MICROFLUIDIC ELECTROCHEMICAL DEVICE FOR MEASURING A VOLUME FLOW RATE

Abstract

A microfluidic electrochemical device has a microfluidic channel and an electrochemical cell having a pair of working electrodes separated by an inter-electrode distance in a flow direction of the fluid in the microfluidic channel, a counter-electrode and a reference electrode. The microfluidic electrochemical device has an electrochemical amperometry measurement system configured to bias the pair of working electrodes so that each electrode produces an amperometric signal by oxidation reaction or by reduction reaction with the electroreactive fluid or with a chemical species associated with a redox couple intended for the fluid. The microfluidic electrochemical device determines the volume flow rate of the fluid in the microfluidic channel, notably based on the inter-electrode distance and a time delay between the amperometric signals produced by the pair of working electrodes.

Claims

1. A microfluidic electrochemical device (100) for measuring a volume flow rate (Q) of a fluid, the fluid comprising a solvent, the microfluidic electrochemical device (100) comprising: at least one microfluidic channel (11, 11a, 11b, 11c) configured to allow the fluid to flow in a flow direction (13); at least one electrochemical cell (14, 14a, 14b, 14c) disposed in the at least one microfluidic channel (11, 11a, 11b, 11c), the electrochemical cell (14, 14a, 14b, 14c) comprising a first working electrode (WE1) and at least one second working electrode (WE2, WE2(1), WE2(2)), with said at least one second working electrode (WE2, WE2(1), WE2(2)) being spaced apart from the first working electrode (WE1) by an inter-electrode distance (La, Lb, Lc, L(1), L(2)) in the flow direction (13), at least one counter-electrode (CE) and at least one reference electrode (REF); an electrochemical amperometry measurement system (15) configured to bias the first working electrode (WE1) at a first electrode potential (E1) and the second working electrode (WE2, WE2(1), WE2(2)) at a second electrode potential (E2), so that each of said first and second working electrodes produces an amperometric signal by oxidation reaction or by reduction reaction of the solvent or with at least one chemical species forming a redox couple with the solvent; and the electrochemical amperometry measurement system (15) being configured to determine the volume flow rate (Q) of the fluid in the microfluidic channel (11, 11a, 11b, 11c) based on the inter-electrode distance (La, Lb, Lc, L(1), L(2)) and a time delay (t) between a variation in the amperometric signal produced by the first working electrode (WE1) and a variation in the amperometric signal produced by the second working electrode (WE2, WE2(1), WE2(2)).

2. The microfluidic electrochemical device (100) as claimed in claim 1, wherein the solvent is water H2O.

3. The microfluidic electrochemical device (100) as claimed in claim 2, wherein the fluid is sweat from a human or animal subject.

4. The microfluidic electrochemical device (100) as claimed in claim 2, wherein the first electrode potential (E1) allows the oxidation of water H2O to dioxygen O2 and the second electrode potential E2 allows the reduction of the dioxygen O2 dissolved in the produced water H2O to water H2O.

5. The microfluidic electrochemical device (100) as claimed in claim 2, wherein the first electrode potential (E1) allows the reduction of water H2 O to dihydrogen H2 and the second electrode potential (E2) allows the reduction of water H2O to dihydrogen H2.

6. The microfluidic electrochemical device (100) as claimed in claim 2, wherein the first electrode potential (E1) allows the reduction of dioxygen O2 dissolved in water H2O to water H2O and the second electrode potential (E2) allows the reduction of dioxygen O2 dissolved in water H2O to water H2O.

7. The microfluidic electrochemical device (100) as claimed in claim 1, wherein the electrochemical amperometry measurement system (15) is also configured to: during a first step, bias the first working electrode (WE1) at the first electrode potential (E1) and the second working electrode (WE2) at the second electrode potential (E2); and during a second step, disconnect the first working electrode (WE1) or set the first electrode potential (E1) at a potential close to or equal to a zero-current equilibrium potential.

8. The microfluidic electrochemical device (100) as claimed in claim 1, further comprising an isolating support (4), said at least one microfluidic channel (11, 11a, 11b, 11c) being formed in the isolating support (4), the first working electrode (WE1) and said at least one second working electrode (WE2, WE2(1), WE2(2)) being formed by metal deposits of platinum or platinum black on said isolating support (6).

9. The microfluidic electrochemical device (100) as claimed in claim 1, wherein the counter-electrode (CE) is positioned downstream of the working electrodes (WE1, WE2, WE2(1), WE2(2)) in the flow direction (13), and wherein the reference electrode (REF) is positioned upstream of said working electrodes (WE1, WE2, WE2(1), WE2(2)) in said flow direction (13).

10. The microfluidic electrochemical device (100) as claimed in claim 1, comprising a first and a second microfluidic channel (11a, 11b), with the first, respectively, the second, electrochemical cell (14a, 14b) being disposed in the first, respectively, the second, microfluidic channel (11a, 11b), with the inter-electrode distance (La) of the first electrochemical cell (14a) being different from the inter-electrode distance (Lb) of the second electrochemical cell (14b).

11. The microfluidic electrochemical device (3) as claimed in claim 1, wherein said at least one electrochemical cell (14a) comprises two second working electrodes (WE2(1), WE2(2)) respectively separated from the first working electrode (WE1) by a first inter-electrode distance (L(1)) and by a second inter-electrode distance (L(2)), with the first inter-electrode distance (La (1)) being different from the second inter-electrode distance (L(2)).

