Development of human machine interfaces
The most successful method for controlling a prosthesis to date is a surgical procedure in which nerves that were previously attached to muscles in a lost arm and hand are transplanted into the chest. When the wearer thinks about moving the hand, chest muscles contract, and those signals are used to control the limb. While a vast improvement over existing methods, this approach still provides a limited level of control–only about five nerves can be transplanted to the chest.
The new interface, developed by plastic surgeon Paul Cederna and colleagues, builds on this concept, using transplanted muscle cells as targets rather than intact muscle. After a limb is severed, the nerves that originally attached to it continue to sprout, searching for a new muscle with which to connect. (This biological process can sometimes create painful tangles of nerve tissue, called neuromas, at the tip of the severed limb.) “The nerve is constantly sending signals downstream to tell the hand what to do, even if the hand isn’t there,” says Cederna. “We can interpret those signals and use them to run a prosthesis.”
The interface consists of a small cuplike structure about one-tenth of a millimeter in diameter that is surgically implanted at the end of the nerve, relaying both motor and sensory signals from the nerve to the prosthesis. Inside the cup is a scaffold of biological tissue seeded with muscle cells–because motor and sensory nerves make connections onto muscle in healthy tissue, the muscle cells provide a natural target for wandering nerve endings. The severed nerve grows into the cup and connects to the cells, transmitting electrical signals from the brain. Because it is coated with an electrically active polymer, the cup acts as a wire to pick up electrical signals and transmit them to a robotic limb. Cederna’s team doesn’t develop prostheses themselves, but he says the signals could be transmitted via existing wireless technology.