US20100104459A1 - Micropump actuated by droplets - Google Patents

Abstract

The invention relates to a micropump for which the pressure force is not limited by the electrowetting saturation angle, while being simple to manufacture. According to the invention, the microchannel (10) comprises an inlet orifice (11) and has a hydrophilic wall (12) extending from said inlet orifice (11). Displacement means are provided to displace a droplet (51) of liquid (L1) by electrowetting on a hydrophobic surface (22) until said droplet (51) is brought into contact with said hydrophilic wall (12), such that said droplet (51) enters into said microchannel (10) through said inlet orifice (11) by wetting, causing displacement of said fluid (F1).

Description

TECHNICAL FIELD
• This invention relates to the general field of microfluidics and more particularly micropumps, and concerns a micropump actuated by droplets.
STATE OF PRIOR ART
• Micropumps can be used to control a fluid flow, particularly in a microchannel, and are used in many microfluidics systems.
• For example, micropumps may be used in laboratories on chip, medical substance injection systems or hydraulic circuits for cooling electronic chips.
• Micropumps may be actuated in different ways, for example using a piezoelectric, electrostatic, thermopneumatic or even electromagnetic device. These different actuation devices are presented in the document by D. J. Laser and J. G. Santiago entitled “A review of micropumps”, J. Micromech. Microeng., 14 (2004), R35-R64.
• However, these actuation devices have some disadvantages such as the presence of deformable membranes or valves, the use of high voltages, for example for piezoelectric or electrostatic devices, or high electrical consumption for example with thermopneumatic or electromagnetic devices.
• Another approach that at least partly avoids the disadvantages mentioned above, consists of activating the micropump by electrowetting, and more precisely by electrowetting on dielectric.
• Thus, patent application WO2002/07503A1 describes a micropump shown inFIG. 1, comprising a substrate in which amicrochannel10 is formed, and an actuation device to allow flow of a fluid F1 in themicrochannel10. The operating principle of the actuation device is based on displacement of a conducting liquid L1 by electrowetting in themicrochannel10 starting from areservoir41.
• The actuation device comprises a linear network of displacement electrodes31(1),31(2),31(3) . . . integrated into the substrate and arranged in themicrochannel10 starting from thereservoir41. Acounter-electrode43 is placed in thereservoir41 and provides electrical contact with the conducting liquid L1. The displacement electrodes are covered by a hydrophobic dielectric layer (not shown).
• A voltage generator (not shown) is connected to thedisplacement electrodes network31 and to thecounter-electrode43, and applies a voltage U between the electrodes.
• The conducting liquid L1 forms an interface I1 with the fluid F1 filling themicrochannel10.
• When the displacement electrode31(i) located facing the interface I1 is activated by switching means (not shown) which on closure creates a contact between this electrode and the voltage source through a common conductor, the assembly comprising the liquid at voltage L1, dielectric layer and activated electrode31(i) acts like a capacitor.
• As indicated in the article by Berge entitled “Electrocapillarité et mouillage de films isolants par l'eau”, C. R. Acad. Sci., 317,series 2, 1993, 157-163, the contact angle θ of the liquid interface I1 is expressed according to the following relation:
• $\begin{array}{cc}\cos \theta \left(U\right)=\cos \theta \left(0\right)+\frac{{\varepsilon }_{\mathrm{<figure-callout\; label="r"\; filenames="US20100104459A1-20100429-D00000.png,US20100104459A1-20100429-D00001.png"\; state="\{\{state\}\}">r</figure-callout>}}}{2e\sigma }U^2& \left(1\right)\end{array}$
• where e is the thickness of the dielectric layer, ∈_ris the permittivity of this layer and σ is the surface tension of the interface I1 of the liquid L1.
• When an AC polarisation voltage U is used, the liquid L1 behaves like a conductor to the extent that the frequency of the polarisation voltage is significantly less than a cutoff frequency. This cutoff frequency, that depends particularly on the electrical conductivity of the liquid, is typically of the order of few tens of kilohertz (for example see the article by Mugele and Baret entitled “Electrowetting: from basics to applications”, J. Phys. Condens. Matter, 17 (2005), R705-R774). Furthermore, the frequency is preferably significantly greater than the frequency corresponding to the hydrodynamic response time of the liquid that depends on physical parameters such as the surface tension, viscosity or size of the microchannel and that is of the order of a few tens or hundreds of Hertz. The liquid response then depends on the effective value of the voltage, because the contact angle depends on the voltage in proportion to U², according to the relation (1).
• An electrostatic pressure acting on the interface I1 then arises close to the contact line, as explained in the article by Bavière et al. entitled “Dynamics of droplet transport induced by electrowetting actuation”, Microfluid Nanofluid, 4, 2008, 287-294. The liquid can then be displaced step by step on the hydrophobic surface by successive activation of electrodes31(1),31(2), etc. As it displaces, the liquid L1 “pushes” the fluid F1 along themicrochannel10.
• However, the micropump according to prior art has some disadvantages.
• The pressure force applied by the liquid on the fluid is proportional to cos θ(U). Thus, as the size of the angle θ(U) reduces, the pressure force will increase and the flow will also increase. In practice, the contact angle decreases with the increase in the polarisation voltage U up to a saturation angle that is normally between about 30° and 80°. The pressure force and therefore the fluid flow are then limited by this saturation angle.
• Furthermore, the displacement length of the liquid in the microchannel corresponds to the length of the actuation electrodes network. Thus displacement of the liquid along the entire length of the microchannel requires that the network of electrodes should be extended along the entire length of the microchannel. Manufacturing then becomes particularly complex, particularly in the case in which the cross-section of the microchannel is non-rectangular, for example if it is circular, or if it includes direction changes.
