensor system based on compound with solubility depending on analyte concentration

Info

Publication number: WO2008135930A1
Application number: PCT/IB2008/051709
Authority: WO
Inventors: Hans Zou; Lucian Remus Albu; Jeff Shimizu
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-05-03
Filing date: 2008-05-02
Publication date: 2008-11-13
Also published as: TW200914822A

Abstract

A biochemical sensing device for measuring an analyte level, e.g., for in vivo applications. An exemplary device includes an electrode (12), a power supply (18) connected to the electrode, and a compound/polymer coating (14) on the electrode. The compound/polymer is soluble in a solution, and its rate of solubility is dependent on an analyte-related property of the solution, e.g., the solution pH. The biochemical sensing device can further include an electrical switch (16) in the connection between the power supply and the electrode. The biochemical sensing device can further include a data processor (22) connected to the power supply (18), and a data memory component (24) connected to the data processor. A process for measuring analyte-related property of a solution is also provided. The process includes providing an electrode coated with a compound/polymer, wherein the compound/polymer has an analyte- dependent solubility profile, exposing the compound/polymer coated electrode to the solution, measuring the conductance at the electrode as a function of time, and correlating the rate of conductance to the solubility of the compound/polymer to determine the pH of the solution.

Description

SENSOR SYSTEM BASED ON COMPOUND WITH SOLUBILITY DEPENDING ON ANALYTE CONCENTRATION

The present invention relates to a method of measuring the concentration of analytes present in a fluid and, more particularly, to sensor systems that utilize or include compounds that exhibit variable solubility based on analyte concentration.

Concentration of an analyte in biological fluid can be measured either by quantifying a chemical interaction that is specific to the target analyte or transducing presence of an analyte into measurable physical parameters. For example, a fluid's pH can be measured by titration (chemical reaction) or using a pH meter that translates concentration of H+ into electrical potential.

A traditional pH meter uses two glass electrodes: the indicator electrode and the reference electrode. When the two electrodes are immersed in a solution, a small galvanic cell is established. The potential developed is dependent on both electrodes. The response is caused by an exchange at both surfaces of the swollen membrane between the ions of the glass and the H+ of the solution in an ion exchange which is controlled by the concentration of H+ in both solutions. This traditional pH-sensing technology could be miniaturized to a certain size to measures pH values in vivo and report data telemetrically.

Another pH-sensing technology is based on use of an ion sensitive field effect transistor (ISFET). In an ISFET, an H+ sensitive buffer coating is applied to the gate electrode. Therefore, the voltage drop between the drain and source electrodes becomes a function of H+ concentration to which the gate electrode is exposed. An ISFET-based pH-sensor can be built into a small volume. Laboratory ISFET pH-sensors have been commercially available and are generally more expensive than traditional glass-electrode pH-sensors.

Traditional pH-sensing technology based on glass electrodes can not be practically miniaturized to a size appropriate for an electronic pill. Both traditional and ISFET-based pH sensors require calibration and a reference electrode, making their use complicated. The pH accuracy measured by an ISFET pH-sensor can be negatively impacted by the presence of ions other than H+, for example, ions of table salt. Current cost of ISFET is relatively high. Additionally, the accuracy of absolute pH value and reproducibility of all currently available pH- sensors depend on accurate and timely calibration, which make their deployment expensive and cumbersome. For application to real-time diagnosis of the gastrointestinal (GI) tract, a pH sensor must operate in vivo during transit in the GI tract and occupy a very small volume, while absolute precision is less critical and recoverability is not a requirement.

Besides pH, concentrations of other analytes, such as proteins, amino acids, glucose, enzymes and the like, are also of great diagnostic value. Although, such analytes can be measured in vitro with laboratory devices, no practical in vivo method and/or apparatus to measure concentration of proteins and enzymes is commercially available.

In the biomedical area, in vivo measurement can provide real-time and in situ information. However, it is a requirement that an in vivo method be biocompatible so that the process and byproduct of the method does not interfere with the natural operation of the biological system. In addition, due to the natural size of organs, any devices and/or components to be introduced in vivo must be small enough to fit into biological sensing areas.

