For decades, amputees have been able to open and close prosthetic hands by twitching muscles and activating a superficial electromyogram (sEMG), but they had no way of feeling what the prosthetic was encountering and little control over the strength of their grasping grip. It was a clunky, incomplete solution.
But recently, an amputee who allowed European researchers to plug electrodes into a bundle of his wrist nerves was able to control the strength of a prosthetic hand's grip and to distinguish the shapes and stiffness of three kinds of objects. The 30-day trial marks a success for one of several new experimental ways of giving amputees a better sense of touch and control over their prosthetics. A different group has conveyed sensations of temperature and vibrations by moving arm nerves into intact muscles of the chest, which act as biological amplifiers of the nerves' tiny signals. Another team tapped into nerves in the lower spine of cats to control their limbs. The Defense Advanced Research Projects Agency, is also seeking to improve sensation and control of its advanced prosthetics.
In the latest trial, appearing in Science Translational Medicine today, biomedical engineer Silvestro Micera of the Swiss Federal Institute of Technology in Lausanne (EPFL) and a large team surgically attached electrodes from a robotic hand to a 36-year-old volunteer's median and ulnar nerves. Those nerves carry sensations that correspond with the volunteer's index finger and thumb, and with his pinky finger and the edge of his hand, respectively. The volunteer controlled the prosthetic with small muscle movements detected by sEMG, a method that dates to the 1970s and measures electrical signals through the skin—unlike the electrodes attached to his nerves, sEMG is not invasive.
"They make an important contribution to show that direct electrical feedback through the peripheral nerve is important for improved control of a prosthetic hand," says biomedical engineer Dustin Tyler of Case Western Reserve University in Cleveland, Ohio, who is working on other neural prosthetics.
Over the course of seven days, Micera's team asked the volunteer to grasp something with a light grip, a medium grip, and a hard grip. They also asked him to evaluate the shape and stiffness of three kinds of objects. During the 710 tests the volunteer completed, he wore a blindfold and earphones so that he could not use his vision or sound to guide the prosthetic. The researchers also sometimes turned off the sensory feedback to test whether he was just using time to modulate his grip.
The volunteer was able to complete the requested tasks with his prosthetic thumb and index finger 67 percent of the time the first day and 93 percent of the time by the seventh day of the experiment, Micera and colleagues report. He found the pinky finger harder to control: he was only able to accomplish the requested grip 83 percent of the time by the end of the experiment. In both the grip strength tests and in detecting the stiffness of objects, the volunteer made mistakes with the medium setting and object, but he never confused the softest and hardest objects. The ability to modulate his grip strength is this study's main progress over previous work by the same group (as covered in a 2010 Discovery News video and 2014 EPFL video.)
Case Western's Tyler and his colleagues take a different approach: they wrap the median and ulnar nerves, together with the radial nerve, in a cuff. That makes it less invasive, which is beneficial for long-term implants, but may be less precise. Late last year, Tyler's team completed an 18-month trial, as reported Technology Review, and their study is now undergoing peer review. The cuff electrode, which slowly presses the enclosed bundle of nerves until even the inner fibers are available for contact with electrodes, even managed to convey texture sensations. "The human hand is capable of many different sensory perceptions," says Tyler, and he and other groups are trying to convey as many of those as possible.
Biomedical engineering doctoral student Max Ortiz of Chalmers University in Gothenburg, Sweden, who works on prosthetics attached to human bones, says that Micera's method offers promise for fine-tuning prosthetics control. The next step, say both Tyler and Ortiz, is to determine whether the more invasive electrode attachment can endure longer in a larger number of patients. If so, Ortiz says, "we would be happy to use them."
Photos: Patrizia Tocci/LifeHand 2