Saturday science subject: The bionic woman

Claudia Mitchell, a former Marine who lost her left arm in a motorcycle accident two years ago, has become the first woman to be outfitted with a bionic arm that can be controlled like a normal appendage. The arm is a prototype developed by the Rehabilitation Institute of Chicago. It uses electrodes to detect muscle twitches from an area on Mitchell's chest that has been "rewired" with nerves from her missing arm.
[Surgeons] cut the nerves to two chest muscles, the pectoralis and serratus, at a point where those nerves have branched to go to different parts of the muscles, but far "upstream" from the point where the nerves divide into tiny fibers that attach to individual bundles of muscle fiber. . . . They then sew the stumps of the large nerves that once went to the arm and hand to the cut ends of the chest-muscle nerves. In the same operation, the nerves carrying sensation from the skin over the pectoral muscle are also sewn into the arm nerves.

Over several months, the arm nerves grow down the sheaths of the motor fibers and attach to the muscles. . . . Simultaneously, the sensory nerves grow down the sensory sheaths and into the skin. . . . If all goes well, a person is left with chest muscles that twitch in different places in response to such thoughts as "bend the wrist back," "move the thumb" and "clench the fingers." The person also ends up with a patch of skin about the width of a baseball that, when stroked, warmed or pricked, feels like a hand rather than part of the chest.

Thanks to electrodes positioned over the area and wired to the prosthesis, when Mitchell attempts to move her missing arm, the prosthetic arm moves in its place (a video of the arm in action can be seen here.) Future iterations of the arm will have more motors in addition to electrodes in the hand that send signals to the user's chest skin, emulating sensory input from the missing hand.
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