Despite efforts to date, a need remains for effective methods and systems for measurement of analyte concentration, particularly in biomedical applications, e.g., in vivo applications. These and other needs are satisfied by the methods and systems disclosed herein.

Biochemical sensing devices and methods for biochemical measurement are provided according to the present disclosure. The disclosed sensing devices and methods may be employed in vivo for measurement of analyte levels. For example, the disclosed sensing devices and methods may be employed to measure H+ concentration (i.e., pH), proteins, amino acids, glucose, enzymes and other analytes of interest.

In an exemplary embodiment of the present disclosure, a sensing device is provided that includes an electrode (12), a power supply (18) connected to the electrode, and a compound coating (14) on the electrode. The compound is soluble in a solution of an analyte and its rate of solubility is dependent on the concentration of the analyte, thereby permitting quantification of the analyte level in a particular environment. The biochemical sensing device can further include an electrical switch (16) in the connection between the power supply and the electrode. The biochemical device can further include a data processor (22) connected to the power supply (18), and a data memory component (24) connected to the data processor.

A process for measuring the analyte concentration of a solution is also provided. In exemplary embodiments of the present disclosure, the disclosed process includes providing an electrode coated with a compound, wherein the compound has an analyte-concentration- dependent solubility profile, exposing the compound coated electrode to the solution, measuring the rate of impedance at the electrode as a function of time, and correlating the rate of impedance to the solubility of the compound to determine the analyte concentration of the solution.

An example of biochemical sensing device in accordance with the present disclosure is a pH-sensing device. Additional advantageous features, functions and applications of the disclosed sensing device and methods will be apparent from the description which follows.

To assist those of skill in the art in making and using the disclosed systems and methods, reference is made to the accompanying figures, wherein:

FIG. 1 is a schematic representation of one embodiment of a pH sensor in accordance with the present disclosure.

FIG. 2 is a schematic representation of another embodiment of a pH sensor in accordance with the present disclosure.

FIG. 3 is a is a schematic representation of another embodiment of a pH sensor in accordance with the present disclosure.

FIG. 4 is a schematic representation of a coated electrode in accordance with one embodiment of the present disclosure. .

FIG. 5 is a schematic representation of a pH sensor containing an array of electrodes in accordance with one embodiment of the present disclosure.

FIG. 6A is a representation of a pH sensor containing an array of electrodes in accordance with one embodiment of the present disclosure.

FIG. 6B is a representation the side view of the pH sensor shown in FIG. 6A.

A biochemical sensing system is provided that can be adapted to be compact in size, economic to manufacture, convenient to deploy, and biocompatible. Exemplary embodiments of the disclosed biochemical sensor system includes at least one pair of electrodes covered by a compound, for example a polymer, with solubility that depends on concentration of a biochemical analyte. When the electrodes, which are coated by the noted compound/polymer, are exposed to a solution, the conductance impedance between the electrodes changes as a function of the solubility of the compound/polymer in the solution. To the extent that the compound/polymer dissolves in the solution, the sample solution fills the void generated by the dissolving compound/polymer. Because the compounds/polymers are poorer conductors compared to solutions, dissolution of the compound/polymer coating on the electrode(s) leads to an increase of conductance between the electrodes. The rate of conductance change further depends on the compound/polymer properties and the actual analyte concentration of the sample solution.

Materials suitable for use in coating electrode(s) according to the present disclosure include commercially available materials that exhibit solubilities dependent on the concentration of a biochemical substance of interest, e.g., H+ concentration (i.e., pH), proteins, amino acids, glucose, enzymes and other analytes of interest. Exemplary materials for use in coating electrode(s) according to the present disclosure include polymers that exhibit a pH- dependent dissolution rate, such as EUDRAGIT acrylic polymers manufactured by Degussa GmbH, and polymers that exhibit dissolution rates that are dependent on the presence of colon enzyme, such as azo polymers used by Alizyme pic (Cambridge, United Kingdom). The noted polymers have been used as coatings on pills for targeted drug delivery. However, according to the present disclosure, use of materials having the noted properties are advantageously employed as electrode coatings.