12. The microfluidic electrochemical device (100) as claimed in claim 1, wherein the electrochemical amperometry measurement system (15) is configured to determine the volume flow rate (Q) as a function of a cross-sectional surface area(S) of said microfluidic channel (11, 11a, 11b, 11c) in the flow direction (13).

13. An apparatus (1) intended to be placed on an investigation zone (8) of an epidermis of a human or animal subject in order to measure a quantitative sweating parameter of the subject, said apparatus (1) comprising: a structure defining a microfluidic electrochemical device (100) as claimed in claim 1, the structure comprising an inlet orifice (6) defining the investigation zone (8) and allowing through sweat from the epidermis, the at least one microfluidic channel (11, 11a, 11b, 11c) of the microfluidic electrochemical device (100) being connected to the inlet orifice (6); and an electronic processing device (16) configured to determine the quantitative sweating parameter of said human or animal subject based on measurements of the volume flow rate (Q) of sweat carried out by the microfluidic electrochemical device (100).

14. The apparatus (1) as claimed in claim 13, wherein the quantitative sweating parameter of said human or animal subject is a sweating rate.

15. The apparatus (1) as claimed in claim 13, wherein the structure is a multi-layer structure comprising a lower layer (3) and at least one layer superimposed on the lower layer (3), with the microfluidic electrochemical device (100) extending parallel to the lower layer (3), the lower layer (3) comprising said inlet orifice (6).

16. The apparatus (1) as claimed in claim 15, wherein the multi-layer structure further comprises an upper layer (10) and at least one intermediate layer (4) located between the lower layer (3) and the upper layer (10), with the microfluidic electrochemical device (100) being formed within the thickness of the at least one intermediate layer (6).

17. The apparatus (1) as claimed in claim 16, wherein the upper layer (10) has an outlet orifice (22) passing through the upper layer (10), and wherein the at least one microfluidic channel (11, 11a, 11b, 11c) is connected to the outlet orifice (22).

18. The apparatus (1) as claimed in claim 16, wherein the first working electrode (WE1), the at least one second working electrode (WE2, WE2(1), WE2(2)), the at least one counter-electrode (CE) and the at least one reference electrode (REF) are disposed on an inner face of the upper layer (10) closing the at least one microfluidic channel (11, 11a, 11b, 11c) from above and/or are disposed on an upper face of the lower layer (3) closing said at least one microfluidic channel (11, 11a, 11b, 11c) from below.

19. The apparatus (1) as claimed in claim 13, further comprising a communication device (21) configured to transmit one or more measurement signals produced by the microfluidic electrochemical device (100).

20. The apparatus (1) as claimed in claim 13, further comprising a gyroscopic module and/or at least one accelerometer for detecting a state of activity of said human or animal subject.

21. The apparatus (1) as claimed in claim 13, further comprising a temperature sensor configured to measure the temperature of the epidermis (2) of said human or animal subject.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] The invention will be better understood, and further aims, details, features and advantages thereof will become more clearly apparent throughout the following description of several particular embodiments of the invention, which are provided solely by way of a non-limiting illustration, with reference to the appended drawings.

[0060] FIG. 1 is a schematic back view of a subject on which an apparatus according to one embodiment has been placed;

[0061] FIG. 2 is a perspective view partially showing a multilayer structure for an apparatus according to one embodiment;

[0062] FIG. 3 is a cross-sectional view along line III-III of FIG. 2;

[0063] FIG. 4 is an exploded view of the multilayer structure according to one embodiment;

[0064] FIG. 5 is a partial functional schematic representation of a multilayer structure defining an electrochemical device in an apparatus;

[0065] FIG. 6 is a partial functional schematic representation of a microfluidic electrochemical device that can be used in an apparatus;

[0066] FIG. 7 is a schematic top view of an electrochemical cell according to a first embodiment;

[0067] FIG. 8 is a schematic view similar to that of FIG. 7, according to a second embodiment;

[0068] FIG. 9 is a functional schematic cross-sectional representation of an electrochemical cell along a microfluidic channel according to a first embodiment;

[0069] FIG. 10 is a set of chronoamperograms illustrating a method that can be implemented, according to the first embodiment, with the microfluidic electrochemical device of FIG. 7;

[0070] FIG. 11 is a functional schematic representation similar to that of FIG. 9 according to a second embodiment;

[0071] FIG. 12 is a set of chronoamperograms similar to those of FIG. 10 according to the second embodiment;

[0072] FIG. 13 is a functional schematic representation similar to those of FIGS. 9 and 11, according to a third embodiment;

[0073] FIG. 14 is a set of chronoamperograms similar to those in FIGS. 10 and 12, according to a third embodiment;

[0074] FIG. 15 is a schematic functional representation of an electronic control device that can be implemented with the apparatus.

DESCRIPTION OF THE EMBODIMENTS

[0075] The embodiments described hereafter relate to an apparatus for determining a quantitative sweating parameter of a subject by means of a microfluidic electrochemical device for continuously measuring the volume flow rate of sweat in a microfluidic channel. More generally, such a microfluidic electrochemical device can be incorporated into numerous microsystems for the in situ measurement of the average flow speed of an electroactive fluid in a microfluidic channel. These microsystems can be, for example, lab-on-a-chip microfluidic platforms or micro-Total Analysis Systems (TAS).

[0076] With reference to FIG. 1, the apparatus 1 for determining a quantitative sweating parameter is disposed on the skin 2 of a human subject, for example, on their back. In a variant, not shown, the apparatus 1 can be disposed on the skin of an animal subject.

