Goldfarb reckoned that current battery tech was never going to offer an arm with decent strength and endurance at a reasonable weight. He’d previously run up against these limitations working on a DARPA exoskeleton project, and he reckoned he had the solution: the arm needed to be powered by a small rocket motor.
No, really. Goldfarb’s design uses high-test hydrogen peroxide fuel, formerly used mainly in rockets and torpedoes. Concentrated peroxide breaks down in the presence of a catalyst to produce high-pressure steam and oxygen. In Goldfarb’s arm, the vapour is used to drive pistons, just as in a steam engine: but rather than turning a wheel, the various pistons heave in or let out tension belts against springs so as to bend or straighten the elbow, wrist, fingers, etc.
According to Goldfarb, 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.
There have been a few little problems, though. The steam comes out of the generator unit at oven-like 230-degrees-C temperatures, making parts of the arm baking hot. But Goldfarb and his team have used strategically-placed insulation “to reduce surface temperatures enough so they are safe to touch”. An initial tendency to hiss and chuff noisily was fixed, too.