For illustration purposes, the disclosed systems and methods are described in greater detail herein with reference to a pH-related implementation. However, the disclosed systems and methods have wide ranging applicability, as will be readily apparent to persons skilled in the art, including implementations directed to a variety of analytes. Thus, in one exemplary embodiment of the present disclosure, the system includes an electrode-coating compound/polymer that does not dissolve until the pH is above a threshold value and, as a result, the conductance between the electrodes does not increase until the sample solution has a pH above this threshold. If the pH of the sample solution is above the applicable threshold, the conductance between electrodes will advantageously increase proportionally to the difference between the sample's actual pH value and the threshold pH of the polymer.

Additionally, if the compound/polymer coated on the electrodes does not dissolve until the pH of the solution is below a threshold value, the conductance between the electrodes does not increase until the solution has a pH below this threshold, and the conductance will increase proportionally to the difference between solution's actual pH value and the threshold if the solution's pH value is below the threshold. Therefore, by monitoring the rate of change in conductance between the electrodes that are covered with the compound/polymer, either a pH limit value or an actual pH value can be derived. When a pH-sensor based on this principle of pH-dependent solubility is used, conductance between each pair of electrodes is measured as a function of time, and the rate of conductance change is used to derive the pH value of the solution. One unique advantage of such a pH sensor system is that the sensor can be operated without calibration. Variation in manufacturing process and environmental conditions, such as overall conductivity of the sample solutions, can cause variation in absolute conductance between electrodes. These variations, however, do not interfere with derivation of the pH value of a sample solution because the pH value is determined by the change rate of conductance, not by the absolute value of conductance. Of course, such systems can be used in conjunction with a reference electrode to account for environmental changes in the rate of conductance.

As shown in FIG. 1, an exemplary pH sensor according to the present disclosure includes a pair of electrodes 12 and 12' covered with a compound/polymer 14, a timed switch 16, an AC power source 18 with an alternating frequency between 100Hz and lMhz, a data- processor 22, a memory 24 that stores values of measured conductance between the electrodes and a look-up table for conductance change gradient and its corresponding pH value, and a component 26 for displaying or reporting a pH value. The switch 16 turns a connection between the AC power source and electrodes on and off at a given temporal interval. Current passing through the electrodes is measured, converted to corresponding conductance by the data- processor and stored in the memory. The data-processor calculates the gradient of conductance change, compares it with values in the look -up tables for the compound/polymer, and reports the current pH value of the sample solution 10.

The electrodes can be any appropriate configuration, and do not need to be flat plates as shown, but rather can take any convenient shape and layout. Additionally, a system clock can be used to synchronize data flow and processing.

In one embodiment, the pH sensing device includes an electrode pair 12, 12', a power supply 18 connected 15 to the electrode pair 12, 12', and a compound/polymer coating 14 on at least one electrode. The polymer can be soluble in a solution, and its rate of solubility can be dependent on the pH of the solution. In another embodiment, the pH sensing device can further include an electrical switch 16 in the connection between the power supply 18 and the electrode pair 12, 12'. Additionally, the pH sensing device can include a data processor 22 connected 20 to the power supply 18, and a data memory component 24 connected to the data processor. In an exemplary embodiment, the pH sensor includes one pair of electrodes covered with a compound/polymer that dissolves in solutions with pH between about 2 and about 6.5. The presence of H+ ions can affect the solubility of particular compounds/polymers, for example polymers that are commonly used as enteric coatings on pharmaceuticals. Examples of such polymers are synthetic acrylic polymers for coating pharmaceutical dosage forms under brand name EUDRAGIT (Rohm GmbH, Germany). The solubility properties of EUDRAGIT polymer depend on conditions, particularly pH value of the solution. Enteric coatings are specifically designed to have solubility properties compatible with the desired stage of the digestive tract. For example, EUDRAGIT LlOO does not dissolve in aqueous fluid until pH value is above 6.0 (threshold pH value for dissolution) and its dissolution rate can increase by 10-fold from pH 6.1 to pH 7.1. Further characteristics of such polymers are the high stability to environmental influences during storage and skin friendliness, i.e., indifference to bodily tissue and fluids (see, "Practical Course in Film Coating of Pharmaceutical Dosage Forms with EUDRAGIT," Rohm GmbH, Germany, 2001). These polymers are poor electrical conductors when they are in solid state.

