In this month's issue of Spectrum, Sally Adee describes the Revolutionizing Prosthetics 2009 program, a "Manhattan Project" for prosthetic arms organized by the U.S. Defense Advanced Research Projects Agency. Here's an interesting excerpt:
But the most power-efficient arm in the world is no good if you can''t control it. So APL [Johns Hopkins University Applied Physics Laboratory] partnered with RIC [Rehabilitation Institute of Chicago] to develop a groundbreaking technique for controlling a mechanical arm with an amputee''s own nervous system.
Todd Kuiken, director of RIC''s neural engineering center, created a portal to the nervous system by rewiring the shoulder. The procedure, called targeted muscle reinnervation surgery, redirects the residual nerve bundles that once connected the spinal cord to the 70 000 nerve fibers in the arm. After an amputation, these nerves remain in place, and they continue to work''they just aren''t connected to anything functional. The pectoral muscles also remain intact, but they too are no longer driving an arm. So Kuiken surgically threaded the residual nerves from their original locations into the chest, where they innervated, or grew into, an area of pectoral muscle slightly smaller than a compact disc.
Next, Kuiken taped electrodes over the chest patch, where they could pick up the electromagnetic signals from muscles and send them to paired electrodes inside the prosthesis. There, signal-processing algorithms could translate the signals into the user''s intended movements. With Kuiken''s surgery, amputees were able to control the DARPA prototype with their own muscles, as if it were an extension of their own flesh.
Then came an unexpected and very lucky break: the researchers found that the redirected nerves restored not only muscle function but also sensation. The skin on that patch had been rewired with the nerves from the arm and hand; therefore, the patients sensed a touch on their chests as if someone were touching their missing hands''even if it was just a tap. The Hopkins team exploited this discovery with a device called a tactor, built at Northwestern University. By placing the tactor on the chest next to the electrodes, they created a complete feedback loop. In the end, the combination of Kuiken''s complex rewiring scheme and the brain''s natural plasticity simulated a real arm''s ability to sense touch, heat, and cold.
Read the full article here.