Patent Application
20090134009
The present invention relates to the displacing and controlling of conductive and optionally polar microdrops in a dielectric or weakly conductive liquid medium which is immiscible with the drops, using electrohydrodynamic forces, between two or more positions that may correspond to specific contact zones on solid substrates.
A commonly encountered technique for displacing drops using electrostatic forces is electrowetting on dielectric (EWOD). Devices based on this technique consist of a matrix of insulated electrodes coated with a hydrophobic layer, the wetting of which exhibits very weak hysteresis. The drops to be displaced are placed on this hydrophobic surface and in contact with a bare electrode which polarizes them. The displacement of a drop above an insulated electrode adjacent to the drop is carried out by applying a voltage between the electrode in question and the electrode which polarizes the drop. This technique is very sound, but the quality of the hydrophobic surface in contact with the drops to be displaced is an essential point to be controlled in the fabrication of the systems.
Another technique for displacing drops is based on dielectrophoretic forces. The device is similar to that used to displace the drops by electrowetting, but does not comprise a bare electrode in contact with the drops, and therefore the electric potential of the drops is not fixed. The displacement is brought about by applying electrical voltages between the electrodes of the matrix so as to create nonuniform field zones. The drops displaced according to this technique may once again be in contact with a solid surface and the quality of this surface remains an essential point, or else may be in suspension at the interface between two immiscible fluids. In this case, the drops are in levitation. In the latter case, it is necessary to maintain the fluid/fluid interface along which the drop is displaced, parallel to the matrix of electrodes, which amounts to imposing the condition that the microsystem remains horizontal.
Particles or cells may also be manipulated using electric fields by dielectrophorosis. The particles are trapped in wells having electrostatic potentials. The particles can be maintained in levitation, out of contact with a solid surface, by this technique. The electric fields used are alternating electric fields of quite high frequency.
U.S. Pat. No. 5,486,337 A describes a device for displacing drops from one electrode to another by applying a voltage between the two electrodes. In contact with the first and under the effect of the electric field, the drop to be displaced is charged and then, under the effect of the electrostatic force to which it is subjected, leaves the electrode to move to the second. The drawback of this method once again lies in the prolonged contacts with solid elements, in particular wetting elements.
An advantageous technique for displacing drops without contact consists in causing them to float at the interface between two superimposed fluids, the densities of which flank that of the drop. A matrix of electrodes makes it possible to exert electrostatic forces on the drops in order to displace them (Orlin D. Velev, Nature, 4 Dec. 2003). However this technique is difficult to implement. It is necessary for the interface between the two fluids to be horizontal and parallel to the electrodes of the matrix.
The objective of the present invention is therefore to find a method for doing away with the constraints hanging over the quality of the solid surfaces.
An additional problem concerns the displacement of drops in which certain constituents might attach to the solid surfaces.
The present invention therefore relates to a device for displacing a volume of liquid under the action of an electrical voltage, comprising:
The present invention also relates to a method for displacing a volume of liquid, in which said volume is initially in contact with a first substrate with the first electrode, comprising the following steps:
The volume of liquid is generally immersed in a dielectric medium.
The voltage between the first and the second electrode is in general an alternating voltage.
Other features and advantages of the invention will emerge from the description which follows, with reference to the figures of the attached drawings. The exemplary embodiments described with reference to the drawings attached hereto are in no way limiting.
In the figures, identical reference numbers are used to denote identical parts.
However, the surface of the electrodes, in particular lower electrodes, is advantageously textured. These surfaces may, for example, consist of pads or a network of walls. Furthermore, the surface of the lower substrate is advantageously hydrophobic. A very thin layer (a few hundred angstroms) of Teflon is, for example, deposited.
The gap between the electrodes 4 and 5 is greater than the diameter of the drop and can vary in the present invention. In general, the gap between the microelectrodes is less than the diameter of the volume of liquid (of the drops) to be displaced. In the present invention, it is preferred for the gap between the microelectrodes to be from 1 to 500 μm.
