So far, scientists have tested the interface in rodents with a severed peripheral nerve, showing that the nerve will grow into the cup and make connections with the muscle cells. “If they can keep the end of the neuron intact in that area, that’s a major breakthrough,” says Ellis-Behnke. The nerves in rats are about the same size as those that would be targeted in humans. The research was presented today at a conference of the American College of Surgeons in Chicago.
The device can also feed sensation back into sensory nerves, which relay heat, pressure, and other information from the skin to the brain. Like motor nerves, sensory nerves make connections onto the muscle cells in the cup. In rodent tests, scientists capped two nerves in a single animal–one motor and one sensory. While the rat did not have a prosthesis, scientists were able to show that the implant could bridge the severed nerve, transmitting neural messages across it; tickling the rat’s foot triggered muscle cell activity in the implant.
Sensory capability is a major missing component of today’s prostheses–tactile, pressure, and temperature feedback is vital for picking up a fragile egg or a hot pan. Ultimately, prosthetic limbs could be outfitted with heat or pressure sensors that could transmit that information to muscle cells in the interface and allow this information to be sent to the brain.
The research is still in its early stages, and a number of questions remain to be answered. “We need to find out how long it takes for the connections to become functional, and what the durability and robustness will be,” says Joseph Pancrazio, a program director at the National Institute for Neurological Disorders and Stroke, who was not involved in the research. “But it looks very exciting.” The research is funded by the Department of Defense.
One of the major issues with neural implants to date has been the stability of the devices, because implanted electrodes often become coated in scar tissue and stop working. So far, for the six months that the scientists have been assessing the interfaces in rats, there have been no signs of scarring. While scientists aren’t sure why, it may be that the cup protects the implant from the inflammatory reactions that lead to scarring, or that providing a target for the nerve cells dampens these reactions altogether by recreating a more normal environment for the severed nerve. The researchers are now monitoring the implants on a daily basis to determine their durability over time.
One particularly promising early finding, however, is that the tissue surrounding the interface grows new blood vessels to feed the implanted muscle cells, supplying them the nutrients they need to survive.
It’s not yet clear how many of these nerve caps patients would need for adequate control over a sophisticated artificial limb. Someone who has lost their arm at shoulder level, for example, would need enough nerve caps to flex and extend the elbow, wrist, and fingers, as well as those for sensory nerves. “The only limit,” says Cederna, “is going to be how high-tech they can make the prosthetics.”