BODY MONITORING DEVICE AND ASSOCIATED METHOD

Abstract

The present invention relates to a method of a method of measuring a human body analyte concentration in an interstitial fluid, comprising the step of measuring a first electrical current I.sub.1 between a working electrode and a pseudo-reference electrode while applying a first potential between the working electrode and the pseudo-reference electrode, the first potential being less than a threshold potential, the magnitude of the first electrical current being greater than the magnitude of a predetermined first threshold current, and further measuring an output electrical current between the working electrode and the pseudo-reference electrode while applying a second electrical potential, the second electrical potential being greater than the first electrical potential.

Claims

1. A method of measuring a human body analyte concentration in an interstitial fluid of a user by a device comprising: a control unit, at least a working electrode and a pseudo-reference electrode, the working electrode and the pseudo-reference electrode being configured for an implantation in the dermis of the user by at least one microneedle so as to be in contact with the interstitial fluid of the user, an enzyme configured for oxidizing the analyte, the enzyme being attached to at least a part of the working electrode, a mediator being fixed on said part of the working electrode, the mediator being a redox mediator having a reduced state and an oxidized state, and being configured for being in a reduced state below the predetermined threshold electrical potential V.sub.t, and in an oxidized state above the predetermined threshold electrical potential V.sub.t, the method comprising the step of: a) measuring a first electrical current I.sub.1 between the working electrode and the pseudo-reference electrode while applying a first potential V.sub.1 between the working electrode and the pseudo-reference electrode, the first potential V.sub.1 being less than a predetermined threshold potential V.sub.t, the magnitude of the first electrical current I.sub.1 being greater than the magnitude of a predetermined first threshold current I.sub.t1, and further measuring an output electrical current I.sub.0 between the working electrode and the pseudo-reference electrode while applying a second electrical potential V.sub.2, the second electrical potential V.sub.2 being greater than the first electrical potential V.sub.1, and/or comprising the step of b) measuring a second electrical current I.sub.2 between the working electrode and the pseudo-reference electrode while applying a third electrical potential V.sub.3 between the working electrode and the pseudo-reference electrode, the third potential V.sub.3 being greater than the threshold potential V.sub.t, the magnitude of the second electrical current I.sub.2 being less than a magnitude of a predetermined second threshold current I.sub.t2, and then measuring the output electrical current I.sub.0 while applying a fourth electrical potential V.sub.4, the fourth electrical potential V.sub.4 being inferior to the third electrical potential V.sub.3.

2. The method of claim 1, the method comprising the step of: a) measuring a first electrical current I.sub.1 between the working electrode and the pseudo-reference electrode while applying a first potential V.sub.1 between the working electrode and the pseudo-reference electrode, the first potential V.sub.1 being less than a predetermined threshold potential V.sub.t, the magnitude of the first electrical current I.sub.1 being greater than the magnitude of a predetermined first threshold current I.sub.t1, and further measuring an output electrical current I.sub.0 between the working electrode and the pseudo-reference electrode while applying a second electrical potential V.sub.2, the second electrical potential V.sub.2 being greater than the first electrical potential V.sub.1, wherein the second electrical potential V.sub.2 is greater than the threshold electrical potential V.sub.t.

3. The method of claim 1, the method comprising the step of: b) measuring a second electrical current I.sub.2 between the working electrode and the pseudo-reference electrode while applying a third electrical potential V.sub.3 between the working electrode and the pseudo-reference electrode, the third potential V.sub.3 being greater than the threshold potential V.sub.t, the magnitude of the second electrical current I.sub.2 being less than a magnitude of a predetermined second threshold current I.sub.t2, and then measuring the output electrical current I.sub.0 while applying a fourth electrical potential V.sub.4, the fourth electrical potential V.sub.4 being inferior to the third electrical potential V.sub.3. Wherein the fourth electrical potential V.sub.4 is under the threshold electrical potential V.sub.t.

4. The method according to claim 1, comprising both step a) and step b).

5. The method according to claim 1, wherein step a) and/or step b) comprise(s) a sub-step of computing a concentration of the analyte in the interstitial fluid from the value of the output electrical current I.sub.o.

6. The method according to claim 1, wherein the first threshold current I.sub.t1 and the second threshold I.sub.t2 current are of opposite signs.

