A contact lens with a built-in pressure sensor that could help monitor conditions such as glaucoma has been made by researchers in the U.S. The device is the result of a new technique that can embed conducting circuits in the organic polymer traditionally used to make contact lenses. Polydimethylsiloxane (PDMS) is an elastic, transparent and gas-permeable organic polymer that can be cast-molded into a range of simple shapes. It is widely used in everything from contact lenses to breast implants.

One problem with the process however is that cast molding severely limits the kinds of structures that can be made with the material. Recently materials scientists Hailin Cong and Tingrui Pan at the University of California, Davis, have come up with a simple method to produce PDMS components without the need for casting molds. Their process can also make the material conduct electricity.

They added a chemical to a liquid PDMS solution which causes the mixture to set, or polymerise, when zapped with UV light. Cong and Pan then exposed the mixture to UV light through a mask containing a circuit pattern. Only the areas hit by the light polymerise and any remaining liquid can be washed away, leaving an imprint of the circuit. The technique can be used to create features just 10 micrometers in size (1mm is 1000 micrometers).

Cong and Pan have also developed a technique to give the polymer another property, electrical conductivity. They did this by adding a solution of silver to the mix. The silver then becomes trapped in the material when it polymerises, creating a path along which electricity can pass.

"This new way of manufacturing contact lens materials can significantly extend its use for biological sensing systems," says Pan.

Working with James Brandt, Director of Glaucoma Services at UC Davis Medical Center, Cong and Pan helped produce a tiny pressure sensor, which they bent into the shape of a contact lens. Such a device could measure the stress on the corneal surface, and the fluid pressure within the eye to monitor glaucoma and ocular hypertension, Pan says.

"The eye always has a certain pressure, which is why the eyeball is a sphere," says Pan. In glaucoma patients, as the pressure rises, the contact lens sensor is triggered and the patient or doctor can be alerted.

Their prototype has an opaque sensor that would impair vision and so would be worn only briefly, but Cong and Pan are designing transparent equivalents that could be worn for long periods to give a continuous pressure read-out. This could allow the data to be read wirelessly. Although this technology is in its infancy the potential is remarkable. It has applications beyond just measuring eye pressure. Virtual images similar to those seen in the movie Terminator can also be produced with this technology. Watch for future Eye To Eye's for updates.

Journal Reference: New Scientist Tech, July 2008

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