Microfluidic electrochemical device for measuring volumetric flow rate

Abstract

The invention relates to a microfluidic electrochemical device for measuring an average volumetric flow rate of an electroactive carrier fluid. The microfluidic electrochemical device comprises at least one microfluidic channel 11a, 11b, 11c in which the carrier fluid flows and an electrochemical cell 14a, 14b, 14c comprising at least one pair of working electrodes WE1, WE2 separated by an inter-electrode distance L a, L b, L c in a direction of carrier fluid flow in the microfluidic channel 11a, 11b, 11c, at least one counter electrode CE and at least one reference electrode REF.The microfluidic electrochemical device also includes an amperometric electrochemical measurement system 15 configured to bias the working electrode pair WE1, WE2 so that each electrode WE1, WE2 produces an amperometric signal through an oxidation or reduction reaction with the electroactive carrier fluid or with a chemical species associated with a redox couple to which the carrier fluid belongs. The microfluidic electrochemical device determines the average volumetric flow rate of the carrier fluid in the microfluidic channel 11a, 11b, 11c based, in particular, on the inter-electrode distance La, Lb, Lc and a time delay between the amperometric signals produced by the working electrode pair WE1, WE2. Figure to be published: 9.

Description

Title of the invention: Microfluidic electrochemical device for measuring volumetric flow rate technical field

[0001] The invention relates to the field of microfluidic electrochemical devices and to methods for measuring the volumetric flow rate of an electroactive fluid or a fluid containing one or more electroactive species in a microfluidic channel. More generally, the invention also relates to an apparatus for determining a quantitative parameter of perspiration in a human or animal subject. Technological background

[0002] Sweat is secreted by the sweat glands in the skin, released through the pores, and evaporates at the level of the epidermis. Sweating plays an important role in the body, as it allows for thermoregulation through perspiration. Excessive sweating can lead to dehydration, impair physical performance, and have detrimental health consequences. Similarly, excessive water consumption can lead to hyponatremia, fatigue, confusion, coma, or even death.

[0003] Various devices for in situ measurement of micro-flow rates are known in the prior art, including thermal flux sensors and Coriolis micro-flowmeters. In practice, these devices are expensive, have unresolved reliability issues, and are generally housed in bulky casings. More specifically, microfluidic devices have been developed to measure the micro-flow rates of sweat perspired by a human or animal subject, circulating in microfluidic channels.Measuring a micro-flow of sweat allows for the evaluation of a quantitative parameter of the subject's perspiration in order, for example, to monitor the subject's hydration level in order to prevent fluid imbalances in the body, particularly in athletes and the elderly, or to diagnose hypohidrosis, a sweating disorder characterized by insufficient sweating and which can be caused by pathologies likely to damage the functioning of the sweat glands (diabetes, alcoholism, Parkinson's disease, Ross syndrome, Sjögren's syndrome, small cell lung cancer, etc.), by cutaneous causes (burns, inflammations, infections, skin pathologies, etc.), by drug causes (e.g., anticholinergic treatments) and by genetic causes (e.g., hypohidrotic ectodermal dysplasia).

[0004] Known microfluidic devices have the advantage of being able to easily collect sweat in microfluidic channels without evaporation, with a very high efficiency. High temporal resolution. Colorimetric techniques are generally preferred due to the ease of manufacturing the associated devices. However, their main drawback is the irreversible nature of the process. After the microfluidic channel fills with sweat, these devices are permanently altered by the dyes and can no longer be used. This situation is comparable to detection techniques based on electrical transduction signals such as resistance, conductance, capacitance, or impedance. To estimate micro-flow rates of sweat, these techniques involve monitoring the channel filling rate using electrodes placed along the microfluidic channels. Once filled, the microfluidic channels are therefore unusable.

[0005] Document FR3103901Al describes in particular a method for measuring a sweat flow velocity based on a delay between the temporal variations of amperometric signals which intrinsically represent the concentration of hydrogen peroxide H2O2, nitrogen monoxide NO or nitrite ion NO2, present in the sweat flow. Summary

[0006] An idea underlying the invention is to provide a microfluidic electrochemical device for measuring the volumetric flow rate of a fluid in a microfluidic channel, without necessarily carrying out the determination of the concentration of electroactive chemical species contained in the fluid flow.

[0007] Another idea underlying the invention is to provide a flexible device to be stuck to the skin to determine a quantitative parameter of perspiration of a subject from the in situ and continuous measurement of the volumetric flow rate of sweat flowing in a microfluidic channel.

[0008] An object of the invention is to provide such a device which also has the advantages of being compact, of simple design and of limited cost.

[0009] According to one embodiment, the invention provides a microfluidic electrochemical device for measuring the flow velocity and / or volumetric flow rate of a fluid, the fluid comprising a solvent, the microfluidic electrochemical device comprising: - at least one microfluidic channel configured to allow the fluid to flow in a direction of flow; - at least one electrochemical cell disposed in at least one microfluidic channel, the electrochemical cell comprising a first working electrode and at least a second working electrode spaced from the first working electrode by an inter-electrode distance in the direction of flow, at least one counter electrode and at least one reference electrode; and - an electrochemical amperometric measurement system configured to polarize the first working electrode at a first electrode potential and the second working electrode at a second electrode potential, such 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; the electrochemical measurement system by amperometry being configured to determine the flow velocity and / or volumetric flow rate of the fluid in the microfluidic channel from the inter-electrode distance between the first and second working electrodes, and a time delay between a variation of the amperometric signal produced by the first working electrode and a variation of the amperometric signal produced by the second working electrode.

[0010] Such a microfluidic electrochemical device can be incorporated into numerous microsystems to measure in situ the volumetric flow rate and / or flow velocity of a fluid in a microfluidic channel. These microsystems can be, for example, lab-on-a-chip type microfluidic platforms or micro-Total Analysis Systems (pTAS).

[0011] The microfluidic electrochemical device is simple to manufacture and easily industrializable, as it has no moving parts and requires no assumptions about the hydrodynamic regime of the fluid flow in the microfluidic channel. The determination of the flow velocity and / or volumetric flow rate is in no way related to the determination of the concentration of chemical species generated or contained in the fluid, but solely to a response time between variations in the amperometric signals from a pair of working electrodes.

[0012] According to embodiments, such a microfluidic electrochemical device may include one or more of the following characteristics.

[0013] According to one embodiment, the solvent is water H2OW.

[0014] Water can act as a reducing chemical species in the O2 / H2O redox couple and as an oxidizing chemical species in the H3O+ / H2 redox couple.

[0015] According to one embodiment, the fluid is sweat from a human or animal subject.

[0016] According to one embodiment, the first electrode potential allows the oxidation of water H2O(i) into dioxygen O2(aq) and the second electrode potential allows the reduction of dioxygen O2(aq) dissolved in the water H2O(i) produced into water H2O(i).

