MICROFLUIDIC ELECTROCHEMICAL DEVICE FOR MEASURING A VOLUME FLOW

Description

Domaine technique

[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. Arrière-plan technologique

[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 measurement devices are known in the state of the art in situ Micro-flow measurement devices, such as thermal flux sensors or Coriolis effect micro-flowmeters, are being developed. In practice, these devices are expensive, have unresolved reliability issues, and are generally housed in bulky enclosures. More specifically, microfluidic devices have been developed to measure the micro-flow rates of sweat perspired by a human or animal subject, circulating through 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, which can be caused by pathologies that may 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] Existing microfluidic devices offer the advantage of easily collecting sweat in microfluidic channels without evaporation, with very 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 is filled 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 FR3103901A1 or WO 2021 / 105122 A1 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. Résumé

[0006] An idea underlying the invention is to provide a microfluidic electrochemical device to measure the volumetric flow rate of a fluid in a microfluidic channel, without necessarily performing 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 that adheres to the skin to determine a quantitative parameter of a subject's perspiration from the measurement in situ and continuously the volumetric flow rate of sweat flowing in a microfluidic channel.

[0008] One aim of the invention is to provide such a device which also has the advantages of being compact, simple in 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 the at least one microfluidic channel, the electrochemical cell comprising a first working electrode and at least one 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 bias 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 many microsystems for measuring 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 ( lab - on-a-chip ) or total analysis microsystems ( micro-Total Analysis System or µTAS).

[0011] The microfluidic electrochemical device is simple to manufacture and easily industrialized, 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 linked 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 H2O (1).

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

[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(1) into dioxygen O2(aq) and the second electrode potential allows the reduction of dioxygen O2(aq) dissolved in the water H2O(1) produced into water H2O(1).

[0017] The choice of the oxidation reactions of water H2O (1) at the first working electrode and of the reduction of dioxygen O2(aq) at the second working electrode, thanks to 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 the said 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 the variations of the amperometric signals.

[0018] According to one embodiment, the first electrode potential allows the reduction of water H2O(1) to dihydrogen H2(aq) and the second electrode potential allows the reduction of water H2O(1) to dihydrogen H2(aq).

[0019] According to one embodiment, the first electrode potential allows the reduction of dioxygen O2(aq) dissolved in water H2O(1) to water H2O(1) and the second electrode potential allows the reduction of dioxygen O2(aq) dissolved in water H2O(1) to water H2O(1).

[0020] According to one embodiment, the electrochemical amperometric measurement system is also configured to: In a first step, polarize the first working electrode at the first electrode potential and the second working electrode at 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] In some embodiments, the microfluidic electrochemical device may include 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 may 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 working electrode pair is chosen to be small enough that changes in the subject's physiological response are negligible during the time lag between changes in 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 at the first working electrode remains inhomogeneous after reaching the second working electrode. This coupling mode limits the temporal resolution of the amperometric signals, thus degrading the accuracy of 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 having 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 sweating parameter of said human or animal subject from measurements of the volumetric flow rate of sweat made by the microfluidic electrochemical device.

[0033] The quantitative parameter of sweating can be a sweating rate determined from the total volume of sweat perspired 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 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 manufacturing, assembly and therefore the industrialization of the device is facilitated.

[0041] Thanks to these features, the device adapts 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. This ensures the device's watertight seal.

[0042] According to one embodiment, the upper layer has an outlet orifice 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 containing these electrodes is simplified because the electrodes can be fabricated on a flat layer while 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 features, 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 volumetric sweat rate and / or quantitative sweating parameters 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 cause.