The EUDRAGIT polymers appropriate for use with the pH sensing device include methacrylic polymers, methacrylate polymers, aminoalkyl methacrylate polymers, and ammonioalkyl methacrylate polymers. In addition to EUDRAGIT polymers, appropriate polymers for use in pH-related implementations of the present disclosure include any polymer(s) that have pH-dependent solubility. Appropriate polymers include polyvinyl acetate phthalate polymers, hydroxypropyl methylcellulose phthalate polymers, cellulose acetate trimelliate polymers, and cellulose acetate phthalate polymers.

Also appropriate for use with the disclosed pH sensing devices of the present disclosure are compounds/polymers that have a threshold pH between about 5 and about 7, which means that the compounds/polymers dissolve in solutions with pH between about 5 and about 7. Alternatively, compounds/polymers that have a threshold pH between about 4 and about 6 are also appropriate for use, and such compounds/polymers dissolve in solutions with pH between about 4 and about 6. Additionally, the compounds/polymers used can dissolve in solutions with pH between about 2 and about 6.5. The compounds/polymers can have solubility rates that are appropriate for the application, such that the rates are not so fast that it is difficult to measure the rate and/or that would require a particularly thick coating of compound/polymer, and are not so slow to dissolve that it is difficult to measure a change in conductance in a short period of time. Preferably, the compounds/polymers have maximum solubility rates of greater than about 1 mm per 100 minutes. Similarly, the compounds/polymers preferably have a range of solubility rates that span over 10 fold, such that at the threshold pH, the compound/polymer dissolves at a rate of 1 μm per minute and the rate increases to a maximum of about 20 μm per minute.

To cover a wider pH range than available with a single compound/polymer, multiple pairs of electrodes, each covered with a compound/polymer of different threshold dissolving pH, may be advantageously employed according to the present disclosure in a single pH sensor system. In another embodiment according to the present disclosure, a pH sensor is provided that includes more than one pair of electrodes and each pair of electrodes is coated with a compound/polymer having a different threshold pH value for dissolution.

Thus, a pH sensor is disclosed herein in which two or more pairs of electrodes are coated with compounds/polymers having a threshold pH value different from the other pairs of electrodes. With more than one pair of electrodes coated with compounds/polymers having different threshold pH values, the disclosed pH sensor can sense a wider range of pH and achieve better accuracy. For example, electrodes can be designated for detecting pH in different regions of the digestive tract, including the stomach and the intestines, which have different pH ranges. In an application such as an ingestible pill, the pH sensor could have electrodes dedicated to detecting the low pH values present in the stomach, while electrodes with compounds/polymers that are insoluble at such low pH values remain intact until the sensor reaches the higher pH regions of the intestine.

As shown in FIG. 2, an exemplary pH sensor with a first pair of electrodes 25, 25' that are coated with a first compound/polymer 28. The device further includes a second pair of electrodes 29, 29' that are coated with a second compound/polymer 30. The second compound/polymer is soluble in a solution, and its rate of solubility is dependent on the pH of the solution. In one embodiment, the first compound/polymer is insoluble below a threshold pH value, and the second compound/polymer is insoluble below a second threshold pH value. In another embodiment, the first compound/polymer is insoluble above a threshold pH value, and the second compound/polymer is insoluble below a second threshold pH value. In yet another embodiment, the first compound/polymer is insoluble below a threshold pH value, and the second compound/polymer is insoluble below a second threshold pH value.

