Thanks to a new generation of prosthetic arms, patients can take a sensory step forward and “feel” again.
With approximately 10 to 15 million amputees in the world, those without arms or legs make up a significant percentage of our society. But these numbers don’t make everyday existence any less of a challenge. Thanks to modern prosthetics, the loss of a limb is now far less devastating than it once was. In fact, the burgeoning capabilities of today’s artificial limbs—especially arms—mean that many amputees are now enjoying the sensation of touch for the very first time.
Most prosthetic arms on the market today fall into two categories. Some are powered by the body, in which case the wearer controls the prosthetic through cables and harnesses. Myoelectric devices, which are far more sophisticated, are controlled by electrical signals from muscles attached to the prosthetic. Although they cost a lot more than cable-and-harness systems, many users find that myoelectric prostheses provide a greater degree of flexibility and control, as well as a more natural appearance.
Myoelectrics in Motion
Based in Leeds, U.K. Steeper Group offers one of the most advanced myoelectric prostheses on the market. The company’s battery-powered bebionic small hand is the latest model in a long line of prosthetic hands, and is the first advanced myoelectric hand to be specially designed for women and teenagers, with small proportions and a lightweight structure.
Trained users can type on a keyboard, peel vegetables, and even dress themselves.
As with other myoelectric devices, the bebionic small hand is operated through muscle contraction. Muscle twitches in the upper arm are detected by sensors that trigger one of 14 pre-programmed grips, mimicking natural human movement.
Such grips include a clenched fist, a pointed finger and a pincer, and are stronger or weaker according to how the muscle is flexed. Trained users can type on a keyboard, peel vegetables, and even dress themselves.
“With its human-centered profile, the bebionic small is the most lifelike and anatomically correct multi-articulating myoelectric hand on the market,” said Ted Varley, Steeper’s technical director.
Mind Over Matter
Thanks to constantly advancing technology, the latest myoelectric prostheses boast astonishing versatility. Elbows that flex and extend. Wrists that bend and rotate. Hands that can hold a heavy suitcase or grip an egg without cracking it. Thumbs that can change orientation to create numerous hand positions.
Yet despite this level of kinetic complexity, myoelectric devices are still constrained by one overriding deficiency—an absence of sensory feedback. Forced to rely on other senses (primarily vision), users must keep the prosthetic limb in sight to control it. It may be a useful technical aid, but the limb is not perceived as a natural and true part of the body by the brain.
“This is the major challenge facing developers of prosthetic limbs today,” explained Varley. “How to empower the user with a real sense of touch. Non-invasive feedback—enabling a seamless connection and controllable flow of signals between limb, body and brain—is critical.”
Around the world, various research teams are now working on mind-controlled prosthetic arms. Last year, DARPA—the U.S. government’s defense research agency—announced the latest breakthrough: a robotic arm that the wearer can actually feel. Thanks to electrodes attached to the sensory and motor cortexes of the brain, paralyzed volunteers testing the limb were able to tell which mechanical finger had been touched, even if two were touched simultaneously.
Despite these advances, the commercialization of mind-controlled, touch-enabled prosthetics is still some way away. Scientists are only now beginning to understand how the brain generates movement signals. Further developments in processing power, software sophistication and miniaturization are also essential.
When it comes to getting prosthetic technology out of the laboratory and into the hands of people that need it, cost is another critical consideration. Some of the most advanced limb prototypes can cost nearly $500,000 each to develop. Even a commercially available myoelectric arm may have a piece tag of $50,000 or more.
Japanese company Exiii is now working hard to make advanced prosthetics more affordable. Prototypes of their HACKberry bionic limb, which hasn’t hit the market yet, only cost a few hundred dollars each to produce.
The HACKberry, whose blueprints and source code have been made available online, is mostly made up of easily replaceable 3-D-printed parts, while each finger contains just a single motor. Exiii began by experimenting with app-based smartphone technology to control their hands, but have since moved on to infrared muscle sensors.
Thanks to innovative materials and production techniques, engineering breakthroughs and advances in microtechnology, we are now witnessing prosthetics that are lighter, cheaper, more comfortable and more real than ever before. As the bionic boundaries continue to shift, both limbs and lives are sure to benefit.