Thought-controlled robotic limbs were only the beginning.

Scientists have had a string of remarkable successes lately, taking signals from the brains of monkeys and men, and using them to move mechanical arms.

Darpa, the Pentagon's blue-sky research division, now wants to ratchet that work up about ten notches, by developing a "neurally controlled artificial limb that will restore full motor and sensory capability to upper extremity amputee patients. This revolutionary prosthesis will be controlled, feel, look and perform like the native limb."

So, basically, what Luke Skywalker gets in Empire Strikes Back, after Darth chops off his hand. Except, researchers won't have a long, long time to get this limb ready. Darpa wants the robo-arm stat -- in four years or less.

The limb would have to be wired directly into the peripheral nervous system, instead of the brain-controlled arms being demonstrated today, Darpa tells researchers interested in working on this "Revolutionizing Prosthetics" project. Under agency guidelines, the arm will need enough finesse to pick up a raisin or to write in longhand. It needs to be sensitive enough for the wearer to handle day-to-day tasks in the dark. And the limb will have to be strong enough to lift 60 pounds at a time.

These are beyond ambitious goals, and even the even the big thinkers at Darpa acknowledge it. Breakthrough research in "neural control, sensory input, advanced mechanics and actuators, and prosthesis design and integration" will all be needed, the agency says in a call for proposals. Neuroscientists, roboticists, engineers, occupational therapists, and surgeons in the neural, orthopedic, reconstructive subspecialties will have to chip in.

"Revolutionizing Prosthetics" is so far-out that Darpa is taking the unusual step of hedging its bets, and running a parallel, more down-to-earth program.

The vision of the Prosthesis 2007 program is to leverage recent research advances in neural sensing, control systems, actuation, power storage and distribution, freeform manufacturing, neural control, microfabrication, sensory feedback, flexure and transmission design, signal processing, and information science to dramatically improve the capability of upper extremity prosthetic limbs beyond those that are currently available commercially. This vision will be realized by increasing the range of motion, strength, endurance, and dexterity of upper extremity prosthetic devices. The final product [an above-the-elbow prosthetic arm] must be ready for human clinical trials… [and] sufficiently mature to enter the appropriate approval processes for general medical use by the end of 24 months."

Sounds like a snap.