The advantage of surface electrodes over implanted ones is that they don’t cause scarring, says Andrew Schwartz, professor of neurobiology at the University of Pittsburgh. In 2008, Schwartz demonstrated that a monkey with an electrode in its brain can control a prosthetic arm to feed itself. “This design is even better because it has a relatively small feature size and is flexible–it could make these implants less traumatic,” he says. “What would really be nice is if you could amplify the signal near where you pick it up to reduce noise, and multiplex the signal to cut down on the number of wires needed,” says Schwartz.
The silk electronics researchers say this is their next step, and one of the major promises of the technology. They’ve already demonstrated thin, flexible silicon transistor arrays built on silk, and tested them in animals–just not in the brain yet. Schwartz says other groups have recognized the importance of multiplexing and signal amplification, but have been working with rigid circuit boards that are not as biocompatible. Adding these active components would reduce the number of wires needed in these implants, which today require one wire per sensor. And active devices could respond to brain activity to provide electrical stimuli, or release drugs. (One of the collaborators on the silk project, David Kaplan at Tufts University, has demonstrated that silk devices implanted in the brain in small animals can deliver anti-epilepsy drugs.)
Adding transistors to the electronics is currently a design challenge, says John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign. The electrode-array design his group found to be most compatible with brain tissue is a mesh–solid sheets won’t wrap around brain tissue as effectively. And adding silicon transistors to the mesh is more difficult than doing so on a solid substrate. Still, says Rogers, all the major pieces are in place and just need to be integrated. With further development and testing to prove the devices are safe, says Rogers, “we hope this will be the foundation for new higher quality brain-machine interfaces.”