In February, the Defense Advanced Research Projects Agency (DARPA) authorized the next phase of a four-year program to create prosthetic arms that can better emulate natural limbs. They will more closely match the real thing in appearance and function. And the user's ability to feel with them and control them will be vast improvements over anything currently available. The Revolutionizing Prosthetics Program is spread over 30 different organizations, including 10 universities across Canada, Europe, and the United States: the University of New Brunswick, Fredericton, is working on signal processing and pattern recognition for natural arm control; the University of Utah, in Salt Lake City, is working on electrodes for brain implants. The Johns Hopkins University Applied Physics Laboratory, in Laurel, Md., is ”herding the cats,” according to DARPA project manager Colonel Geoffrey Ling, ensuring that these far-flung research partners work together to make the bionic arm a near-term reality. Scientists involved say that this Manhattan Project-like system--on which DARPA has already spent US $30.4 million--is the only way to bring technology this advanced into the world by 2009.

The program was conceived in 2005 to create prosthetic arms that would leapfrog the stagnant hook-and-cable technology that has improved little since World War II. DARPA split the program into two separate projects--one of them a two-year effort that would yield, by 2007, the most sophisticated mechanical arm possible with currently available technologies (that contract went to New Hampshire-based Deka Research and Development Corp.). The international Applied Physics Laboratory, the longer effort, also had a mandate to produce an arm with state-of-the-art mechanics by 2007. Called the Proto-1 this first APL arm, completed in 2007, had approximately eight joints or degrees of freedom.

APL's second-generation prototype, called Proto-2, begun in 2007, has 25 degrees of freedom--almost as much dexterity as a human limb. APL project leader Stuart Harshbarger says the Proto-2, with 15 motors in the hand alone, is capable of unprecedented mechanical agility and shows that we will be able to develop a viable mechanical limb system, including the finger movements of the native limb, over the next two years. But that's not really the point: making a truly bionic arm requires far more than mechanical breakthroughs, better processing power, or longer batteries. None of these enable the prosthetic to respond to the wearer's intent with a natural limb's unthinking grace.

”Think about taking a sip from a can of soda,” Harshbarger says. The complex neural feedback system connecting a native limb to its user lets that user ignore an entire series of complicated steps. The nervous system makes constant automatic adjustments to ensure, for example, that the tilt of the wrist adjusts to compensate for the changing fluid level inside the can. The action requires little to no attention. Not so for the wearer of current prosthetic arms, for whom the act of taking a sip of soda precludes any other activity. The wearer must first consciously direct the arm to extend it to the correct point in space, then switch modes to rotate the wrist into proper position. Then he must open the hand, close it to grasp the soda can (not so weakly as to drop it but not so hard as to crush it), switch modes to bend the elbow to correctly place the can in front of his mouth, rotate the wrist into position, and then concentrate on drinking from the can of soda without spilling it.

All of today's prosthetics rely on the user to control them with a linear series of steps. The best prosthetic arm on the market today allows three degrees of freedom--moving the elbow, rotating the wrist, and opening and closing a claw. You just can't do all three at once. The Deka arm, which should go to clinical trials this year, allows for simultaneous motion of several joints with 18 degrees of freedom. But without a direct neural interface, controlling even the most sophisticated arm still takes the attention and concentration required to control any machine. This is the fundamental difference between the intuitive grace of a native limb and the strained, hesitant movements of a prosthesis. Sensory feedback is a crucial component of mimicking the feedback system that makes a real arm work like a real arm.