As shown in FIG. 2, the electrodes are exposed to a sample solution 27 and impedance or conductance between each pair of electrodes is monitored by applying an AC voltage 18 to the electrodes at a constant time interval. The impedance values of at least two successive measurements are used to calculate the rate of conductance change. The calculated rate of conductance change is compared with the stored data to determine the current pH value limit or actual pH value of the sample solution. According to the present disclosure, the pH sensor can have more than one pair of electrodes coated with a compound/polymer having a threshold pH value between 2 and 6.5, but the threshold pH can be different from that of any other pairs of electrodes. It should be noted that the electrodes do not have to be flat plates as shown in FIG. 2, but can take any convenient shape and layout as needed. Further, all electrodes do not have to be made on separate carriers as shown in FIG. 2, but can be built on the same physical carrier. A system clock (not shown here) can be used to synchronize data flow and processing.

With reference to FIG. 3, the pH sensing device includes a second compound/polymer 38 covering a first compound/polymer 36 that coats the electrode pair 39, 39'. The second compound/polymer can serve to protect the first compound/polymer from dissolving until after the second compound/polymer has dissolved. The combination can be designed to allow detection of pH in a region of the digestive tract that requires passage through an earlier region of similar pH. In one embodiment, the first compound/polymer 36 is insoluble below a threshold pH value, and the second compound/polymer 38 is insoluble below a second threshold pH value. In another embodiment, the first compound/polymer 36 is insoluble above a threshold pH value, and the second compound/polymer 38 is insoluble below a second threshold pH value. In yet another embodiment, the first compound/polymer 36 is insoluble above a threshold pH value, and the second compound/polymer 38 is insoluble above a second threshold pH value. In still another embodiment, the first compound/polymer 36 is insoluble below a threshold pH value, and the second compound/polymer is insoluble above a second threshold pH value.

The pH sensor can include at least one pair of electrodes coated with a first thick compound/polymer having a defined threshold pH value and a second thin compound/polymer having a threshold pH value different from that of the first one. In this configuration, the first compound/polymer is protected from unintended exposure. For example, to build a pH sensor to measure pH in the colon, an appropriate compound/polymer is one that does not dissolve until the pH value is above 6. However, before and shortly after a pH sensor in the form of an ingestible pill is swallowed by a patient, the sensor may have a chance to be exposed to fluids with pH above 6. To protect the sensor from unintended or premature exposure, a compound/polymer that does not dissolve until pH is below 5 can be used as a thin coating on top of the compound/polymer with threshold pH 6. This thin top coating will dissolve quickly in the low pH environment of the stomach, and the colon pH sensor will be ready for deployment. The sensed dissolution of this thin coating may be too fast to provide exact pH value, but it can be used as a landmark for the pH sensor pill's passage.

Referring now to FIG. 4, the disclosed pH sensing device can include a protection layer 46 between the electrode 42 and the compound/polymer coating 48. As shown in FIG. 4, exemplary electrode 42 is mounted on a substrate 44 and first coated with a protection layer 46 before it is covered by the compound/polymer 48. The protection layer can be formed out of a suitable material, e.g., chromium, gold, platinum, metal-oxide, or polyurethane. This protection layer is intended to protect the electrode from erosion in the sample solution, such as a gastrointestinal fluid.

Referring to FIG. 5, the cross-section of an array 50 of electrodes is shown. This array of electrode pairs is part of a single pH sensing device. As shown, the array 50 includes six pairs of electrodes, each pair having one electrode coated with a different compound/polymer than the other covered electrodes. While only one pair 52 of electrodes is illustrated as connected to an electrical circuit 54, each of the electrodes in the array is connected to electrical circuitry which includes a power supply, a switch mechanism and associated componentry for determining the rate of change of impedance.

