Johns Hopkins University Applied Physics Laboratory (APL) researchers lead a nationwide effort to make a bionic arm that wires directly into the brain to let amputees regain motor control—and feeling.
The second prototype, demonstrated at DARPA Tech 2007 last August, has 25 individual joints that approach the natural speed and range of motion of the human limb. These mechanical limb systems are complemented by a range of emerging neural integration strategies that promise to restore near-natural control and important sensory feedback capabilities.
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.
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.
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. 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. 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. They reroute the nerves to the chest muscles.
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.
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.