Sci-Fi Arm for Amputees

The project is called the ‘‘Revolutionizing Prosthetics Program’’. Although it sounds like science fiction, team members have already taught a mechanical hand to play Rock-Paper-Scissors. And a Tennessee amputee wrecked a prototype thought-controlled arm trying to pull-start his lawnmower. So engineers know the concept works.

The Defense Advanced Research Projects Agency (DARPA) has broken the job into two parts. APL hopes to have the arm’s thought-control and sensor systems ready for clinical trials in four years with an international team of 30 corporate, government and academic partners. In a parallel, two-year effort under an $18.1 million contract, DEKA Research and Development Corp of Manchester, New Hampshire, will lead a team developing the arm’s mechanical and cosmetic components.

The target price for the final product is $30,000 to $50,000 a copy. ‘‘It’s going to cost about the same as a new car, ’’ Harshbarger says. There’s a growing market for these though. Through January 2006, the Pentagon says, 387 military personnel had been treated at Army hospitals for the loss of hands, feet, arms or legs. Some are multiple amputees. ‘‘We suddenly found ourselves with a number of young Americans who are badly injured and didn’t have the prostheses they would need, or we would want them to have, in an ideal world,’’ says Col. Geoffrey S F Ling, a physician and veteran of Iraq and Afghanistan managing the programme for DARPA.

While today’s best prosthetics offer amputees more than simple hooks or cosmetic hands, they’re far from ideal. For example, a German company called Otto-Bock Healthcare (a partner in the new project) makes a three-fingered hand with a simple pinch grasp. The wrist can rotate and the elbow can open and close. Its movements are controlled by myoelectric signals that originate in the patient’s own muscles, picked up by electrodes on the skin. Users contract biceps if they have them (or back or chest muscles if they don’t) to generate an electrical signal that activates the artificial limbs. To tighten a grip, they repeat the contraction. ‘It’s not very natural to open and close your hand with your pectoral muscle or back muscle. But that’s what (today’s) standard of care is,’’ says Dr Todd Kuiken, director of neuroengineering at the Center for Artificial Limbs at the Rehabilitation Institute of Chicago — a partner in the APL program.

Many amputees don’t like the cable-operated hooks they’re offered. They opt instead for a nonfunctional but more cosmetically appealing prosthesis. So the goal for developers is an arm and hand as much like the real thing as possible. Weight, too, is critical. DARPA’s limit is 8 pounds, the weight of a natural arm. But ‘‘the perception of weight is much different for an amputee,’’ Harshbarger says. The team’s goal is an arm with 22 ‘‘degrees of freedom’’. That means 22 ways to move the shoulder, elbow, wrist and fingers. The best prosthetics now in development offer only six. ‘‘Ideally, the device would grant an amputee the fine motor control necessary to thread a needle, use a computer keyboard, play a piano or perform fretwork on a guitar,’’ the agency’s planning documents declare.

The biggest nut to crack, Ling said, will be wiring the mind directly into the prosthesis. Engineers will have to interpret motor signals from the brain, then translate them instantly and accurately into deliberate movements. ‘‘A lot of these folks are still in their 20s ,’’ says Harshbarger. ‘‘The Pentagon felt it was important to do whatever is possible to give them back a full range of abilities.’’ (Frank D. Roylance)