The upper electrode 4 is advantageously made of glass or polycarbonate coated with a transparent conductive film such as ITO.
A surface treatment is not necessary. However, it could be advantageous for the surface(s) of the electrodes to be textured. The texture advantageously provides pads or a network of walls.
The dielectric 2 in general has a density lower than and close to the liquid 1 to be displaced, thus reducing the amplitude of the first voltage. It is advantageous for the difference in density between the liquid 1 and the liquid dielectric 2 to be approximately 0.1.
The second voltage, applied between two or more electrodes of the matrix, makes it possible to displace the drop laterally. It has, for example, a frequency equal to or in the vicinity of the first voltage and its phase can be adjusted with respect to that of the first voltage.
In a configuration in which the displaced drops come into contact with the matrix of electrodes, in order to prevent electrolysis, the electrodes of the matrix are, for example, spaced out in such a way that a drop cannot overlap two electrodes with different potentials at the same time. It is also possible to prevent electrolysis by taking care not to place an electrode adjacent to that above which a drop jumps at a potential different than the potential of the electrode above which the drop jumps.
The amplitude of the first voltage U1 is such that the electrostatic forces exerted on the drop to be displaced make it possible to detach it from the first electrode and the half-period τ of this voltage is less than the time of flight of the drop between the two electrode systems. As soon as this first voltage is applied, the drop continually detaches from an electrode and returns thereto: the electrostatic forces oppose wetting and the drop is therefore virtually never in contact with a solid surface.
The device of the present invention advantageously comprises a dielectric liquid between the first and the second substrate, the density of the dielectric preferably being close to the liquid to be displaced.
The supports of the device of the present invention may consist of various materials. The matrix of microelectrodes is preferably a printed circuit.
The electrodes preferably consist of a metal.
In the device of the present invention, it is preferable for one of the supports to consist essentially of glass or of a polymeric material. More particularly, the electrode produced on this support should be transparent.
The matrix of electrodes may extend along one or two dimensions.
In general, the second voltage has a frequency equal to or close to the first voltage. The phase and/or the amplitude of the second voltage can be adjusted with respect to that of the first voltage.
Preferably, the volume of liquid is, after displacement to the second substrate, maintained against the second electrode.
It is possible for the potential applied to two adjacent microelectrodes to be equal.
In the method according to the present invention, it is advantageously possible to cause two volumes of liquid to be displaced to coalesce by applying opposite charges when they are being brought closer together.
The present invention provides several specific advantages.
A volume of liquid (drop) displaced according to the method of the present invention is almost never in contact with a solid surface and the surface finishes are therefore much less critical than in the prior art devices. In particular, the lateral displacement of a drop takes place during a flight phase, during which only forces of inertia and viscous forces oppose the movement; the finishes of the solid surfaces have little influence.
The device and the method of the present invention have the ability to very easily cause two drops to coalesce during a protocol. In fact, it is sufficient for the two drops in question to have opposite charges when they are being brought closer together.
In addition, the device and the method of the present invention are compatible with French patent application FR 0450276 (corresponding to European patent application 05101090-8) which proposes a device for 3D displacement of conductive drops by electrostatic forces.
The following exemplary embodiments are in no way limiting. The embodiment described may be used in biological and/or chemical applications.
The matrix of electrodes can be produced on a printed circuit, the gap between the electrodes being of the order of 200 μm. The printed circuit is coated with a very thin layer of Teflon (a few tens of nanometers). The cover is made of glass coated successively with a thin conductive layer made of ITO (indium tin oxide) (140 nm), with a dielectric layer of Si3N4 (300 nm), with a photosensitive dry film (100 μm) into which structures, for example wells, can be etched and, finally, a thin layer of a plasma-deposited hydrophobic material. The cover is separated from the matrix of electrodes by a distance of 2 to 4 mm. The dielectric used is, for example, a mineral oil, the density of which is close to 0.9 and the drops displaced are of water or milk having a volume of 1 μl.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0508220 | Aug 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP06/07281 | 7/24/2006 | WO | 00 | 7/28/2008 |
