The timing was good: microprocessors had gotten small enough, and power consumption efficient enough, to make it possible to cram the control electronics, lithium batteries, motors, and wiring into a package the size, shape, and weight of a human arm—about 3.6 kilograms. Still, the engineering was tough, says program manager Ling. “You’re asking an engineer to build an arm that can do what your arm can do, but they’re confined to a package the size of—an arm. In addition to being the right size and weight, it also has to look like an arm!”

In order to make a better arm, Kamen first had to figure out what was wrong with the old one. Part of the reason the technology was still in “the Flintstones” was a lack of agility: a human arm has 22 degrees of freedom, not three. The Luke Arm prosthetic is agile because of the fine motor control imparted by the enormous amount of circuitry inside the arm, which enables 18 degrees of freedom. The engineers fought for space inside the arm and created workarounds when they couldn’t have the space they needed, such as using rigid-to-flex circuit boards folded into origami-like shapes inside the tiny spaces, which are connected by a dense thicket of wiring.

The arm has motor control fine enough for test subjects to pluck chocolate-covered coffee beans one by one, pick up a power drill, unlock a door, and shake a hand. Six preconfigured grip settings make this possible, with names like chuck grip, key grip, and power grip. The different grips are shortcuts for the main operations humans perform daily.

The Luke arm also had to be modular, usable by anyone with any level of amputation. The arm works as though it had a very complicated set of vacuum cleaner attachments; the hand contains separate electronics, as does the forearm. The elbow is powered, and the electronics that power it are contained in the upper arm. The shoulder is also powered and can accomplish the never-before-seen feat of reaching up as if to pick an apple off a tree.

It must be less than what a native limb would have weighed, because in an amputee the human skeletal system can no longer be used as a method of attachment. Instead, for amputations above the elbow, a user is strapped into a kind of harness. Deka engineers modeled the arm based on the weight of a statistically average female arm (about 3.6 kg), including all the electronics and the lithium battery. Amazingly, titanium, the legendarily light material, is too heavy to keep the arm under its weight limit—it can’t be made thin enough without bending—so the arm is mostly aluminum.

Kamen’s group found that the discomfort caused by the arm socket, where the prosthesis connects to the body, is one of the crucial reasons Hildreth and others stop wearing their prosthetics. The traditional connection method is designed to create the greatest possible surface area connecting the native limb to the prosthetic: basically, the residuum—the amputee’s stump—is stuffed into the prosthesis. But the strain of normal use often results in a sweaty, slippery connection that makes proper use of the prosthesis nearly impossible. It can also be painful. Deka’s new socket was designed to be used with the Luke arm, but it can also improve traditional prostheses.

The last piece of the puzzle was the user interface for controlling the arm. DARPA stipulated in Deka’s contract that the interface must be completely noninvasive. However, Kamen says, his engineers created the arm to support any means of control. When a Deka engineer tests the arm via a linked exoskeleton, the arm can replicate almost every subtlety of human movement. Of course, real users will not be operating a prosthetic with an existing limb: the exoskeleton merely showcases the arm’s potential.

Deka worked closely with the Rehabilitation Institute of Chicago, where neuroscientist Todd Kuiken has had recent successes in surgically rerouting amputees’ residual nerves—which connect the upper spinal cord to the 70 000 nerve fibers in the arm—to impart the ability to “feel” the stimulation of a phantom limb. Normally, the nerves travel from the upper spinal cord across the shoulder, down into the armpit, and into the arm. Kuiken pulled them away from the armpit and under the clavicle to connect to the pectoral muscles. The patient thinks about moving the arm, and signals travel down nerves that were formerly connected to the native arm but are now connected to the chest. The chest muscles then contract in response to the nerve signals. The contractions are sensed by electrodes on the chest, the electrodes send signals to the motors of the prosthetic arm—and the arm moves. With Kuiken’s surgery, a user can control the Luke arm with his or her own muscles, as if the arm were an extension of the person’s flesh. However, the Luke arm also provides feedback to the user without surgery.

Instead, the feedback is given by a tactor. A tactor is a small vibrating motor—about the size of a bite-size candy bar—secured against the user’s skin. A sensor on the Luke hand, connected to a microprocessor, sends a signal to the tactor, and that signal changes with grip strength. When a user grips something lightly, the tactor vibrates slightly. As the user’s grip tightens, the frequency of the vibration increases. This enables Hildreth to pick up and drink out of a flimsy paper cup without crushing it, or firmly hold a heavy cordless drill without dropping it. “I can do things I haven’t done in 26 years,” he says, looking at his hand. “I can peel a banana without squishing it.” Hildreth steers the Luke arm with joystick-like controllers embedded in the soles of his shoes. These customizable foot pedals are connected to the arm by long, flat cords. “When I push down with my left big toe, the arm moves out,” he says, shifting to demonstrate. “When I move my right big toe, it moves back in.” He shifts again, and the arm dutifully obeys. A wireless version is in the works.

In the United States, there are about 6000 upper extremity amputees in a given year. That number has risen due to the war in Iraq. The Deka arm is the earliest hope for the increasing number of Iraq war veterans who are coming home without arms.

At press time, Ling was sanguine about the Luke arm’s future. “We’re trying to get a transition partner so it can go into clinical use and a commercial partner to get it out to the patients,” he says. “This is no longer a science fair project.” The costly research and development, Kamen says, means that any company can now take over the Luke arm and look for ways to manufacture it cost-effectively. Depending on the degree of amputation, today’s state-of-the-art prosthetic arms can cost patients about $100 000 or more. Luke project manager Rick Needham says that the goal is to keep as close to that cost as possible.

But before the arm can be commercialized, it needs to be approved by the FDA, and that can’t happen without clinical trials. And right now it’s not clear who will fund those clinical trials. DARPA’s funding often ends after a project’s funding is picked up by some other organization. Deka doesn’t yet have such a transition partner.

“Clinical trials certainly have a cost,” says DARPA spokesperson Jan Walker. “If no one funds the costs, then trials obviously can’t happen.” But she says DARPA’s funding procedures are not set in stone. Sometimes DARPA funding ends completely; sometimes the agency continues a low level of funding as the new organization ramps up its own funding. Walker declined to comment on specific plans for the Luke arm.

If DARPA continues funding the project, Kamen’s group would like to start clinical take-home trials sometime this year. Kamen hints that he has been in talks with Walter Reed Army Medical Center in Washington, D.C., and with other Veterans Affairs hospitals. “Certainly within the next two years we hope to submit to the FDA for approval to sell the arm,” says Needham.

Hildreth says he can’t wait to get one of the Luke arm prostheses home. “My wife can’t wait either,” he says. “She says, ‘Oh yeah, I got lots of stuff for you to do around the house.’ ”