Artificial Limbs That Feel

Major innovations in prosthetic technology have greatly expanded the functionality of anatomical replacements. Some of the most exciting advances have resulted from interfaces that allow commands from the brain to be delivered directly to artificial limbs. This research could deliver sensation to prosthetics, as if they are living parts of the body.

Biomedical research led by the Revolutionizing Prosthetics program of the Defense Advanced Research Projects Agency (DARPA), and other programs, is helping those with lost limbs make a giant reach forward. According to findings reported in September 2015, a man who had been paralyzed for more than ten years by a spinal injury was able to feel things with a robotic hand. This was made possible by sensory receptors developed by the Applied Physics Laboratory at Johns Hopkins University. The man could identify which fingers of his artificial hand were being touched, with almost 100 percent accuracy. Moreover, he claimed that the sensations he felt were the same as what he would feel with his own hand.

This capability is made possible by a sequence of communications from brain, to computer, to prosthetic hand, and then back to the brain. Electrodes are placed over two areas of the brain: 1) the sensory cortex, which identifies tactile sensations, and 2) the motor cortex, which directs body movements. Wires connecting the electrodes to the mechanical hand then enable users to direct the hand’s movements with their own thoughts.

Sensors put into the prosthetic allow detection of pressure applied to the fingers. The detected sensations are then translated into electrical signals transmitted to the brain. As reported in The Washington Post, DARPA program manager Justin Sanchez says, “By wiring a sense of touch from a mechanical hand directly into the brain, this work shows the potential for seamless bio-technological restoration of near-natural function.”

Other research teams have also developed prosthetics that can feel. In 2014, scientists at Case Western Reserve University and the Louis Stokes Cleveland Veterans Affairs Medical Center created a robotic hand that established sensation over multiple points across the prosthetic. This extensive sensory landscape helped simulate the feeling of a real hand. In one experiment, a man who was blindfolded could feel a cotton ball brushing the back of his prosthetic hand. It was much more than a vague sensation – he could tell right away that it was cotton.

Adding genuine sensation also helps enhance the functional value of a prosthetic because users knows how much pressure they are exerting on an object. It also helps people feel that the limb replacements are more a part of themselves. Dustin Tyler – associate professor of biomedical engineering at Case Western Reserve, who directed the research – says, “Our goal is not just to restore function, but to build a reconnection to the world.”

Prosthetic limbs with sensory capability have also been shown to alleviate phantom limb pain, a common problem following amputations. Phantom limb pain may occur immediately after a limb is lost or even later, and may include tingling, throbbing, piercing, and pins and needles. This pain is thought to be the result of the brain seeking signals from a missing limb. The prosthetic may help overcome this pain, by providing familiar sensations based on what is actually being touched. In one experiment, Austrian scientists reported that a man had no more phantom limb pain after being equipped with an artificial leg that had sensory capacity.

Feeling prosthetics may also have capacity to learn

A key capability under further development is pattern recognition, which equips prosthetics to learn based on the user’s daily experience. This can allow the artificial limb to better interpret signals and improve its response to the user’s intentions over time; in effect, the prosthetic becomes customized to the person. Neuroscientist Dr. Paul Marasco – who specializes in sensation and prosthetics at the Cleveland Clinic’s Lerner Research Institute – explains, “We can get people to view the limb as if it actually belongs to them,” National Geographic reports.

DARPA and other research teams are continuing to develop neurotechnologies that overcome challenges faced by those with different forms of paralysis. These efforts will lead to a future of reclaimed function and feeling for war veterans and others who have lost one or more limbs – a future where they can feel more connected to the world.