PRESENTATION OF THE INVENTION
• The purpose of this invention is to disclose a micropump in which the pressure force is not limited by the electrowetting saturation angle, while being simpler to manufacture.
• To achieve this, the purpose of the invention is a micropump to displace a fluid in a microchannel.
• According to the invention, the microchannel comprises an inlet orifice and has an hydrophilic wall extending from said inlet orifice, and the micropump comprises means of displacing a liquid droplet by electrowetting on a hydrophobic surface until said droplet comes into contact with said hydrophilic wall, such that said droplet is introduced by wetting in said microchannel through said inlet orifice, causing displacement of said fluid.
• Thus, the pressure force exerted by the liquid on the fluid in the microchannel is not limited by the electrowetting saturation angle, as it is in the micropump according to prior art.
• According to this invention, electrowetting is used to bring liquid droplets as far as the inlet orifice of the microchannel, but it is not the micropump driving phenomenon.
• The fluid flow is obtained by introducing the liquid droplet into the microchannel through the inlet orifice. This takes place naturally due the difference in wettability associated with the droplet. When the droplet is brought into contact with the hydrophilic wall through the inlet orifice, it wets the hydrophobic surface and the hydrophilic wall of the microchannel at the same time. The difference in wettability between these two surfaces causes migration of the entire droplet from the hydrophobic surface towards the hydrophilic wall. The liquid droplet then enters the microchannel and simultaneously “pushes” the fluid.
• Furthermore, manufacturing of the micropump is simplified because it is no longer necessary to have electrowetting electrodes over the entire length of the microchannel.
• Advantageously, the contact angle formed between said droplet and said hydrophilic wall is significantly less than the contact angle formed by electrowetting on said hydrophobic surface.
• Said displacement means preferably comprise at least one displacement electrode and a counter-electrode in electrical contact with the droplet, and a voltage generator to apply a potential difference between one or several displacement electrodes and said counter-electrode.
• Said displacement electrodes may be arranged along a determined path.
• One of said displacement electrodes, called the contact making electrode, is advantageously arranged such that a liquid droplet recovering it is in contact with said hydrophilic wall through said inlet orifice.
• Said displacement means may comprise a single displacement electrode, which is then said contact making electrode.
• Said hydrophilic wall may have a nanotextured or microtextured surface.
• Said hydrophilic wall may be a hydrophilic material.
• Said hydrophilic wall may comprise a layer of a hydrophilic material.
• Advantageously, said hydrophilic wall extends along the entire length of the microchannel.
• A layer of dielectric material is preferably placed between said hydrophobic surface and said electrodes.
• Advantageously, the microchannel comprises a connection portion defining an upstream portion and a downstream portion, said connection portion having a significantly larger cross section than the upstream portion.
• The size of the connection portion is preferably between 5 and 50 times the size of the upstream portion.
• A second fluid may be located downstream from the first fluid so as to form an interface with the first fluid, said interface being located in said connection portion.
• The upstream portion may comprise a first upstream portion extending from the inlet orifice and a plurality of second elementary upstream portions arranged in parallel each communicating with said first upstream portion.
• Each second elementary upstream portion may communicate with said connection portion.
• Each second elementary upstream portion may at least be partially filled with said fluid.
• The micropump advantageously comprises means of forming said droplet on said hydrophobic surface by electrowetting.
• The droplet formation means may comprise a plurality of droplet formation electrodes, one of which is adjacent to a displacement electrode.
• A second hydrophobic surface may be arranged facing the first hydrophobic surface so as to form a closed or confined device for said droplet.
• Other advantages and characteristics of the invention will become clear after reading the non-limitative detailed description given below.
BRIEF DESCRIPTION OF THE DRAWINGS
• We will now describe embodiments of the invention as non-limitative examples with reference to the appended drawings among which:
• FIG. 1, already described, is a diagrammatic representation of a top view of a micropump according to prior art;
• FIGS. 2A and 2B are diagrammatic representations of a top view of a micropump according to a first embodiment of the invention, for two operating steps in which the configuration of the droplet formation and displacement means is said to be open or not confined;
• FIGS. 3A to 3C show the formation of droplets by electrowetting in the case of a micropump according to the first embodiment;
• FIGS. 4A and 4B are diagrammatic representations of a micropump according to a second embodiment, in which the configuration of the droplet formation and displacement means is said to be confined,FIG. 4A being a top view andFIG. 4B a longitudinal sectional view ofFIG. 4A along axis I-I;
• FIG. 5 is a diagrammatic representation of a top view of a micropump according to a third embodiment of the invention, in which the microchannel comprises a connection portion;
• FIG. 6 is a diagrammatic representation of a micropump according to a fourth embodiment of the invention showing a longitudinal section in which the microchannel comprises a plurality of elementary portions arranged in parallel;
• FIG. 7 is a diagrammatic representation of a micropump according to a variant of the fourth embodiment shown inFIG. 6, showing a longitudinal section comprising two elementary micropumps arranged in parallel.
DETAIL PRESENTATION OF A PREFERRED EMBODIMENT
• A first embodiment of the invention is represented diagrammatically in a top view shown inFIGS. 2A and 2B.