The sensor array 50 can include several pairs of electrodes. Each of the electrode pairs can be coated with a compound/polymer whose dissolution rate varies as a function of concentration of substances in a fluid, such as H+, Na+, enzyme, glucose, protein, virus, bacteria, amino acid, or other factors. Accordingly, the array can be used to detect pH as well as the presence and concentration of other components in the sample solution. In one embodiment, at least two pairs of electrodes are coated with different compounds/polymers that respond to different substances and/or different concentration ranges. The grid-well structure can facilitate filling polymers and the depth of each well can be tailored for desired operation time of the sensor.

The pH can be determined by knowing the rate of solubility in a given solution. The disclosed pH sensing device allows measurement of the rate of solubility as the time derivative of fluid conductivity. Because the coating compounds/polymers have a different conductivity than the solution, as the compounds/polymers dissolve, the fluid fills in the gaps between the electrodes and causes an increase in the conductivity. The change in conductivity can be related to the rate of solubility of the polymer approximately through the following equation that is applicable to the case when analyte concentration is above the threshold pH value.

In EQN 1 , C is the measured rate of solubility, where S_p is the proportional coefficient , d_eι_ectrθde_S is the distance between the electrodes, Pβ_uld is the resistivity of the solution, Cthreshoid is solubility rate at the threshold pH, and Y is the conductivity between electrodes

Also provided is a system for detecting the pH in a solution. The system includes a compound/polymer coated electrode and means for measuring a solution's conductance at the electrode. The means for measuring includes any appropriate electronic system, including electrical circuitry for measuring the rate of conductance change, as known in the art. In the disclosed system, the compound/polymer is soluble in the solution, and its rate of solubility is dependent on the pH of the solution.

Also provided is a process for measuring the pH of a solution. The process includes providing an electrode that is coated in or by a compound/polymer. The compound/polymer has a pH-dependent solubility profile. This pH-dependent solubility profile can be known in the art, or it can be determined empirically. The process further includes exposing the compound/polymer coated electrode to the solution, and measuring the conductance at the electrode as a function of time. With the rate of conductance measured, the pH of the solution can be determined by correlating the rate of conductance to the solubility of the compound/polymer to determine the pH of the solution.

The present disclosure provides an approach and/or technique for building a compact, low-cost, biocompatible, calibration- free and sufficiently reliable pH sensor to monitor pH along the passage in the GI tract. Further, the disclosed pH sensor may be used as a disposable sensor to monitor sequential pH changes in other environment, e.g., within 24 hours. Based on the same principle, a sensor array capable of sensing multiple substances and/or properties can be built and implemented in a variety of environments.

One example of a device that includes multiple electrodes is shown in FIGS. 6 A and 6B. This pH sensing device contains multiple electrode arrays dispersed among three stacked layers. A device configured in this manner is dimensioned so as to permit the device to be ingested and can provide pH information and concentration information for other ions and biological factors. EXAMPLES

Example 1 : Measuring pH using polymer-covered electrodes

The impedance of electrodes immersed in a sample fluid were measured. An interdigitated electrode array coated with EUDRAGIT polymer L30-D55, or ElOO was used. Three sample solutions were used. Two solutions were salt water with salt content from 0.2% to 0.6% w/v, and pH 5.7. Hydrochloric acid was used to adjust pH and the sample pH was calibrated with a Corning (glass electrode) pH-meter CHEKMITE pH-15.

Simulated gastric fluid (SGF) without protein (Ricca Chemical Part# 7108-32) was used, which was 0.2% w/v NaCl in 0.7% v/v HCl at pH 1.1. Simulated intestinal fluid (SIF) USPXXII (Ricca Chemical Part# 7109.75 - 16) was also used, which was 0.68% monobasic potassium phosphate, and sodium hydroxide. The final solution was pH 7.4.

Three electrodes coated with the same polymer L30-D55, that does not dissolve until pH is above 5.5, were separately immersed in the three sample solutions. Conductance between the electrodes changed with time as a result of dissolution of the polymer. The rate of conductance change was well differentiated in fluids of different pH values. The impedance, reported as the inverse of resistance in microsiemans (ms), increased rapidly to 63 ms for the pH 7.4 solution, with a capacitance of -3.3 nanofarad (nf). The pH 5.7 solution demonstrated a much less dramatic increase in impedance, reaching only 36 ms and -0.6 nf in capacitance, and the pH 1.1 solution should very little impedance at 48 ms and -9.1 capacitance.