[0051] Other applications are possible in various technological or environmental fields where flow measurement is required in a device or process involving an electroactive fluid or containing one or more electroactive species. Brève description des 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. [ Fig.1 ] There [ Fig.1 ] is a schematic view of a subject seen from behind, on which a device has been placed according to a particular embodiment. Fig.2 ] There [ Fig.2 ] is a perspective view partially representing a multilayer structure for a device according to one embodiment. Fig.3 ] There [ Fig.3 ] is a cross-sectional view along line III-III of the [ Fig.2 ]. Fig.4 ] There [ Fig.4 ] is an exploded view of the multilayer structure according to one embodiment. Fig.5 ] There [ Fig.5 ] is a partial functional schematic representation of a multilayer structure defining an electrochemical device in an apparatus. Fig.6 ] There [ Fig.6 ] is a partial functional schematic representation of a microfluidic electrochemical device that can be used in an apparatus. Fig.7 ] There [ Fig.7 [ ] is a schematic top view of an electrochemical cell according to a first embodiment. ] Fig.8 ] There [ Fig.8 ] is a schematic view analogous to that of the [ Fig.7 ], according to a second embodiment. [ Fig.9 ] There [ Fig.9 ] is a functional schematic cross-sectional representation of an electrochemical cell along a microfluidic channel according to a first embodiment. Fig.10 ] There [ 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 ]. Fig.11 ] There [ Fig.11 ] is a functional schematic representation analogous to that of the [ Fig.9 ] according to a second embodiment. [ Fig.12 ] There [ Fig.12 ] is a set of chronoamperograms analogous to those of the [ Fig.10 ] according to the second embodiment. [ Fig.13 ] There [ Fig.13 ] is a functional schematic representation analogous to those of figures 9 And 11 , according to a third embodiment. [ Fig.14 ] There [ Fig.14 ] is a set of chronoamperograms analogous to those of figures 10 And12 , according to a third embodiment. [ Fig.15 ] There [ Fig.15 ] is the functional schematic representation of an electronic control device that can be implemented with the device. Description des modes de réalisation

[0053] 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 measuring in situ The average flow velocity of an electroactive fluid in a microfluidic channel. These microsystems can be, for example, lab-on-a-chip microfluidic platforms. (lab-on-a-chip) or total analysis microsystems ( micro-Total Analysis System or µTAS).

[0054] With reference to the [ Fig.1 [ ], Device 1 for determining a quantitative parameter of perspiration is placed on the skin 2 of a human subject, for example on their back. In a variant not shown, Device 1 can be placed on the skin of an animal subject.

[0055] With reference to the [ Fig.2 [ ], the device 1 is presented, for example, in the form of a compact, multi-layered structure made of waterproof materials, for example, a polymer material. The multi-layered 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 subject's skin 2, and an insulating support 4 superimposed on the lower layer 3.

[0056] 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.

[0057] With reference to the [ 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 a circular opening 6, defining an investigation area 8 on the subject's skin 2, which may be, for example, a few millimeters to a few centimeters in diameter. The circular opening 6 can take another shape, such as 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.

[0058] 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.

[0059] According to a first embodiment, with reference to the [ Fig.4 ], the multilayer structure of the device 1 comprises a lower layer 3 including an inlet orifice 6 allowing sweat to pass through, 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.

[0060] The microfluidic electrochemical device 100 comprises a microfluidic channel 11 which is connected to the inlet port 6 at one end and to the outlet port 22 at the other end. Thus, the sweat flow 9 from the subject's skin 2 is brought into the microfluidic channel 11, which guides the sweat from the inlet port 6 to the outlet port 22 by capillary action.

[0061] The microfluidic channel 11 is equipped with an electrochemical cell 14 described below, represented in [ Fig.7 ] or in [ 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.

[0062] In terms of dimensions, 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 µm and 1000 µm, the intermediate layer 4 has a thickness between 10 µm and 500 µm, 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.

[0063] 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 µm, the intermediate layer 4 has a thickness of 150 µm, the layers 3, 4, 10 of the multilayer structure have a width of 3 cm and a length of 9 cm.

[0064] According to a second embodiment, with reference to figures 5 And 6The 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 11a, 11b, 11c, parallel to each other, formed within the thickness of the insulating 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 flow independently. The number of microfluidic channels may be higher or lower than that shown in the diagrams. figures 5 And 6 .

[0065] In terms of design, the microfluidic channels 11a, 11b, 11c 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 11a, 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 11a, 11b, 11c are assumed to have the same area S, i.e., Sa = Sb = Sc = S.

[0066] 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 circulate independently in the microfluidic channels 11a, 11b, 11c of the microfluidic electrochemical device 100. The arrows 13 illustrate the direction of sweat flow in the microfluidic channels 11a, 11b, 11c. Each microfluidic channel 11a, 11b, 11c is respectively equipped with an electrochemical cell 14a, 14b, 14c indicated in [ Fig.6 ] and detailed in [ Fig.7 ] or in [ Fig.8 Preferably, the microfluidic channels 11a, 11b, 11c terminate in an outlet reservoir (not shown) in communication with the outlet port 22. The outlet reservoir retains the sweat to prevent it from coming back into contact with the skin 2.

