The bionic arm is one of three projects under way to create a good-as-original prosthetic limbs under the Defense Advanced Research Projects Agency's Revolutionizing Prosthetics program.

Vanderbilt's project accounts for $2.7 million of the $30 million Darpa initiative. Program leader Johns Hopkins University (Baltimore) is developing two more-conventional, battery- and electric-motor-powered prosthetic designs. And a team from the University of Utah (Salt Lake City), the California Institute of Technology (Pasadena) and the Rehabilitation Institute of Chicago, is developing implanted electrodes that connect to the same nerve endings in amputee's stumps that would ordinarily drive their arm and hand muscles.

Vanderbilt professor Michael Goldfarb inspects rocket-powered prosthetic arm.

This fall, Darpa will evaluate Vanderbilt's rocket-powered arm against the more-conventional approaches from Johns Hopkins and decide which of the designs will receive phase two funding for two more years, with the goal of having a working bionic arm on the commercial market by 2009. Though Vanderbilt's invention may be truer to life in its functioning, even its developers concede that the hurdle of obtaining regulatory approval to develop a distribution channel for the fuel could sway favor to Johns Hopkins' approach.

Prosthetic limbs today use electric motors and surface electrodes. Optimal functioning lets the wearer slowly lift about 6 pounds. By switching to on-demand rocket-engine power and an array of implanted muscle sensors, Vanderbilt claims, bionic arms can be made to respond faster, with more flexibility and strength. Professor Michael Goldfarb's lab at Vanderbilt has been working on a rocket propellant scheme that lifts four times as much weight—24 pounds—2.5 times faster than an electric motor, for about 10 times as much power as battery-powered arms.

"We think our rocket-powered version will offer the closest thing to a real arm," said Michael Goldfarb, a professor at Vanderbilt. "But it also offers the greatest obstacles to commercialization."