[0017] The choice of the oxidation reactions of water H2O(i) at the first working electrode and of the reduction of dioxygen O2(aq) at the second working electrode, by means of the appropriate potentials applied to the first and second working electrodes, makes it possible to control the amplitude of the amperometric signals detected at each of said electrodes working electrodes without necessarily measuring these amplitudes but in such a way as to maintain a signal-to-noise ratio sufficient to allow easy detection of variations in amperometric signals.

[0018] According to one embodiment, the first electrode potential allows the reduction of water H2O(i) to dihydrogen H2(aq) and the second electrode potential allows the reduction of water H2O(ij) to dihydrogen H2(aq).

[0019] According to one embodiment, the first electrode potential allows the reduction of dioxygen O2(aq) dissolved in water H2O(ij) to water H2O(ij and the second electrode potential allows the reduction of dioxygen O2(aq) dissolved in water H2OW to water H2 O®.

[0020] According to one embodiment, the electrochemical measurement system by amperometry is also configured to: - in a first step, polarize the first working electrode to the first electrode potential and the second working electrode to the second electrode potential; - in a second step, disconnect the first working electrode or fix the first electrode potential at a potential close to or equal to a zero current equilibrium potential.

[0021] According to one embodiment, the microfluidic electrochemical device further comprises an insulating support, said at least one microfluidic channel being formed in the insulating support, the first and second working electrodes being formed by metallic deposits of platinum or platinum black on said insulating support.

[0022] According to one embodiment, the counter electrode is positioned downstream of the working electrodes in the direction of flow, and in which the reference electrode is positioned upstream of said working electrodes in said direction of flow.

[0023] Thus, the reference electrode is located upstream of the working electrode pair in order to preserve the stability of the reference electrode potential over time; and the counter electrode is located downstream of the working electrode pair and, therefore, of the reference electrode, so that the chemical species generated on its surface do not disturb either the working electrodes or the reference electrode.

[0024] Advantageously, the surface area of ​​the counter electrode is two to three times larger than those of the other electrodes.

[0025] According to some embodiments, the microfluidic electrochemical device may comprise one or more microfluidic channels. If applicable, an electrochemical cell may be disposed in one or each microfluidic channel, or in some or all of the microfluidic channels. The electrochemical cells disposed in different channels may be different or identical. The redox reactions carried out in the electrochemical cells disposed in Different channels can be different or identical.

[0026] According to one embodiment, the microfluidic electrochemical device comprises a first and a second microfluidic channel, the first, respectively, the second, electrochemical cell being disposed in the first, respectively, the second, microfluidic channel, the inter-electrode distance of the first electrochemical cell differing from the inter-electrode distance of the second electrochemical cell.

[0027] According to one embodiment, said at least one electrochemical cell comprises two second working electrodes respectively separated from the first working electrode by a first inter-electrode distance and by a second inter-electrode distance, the first inter-electrode distance being different from the second inter-electrode distance.

[0028] Preferably, the inter-electrode distance separating the pair of working electrodes is chosen to be small enough so that changes in the physiological response of the subject are negligible during the duration of the time delay between the variations of the amperometric signals of the working electrodes, and large enough to allow, at least in one of the microfluidic channels, a decoupled operating regime of the working electrodes.

[0029] Indeed, the operating mode—coupled or decoupled—of the working electrodes depends on the average fluid flow velocity in the microfluidic channel and the inter-electrode distance. At high flow velocities, if the inter-electrode distance is too small, the fluid flow that reacted to the first working electrode remains inhomogeneous after reaching the second working electrode. This coupling mode limits the temporal resolution of the amperometric signals, which degrades the accuracy of the sweat volume flow measurements.

[0030] According to one embodiment, the electrochemical measurement system by amperometry is configured to determine the volumetric flow rate as a function of a cross-sectional area of ​​said microfluidic channel according to the direction of flow.

[0031] By configuring the inter-electrode distance differently depending on the electrochemical cell considered, it is thus possible to measure a volumetric flow rate over a range of values ​​covering all conceivable physiological flow rates.

[0032] According to one embodiment, the invention provides a device intended to be placed on an area of ​​investigation of the epidermis of a human or animal subject to measure a quantitative parameter of the subject's perspiration, said device comprising: - a structure defining a microfluidic electrochemical device, the structure comprising an inlet orifice defining the area of ​​investigation and allowing sweat to pass from the epidermis, at least one microfluidic channel of the microfluidic electrochemical device being in communication with the inlet orifice; and - an electronic processing device configured to determine the quantitative parameter of perspiration of said human or animal subject from measurements of the volumetric flow rate of perspiration carried out by the microfluidic electrochemical device.

[0033] The quantitative sweating parameter can be a sweating rate determined from the total volume of sweat sweated by the subject over a given time range, reported to the surface area of ​​the investigation area.

[0034] According to one embodiment, the quantitative parameter of perspiration of said human or animal subject is a perspiration rate.

[0035] The term epidermis refers to the superficial layer of skin in humans and animals.

[0036] According to embodiments, such a device may include one or more of the following characteristics.

[0037] According to one embodiment, the structure is a multilayer structure comprising a lower layer and at least one layer superimposed on the lower layer, the microfluidic electrochemical device extending parallel to the lower layer, the lower layer comprising said inlet orifice.

[0038] According to one embodiment, the multilayer structure further comprises a top layer and at least one intermediate layer located between the bottom layer and the top layer, the microfluidic electrochemical device being formed in the thickness of at least one intermediate layer.

[0039] The layers can be fixed to each other by any suitable method, for example by adhesives, by welding, by mechanical clamping etc.

[0040] Thanks to these characteristics, the manufacture, assembly and therefore the industrialization of the device is facilitated.

[0041] Thanks to these characteristics, the device is adapted to any curvature when applied to the epidermis. Furthermore, the intermediate layer(s) also create a thickness that compensates for the thickness of the electrodes of the microfluidic electrochemical device. Thus, the device's seal is ensured.

[0042] According to one embodiment, the upper layer has an outlet orifice passing through the upper layer, and in which at least one microfluidic channel is in communication with the outlet orifice.

[0043] According to one embodiment, the first working electrode, at least one second working electrode, said at least one second working electrode, at least one counter electrode and at least one reference electrode are arranged on an inner face of the upper layer closing at least one microfluidic channel from the top and / or on an upper face of the lower layer closing said at least one microfluidic channel from the bottom.

[0044] Thanks to these characteristics, the electrodes are reliably arranged. Furthermore, the fabrication of the multilayer structure comprising these electrodes is facilitated because it is possible to fabricate the electrodes on a flat layer when the microfluidic electrochemical device is formed in an intermediate layer.