[0067] With reference to the [ 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, 11c. The electrodes WE1, WE2, CE, and REF are fabricated as parallel microelectrode strips, perpendicular to the direction of sweat flow in the microfluidic channel 11, 11a, 11b, 11c, and implanted by microfabrication, for example, by chemical vapor deposition (C₂O₅). hemical Vapor Deposition or CVD) and / or by lithography. The length of the microelectrode strips thus corresponds to the width of the microfluidic channel 11, 11a, 11b, 11c.

[0068] The WE 1 and WE 2 working electrodes can be made from microstrips of platinum or platinum-plated platinum—also called platinum black—with a thickness of nanometers, for example, on the order of a few tens of nanometers to a hundred nanometers, typically 200 nm. The WE 1 and WE 2 working electrodes are spaced at an inter-electrode distance L, L a , L b , L c in the direction of sweat flow in the microfluidic channel 11, 11a, 11b, 11c. As will be explained later, the inter-electrode distance L, L a , L b , L c varies depending on the microfluidic channel 11, 11a, 11b, 11c considered. The first working electrode will be designated by the reference WE 1 and the second working electrode by the reference WE 2. By convention, the first working electrode WE 1 is located upstream of the second working electrode WE 2 with respect to the direction of sweat flow from subject 2 in the microfluidic channel 11, 11a, 11b, 11c.

[0069] The reference electrode REF, made for example in the form of a nanometrically thick Ag / AgCl reference micro-electrode strip, for example 500 nm, is located upstream of the working electrode pair WE 1 and WE 2 in order to preserve the stability of the reference electrode potential over time.

[0070] The counter electrode (CE), made for example in the form of a microstrip of platinum, platinized or not, with a nanometer 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 working electrode pair WE1 and WE2 and, consequently, of the reference electrode REF, so that the chemical species generated on its surface do not interfere with either the working electrodes WE1 and WE2 or the reference electrode REF. Advantageously, the surface area of ​​the counter electrode CE is two to three times larger than those of the other electrodes.

[0071] Advantageously, the microelectrodes made in the form of microstrips are all deposited on a sub-nanometric bonding 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.

[0072] The microfluidic electrochemical device 100 also includes an electrochemical amperometric measurement system 15. Each of the electrodes WE 1 , WE 2 , CE and REF is connected to the electrochemical amperometric measurement system 15 via electrical contacts (not shown) electrically isolated from the subject's sweat.

[0073] 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 WE1 and WE2 of an electrochemical cell 14, 14a, 14b, 14c respectively at the first and second electrode potentials E1 and E2 so as to generate in the sweat an oxidation reaction or a reduction reaction associated with water H2O (1) (examples 1 and 2) or the reaction of a chemical species of a redox couple associated with water H2O (1), in particular dioxygen O2(aq) previously dissolved in the sweat (example 3).

[0074] The average volumetric flow rate Q of sweat circulating in a microfluidic channel 11, 11a, 11b or 11c is determined based on the principle of the so-called "time of flight" technique (in English: " time of flight "), that is, from the measurement of the time required for an electroactive chemical species, detected by amperometry, to travel the inter-electrode distance L, L a , L b , L c included between the first and second working electrodes WE 1 and WE 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 1, WE 2 are set, located at the most upstream limit of each.

[0075] In the three examples that follow, the working electrodes WE 1 and WE 2 are respectively polarized according to the processes illustrated in figures 9 et 10 (Example 1), 11 and 12 (Example 2), and 13 and 14 (Example 3). These processes 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 E1 applied to the first working electrode WE1 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 E1 and E2 reach the value "0" on graphs 101, 102, 121, 122, 141, and 142, this corresponds to the disconnection of the corresponding working electrode WE1 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 WE1 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.

