Fluid extraction from a microfluidic device

1. A method of extracting assay fluid from an electrowetting on dielectric (EWOD) device, the EWOD device comprising two opposing substrates defining a fluid space there between and an aperture for extraction of fluid from the fluid space;

the method comprising:

providing, in the fluid space of the EWOD device, an assay fluid droplet adjacent to the aperture such that the assay fluid droplet blocks extraction of filler fluid contained in the fluid space of the EWOD device, and

extracting, via the aperture, at least some of the assay fluid of the assay fluid droplet from the fluid space;

wherein the method comprises, during the extracting, controlling the assay fluid droplet by electrowetting to maintain the blocking of extraction of filler fluid;

wherein the EWOD device comprises a plurality of electrowetting electrodes, and the method comprises selectively actuating the electrowetting electrodes in a pattern of actuated electrodes during the extracting to maintain the blocking of extraction of filler fluid; and

the method further comprises, during the extracting, varying the pattern of actuated electrodes over time to maintain the blocking of extraction of filler fluid.

2. A method as claimed in claim 1 and comprising, during the extracting, controlling the assay fluid droplet by electrowetting so as to keep the centre of the assay fluid droplet substantially at a predetermined position relative to the aperture.

3. A method as claimed in claim 1 and comprising, during the extracting, varying the pattern of actuated electrodes over time in a predetermined manner.

4. A method as claimed in claim 1 and comprising, during the extracting, varying the pattern of actuated electrodes over time in dependence on a sensed position and/or size of the assay fluid droplet.

5. A method as claimed in claim 1 wherein providing the assay fluid droplet comprises moving assay fluid to the vicinity of the aperture by electrowetting.

6. A method as claimed in claim 5 , and comprising moving the assay fluid by actuating electrodes within an area that is smaller than the area of the assay fluid droplet.

7. A method as claimed in claim 5 , and comprising moving the assay fluid by actuating electrodes within an area having a dimension parallel to a direction of movement of the assay fluid that is greater than a dimension perpendicular to the direction of movement of the assay fluid.

8. A method as claimed in claim 5 wherein providing the assay fluid droplet comprises moving a plurality of sub-droplets of assay fluid to the vicinity of the aperture and merging the sub-droplets to form the assay fluid droplet for blocking extraction of filler fluid via the aperture.

9. A method as claimed in claim 1 and further comprising introducing additional filler fluid into the fluid space of the EWOD device before the extracting.

10. A method of extracting assay fluid from an electrowetting on dielectric (EWOD) device, the EWOD device comprising two opposing substrates defining a fluid space there between and an aperture for extraction of fluid from the fluid space;

the method comprising:

providing, in the fluid space of the EWOD device, an assay fluid droplet adjacent to the aperture such that the assay fluid droplet blocks extraction of filler fluid contained in the fluid space of the EWOD device, and

extracting, via the aperture, at least some of the assay fluid of the assay fluid droplet from the fluid space;

wherein the method comprises, during the extracting, controlling the assay fluid droplet by electrowetting to maintain the blocking of extraction of filler fluid, and

wherein the opposing substrates include an upper substrate and a lower substrate separated by a spacer, and the aperture is formed in a region beyond an edge of the upper substrate and inside the spacer, the method further comprising transporting by electrowetting the assay fluid droplet to the region.

11. A method as claimed in claim 10 , wherein transporting by electrowetting the assay fluid droplet to the region comprises moving an actuation pattern of electrowetting electrodes beyond the edge of the upper substrate and into the region.

12. A method as claimed in claim 11 , wherein the actuation pattern is moved until the actuation pattern reaches the spacer.

13. An electrowetting on dielectric (EWOD) device comprising:

two opposing substrates defining a fluid space therebetween;

an aperture for extraction of fluid from the fluid space;

a plurality of independently actuatable electrowetting electrodes; and

an electrode control unit configured to apply actuation voltages to the independently actuatable electrowetting electrodes;

wherein the independently actuatable electrowetting electrodes comprise active matrix electrowetting electrodes;

wherein the electrode control unit is configured to apply actuation voltages to actuate the independently actuatable electrowetting electrodes so as to:

provide, in the fluid space of the EWOD device, a droplet of assay fluid adjacent to the aperture such that the droplet blocks extraction of filler fluid contained in the fluid space of the EWOD device, and

control the assay fluid droplet by electrowetting during extraction, via the aperture, of at least some of the assay fluid of the droplet from the fluid space so as to maintain the blocking of extraction of filler fluid.

14. An EWOD device as claimed in claim 13 wherein the aperture is in one of the substrates of the EWOD device.

15. An EWOD device of claim 13 , wherein the aperture is at a side of the EWOD device.

16. An EWOD device of claim 15 , wherein the opposing substrates include an upper substrate and a lower substrate separated by a spacer, and the aperture is formed in a region beyond an edge of the upper substrate and inside the spacer.