[0045] According to one embodiment, the device further comprises a wired or wireless communication device configured to transmit one or more measurement signals produced by the microfluidic electrochemical device.

[0046] According to one embodiment, the device further comprises a gyroscopic module and / or at least one accelerometer to detect a state of activity of said human or animal subject.

[0047] According to one embodiment, the device further comprises a temperature sensor configured to measure the temperature of the epidermis of said human or animal subject.

[0048] According to one embodiment, the device includes a geolocation module.

[0049] Thanks to these characteristics, the device is configured to periodically perform and transmit measurements, for example at a configurable frequency or at a frequency dependent on an activity state detected by the device in order to facilitate an analysis of the correlations between the subject's activity state and the quantitative parameter of sweating measured by the device.

[0050] Measurements of the volumetric flow rate of sweat and / or the quantitative parameter of perspiration can be used in various applications, for example to monitor the subject's hydration level in order to prevent bodily fluid imbalances, particularly in athletes during exertion or in the elderly, especially in cases of high temperatures, or to diagnose hypohidrosis, whatever its causes.

[0051] Other applications are possible in various technological or environmental fields where the measurement of a flow rate is necessary in a device or process involving an electroactive fluid or containing one or more electroactive species. Brief description of the figures

[0052] The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent from the following description of several particular embodiments of the invention, given solely by way of illustration and not limitation, with reference to the accompanying drawings.

[0053] [Fig.1] Fig.1 is a schematic view of a subject seen from behind on which a device has been placed according to an embodiment.

[0054] [Fig.2] Fig.2 is a perspective view partially representing a structure multilayer for a device according to an embodiment.

[0055] [Fig.3] The [Fig.3] is a cross-sectional view along line III-III of the [Fig.2].

[0056] [Fig.4] Fig.4 is an exploded view of the multilayer structure according to a mode of realization.

[0057] [Fig.5] The [Fig.5] is a partial functional schematic representation of a multilayer structure defining an electrochemical device in an apparatus.

[0058] [Fig.6] The [Fig.6] is a partial functional schematic representation of a microfluidic electrochemical device that can be used in an apparatus.

[0059] [Fig.7] The [Fig.7] is a schematic top view of an electrochemical cell according to a first embodiment.

[0060] [Fig.8] The [Fig.8] is a schematic view analogous to that of the [Fig.7], according to a second embodiment.

[0061] [Fig.9] The [Fig.9] is a functional schematic cross-sectional representation of an electrochemical cell along a microfluidic channel according to a first embodiment.

[0062] [Fig. 10] The [Fig. 10] is a set of chronoamperograms illustrating a process that can be implemented, according to the first embodiment, with the microfluidic electrochemical device of the [Fig.7].

[0063] [Fig. 11] The [Fig. 11] is a functional schematic representation analogous to that of the [Fig.9] according to a second embodiment.

[0064] [Fig. 12] The [Fig. 12] is a set of chronoamperograms analogous to those of the [Fig. 10] according to the second embodiment.

[0065] [Fig. 13] The [Fig. 13] is a functional schematic representation analogous to those of figures 9 and 11, according to a third embodiment.

[0066] [Fig. 14] The [Fig. 14] is a set of chronoamperograms similar to those in Figures 10 and 12, according to a third embodiment.

[0067] [Fig. 15] The [Fig. 15] is the functional schematic representation of an electronic control device that can be implemented with the apparatus. Description of the implementation methods

[0068] The embodiments described below relate to an apparatus for determining a quantitative parameter of a subject's perspiration by means of a microfluidic electrochemical device for continuously measuring the volumetric flow rate of perspiration in a microfluidic channel. More generally, such a microfluidic electrochemical device can be incorporated into numerous microsystems for in situ measuring the average flow velocity 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 (pTAS).

[0069] With reference to [Fig. 1], the device 1 for determining a quantitative sweating parameter is placed on the skin 2 of a human subject, for example on their back. In an alternative embodiment not shown, the device 1 can be placed on the skin of an animal subject.

[0070] With reference to [Fig. 2], the device 1 is, for example, in the form of a compact, multilayer structure made of waterproof 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 insulating support 4 superimposed on the lower layer 3.

[0071] A microfluidic channel 11 is cut into the thickness of the insulating support 4. A sampling cup 5, located at the right of a circular opening 6, is made in the lower layer 3.

[0072] With reference to [Fig. 3], the lower layer 3 is adhered to the skin 2 by an adhesive layer 7. A central portion of the lower layer 3 and the adhesive layer 7 comprises the circular opening 6, which defines an investigation area 8 on the subject's skin 2, for example, a few millimeters to a few centimeters in diameter. The circular opening 6 may take another shape, for example, an ellipse, a triangle, a rectangle, a square, a polygon, or other. The circular opening 6 forms an inlet orifice 6, allowing the flow of sweat 9 to be guided, in particular to bring the sweat into the microfluidic channel 11. The sweat flow 9 passes from the subject's skin 2 into the microfluidic channel 11 through the circular opening 6.

[0073] A hydrophilic collecting element (not shown), for example a fibrous body such as cotton or a non-woven material, can be placed in the circular opening 6 and the sampling cup 5. The collecting element fulfills the function of bringing the sweat produced in the investigation area 8 to the microfluidic electrochemical device 100.

[0074] According to a first embodiment, with reference to [Fig.4], the multilayer structure of the device 1 comprises a lower layer 3 including an inlet orifice 6 allowing the passage of sweat, an upper layer 10 including an outlet orifice 22, an intermediate layer 4 located between the lower layer 3 and the upper layer 10, the microfluidic electrochemical device 100 being formed in the thickness of the insulating support 4 constituting an intermediate layer 4 extending parallel to the lower layer 3.

[0075] The microfluidic electrochemical device 100 comprises a microfluidic channel 11 which is in communication with the inlet port 6 at a first end and in communication with the outlet port 22 at a second end. Thus, the sweat flow 9 from the skin 2 of the subject is brought into the micro-fluidic channel 11 which guides the sweat from the inlet orifice 6 to the outlet orifice 22 by capillarity.

[0076] The microfluidic channel 11 is provided with an electrochemical cell 14, described below, shown in [Fig. 7] or [Fig. 8]. The electrochemical cell 14 is arranged on the inner face of the upper layer 10 closing the microfluidic channel 11 from the top so as to be located in the internal space of the microfluidic channel 11.

[0077] In dimensioning, the inlet orifice 6 has a diameter of a few millimeters, the microfluidic channel 11 has a length between 0.5 cm and 5 cm and a width between 20 qm and 1000 qm, the intermediate layer 4 has a thickness between 10 qm and 500 qm, the layers 3, 4, 10 of the multilayer structure have a width between 1 cm and 5 cm and a length between 2 cm and 10 cm.