[0076] The values ​​of the electrode potentials E1 and E2 shown as examples are given in volts relative to the standard hydrogen electrode (V / ESH). By convention, the anodic intensity of the faradaic current takes on positive values, while the cathodic intensity of the faradaic current takes on negative values. Exemple 1

[0077] In a first example, with reference to the [ Fig.9 The potential difference between the first working electrode WE1 and the counter electrode CE is fixed so that the first working electrode WE1 is biased at a first electrode potential E1, typically 1.6 V / ESH, allowing the oxidation of water H2O(1) to dioxygen O2(aq) according to the half-reaction: 6 H2O(1) → O2(aq) + 4 H3O+(aq) + 4 e-

[0078] Correspondingly, the electrode potential difference between the second working electrode WE 2 and the counter electrode CE is fixed so that the second working electrode WE 2 is biased at a second electrode potential E 2, typically -0.3 V / ESH, allowing the oxygen O 2(aq) produced at the surface of the working electrode WE 1 dissolved in sweat to be reduced to water H 2 O (1) according to the redox half-equation: O 2(aq) +4 H 3 O +< (aq) + 4 e → 6 H 2 O (1)

[0079] With reference to the [ Fig.10 ], the polarization process of the first and second working electrodes WE 1 and WE 2 comprises two successive steps in time t.

[0080] During a first stage, before a moment t 0 (i.e. for t < t 0 The first electrode potential E1 applied to the first working electrode WE1 may be close to the initial equilibrium potential, or the first working electrode WE1 may be disconnected. In the latter case, illustrated in Figure 101, the first electrode potential E1 conventionally takes the value zero, "0". The detected anodic current iox is zero, as shown in Figure 103.

[0081] Simultaneously, the second electrode potential E2 applied to the second working electrode WE2, illustrated in Figure 102, is set to a value lower than the initial equilibrium potential but sufficient to reduce the dissolved oxygen O2(aq). The cathodic intensity ired is proportional to the concentration of oxygen O2(aq) previously dissolved in the sweat, as illustrated in Figure 104; if this initial concentration is zero, the cathodic intensity ired is zero.

[0082] During a second stage beginning from the moment t 0 ,The first electrode potential E1 applied to the first working electrode WE1, illustrated in Figure 101, is fixed to the oxidation wave of water H2O (1) so as to initiate the production of dioxygen O2(aq) in appreciable quantities; in other words, so that the total concentration of dioxygen O2(aq) in the vicinity of the surface of the first working electrode WE1 is much greater than the concentration of dioxygen O2(aq) previously dissolved in sweat. The anodic current iox, a quantity representative of the amount of dioxygen O2(aq) generated at the surface of the first working electrode WE1, is thus fixed by the first electrode potential E1 applied to the first working electrode WE1.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 WE1 is not limited by mass transport since the reactant is water H2O (1). The capacitive currents associated with potential switching are not shown in the diagrams, which only consider Faradic currents.

[0083] As soon as the first working electrode WE1 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 WE1. The anodic current iox measured at the first working electrode WE1 increases due to the oxidation of water H2O (1). In Figure 103, the increase in the anodic current 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 WE1 by the flow of sweat.

[0084] 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 H2O (1). Thus, at time t 0 + Δt, When the oxygen concentration front O2(aq) generated at the surface of the first working electrode WE1 passes over the surface of the second working electrode WE2, the detected cathodic intensity i_red decreases (in relative value) due to the reduction of oxygen O2(aq) to water H2O (1), as shown in graph 104. The duration Δt corresponds to the time required for the oxygen front O 2(aq) generated at the first working electrode WE 1 to pass to the second working electrode WE 2 under the effect of the flow of sweat in the microfluidic channel 11, 11a, 11b or 11c at the flow velocity V.

[0085] The second stage ends at a moment t 1 subsequent to the moment t 0 ,from which the first working electrode WE 1 is again polarized to a first electrode potential E 1 close to the initial equilibrium potential, or is disconnected as illustrated in Figure 101. The first working electrode WE 1 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 successive times more or less close together.

[0086] The method described here is simple and easily industrialized, as it involves no moving parts and requires no assumptions about the hydrodynamic regime of sweat flow in the microfluidic channel. The solution is not related to determining the concentration of chemical species generated or contained in sweat, but solely to a response time. Δt between the amperometric signals a pair of working electrodes WE 1 and WE 2. In particular, the choice of the oxidation reactions of water H 2 O (1) and of reduction of dioxygen O 2(aq) makes it possible to control the amplitude of the amperometric signals detected at each of the working electrodes WE 1 , WE 2 so as to maintain a signal-to-noise ratio sufficient to allow easy detection of the variation of the amperometric signals. Exemple 2

