Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In vivo, neural mapping experiments on feline animal models illustrate one mode of use for this class of technology. These concepts provide new capabilities for implantable and surgical devices.
“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.
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 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.”