• The micropump comprises a microchannel10 at least partially filled with a fluid F1 and an actuation device used to control flow of said fluid F1 in themicrochannel10.
• FIG. 2A shows a direct orthonormal coordinate system (i,j,k). In the first embodiment of the invention, adroplet51 may be displaced in a plane approximately parallel to the (i,j) plane.
• The longitudinal axis of themicrochannel10 is defined as being the median line of the microchannel. The longitudinal axis may be straight or curved, and it may have direction changes.
• Themicrochannel10 may have a convex polygonal cross-section, for example square, rectangular, hexagonal, a square section being a special case of the more general rectangular shape. It may also have a circular cross-section. The term microchannel in this description is considered in the general sense and in particular it includes the special case of the microtube with a circular cross-section. The microchannel may also be the catheter of a medicine dispenser system.
• The transverse characteristic size of themicrochannel10 will be called the height. The height of a microtube refers to the diameter.
• According to the invention, themicrochannel10 comprises aninlet orifice11 through which a liquid L1 can pass from the outside of the microchannel10 to the inside.
• Preferably, theinlet orifice11 is located at one end of themicrochannel10.
• Themicrochannel10 comprises ahydrophilic wall12 that extends from saidinlet orifice11 onto part of the transverse contour, or preferably onto the entire transverse contour.
• Thehydrophilic wall12 may extend over a defined length along the longitudinal axis of the microchannel, or preferably extend over the entire length of the microchannel.
• The actuation device of the micropump controls flow of the fluid F1 in themicrochannel10.
• It comprises means of displacing at least onedroplet51 of liquid L1 by electrowetting on a hydrophobic surface as far as theinlet orifice11 of themicrochannel10.
• In this case the displacement means comprise asingle displacement electrode31 integrated into or located on asupport substrate21 and covered by the hydrophobic surface.
• Thedisplacement electrode31, called the contact making electrode, is arranged such that adroplet51 of liquid L1 covering it is in contact with thehydrophilic wall12 through saidinlet orifice11.
• According to one variant not shown, a series of displacement electrodes may be arranged along a determined path terminating by acontact making electrode31 placed so as to put adroplet51 covering it into contact with thehydrophilic wall12 through theinlet orifice11 of themicrochannel10.
• It will be noted that the verbs “cover”, “be located on”, and “be arranged on” do not necessarily mean direct contact in this description. Thus, theliquid droplet51 may cover thedisplacement electrode31 without there being any direct contact, because there is a hydrophobic surface between thedroplet51 and theelectrode31.
• Note also that the droplet displacement means in this description are in a so-called open or non-confined configuration, in that said liquid droplets are not confined between two support substrates or two hydrophobic surfaces parallel to each other, but are supported solely on thesupport substrate21.
• InFIGS. 2A and 2B, theinlet orifice11 is arranged approximately facing thedisplacement electrode31. More precisely, the inlet axis through theorifice11, in this case along i, is approximately parallel to the (i,j) plane of thedisplacement electrode31. Other arrangements are possible as shown inFIG. 5 (described in detail below), in which theinlet orifice11 is formed approximately in the same plane as thedisplacement electrode31. The inlet axis through the orifice, in this case along k, is approximately perpendicular to the (i,j) plane of thedisplacement electrode31. In this example, theinlet orifice11 is surrounded by thedisplacement electrode31 such that adroplet51 that covers theelectrode31 is brought into contact with saidhydrophilic wall12 through saidinlet orifice11.
• The hydrophobic surface may be a layer of hydrophobic material.
• Preferably, a layer of a dielectric material is arranged between the displacement electrode(s)31 and the hydrophobic surface.
• The dielectric and hydrophobic layers may be a single layer comprising these two functions, for example a parylene layer.
• Preferably, a counter-electrode (not shown) is provided to make electrical contact with theliquid droplet51. It is arranged at least facing thedisplacement electrode31. This counter-electrode may be either a catenary, or a wire buried between the dielectric layer and the hydrophobic layer, or a planar electrode integrated into a cover of the micropump (such a cover is described later). In the latter case, an electrically conducting hydrophobic layer may cover the counter-electrode.
• Thedisplacement electrode31 and the counter-electrode may be connected to a DC voltage generator (not shown) or preferably an AC voltage generator to move thedroplet51 by electrowetting as described above.
• In the case of AC voltage, the frequency is advantageously between 100 Hz and 10 kHz, preferably of the order of 1 kHz, so as to keep the electrical conducting properties of the liquid and to exceed the hydrodynamic response time of thedroplet51. The response of thedroplet51 then depends on the RMS value of the voltage applied. The RMS value may vary between a few volts and a few hundred volts, for example 200V. It will preferably be of the order of a few tens of volts.
• It is particularly advantageous if the micropump has droplet formation means51 by electrowetting starting from areservoir41 containing said liquid L1.
• As shown inFIG. 2A, the droplet formation means preferably comprise at least three formation electrodes42(1),42(2),42(3) integrated into or deposited on saidsupport substrate21 and covered by said hydrophobic surface.
• Preferably, said dielectric layer is also arranged between the hydrophobic surface and theformation electrodes42.
• A first formation electrode42(1) is arranged approximately facing or close to thereservoir41 containing the liquid. A second formation electrode42(2) is adjacent to the first42(1) and is followed by a third electrode42(3). The third electrode42(3) is preferably adjacent to thedisplacement electrode31.
• Advantageously, the counter-electrode and the voltage generator described above are common to the droplet formation means and the displacement means. The counter-electrode is then arranged such that it is also facing theformation electrodes42.
