'It's Like You Have a Hand Again'

While study participants aren't yet allowed to take the arm home, in the lab, they 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 even play in a version of Rock, Paper, Scissors called Rock, Paper, Pliers. 

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"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."

Turning a tiny muscle graft into a nerve signal amplifier

One of the biggest hurdles in mind controlled prosthetics is tapping into a strong and stable nerve signal to feed the bionic limb. Some research groups—those working in the brain machine interface field—go all the way to the primary source, the brain. This is necessary when working with people who are paralyzed. But it's invasive and high-risk.  

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 for a couple of reasons: The nerve signals they carry are small. And other approaches to picking up those signals involved probes that eavesdropped by force. These "nails in nerves," as researchers sometimes refer to them, lead to scar tissue, which muddles that already faint signal over time. 

The U-M team came up with a better way. They wrapped tiny muscle grafts around the nerve endings in the participants' arms. These "regenerative peripheral nerve interfaces," or RPNIs, offer severed nerves new tissue to latch on to. This prevents the growth of nerve masses called neuromas that lead to phantom limb pain. And it gives the nerves a megaphone. The muscle grafts amplify 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, multidegree of freedom thumb movement, individual fingers. This opens up a whole new world for people who are upper limb prosthesis users."

And their interface has already lasted years. Others degrade within months due to scar tissue. 

The future of prosthetics research and industry

The findings also open up new possibilities for the field, said Chestek, whose expertise is on real time machine learning algorithms to translate neural signals into movement intent.

"What we found is now the nerve signals are good enough to apply the whole world of things we learned in brain control algorithms to nerve control," she said.