The same electrode coated with the same polymer L30-D55 is sequentially immersed in fluids of different pH value. The pH-dependent rate of conductance change remains consistent despite historical exposures to solutions of different pH values.

Additional experiments have been conducted to verify that salt content from 0.2% to 1% does not affect the response of the polymers. Also the similar repeatability of other polymers has been confirmed, including polymers which do not dissolve until the solution is below pH 5.

Additional compounds and/or polymers with other threshold pH values may be employed according to the present disclosure. Therefore, by using multiple pairs of electrodes coated with compounds/polymers of different threshold pH values, a wide range pH can be monitored with the disclosed pH sensor. Similarly, a host of analyte levels may be determined, as described herein. Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

Claims

1. A biochemical sensing device comprising: a. at least one electrode (12); b. a power supply connected to the electrode (18); and c. a compound coating on the electrode (14), wherein the compound is soluble in a solution of an analyte, and its rate of solubility is dependent on the analyte concentration of the solution.

2. A pH sensing device comprising: a. an electrode (12); b. a power supply connected to the electrode (18); and c. a polymer coating on the electrode (14), wherein the polymer is soluble in a solution, and its rate of solubility is dependent on the pH of the solution.

3. The biochemical/pH sensing device of claim 1 or 2, further comprising an electrical switch (16) in the connection between the power supply and the electrode.

4. The biochemical/pH sensing device of claim 1 or 2, further comprising a data processor (22) connected to the power supply (18).

5. The biochemical/pH sensing device of claim 4, further comprising a data memory component (24) connected to the data processor.

6. The pH sensing device of claim 2, wherein the polymer (28) is insoluble below a threshold pH value, and the device further comprises a second electrode (29) coated with a second polymer (30), wherein the second polymer is soluble in a solution, and its rate of solubility is dependent on the pH of the solution, and further wherein the second polymer is insoluble below a second threshold pH value.

7. The pH sensing device of claim 2, wherein the polymer is insoluble above a threshold pH value, and the device further comprises a second electrode coated with a second polymer, wherein the second polymer is soluble in a solution, and its rate of solubility is dependent on the pH of the solution, and further wherein the second polymer is insoluble below a second threshold pH value.

8. The pH sensing device of claim 2, wherein the device further comprises a second polymer (38) covering the polymer (36) coating the electrode (39).

9. The biochemical sensing device of claim 1, further comprising a reference electrode connected to the power supply, wherein no soluble compound is coated to the reference electrode.

10. The biochemical sensing device of claim 1, further comprising a protection layer (46) residing between the electrode (42) and the polymer coating (48).

11. The pH sensing device of claim 2, wherein the polymer is selected from the group consisting of methacrylic polymers, methacrylate polymers, aminoalkyl methacrylate polymers, ammonioalkyl methacrylate polymers, poly vinyl acetate phthalate polymers, hydroxypropyl methylcellulose phthalate polymers, cellulose acetate trimelliate polymers, and cellulose acetate phthalate polymers.

12. The pH sensing device of claim 2, wherein the polymer dissolves in solutions with pH between about 5 and about 7.

13. The pH sensing device of claim 2, wherein the polymer dissolves in solutions with pH between about 2 and about 6.5.

14. A system for detecting pH in a solution, comprising a polymer coated electrode and means for measuring the rate of conductance change at the electrode, wherein the polymer is soluble in the solution, and its rate of solubility is dependent on the pH of the solution.

15. A process for measuring the pH of a solution, the process comprising: a. providing an electrode coated in a polymer, wherein the polymer has a pH-dependent solubility profile; b. exposing the polymer coated electrode to the solution; c. measuring the rate of conductance at the electrode as a function of time; d. correlating the rate of conductance to the solubility of the polymer to determine the pH of the solution.