[0078] 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 sqm, the intermediate layer 4 has a thickness of 150 sqm, the layers 3, 4, 10 of the multilayer structure have a width of 3 cm and a length of 9 cm.

[0079] According to a second embodiment, with reference to Figures 5 and 6, the device 1 comprises a main channel 23 dividing in the direction of the sweat flow 9 into one or more microfluidic channels, here three parallelepiped microfluidic channels 1a, 11b, 11c, parallel to each other, formed within the thickness of the insulating support 6 and separated by partitions 12. Each microfluidic channel 1a, 11b, 11c is thus respectively separated from the other microfluidic channels 1a, 11b, 1le within which the sweat can flow independently. The number of microfluidic channels may be higher or lower than that shown in Figures 5 and 6.

[0080] In terms of dimensioning, the microfluidic channels 1a, 11b, 1le preferably have a height between 10 µm and 500 µm, a width between 20 µm and 1000 µm, and a length between 0.5 cm and 5 cm. The microfluidic channels 1a, 11b, 11c have a constant cross-sectional area Sa, Sb, Sc. For simplicity, but without loss of generality, the cross-sectional areas of the microfluidic channels 1a, 11b, 11c are assumed to have the same surface area S, i.e., Sa = Sb = Sc = S.

[0081] The sweat perspired by subject 2 in the investigation area 8 is collected by the part of the collecting element in contact with the subject's skin 2 and then transferred by capillary action to the main channel 8 in order to flow independently in the microfluidic channels 1a, 11b, 11c of the microfluidic electrochemical device 100. The arrows 13 illustrate the direction of sweat flow in the microfluidic channels 1a, 11b, 11c. Each microfluidic channel 1a, 11b, 11c is respectively equipped with an electrochemical cell 14a, 14b, 14c shown in [Fig. 6] and detailed in [Fig. 7] or [Fig. 8]. Preferably, the microfluidic channels 11a, 11b, 1 lead to an outlet reservoir (not shown) in communication with the outlet port 22. The outlet reservoir retains the sweat to prevent it from coming into contact with the skin 2.

[0082] 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 WE1 and WE2, a counter electrode CE, and a reference electrode REF, arranged in the microfluidic channel 11, 11a, 11b, 11e. The WE2, CE, and REF electrodes are made in the form of parallel microelectrode strips, perpendicular to the direction of sweat flow in the microfluidic channel 11, 11a, 11b, 11e, and implanted by microfabrication, for example, by chemical vapor deposition (CVD) and / or lithography. The length of the microelectrode strips thus corresponds to the width of the microfluidic channel 11, lia, 11b, 11c.

[0083] The working electrodes WEi and WE2 can be made of platinum or platinum-plated microstrips—also called platinum black—with a nanometer thickness, for example, on the order of a few tens of nanometers to a hundred nanometers, typically 200 nm. The working electrodes WEi and WE2 are spaced at an inter-electrode distance L, La, Lh, Lc in the direction of sweat flow in the microfluidic channel 11, lia, 11b, 1le. As will be explained later, the inter-electrode distance L, La, Lh, Lc varies depending on the microfluidic channel 11, lia, 11b, 1le considered. The first working electrode will be designated by the reference WEi and the second working electrode by the reference WE2. By convention, the first working electrode WEi is located upstream of the second working electrode WE2 with respect to the direction of sweat flow from subject 2 in the microfluidic channel 11, lia, 11b, 11c.

[0084] The reference electrode REF, made for example in the form of a nanometrically thick Ag / AgCl reference microelectrode strip, for example 500 nm, is located upstream of the working electrode pair WEi and WE2 in order to preserve the stability of the reference electrode potential over time.

[0085] The counter electrode CE, made for example in the form of a microstrip for example in platinum, platinated or not, of nanometric thickness, for example on the order of a few tens of nanometers, to a few hundred nanometers, typically 100 nm, is located downstream of the pair of working electrodes WEi and WE2 and, therefore, of the reference electrode REF so that the chemical species generated on its surface do not disturb either the working electrodes WEi and WE2 or the reference electrode REF. Advantageously, the surface area of ​​the CE counter electrode is two to three times larger than those of the other electrodes.

[0086] Advantageously, the microelectrodes made in the form of microstrips are all deposited on a sub-nanometric adhesion layer (not shown), for example in titanium or chromium or other depending on the nature of the insulating support 4, to provide good adhesion of the microstrips to the insulating support 4.

[0087] The microfluidic electrochemical device 100 also includes an electrochemical amperometric measurement system 15. Each of the electrodes WEb WE2, CE and REF is connected to the electrochemical amperometric measurement system 15 via electrical contacts (not shown) electrically isolated from the subject's sweat.

[0088] The electrochemical amperometric measurement system 15 includes, 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 amperometric measurement system 15 is configured to bias the first and second working electrodes WEi and WE2 of an electrochemical cell 14, 14a, 14b, 14c respectively at the first and second electrode potentials El and E2 so as to generate in the sweat an oxidation reaction or a reduction reaction associated with water H2O(i) (examples 1 and 2) or the reaction of a chemical species of a redox couple associated with water H2O(i), in particular dioxygen O2(aq) previously dissolved in the sweat (example 3).

[0089] The average volumetric flow rate Q of sweat circulating in a microfluidic channel 11, 1la, 11b or 1le is determined based on the principle of the so-called "time offlight" technique, that is to say from the measurement of the time required for an electroactive chemical species, detected by amperometry, to travel the inter-electrode distance L, La, Lh, Lc between the first and second working electrodes WEi and WE2, and the volume of the microfluidic channel 11, 1la, 11b, 1le delimited by the planes perpendicular to the plane in which the working electrodes WEB WE2 are set, located at the most upstream limit of each.

[0090] In the three examples that follow, the working electrodes WEi and WE2 are polarized respectively according to the methods illustrated in Figures 9 and 10 (Example 1), 11 and 12 (Example 2), and 13 and 14 (Example 3). These methods involve several steps, which are explained in more detail for each of the specific examples described below. Graphs 101, 121, and 141 represent the first electrode potential Ei applied to the first working electrode WEi as a function of time t. Graphs 102, 122, and 142 represent the second electrode potential E2 applied to the second working electrode WE2 as a function of time t. When the first and second electrode potentials Ei and E2 take the value "0" in Graphs 101, 102, and 121, Graphs 122, 141, and 142 correspond to the disconnection of the corresponding working electrode WEi or WE2 (open circuit) or to the application of a potential close to or equal to the equilibrium potential. Graphs 103, 123, and 143 represent the faradaic current measured at the first working electrode WEi as a function of time t. Finally, graphs 104, 124, and 144 represent the faradaic current measured at the second working electrode WE2 as a function of time t.