[0077] With reference to FIG. 2, the apparatus 1 is, for example, in the form of a space-saving multilayer structure made of watertight materials, for example, a polymer material. The multilayer structure comprises a lower layer 3 made of a flexible and biocompatible material, preferably self-adhesive, for example, polyethylene terephthalate (PET), which can be positioned directly on the skin 2 of the subject, and an isolating support 4 superimposed on the lower layer 3.

[0078] A microfluidic channel 11 is hollowed out of the thickness of the isolating support 4. A sampling cup 5, located in line with a circular opening 6, is made in the lower layer 3.

[0079] With reference to FIG. 3, the lower layer 3 is adhered to the skin 2 by an adhesive layer 7. A central part of the lower layer 3 and of the adhesive layer 7 comprises the circular opening 6 delimiting an investigation zone 8 on the skin 2 of the subject, for example, with a diameter of a few millimetres to a few centimetres. The circular opening 6 can assume any other shape, for example, an ellipse, a triangle, a rectangle, a square, a polygon or the like. The circular opening 6 forms an inlet orifice 6 for guiding a flow of sweat 9, notably in order to convey the sweat into the microfluidic channel 11. The flow of sweat 9 passes from the skin 2 of the subject through the circular opening 6 into the microfluidic channel 11.

[0080] A hydrophilic collection element (not shown), for example, a fibrous body such as cotton or a non-woven material, can be disposed in the circular opening 6 and the sampling cup 5. The collection element conveys the sweat produced in the investigation zone 8 to the microfluidic electrochemical device 100.

[0081] According to a first embodiment, with reference to FIG. 4, the multilayer structure of the apparatus 1 comprises a lower layer 3 comprising an inlet orifice 6 allowing through sweat, an upper layer 10 comprising an outlet orifice 22, an intermediate layer 4 located between the lower layer 3 and the upper layer 10, with the microfluidic electrochemical device 100 being formed in the thickness of the isolating support 4 forming an intermediate layer 4 extending parallel to the lower layer 3.

[0082] The microfluidic electrochemical device 100 comprises a microfluidic channel 11 that is connected to the inlet orifice 6 at a first end and is connected to the outlet orifice 22 at a second end. Thus, the flow of sweat 9 from the skin 2 of the subject is conveyed into the microfluidic channel 11, which guides the sweat from the inlet orifice 6 to the outlet orifice 22 by capillary action.

[0083] The microfluidic channel 11 is provided with an electrochemical cell 14 described hereafter, shown in FIG. 7 or FIG. 8. The electrochemical cell 14 is disposed on the inner face of the upper layer 10 closing the microfluidic channel 11 from above so as to be located in the internal space of the microfluidic channel 11.

[0084] In terms of dimensions, the diameter of the inlet orifice 6 is a few millimetres, the length of the microfluidic channel 11 ranges between 0.5 cm and 5 cm and its width ranges between 20 m and 1,000 m, the thickness of the intermediate layer 4 ranges between 10 m and 500 m, the width of the layers 3, 4, 10 of the multilayer structure ranges between 1 cm and 5 cm and the length of said layers ranges between 2 cm and 10 cm.

[0085] For example, the inlet orifice 6 has a diameter of 5 mm, the microfluidic channel 11 has a length of 3 cm and a width of 200 m, the intermediate layer 4 has a thickness of 150 m, the layers 3, 4, 10 of the multilayer structure have a width of 3 cm and a length of 9 cm.

[0086] According to a second embodiment, with reference to FIGS. 5 and 6, the apparatus 1 comprises a main channel 23 dividing, in the flow direction of the flow of sweat 9, into one or more microfluidic channels, in this case three parallelepiped microfluidic channels 11a, 11b, 11c, parallel to each other, formed in the thickness of the isolating support 6 and separated by partitions 12. Each microfluidic channel 11a, 11b, 11c is thus respectively separated from the other microfluidic channels 11a, 11b, 11c within which the sweat can circulate independently. The number of microfluidic channels can be higher or lower than that shown in FIGS. 5 and 6.

[0087] In terms of dimensioning, the microfluidic channels 11a, 11b, 11c preferably have a height ranging between 10 m and 500 m, a width ranging between 20 m and 1,000 m, and a length ranging between 0.5 cm and 5 cm. The microfluidic channels 11a, 11b, 11c have a constant cross-sectional surface area S.sub.a, S.sub.b, S.sub.c. For the sake of simplicity, but without loss of generality, the cross-sections of the microfluidic channels 11a, 11b, 11c are assumed to have the same surface area S, i.e., S.sub.a=S.sub.b=S.sub.c=S.

[0088] The sweat perspired by the subject 2 in the investigation zone 8 is collected by the part of the collection element in contact with the skin 2 of the subject and is then transferred by capillary action to the main channel 23 in order to independently circulate in the microfluidic channels 11a, 11b, 11c of the microfluidic electrochemical device 100. The arrows 13 illustrate the flow direction of the sweat in the microfluidic channels 11a, 11b, 11c. Each microfluidic channel 11a, 11b, 11c is respectively provided with an electrochemical cell 14a, 14b, 14c, indicated in FIG. 6 and shown in detail in FIG. 7 or FIG. 8. Preferably, the microfluidic channels 11a, 11b, 11c lead to an outlet reservoir (not shown) connected to the outlet orifice 22. The outlet reservoir retains the sweat to prevent it from coming back into contact with the skin 2.