• Switching means (not shown) are provided to activate the different electrodes42(1),42(2),42(3),31 one after the other and thus firstly control the formation and secondly the displacement of the droplet as far as theinlet orifice11 of themicrochannel10.
• FIGS. 3A to 3C show an example of the formation of a droplet by electrowetting starting from areservoir41 containing said liquid L1, in the case of an open configuration. Patent application WO2006/070162 filed in the name of the applicant describes details of the droplet formation principle used herein, and also gives an example of droplet formation in a confined configuration.
• As shown inFIG. 3A, saidreservoir41 may be a reservoir electrode at which there is areservoir droplet53 of liquid L1. This reservoir electrode defines a liquid retention micro-reservoir and it may be similar or identical to thereservoir electrode46 described later with reference to the second embodiment of the invention. Saidreservoir electrode41 may be circular in shape as shown inFIGS. 2A and 2B, square as shown inFIG. 4A or it may be any other shape.
• Three electrodes42(1),42(2),42(3) are shown inFIGS. 3A to 3C.
• Activation of this series of electrodes42(1),42(2),42(3) causes the liquid to be spread by electrowetting starting from thereservoir droplet53 in the form of aliquid segment52 as shown inFIG. 3B.
• Thisliquid segment52 is then cut into two parts by deactivating the electrode42(2). The result obtained is adroplet51 as shown inFIG. 3C.
• Therefore, a series of electrodes42(1),42(2),42(3) is used to draw liquid L1 from thereservoir droplet53 into a liquid segment52 (FIG. 3B) and then to cut this liquid segment52 (FIG. 3C) and form adroplet51 that can be displaced by the displacement means.
• The micropump according to the first embodiment of the invention operates as follows with reference toFIGS. 2A and 2B.
• The droplet formation means are activated so as to form adroplet51 of liquid L1 on the hydrophobic surface by electrowetting, as described above.
• The displacement means are then activated to displace thedroplet51 formed as far as theinlet orifice11 by electrowetting, and thus bring it into contact with thehydrophilic wall12.
• When the droplet is in contact with thehydrophilic wall12 through theinlet orifice11, it spontaneously enters themicrochannel10 by wetting. More precisely, the droplet migrates from the hydrophobic surface of the actuation device towards thehydrophilic wall12 of themicrochannel10. In doing so, it “pushes” the fluid F1 contained in themicrochannel10 and thus assures controlled flow of the fluid.
• When thedroplet51 has fully entered themicrochannel10, the procedure may be repeated. Asecond droplet51 may be brought as far as theinlet orifice11 by electrowetting and then introduced into themicrochannel10 by wetting. More precisely, thesecond droplet51 coalesces with the liquid L1 already present in themicrochannel10 starting from theinlet orifice11. The result obtained is then a larger volume droplet, part of which wets the hydrophobic surface and the other part wets thehydrophilic wall12. The phenomenon remains exactly the same. The new droplet will move to dewet the hydrophobic surface and further wet thehydrophilic wall12 of themicrochannel10. In doing so, it “pushes” the fluid F1 and thus causes the fluid to flow.
• Therefore the micropump according to the invention has the advantage that it is not limited by the electrowetting saturation angle. The driving force is then the wetting force that appears spontaneously when theliquid droplet51 is in contact with thehydrophilic wall12 of themicrochannel10. This wetting force depends on the contact angle formed by the liquid L1 on the hydrophilic wall. This may be very small, for example of the order of or less than 10°. The pressure force and therefore the fluid flow in the microchannel are then greater than in the micropump according to prior art.
• Furthermore, the flow of fluid F1 is assured provided that themicrochannel10 is supplied withliquid droplets51 through the displacement means. The liquid L1 may spread in themicrochannel10 over the entire length of thehydrophilic wall12. It is thus not necessary to havedisplacement electrodes31 along themicrochannel10. Manufacturing of the micropump is then particularly simplified.
• A second embodiment of the invention is shown inFIGS. 4A and 4B, in whichFIG. 4A shows a top view andFIG. 4B shows a longitudinal section of the top view along axis I-I.
• Identical numeric references as inFIG. 2A denote identical or similar elements.
• In this embodiment, droplet formation and displacement means confine the liquid droplet.
• A secondhydrophobic surface26 is arranged facing the firsthydrophobic surface22 and approximately parallel to it, and integrated into or placed on anupper cover25.
• Thus, adroplet51 may be formed by the droplet formation means and displaced between the first and secondhydrophobic surfaces22,26 by the displacement means.
• Preferably, the counter-electrode43 is integrated into thecover25 or placed on it, and is covered by the secondhydrophobic surface26.
• The droplet formation means are advantageously similar to those described in patent application WO2006/070162 filed in the name of the applicant.
• Thus, a well27 is formed in theupper cover25.
• This well27 is placed at least partially facing atransfer electrode47 that is integrated into thesubstrate21 or located on it.
• Beyond thetransfer electrode47, there is areservoir electrode46 that will define a liquid retention micro-reservoir.
• Thedroplet formation electrodes42 are then arranged followed by at least one displacement electrode, in other words a single electrode called acontact making electrode31.
• Note that although the dielectric layer is distinct from thehydrophobic layer22, it is not shown inFIGS. 4A and 4B.
• As described in patent application WO2006/070162, thetransfer electrode47 pumps liquid from the reservoir (not shown) communicating with the well, so as to bring it close to thereservoir electrode46.
• A certain quantity of liquid may be accumulated on this reservoir electrode. It is shown inFIG. 4A as being square or rectangular, but its shape may be arbitrary. Preferably, it may accumulate at least three or four times the volume ofdroplets51 to be dispensed, and preferably at least 10 times or 20 times the volume of each dispenseddroplet51.
