Droplet interfaces in electro-wetting devices
Droplet interfaces are formed between droplets in an electro-wetting device comprising an array of actuation electrodes. Actuation signals are applied to selected actuation electrodes to place the droplets into an energised state in which the shape of the droplets is modified compared to a shape of the droplets in a lower energy state and to bring the two droplets into proximity. The actuation signals are then changed to lower the energy of the droplets into the lower energy state so that the droplets relax into the gap and the two droplets contact each other thereby forming a droplet interface. The use of sensing electrodes in the device permit electrical current measurements across the droplet interface. The sensing electrodes can be used for either (i) applying a reference signal during droplet actuation or (ii) recording electrical current measurements. Two or more electrodes are configurable to lyse cells within a droplet positioned over said electrodes.
1. A method of forming a droplet interface in an electro-wetting device,
the electro-wetting device comprising:
an array of actuation electrodes;
an insulator layer covering the actuation electrodes and having an outermost hydrophobic surface;
disposed on the hydrophobic surface, a fluid medium and two droplets comprising liquid in the fluid medium, one of the liquid and the fluid medium being polar, and the other of the liquid and the fluid medium being apolar,
whereby the actuation electrodes are configured to electro-wet the droplets when actuation signals are applied thereto,
the method comprising:
applying actuation signals to selected actuation electrodes to place one or both of the two droplets in an energised state in which the shape of said one or both droplets is modified compared to when in a lower energy state and to bring the two droplets into proximity with a gap therebetween, the gap being chosen such that the two droplets do not contact each other when one or both are in the energised state and contact each other to form a droplet interface when in the lower energy state; and
changing the actuation signals applied to the actuation electrodes to lower the energy of said one or both droplets into the lower energy state so that said one or both droplets relax into the gap and the two droplets contact each other thereby forming a droplet interface.
2. A method according to claim 1 , wherein both droplets are placed in the energised state.
3. A method according to claim 1 , wherein the shape of the droplet is elongate, said gap extending along a major length of the elongate shape.
4. A method according to claim 3 , wherein the shape of the droplet has an aspect ratio of at least 2:1.
5. A method according to claim 1 , wherein, during the step of applying actuation signals to the actuation electrodes, the two droplets are brought into proximity with the centroids of the two droplets separated by a distance less than the combined radii of the droplets along a line between the two centroids in the lower energy state of the droplets.
6. A method according to claim 1 , wherein the area enclosed by the contact line of the droplets in the lower-energy state covers at least two actuation electrodes, preferably at least 5 actuation electrodes, at least 10 actuation electrodes or at least 20 actuation electrodes.
7. A method according to claim 6 , further comprising taking electrical measurements between the droplets across the droplet interface.
8. A method according to claim 7 , wherein the step of taking electrical measurements is carried out while not applying actuation signals to the actuation electrodes.
9. A method according to claim 7 , wherein the electrical measurements are measurements of ion flow between droplets through a transmembrane pore.
10. A method according to claim 9 , wherein the transmembrane pore is a protein.
11. A method according to claim 7 , wherein the electrical measurements are taken while applying a potential difference between the droplets.
12. A method according to claim 1 , wherein the actuation signals applied to the selected actuation electrodes are AC actuation signals.
13. A method according to claim 12 , wherein the step of changing the actuation signals applied to the selected actuation electrodes comprises applying DC potentials or floating potentials to the selected actuation electrodes in place of the AC actuation signals.
14. A method according to claim 1 , wherein the fluid medium is a liquid medium.
15. A method according to claim 1 , wherein the liquid is polar, and the fluid medium is apolar.
16. A method according to claim 1 , wherein the droplets further comprise amphipathic molecules at the interface between the liquid of the droplets and the fluid medium, and the droplet interface comprises a membrane of amphipathic molecules.
17. A method according to claim 16 , wherein at least one of the droplets comprises a transmembrane pore and the method further comprises allowing the transmembrane pore to insert into the membrane.
18. A method according to claim 1 , wherein the two droplets have respective volumes between 1 nL and 10 μL.
19. A method according to claim 1 , wherein the insulator layer comprises a layer of electrically insulating material coated by a hydrophobic material that forms said hydrophobic surface.
20. A method according to claim 1 , further comprising a second substrate facing the hydrophobic surface of the insulator layer, wherein the second substrate is coated by a hydrophobic material forming a further hydrophobic surface facing the hydrophobic surface of the insulator layer, the droplets being disposed on the further hydrophobic surface of the hydrophobic layer as well as the hydrophobic surface of the insulator layer.