Andrew Rubin sits with a Surface tablet, watching a white skeletal hand open and close on its screen. Rubin's right hand was amputated a year ago, but he follows these motions with a special device fitted to his upper arm. Electrodes on his arm connect to a box that records the patterns of nerve signals firing, allowing Rubin to train a prosthetic limb to act like a real hand. "When I think of closing a hand, it's going to contract certain muscles in my forearm," he says. "The software recognizes the patterns created when I flex or extend a hand that I do not have."