Tear fluid conductivity sensor

1. An eye-mountable device comprising:

a transparent polymeric material;

a substrate at least partially embedded within the polymeric material;

an antenna disposed on the substrate;

a conductivity sensor disposed on the substrate, wherein the conductivity sensor includes:

a frame having a trench formed therein that defines a fluid sample cell, wherein the trench includes a first sidewall and a second sidewall,

a first electrode formed on the first sidewall, and

a second electrode formed on the second sidewall such that the first and second electrodes are on opposite sides of the fluid sample cell; and

a controller electrically connected to the sensor electrodes and the antenna, wherein the controller is configured to: (i) apply a voltage to the electrodes sufficient to generate a current through the electrodes related to a conductance of a fluid occupying the fluid sample cell; (ii) measure the generated current; and (iii) use the antenna to indicate the measured current.

2. The eye-mountable device of claim 1 , wherein the first electrode and the second electrode are separated by a substantially constant separation distance throughout the fluid sample cell.

3. The eye-mountable device of claim 2 , wherein the separation distance between the two electrodes is less than about 100 micrometers.

4. The eye-mountable device of claim 1 , wherein the transparent polymeric material has a concave surface and a convex surface, and wherein the concave surface is configured to be removably mounted over a corneal surface and the convex surface is configured to be compatible with eyelid motion when the concave surface is so mounted.

5. The eye-mountable device of claim 4 ,

wherein the first and second electrodes each include a width dimension and a length dimension;

wherein the width dimension of each electrode extends along a depth of the fluid sample cell, which depth is oriented approximately perpendicular to a locally proximate region of the convex surface of the polymeric material; and

wherein the length dimension of each electrode extends within a plane approximately co-planar with the locally proximate region of the convex surface.

6. The eye-mountable device of claim 4 ,

wherein the two electrodes each substantially span a depth of the fluid sample cell that extends transverse to a locally proximate region of the convex surface of the polymeric material; and

wherein the depth of the fluid sample cell is greater than about 20 micrometers.

7. The eye-mountable device of claim 6 , wherein the depth of the fluid sample cell is between about 50 micrometers and about 100 micrometers.

8. The eye-mountable device of claim 4 , wherein the polymeric material includes a channel situated so as to expose the electrodes to tear fluid, via the channel, when the concave surface is mounted over an eye.

9. The eye-mountable device of claim 4 , wherein the polymeric material includes an electrolyte-absorbant silicon hydrogel in the vicinity of the electrodes so as to expose the electrodes to electrolytes in tear fluid, via diffusion through the silicon hydrogel, when the concave surface is mounted over an eye.

10. The eye-mountable device of claim 1 , wherein the fluid sample cell has a volume less than about 100 nano liters.

11. The eye-mountable device of claim 1 , wherein the voltage applied to the sensor electrodes is a periodically varying AC voltage which causes the generated current to be an AC current, and wherein the controller is further configured to generate the AC voltage and to measure the AC current.

12. The eye-mountable device of claim 1 , wherein the frame of the conductivity sensor includes a silicon wafer.

13. The eye-mountable device of claim 1 , wherein the transparent polymeric material includes a silicone elastomer.

14. The eye-mountable device of claim 1 , further comprising an energy harvesting system that captures electrical energy from incident radiation to power the controller.