[0091] The values ​​of the electrode potentials Ei and E2 indicated by way of example are given in volts relative to the standard hydrogen electrode (V / ESH). By convention, the anodic intensity of the Faradic current takes on positive values, while the cathodic intensity of the Faradic current takes on negative values.

[0092] Example 1

[0093] In a first example, with reference to [Fig.9], the electrode potential difference between the first working electrode WEi and the counter electrode CE is fixed so that the first working electrode WEi is biased at a first electrode potential Eb typically 1.6 V / ESH, allowing the oxidization of water H2O(i) to dioxygen O2(aq) according to the redox half-equation: 6 HoOd; —> CFaq) +4 H3O+(aq) + 4 C

[0094] Correspondingly, the electrode potential difference between the second working electrode WE2 and the counter electrode CE is fixed so that the second working electrode WE2 is polarized at a second electrode potential E2, typically -0.3 V / ESH, allowing the oxygen O2(aq) produced on the surface of the working electrode WEi dissolved in sweat to be reduced to water H2O(i) according to the redox half-equation: Û2(aq) +4 H3O+(aq) + 4 6 —> 6 H2O(1)

[0095] With reference to [Fig. 10], the polarization process of the first and second working electrodes WEi and WE2 comprises two successive steps in time t.

[0096] During a first step, before a time t0 (i.e., for t < t0), the first electrode potential Ei applied to the first working electrode WEi may be close to the initial equilibrium potential, or the first working electrode WEi may be disconnected. In this latter case, illustrated in Figure 101, the first electrode potential Ei conventionally takes the value zero, "0". The detected anodic current iox is zero, as shown in Figure 103.

[0097] In parallel, the second electrode potential E2 applied to the second working electrode WE2, illustrated in Figure 102, is imposed at a value lower than the initial equilibrium potential and sufficient to reduce dissolved dioxygen O2(aq). The cathodic intensity ired is proportional to the concentration of dioxygen O2(aq) previously dissolved in the sweat, as illustrated in graph 104; if this initial concentration is zero, the cathodic intensity ired is zero.

[0098] During a second step starting from time t0, the first electrode potential Ei applied to the first working electrode WEi, illustrated in Figure 101, is fixed on the oxidation wave of water H2O(ij) so as to initiate the production of dioxygen O2(aq) in appreciable quantity, in other words so that the total concentration of dioxygen O2(aq) in the vicinity of the surface of the first working electrode WEi is much greater than the concentration of dioxygen O2(aq) previously dissolved in the sweat.The anodic current iox, a quantity representing the amount of dioxygen O2(aq) generated at the surface of the first working electrode WEi, is thus determined by the first electrode potential Ei applied to the first working electrode WEb. The anodic current iox is constant over time as soon as the capacitive current associated with the potential jump becomes zero because the redox reaction considered at the first working electrode WEi is not limited by mass transport since the reactant is water H2O(ij). The capacitive currents associated with potential switching are not represented in the diagrams, which only consider Faradic currents.

[0099] As soon as the first working electrode WEi is polarized at time t0, a gradient in the concentration of dissolved oxygen O2(aq) is created in the vicinity of the first working electrode WEi. The anodic intensity iox measured at the first working electrode WEi increases due to the oxidation of water H2O(i). In Figure 103, the increase in the anodic intensity iox is represented schematically by a step function or a Heaviside function. The oxygen concentration gradient O2(aq) forms a concentration front driven by convection downstream of the first working electrode WEi by the flow of sweat.

[0100] Simultaneously, the second working electrode WE2 remains polarized at the constant second electrode potential E2. The second working electrode WE2 continuously records the cathodic intensity ired of the faradaic current generated by the reduction of dioxygen O2(aq) to water H2OW. Thus, at time t0 + At, when the oxygen concentration front O2(aq) generated at the surface of the first working electrode WEi passes over the surface of the second working electrode WE2, the detected cathodic intensity ired decreases (in relative value) due to the reduction of oxygen O2(aq) to water H2O(ij), as shown in graph 104. The duration At corresponds to the time required for the oxygen front O2(aq) generated at the first working electrode WEi to pass to the second working electrode WE2 under the effect of the flow of sweat in the microfluidic channel 11, lia, 11b ou le at the flow velocity V.

[0101] The second step ends at a time h after time to, from which point the first working electrode WEi is again polarized to a first electrode potential Ei close to the initial equilibrium potential, or is disconnected as illustrated in Figure 101. The first working electrode WEi may be subsequently repolarized so that the process can be repeated as many times as necessary to determine the average volumetric flow rate Q at successive times more or less close together.

[0102] The method described here is simple and easily industrialized, as it has no moving parts and makes no assumptions about the hydrodynamic regime of the sweat flow in the microfluidic channel. The solution is in no way related to determining the concentration of chemical species generated or contained in the sweat, but solely to a response time rit between the amperometric signals from a pair of working electrodes WE1 and WE2. In particular, the choice of the oxidation reactions of water H2O(ij) and the reduction of dioxygen O2(aq) allows the amplitude of the amperometric signals detected at each of the working electrodes WE2 to be controlled so as to maintain a signal-to-noise ratio sufficient to allow easy detection of the variation in the amperometric signals.

[0103] Example 2

[0104] In a second example, with reference to [Fig. 11], the electrode potential difference between the first working electrode WEI and the counter electrode CE is fixed so that the first working electrode WEI is biased at a first electrode potential El, typically -0.8 V / ESH, allowing the reduction of water H2O(l) to dihydrogen H2(aq) according to the redox half-equation 2 H2O0) + 2 e —> H2(aq) + 2 OH (aq)

[0105] Correspondingly, the electrode potential difference between the second working electrode WE2 and the counter electrode CE is fixed so that the second working electrode WE2 is polarized at a second electrode potential E2 equal to the first electrode potential Eb. Like the first working electrode WEh, the second working electrode WE2 is thus configured to reduce water flOn, from sweat to dihydrogen H2(aq).

[0106] With reference to [Fig. 12], the polarization process of the first and second working electrodes WEI and WE2 comprises two successive steps in time t.

[0107] During a first step, before a time t0, i.e., for t < t0, the first electrode potential Ei applied to the first working electrode WEi may be close to the initial equilibrium potential, or the first working electrode WEi may be disconnected. In this latter case, illustrated in Figure 121, the first electrode potential Ei conventionally takes the value zero, so that the detected cathode current ired is zero, as shown in Figure 123.

[0108] Concurrently, the second electrode potential E2 applied to the second working electrode WE2, illustrated in Figure 112, is lower than the initial equilibrium potential for reducing sweat water to dihydrogen H2(aq). The cathodic current ired is constant, as illustrated in Figure 124.

