At a lab at Johns Hopkins University, researchers are building a prosthetic hand unlike any other: It can sense pain.
It’s easy to understand why you might want a prosthesis that can feel the squishiness of a grape or the warmth of another person’s hand. But pain? Well, pain could be useful, too. “If you think about how we humans use pain, it’s to protect our bodies, to prevent damage,” says Luke Osborn, a graduate student in Nitish Thakor’s lab at Hopkins, who co-authored a new paper on the pain-sensitive hand.
People born without the ability to feel pain stumble through life with dangerous freedom. Babies who do not feel pain are known to chew their fingers raw; children without pain will plunge their hands into boiling water. Pain is a signal that says, Hey, watch out. “People do damage their prosthetic limbs a lot. They use them as tools they weren’t designed to be used as,” says Levi Hargrove, a bioengineer at the Shirley Ryan AbilityLab, who was not involved in the study. It’s easy, for example, to bang an unfeeling piece of plastic and metal against a table. Pain could make a prosthesis feel more real, more lifelike—less a tool and more like a natural part of the body.
So Osborn and his collaborators set about making an “e-dermis,” an electronic skin that fits over the fingertips of a prosthetic hand. The e-dermis is a thin layer of rubber and fabric that senses pressure. It can “feel” the difference between a small round object and a sharp, pointy one, and converts the signal to a series of electric pulses.
The man testing out this device was 29-year-old volunteer, who lost both arms after a blood infection 5 years ago. By previously stimulating different parts of his left stump, Osborn’s team figured out that an electric pulse in one place “feels” like pressure on the thumb and another like pressure on the pointer finger. Alter the kind of pulse a bit and the sensation changes from pressure to mild discomfort. (The researchers never went above three on a pain scale of one to 10, with one being pleasant and 10 being disabling pain.) And with that, the man was able to pick up round and sharp objects and distinguish between them.
“After many years I felt my hand, as if a hollow shell, got filled with life again,” the volunteer told Osborn. “I can differentiate between pain and not-pain without thinking, instinctively knowing if my arm is in danger.”
Paul Marasco, a biomedical engineer at the Cleveland Clinic, says he’s also recently heard an amputee comment that pain would help prevent accidental damage to a prosthesis. (The devices are, after all, expensive, and they cannot heal on their own like an arm.) “It certainly would contribute to the richness of the sensation,” says Marasco, and touch specifically is “crazy complex.” On a cellular level, many different types of receptors contribute to the pressure, texture, temperature, pleasure, and pain that all fall under the umbrella of touch.
Osborn and his team added one more feature to make the prosthetic hand, as he puts it, “more lifelike, more self-aware”: When it grasps something too sharp, it’ll open its fingers and immediately drop it—no human control necessary. The fingers react in just 100 milliseconds, the speed of a human reflex. Existing prosthetic hands have a similar degree of theoretically helpful autonomy: If an object starts slipping, the hand will grasp more tightly. Ideally, users would have a way to override a prosthesis’s reflex, like how you can hold your hand on a stove if you really, really want to. After all, the whole point of having a hand is being able to tell it what to do.
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