[0087] In a second example, with reference to the [ Fig.1 ]1, the electrode potential difference between the first working electrode WE 1 and the counter electrode CE is fixed so that the first working electrode WE 1 is biased at a first electrode potential E 1, typically -0.8 V / ESH, allowing the reduction of water H 2 O (1) to dihydrogen H 2(aq) according to the redox half-equation 2 H 2 O (1) + 2 e → H 2(aq) + 2 OH -(aq)

[0088] Correspondingly, the electrode potential difference between the second working electrode WE 2 and the counter electrode CE is fixed so that the second working electrode WE 2 is biased at a second electrode potential E 2 equal to the first electrode potential E 1. Like the first working electrode WE 1, the second working electrode WE 2 is thus configured to reduce the water H 2 O (1) of sweat to dihydrogen H 2(aq) .

[0089] With reference to the [ Fig.12 ], the polarization process of the first and second working electrodes WE 1 and WE2 comprises two successive steps in time t.

[0090] During a first stage, before a moment t 0 , i.e. for t < t 0 , The first electrode potential E1 applied to the first working electrode WE1 may be close to the initial equilibrium potential, or the first working electrode WE1 may be disconnected. In the latter case, illustrated in Figure 121, the first electrode potential E1 conventionally takes the value zero, "0", so that the detected cathode current ired is zero, as shown in Figure 123.

[0091] 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 intensity ired is constant, as illustrated in Figure 124.

[0092] During a second stage beginning from the moment t 0 ,The first electrode potential E1 applied to the first working electrode WE1, illustrated in Figure 111, is lower than the initial equilibrium potential in order to initiate the reduction of sweat water to dihydrogen H2(aq). The cathodic intensity ired detected at the first working electrode WE1 decreases (in relative value) due to the increase in pH imposed by the reduction of water H2O(1) at the first working electrode WE1. In Figure 123, the decrease in the growth of the cathodic intensity ired is represented by a step function or a Heaviside function. The hydrolyzed portion of sweat is carried by convection downstream of the first working electrode WE1 by the flow.

[0093] 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 sweat water to dihydrogen H2(aq). Thus, at time t 0 + Δt, When the partially hydrolyzed sweat flow passes over the surface of the second working electrode WE2, the detected cathodic intensity i_red increases (in relative value), i.e., since the concentration of hydronium ions H3O+(aq) is lower than upstream of the first working electrode WE1, as shown in graph 114. The duration Δt corresponds to the time required for the hydronium ion-depleted sweat flow front H3O+<(aq) generated at the first working electrode WE1 to pass to the second working electrode WE2 under the effect of the sweat flow in the microfluidic channel 11, 11a, 11b, 11c at the flow velocity V.

[0094] The volume of sweat whose pH has been increased by the action of the first electrode passes from the first working electrode WE 1 to the second working electrode WE 2 under the effect of the flow of sweat in the microfluidic channel 11, 11a, 11b, 11c at the flow velocity V.

[0095] The second stage ends at a moment t 1 subsequent to the moment t 0 ,from which the first working electrode WE 1 is again polarized to a first electrode potential E 1 close to the initial equilibrium potential, or is disconnected as illustrated in Figure 121. The first working electrode WE 1 can subsequently be repolarized so that the process can be repeated as many times as necessary to determine the mean volumetric flow rate Q at closely spaced successive times.

[0096] The method described here is simple and easily industrialized, 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. Δt between the variations of the amperometric signals of the working electrode pair WE 1 and WE 2. Exemple 3

[0097] In a third example, with reference to the [ Fig.13 ], the electrode potential difference between the first working electrode WE 1 and the counter electrode CE is fixed so that the first working electrode WE 1 is biased at a first electrode potential E 1, typically -0.3 V / ESH, allowing the reduction of only the oxygen O 2(aq) initially dissolved in the aqueous solution under examination, when it contains it, to water H 2 O (1) according to the redox half-equation O 2(aq) +4 H 3 O +< (aq) + 4 e → 6 H 2 O (1)

[0098] Correspondingly, the electrode potential difference between the second working electrode WE 2 and the counter electrode CE is fixed so that the second working electrode WE 2 is biased at a second electrode potential E 2 equal to the first electrode potential E 1. Like the first working electrode WE 1, the second working electrode WE 2 is thus configured to reduce the fraction of dissolved oxygen O 2(aq) in sweat to water H 2 O (1) that was not reduced at the first working electrode WE 1.