• Since the distance between the twosubstrates21, is approximately constant (as can be seen inFIG. 4B), the surface area of theelectrode46 is equal to at last three or four times, or at least 10 or 20 times, the surface area of each of thedroplet formation electrodes42.
• When the transfer electrode is activated, it brings a portion of liquid located in the well27, close to thereservoir electrode46.
• When the reservoir electrode is also activated, the liquid is transferred into the zone above thereservoir electrode46.
• If it is required to continue supplying the zone located above thereservoir electrode46, thetransfer electrode47 and then thereservoir electrode46 can be reactivated to continue to accumulate liquid in this reservoir zone.
• A large volume ofliquid53 can thus be accumulated (FIG. 4B). One important advantage is that the pressure in this liquid volume accumulated above theelectrode46 is independent of the pressure of the liquid in the well27 by deactivating thetransfer electrode47.
• As long as thetransfer electrode47 is not activated, the liquid defined by thereservoir electrode46 is not in contact with the well27. The droplets that can be formed starting from the liquid stored above thereservoir electrode46 can then be created in a calibrated manner, while using a well27, independently of the pressure in the well, to fill the component.
• Note that the twohydrophobic surfaces22,26 form two approximately parallel planes and do not form a microchannel. Thus, displacement of adroplet51 does not cause displacement of all the surrounding fluid in the same direction. This surrounding fluid goes around thedroplet51 in its displacement. Thus, adroplet51 can be brought as far as theinlet orifice11 without introducing the surrounding fluid in the microchannel.
• This arrangement means thatdroplets51 can be dispensed reproducibly with a very high precision in volume. Volume variation coefficients (CV) (CV=2×standard deviation/average×100) less than 3% are typically measured.
• Furthermore, the micropump according to this embodiment of the invention can give precise control over the flow of fluid F1 in themicrochannel10. The fluid F1 is “pushed” by theliquid droplet51 over a distance that depends particularly on the volume of thedroplet51. Thus, the formation of a droplet with a calibrated volume can displace the fluid F1 over a precise distance.
• In this embodiment of the invention, the distance between twohydrophobic surfaces22,26 is of the order of a few hundred micrometres, and preferably 100 μm. The volume of thedroplets51 obtained is between a few nanolitres and a few microlitres, for example 64 nl.
• According to variants not shown, thereservoir droplet53 located at thereservoir electrode46 may be formed while the micropump is being manufactured. Thus, the droplet formation means do not include a well communicating with a reservoir, nor a transfer electrode, but simply a reservoir droplet located at the reservoir electrode. It is then advantageous if thecover25 comprises a cavity at thereservoir electrode46, so as to hold a large volume reservoir droplet.
• It is also possible that the space located at thereservoir electrode46, or said cavity, communicates with the exterior, such that the liquid can for example be introduced manually with a pipette, to reform or resupply the reservoir droplet. The space located at the reservoir electrode and said cavity when the liquid L1 is present in it, then form a reservoir.
• Thesupport substrate21 and thecover25 may be made of silicon or glass, polycarbonate, polymer or ceramic.
• Themicrochannel10 may for example be made by lithography and selective etching. It may be possible to use dry etching (gas etching, for example with SF₆in a plasma), depending on the required dimensions. Wet etching may also be used. Hydrofluoric acid or phosphoric acid etchings may be used for glass (mostly SiO₂) or silicon nitrides (these etchings are selective but isotropic). Etching may be done by laser ablation or by ultrasounds. Micro-machining may also be used, particularly for polycarbonate. Themicrochannel10 may also be made of a flexible molten silica capillary.
• The height of themicrochannel10 is typically between a few tens of nanometres and 200 μm, and preferably between 1 μm and 100 μm, and even more preferably 30 μm. The length of themicrochannel10 may be few hundred microns to a few centimetres, for example 50 cm.
• The displacement andformation electrodes31,42 and thetransfer electrode47 and thereservoir electrode46, and the counter-electrode43, may be made by deposition of a thin layer of a metal chosen from among Au, Al, ITO, Pt, Cu, Cr . . . or Al—Si alloy, using classical microelectronics microtechnologies, for example by photolithography. Theelectrodes31,42,46,47 are then etched according to an appropriate pattern, for example by wet etching.
• The thickness of theelectrodes31,42,46,47 may be between 10 nm and 1 μm, and preferably of the order of 300 nm. The length of theelectrodes31 and42 may be between a few micrometres to a few millimetres, preferably between 50 μm and 1 mm, and preferably 800 μm. The surface area of these electrodes depends on the size of the droplets to be formed and displaced.
• The spacing between adjacent electrodes may be between 1 μm and 20 μm.
• Note that in the different embodiments, the droplet displacement andformation electrodes31 and42 may be approximately square or rectangular as shown on the figures.
• However, the inter-electrode space may be curved or angular. In the case of an angular shape, the shape of the edge of an electrode may be a sawtooth approximately parallel to the edge of the adjacent electrode that has a corresponding shape. This electrode shape facilitates the passage of the liquid droplet from one electrode to the next.
• As described in patent application WO2006/07162, thereservoir electrode46 may be in the shape of a comb or half-star, or even a tip, to guarantee an electrode surface gradient. The shape of thetransfer electrode47 is adapted to the shape of thereservoir electrode46.
• A dielectric layer may cover thedifferent electrodes31,42,46,47. It may be made of Si₃N₄, SiO₂, in SiN, or strontium barium titanate (BST) or other materials with high permittivity such as HFO₂, Al₂O₃, Ta₂O₅[29], Ta₂O₅—TiO₂, SrTiO₃or Ba_1-xSr_xTiO₃. The thickness of this layer may be between 100 nm and 3 μm, generally between 100 nm and 1 μm, and preferably 300 nm. The dielectric layer made of SiO₂may be obtained by thermal oxidation. A plasma enhanced chemical vapour deposition (PECVD) process is preferred to the low pressure chemical vapour deposition (LPCVD), for thermal stress reasons. The temperature of the substrate is only increased from 150° C. to 350° C. (depending on the required properties), compared with about 750° C. for the LPCVD deposit.
