Brain prostheses sound eerie, but they’re actually quite useful. More and more, implants are being planted into the skulls of people worldwide in order to record their encephalic activities, or help them cope with ailments such as epilepsy or Alzheimer’s via low doses of electric stimulation.
Their potential applications go much further: The US military is pouring millions into research on brain implants that would treat soldiers’ PTSD, or allow them to engage in “brain-to-brain” communication; transhumanists like Ray Kurzweil see brain prosthetics as the next step towards transcendence.
Developing these devices naturally poses a challenge in terms of engineering and electronics, but there’s another, fundamental component: finding the best materials to make them. You can’t just stuff anything on top of your brain.
Implants are made up of two parts: a circuit with electrodes that are in contact with the brain, and a softer coat enveloping it. Materials composing the coating must first of all be biologically compatible, neurobiologist Greg Gerhardt of the University of Kentucky told me in a Skype call. Gerhardt is part of a team led by brain-prosthetics supremo Theodore Berger, working to design memory-restoring implants.
“Every time you put something inside the brain, there will always be a small amount of damage, no matter how good the device is,” he said. “Biologically compatible materials are able to minimise that damage to the brain, and minimise the risk that the body attacks the implant as a foreign object.”
There are two main candidates up to the task: silicone and polymers. Choosing the best option between them, and among different polymers, is a matter of trade-offs. For instance, you don’t want too squishy a device, as it’s harder to plant in the brain. That would suggest that polymers win out over silicone—the latter is softer and more difficult to manipulate, unless you add a temporary stiffener to steer it in.
But manageability can backfire. Researchers at the Ecole Polytechnique Fédérale in Lausanne (EPFL) published a paper in Science in January, backing the use of soft neural implants. One of the researchers, Stéphanie Lacour, told me on the phone that the team made and compared two models of prosthetic dura mater, the external membrane enclosing the brain and the spinal cord. One of them was made of polyamide (a frequently used polymer); the other was made of silicone. Both were implanted in healthy mice.
After a couple of weeks, the mice with the silicone devices were scurrying in their cages, whereas the polymer-equipped ones stumbled around and generally fared quite badly. A later examination revealed that the polyamide implants had almost crushed the spinal cord, and triggered a foreign-body reaction.
“The reason is that polymers, unlike silicone, are flexible without being elastic. When there’s movement, and rubbing, stiffer implants are problematic,” Lacour said. The difficulty was likely more marked at the level of spinal cord, as it’s in constant motion, but Lacour underlined that elasticity is also key in the more stable environment of the brain. “Just the blood pulse is enough to make the brain move, and implants moving along with the tissue would reduce tearing and immune reaction over time,” she said. “Stiff prostheses, on the other hand, are like fingernails to the human brain’s Jell-O.”
In Gerhardt’s words, “there is a lot of voodoo” around the choice of the right material. But at the end of the day, it still boils down to silicone and plastic polymers.
The contenders for materials for the circuit and electrode components are many more. One property all of them must have? Longevity.
“Putting something in the brain is like soaking it in the ocean. Brains have a high salt concentration and plenty of fluid. You want materials that do not rust quickly,” Gerhardt said. That makes noble metals like platinum, gold, or iridium particularly apt, as they oxidate slowly, and they’re also good conductors. However, there’s something they can’t do: interact with light.
Increasingly, brain researchers are getting interested in the potential of optogenetics, which uses light to activate neurons. It’s quicker and less invasive than electricity-based systems, but metals are pretty bad at that: If you pointed a light beam on platinum electrodes, you’d just get hot metal in the brain.
Last September, scientists at the University of Wisconsin came up with graphene implants as part of a DARPA-funded project. Graphene is a one-atom-thick carbon wafer that has earned a reputation as a quasi-sorcerous stuff. It’s the most conductive known material, and—being made of carbon, like organic compounds—it also scores high in biocompatibilty. Most of all, its thinness makes it virtually transparent. In Wisconsin, graphene implants allowed the researchers to use light to prompt limb movements in mice.
This is far from an isolated attempt to introduce new materials in the sector. In July, researchers from Michigan University even floated the idea of loading up the brain with “liquid hard drives.”
“People are increasingly getting into high-tech materials,” Gerhardt said. “Some of them come straight from the military, like for instance kevlar implants. They sound bold, but we still don’t know much about their biological compatibility. It’ll take years.”
