In the silk-silicon electronics, the silk plays a passive but important role. “Silk is mechanically strong enough to act as a support, but if you pour water on it, it conforms to the tissue surface,” says Omenetto. Silk is already approved by the U.S. Food and Drug Administration for medical implants and is broken down completely by the body into harmless by-products. The silk sheets are flexible, and can be rolled up and then unfurled during surgery, making them easier for surgeons to work with. By adjusting the processing conditions used to fabricate the films, the Tufts researchers can control the rate at which the films will degrade, from immediately after implantation to years.
The biocompatibility of silicon is not as well established as that of silk, though all studies so far have shown the material to be safe. It seems to depend on the size and shape of the silicon pieces, so the group is working to minimize them. These devices also require electrical connections of gold and titanium, which are biocompatible but not biodegradable. Rogers is developing biodegradable electrical contacts so that all that would remain is the silicon.
The group is currently designing electrodes built on silk as interfaces for the nervous system. Electrodes built on silk could, Litt says, integrate much better with biological tissues than existing electrodes, which either pierce the tissue or sit on top of it. The electrodes might be wrapped around individual peripheral nerves to help control prostheses. Arrays of silk electrodes for applications such as deep-brain stimulation, which is used to control Parkinson’s symptoms, could conform to the brain’s crevices to reach otherwise inaccessible regions. “It would be nice to see the sophistication of devices start to catch up with the sophistication of our basic science, and this technology could really close that gap,” says Litt.