7. The method according to claim 1, comprising a repetition of the steps of measuring the current between the working electrode and the pseudo-reference electrode and determining a concentration of the analyte in the interstitial fluid from the value of the output electrical current I.sub.0.

8. The method according to claim 7, wherein each repetition is separated by a time between 0.5 s and 30 min, and preferably between 15 s and 15 min.

9. The method according to claim 8, wherein the threshold potential V.sub.t is comprised between 10 mV and 650 mV.

10. The method according to claim 1, wherein the first threshold current I.sub.t1 is comprised in the range from −500 nA and −0.1 nA and preferably between −200 nA and −0.1 nA.

11. The method according to claim 1, wherein the enzyme configured for oxidizing the analyte is at least chosen between an enzyme configured for oxidizing glucose and an enzyme configured for oxidizing lactate.

12. The method according to claim 1, wherein the mediator is Prussian blue.

13. A device for measuring a human body analyte in an interstitial fluid of a user by a device comprising: a control unit, at least a working electrode and a pseudo-reference electrode, the working electrode and the pseudo-reference electrode being configured for an implantation in the dermis of the user by at least one microneedles so as to be in contact with the interstitial fluid of the user, an enzyme configured for oxidizing the analyte, the enzyme being attached over at least a part of the working electrode, a mediator being fixed on said part of the working electrode, the mediator being a redox mediator having a reduced state and an oxidized state, and being configured for being in a reduced state below the predetermined threshold electrical potential V.sub.t, and in an oxidized state above the predetermined threshold electrical potential V.sub.t, the control unit being configured for: a) measuring a first electrical current I.sub.1 between the working electrode and the pseudo-reference electrode while applying a first potential V.sub.1 between the working electrode and the pseudo-reference electrode, the first potential V.sub.1 being less than a predetermined threshold potential V.sub.t, the magnitude of the first electrical current I.sub.1 being greater than the magnitude of a predetermined first threshold current I.sub.t1, and further measuring an output electrical current I.sub.0 between the working electrode and the pseudo-reference electrode while applying a second electrical potential V.sub.2, the second electrical potential V.sub.2 being greater than the first electrical potential V.sub.1, and/or for b) measuring a second electrical current I.sub.2 between the working electrode and the pseudo-reference electrode while applying a third electrical potential V.sub.3 between the working electrode and the pseudo-reference electrode, the third potential V.sub.3 being greater than the threshold potential V.sub.t, the magnitude of the second electrical current I.sub.2 being less than a magnitude of a predetermined second threshold current I.sub.t2, and then measuring the output electrical current I.sub.0 while applying a fourth electrical potential V.sub.4, the fourth electrical potential V.sub.4 being inferior to the third electrical potential V.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The invention will be described by way of example, with reference to the accompanying drawings in which:

[0042] FIG. 1 diagrammatically shows a reaction scheme of a first-generation glucose biosensor of the prior art, in which electrons are transferred from the enzyme to the electrode by the oxidation of H.sub.2O.sub.2,

[0043] FIG. 2 diagrammatically shows a reaction scheme of a second-second generation glucose biosensor of the prior art, in which electrons are transferred from the enzyme to the electrode by the oxidoreduction of a mediator,

[0044] FIG. 3 diagrammatically shows a device according to an embodiment of the invention,

[0045] FIG. 4 diagrammatically shows a device according to an embodiment of the invention,

[0046] FIG. 5A diagrammatically shows a reaction scheme adapted to detect an analyte according to an embodiment of the invention,

[0047] FIG. 5B diagrammatically shows a reaction scheme adapted to detect an analyte according to an embodiment of the invention,

[0048] FIG. 6 diagrammatically shows a method according to an embodiment of the invention,

[0049] FIG. 7 diagrammatically shows two reaction schemes adapted to detect an analyte according to an embodiment of the invention

[0050] FIG. 8 diagrammatical three preferred work zones of a device according to an embodiment of the invention, as a function of the glucose concentration to be detected in an individual.

DETAILED DESCRIPTION OF PREFERRED ASPECTS OF THE INVENTION

General Architecture of the Device 1

[0051] In reference with FIG. 3 and FIG. 4, a device 1 for measuring a human body analyte 2 in an interstitial fluid of a user comprises a working electrode 4, a counter electrode 5 (also called auxiliary electrode) and preferably a reference electrode 6.

