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August 31st, 2007 at 9:41 am

Prosthetic Arm Powered by Rocket Fuel

An artificial arm that uses rocket propellant to power motorized muscles is being touted by its developers as the closest thing yet to a bionic limb.


Vanderbilt Arm
Michael Goldfarb, Vanderbilt University

Weighing in at about 4 pounds and able to move in 21 directions, the Vanderbilt Arm works remarkably similar to a human arm, which weighs about 7 pounds and can move in 26 directions. The idea is to eventually hardwire the prosthetic to a person’s nervous system for thought-controlled motion.

"As far as the user is concerned, it would almost be no different than the native limb," said Michael Goldfarb, professor of mechanical engineering at Vanderbilt University in Nashville, Tenn.

All prosthetic arms must deal with two factors, said Goldfarb: power and function. Power allows a person to lift an object, like a gallon of milk, or throw something, such as a baseball.

Artificial arms usually sacrifice speed for force and so amputees are often stuck with mechanical limbs that move too slowly.

Function is also sorely lacking in prosthetic arms, which typically have just two joints, an elbow and a pincher-like claw.

"We have something that is nearly as fast and powerful as the human arm and can move in almost as many ways," said Goldfarb.

The Vanderbilt Arm achieves its power and speed from an unconventional source: rocket fuel. Other mechanical arms rely on lithium polymer batteries. But as power and speed demands rise, so does battery size. And this adds weight to the limb.

Goldfarb and his team decided to miniaturize a motor system used on rockets and torpedoes. The fuel, hydrogen peroxide, is contained within a small cartridge. A chemical compound serves as a catalyst, and is held in a tube about the size of a pencil.

When the fuel passes over the catalyst, a chemical reaction causes the liquid to expand into steam. The steam is used to open and close valves connected to spring-loaded joints.

When the joints move, they do so three to four times faster than those in conventional prosthetic arms and can lift 20 to 25 pounds.

A lot of energy can be stored in the liquid fuel, "and that’s the attraction of this system," said Richard Weir, research associate professor at Northwestern University and an expert in designing prosthetic hands and arms.

But the fuel presents some regulatory and logistical hurdles that research cannot overcome.

For example, "The FAA is squeamish about letting hydrogen peroxide onto airplanes," said Weir, because it can be used to make a bomb. And buying the fuel is not something one can do easily. "You can’t walk into a pharmacy and ask for a cylinder of hydrogen peroxide to power your arm," he said.

Goldfarb acknowledges those problems. "If the FAA won’t let you travel with four ounces of toothpaste, they certainly won’t let you travel with four ounces of peroxide," he said. An alternative, he said, could be to power the arm using a CO2 cartridge while in flight and switch over to the hydrogen peroxide when back on the ground.

His team will be moving into phase two of the research later this fall.

Via: Discovery Channel


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