Silicon on silk: This clear silk film, about one centimeter squared, has six silicon transistors on its surface. These flexible devices can be implanted in mice like the one in this image without causing any harm, and the silk degrades over time. The orange liquid on the hair is a disinfectant used during the surgery.
MIT Technology Review reports researchers from several universties have demonstrated arrays of transistors made on thin films of silk. While electronics must usually be encased to protect them from the body, these electronics don’t need protection, and the silk means the electronics conform to biological tissue. The silk melts away over time and the thin silicon circuits left behind don’t cause irritation because they are just nanometers thick.
Many existing and envisioned classes of implantable biomedical devices require high performance electronics/sensors. An approach that avoids some of the longer term challenges in biocompatibility involves a construction in which some parts or all of the system resorbs in the body over time. This paper describes strategies for integrating single crystalline silicon electronics, where the silicon is in the form of nanomembranes, onto water soluble and biocompatible silk substrates. Electrical, bending, water dissolution, and animal toxicity studies suggest that this approach might provide many opportunities for future biomedical devices and clinical applications.
“Current medical devices are very limited by the fact that the active electronics have to be ‘canned,’ or isolated from the body, and are on rigid silicon,” says Brian Litt, associate professor of neurology and bioengineering at the University of Pennsylvania. Litt, who is working with the silk-silicon group to develop medical applications for the new devices, says they could interact with tissues in new ways. The group is developing silk-silicon LEDs that might act as photonic tattoos that can show blood-sugar readings, as well as arrays of conformable electrodes that might interface with the nervous system.
Last year, John Rogers, professor of materials science and engineering at the Beckman Institute at the University of Illinois at Champaign-Urbana, developed flexible, stretchable silicon circuits whose performance matches that of their rigid counterparts. To make these devices biocompatible, Rogers’s lab collaborated with Fiorenzo Omenetto and David Kaplan, professors of bioengineering at Tufts University in Medford, MA, who last year reported making nanopatterned optical devices from silkworm-cocoon proteins.
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.