In a technological advance for mind-controlled upper-limb prostheses, University of Michigan (U-M) researchers have tapped faint, latent signals from arm nerves and amplified them to enable real-time, intuitive, finger-level control of a robotic hand.
To achieve this, the researchers developed a way to tame nerve endings, separate thick nerve bundles into smaller fibers that enable more precise control, and amplify the signals coming through those nerves. The approach involves tiny muscle grafts and machine learning algorithms borrowed from the brain-machine interface field.
“This is the biggest advance in motor control for people with amputations in many years,” said Paul Cederna, MD, who is a professor of plastic surgery at the U-M Medical School and a professor of biomedical engineering. “We have developed a technique to provide individual finger control of prosthetic devices using the nerves in a patient’s residual limb. With it, we have been able to provide some of the most advanced prosthetic control that the world has seen.”
Cederna co-leads the research with Cindy Chestek, PhD, associate professor of biomedical engineering at U-M. In a paper published March 4 in Science Translational Medicine, they describe results with four study participants using the Mobius Bionics LUKE arm.
“You can make a prosthetic hand do a lot of things, but that doesn’t mean that the person is intuitively controlling it. The difference is when it works on the first try just by thinking about it, and that’s what our approach offers,” Chestek said. “This worked the very first time we tried it. There’s no learning for the participants. All of the learning happens in our algorithms. That’s different from other approaches.”
In a lab setting, participants were able to pick up blocks with a pincer grasp; move their thumb in a continuous motion, rather than have to choose from two positions; lift spherically shaped objects; and play a version of Rock, Paper, Scissors called Rock, Paper, Pliers.
“It’s like you have a hand again,” said study participant Joe Hamilton, who lost his arm in a fireworks accident in 2013. “You can pretty much do anything you can do with a real hand with that hand. It brings you back to a sense of normalcy.”
One of the biggest hurdles in mind-controlled prosthetic devices is tapping into a strong and stable nerve signal to feed the bionic limb. For people with amputations, peripheral nerves—the network that fans out from the brain and spinal cord—have been interesting, but they hadn’t yet led to a long-term solution because the nerve signals they carry are small, and other approaches lead to scar tissue, which muddles that already faint signal over time.
Instead, the U-M team wrapped tiny muscle grafts around the nerve endings in the participants’ arms. The regenerative peripheral nerve interfaces (RPNIs) offer severed nerves new tissue to latch on to, which prevents the growth of neuromas and amplifies the nerve signals. Two patients had electrodes implanted in their muscle grafts, and the electrodes were able to record these nerve signals and pass them on to a prosthetic hand in real time.
“To my knowledge, we’ve seen the largest voltage recorded from a nerve compared to all previous results,” Chestek said. “In previous approaches, you might get 5 microvolts or 50 microvolts—very, very small signals. We’ve seen the first ever millivolt signals.
“So now we can access the signals associated with individual thumb movement, multi-degree of freedom thumb movement, individual fingers. This opens up a whole new world for people who are upper-limb prosthesis users.”
The U-M team’s interface has already lasted years. Scar tissue has caused other interfaces to degrade within months.
“Other research groups have contributed to this as well, but we’ve leapfrogged the capabilities of the prosthetic hands that are currently available. I think this is strong motivation for further developments from prosthetic hand companies,” said Philip Vu, a research fellow in biomedical engineering and first author of the paper.
The researchers, whose work is funded by the Defense Advanced Research Projects Agency and the National Institutes of Health, have begun seeking participants for its ongoing clinical trial.