DIY Friday: rocket-powered bionic arm

What used to be just a science-fiction plot-line is quickly becoming reality.

Wired profiles what it calls "The World’s Most Advanced Bionic Arm" — a new design by Johns Hopkins University’s Applied Physics Laboratory (APL).

Jonathan Kuniholm’s right arm terminates in a carbon-fiber sleeve trailing cables connected to a PC. He has no right hand, unless you count the virtual one on a display in front of him. The CG hand, programmed to look like silvery stainless steel, moves through a sequence of motions: spherical grasp, cylindrical grasp, thumb to forefinger — all in response to signals from Kuniholm’s muscles picked up by electrodes in the sleeve.

 

One of the largest challenges in this field is mimicing the strength of the human arm. This is where rockets come into play (link):

Mechanical engineers at Vanderbilt University are showing off a working prototype of a prosthetic arm whose "monopropellant rocket motor system" allows it to lift up to 25 pounds, more than three times as much as today’s prosthetic limbs.

The device, whose research and design was funded by the Defense Department, runs on a miniature version of the same motor system used to maneuver the Space Shuttle in orbit; the system works by mixing a chemical catalyst with hydrogen peroxide, producing steam, which is then pushed through a series of valves to cause the arm to move.

The researchers say their fuel system is superior to the traditional method of powering prostheses, batteries. Batteries are heavy relative to the power they produce; the rocket-powered arm, says Michael Goldfarb, the professor who led the team, produces more power with less weight than limbs that use other power sources.

The prototype also produces more natural movement that conventional prosthetic arms. Instead of two joints — typical arms only move at the elbow and at the "claw" — the new device has fingers that can open and close independently of each other, and a wrist that twists and bends.

But who wants a rocket attached to their body? What about the heat? The Vanderbilt team figured it out:

One of their immediate concerns was protecting the wearer and others in close proximity from the heat generated by the device. They covered the hottest part, the catalyst pack, with a millimeter-thick coating of a special insulating plastic that reduced the surface temperature enough so it was safe to touch. The hot 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 condenses and turns into water droplets. “The amount of water involved is about the same as a person would normally sweat from their arm in a warm day,” Goldfarb says.

To allow for thermal expansion, the engineers replaced the arm’s nine valves with a set machined to a slightly lower tolerance, approximately 100 millionths of an inch. But when they began operating the rebuilt arm, they found that it hissed and leaked. At first, they thought that the arm had only a single leak, and spent several weeks trying to track it down. Finally, they realized that the noise and leakage were coming from all the valves. Replacing the high-precision valve set solved the problem. “We were astonished at by the difference between 50-millionths and 100-millionths: It made all the difference in the world,” says Goldfarb.

Their biggest problem operating with hot gas turned out to be finding belt material that was strong enough and could withstand the high temperatures involved. They tried silk surgical sutures, but found that silk wasn’t strong enough. They tried nylon monofilament, which is stronger than steel, but it couldn’t take the heat. Finally, after a long process of trial and error, they found a material that works: the engineering thermoplastic polyether ether ketone.

A video on the project is available here.

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