[0089] With reference to FIG. 7, the electrochemical cell 14, 14a, 14b, 14c is based on a four-electrode configuration. More specifically, the electrochemical cell 14, 14a, 14b, 14c comprises a pair of independent working electrodes WE.sub.1 and WE.sub.2, a counter electrode CE and a reference electrode REF, disposed in the microfluidic channel 11, 11a, 11b, 11c. The electrodes WE.sub.1, WE.sub.2, CE and REF are produced in the form of strips of microelectrodes parallel to each other, perpendicular to the flow direction of the sweat in the microfluidic channel 11, 11a, 11b, 11c, and are implanted by microfabrication, for example, by Chemical Vapour Deposition (CVD) and/or by lithography. The length of the microelectrode strips thus corresponds to the width of the microfluidic channel 11, 11a, 11b, 11c.

[0090] The working electrodes WE.sub.1 and WE.sub.2 can be produced in the form of microstrips made of platinum or platinized platinum, also known as platinum black, with a nanometric thickness, for example, of the order of a few tens of nanometres to a hundred nanometres, typically 200 nm. The working electrodes WE.sub.1 and WE.sub.2 are spaced apart by an inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c in the flow direction of the sweat in the microfluidic channel 11, 11a, 11b, 11c. As will be explained hereafter, the inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c varies as a function of the considered microfluidic channel 11, 11a, 11b, 11c. The first working electrode is denoted using reference sign WE.sub.1 and the second working electrode is denoted using reference sign WE.sub.2. By convention, the first working electrode WE.sub.1 is located upstream of the second working electrode WE.sub.2 relative to the flow direction of the sweat of the subject 2 in the microfluidic channel 11, 11a, 11b, 11c.

[0091] The reference electrode REF, which is produced, for example, in the form of a strip of Ag/AgCl reference microelectrode with a nanometric thickness, for example, 500 nm, is located upstream of the pair of working electrodes WE.sub.1 and WE.sub.2 in order to maintain the stability of the reference electrode potential over time.

[0092] The counter-electrode CE, which is produced, for example, in the form of a microstrip, for example, made of platinum, which may or may not be platinized, with a nanometric thickness, for example, of the order of a few tens of nanometres to a few hundred nanometres, typically 100 nm, is located downstream of the pair of working electrodes WE.sub.1 and WE.sub.2 and, therefore, of the reference electrode REF so that the chemical species generated on its surface disrupt neither the working electrodes WE.sub.1 and WE.sub.2 nor the reference electrode REF. Advantageously, the surface of the counter-electrode CE is two to three times larger than those of the other electrodes.

[0093] Advantageously, the microelectrodes produced in the form of microstrips are all deposited on a sub-nanometric adhesion layer (not shown), for example, made of titanium or chromium or the like depending on the nature of the isolating support 4, in order to properly adhere the microstrips on the isolating support 4.

[0094] The microfluidic electrochemical device 100 also comprises an electrochemical amperometry measurement system 15. Each of the electrodes WE.sub.1, WE.sub.2, CE and REF is connected to the electrochemical amperometry measurement system 15 by means of electrical contacts (not shown) electrically isolated from the sweat of the subject.

[0095] The electrochemical amperometry measurement system 15 comprises, for example, a potentiostat or a multipotentiostat (not shown) configured to control one or all or some of the electrochemical cells 14, 14a, 14b and 14c. More specifically, the electrochemical amperometry measurement system 15 is configured to bias the first and second working electrodes WE.sub.1 and WE.sub.2 of an electrochemical cell 14, 14a, 14b, 14c, respectively, at the first and second electrode potentials E.sub.1 and E.sub.2 so as to generate an oxidation reaction or a reduction reaction in the sweat that is associated with water H.sub.2O.sub.(l) (examples 1 and 2) or even the reaction of a chemical species of a redox couple associated with water H.sub.2O.sub.(l), in particular dioxygen O.sub.2(aq) previously dissolved in the sweat (example 3).

[0096] The average volumetric flow rate Q of sweat circulating in a microfluidic channel 11, 11a, 11b or 11c is determined on the basis of the principle of the time of flight technique, i.e., based on measuring the time required for an electroactive chemical species, detected by amperometry, to travel the inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c between the first and second working electrodes WE.sub.1 and WE.sub.2, and the volume of the microfluidic channel 11, 11a, 11b, 11c delimited by the planes perpendicular to the plane in which the working electrodes WE.sub.1, WE.sub.2 are crimped and located at the most upstream limit of each.

[0097] In the following three examples, the working electrodes WE.sub.1 and WE.sub.2 are respectively biased according to the methods illustrated in FIGS. 9 and 10 (example 1), 11 and 12 (example 2), and 13 and 14 (example 3). These methods comprise several steps that are explained in more detail for each of the particular examples described hereafter. The graphs 101, 121 and 141 show the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1 as a function of time t. The graphs 102, 122 and 142 show the second electrode potential E.sub.2 applied to the second working electrode WE.sub.2 as a function of time t. When the first and second electrode potentials E.sub.1 and E.sub.2 assume the value of 0 on graphs 101, 102, 121, 122, 141 and 142, this corresponds to the disconnection of the corresponding working electrode WE.sub.1 or WE.sub.2 (open circuit) or even to the application of a potential close to or equal to the equilibrium potential. The graphs 103, 123 and 143 show the intensity of the Faradic current measured at the first working electrode WE.sub.1 as a function of time t. Finally, the graphs 104, 124 and 144 show the Faradic current intensity measured at the second working electrode WE.sub.2 as a function of time t.

[0098] The values of the electrode potentials E.sub.1 and E.sub.2 that are provided by way of an example are provided in volts with respect to the standard hydrogen electrode (V/SHE). By convention, the anodic intensity of the Faradic current assumes positive values, while the cathodic intensity of the Faradic current assumes negative values.

