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Eventually, Xenofon Strakosas, an assistant professor working in Berggren’s lab, figured out the problem: In plants, hydrogen peroxide helps the injected material bond together, but there isn’t enough peroxide in animals for the reaction to work. So Strakosas added some additional elements to the mix: an enzyme that uses glucose or lactate, which are common in animal tissues, to produce peroxide, and another enzyme that breaks down the peroxide. Suddenly, the electrodes formed perfectly.
For experts like Maria Asplund, a professor of bioelectronic microtechnology at Chalmers University of Technology in Sweden, the idea of forging electrodes inside the body is totally new. “Chemists can make things happen that I would never have imagined,” she says. But Asplund, who has spent over a decade working to create more brain-friendly electrodes, isn’t planning on abandoning her tried-and-tested methods for creating electrodes just yet. For one, this new tool hasn’t been tested in mammals—and no one knows how long it will last inside the body. Most important, though the electrodes might be able to successfully conduct electrical signals, Berggren and his colleagues don’t have a solution for getting those signals out of the brain so that scientists can actually see them, or for sending in current so the electrodes can be used for brain stimulation.
They have a number of options. One would be to stick an insulated wire directly into the electrode to carry its signals from deep within the brain to the surface of the skull, where scientists could measure them. That wire, though, could do damage to brain tissue, which is exactly what the team is trying to avoid. Instead, they may try to design other components that, like the electrode, could self-assemble within the brain, so that a signal could be wirelessly read from the outside.
If Berggren and his colleagues figure how to communicate with their electrodes, they will still struggle to compete with state-of-the-art devices like Neuropixels, which can record from hundreds of neurons at once. Achieving that degree of precision with a soft electrode could prove difficult, says Jacob Robinson, associate professor of electrical and computer engineering at Rice University in Texas. “There’s usually a trade-off between performance and invasiveness,” he says. “The engineering challenge is to push that envelope.”
At least to begin with, brain stimulation might be a better application for the soft electrodes, since it doesn’t require being quite so precise. And even imprecise recordings could benefit people who are fully paralyzed, says Aaron Batista, a professor of bioengineering at the University of Pittsburgh who researches brain-computer interfaces in monkeys. Soft electrodes might not be able to produce fluent speech by directly measuring someone’s brain signals—but for patients who can’t move at all, simply being able to convey “yes” or “no” would make an enormous difference.
Polymer electrodes aren’t just a safer, messier version of traditional electrodes, however. Because they form only in the presence of specific substances, they could be used to target parts of the brain with particular chemical profiles. Berggren and Strakosas plan to fine-tune their recipe so that the gel solidifies only in areas of the brain where there’s lots of lactate available—that is, areas that are extremely active. Using that strategy, they could specifically target the brain region where someone’s seizures originate. They’ll soon test that approach in epileptic mice. In principle, they could also create a material that uses not glucose nor lactate but some other substance to help the electrode form—a specific neurotransmitter, for example. That way, the electrodes would end up only in parts of the brain high in that specific neurotransmitter, which would allow neuroscientists to precisely target particular brain regions.
If Berggren and his team do manage to surmount the scientific obstacles ahead of them, their final task will be to navigate the thicket of regulations that govern devices that are used in medical settings. It’s impossible to anticipate how long that might take, especially for so novel a material. But Batista nonetheless thinks this discovery heralds a new era in electrode technology, no matter how far off it may be.
“I can’t be sure anybody living today will receive a flexible electronic neural implant,” he says. “But it seems likely now that someday somebody will.”
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