Most of us equipped with complete limbs often take for granted our ability to distinguish between the feather-lightness of a cotton ball and the rough surface of sandpaper. For amputees who have had their hands or arms removed, the sensation is something they long for.
Two new studies, however, offer hope. A group of researchers in Ohio and another in Sweden were able to develop a prosthetic arm that gives its user an almost natural sense of touch and the ability to control the movements of the prosthetic with his mind.
Igor Spetic, who lost his hand in an industrial accident four years ago, became the first man to receive a prosthetic arm that allowed him to recreate tactile sensations in his hand. The prosthesis was implanted by biomedical engineer Dustin Tyler and his team at the Case Western Reserve University and the Louis Stokes Cleveland Veterans Affairs Medical Center by connecting three cuffs of electrodes around the remaining nerve bundles in his stump. Each cuff had eight electrodes, which were assigned to different parts of the nerves and connected to a machine that provided stimulation. This was then connected to Spetic's existing prosthesis.
"As soon as we stimulated the nerves in the first subject, he immediately said, 'That's the first time I've felt my hand since it was removed,'" says Tyler.
The team went on to stimulate different electrodes, producing sensation in different parts of Spetic's hands, including the thumb, the tip of the index finger, the palm and so on. All in all, Spetic was able to feel on 19 different touch points on his hand. Keith Vonderhuevel, the second subject who received his prosthesis in 2013, felt 16 different touch points.
"It was really exciting for us," says Tyler. "In the end, we managed to produce the perception of touch across the whole of the hand."
This isn't the first time a prosthesis meant to produce the feeling of touch was developed, but previous attempts had users losing the tactile sensations after a month or less of using their prosthetic arm. Spetic has been using his prosthesis for two and a half years, and Vonderhuevel for one and a half years.
Tyler's team went further to adjust the stimulation and fine-tune the sensations the subjects would feel. They were able to go as far as allowing the users to tell the differences between a sandpaper, smooth surface and ridged surface. In fact, using their prosthetic arm, the subjects were able to pluck a bunch of grapes and cherries from their stems.
"When the sensation's on, it's not too hard," says Vonderhuevel. "When it's off, you make a lot of grape juice."
The prosthesis also came with one unexpected benefit: relief from the phantom pain that usually plagues persons with their limbs cut off. Spetic, who describes the pain "like having a clenched fist put into a vice and crushed 24/7," says that he no longer feels 95 percent of the pain. Vonderhuevel agrees.
"We don't know if it's because now that they're getting natural feedback the brain is reincorporating the [prosthetic] hand, or because of other mechanisms," says Tyler.
The study is published as the cover story in the Oct. 8 issue of the Science Translational Medicine journal.
Also making it to the same edition is a related study conducted by a team led by Max Ortiz-Catalan at Chalmers University of Technology in Gothenburg. The team successfully performed a procedure called osseointegration. The procedure involves implanting a titanium rod directly into the bone and stitching electrodes attached to the implant into the nerves and muscles. These electrodes transmit electric signals to the brain, which interprets them and sends back its own signals that are translated as movements.
As a result, the Swedish man who received the prosthesis can do a variety of day-to-day activities he previously wasn't able to do with his old prosthetics, including tying the shoelaces on his children's skates, handling eggs, clamping down the load on his trailer and operating machinery.
Both teams of researchers consider their work as "investigational devices." Tyler hopes the prosthetic device can develop into something like a pacemaker, where it can be implanted in a one-day out-patient procedure.
"One day, it will be just an app on the phone," he says.
