Four years ago, Igor Spetic lost his right arm in an industrial accident. Doctors outfitted him with a prosthetic arm that restored some function, but they couldn't restore his sense of touch. Without it, simple tasks like picking up a glass or shaking hands became hit-or-miss propositions. The lack of touch also robs Spetic of basic pleasures. “I would love to feel my wife’s hand,” he says. In time, he may regain that pleasure: Two independent research teams have now equipped artificial hands with sensors that send signals to the wearer’s nerves to recreate this missing sense.
The sensing technologies work only in the lab, but they have proved durable, and amputees who have tried them, including Spetic, say that they are effective. One technology advances the range of touch sensations available, while the other promises to enable touch through a better way to attach the prosthesis. “All of these results are very positive,” says Mandayam Srinivasan, a neuroengineer at the Massachusetts Institute of Technology in Cambridge, who was not involved in either project. “Each of them fills a piece of the puzzle in terms of [prosthesis] development.”
Almost 40 years ago, researchers tried to provide sensory feedback by adding pressure sensors to prostheses that relayed the sensation through electrodes attached to nerves. But for the most part, they just made it seem like the hand was tingling. And durability has been an issue in such efforts, too. In February, Silvestro Micera, a neuroengineer at the Sant'Anna School of Advanced Studies in Pisa, Italy, and the Swiss Federal Institute of Technology in Lausanne and his team showed that it was possible for sensor-equipped prosthetic arms to gently or powerfully grab objects and even to distinguish a round from a square object. But the study lasted just 4 weeks, in part because of the delicate interface with the body.
Dustin Tyler thought he could do better. The biomedical engineer at Case Western Reserve University in Cleveland, Ohio, and his colleagues outfitted prostheses worn by Spetic and a second amputee with more than a dozen pressure sensors, the outputs of which were conveyed by wires to a computer. The computer processes all incoming signals to create specific patterns of electrical impulses that vary in duration and intensity. For Spetic, more wires relay those electrical impulses to nerves in the arm via three electrodes built into cuffs implanted under the skin. Each electrode goes to a different nerve and from there branches for a total of 20 potential points of connection. The other amputee was similarly fitted but with fewer points of connection. Depending on which point gets the signal, the brain “feels” something on a different place on the hand, say the thumb or pinkie. Tyler thinks the signals are not exactly the same as would come from a real hand, but must be close enough to trigger the particular sensation.
That ability to “feel” greatly sharpened the amputees’ ability to pull a stem off a cherry without squashing it or put toothpaste on a toothbrush, Tyler and his colleagues report today in Science Translational Medicine. And the sensor connections have lasted more than 2 years in Spetic and 18 months in the other amputee. Spetic is working on distinguishing sandpaper from smooth and ridged textures and even to tell what finger is sensing which texture. And as the researchers refine the computer processing and put more “points” on the nerves, the diversity of sensations conveyed should improve. (Watch a related video.) “They managed to move from that tingling sensation to a more natural sensation,” says Max Ortiz-Catalan, a research scientist at Chalmers University of Technology in Gothenburg, Sweden, who was not involved with the work.
Besides restoring some touch, the sensor-equipped hand also gave the amputees the sense that it was part of their body. Spetic told researchers that it eliminated the phantom pain lingering from his lost hand—which he likened to the closed fist being crushed in a vice.
Ortiz-Catalan has taken a different tack in improving prosthetic arms. Most artificial limbs are challenging to attach securely and comfortably to the body. And the wires that control the limb's movement typically enter the upper arm through the skin and are implanted directly into the nerves. This setup can be jostled out of connection and prone to infection, so that sometimes amputees just give up wearing their prostheses.
Ortiz-Catalan and his colleagues developed a way to secure these wires by taking advantage of “osseointegration,” a technique for anchoring the prosthesis directly to the bone. The prosthesis screws into a titanium post surgically implanted in the bone, much as in a dental implant. And the electrical connections are made within that coupling through connectors that go to the nerves themselves. An amputee has used this connection scheme successfully since January 2013 to go about his daily life as a truck driver and a father, the researchers report today in Science Translational Medicine. (Click here for a related video.) In the lab, Ortiz-Catalan’s team has shown that these connections can transmit touch sensations as well. This long-term success bodes well for amputees, Tyler says. “By having this direct connection [into] the bone, that’s going to be a major advance," he says.
Ultimately, Tyler's and Ortiz-Catalan's approaches might be combined. “Now we can borrow their findings to improve our sensory part,” Ortiz-Catalan says. “We want to make [the sensing technology] reliable and small enough that the patient can use it at home.”
But don’t expect a touch-sensitive artificial arm to hit the market any time soon, Micera says. “If you look at both papers, there’s no evidence that they can use their approach to do sophisticated tasks in real time.” he says. But the work “is going in the right direction in improving the quality of life of these people," he says, "which is the most important thing.”