[0052] The electrodes (i.e. the working electrode 3, the counter electrode 4 and preferably the reference electrode 6) are configured to be implanted in the dermis of a user by one or several microneedles 7 so as to be in contact with the interstitial fluid of the user. For example, the microneedle 7 can be metallic, so that the microneedle is the electrode. Metallic track can also be deposited on a microfabricated microneedle 7 so as to form the electrode. In general, the microneedle 7 has a metallic surface S which can be electrically connected outside of the microneedle 7. Several electrodes can be fabricated on the same microneedle 7, by electrically insulating the metallic surface of each electrode from the other metallic surface(s). In another embodiment of the invention, each electrode can be mounted on a different microneedle 7. With reference to FIG. 4, each electrode can have a basis and a top. The metallic surface of each electrode can at least have a part arranged at a distance superior to 100 μm and preferably 300 μm from the basis of the electrode. Therefore, a part of the electrode is not in contact with the epidermis, which does not contain available interstitial fluid, but with the dermis, which contains interstitial fluid. Preferably, the top of each microneedle 7 is arranged at a distance inferior to 1 mm from the basis of the electrode. Therefore, the microneedle is short enough to avoid contacting a nerve and causing pain to the user.

[0053] The device 1 also comprises a control unit 3. The control unit 3 is electrically connected to the electrodes, particularly to the working electrode 4, to the counter electrode 5, and preferably to the reference electrode 6. The control unit 3 can comprise processor, a memory, an electrical acquisition module and an electrical control module. Preferably, every working electrode 4 and counter electrode 5 is independently electrically connected to the electrical acquisition module and to the electrical control module. The electrical connections are illustrated in FIG. 4 by dashed lines.

Electrode Surface Treatment

[0054] The device 1 comprises enzymes 8 configured for oxidizing the analyte 2. Each enzyme 8 comprises a cofactor which is responsible for the oxidation of the analyte, preferably glucose or lactate. The enzyme 8 is preferably an enzyme chosen between glucose oxidase and dehydrogenase, such as PQQ-glucose dehydrogenase, NAD-glucose dehydrogenase or FAD-glucose dehydrogenase. The enzymes 8 are attached to (or attached over, which is strictly equivalent) at least a part of the working electrode. The enzymes 8 can be attached over the part of the electrode for example by adsorption, by covalent grafting, by cross-linking and/or by encapsulation in a matrix.

[0055] The device 1 comprises mediators. The mediator is attached at least over the part of the working electrode on which the enzymes are attached. The mediator is preferably chosen between Prussian blue, a conducting polymer, an osmium complex, and a cobalt complex. In a preferred embodiment of the invention, the mediator is Prussian blue (having the chemical formula Fe.sup.III.sub.4[Fe.sup.II(CN).sub.6].sub.3).

[0056] With reference to FIG. 5A and FIG. 5B, the mediator, preferably Prussian blue, allows both oxidation and reduction of H.sub.2O.sub.2 for respectively transferring electrons or capturing electrons from the electrode it is fixed on.

[0057] With reference to FIG. 5A, for an applied voltage being under an applied threshold potential V.sub.t between the working electrode 4 and a pseudo-reference electrode, said threshold potential being chosen between 20 mV and 650 mV, the mediator 9 is configured for reducing H.sub.2O.sub.2 which is produced by the enzyme 8. Then, the mediator 9, which is in an oxidized state, can be reduced at the working electrode 4. The working electrode gives electrons to the mediator 9, and a negative current takes place between the working electrode 4 and the pseudo-reference electrode.

[0058] With reference to FIG. 5B, for an applied voltage being over an applied threshold potential V.sub.t between the working electrode and a pseudo-reference electrode, said threshold potential being chosen between 20 mV and 650 mV, the mediator 9 is configured for oxidizing H.sub.2O.sub.2 which is produced by the enzyme 8. Then, the mediator 9, which is in a reduced state, can be oxidized at the working electrode 4. The working electrode 4 takes electrons from the mediator 9, and a positive current takes place between the working electrode 4 and the pseudo-reference electrode.

[0059] A porous membrane can, for example, be deposited on the metallic surface of the electrode. Glucose oxidase enzymes 8 can be, for example, grafter on the membrane. Then, Prussian blue can be deposited on both the membrane and the glucose oxidase enzymes 8.

