Improved Prosthetic Hand Has A Lighter Touch And Easy Grip

Enlarge this image toggle caption Dan Hixson/University of Utah College of Engineering Dan Hixson/University of Utah College of Engineering

Keven Walgamott, a real estate agent who lost his left hand and part of his arm in an electrical accident in 2002, got emotional when he was able to clasp his hands together and "feel" the space between his thumb and his index finger using a 3D-printed prosthetic hand in 2016.

"That was the first time I ever felt anything in my left hand since my amputation," Walgamott says. He got emotional a second time when he was able to grasp his wife's hand and "feel" her touch, this time through a newly designed, experimental prosthetic hand that can be directed with the user's mind.

Walgamott was the sole volunteer for a 14-month study conducted by a multidisciplinary team based at the University of Utah, and also included collaborators from the University of Chicago and the Cleveland Clinic. The findings were published Wednesday in the journal Science Robotics.

The research team adapted a commercially developed prosthetic named the "LUKE arm" — after the robotic hand the character Luke Skywalker got in the Star Wars film The Empire Strikes Back.

The original LUKE arm had been designed to perform a variety of movements, but the Utah collaboration significantly improved it by figuring out how to mimic the way the human brain sends a signal to the hand to touch an object, and how the hand sends a signal back in response to touching it.

"The nerves that communicate and control the hand and the nerves that send information back up to the brain still exist even after the hand has been amputated," explains Jacob George, a doctoral student of biomedical engineering at the University of Utah and one of the lead authors of the study.

The research team implanted electrodes in some of those nerves in Walgamott's forearm and then recorded the signals that occurred when he thought of moving his hand to grab or touch an object. The prosthetic hand moved as commanded by Walgamott's brain.

Better feedback between prosthetic and brain

"The whole 'using feedback for prosthetics' idea has been floating around for a while and people just haven't shown whether or not it works," says Sharlene Flesher, postdoctoral research associate at Stanford University, who also studies tactile feedback in prosthetic robotic limbs, but was not involved in this study. "The fact that they got everything to work is not a trivial result."

Other existing prosthetics can also convert brain signals into movement. But doing daily life tasks depends not only on our ability to grab objects but also on our ability to sense through touch what those objects are without looking at them.

For instance, if you try holding an egg in your hand while blindfolded, you'll likely know right away that you have to handle it carefully to avoid breaking it. This is because the nerves in your hand send signals to your brain that tell you — based on past experience and characteristics such as shape, texture and heft — that you're holding an egg. But with most prosthetic hands you'd need visual clues to tell you how strongly you could grip the object to avoid dropping it or crushing it.

The University of Utah researchers and their colleagues were able to restore Walgamott's ability to "feel" what he was touching by providing stimuli through electrodes implanted in his arm; the information was quickly transmitted back up to the brain as if it were coming directly from the hand.

By recovering some of his sense of touch in this way, Walgamott was able to precisely distinguish between large and small and soft and hard objects after roughly 10 seconds of handling them — even when his eyes or ears were covered.

"We're tapping into the same [mechanism] that's used in my body and your body and everyone's body, and we're trying to just activate it in the way it would have normally been activated. So the sensations feel like they're coming from their hand," says George.

Walgamott says the sensations he experienced with the robotic hand didn't feel exactly the same as those from his other hand, but he still found the ability to touch, feel and manipulate fragile objects like grapes and eggs astounding.

"Being able to pick up an egg without crushing it was a remarkable experience," he says, and the skill translated to challenges he faces in daily life. Using the prosthetic in the lab, he was able to feed himself and load a pillow into a pillow case, and put on his wedding ring.

"This study kind of pushes the importance of why we should have sensorized prosthetics," says Flesher. Beyond all the remaining technical challenges, she says, getting insurance companies to recognize the importance of such devices will be a key hurdle to overcome.

Insurance companies "think of it as a luxury," Flesher says. "But [a sense of touch] really is so critical to how you control your arm."

Prosthetic hands that can restore one's sense of touch are currently not available outside the lab — Walgamott participated in the study for 14 months, wearing the prosthetic device for only a few hours per session during that time, and only in the lab. It connected to his arm via surgically implanted wires that were removed when the 14-month-long study was completed.

Using the device required a researcher's direct supervision. For the LUKE Arm to be routinely usable at home, researchers would like to come up with a device that can be implanted inside the body, like a pacemaker. Though Utah companies are working on a wireless version of the device, it has only been tested in animals so far.

George and his collaborators hope next to get approval from the Food and Drug Administration to begin testing their modified LUKE arm in larger groups of people, and in the participants' own homes instead of the lab. They are now testing a prosthetic arm whose electronic controls can be strapped to the waist, which would make it easier for patients to use without supervision.

'Phantom pain' temporarily eased

Walgamott's burning phantom pain — a pain coming from the hand that is no longer there – also was reduced while his prosthetic hand was turned on.

"[We are] recreating that sense of touch that they would normally be getting," George explains. "When we start to do that, we think that we're recreating the sense associated with that hand, [which] can kind of get rid of the pain."

Unfortunately, Walgamott says that the pain was reduced for only about an hour after taking the prosthetic off, and then the pain returned with the same intensity as before. He continues to feel that pain today.

"Getting rid of phantom pain is one of the hopes," a member of the research team told Walgamott during the experimental trial, according to a study transcript. "It's not a promise. But if we restore sensory and motor function, then the prosthetic hand will become your hand, and the phantom will not have a place anymore."

Luisa Torres is a AAAS Mass Media Fellow on NPR's science desk. She's on Twitter at @luisatorresduq.