Example 1

[0099] In a first example, with reference to FIG. 9, the difference in electrode potentials between the first working electrode WE.sub.1 and the counter-electrode CE is set so that the first working electrode WE.sub.1 is biased at a first electrode potential E.sub.1, typically 1.6 V/SHE, allowing water H.sub.2O.sub.(l) to oxidize to dioxygen O.sub.2(aq) according to the following redox half-equation:

##STR00001##

[0100] Correspondingly, the difference in electrode potentials between the second working electrode WE.sub.2 and the counter-electrode CE is set so that the second working electrode WE.sub.2 is biased at a second electrode potential E.sub.2, typically 0.3 V/SHE, allowing the oxygen O.sub.2(aq) produced on the surface of the working electrode WE.sub.1 that is dissolved in the sweat to be reduced to water H.sub.2O.sub.(l) according to the following redox half-equation:

##STR00002##

[0101] With reference to FIG. 10, the method for biasing the first and second working electrodes WE.sub.1 and WE.sub.2 comprises two successive steps over time t.

[0102] During a first step, before an instant t.sub.0 (i.e., for t<t.sub.0) the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1 can be close to the initial equilibrium potential or the first working electrode WE.sub.1 even can be disconnected. In this latter hypothesis, illustrated in graph 101, the first electrode potential E.sub.1 conventionally assumes the value of zero 0. The detected anodic intensity i.sub.ox is zero, as shown in graph 103.

[0103] t the same time, the second electrode potential E.sub.2 applied to the second working electrode WE.sub.2, illustrated in graph 102, is set to a value that is lower than the initial equilibrium potential and that is sufficient to reduce the dissolved oxygen O.sub.2(aq). The cathodic intensity i.sub.red is proportional to the concentration of dioxygen O.sub.2(aq) previously dissolved in the sweat, as illustrated in graph 104; if this initial concentration is zero, the cathodic intensity i.sub.red is zero.

[0104] During a second step starting from the instant to, the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1, illustrated in graph 101, is fixed on the oxidation wave of the water H.sub.2O.sub.(l) so as to initiate the production of an appreciable amount of dioxygen O.sub.2(aq), in other words, so that the total concentration of dioxygen O.sub.2(aq) in the vicinity of the surface of the first working electrode WE.sub.1 is much greater than the concentration of dioxygen O.sub.2(aq) previously dissolved in the sweat. The anodic intensity i.sub.ox, which is a quantity that represents the amount of oxygen O.sub.2(aq) generated on the surface of the first working electrode WE.sub.1, is thus set by the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1. The anodic intensity i.sub.ox is constant over time as soon as the capacitive current linked to the potential jump is cancelled because the redox reaction contemplated at the first working electrode WE.sub.1 is not limited by the conveyance of matter since the reactant is water H.sub.2O.sub.(l) The capacitive currents associated with potential switching are not shown in the diagrams, which only consider Faradic currents.

[0105] As soon as the first working electrode WE.sub.1 is biased at the instant to, a gradient in the concentration of dissolved oxygen O.sub.2(aq) is created in the vicinity of the first working electrode WE.sub.1. The anodic intensity i.sub.ox measured at the first working electrode WE.sub.1 increases due to the oxidation of the water H.sub.2O.sub.(l) In graph 103, the increase in anodic intensity i.sub.ox is schematically shown by a step or a Heaviside function. The dioxygen O.sub.2(aq) concentration gradient forms a concentration front driven by convection downstream of the first working electrode WE.sub.1 under the effect of the flow of sweat.

[0106] t the same time, the second working electrode WE.sub.2 remains biased at a constant second electrode potential E.sub.2. The second working electrode WE.sub.2 continuously records the cathodic intensity i.sub.red of the Faradic current generated by the reduction of dioxygen O.sub.2(aq) to water H.sub.2O.sub.(l). Thus, at the instant t.sub.0+t, when the dioxygen O.sub.2(aq) concentration front generated on the surface of the first working electrode WE.sub.1 passes above the surface of the second working electrode WE.sub.2, the detected cathodic intensity i.sub.red decreases (by relative value) due to the reduction of oxygen O.sub.2(aq) to water H.sub.2O.sub.(l) which is indicated in graph 104. The duration t corresponds to the time requi.sub.red for the dioxygen O.sub.2(aq) front generated at the first working electrode WE.sub.1 to travel to the second working electrode WE.sub.2 under the effect of the flow of sweat through the microfluidic channel 11, 11a, 11b or 11c at the flow speed V.

[0107] The second step ends at an instant t.sub.1 after the instant t.sub.0, from which the first working electrode WE.sub.1 is again biased at a first electrode potential E.sub.1 close to the initial equilibrium potential, or is even disconnected, as illustrated in graph 101. The first working electrode WE.sub.1 can be subsequently rebiased so that the method can be repeated as often as necessary in order to determine the average volume flow rate Q at successive instants that are more or less close together.

[0108] The method described herein is simple and easy to industrialize, as it requires no moving parts and no assumptions concerning the hydrodynamic regime of the flow of sweat in the microfluidic channel. The solution is by no means associated with determining the concentration of chemical species generated or contained in the sweat, but only to a response time t between the amperometric signals of a pair of working electrodes WE.sub.1 and WE.sub.2. In particular, the selection of the oxidation reactions of water H.sub.2O.sub.(l) and of the reduction of dioxygen O.sub.2(aq) allows the amplitude of the detected amperometric signals to be guided to each of the working electrodes WE.sub.1, WE.sub.2 so as to maintain a sufficient signal-to-noise ratio to allow easy detection of the variation in the amperometric signals.

