WO2005055821A1

WO2005055821A1 - Reduction of settling time for an electrochemical sensor

Info

Publication number: WO2005055821A1
Application number: PCT/DK2004/000854
Authority: WO
Inventors: Mads Christian Sabra
Original assignee: Novo Nordisk A/S
Priority date: 2003-12-11
Filing date: 2004-12-09
Publication date: 2005-06-23

Abstract

One of the major impediments to the commercialization of an amperometric sensor, particular an enzymatic glucose sensor, is its settling time. This problem can be dealt with by applying a potential difference between the working electrode and its reference electrode and/or counter electrode, determine when the electrode is immersed in the medium wherein the concentration of a certain compound is to be measured followed by a preferred combination of regulation of the potential difference.

Description

Delayed Electrode Pulsing

FIELD OF THIS INVENTION

This invention relates to the production of electrode assemblies suitable for use in electrochemical sensors, in particular transcutaneous electrochemical sensors suitable for in vivo measurement of metabolites. In other words, this invention generally relates to a device for measuring the concentration of a certain compound in a medium.

BACKGROUND OF THIS INVENTION

In recent years, a variety of electrochemical sensors have been developed for in vivo measurements of metabolites. Most prominent among these, glucose sensors have been developed for use in obtaining an indication of blood glucose levels in a diabetic patient.

BLOOD GLUCOSE information is of the utmost importance to diabetics, as these readings are instrumental in the adjustment of the treatment regimen. The conventional way to obtain blood glucose information is by applying minute amounts of blood to test strips. A new development is transcutaneous sensors where the sensor is implanted under the skin. As the sensor is in contact with biological fluids for a prolonged period of time, the possibility for continuous measurements is opened. Continuous blood glucose readings obtained with little or no delay is particularly useful in numerous ways. First of all, the continuous monitoring will help preventing hypoglycaemic incidents and thus contribute to a vast increase in the quality of life for the diabetic patient. Furthermore, continuous blood glucose readings may, for example, be used in conjunction with semi automated medication infusion pumps of the external type or automated implantable medication infusion pumps, as generally described in U.S. patent Nos. 3,837,339; 4,245,634; and 4,515,584. This will allow the patient to have a near normal lifestyle, thus eliminating or greatly minimizing the problems normally associated with diabetes.

The sensors utilised for blood glucose measurements can be made in a number of different ways. In the simplest form the sensor is made by two separate electrodes placed transcutaneously, near each other. The two electrodes typically designated working electrode and reference electrode serve different purposes, respectively. The function of the working electrode is to detect the metabolite of interest, thus this electrode is often covered with an enzyme and/or a catalytic coating to facilitate creation of charge due to reduction or oxidation of the metabolite of interest.

The function of the reference electrode is to have a stable reference point in order to keep a constant applied potential. In an amperometric system, a fixed potential difference is applied between the working electrode and the reference electrode. This potential drives the electrochemical reaction at the working electrode's surface.

When a more controlled applied potential on the working electrode or a longer lifetime of the reference electrode is needed, a so called three-electrode system is used instead. In this slightly more complicated set-up, the reference electrode of the two-electrode system is substituted with two electrodes, a reference electrode and a counter electrode. The counter electrode is responsible for the transfer of the current and the reference electrode's function is to act as a reference point for the applied potential.

If used for clinical purposes, it is clearly not convenient to implant several electrodes near each other, thus the electrodes are assembled in one unit defining an electrode assembly or electrode array (forth simply denoted electrode assembly or assembly). An electrode assembly comprises at least the three (or at least the two) electrodes mentioned above working electrode, reference electrode and counter electrode (or working electrode and reference electrode) but can additionally contain electrodes for temperature measurements, differential measurements or other purposes.

Different strategies exist for production of electrode assemblies, for example, as described in Urban and Jobst, in D.M. Fraser (Editor), Biosensors in the body, John Wiley & Sons, Chichester, UK, 1997, pages 197-216. One common used strategy is to dispose electrical conducting tracks on flexible foils made by a dielectric material. Several methods exist for deposition of conducting tracks, including printing, etching of conducting layers covering the flexible foils or by direct vacuum plating of conducting structures. The conventional technologies have in common that the conducting material is usually deposited in a 2D pattern. The method involves either (I) (see, for example, Fiaccabrino and Koudelka-Hep, Electroanalysis 10 (1998), 217-222) the steps of first applying a conducting layer (thin-film technology, sputtering, electroplating, screen printing etc.) onto a dielectric substrate foil and then partial removal of layer (etching, laser ablation etc.) to generate the pattern; or (II) the step of applying metal/metals in a pattern/patterns (screen printing, ink jet printing etc.) onto a dielectric substrate foil. In other words, in method (I) the material that is not wanted is removed and in method (II) only the wanted material is added.

