A new robotic prosthetic leg prototype offers a more natural gait while also being quieter and more energy efficient than other designs, researchers report.

The key is the use of new small and powerful motors, originally designed for a robotic arm on the International Space Station.

The streamlined design offers a free-swinging knee and regenerative braking, which charges the battery with energy captured when the foot hits the ground. This feature enables the leg to more than double a typical prosthetic user’s walking needs with one charge per day.

“Our prosthetic leg consumes approximately half the battery power of state-of-art robotic legs, yet can produce more force,” says Robert Gregg, an associate professor of electrical and computer engineering at the University of Michigan and a member of the university’s Robotics Institute, who led the study while at the University of Texas at Dallas.

Using conventional prosthetics, amputees must raise their hips to lift the prosthetic foot from the floor and swing the leg forward. This unnatural gait takes more energy than ordinary walking, causes extra stress and pain in the hips and lower back, and eventually damages the joints. Robotic legs have the potential to provide a much more comfortable gait, but one of their drawbacks is stiffness in the joints.

“We designed our joints to be as compliant, or flexible, as possible,” says Toby Elery, recent doctoral graduate from UT Dallas and first author of the study on the work in IEEE Transactions on Robotics.

“Our robotic leg can perform and even react like a human joint would, enabling a naturally free-swinging knee and shock absorption when contacting the ground.”

Motors in robotic legs need to fit into the space that an ordinary limb would take up. In the past, this has meant using small motors that spin quickly, and then using a series of gears to convert the fast spin into a more powerful force.

The problem is that the gears are noisy, inefficient, add weight, and make it harder for the joints to swing. Gregg’s group surmounted this by incorporating two of those stronger space station motors, one powering the knee and the other powering the ankle.

There are many benefits to using fewer gears. In addition to enabling the free-swinging knee, removing gears brought the noise level down from the scale of a vacuum cleaner to a refrigerator. Also, the regenerative braking absorbs some of the shock when the prosthetic foot hits the ground.

“If the joints are stiff or rigid, the force is transferred to the residual limb, and that can be painful,” Gregg says. “Instead, we use that force to charge the battery.”

The amputees who test drive the prosthetics in Gregg’s lab say they can feel the leg helping them push off the ground as they walk.

“In some cases, they have observed that they feel like muscles in their hips and back are working less with our leg, compared to their conventional leg,” Gregg says. “We’re able to reduce compensations at the hips.”

The team’s next step is to improve the control algorithms that can help the leg automatically adjust to different terrain, changes in pace and transitions between different types of activity.

Funding for the study came from the National Institutes of Health, the National Science Foundation, and the Burroughs Wellcome Fund.

UT Dallas and the University of Michigan are jointly pursuing patent protection. As Gregg continues his work, UM Tech Transfer is actively seeking commercial partners to help bring the technology to market.

Source: University of Michigan

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Prosthetic offers real-time mind control of robotic hand

Researchers have tapped faint, latent signals from arm nerves and amplified them to create a prosthetic that enables real-time, intuitive, finger-level control of a robotic hand.

To achieve this, the researchers developed a way to tame temperamental 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,” says Paul Cederna, professor of plastic surgery at the University of Michigan 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.”

 “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.”

“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,” says Cindy Chestek, associate professor of biomedical engineering in the College of Engineering.

“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.”

The researchers report their results with four study participants using the Mobius Bionics LUKE arm in Science Translational Medicine.

Nerves get a ‘megaphone’

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 a version of Rock, Paper, Scissors called Rock, Paper, Pliers.

“It’s like you have a hand again,” says 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 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 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.

It also 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 says. “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.”

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

The future of prosthetics

The findings also open up new possibilities for the field, says 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 says.

The approach generates signals for finer movements than what today’s prosthetic hands are capable of.

“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,” says Philip Vu, a research fellow in biomedical engineering and first author of the paper.

A clinical trial is ongoing and the team is looking for participants.

“So many times, the things we do in a research lab add to the knowledge in the field, but you never actually get a chance to see how that impacts a person,” Cederna says. “When you can sit and watch one person with a prosthetic device do something that was unthinkable 10 years ago, it is so gratifying. I’m so happy for our participants, and even more happy for all the people in the future that this will help.”

“It’s going to be a ways from here, but we’re not going to stop working on this until we can completely restore able-bodied hand movements. That’s the dream of neuroprosthetics,” says Chestek.

DARPA and the National Institutes of Health funded the work.

Source: University of Michigan

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