Example 2

[0109] In a second example, with reference to FIG. 11, the difference in electrode potentials between the first working electrode WE.sub.1 and the counter-electrode CE is set so that the first working electrode WE.sub.1 is biased at a first electrode potential E.sub.1, typically 0.8 V/SHE, allowing the reduction of water H.sub.2O.sub.(l) to dihydrogen H.sub.2(aq) according to the following redox half-equation:

##STR00003##

[0110] Correspondingly, the difference in electrode potentials between the second working electrode WE.sub.2 and the counter-electrode CE is set so that the second working electrode WE.sub.2 is biased at a second electrode potential E.sub.2 that is equal to the first electrode potential E.sub.1. Like the first working electrode WE.sub.1, the second working electrode WE.sub.2 is thus configured to reduce the water H.sub.2O.sub.(l) in the sweat to dihydrogen H.sub.2(aq).

[0111] With reference to FIG. 12, the method for biasing the first and second working electrodes WE.sub.1 and WE.sub.2 comprises two successive steps over time t.

[0112] During a first step, before an instant t.sub.0, i.e., for t<t.sub.0, the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1 can be close to the initial equilibrium potential or the first working electrode WE.sub.1 even can be disconnected. In this latter hypothesis, illustrated in graph 121, the first electrode potential E.sub.1 conventionally assumes the value of zero 0, such that the detected cathodic intensity i.sub.red is zero, as shown in graph 123.

[0113] t the same time, the second electrode potential E.sub.2 applied to the second working electrode WE.sub.2, shown in graph 112, is lower than the initial equilibrium potential to reduce the water in the sweat to dihydrogen H.sub.2(aq). The cathodic intensity i.sub.red is constant, as illustrated in graph 124.

[0114] During a second step starting from the instant to, the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1, illustrated in graph 111, is lower than the initial equilibrium potential so as to initiate the reduction of the water in the sweat to dihydrogen H.sub.2(aq). The cathodic intensity i.sub.red detected at the first working electrode WE.sub.1 decreases (by relative value) due to the increase in pH imposed by the reduction of water H.sub.2O.sub.(l) at the first working electrode WE.sub.1. In graph 123, the decrease in the growth of the cathodic intensity i.sub.red is schematically shown by a step or a Heaviside function. The portion of the hydrolyzed sweat is carried by convection downstream of the first working electrode WE.sub.1 under the effect of the flow.

[0115] t the same time, the second working electrode WE.sub.2 remains biased at a constant second electrode potential E.sub.2. The second working electrode WE.sub.2 continuously records the cathodic intensity i.sub.red of the faradic current generated by reducing the water in the sweat to dihydrogen H.sub.2(aq). Thus, at the instant t.sub.0+t, when the flow of partially hydrolyzed sweat passes above the surface of the second working electrode WE.sub.2, the detected cathodic intensity i.sub.red increases (by relative value), i.e., since the concentration of hydronium ions H.sub.3O.sup.+.sub.(aq) is lower than upstream of the first working electrode WE.sub.1, which is shown in graph 114. The duration t corresponds to the time requi.sub.red for the hydronium ion-depleted H.sub.3O.sup.+.sub.(aq) sweat flow front generated at the first working electrode WE.sub.1 to travel to the second working electrode WE.sub.2 under the effect of the flow of sweat through the microfluidic channel 11, 11a, 11b, 11c at the flow speed V.

[0116] The volume of sweat that has experienced an increase in pH due to the action of the first electrode travels from the first working electrode WE.sub.1 to the second working electrode WE.sub.2 under the effect of the flow of sweat in the microfluidic channel 11, 11a, 11b, 11c at the flow speed V.

[0117] The second step ends at an instant t.sub.1 after the instant t.sub.0, from which the first working electrode WE.sub.1 is again biased at a first electrode potential E.sub.1 close to the initial equilibrium potential, or is even disconnected as illustrated in graph 121. The first working electrode WE.sub.1 can be subsequently rebiased so that the method can be repeated as often as necessary in order to determine the average volume flow rate Q at close successive instants.

[0118] The method described herein is simple and easy to industrialize, as it requires no moving parts and no assumptions concerning the hydrodynamic regime. The solution is by no means associated with determining the concentration of chemical species generated or contained in the sweat, but only to a response time t between the variations in the amperometric signals of the pair of working electrodes WE.sub.1 and WE.sub.2.

Example 3

[0119] In a third example, with reference to FIG. 13, the difference in electrode potentials between the first working electrode WE.sub.1 and the counter-electrode CE is set so that the first working electrode WE.sub.1 is biased at a first electrode potential E.sub.1, typically 0.3 V/SHE, allowing only the oxygen O.sub.2(aq) initially dissolved in the examined aqueous solution, when it contains said solution, to be reduced to water H.sub.2O.sub.(l) according to the following redox half-equation:

##STR00004##

[0120] Correspondingly, the difference in electrode potentials between the second working electrode WE.sub.2 and the counter-electrode CE is set so that the second working electrode WE.sub.2 is biased at a second electrode potential E.sub.2 that is equal to the first electrode potential E.sub.1. Like the first working electrode WE.sub.1, the second working electrode WE.sub.2 is thus configured to reduce the fraction of oxygen O.sub.2(aq) dissolved in the sweat to water H.sub.2O.sub.(l) that has not been reduced at the first working electrode WE.sub.1.

[0121] With reference to FIG. 14, the method for biasing the first and second working electrodes WE.sub.1 and WE.sub.2 comprises two successive steps over time t.