Screen printing or thick film technology has normally been used since the 1950s for the production of hybrid circuits in the electronics industry. Thick film devices consist of one or more layers of material on a dielectric substrate, which are conventionally deposited by screen printing (Albareda-Sirvent et al., Sensors and Actuators B, 69 (2000), 153-163). Screen printing is performed by pressing paste through a screen (for example formed by a woven screen or a metal mask, having the layout of the desired device) by means of a moving rubber squeegee. The squeegee brings the screen into contact with the substrate surface dependent on screen tension and squeegee pressure, hardness and speed. The paste remaining in the screen aperture is then transferred to the substrate resulting in the desired layout. After deposition of the pattern onto the dielectric substrate the paste is cured by temperature rise to remove solvents and allow tight fusion to the substrate alternatively by UV light exposure.

Common for most electrode assemblies is that electrical contact is preferred at the two ends of each conductor track. The conductor tracks are covered with a layer of insulating material (dielectric). At one end of the conductor track, an area remains naked such that contact can be established to the supporting electrical circuits; such an end is in the following designated CPE (contact pad for electronics). At the other end, an area is also left naked and serves as the electrode surface.

One of the major impediments to the commercialization of an amperometric sensor, particular an enzymatic glucose sensor, is its settling time. A method of reducing the settling time of an electrochemical sensor having one or more electrodes is described in US 5,411 ,647 and comprises pretreating the electrochemical sensor after placement in a medium by applying a controlled electric current to one or more of the electrodes of the sensor before using the sensor to measure the presence or concentration of a substance of interest, wherein the controlled current is applied at a density and for a time to reduce the settling time of the electrode to less than about 25 minutes. One disadvantage of such a method by which the current is controlled lies in the practical way of applying a current. A current is always a result of a potential difference. In order to control a current, a voltage must be applied and the resulting current measured.

One object of this invention is to improve the prior art. Another object of this invention is to furnish a measuring device having a short settling time. A still further object of this invention is to furnish a measuring device providing accurate readings of one desired compound, even in the presence of interfering compounds in the medium wherein the concentration of the desired compound is to be determined. Another object of this invention is to ensure a high signal to noise ratio. A still further object of this invention is to ensure a reproducible signal to noise ratio. Further objects and advantages of this invention will be apparent from this description.

SUMMARY OF THIS INVENTION

The present invention relates to a device for measuring the concentration of a certain compound in a medium by applying a potential difference between the working electrode and its reference electrode and/or counter electrode, determine when the electrode is immersed in the medium wherein the concentration of a certain compound is to be measured followed by a preferred combination of regulation of the potential difference.

DRAWINGS

Figure 1 is a drawing of a sensor system comprising an electrochemical sensor assembly (1) with a working electrode (2) and a reference electrode (3) connected to a potentiostat (4) with control electronics.

Figure 2 shows, at the left side, a person having a sensor placed on the body. On the right side in figure 2 is, in the circle, shown an enlargement of a segment which is, in smaller scale, shown in the circle on the person. What is present in the circle is a sensor system mounted on a person. The sensor itself penetrates into the subcutaneous tissue. Potentiostat and electronics (5) is placed on the skin (6) of the person. A sensor assembly (7) is subcutaneously inserted in the tissue (8).

Figure 3a, 3b, and 3c illustrates one way of mechanically detecting when the sensor is inserted into the medium, in this case the subcutaneous tissue of a person. Fig 3a illustrates the device before it is placed on the skin and the mechanical detector (9) is protruding from the device. Figure 3b shows that when the device is placed on the skin, the mechanical detector (9) is pushed into the device thereby triggering a timer (10). Figure 3c illustrates that after a predetermined period of time, the sensor is pre-treated according to this invention.

DEFINITIONS

Settling time is the amount of time necessary for current output from the sensor to settle to a stable value, assuming a constant analyte concentration, following the initial application of the potential to the sensor.

Herein, all references to applied potential or potential differences relates to an Ag/AgCI reference electrode.

