In context: Prosthetic limbs have made enormous advancements in the last several years. The field of Neuroprosthetics aims to give amputees limbs that provide the natural function and movement of arms and legs, but current methods are invasive and require patients to relearn impulses to manipulate the prosthetic.

Neuroprosthetic arms are a particular challenge because of the complex ways the hand and fingers can move. Since the artificial limbs are controlled by muscle movement, nerve impulses, or a combination of both, most systems require quite a bit of practice to learn how to use. Scientists think they have the solution. They have created a system that is so intuitive that users can use the limb with no training.

University of Michigan Bioengineering Professors Paul Cederna and Cindy Chestek, co-lead a team that has figured out a way to not only make artificial limbs more intuitive but also increase the voltage sent from nerve endings. The technique involves separating large nerve clusters, then using tiny muscle grafts to boost the impulses, which are then interpreted by a “brain-machine interface.”

In tests, the prosthetics have worked on the first try. Participants can seemingly think about what they want the hand to do, and the bionic limb does it.

“You can make a prosthetic hand do a lot of things, but that doesn’t mean that the person is intuitively controlling it,” said Chestek. “The difference is when it works on the first try just by thinking about it, and that’s what our approach offers. There’s no learning for the participants. All of the learning happens in our algorithms.”

Typically researchers working on neural interfaces tap straight into the brain because that is where they can get the strongest and most stable signals. However, it is a risky and invasive procedure mainly reserved for people living with paralysis.

Using peripheral nerves in amputees is safer but comes with some hurdles, the primary being that nerve impulses are too weak. It requires the use of probes, referred to as “nails in nerves,” which are inserted into the nerve endings. This approach leaves scar tissue, which further weakens the electrical impulse over time.

The new technique grafts small muscle fibers to the nerves, which converts the tiny nerve signals into a much larger muscle impulse. The “regenerative peripheral nerve interfaces” (RPNI) do not only amplify the voltage but also give nerves new tissue to connect to and help prevent neuromas, which cause phantom limb pain.

“This is the biggest advance in motor control for people with amputations in many years,” said Cederna. “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.”

The technology is not ready for commercial use, and the participants cannot yet keep the prosthetics. There are already other products one the market like the Hero Arm that perform similar functions using different technology.

The team’s paper, “A regenerative peripheral nerve interface allows real-time control of an artificial hand in upper limb amputees,” has already been published in Science Translational Medicine. However, the study is ongoing and is looking for more participants.