Kuiken knew this would be the best chance at controlling the arm with intent, because the surface electrodes can't tell the difference between a muscle twitch in the forearm and a muscle twitch in the pectoral muscle. The result—the ability to move the artificial arm as if it were your own—is always the same. At DARPATech, Jesse Sullivan (an electrician whose arms were amputated at the shoulder in 2001 after he was nearly electrocuted by a high-voltage wire) used the Proto-1 arm to gesture unconsciously while talking, shake a reporter's hand without trying to adjust for grip strength, and eat small pieces of candy. The movements of the arm were totally—almost eerily—natural.

When Sullivan thinks about opening his hand or moving his elbow, instead of causing a twitch in the muscle of his hand, those spikes cause a twitch in his chest muscles. A surface electrode picks up those signals and translates them into mechanical motion. “When Jesse flexes his wrist, he's doing it by thinking about making that motion,” says RIC's Kuiken. “The limb figures out the signals and what those patterns are, and then it causes the limb to move the way he intended. He's not thinking about some series of steps; he's thinking about moving his normal, regular arm.”

Most notably, Sullivan was able to “feel” pressure when it was applied to his index finger and not on some other part of his body, where a small vibrating motor might vibrate. When the reporter squeezed the tip of the Proto-1 arm's thumb, Sullivan showed her the location on the hand where he could “feel” the pressure in his phantom limb: at the base of the thumb. Early last year, Kuiken had discovered that in addition to allowing intuitive muscle control, rerouted nerves unexpectedly sensitized the skin on the chest. But the volunteers didn't feel the sensation in the chest—they felt it in the phantom limb. Kuiken has been working to refine the map of the phantom limb to determine the connection between the exact location on a prosthetic finger and the corresponding feeling perceived in the phantom limb. He's hoping for a one-to-one match soon, so that when the tip of the thumb is squeezed, the corresponding feeling will be in the phantom thumb's tip. Kuiken says the patients were also able to feel hot and cold. Harshbarger says that some volunteers have been able to use sensory-encoding tactors developed at Northwestern University for the Proto-2 arm to distinguish paper from sandpaper.

But what if for whatever reason these unused areas of muscle are unavailable or damaged? Another way to access the peripheral nerves is with penetrating electrodes that intersect the nerves with what are essentially needles. Researchers at the University of Utah developed an implantable device called the Utah Slant Electrode Array (USEA), a 5-millimeter-square grid of 100 needlelike electrodes. These electrodes hold hundreds of different mechanisms, among them signal amplifiers, storage registers, and a multiplexing scheme to transmit to a receiver on the skin. Like the injectable sensors, these can be powered wirelessly and extract a signal in real time. Unlike IMES, however, the electrode arrays access the nerves directly, instead of the muscles obeying the nerves. That subtracts, in theory, another layer of signal interference. The electrode arrays are still in experimental stages. Harshbarger says they will be used this year.

Finally, the most extreme solution is meant for people whose bodies no longer offer any means for interfacing to the artificial limb, for whom even nerve-rerouting surgery may not be an option. In such cases, the Utah electrode arrays are relocated to the source of all neural signals—the brain's motor cortex, which is right at the top of the head, toward the back of the frontal lobe. The electrode arrays are either placed on the inside surface of the top of the skull near the motor cortex or penetrate directly into the motor cortex. A device very much like the skull-mounted USEA has already been proven to pick up the brain's electrical signals and is currently used to warn epileptic patients of impending seizures. When electrodes penetrate directly into the motor cortex, embedded electronic circuits intercept the motor neurons firing their instructions and, with the help of complex algorithms, translate the related signals into a language that can control the mechanics of the arms.

It may take combinations of these methods to offer the most natural control and feedback. As a result, Harshbarger wants to push the limits of all four tiers of interface. Rather than expecting the user to learn how to control the prosthetic limb, Harshbarger wants electronics that learn how to figure out what the user wants the arm to do. “We don't want the user to have to learn a new strategy for activating muscles in order to control the limb,” he says. So far, Sullivan has been able to put on a hat without looking in a mirror. Harshbarger says his next goal is for Sullivan to start typing. “Well,” he qualifies, making hunting and pecking motions, “more like the way I type.”