Herein, the term potentiostat covers a devise which can apply a constant potential difference for a certain period of time as well as more advanced devices which can vary the potential difference over time.

DESCRIPTION OF THIS INVENTION

This invention relates to a device comprising at least one working electrode (2) and its reference electrode (3) and/or counter electrode, capable of giving a measuring depending on the concentration of a certain compound in a medium (8), which is worked out so that it can perform the following activities in the order given here: a) a potential difference within a certain range (including zero volts) is applied between the working electrode (2) and its reference electrode (3) b) determine when the electrode is immersed in the medium (8), c) after a certain period of time, increase the potential difference applied between the above electrodes (2 & 3), d) maintain an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time, and e) decrease the potential difference applied to a value appropriate for measuring, and f) perform measurements in order to determine the concentration of a certain compound in the medium (8).

In a preferred embodiment of this invention, the device has a sensor which can be used to measure glucose. In a preferred embodiment of this invention, the compound to be measured is glucose. In a preferred embodiment of this invention, the medium containing the compound to be measured is subcutis of a mammal, preferably a human.

In a preferred embodiment of this invention, the electrochemical sensor has a working electrode, a counter electrode, and a reference electrode. In a preferred embodiment of this invention, the working electrode is made of a platinum based material, for example, platinum black. In another preferred embodiment of this invention, the counter electrode is made of a platinum based material, for example, black electrode. In another preferred embodiment of this invention, the reference electrode is an Ag/AgCI electrode. In a preferred embodiment of this invention, the working electrode is the anode. In another preferred embodiment of this invention, the counter electrode is the cathode.

Re: Step a):

In a preferred embodiment of this invention, the range of the potential difference in step a) is from about 0.4 to about 0.9 volt. In a further preferred embodiment of this invention, the lower limit of the range of the potential difference in step a) is about 0.5 volt and the upper limit of said range is about 0.8 volt, preferably about 0.7 volt. If there are three electrodes, the potential difference is measured and applied between the working electrode and the reference electrode and the current is measured between the working electrode and the counter electrode. Re: Step b):

In a preferred embodiment of this invention, the immersion of the electrode into the medium according to step b) is determined by measuring a current between the working electrode and its reference and/or counter electrode. In another preferred embodiment of this invention, the immersion of the electrode into the medium according to step b) is determined mechanically by a part of the system. In another preferred embodiment of this invention, the device is worked out so that it can determine when at least about 80 % of the electrode has been wetted.

Re: Step c):

In a preferred embodiment of this invention, the potential difference is increased as mentioned in step c) when at least about 80 % of the electrode has been wet. In another preferred embodiment of this invention, the period of time in step c) is in the range from about 1 to about 10 minutes. In a preferred embodiment of this invention, the potential difference according to step c) is in the range from about 0.8 volt to about 1.5 volt. In a further preferred embodiment of this invention, the lower limit of the range of potential difference according to step c) is 2 minutes, preferably 3 minutes, and the upper limit of the range is 7 minutes, preferably 5 minutes, more preferred 4 minutes. In a preferred embodiment of this invention, the lower limit of the potential difference according to step c) is about 0.9 volt, preferably about 1 volt, and the upper limit of said range is 1.3 volt, preferably about 1.2 volt.

Re: Step d):

In a preferred embodiment of this invention, the potential difference according to step d) is repeatedly increased and decreased instead of being maintained increased. Hence, it may be desired to use several pulses, for example, repeatedly increasing and decreasing the potential. In a preferred embodiment of this invention, the period of time in step d) is in the range from about 1 to about 10 minutes. In a further preferred embodiment of this invention, the lower limit of the period of time in step d) is about 2 minutes, and the upper limit of said range is about 7 minutes, preferably about 5 minutes. Re: Step e):

In a preferred embodiment of this invention, the range of the potential difference in step e) is from about 0.4 to about 0.9 volt. In a further preferred embodiment of this invention, the lower limit of the potential difference in step e) is about 0.5 volt and the upper limit of said range is about 0.8 volt, preferably about 0.7 volt.