[0122] During a first step, before an instant t.sub.0, i.e., for t <to, the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1 can be close to the initial equilibrium potential or the first working electrode WE.sub.1 even can be disconnected. In this latter hypothesis, illustrated in graph 141, the first electrode potential E.sub.1 conventionally assumes the value of zero 0, such that the detected cathodic intensity i.sub.red is zero, as shown in graph 143.

[0123] t the same time, the second electrode potential E.sub.2 applied to the second working electrode WE.sub.2, shown in graph 142, is lower than the initial equilibrium potential for reducing the dioxygen O.sub.2(aq) to water H.sub.2O.sub.(l) The cathodic intensity i.sub.red is constant since it is proportional to the concentration of dioxygen O.sub.2(aq) previously dissolved in the sweat, as illustrated in graph 144.

[0124] During a second step starting from the instant t.sub.0, the first electrode potential E.sub.1 applied to the first working electrode WE.sub.1, illustrated in graph 141, is lower than the initial equilibrium potential so as to initiate the reduction of all or some of the dioxygen O.sub.2(aq) dissolved in the sweat. The cathodic intensity i.sub.red detected at the first working electrode WE.sub.1 decreases (by relative value) as a result of the reduction of oxygen O.sub.2(aq) to water H.sub.2O.sub.(l) In graph 143, the decrease in the growth of the cathodic intensity i.sub.red is schematically shown by a step or a Heaviside function. The hydrolyzed portion of the dissolved dioxygen-depleted O.sub.2(aq) sweat is carried by convection downstream of the first working electrode WE.sub.1 under the effect of the flow.

[0125] t the same time, the second working electrode WE.sub.2 remains biased at a constant second electrode potential E.sub.2. The second working electrode WE.sub.2 continuously records the cathodic intensity i.sub.red of the Faradic current generated by the reduction of dioxygen O.sub.2(aq) to water H.sub.2O.sub.(l). Thus, at the instant t.sub.0+t, when the flow of dioxygen-depleted O.sub.2(aq) sweat passes above the surface of the second working electrode WE.sub.2, the detected cathodic intensity i.sub.red increases (by relative value), i.e., it approaches the zero value, since the concentration of dioxygen O.sub.2(aq) dissolved in the sweat is zero or, at least, lower than upstream of the first working electrode WE.sub.1, which is shown in graph 144. The duration t corresponds to the time requi.sub.red for the flow of dissolved dioxygen-depleted O.sub.2(aq) sweat to travel from the first working electrode WE.sub.1 to the second working electrode WE.sub.2 under the effect of the flow of sweat through the microfluidic channel 11, 11a, 11b or 11c at the flow speed V.

[0126] The second step ends at an instant t1 after the instant t.sub.0, from which the first working electrode WE.sub.1 is again biased at a first electrode potential E.sub.1 close to the initial equilibrium potential, or is even disconnected, as illustrated in graph 141. The first working electrode WE.sub.1 can be subsequently rebiased so that the method can be repeated as often as necessary in order to determine the average volume flow rate Q at close successive instants.

[0127] The method described herein is simple and easy to industrialize, as it requires no moving parts and no assumptions concerning the hydrodynamic regime. The solution is by no means associated with determining the concentration of chemical species generated or contained in the sweat, but only to a response time t between the variations in the amperometric signals of the pair of working electrodes WE.sub.1 and WE.sub.2.

[0128] In the three examples described above, the flow speed V and the volume flow rate Q of sweat flowing through a microfluidic channel 11, 11a, 11b or 11c of rectangular parallelepiped shape with a constant cross-sectional surface area S in the flow direction 13 can be determined based on the inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c separating the working electrodes WE.sub.1 and WE.sub.2, and based on the duration t characteristic of the delay in the response of the second working electrode WE.sub.2, monitored by chronoamperometry, relative to the instantaneous response of the first working electrode WE.sub.1.

Subject to the reservations that will be described hereafter, the average linear flow speed V and the average volumetric flow rate Q of the flow of sweat circulating in a microfluidic channel, for example, the microfluidic channel referenced 11 and for which the inter-electrode distance is referenced L, can be estimated according to the following equations:

[00001]$V=L/tQ=SV=LS/t$

With the same reservations, these equations are also valid, respectively, in the microfluidic channels 11a, 11b, 11c by substituting the inter-electrode distance L with the inter-electrode distance L.sub.a, L.sub.b, L.sub.c.

[0129] Within the context of the contemplated dynamic applications, for temporally monitoring the physiological state of the subject, it is beneficial for the apparatus 1, in order to determine the quantitative sweating parameter of a subject, to measure the value of the volumetric flow rate Q of sweat at successive instants in close succession, consistent with the expected sweating rate, for example, once a minute. Integrating any temporal variations in the volume flow rate Q of sweat then allows the value of the total flow of sweat perspired by the subject to be determined over a given time range t.

[0130] The quantitative sweating parameter can be a sweating rate that is determined based on the total volume of sweat perspired by the subject over a given time range t, in relation to the surface area of the investigation zone 8.

[0131] The inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c separating the working electrodes WE.sub.1 and WE.sub.2 is selected so to be small enough for changes in the physiological response of the subject to be negligible over the duration t and large enough to allow a decoupled operating regime for the working electrodes WE.sub.1 and WE.sub.2 in the one or each microfluidic channel 11, 11a, 11b, 11c where the volume flow rate Q is measured.