[0109] During a second step beginning at time t0, the first electrode potential Ei applied to the first working electrode WEi, illustrated in Figure 111, is lower than the initial equilibrium potential so as to initiate the reduction of sweat water to dihydrogen H2(aq). The cathodic intensity ired detected at the first working electrode WEi decreases (in relative value) due to the increase in pH imposed by the reduction of water H2O(ij) at the first working electrode WEb. In Figure 123, the decrease in the growth of the cathodic intensity ired is represented schematically by a step function or a Heaviside function. The hydrolyzed portion of sweat is carried by convection downstream of the first working electrode WEi by the flow.

[0110] In parallel, the second working electrode WE2 remains polarized at the constant second electrode potential E2. The second working electrode WE2 continuously records the cathodic intensity ired of the faradaic current generated by the reduction of sweat water to dihydrogen H2(aq). Thus, at time t0 + At, when the partially hydrolyzed sweat flow passes over the surface of the second working electrode WE2, the detected cathodic intensity ired increases (in relative value), i.e., since the concentration of hydronium ions H3O+(aq) is lower than upstream of the first working electrode WEi, as shown in Figure 114.The duration At corresponds to the time required for the sweat flow front depleted into hydronium ions H3O+(aq) generated at the first working electrode WEi to pass to the second working electrode WE2 under the effect of the flow of sweat in the microfluidic channel 11, lia, 11b, 11c at the flow velocity V. .

[0111] The volume of sweat whose pH has been increased by the action of the first electrode passes from the first working electrode WEi to the second working electrode WE2 under the effect of the flow of sweat in the microfluidic channel 11, 1 la, 11b, 1 le at the flow velocity V.

[0112] The second step ends at a time tj later than time t0, from which the first working electrode WEi is again polarized to a first electrode potential Ei close to the initial equilibrium potential, or is disconnected as illustrated in Figure 121. The first working electrode WEi can subsequently be repolarized so that the process can be repeated as many times as necessary to determine the average volumetric flow rate Q at closely spaced successive times.

[0113] The method described here is simple and easily industrializable, as it has no moving parts and makes no assumptions about the hydrodynamic regime. The solution is in no way related to determining the concentration of chemical species generated or contained in sweat, but solely to a response time At between the variations of the amperometric signals of the working electrode pair WE1 and WE2.

[0114] Example 3

[0115] In a third example, with reference to [Fig. 13], the electrode potential difference between the first working electrode WEi and the counter electrode CE is fixed so that the first working electrode WEi is biased at a first electrode potential EB typically -0.3 V / ESH, allowing only the oxygen O2(aq) initially dissolved in the aqueous solution under examination, when it contains it, to be reduced to water H2O(ij) according to the redox half-equation O2(aq) +4 H3O+(aq) + 4 e- —> 6 H2O(i)

[0116] Correspondingly, the electrode potential difference between the second working electrode WE2 and the counter electrode CE is fixed so that the second working electrode WE2 is polarized at a second electrode potential E2 equal to the first electrode potential Eb. Like the first working electrode WEh, the second working electrode WE2 is thus configured to reduce the fraction of dissolved oxygen O2(aq) in sweat to water H2O(i) which has not been reduced at the first working electrode WEi.

[0117] With reference to [Fig. 14], the polarization process of the first and second working electrodes WEI and WE2 comprises two successive steps in time t.

[0118] During a first step, before an instant t0, i.e., for t < t0, the first electrode potential Ei applied to the first working electrode WEi may be close to the initial equilibrium potential, or the first working electrode WEi may be disconnected. In the latter case, illustrated in Figure 141, the first electrode potential Ei conventionally takes the value zero, so that the detected cathode current ired is zero, as shown in Figure 143.

[0119] In parallel, the second electrode potential E2 applied to the second working electrode WE2, illustrated in Figure 142, is lower than the initial equilibrium potential for reducing dioxygen O2(aq) to water fLOn. The cathodic current ired is constant since it is proportional to the concentration of dioxygen O2(aq) previously dissolved in sweat, as illustrated in Figure 144.

[0120] During a second step beginning at time t0, the first electrode potential Ei applied to the first working electrode WEB, illustrated in Figure 141, is lower than the initial equilibrium potential so as to initiate the reduction of all or part of the dissolved oxygen O2(aq) in the sweat. The cathodic intensity ired detected at the first working electrode WEi decreases (in relative value) due to the reduction of oxygen O2(aq) to water H2O(i). In Figure 143, the decrease in the growth of the cathodic intensity ired is represented schematically by a step function or a Heaviside function. The hydrolyzed portion of the sweat, depleted of dissolved oxygen O2(aq), is carried by convection downstream of the first working electrode WEi by the flow.

[0121] In parallel, the second working electrode WE2 remains polarized at the constant second electrode potential E2. The second working electrode WE2 continuously records the cathodic intensity ired of the faradaic current generated by the reduction of dioxygen O2(aq) to water H2OW. Thus, at time t0 + At, when the flow of sweat depleted into dioxygen O2(aq) passes over the surface of the second working electrode WE2, the detected cathodic intensity ired increases (in relative value), i.e., it approaches zero, since the concentration of dioxygen O2(aq) dissolved in the sweat is zero or, at the very least, lower than upstream of the first working electrode WEB, as shown in Figure 144.The duration At corresponds to the time required for the flow of sweat depleted of dissolved dioxygen O2(aq) to pass from the first working electrode WEi to the second working electrode WE2 under the effect of the flow of sweat in the microfluidic channel 11, 1 la, 11b, 1 le at the flow velocity V. .

[0122] The second step ends at a time tj after time to, from which the first working electrode WEi is again polarized to a first electrode potential Ei close to the initial equilibrium potential, or is disconnected as illustrated in Figure 141. The first working electrode WEi can subsequently be repolarized so that the process can be repeated as many times as necessary to determine the average volumetric flow rate Q at closely spaced successive times.

[0123] The method described here is simple and easily industrializable, as it has no moving parts and makes no assumptions about the hydrodynamic regime. The solution is in no way related to determining the concentration of chemical species generated or contained in sweat, but solely to a response time At between the variations of the amperometric signals of the working electrode pair WE1 and WE2.

[0124] In the three examples described above, the flow velocity V and the volumetric flow rate Q of sweat flowing in a rectangular parallelepiped-shaped microfluidic channel 11, 11a, 11b, or 11c with a constant cross-sectional area S in the flow direction 13 can be determined from the inter-electrode distance L, 1a, 11h, 11c separating the working electrodes WEi and WE2, and from the characteristic time At of the response delay of the second working electrode WE2, monitored by chronoamperometry, relative to the instantaneous response of the first working electrode WEh

[0125] Subject to the reservations that will be expressed below, the average linear flow velocity V and the average volumetric flow rate Q of the sweat flow 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: V=L / At Q = SxV = LxS / At Subject to the same reservations, these equations are also valid, respectively in the microfluidic channels lia, 11b, 11c by substituting the inter-electrode distance L, by the inter-electrode distance La, Lh, Lc.

