Movement and touch are parts of everyday life that most of us take for granted. We catch a ball and feel its rubbery exterior. We reach for a glass on the top shelf and feel its smooth surface without even thinking about it. All that changes, however, when a terrible accident occurs and a limb must be amputated. Current prosthetic limbs replicate the functions of a real limb to a certain extent, but the wearer cannot effortlessly coordinate the limb, nor can he feel different textures. Sensations relayed to the wearer, if any, feel unnatural, and last for no more than one month after surgery.
Researchers at Case Western Reserve University are combatting these issues with their development of what are essentially mind-controlled prosthetics. The main difference between these prosthetics and traditional prosthetics is that the biotechnology is more directly integrated with the nervous system via implanted electrodes. The new prosthetics allow for accurate movement regardless of limb position, as well refined manipulation of objects and the experience of different sensations for different textures. As Dustin Tyler, associate professor of biomedical engineering at Case Western, described, the goal was not only to restore normal limb function, but to build a reconnection to the physical world for patients who undergo amputations.
Image courtesy of Case Western Reserve University
Igor Spetic and Keith Vonderhuevel both lost their arms in accidents years ago. Now, they are the first two patients to test out the new mind-controlled prosthetics in a lab setting at Case Western. Fitted with these state-of-the-art prosthetic arms, each has been able to feel a wide range of textures and has been able to manipulate his arm with a degree of dexterity that current prosthetics do not allow. When the researchers attached the artificial arms, the patients said it was the first time they actually “felt” their hands since the accidents. Spetic distinguishes easily between sandpaper, rigid surfaces, and smooth metal, and can tell which part of the hand is in contact with each texture. Vonderhuevel can pull out the stems of grapes and cherries while blindfolded, a feat of dexterity that was nearly impossible when the researchers turned off the sensation in the artificial arm to observe the difference.
The key to the precision and consistency lies in the technological concepts behind the mind-controlled prosthetic’s development. The artificial arm uses a technology called osseointegration – the direct attachment of the prosthetic to the skeleton – which provides mechanical stability. Neuromuscular cuff electrodes are implanted so they enclose the main nerve bundles in the arm, whereas formerly, electrodes were placed on the surface of the skin. Mind-controlled prosthetics can interface with the natural neuromuscular system of the body. In this way, the prosthetics are more integrated with the body than ever before.
Image courtesy of Case Western Reserve University
The technology also incorporates patterned stimulation intensity, which approximates different types of tactile stimuli through algorithms. Because of this, neither Spetic nor Vonderhuevel experience paresthesia, or the tingling often associated with current prosthetics. The implanted cuff electrodes create 16 to 19 sensory points on the prosthetic hand, which relay stable responses to the wearer, and computer algorithms approximate neural responses to each distinct texture. In the two patients, sensations have lasted for more than one year – well over the one-month duration of ordinary prosthetics.
This mind-controlled technology went through several stages of development before it reached its current sophistication. The first sensors caused tingling, so researchers began programming algorithms intended to match textures to specific patterns and intensities of electric signals. Then, researchers started to fine-tune algorithms to develop electrical impulses closer to the neural impulses one would feel naturally when touching an object. The sensors themselves only gauge pressure, so the algorithms are key to the creation of a sensory interface.
Beyond improved tactile perception, the implanted electrodes of these prosthetics provide Spetic and Vonderhuevel with better overall coordination. In most prosthetic arms that use surface electrodes, several conditions can cause the prosthetic to move involuntarily. Moving or raising the arm quickly can cause the hand to open, and electromagnetic interference from equipment such as power tools can cause the arm to move. Implanted electrodes bypass these issues because they are more directly connected to the neurons sending the signals through the arm. Additionally, Spetic and Vonderhuevel do not have to exert as much effort to move the arm because these implanted electrodes can more accurately detect signals from the nervous system.
Image courtesy of Oritz Catalan et al., Science Translational Medicine, 2014
An unexpected benefit of these new prosthetic arms is that they almost completely relieve phantom pain. The sensation of phantom limbs occurs because of lingering nerve endings that were once connected to the amputated limb. These nerve endings continue to send signals to the brain, and the brain sometimes interprets this as pain – what neuroscientists call phantom pain. Spetic had described this pain as a vice crushing his fist – pain which the prosthetic arm now essentially eradicates. Although Vonderhuevel’s phantom pain was not as severe to start, he also reported that his pain was almost entirely gone. The researchers do not know the exact explanation behind this unexpected benefit, but they were happily surprised.
Mind-controlled prosthetics are, without a doubt, a ground-breaking development in the field of neuroprosthetics. They have the potential to improve the lives of many amputees. The sensations have lasted for two-and-a-half years for Spetic and one-and-a-half years for Vonderhuevel, meaning these prosthetics are becoming increasingly sustainable as devices for long-term sensory restoration. Currently, the prosthetics are still in the lab phase, and are years away from commercial availability. Furthermore, by nature of being cutting-edge technology, the new prosthetics are bound to be expensive.
There is also much to celebrate, as mind-controlled prosthetic arms open new doors in the field of biomedical engineering. Researchers are already looking ahead to how their development of a sensitive prosthetic arm could be translated to prosthetic legs, which would benefit greatly from a feedback system that would allow the wearer to adjust naturally to uneven ground. Moreover, the integrated system of the neuromuscular interface could be implemented in more refined deep brain stimulation, which would help control tremors in people with Parkinson’s disease.
Tyler is optimistic about this technology extending to lifetime use. Within the next five years, the team hopes to develop a home system that can be used every day, outside of a lab setting. Spetic is a truck driver in Sweden, and can do everything he needs to do at his job and at home with the new prosthetic arm. He can clamp down his trailer load, and he can tie his children’s skates.
The technology of refined, sensitive prosthetics does not only improve the mechanical ability of the wearers, but also restores a sense of self and a connection to the world. It is just as much an advancement in humanity as it is in science. Almost three years ago, Spetic lost his hand and thought he would never feel it again. This research has given him hope. “I would love to feel my wife’s hand,” Spetic said. “Just to hold hands would be the ultimate.”