• Finally, thehydrophobic surface22 may be deposited on the dielectric layer. A Teflon deposit by dipping or spray or a plasma deposited SiOC deposit may be made for this purpose. A hydrophobic vapour or liquid phase silane deposit may be made. Its thickness will be between 100 nm and 5 μm, and preferably 1 μm. In particular, this layer can reduce or even eliminate hysteresis effects of the wetting angle.
• In the case of a confined configuration, ahydrophobic layer26 covers the counter-electrode43.
• Themicrochannel10 is at least partially filled with fluid F1, preferably an insulating fluid that may be air, a mineral oil or silicon, a perfluorinated solvent such as FC-40 or FC-70, or an alkane like undecane.
• The liquid L1 is electrically conducting and it may be an aqueous solution charged with ions, for example Cl⁻, K⁺, Na⁺, Ca²⁺, Mg²⁺, Zn²⁺, Mn²⁺ ions. The liquid may also be mercury, gallium, eutectic gallium or ionic liquids such as bmim PF6, bmim BF4 or tmba NTf2.
• The volume of theliquid droplets51 varies between a few nanolitres and a few microlitres, for example about 64 nl.
• The fluid F1 is not miscible with the conducting liquid L1.
• The hydrophilic nature of saidwall12 may be obtained using a material that is naturally hydrophilic for thesubstrate21 in which themicrochannel10 is formed, such as aluminium, silica or hydrogel.
• The substrate may also be a hydrated porous environment such as hydrated Nafion.
• Thehydrophilic wall12 may also comprise a silica layer. In the case of asubstrate21 made of silicon, the silica layer may be obtained by thermal oxidation of the silicon.
• The surface of thehydrophilic wall12 may also be microtextured or nanotextured so as to amplify the wetting effects and increase the capillarity force as described in the publication by J. Bico et al. entitled “Wetting of textured surfaces” Colloids and Surfaces A, Physicochem. Eng. Aspects, 206 (2002), 41-46.
• A surface is said to be or nanotextured or microtextured when the characteristic scale of its relief is between a few nanometres or micrometres and a few hundred nanometres or micrometres respectively. The textured surface may have a network of surface roughnesses, for example pins, pads or nanometric or micrometric grooves.
• The hydrophilic or even super-hydrophilic nature of the wall is then obtained by a liquid film between the roughnesses. The thickness of this so-called impregnation film is comparable to the height of the roughnesses but is negligible relative to the characteristic size of the droplet. Thus, as explained by P.-G. de Gennes et al. in the book entitled “Capillarity and Wetting Phenomena: props, Bubbles, Pearls, Waves”, Springer, 2003, the droplet is eventually placed on a wetted surface that is a sort of patchwork of solids and liquids. Thus, the wall is highly hydrophilic in nature.
• Different techniques known by those skilled in the art may be used to obtain a textured surface and are described particularly in the thesis by M. Callies Reyssat entitled “Splendeur et misère de l'effet lotus” (Splendour and misery of the lotus effect), 2007, Paris VI University.
• Chemical surface treatment techniques may be used to make thewall12 of the microchannel10 hydrophilic. A chemical layer or a film is usually placed on thewall12, the thickness of which may vary between a few nanometres and a few hundred microns.
• For example, silanisation of a metallic oxide or semiconducting surface (for example SiO₂, HfO₂, ITO, TiO₂, SnO₂) or polymers (for example PDMS, the COC) in the vapour phase or liquid phase can make the wall of the microchannel hydrophilic. A large variety of silanes can be used to obtain a hydrophilic surface. In order to be as hydrophilic as possible, silanes preferably comprise an ionic group for example such as a carboxylate, a phosphate, a phosphonate, imidazolium, protonated amine, quaternary amine or a sulfonate. A number of these functions, the synthesis of associated molecules and surface functionalisation methods are described in patent application WO2007/088187.
• For other oxide surfaces such as TiO₂or SnO₂, it is advantageous to graft the molecule by phosphatation or phosphanation, to improve the strength of the layer. In this case, the group conferring the hydrophilic property may be of the same type as that described above. Preparation of such compounds and their use on surfaces are described particularly in the publication by F. Durmaz et al. entitled “New phosphates/phosphonates; A modular approach to functional sams”, European Cells and Materials, Vol. 6, Suppl. 1, 2003, 55.
• These two methods described above may be used in different ways depending on the thickness of the layer to be obtained. Thus, the result in an anhydrous and only slightly concentrated environment is a thin layer a few nanometres thick. A thicker layer is obtained in the presence of water and alcohol (for example ethanol), varying from a few hundred nanometres to about a hundred microns using sol-gel type processes.
• Note that a hydrophilic surface can also be obtained by grafting molecules in the polysaccharides family, as described in patent application WO2002/100559.
• Polymer families can be used to obtain a strong hydrophilic layer about a few hundred nanometres thick, such as polyhydroxystyrenes.
• Patent application WO2007/053326 also describes hydrophilic groups, for example silanols, introduced into a polymer matrix to be deposited to form the hydrophilic layer.
• All techniques mentioned above are known by those skilled in the art, and can make the wall of the microchannel hydrophilic starting from the inlet orifice.
• A third embodiment of the invention is shown on the top view inFIG. 5.
• Numeric references identical to those inFIG. 4A refer to identical or similar elements.
• In this embodiment, themicrochannel10 may comprise a second fluid F2 downstream from the first fluid F1 so as to form an interface I2 with it. Preferably, the first and second fluids F1 and F2 are not miscible with each other.
• The interface I2 is preferably located in aconnection portion17.
• Theconnection portion17 defines anupstream portion13 extending from theinlet orifice11 as far as theconnection portion17, and adownstream portion16 that extends downstream from theconnection portion17.
