Scientists at Vanderbilt University are developing a prosthetic arm with 10 times the power of currently available devices. Powered by a rocket motor, the prototype arm weighs about the same as a natural arm and can curl 20 to 25 pounds.
Unlike conventional prostheses, which can bend only at the elbow and the clawlike hand, the new arm has independently movable fingers and a bendable wrist. But that additional functionality requires a more efficient energy source. According to a press release from Vanderbilt,
At a certain point, the weight of the batteries required to provide the energy to operate the arm for a reasonable period becomes a problem. It was the poor power-to-weight ratio of the batteries that drove [Michael] Goldfarb to look for alternatives in 2000 while he was working on a previous exoskeleton project for DARPA. He decided to miniaturize the monopropellant rocket motor system that is used by the space shuttle for maneuvering in orbit. His adaptation impressed the Johns Hopkins researchers, so they offered him $2.7 million in research funding to apply this approach to the development of a prosthetic arm.
Goldfarb’s power source is about the size of a pencil and contains a special catalyst that causes hydrogen peroxide to burn. When hydrogen peroxide burns, it produces pure steam. The steam is used to open and close a series of valves. The valves are connected to the spring-loaded joints by belts made of a special monofilament used in appliance handles and aircraft parts. A small sealed canister of hydrogen peroxide that easily fits in the upper arm can provide enough energy to power the device for 18 hours of normal activity.
While it may sound scary to walk around with a rocket motor, the scientists say that they have addressed those concerns.
By covering the hottest parts with special insulating plastic, they were able to reduce surface temperatures enough so they are safe to touch. The steam exhaust was also a problem, which they decided to handle in as natural a fashion as possible: by venting it through a porous cover, where it evaporates like natural perspiration. “The amount of water involved is about the same as a person would normally sweat from their arm in a warm day,” Goldfarb says.