[0060] The surface area S of the working electrode 4 covered with both the enzymes 8 and the mediators 9 is preferably comprised between 20 000 μm.sup.2 and 600 000 μm.sup.2.

Analyte 2 Measurement

[0061] With reference to FIG. 5 and FIG. 6, an aspect of the invention is a method of measuring a human body analyte 2 in an interstitial fluid of a user by a device, and preferably measuring the concentration of said body analyte 2.

[0062] After inserting the microneedles 7 of the device 1 in the skin of the user, more specifically in the dermis of the user, the method comprises: [0063] a step 601 of measuring a first electrical current I.sub.1 between the working electrode 4 and the pseudo-reference electrode 5 while applying a first potential V.sub.1 between the working electrode 4 and the pseudo-reference electrode 5, the first potential V.sub.1 being less than the threshold potential V.sub.t, the magnitude of the first electrical current I.sub.1 being greater than the magnitude of a predetermined first threshold current I.sub.t1, and further measuring an output electrical current I.sub.0 between the working electrode 4 and the pseudo-reference electrode 5 while applying a second electrical potential V.sub.2, the second electrical potential V.sub.2 being greater than the first electrical potential V.sub.1, or [0064] a step 602 of b) measuring a second electrical current I.sub.2 between the working electrode 4 and the pseudo-reference electrode 5 while applying a third electrical potential V.sub.3 between the working electrode 4 and the pseudo-reference electrode 5, the third potential V.sub.3 being greater than the threshold potential V.sub.t, the magnitude of the second electrical current I.sub.2 being less than a magnitude of a predetermined second threshold current I.sub.t2, and then measuring the output electrical current I.sub.0 while applying a fourth electrical potential V.sub.4, the fourth electrical potential V.sub.4 being inferior to the third electrical potential V.sub.3.

[0065] Therefore, it is possible to switch the predominant reaction path in order to transfer electrons from the enzymes 8 to the working electrode 4, depending on the concentration of analyte 2 in the interstitial fluid.

[0066] Specifically, step 601 allows to switch from a first potential V.sub.1 adapted to measure low concentrations of analytes 2 to a second potential V.sub.2 adapted to measure higher concentrations of analyte 2. The terms “low concentration” are used herein to define a concentration under 11 mM, and preferably than 7 mM. Typically, in order to be accurate when measuring low concentrations of analyte 2, the electron transfer implying a reduction of H.sub.2O.sub.2 and a reduction of the mediator at the electrode is adapted because it allows to avoid a contribution of interferent species in the measurement. When the concentration increases, it leads the magnitude of the current to increase. Over a magnitude of a predetermined first threshold current I.sub.t1, this chemical pathway suffers from non-linearity and saturation. Then, a second potential V.sub.2, greater than V.sub.1, is applied between the working electrode and the pseudo-reference electrode in order to foster the chemical pathway adapted to high concentrations of analyte 2. In step 601, the current I.sub.1 is measured when the first potential V.sub.1 is applied between the working electrode 4 and the pseudo-reference electrode 5. In step 602, the current I.sub.2 is measured when the third potential V.sub.3 is applied between the working electrode 4 and the pseudo-reference electrode 5.

[0067] In reference with FIG. 7, the potential V.sub.2 is preferably higher than the potential V.sub.t. Therefore, it is possible to impose in a majority way the chemical pathway adapted to the high concentrations of analyte 2.

[0068] Step 602 allows to switch from a third potential V.sub.3 adapted to measure high concentrations of analytes 2 to a fourth electrical potential V.sub.4, adapted to measure lower concentrations of analyte 2. The terms “high concentration” are used herein to define a concentration of analyte 2 greater than 11 mM, and preferably than 13 mM. Typically, in order to be accurate when measuring high concentrations of analyte 2, the electron transfer implying a oxidation of H.sub.2O.sub.2 and an oxidation of the mediator 9 at the electrode, or a direct oxidation of H.sub.2O.sub.2 at the working electrode 4, is adapted because it avoids limiting the quantity of current due to the mediator 9 quantity at the electrode. When the concentration of analyte 2 decreases, it leads the magnitude of the current to decrease. Under a magnitude of a predetermined second threshold current I.sub.t2, this chemical pathway suffers from noise of the interferent species also reacting at this potential. Then, a fourth potential V.sub.4, lower than V.sub.3, is applied between the working electrode and the pseudo-reference electrode in order to foster the chemical pathway adapted to low concentrations of analyte 2.

