A bionic arm: After undergoing a novel surgical procedure, Claudia Mitchell can control a prosthetic arm much as she once controlled her real arm, performing complex movements such as picking up small objects and dropping them into a cup. Credit: The Rehabilitation Institute of Chicago and DEKA Research

People who have lost an arm have not traditionally had much hope of ever regaining meaningful function. Prosthetic arms have been controlled in a rudimentary way, by transforming residual shoulder movements or muscle signals into the ­simplest movement commands. These artificial arms cannot do two things at once, much less three or four. Amputees often toss them in the closet out of sheer frustration, somewhat stung by the fact that leg amputees have far better products available to them.

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