PHOTO: The Johns Hopkins University Applied Physics Laboratory

Sensory feedback for prosthetics is in the embryonic stages. The best mechanism on the market today consists of a vibrating motor that buzzes against the skin more or less intensely to reflect, for instance, such force factors as grip strength. The DARPA project is gunning for much more than that: researchers want an arm that transmits sensation to the user—pressure, texture, even temperature. The Proto-1 arm already has integrated force sensors in the artificial hand that give the wearer a sensation of feeling. Harshbarger says Proto-2 builds on that breakthrough with 100 sensors that connect the body's natural neural signals to the mechanical prosthetic arm to create a sensory feedback loop: the wearer interacts with an object and the arm feeds back, in real time, where the arm is in space, what object it is touching, whether that object is smooth or rough, how hard the hand is holding it, and what temperature the object is. With that information, the user can react in split-second real time.

As it turns out, the degree of control is directly proportional to the invasiveness of the method. Harshbarger's team is working with four tiers of neural interface. Each tier adds a level of magnitude to the control and sensory capability of the prosthesis—but also a level of magnitude in required surgery.

For simple activities, like grasping a ball, you don't need surgery. The most basic interface (for low-level amputation) uses electrodes taped to the surface of the residual limb's skin. After all, the hand is missing, but not the muscles and nerves that once controlled it. The APL researchers figured out a way to tap the signals still being transmitted to the nonexistent hand from the residual muscles. They used the surface electrodes to detect and amplify those signals. Then, with complex signal-processing and pattern-recognition algorithms, the electrical impulses were translated into instructions for the arm's motors and microprocessors. But while the electrodes can amplify the signal, they can't clean it up: by the time that signal has traveled from the originating muscles through layers of flesh and skin, a lot of noise has been introduced, and some of the impulses may have crossed. So users can open and close the artificial hands at will, but they probably can't move individual fingers the way they want to.

To move individual fingers, which is necessary, for example, to statically hold a key or a pen, you need to access the muscle firings directly. The next level (of invasiveness and control) bypasses these interfering layers of flesh and skin by using small wireless devices called injectable myoelectric sensors (IMES). These tiny, rice grain-like devices are injected into the muscle tissue of the residual arm and work just like the surface electrodes to tap the muscle signals right at the source. But because IMES pick up and transmit a cleaner and higher fidelity signal, they allow finer motor control of the arm. “Instead of picking up the sum of the signals at the surface,” says Harshbarger, “we can pick them up at the source, in the muscles that are being excited.” That means, depending on the nature of the injury, a wearer could even control individual fingers. The little devices are perpetually powered by a coil in the prosthetic limb, so they never need batteries. At this point, Harshbarger says, nine have been implanted in trained primates for six months without harmful effects. “It's going incredibly,” he says. “These are very low-risk devices, and they have posed no risk to the animals.” But the IMES system depends on the nature of the injury and the availability of implantation sites—which is to say, if you don't have an arm with residual muscles to put IMES devices into, you're out of luck.

For more severe amputations (for example, having both arms removed at the shoulder), there may not be much arm—or muscle—for IMES or surface electrodes to work with. So the next level of interface bypasses the residual muscle to tap into the peripheral nerves either with surgery or implanted electrodes. So far the team has had great success with the former, a technique called targeted muscle reinnervation. Pioneered at one of APL's partners, the Rehabilitation Institute of Chicago (RIC), this surgery reroutes nerves that once led to the muscles controlling the native arm and opens a direct line between those nerves and the mechanical arm. To move a real arm, a nerve signal travels down the nerve as a result of an intention, and that spike causes twitches in the terminal muscle, which results in electric signals on the surface of the skin that are directed to engage the features of the hand and arm. In a an individual with both limbs, those nerves travel from the spinal cord down the shoulder over the clavicle and then into the armpit, where they connect to about 80 000 nerve fibers that allow the brain to communicate with the arm. When the arm is amputated at the shoulder, the residual nerves are still there, but the muscles they influence are not. So RIC's Todd Kuiken developed a surgical method to take those nerves and give them new “home base” muscles—those still available in the chest. The surgery threads the nerves down under the clavicle, so that instead of extending to the armpit they now extend to the chest. After about six months, those nerves will spread into a saucer-size area of the chest muscle. That means that when the person tries to move his bicep, for example, a muscle in the chest will twitch in response.