[0126] In the context of the envisaged dynamic applications, for the temporal monitoring of the subject's physiological state, it is desirable that the device 1, in order to determine the quantitative parameter of perspiration of a subject, measure the value of the volumetric flow rate Q of perspiration at successive closely spaced instants consistent with the expected perspiration rate, for example, once per minute. The integration of the temporal variations of the volumetric flow rate Q of perspiration then makes it possible to determine the value of the total perspiration flux of the subject over a given time range t.

[0127] The quantitative sweating parameter can be a sweating rate determined from the total volume of sweat perspired by the subject over a given time range t, related to the surface area of ​​the investigation zone 8.

[0128] The inter-electrode distance L, La, Lh, Lc separating the working electrodes WEi and WE2 is chosen small enough so that changes in the physiological response of the subject are negligible during the duration At and large enough to allow a decoupled operating regime of the working electrodes WEi and WE2 in the or each microfluidic channel 11, lia, 11b, 1 le where the volumetric flow measurement Q is carried out.

[0129] Indeed, depending on the average linear flow velocity V of the sweat in the microfluidic channel 11, 1la, 11b or 1le, the concentration gradient created in the vicinity of the first working electrode, by generation of electroactive chemical species (example 1) or by depletion of electroactive chemical species already present in the sweat (examples 2 and 3), may or may not become homogeneous along the height of the microfluidic channel 11, 1lia, 11b or 11c after being carried over the inter-electrode distance L, La, Lh, Lc. In particular, when, given the linear flow velocity V of the sweat, the concentration gradient does not have time to dissipate along the height of the microfluidic channel 1la, 11b or 1le before reaching the second working electrode WE2, the operation of the two working electrodes WE1 and WE2 is linked.This coupling regime limits the temporal resolution of amperometric signals, which disrupts the volumetric flow rate measurements Q of sweat flowing in the microfluidic channel 1la, 11b or 1le. This pitfall is easily avoided by adjusting the relative values ​​of the inter-electrode distance L, La, Lh, Lc and the time interval h - to to the expected values ​​of the average linear flow velocity V.

[0130] According to a first embodiment, illustrated in [Fig. 6], pairs of first and second working electrodes WE1 and WE2 separated by inter-distances Different electrodes La, Lb, Lc can be used in separate parallel microfluidic channels 1a, 11b, 11c. Preferably, the inter-electrode distance La, Lh, Lc separating the working electrodes WEi and WE2 differs depending on the microfluidic channel 1a, 11b or 1le considered, for example such as La <Lh< Lc.

[0131] Alternatively, according to a second embodiment illustrated in [Fig. 8], a network of second working electrodes, here a first second working electrode WE2(1) and a second second working electrode WE2(2), can be implemented in the same microfluidic channel 11, 11a, 11b, 11c. According to embodiments not shown, the network of second working electrodes can comprise more than two second working electrodes. In the following, we will limit ourselves to describing the network of second working electrodes implemented in the referenced microfluidic channel 11. Such a network could also be implemented in the microfluidic channels 11a, 11b, 11c.

[0132] In the microfluidic channel 11, each second working electrode WE2(1), WE2(2) is respectively arranged at a different inter-electrode distance L(1>, L<2) from the first working electrode WEi. The working electrodes WEi, WE2(1), WE2(2) are electronically switchable. The duration h - to is electronically adjusted by feedback of the average linear flow velocity value V measured at previous measurement times.

[0133] The volumetric flow rate Q can thus be determined over a wide range of values, since the volumetric flow rate Q measurement can be carried out in each of the microfluidic channels 11, 1 la, 11b and 1 le or in several of them, retaining only the volumetric flow rate Q measurements consistent with the inter-electrode distances La, Lh, L„ L^\ LP\ Advantageously, the inter-electrode distances La, Lh and Lc,Lm, LP2* are on the order of the millimeter.

[0134] The methods for measuring the volumetric flow rate Q of sweat described above can be implemented in an automated manner using an electronic processing device 16, preferably integrated into the device 1.

[0135] With reference to [Fig. 15], an embodiment of the electronic processing device 16 that can be integrated into the device 1, for example in the form of an electronic card 17, is now described.

[0136] In the embodiment with a plurality of microfluidic channels 1a, 11b, 1le shown in Figures 5 and 6, the electrochemical cells 14a, 14b, 14c are connected to an analog-to-digital converter 18, which itself feeds a processor 19. The processor 19 is, for example, programmed to implement the sweat volume flow rate measurement methods described above.

[0137] In the embodiment shown in [Fig. 8], where a first working electrode WEi and several second working electrodes WE2(1), WE2(2) located respectively at different inter-electrode distances, each pair consisting of the first working electrode WEi and one of the second working electrodes WE2(1), WE2(2) forms an electrochemical cell 14 connected to an analog-to-digital converter 18, which itself feeds a processor 19. The processor 19 is programmed for example to implement the sweat volume flow rate measurement processes Q described above.

[0138] A power source 20, for example a battery, powers the electronic processing device 16. A communication module 21, wired or wireless, may also be provided to communicate the results of the sweat volumetric flow rate measurements Q to a storage or post-processing device.

[0139] The electronic processing device 16 may include other functional modules, for example a gyroscopic and / or accelerometric module to detect the orientation and movements of the subject 2, as well as to quantify its level of activity, and / or a temperature sensor to measure the temperature of the epidermis of the subject 2. Indeed, it is useful to know the skin temperature because of the correlations between temperature and the rate of perspiration.

[0140] Certain elements of the apparatus 1, in particular the electronic processing device 16, can be implemented in various forms, either individually or in a distributed manner, using hardware and / or software components. Usable hardware components 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.

[0141] Although the invention has been described in connection with several particular embodiments, it is clearly evident that it is by no means limited to them and that it includes all technical equivalents of the means described as well as their combinations if these fall within the scope of the invention.

[0142] The use of the verb "comprise", "comprendre" or "include" and its conjugated forms does not exclude the presence of other elements or steps than those stated in a claim.

[0143] In the claims, any reference sign in parentheses shall not be interpreted as a limitation of the claim.

