PROSTHETIC body parts are nothing new. In 2000 researchers published in the Lancet an account of a 3,000 year old Egyptian mummy with a wooden big toe grafted to her right foot. It had been expertly carved, and painted to match her skin tone. Making limbs that look natural, though, is one thing. Making them feel real is much harder, because a real limb is constantly talking to the brain that controls it. That lack of feedback, and the discomfort it causes, is one reason why between a quarter and a half of people fitted with an artificial limb end up abandoning it. But two papers just published in Science Translational Medicine report progress towards dealing with the problem of feedback.

Daniel Tan and his colleagues at the Louis Stokes Veterans Affairs Medical Centre, in Cleveland, Ohio, created signals that appeared to come from the prosthetic arms of two volunteers by implanting electrodes around nerves in the amputees’ stumps (see picture). When they connected these electrodes to a machine that generated electrical signals, both volunteers reported sensations which seemed, to them, to be coming from their hands.

The nature of the sensations depended on what sort of current Dr Tan applied. The simplest stimulation—a repetitive square-wave—produced an unnatural, vaguely electrical feeling. Using more elaborate patterns, though, the researchers could recreate everything from simple sensations such as pressure, vibration and tapping to more complicated feelings, as of a pen brushing lightly against the skin or of the hand rubbing across a texture.

Restoring sensation has practical uses. Modern prostheses are able, by reading electrical signals from muscles using electrodes attached to the skin of the missing limb’s stump, to perform tasks such as picking things up. Delicate tasks, however, can be tricky, since the user must rely on a combination of sight and experience to work out how much pressure to apply. For example, when Dr Tan blindfolded his volunteers and asked them to pluck the stalks from cherries without crushing the fruit, they succeeded only 43% of the time. But when he connected pressure sensors attached to the protheses’s fingers to the signal-generating machine, and gave them appropriate feedback, the success rate jumped to 92%.

Intriguingly, one unexpected benefit was that the device’s feedback banished the phenomenon of phantom limbs, in which an amputee perceives that his missing appendage is still present. Without the computer-generated sensations, both volunteers reported that their prosthetic hands felt like external tools (one described it as like an artificial hand that he was holding with his phantom hand). Switching the sensations on made the hand feel like an integral part of the body.

The main drawback of Dr Tan’s approach is that the wires to the implanted electrodes must penetrate a patient’s skin. Besides being ugly and clumsy, that opens a route to infection. As they describe in their paper, Max Ortiz-Catalan of Chalmers University of Technology in Sweden and his colleagues hope to rectify this by combining implanted electrodes with a state-of-the-art prosthetic-limb-mounting system.

Standard prostheses are held in place by a compression socket, which squeezes the artificial limb onto the stump of the natural one. That can be uncomfortable, and the socket is a constant infection risk. So Dr Ortiz-Catalan turned to a procedure called osseointegration, in which an implant is fixed directly into the patient’s bones, leaving a small, protruding metal “abutment” onto which the limb can be attached. He has managed to combine a set of implanted electrodes with a custom-built osseointegrated socket.

One advantage of such a socket is that it allows implanted electrodes to control the prosthesis, instead of relying on second-hand signals detected through the skin. That improves performance. Previously, for example, Dr Ortiz-Catalan’s patient had been unable to lift his artificial arm by more than about 80°: any further and the control mechanism got confused by electrical noise from the shoulder muscles. And Dr Ortiz-Catalan has also been able, by running current into the electrodes instead of just reading from them, to induce in his patient sensations that appear to be coming from the artificial hand.

Combining these two approaches, then, should produce a big advance in the efficacy of artificial limbs. And the subjective benefits could be just as important as the practical ones. Part of the problem of phantom limbs is that they often hurt. By abolishing the phantoms, Dr Tan’s technique also abolishes the pain. Asked about the comparison between a limb with feedback and one without, the participants were unequivocal. As one put it, “I’d rather have it in a heartbeat.”