For people who use an artificial arm or leg—and the designers who build them—natural movement and effortlessness are the goal.
You can choose an electric wrist that turns 360 degrees or a smooth, flesh-colored, look-alike limb, but prosthetist Ryan Spill says, either way, amputees often ask for the same thing.
“Faster, lighter, stronger. That’s what people want. They want everything to be faster, lighter, stronger,” Spill said.
Spill custom fits artificial limbs at Advanced Arm Dynamics in Philadelphia. For one client born without her right forearm, he says a basic hook, harness and loop device feels most natural.
“When she kind of shrugs her shoulder, it pulls on a cable that opens the hook, and when she brings her shoulders back, it relaxes the cable and allows the hook to close on something,” Spill said.
Newer, souped-up prosthetics can detect the small spark of electricity that’s produced when a muscle contracts.
“It’s transmitted into a signal that tells an artificial hand whether to open or close, an elbow to go up or down, or a wrist to twist in or twist out,” Spill said.
Tightening a bicep muscle to move a hand is state of the art but not very natural.
“You can imagine how unintuitive that is to contract your chest muscle, or upper back muscle to contract an elbow or hand. People learn how to do it and they do great with it, but it’s just not that intuitive,” Spill said.
Future replacement limbs—some call them ‘smart’ prosthetics—could be hard-wired to the brain.
“That people can think: ‘Elbow up. Elbow down.’ and it responds that way. That’s the major benefit of having something controlled by the brain,” Spill said.
Right now, most replacement limbs rely on muscles to get them moving. Engineers at Drexel University are working with three other universities to design an arm device that amputees can control directly with their brain.
Movement scientist Patricia Shewokis says the first step is to understand how the brain communicates before a limb is lost.
“You know you have measures of heart rate, you have measures of blood pressure, but what we don’t have is a baseline measure of the brain,” she said.
The Drexel team developed brain imaging technology that maps the prefrontal cortex. When we put forth effort—think or move—different areas of the brain light up. Shewokis’ team uses near-infrared light to chart brain activity linked to decision making and initiating motion.
“Prosthetics that are in place typically use muscles, let’s say, from the back or from the opposing limb to control this device. We want, eventually, to have the brain take over and essentially become in the loop,” she said.
The collaboration, funded by National Science Foundation, may also give artificial limbs the ability to perceive temperature or texture. Prosthetist Ryan Spill says that’s on his clients’ wish list, too.