[0099] With reference to the [ Fig.14 ], the polarization process of the first and second working electrodes WE 1 and WE2 comprises two successive steps in time t.

[0100] During a first stage, before a moment t 0 , i.e. for t < t 0 , The first electrode potential E1 applied to the first working electrode WE1 may be close to the initial equilibrium potential, or the first working electrode WE1 may be disconnected. In the latter case, illustrated in Figure 141, the first electrode potential E1 conventionally takes the value zero, "0", so that the detected cathode current ired is zero, as shown in Figure 143.

[0101] Simultaneously, 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 H2O (1). The cathodic intensity ired is constant since it is proportional to the concentration of dioxygen O2(aq) previously dissolved in sweat, as illustrated in Figure 144.

[0102] During a second stage beginning from the moment t 0 ,The first electrode potential E1 applied to the first working electrode WE1, illustrated in Figure 141, is lower than the initial equilibrium potential in order 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 WE1 decreases (in relative value) due to the reduction of oxygen O2(aq) to water H2O (1). 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 WE1 by the flow.

[0103] 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 H2O (1). Thus, at time t 0 + Δt, When the oxygen-depleted sweat flow (O2(aq)) passes over the surface of the second working electrode WE2, the detected cathodic intensity (i_red) increases (in relative value), i.e., it approaches zero, since the concentration of dissolved oxygen (O2(aq)) in the sweat is zero or, at the very least, lower than upstream of the first working electrode WE1, as shown in Figure 144. The duration Δt corresponds to the time required for the sweat flow depleted of dissolved oxygen O2(aq) to pass from the first working electrode WE1 to the second working electrode WE2 under the effect of the sweat flow in the microfluidic channel 11, 11a, 11b, 11c at the flow velocity V.

[0104] The second stage ends at a moment t 1 subsequent to the moment t 0 , from which the first working electrode WE 1 is again polarized to a first electrode potential E 1 close to the initial equilibrium potential, or is disconnected as illustrated in Figure 141. The first working electrode WE 1 can subsequently be repolarized so that the process can be repeated as many times as necessary to determine the mean volumetric flow rate Q at closely spaced successive times.

[0105] The method described here is simple and easily industrialized, 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. Δt between the variations of the amperometric signals of the working electrode pair WE 1 and WE 2.

[0106] 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 surface area S in the direction of flow 13 can be determined from the inter-electrode distance L, L a , L b , L c separating the working electrodes WE 1 and WE 2, and from the duration Δt characteristic of the response time of the second working electrode WE 2, monitored by chronoamperometry, relative to the instantaneous response of the first working electrode WE 1. Subject to the reservations 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 using the following equations: V = L / Δt Q = S × V = L × S / Δt Subject to the same reservations, these equations are also valid, respectively, in microfluidic channels 11a, 11b, 11c by substituting the inter-electrode distance L , by the inter-electrode distance L a , L b , L c .

[0107] Within the framework of the envisioned dynamic applications, for the temporal monitoring of a subject's physiological state, it is desirable that device 1, in order to determine the quantitative parameter of a subject's perspiration, measure the value of the volumetric perspiration rate Q at successive, closely spaced times consistent with the expected perspiration rate, for example, once per minute. Integrating the temporal variations of the volumetric perspiration rate Q then allows the determination of the total perspiration flux of the subject over a given time range t.

[0108] The quantitative parameter of sweating can be a sweating rate determined from the total volume of sweat sweated by the subject over a given time range t, reported to the surface area of ​​the investigation area 8.

[0109] The inter-electrode distance L, L a , L b , L c The separation between the working electrodes WE 1 and WE 2 is chosen to be sufficiently small so that changes in the subject's physiological response are negligible over the duration Δt and large enough to allow a decoupled operating regime of the working electrodes WE 1 and WE 2 in the or each microfluidic channel 11, 11a, 11b, 11c where the volumetric flow measurement Q is carried out.

[0110] Indeed, depending on the average linear flow velocity V of sweat in the microfluidic channel 11, 11a, 11b or 11c, 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 over the height of the microfluidic channel 11, 11a, 11b or 11c after being carried over the inter-electrode distance L, L a , L b , L c . In particular, when considering the linear flow velocity V Due to the concentration gradient of sweat, there is insufficient time to dissipate along the height of the microfluidic channel 11a, 11b, or 11c before reaching the second working electrode WE2. Consequently, the operation of the two working electrodes WE1 and WE2 is linked. This coupling regime limits the temporal resolution of the amperometric signals, thus disrupting the volumetric flow rate (Q) measurements of sweat flowing through the microfluidic channel 11a, 11b, or 11c. This problem is easily avoided by adjusting the relative values ​​of the inter-electrode distance. L, L a , L b , L c and the duration t 1 - t 0 to the expected values ​​of the average linear flow velocity V.