[0069] In reference with FIG. 7, the potential V.sub.4 is preferably lower than the potential V.sub.t. Therefore, it is possible to impose in a majority way the chemical pathway adapted to the low concentrations of analyte 2.

[0070] The method can preferably comprise both steps 601 and 602, allowing to perform a continuous measurement of analyte 2 with the most favorable chemical pathway.

[0071] The method preferably comprises a repetition of steps 603 and/or steps 604, each step comprising measuring the current I.sub.0 between the working electrode 4 and the pseudo-reference electrode, and determining a concentration of the analyte 2 in the interstitial fluid from the value of the said current. The determination of the concentration can be done using predetermined calibration data, or simultaneous measurement of a reference analyte 2 concertation. Step 603 comprises measuring the output current I.sub.0 while applying an electrical potential V.sub.2, the electrical potential V.sub.2 being greater than the potential V.sub.1. Step 604 comprises measuring the output current I.sub.0 while applying an electrical potential V.sub.3, the electrical potential V.sub.3 being lower than the potential V.sub.4.

[0072] Preferably, step 603 or step 604 is repeated at a time between 10 s and 30 min, and preferably between 15 s and 15 min. Therefore, it is possible to record pseudo-continuously the analyte 2 concentration while both having the most precise reaction pathway to measure the analyte concentration.

[0073] The surface area S of the working electrode 4 is preferably comprised between 20 000 μm.sup.2 and 600 000 μm.sup.2. Therefore, it is possible to limit the electrical noise when measuring the output current I.sub.o while minimizing the size of the microneedle 7 to reduce the pain of the user. Conversely, as the surface area S is limited, the conditions of the step 601 and/or 602 can be expressed in terms of current density.

[0074] Because of different electron pathways, depending on the applied electrical potential between the working electrode 4 and the pseudo-reference electrode, the presence of analyte can induce an oxidation between the metal of the working electrode and another species. Hence, the current measured between the electrodes at step 603 and step 604 can be of opposite signs. Moreover, the first threshold current and the second threshold current can of opposite signs.

[0075] Experimentally, the inventors measured that the first threshold current I.sub.t1 is preferably comprised between −500 nA and −1 nA and, notably between −200 nA and −1 nA. The second current threshold is preferably comprised between 50 nA and 1 μA, notably between 100 nA and 1 μA.

[0076] The threshold potential V.sub.t is preferably comprised between 10 mV and 650 mV, notably between 100 mV and 400 mV.

[0077] With reference to FIG. 8, a method according to an embodiment of the invention may comprise a step a) wherein the second potential V.sub.2 is chosen between a potential comprised between a first threshold potential V.sub.t1 and a second threshold potential V.sub.t2, V.sub.t1 and V.sub.t1 being greater than V.sub.1 and V.sub.t2 being greater than V.sub.t1, and a potential greater than a second potential V.sub.t2, as a function of the measurement of the first electrical current I.sub.1. A method according to an embodiment of the invention may also comprise a step b) wherein the fourth potential V.sub.4 is chosen between a potential comprised between a fourth threshold potential V.sub.t4 and a third threshold potential V.sub.t3, V.sub.t3 and V.sub.t4 being less than V.sub.3 and V.sub.t4 being less than V.sub.t3, and a potential less than a fourth potential V.sub.t4, as a function of the measurement of the first electrical current I.sub.1. Preferably, the first threshold potential V.sub.t1 can be equal to 10 mV, the second threshold potential V.sub.t2 can be equal to 400 mV, the third threshold potential V.sub.t3 can be equal to 400 mV, and/or the fourth threshold potential V.sub.t4 can be equal to 10 mV.

[0078] Therefore, it is possible to define in a predetermined way a number of electrode working modes strictly greater than two, so as to work in a zone with linear current/voltage characteristics and avoiding saturation of the measured current. In the embodiment schematically illustrated in FIG. 8, three preferred working zones are predetermined by four threshold potentials. These zones may be suitable for the detection of hypoglycemia in a first zone (a), normal glucose in a second zone (b), and hyperglycemia in a third zone (c).