• The height of theconnection portion17 is significantly greater than the height of theupstream portion13 of the microchannel. Preferably, the height is of the order of 5 to 50 times of the height of theupstream portion13, and preferably ten times. Preferably, the height of theupstream portion13 and thedownstream portion16 is constant.
• The height of thedownstream portion16 may be identical to, or more or less than the height of theconnection portion17. In the example shown inFIG. 4, the height of thedownstream portion16 is approximately identical to the height of theupstream portion13.
• The presence of theconnection portion17 can reduce the effects of hysteresis of the contact angle that oppose fluid flow. These effects are inversely proportional to the height of theconnection portion17.
• The droplet formation and displacement means are in a confined configuration in this figure, as described in the second embodiment and as shown inFIG. 5. Alternately, they may be in open configuration as described in the first embodiment.
• This third embodiment of the invention has the advantage that it outputs a calibrated fluid flow F2 at the output from thedownstream portion16 of the microchannel.
• A fourth embodiment of the invention is shown inFIG. 6 in a longitudinal section.
• Numeric references identical to those inFIG. 4A refer to identical or similar elements.
• In this embodiment of the invention, theinlet orifice11 is arranged in the same plane as thedisplacement electrode31 and is surrounded by it. The orifice inlet axis, in this case along k, is approximately orthogonal to the plane of the displacement electrode, in this case (i,j). Thus, adroplet51 that covers the displacement electrode is brought into contact with thehydrophilic wall12 through theinlet orifice11.
• In this embodiment, aconnection portion17 is placed between anupstream portion13 and adownstream portion16 of the microchannel.
• Theupstream portion13 also comprises a firstupstream portion14 and a secondupstream portion15. The firstupstream portion14 extends from theinlet orifice11. The secondupstream portion15 extends from the firstupstream portion14 as far as theconnection portion17. Thedownstream portion16 corresponds to athird portion16 of the microchannel.
• More precisely, the secondupstream portion15 comprises a plurality of second elementaryupstream channel portions15′ arranged in parallel, each communicating with the firstupstream portion14 and with theconnection portion17.
• The secondelementary portions15′ may be arranged in a hexagonal network and their diameter may be of the order of a few tens of microns, and preferably 30 μm. Preferably, each secondelementary portion15′ has a circular or hexagonal cross-section or a similar type of shape. The secondelementary portions15′ may be obtained by RIE type plasma etching of thesubstrate21.
• Preferably, the secondelementary portions15′ are filled with liquid L1 and/or the first fluid F1.
• There may be a few hundred secondelementary portions15′, and their height (diameter) may be a few tens of microns, preferably 30 μm, and their length may be a few hundred microns, preferably 700 μm.
• This parallel arrangement of the secondelementary portions15′ can give a high flow of the second fluid F2 in thedownstream portion16.
• In this case, the droplet formation and displacement means are in a confined configuration like that described in the second embodiment and as shown inFIG. 6. Alternately, they may be in an open configuration as described in the first embodiment.
• A variant of the fourth embodiment of the invention is shown inFIG. 7 in the longitudinal cross-section.
• Numeric references identical to those inFIG. 4A refer to identical or similar elements.
• In this variant, two elementary micropumps, each approximately identical to that described in the fourth embodiment, are arranged in parallel and are connected to each other firstly through acommon well27 filled with liquid L1, and secondly through a junction connecting the downstream portions16-1 and16-2. More precisely, the two downstream portions16-1 and16-2 are connected through ajunction18 so as to form only oneportion19.
• The two micropumps may have means of controlling the droplet formation and displacement electrodes independently of each other.
• Furthermore, the second fluids F2-1 and F2-2 manipulated by the two micropumps may be different.
• Thus, the two second fluids F2-1 and F2-2 may be brought into contact at said junction of downstream portions16-1 and16-2, and thus create a mix, or even a two-phase flow.
• The proportions of each second fluid F2-1 and F2-2 may be controlled using the electrode control means.
• The first fluids F1-1 and F1-2 are advantageously identical.
• Obviously, several elementary micropumps may be arranged in parallel, without the number of elementary micropumps being limited to two micropumps as described above.
• Furthermore, the elementary micropumps do not have to be connected to each other at their correspondingdownstream portion16, so that their second fluid F2 can be dispensed independently.
• Finally, note that if electronic programming means are associated with electrode control means in the different embodiments described above, dispensing sequences of calibrated quantities of the first or second fluid can be defined.
• Furthermore, the so-called direct electrowetting phenomenon may be used in the case in which the dielectric layer is not present.
• The capacitance used is then no longer the capacitance of the dielectric layer, but rather the capacitance of a double electrical layer formed in the conducting liquid L1 on the surface of theelectrodes31,41. In this case, the applied voltages must be sufficiently low to prevent electrochemical phenomena such as electrolysis of the water.
• The thickness e used in the relation between the contact angle θ and the applied voltage U, described above, is the thickness of the double layer that is of the order of a few nanometres.
• It is then advantageous to add species with high permittivity into the liquid L1, for example such as zwitterionic species. This can increase the permittivity ∈_rof the double layer. Examples of zwitterions that could be used may include amine sulfonates, amine phosphates, amine carbonates or amine carboxylates, and particularly sulfonate alkanes of ammonium trialkyl, sulfonate alkanes of imidazole alkyl, or sulfonate alkanes of pyridine alkyl.