Claims

Demands

1. Microfluidic electrochemical device (100) for measuring a volumetric flow rate (Q) of a fluid, the fluid comprising a solvent, the microfluidic electrochemical device (100) comprising: - at least one microfluidic channel (11, lia, 11b, 1 le) configured to allow the fluid to flow in a flow direction (13); - at least one electrochemical cell (14, 14a, 14b, 14c) disposed in at least one microfluidic channel (11, 1a, 11b, 1a), the electrochemical cell (14, 14a, 14b, 14c) comprising a first working electrode (WEi) and at least one second working electrode (WE2, WE2(1), WE2(2)), said at least one second working electrode (WE2, WE2(1), WE2(2)) being spaced from the first working electrode (WEi) by an inter-electrode distance (La, Lh, L„ L(1\ U2^ in the direction of flow (13), at least one counter electrode (CE) and at least one reference electrode (REF);and - an electrochemical amperometric 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), such that each of said first and second working electrodes produces an amperometric signal by oxidation or reduction reaction of the solvent or with at least one chemical species forming a redox couple with the solvent; the electrochemical amperometric measurement system (15) being configured to determine the volumetric flow rate (Q) of the fluid in the microfluidic channel (11, 11a, 11b, 11e) from the interelectrode distance (1a, Lh, Lc, L(1), U2^) and a time delay (At) between a change in the amperometric signal produced by the first working electrode (WE1) and a change in the amperometric signal produced by the second working electrode (WE2, WE2(1), WE2(2)).;

2. Microfluidic electrochemical device (100) according to claim 1, wherein the solvent is water H2O.

3. Microfluidic electrochemical device (100) according to claim 2, wherein the fluid is sweat from a human or animal subject.

4. Microfluidic electrochemical device (100) according to claim 2 or 3, wherein the first electrode potential (Ei) allows the oxidation of water H2O into dioxygen O2 and the second electrode potential E2 allows the reduction of dioxygen O2 dissolved in water H2O produced into water H2O.

5. Microfluidic electrochemical device (100) according to claim 2 or 3, wherein the first electrode potential (Ei) enables the reduction of water H2O to dihydrogen H2 and the second electrode potential (E2) enables the reduction of water H2O to dihydrogen H2.

6. Microfluidic electrochemical device (100) according to claim 2 or 3, wherein the first electrode potential (Ei) enables the reduction of dissolved dioxygen O2 in water H2O to water H2O and the second electrode potential (E2) enables the reduction of dissolved dioxygen O2 in water H2O to water H2O.

7. Microfluidic electrochemical device (100) according to any one of claims 1 to 6, wherein the electrochemical amperometric measurement system (15) is also configured to: - in a first step, bias the first working electrode (WEi) at the first electrode potential (Ei) and the second working electrode (WE2) at the second electrode potential (E2); - in a second step, disconnect the first working electrode (WEi) or fix the first electrode potential (Ei) at a potential close to or equal to a zero current equilibrium potential.

8. Microfluidic electrochemical device (100) according to any one of claims 1 to 7, further comprising an insulating support (4), said at least one microfluidic channel (11, 1a, 11b, 1a) being formed in the insulating support (4), the first working electrode (WEi) and said at least one second working electrode (WE2, WE2(1), WE2(2)) being formed by metallic deposits of platinum or platinum black on said insulating support (6).

9. Microfluidic electrochemical device (100) according to any one of claims 1 to 8, wherein the counter electrode (CE) is positioned downstream of the working electrodes (WEB WE2, WE2(1), WE2(2)) in the direction of flow (13), and wherein the reference electrode (REF) is positioned upstream of said working electrodes (WEB WE2, WE2(1), WE2(2)) in said direction of flow (13).

10. Microfluidic electrochemical device (100) according to any one of claims 1 to 9, comprising a first and a second microfluidic channel (1a, 11b), the first, respectively, the second, electrochemical cell (14a, 14b) being disposed in the first, respec- tivement, the second, microfluidic channel (lia, 11b), the inter-electrode distance (La) of the first electrochemical cell (14a) being different from the inter-electrode distance (L^ of the second electrochemical cell (14b).

11. Microfluidic electrochemical device (3) according to any one of claims 1 to 9, wherein said at least one electrochemical cell (14a) comprises two second working electrodes (WE2(1), WE2(2)) respectively separated from the first working electrode (WEi) by a first inter-electrode distance (U1^) and by a second inter-electrode distance (U2^), the first inter-electrode distance (LJ1^) being different from the second inter-electrode distance (U2^).

12. Microfluidic electrochemical device (100) according to any one of claims 1 to 11, wherein the electrochemical amperometric measurement system (15) is configured to determine the volumetric flow rate (Q) as a function of a cross-sectional area (S) of said microfluidic channel (11, 11a, 11b, 11c) according to the direction of flow (13).

13. Apparatus (1) intended to be placed on an investigation area (8) of the epidermis of a human or animal subject for measuring a quantitative parameter of sweating of the subject, said apparatus (1) comprising: - a structure defining a microfluidic electrochemical device (100) according to any one of claims 1 to 12, the structure comprising an inlet orifice (6) defining the investigation area (8) and allowing sweat to pass from the epidermis, at least one microfluidic channel (11, 11a, 11b, 11c) of the microfluidic electrochemical device (100) being in communication with the inlet orifice (6); and - an electronic processing device (16) configured to determine the quantitative parameter of sweating of said human or animal subject from measurements of the volumetric flow rate (Q) of sweat made by the microfluidic electrochemical device (100).

14. Apparatus (1) according to claim 13, wherein the quantitative sweating parameter of said human or animal subject is a sweating rate.

15. Apparatus (1) according to claim 13 or 14, wherein the structure is a multilayer structure comprising a lower layer (3) and at least one layer superimposed on the lower layer (3), the microfluidic electrochemical device (100) extending parallel to the lower layer (3), the lower layer (3) comprising said inlet orifice (6).

16. Apparatus (1) according to claim 15, wherein the multi-layer structure further comprises an upper layer (10) and at least one intermediate layer (4) situated between the lower layer (3) and the upper layer (10), the micro-fluidic electrochemical device (100) being formed in the thickness of at least one intermediate layer (6).

17. Apparatus (1) according to claim 16, wherein the upper layer (10) has an outlet orifice (22) passing through the upper layer (10), and wherein at least one microfluidic channel (11, 1la, 11b, 1le) is in communication with the outlet orifice (22).

18. Apparatus (1) according to any one of claims 16 or 17, wherein the first working electrode (WEi), at least one second working electrode (WE2, WE2(1), WE2(2)), said at least one second working electrode (WE2, WE2(1), WE2(2)), at least one counter electrode (CE) and at least one reference electrode (REF) are disposed on an inner face of the upper layer (10) closing at least one microfluidic channel (11, 1a, 11b, 1a) from above and / or on an upper face of the lower layer (3) closing said at least one microfluidic channel (11, 1a, 11b, 1a) from below.

19. Apparatus (1) according to any one of claims 13 to 18, further comprising a communication device (21) configured to transmit one or more measurement signals produced by the microfluidic electrochemical device (100).

20. Apparatus (1) according to any one of claims 13 to 19, further comprising a gyroscopic module and / or at least one accelerometer for detecting an activity state of said human or animal subject.

21. Apparatus (1) according to any one of claims 13 to 20, further comprising a temperature sensor configured to measure the temperature of the epidermis (2) of said human or animal subject.

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