A contact lens with a built-in pressure sensor could help monitor eye conditions such as glaucoma. The device embeds conducting circuits in an organic polymer often used for contact lenses. Polydimethylsiloxane (PDMS), an elastic, transparent and gas-permeable organic polymer, has been cast-molded into many simple shapes. Cast molding, however, severely limits the structures that can be made with the material.

U.C. Davis material scientists Hailin Cong and Tingrui Pan have devised a simple way to produce PDMS components without molds. First, an additive to liquid PDMS lets the mixture to polymerize when exposed to UV light. They also add a silver solution to the mix. Then it’s exposed to UV light through a mask containing a circuit pattern so only areas hit by the light polymerize and are conductive. Because the resistance of the path changes as the material stretches, the modified PDMS becomes a tiny strain gauge. Remaining liquid is washed away leaving an imprint of the circuit. The technique can create 10 micron features.

The tiny pressure sensor shaped like a contact lens could measure the stress on the cornea surface, and the fluid pressure within the eye to monitor glaucoma and ocular hypertension, says Pan. "The eye always holds some pressure," he adds. “But in glaucoma patients, the pressure rises and changes the shape of the eye, which would deform the contact-lens sensor. The shape change alters electrical resistance in the circuits to generate different electrical reading," he says.

In the accompanying image of several prototypes, opaque sensors would impair vision, making for a short wear time. So the team is designing transparent equivalents that could be worn for longer periods. An RFID tag would allow reading pressure data wirelessly.