[0132] Indeed, depending on the average linear flow speed V of the sweat in the microfluidic channel 11, 11a, 11b or 11c, the concentration gradient created in the vicinity of the first working electrode, by generating electroactive chemical species (example 1) or by depleting electroactive chemical species already present in the sweat (examples 2 and 3), may or may not become homogeneous over the height of the microfluidic channel 11, 11a, 11b or 11c after being carried over the inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c. In particular, when, given the linear flow speed V of the sweat, the concentration gradient does not have time to dissipate over the height of the microfluidic channel 11a, 11b or 11c before reaching the second working electrode WE.sub.2, the operation of the two working electrodes WE.sub.1 and WE.sub.2 is linked. This coupling regime limits the temporal resolution of the amperometric signals, which disrupts the measurements of the volume flow rate Q of the sweat circulating in the microfluidic channel 11a, 11b or 11c. This obstacle is easily avoided by adjusting the relative values of the inter-electrode distance L, L.sub.a, L.sub.b, L.sub.c and the duration t.sub.1-t.sub.0 to the expected values of the average linear flow speed V.

[0133] According to a first embodiment, shown in FIG. 6, pairs of first and second working electrodes WE.sub.1 and WE.sub.2 separated by different inter-electrode distances L.sub.a, L.sub.b, L.sub.c can be used in separate parallel microfluidic channels 11a, 11b, 11c. Preferably, the inter-electrode distance L.sub.a, L.sub.b, L.sub.c separating the working electrodes WE.sub.1 and WE.sub.2 differs as a function of the considered microfluidic channel 11a, 11b or 11c, for example, such that L.sub.a<L.sub.b<L.sub.c.

[0134] As a variant, according to a second embodiment illustrated in FIG. 8, an array of second working electrodes, in this case a first second working electrode WE.sub.2.sup.(1) and a second second working electrode WE.sub.2.sup.(2), can be implemented in the same microfluidic channel 11, 11a, 11b, 11c. According to some embodiments, not shown, the array of second working electrodes can include more than two second working electrodes. The remainder of the description will be limited to describing the array of second working electrodes implemented in the microfluidic channel referenced 11. Such an array also could be implemented in the microfluidic channels 11a, 11b, 11c.

[0135] In the microfluidic channel 11, each second working electrode WE.sub.2.sup.(1) , WE.sub.2.sup.(2) is respectively disposed at a different inter-electrode distance L.sup.(1) , L.sup.(2) from the first working electrode WE.sub.1. The working electrodes WE.sub.1, WE.sub.2.sup.(1) , WE.sub.2.sup.(2) are electronically switchable. The duration t1-to is electronically adjusted by feeding back the average linear flow speed value V measured over the previous measurement instants.

[0136] The volume flow rate Q thus can be determined over a wide range of values, since the volume flow rate Q can be measured in each of the microfluidic channels 11, 11a, 11b and 11c or in several of them, while retaining only the volume flow rate Q measurements consistent with the inter-electrode distances L.sub.a, L.sub.b, L.sub.c, L.sup.(1) , L.sup.(2) . Advantageously, the inter-electrode distances L.sub.a, Lb and L.sub.c, L.sup.(1) , L.sup.(2) are of the order of a millimetre.

[0137] The methods for measuring the volumetric flow rate Q of sweat described above can be implemented automatically using an electronic processing device 16, preferably integrated into the apparatus 1.

[0138] With reference to FIG. 15, an embodiment will now be described of the electronic processing device 16 that can be integrated into the apparatus 1, for example, in the form of an electronic board 17.

[0139] In the embodiment with a plurality of microfluidic channels 11a, 11b, 11c shown in FIGS. 5 and 6, the electrochemical cells 14a, 14b, 14c are connected to an analogue-to-digital converter 18, which itself powers a processor 19. The processor 19 is programmed, for example, to implement the methods for measuring the volume flow rate Q of sweat described above.

[0140] In the embodiment shown in FIG. 8, where a first working electrode WE.sub.1 and several second working electrodes WE.sub.2.sup.(1) , WE.sub.2.sup.(2) respectively located at different inter-electrode distances L.sup.(1) and L.sup.(2) are implemented in the same microfluidic channel 11, each pair formed by the first working electrode WE.sub.1 and one of the second working electrodes WE.sub.2.sup.(1) , WE.sub.2.sup.(2) forms an electrochemical cell 14 connected to an analogue-to-digital converter 18, which itself powers a processor 19. The processor 19 is programmed, for example, to implement the methods for measuring the volume flow rate Q of sweat described above.

[0141] An energy source 20, for example, a battery, powers the electronic processing device 16. A wi.sub.red or wireless communication module 21 also can be provided in order to send the results of the sweat volume flow rate Q measurements to a storage or post-processing apparatus.

[0142] The electronic processing device 16 optionally comprises other functional modules, for example, a gyroscopic and/or an accelerometric module for detecting the orientation and the movements of the subject 2, as well as for quantifying their level of activity, and/or a temperature sensor for measuring the temperature of the epidermis of the subject 2. Indeed, it is worthwhile knowing the skin temperature due to the correlations between the temperature and the sweating rate.

[0143] Some elements of the apparatus 1, notably the electronic processing device 16, can be produced in various forms, in a unitary or distributed manner, by means of hardware and/or software components. Hardware components that can be used include Application-Specific Integrated Circuits (ASICs) and Field-Programmable Gate Arrays (FPGAs). Software components can be written in various programming languages, for example, C, C++, Java or VHDL. This list is not exhaustive.

[0144] Although the invention has been described in conjunction with several particular embodiments, it is obvious that it is by no means limited thereto and that it includes all the technical equivalents of the means described, as well as the combinations thereof, if they fall within the scope of the invention.

[0145] The use of the verbs comprise and include and the conjugated forms thereof does not exclude the presence of elements or steps other than those set forth in a claim.

[0146] In the claims, any reference sign between brackets should not be understood to be a limitation of the claim.