Also, this invention relates to a method for measuring the concentration of a compound in a medium (8) using a device comprising at least one working electrode (2) and its reference electrode (3) and/or counter electrode, capable of giving a measuring depending on the concentration of a certain compound in a medium (8), by performing the following activities in the order given below: a) applying a potential difference within a certain range (including zero volts) between the working electrode (2) and its reference electrode (3) or counter electrode, b) determining when the electrode is immersed in the medium (8), c) increasing, after a certain period of time, the potential difference applied between the above electrodes (2 & 3), d) maintaining an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time, e) decreasing the potential difference applied to a value appropriate for measuring, and performing measurements in order to determine the concentration of a certain compound in the medium (8).

Furthermore, this invention relates to a device comprising at least one working electrode (2) and its reference electrode (3) and/or counter electrode, having means for giving a measuring depending on the concentration of a certain compound in a medium (8), having: a. means for applying a potential difference within a certain range (including zero volts) between the working electrode (2) and its reference electrode (3) or counter electrode, b. means for determining when the electrode (2 & 3) is immersed in the medium (8), c. means for increasing, after a certain period of time, the potential difference applied between the above electrodes (2 & 3), d. means for maintaining an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time, e. means for decreasing the potential difference applied to a value appropriate for measuring, and f. means for performing measurements in order to determine the concentration of a certain compound in the medium (8), and, furthermore, said means, optionally in connection with other means, secures that the activities are performed in the order given here.

Examples of means for applying a potential difference within a certain range (including zero volts) between the working electrode (2) and its reference electrode (3) or counter electrode are a potentiostat or a battery.

Examples of means for determining when the electrode is immersed in the medium (8) is a mechanical detector, vide Fig. 3a & 3b (9) and (10).

Examples of means for increasing, after a certain period of time, the potential difference applied between the above electrodes (2 & 3) are a potentiostat or a battery

Examples of means for maintaining an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time are a potentiostat or a battery.

Examples of means for decreasing the potential difference applied to a value appropriate for measuring are a potentiostat or a battery.

Examples of means for performing measurements in order to determine the concentration of a certain compound in the medium (8) are biosensors.

Described in more details, the present invention can be practised as follows:

A continuous glucose monitoring (hereinafter designated CGM) system consists of an electrochemical glucose sensor and a monitor (a potentiostat, which is controlled by a microprocessor, and a display). The glucose sensor is a conventional three electrode system containing a working electrode, a counter electrode, and a reference electrode. Onto the working electrode are applied glucose oxidase and an outer membrane limiting the glucose flux to the enzyme. Alternatively, the glucose sensor may also be a conventional two electrode system containing a working electrode and a combined reference / counter electrode. The sensor is needle shaped and has to be inserted into the subcutaneous tissue of the user. The steps performed by the user to start measuring glucose continuously are as follows:

1. The sensor is connected to the potentiostat. 2. The sensor is inserted into the subcutaneous tissue. 3. After a while, the device sounds a beep (or a message is shown on the display) and the user calibrates the sensor using a conventional blood glucose measuring strip. 4. The continuous glucose monitoring is now in operation.

The corresponding events happening in the CGM system are

1. A simple contact registers that the sensor is connected and the potentiostat applies a voltage of 600 mV between the working electrode and the reference electrode. As the sensor is dry, no current will run between the electrodes at this time. 2. As the sensor is inserted, the membrane layers of the sensors are wetted and at some point of time, the electrodes will also be wetted, and a certain current runs between the working electrode and the counter electrode. This current is detected by the device. To make sure that the electrodes are completely wetted, the device now waits 5 minutes before the applied voltage is raised to 1100 mV. This voltage is kept for 5 minutes, where after the voltage is reduced back to the working potential of 600 mV.

Note that user steps 1 and 2 can be interchanged, adding to user convenience. In that case, a current may be detected immediately as the senor is connected to the potentiostat. Without wetting detection, the user is limited to connecting the potentiostat after inserting the sensor.

The mentioning herein of a reference is no admission that it constitutes prior art. Herein the word "comprise" is to be interpreted broadly meaning "include", "contain" or "comprehend" {vide, for example EPO guidelines C 4.13). All articles referred to herein are hereby incorporated by reference. The features described in the foregoing description and in the following may, both separately and in any combination thereof, be material for realizing this invention in diverse forms thereof.

Claims

1. A method for measuring the concentration of a compound in a medium (8) using a device comprising at least one working electrode (2) and its reference electrode (3) and/or counter electrode, capable of giving a measuring depending on the concentration of a certain compound in a medium, by performing the following activities in the order given below: a) applying a potential difference within a certain range (including zero volts) between the working electrode (2) and its reference electrode (3) or counter electrode, b) determining when the electrode is immersed in the medium (8), c) increasing, after a certain period of time, the potential difference applied between the above electrodes (2 & 3), d) maintaining an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time, e) decreasing the potential difference applied to a value appropriate for measuring, and f) performing measurements in order to determine the concentration of a certain compound in the medium (8).

