Researchers work to create a sense of touch in prosthetic limbs

A team at the University of Pittsburgh is trying to make prosthetic limbs that work like the one in a Star Wars movie.

After Luke Skywalker loses a hand in a lightsaber fight, “They give him this new hand, and you can’t tell that it’s not his own,” says Lee Fisher, a biomedical engineer.

Luke even says “ouch” when a medical droid prods his prosthetic finger.

“That’s our long-term goal,” Fisher says, “to restore sensory feedback from the missing limb.”

The human brain relies on a constant stream of tactile information to carry out basic tasks, like holding a cup of coffee. Yet some of the most advanced motorized limbs — including those controlled solely by a person’s thoughts — don’t provide this sort of feedback. As a result, even state-of-the-art prosthetics can often frustrate their users.

Fisher is one of more than 80 scientists, staff and trainees at the university’s Rehab Neural Engineering Labs who is working to add the sensation of touch to prosthetics. The goal is to equip artificial hands and feet with sensors that are linked to a person’s own nervous system.

Fisher’s lab, for example, is linking prosthetic arms and legs to a device implanted in a person’s spine.

“It basically looks like a spaghetti noodle,” he says. “They can be inserted through a needle, so it’s a pretty minimally invasive process to put them in.”

The device was originally designed to ease chronic pain by delivering electrical pulses to the spinal cord. But Fisher’s lab is using it to relay information from sensors to a prosthetic hand or foot.

The trick is to stimulate the same nerve fibers that were once connected to the person’s own limb, Fisher says. That requires some trial and error.

“The first thing we do is just try and understand, ‘What did the stimulation feel like?'” he says. “Can we generate a sensation that feels like it’s coming from their missing hand or from their missing foot? Can we change how intense it feels?”

“There’s no hand there, but I can feel it”

A study of four people suggests the answer is yes. Pat Bayne, a participant whose right arm was amputated to stop an infection, describes what the stimulation feels like in a video made by the university: “I know there’s no hand there, but I can feel it,” she says. “They can make the palm of my hand feel like it’s the palm of my hand. It’s pretty exciting.”

Participants also report that the stimulation reduces the perception of pain coming from a missing limb — a common problem after an amputation.

Fisher’s team is now also working to use the spinal implants to provide sensory feedback from artificial legs and feet.

The addition could make these prosthetic limbs more useful, Fisher says, because we rely on constant feedback from our feet just to stay upright. “We’re basically like an upside down pendulum that you have to keep moving around to maintain balance,” he says.

Preliminary results suggest that at least one person using a prosthetic foot was helped by the feedback.

“We saw what look like improvements in her balance control during standing, her stability while she’s walking, and also maybe some improvements in her confidence as well,” Fisher says.

Reaching out with the mind alone

People who are paralyzed could also benefit from artificial limbs with a sense of touch, says Jennifer Collinger, an associate professor in the university’s department of physical medicine and rehabilitation.

For several years, the Pittsburgh group has been working with paralyzed volunteers who have learned to control a robotic arm using just their thoughts.

The goal is to develop technology that would allow them to be more independent, Collinger says. “What we’re moving toward is being able to feed yourself, being able to make a meal, being able to get dressed,” she says.

But tasks like that will be difficult if a person has to rely solely on their eyes to know what the robotic arm is doing. So the Pittsburgh scientists are adding the same kind of touch sensors they use to enhance prosthetic arms. But in this case, the sensory information is being delivered directly to the brain instead of going through the spine.

A study of one person found that a sense of touch makes a big difference, says Robert Gaunt, a biomedical engineer.

“It cuts in half the time it takes somebody to pick up objects and move them around,” he says. And in some cases, the person completed a task nearly as fast as an able-bodied person.

So far, scientists can only offer a very basic sense of touch to people who use prosthetic limbs.

The feedback is good enough to know when a foot has weight on it or a hand has encountered an object, Gaunt says. But users often describe the sensation as a vibration, buzzing, tingling or pressure.

“The information we’re able to provide is definitely not a perfect replacement for what they lost,” Fisher says.

The information will improve as new sensors arrive and scientists find better ways to connect them to a person’s nervous system, Gaunt says. But it won’t match the sensitivity of Luke Skywalker’s prosthetic hand anytime soon.

“Our ability to discriminate [among] different types of objects, textures, surfaces, that’s a hard problem,” Gaunt says. He’s hoping, though, that it’s not unattainable.