Silk used to mould implants to brain

Scientists have developed a brain implant that uses silk to snugly fit to the brain’s surface. The technology could pave the way for better devices to monitor and control seizures, and to transmit signals from the brain past damaged parts of the spinal cord.

“These implants have the potential to maximise the contact between electrodes and brain tissue, while minimising damage to the brain. They could provide a platform for a range of devices with applications in epilepsy, spinal cord injuries and other neurological disorders,” said Walter Koroshetz, deputy director of the US National Institute of Neurological Disorders and Stroke (NINDS), part of the National Institutes of Health.

The study, published in Nature Materials, shows that the ultrathin flexible implants, made partly from silk, can record brain activity more faithfully than thicker implants embedded with similar electronics.

The simplest devices for recording from the brain are needle-like electrodes that can penetrate deep into brain tissue. More complex devices consist of dozens of semi-flexible wire electrodes, usually fixed to rigid silicon grids that do not conform to the brain’s shape.

In people with epilepsy, the arrays could be used to detect when seizures first begin, and deliver pulses to shut the seizures down. In people with spinal cord injuries, the technology has promise for reading complex signals in the brain that direct movement, and routing those signals to healthy muscles or prosthetic devices.

“The focus of our study was to make ultrathin arrays that conform to the complex shape of the brain, and limit the amount of tissue damage and inflammation,” said Brian Litt, an author on the study and an associate professor of neurology at the University of Pennsylvania School of Medicine in Philadelphia. The silk-based implants developed by Litt and his colleagues can hug the brain like shrink wrap, collapsing into its grooves and stretching over its rounded surfaces.

The implants contain metal electrodes that are 500µm thick. The absence of sharp electrodes and rigid surfaces should improve safety, with less damage to brain tissue. Also, the implants’ ability to mould to the brain’s surface could provide better stability; the brain sometimes shifts in the skull and the implant could move with it. Finally, by spreading across the brain, the implants have the potential to capture the activity of large networks of brain cells, Litt said.

Besides its flexibility, silk was chosen as the base material because it is durable enough to undergo patterning of thin metal traces for electrodes and other electronics. It can also be engineered to avoid inflammatory reactions, and to dissolve at controlled time points, from almost immediately after implantation to years later. The electrode arrays can be printed onto layers of polyimide and silk, which can then be positioned on the brain.

To make and test the silk-based implants, Litt collaborated with scientists at the University of Illinois in Urbana-Champaign and at Tufts University outside Boston.

In the current study, the researchers approached the design of a brain implant by first optimising the mechanics of silk films and their ability to hug the brain. They tested electrode arrays of varying thickness on complex objects, brain models and ultimately in the brains of living, anaesthetised animals.

In the future, the researchers hope to design implants that are more densely packed with electrodes to achieve higher resolution recordings.

“It may also be possible to compress the silk-based implants and deliver them to the brain, through a catheter, in forms that are instrumented with a range of high performance, active electronic components,” said John Rogers, a professor of materials science and engineering at the University of Illinois, who devised the flexible electronics

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