2. A device comprising at least one working electrode (2) and its reference electrode (3) and/or counter electrode, having means for giving a measuring depending on the concentration of a certain compound in a medium (8), having: a. means for applying a potential difference within a certain range (including zero volts) between the working electrode (2) and its reference electrode (3) or counter electrode, b. means for determining when the electrode is immersed in the medium (8), c. means for increasing, after a certain period of time, the potential difference applied between the above electrodes (2 & 3), d. means for maintaining an increased (but not necessarily constant) potential difference between the above electrodes (2 & 3) for a certain period of time, e. means for decreasing the potential difference applied to a value appropriate for measuring, and f. means for performing measurements in order to determine the concentration of a certain compound in the medium (8), and, furthermore, said means, optionally in connection with other means, secures that the activities are performed in the order given here.

3. A device or method, according to claim 1 or 2, which can be used to measure glucose.

4. A device or method, according to any one of the preceding claims, wherein the compound to be measured is glucose.

5. A device or method, according to any one of the preceding claims, wherein the medium is subcutis of a mammal, preferably a human.

6. A device or method, according to any one of the preceding claims, wherein the potential difference in step a) is a controlled potential difference.

7. A device or method, according to any one of the preceding claims, wherein the range of the potential difference in step a) is from about 0.4 to about 0.9 volt.

8. A device or method, according to the preceding claim, wherein the lower limit of said range is about 0.5 volt and the upper limit of said range is about 0.8 volt, preferably about 0.7 volt.

9. A device or method according to any of the preceding claims, in which the immersion of the electrode (2 & 3) into the medium (8) according to step b) in claim 1 or 2 is determined by measuring a current between the working electrode (2) and its reference (3) and/or counter electrode.

10. A device or method according to any of the preceding claims, in which the immersion of the electrode (2 & 3) into the medium according to step b) in claim 1 or 2 is determined mechanically by a part of the system.

11. A device or method, according to any one of the preceding claims, wherein the device is worked out so that it can determine when at least about 80 % of the electrode has been wetted.

12. A device or method, according to any one of the preceding claims, wherein the potential difference in step c) is a controlled potential difference.

13. A device or method, according to any one of the preceding claims, wherein the potential difference is increased as mentioned in step c) in claim 1 or 2 when at least about 80 % of the electrode has been wet.

14. A device or method, according to any one of the preceding claims except the claim immediately before this claim, wherein the period of time in step c) is in the range from about 1 to about 10 minutes.

15. A device or method, according to any one of the preceding claims, wherein the potential difference according to step c) in claim 1 or 2 is in the range from about 0.8 volt to about 1.5 volt.

16. A device or method, according to the preceding claim, wherein the lower limit of said range is about 0.9 volt, preferably about 1 volt, and the upper limit of said range is 1.3 volt, preferably about 1.2 volt.

17. A device or method, according to the preceding claim, wherein the lower limit of the range is 2 minutes, preferably 3 minutes, and the upper limit of the range is 7 minutes, preferably 5 minutes, more preferred 4 minutes.

18. A device or method, according to any one of the preceding claims, wherein the potential difference in step d) is a controlled potential difference.

19. A device or method according to any one of the preceding claims in which the potential difference according to step d) in claim 1 or 2 is repeatedly increased and decreased instead of being maintained increased.

20. A device or method, according to any one of the preceding claims, wherein the period of time in step d) of claim 1 or 2 is in the range from about 1 to about 10 minutes.

21. A device or method, according to the preceding claim, wherein the lower limit of said range is about 2 minutes, and the upper limit of said range is about 7 minutes, preferably about 5 minutes.

22. A device or method, according to any one of the preceding claims, wherein the potential difference in step e) is a controlled potential difference.

23. A device or method, according to any one of the preceding claims, wherein the range of the potential difference in step e) in claim 1 or 2 is from about 0.4 to about 0.9 volt.

24. A device or method, according to the preceding claim, wherein the lower limit of said range is about 0.5 volt and the upper limit of said range is about 0.8 volt, preferably about 0.7 volt.

25. Any novel feature or combination of features described herein.