With so few people outfitted with these devices, their longevity is still unknown. So far, the Utah array has lasted up to 10 years in monkeys. In Copeland’s case, his arrays are still working, but not as well as in the first year or so after being implanted, says Robert Gaunt, a biomedical engineer at the University of Pittsburgh and a member of Copeland’s research team. “The body is a very difficult place to put electronics and engineered systems into,” Gaunt says. “It’s an aggressive environment, and the body is always trying to get rid of these things.”
Implanted arrays can provoke an immune response in the neural tissue that surrounds the electrodes—the spiky probes that stick into the brain. Studies have shown that this inflammation can lead to decreased signal quality. And scar tissue can form around brain implants, which also affects their ability to pick up signals from nearby neurons. The less information that a BCI can interpret from neurons, the less effective it is at carrying out its intended functions.
One way scientists are trying to make implants last longer is by experimenting with different kinds of materials. The Utah array is insulated with parylene, a protective polymer coating used in the medical device industry for its stability and low permeability to moisture. But it can corrode and crack over time, and other materials may prove to be more durable.
Florian Solzbacher, co-founder and chairman of Blackrock Neurotech, which manufactures the Utah arrays, says the company is testing one that’s coated with a combination of parylene and silicon carbide, which has been around for more than 100 years as an industrial material. “We’ve seen lifetimes on the benchtop that can reach up to 30 years, and we’ve got some preliminary data in animals right now,” he says. But the company has yet to implant it in people, so the real test will be how human tissue reacts to the new formulation.
Making electrodes more flexible could also help reduce scarring. Angle’s company Paradromics is developing an implant similar to the Utah array, but with thinner electrodes intended to be less disruptive to tissue.
Some researchers are trying out softer materials that may be able to better integrate into the brain than the rigid Utah array. One group, at the Massachusetts Institute of Technology, is experimenting with hydrogel coatings designed to have an elasticity very similar to that of the brain. Scientists at the University of Pennsylvania are also growing “living” electrodes, hairlike microtissues made of neurons and nerve fibers grown from stem cells.
But these approaches have downsides, too. “You can get a rigid thing into a soft thing. But if you’re trying to put a very soft thing into another soft thing, that’s very hard,” Gaunt says.
Another approach is to make the implants smaller, and therefore less invasive. For instance, researchers are testing neurograins, tiny chips the size of a grain of sand that could hypothetically be sprinkled across the cortical surface. But no one has tried dispersing them on a human brain; the system has only been tested in rodents that had their skulls removed.