Claims (16)

1. Micropump to displace a fluid (F1) in a microchannel (10), said micropump being characterised in that:

the microchannel (10) comprises an inlet orifice (11) and has a hydrophilic wall (12) extending from said inlet orifice (11),

and in that it comprises

means (51) for displacement of droplets of liquid (L1) by electrowetting on a hydrophobic surface (22) until said droplet (51) is brought into contact with said hydrophilic wall (12), such that said droplet (51) enters into said microchannel (10) through said inlet orifice (11) by wetting, causing displacement of said fluid (F1).

2. Micropump according toclaim 1, characterised in that the contact angle between said droplet (51) and said hydrophilic wall (12) is significantly less than the contact angle formed by electrowetting on said hydrophobic surface (22).

3. Micropump according toclaim 1, characterised in that said displacement means comprise at least one displacement electrode (31) and a counter-electrode electrically in contact with the droplet (51), and a voltage generator to apply a potential difference between one or several displacement electrodes (31) and said counter-electrode.

4. Micropump according toclaim 3, characterised in that said displacement means comprise a displacement electrode (31) arranged such that a liquid droplet (51) covering it is in contact with said hydrophilic wall (12) through said inlet orifice (11).

5. Micropump according toclaim 1, characterised in that said hydrophilic wall (12) has a nanotextured or microtextured surface.

6. Micropump according toclaim 1, characterised in that said hydrophilic wall (12) is made of a hydrophilic material or comprises a layer of a hydrophilic material.

7. Micropump according toclaim 1, characterised in that said hydrophilic wall (12) extends over the entire length of the microchannel (10).

8. Micropump according toclaim 1, characterised in that the microchannel (10) comprises a connection portion (17) defining an upstream portion (13) and a downstream portion (16), the cross-section of said connection portion (17) being significantly greater than the cross-section of the downstream portion (13).

9. Micropump according toclaim 8, characterised in that the size of the connection portion (17) is between 5 and 50 times the size of the upstream portion (13).

10. Micropump according toclaim 8, characterised in that a second fluid (F2) is located downstream from the first fluid (F1) so as to form an interface (I2) with the first fluid, located in said connection portion (17).

11. Micropump according toclaim 8, characterised in that the upstream portion (13) comprises a first upstream portion (14) extending from the inlet orifice (11) and a plurality of second elementary upstream portions (15′) arranged in parallel, each communicating with said first upstream portion (14).

12. Micropump according toclaim 11, characterised in that each second elementary upstream portion (15′) communicates with said connection portion (17).

13. Micropump according toclaim 12, characterised in that each second elementary upstream portion (15′) is at least partially filled with said fluid (F1).

14. Micropump according toclaim 1, characterised in that it also comprises means of forming said droplet (51) on said hydrophobic surface (22), by electrowetting.

15. Micropump according toclaim 14, characterised in that said displacement means comprising at least one displacement electrode (31), the droplet formation means comprise a plurality of droplet formation electrodes (42), one of which is adjacent to a displacement electrode (31).

16. Micropump according toclaim 14, characterised in that a second hydrophobic surface (26) is arranged facing the first hydrophobic surface (22) so as to form a closed or confined device for said droplet (51).

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