Doctors can put arrays of electrodes on the surface of the brain to pinpoint the source of epileptic seizures; patients can use such electrodes to control a computer cursor. But it's still not safe to leave these devices in the brain over the long term, and that's a quality that needs to be developed before researchers can develop better brain-computer interfaces.
Silk on the brain: Thin, flexible electrodes mounted on top of a biodegradable silk substrate could provide a better brain-machine interface. The device wraps around the crevices in the surface of the brain, as shown on this model.
Now a group of researchers is building biocompatible electronics on thin, flexible substrates. The group hopes to create neural interfaces that take higher-resolution measurements than what's available today without irritating or scarring brain tissue.
"Biocompatibility is a major challenge for new generations of medical implants," says Brian Litt, professor of neurology and bioengineering at the University of Pennsylvania Medical School. "We wanted to make devices that are ultrathin and can be inserted into the brain through small holes in the skull, and be made out of materials that are biocompatible," he says. Litt is working with researchers at the University of Illinois at Urbana-Champaign who are building high-performance flexible electronics from silicon and other conventional materials on substrates of biodegradable, mechanically strong silk films provided by researchers at Tufts University.
This week in the journal Nature Materials, the team reports using a silk electrode device to measure electrical activity from the surface of the brain in cats. Silk is mechanically strong--that means the films can be rolled up and inserted through a small hole in the skull--yet can dissolve into harmless biomolecules over time. When it's placed on brain tissue and wetted with saline, a silk film will shrink-wrap around the surface of the brain, bringing electrodes with it into the wrinkles of the tissue. Conventional surface electrode arrays can't reach these crevices, which make up a large amount of the brain's surface area.
"A device like this would completely open up new avenues in all of neuroscience and clinical applications," says Gerwin Schalk, a researcher at the Wadsworth Center in Albany, NY, who is not affiliated with the silk electrode group. "What I foresee is placing a silk-based device all around the brain and getting a continuous image of brain function for weeks, months, or years, at high spatial and temporal resolution."
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."
By Katherine Bourzac
From Technology Review
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