[0111] According to a first embodiment, illustrated on the [ Fig.6 ], pairs of first and second working electrodes WE 1 and WE 2 separated by inter-electrode distances L a , L b , L c different can be used in separate parallel microfluidic channels 11a, 11b, 11c. Preferably, the inter-electrode distance L a , L b , L c separating the working electrodes WE 1 and WE 2 differs depending on the microfluidic channel 11a, 11b or 11c considered, for example such as L a < L b < L c .

[0112] Alternatively, according to a second embodiment illustrated on the [ Fig.8 A network of second working electrodes, here a first second working electrode WE 2 (1)< and a second second working electrode WE 2 (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 include more than two second working electrodes. In the following, we will limit ourselves to describing the network of second working electrodes implemented in the microfluidic channel referenced 11. Such a network could also be implemented in the microfluidic channels 11a, 11b, 11c.

[0113] In the microfluidic channel 11, each second working electrode WE 2 (1)< , WE 2 (2)< is respectively arranged at an inter-electrode distance L (1)< , L (2)< different from the first working electrode WE 1. The working electrodes WE 1, WE 2 (1)< , WE 2 (2)< are electronically switchable. The duration t 1 - t 0 is electronically adjusted by feedback from the average linear flow velocity value V measured during previous measurement moments.

[0114] 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 channels11, 11a, 11b and 11c or in several of them, retaining only the volumetric flow rate Q measurements consistent with the inter-electrode distances L a , L b , L c , L (1)< , L (2)< . Advantageously, the inter-electrode distances L a , L b And L c , L (1)< , L (2)< are on the order of a millimeter.

[0115] 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.

[0116] With reference to the [ Fig.15 ], we now describe 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.

[0117] In the embodiment with a plurality of microfluidic channels 11a, 11b, 11c shown in the figures 5 And 6 , the electrochemical cells 14a, 14b, 14c are connected to an analog-to-digital converter 18, which in turn powers a processor 19. The processor 19 is programmed, for example, to implement the sweat volume flow rate measurement processes Q described above.

[0118] In the embodiment shown in the [ Fig.8 where a first working electrode WE 1 and several second working electrodes WE 2 (1)< , WE 2 (2)< are implemented in the same microfluidic channel 11, located respectively at inter-electrode distances L (1)< et L (2)< different, each pair consisting of the first working electrode WE 1 and one of the second working electrodes WE 2 (1)< , WE 2 (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.

[0119] 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 Q measurements to a storage or post-processing device.

[0120] 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 subject 2, as well as to quantify its level of activity, and / or a temperature sensor to measure the temperature of the epidermis of subject 2. Indeed, it is useful to know the skin temperature because of the correlations between temperature and the rate of perspiration.

[0121] Certain elements of the apparatus 1, in particular the electronic processing device 16, can be implemented in various forms, either individually or distributedly, using hardware and / or software components. Usable hardware components include specific integrated circuits ( Application-Specific In-tegrated Circuit or ASICs), programmable logic networks ( Field-Programmable Gate Arrayor FPGA). Software components can be written in various programming languages, for example C, C++, Java, or VHDL. This list is not exhaustive.

[0122] 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.

[0123] The use of the verb "comporter", "comprendre" or "include" and its conjugated forms does not exclude the presence of other elements or steps than those stated in a claim.

[0124] In claims, any reference sign in parentheses shall not be interpreted as a limitation of the claim.

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); and - 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; 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 (Δt) 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 or 3, 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 or 3, wherein the first electrode potential (E1) allows the reduction of water H2O 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 or 3, 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 any of claims 1 to 6, 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); - 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 any of claims 1 to 7, 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 any of claims 1 to 8, 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 any of claims 1 to 9, 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 any 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 (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 any of claims 1 to 11, 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 any of claims 1 to 12, 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 or 14, 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 any of claims 16 or 17, 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 any 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. The apparatus (1) as claimed in any of claims 13 to 19, 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 any 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.