PROSTHETIC LIMBS have been around for a long time. The oldest known, a piece of wood carved and painted to replace the lost toe of an Egyptian noblewoman, dates back more than 3,000 years. But prosthetics which behave like the real thing as well as looking like it are still very much a work in progress. And a group at Shanghai Jiao Tong University, in China, have just come up with what looks to be a significant advance—an affordable prosthetic hand that not only responds like a real one to signals from the wearer’s brain, but is also able to signal back to the brain what it is touching and doing.
Gu Guoying and his colleagues describe their invention in Nature Biomedical Engineering. Its fingers are made of rigid tubes connected by soft joints. These are similarly connected to a 3D-printed plastic palm. The whole is covered with a flexible elastomer layer to mimic skin and is attached to the user’s residual limb via a customised plastic socket. In contrast to current models, which are electrically powered, Dr Gu’s hand is powered pneumatically by a pump held in a waist bag, with the connecting air lines running under the user’s clothes alongside communication cables. This reduces its weight below 300 grams—half that of some current models, and less, indeed, than the weight of a real hand—though the waist bag adds a further 444 grams.
The hand uses similar signal-processing algorithms to other prosthetics on the market. The big advance is that it does not require invasive surgery or electronic implants into the residual limb to communicate with the user’s brain. Sensors on the skin record electrical activity from the remaining arm muscles. In an intact arm, this activity would tell those muscles how to operate the hand. Instead, they are interpreted by pattern-recognition software that sends appropriate commands to the pump to move the artificial hand in the same way. Meanwhile, other signals travel in the opposite direction from sensors in the hand’s fingertips to nerves in the arm, whence they are relayed to the brain and provide a sensation of touch. The upshot is something which responds like a hand and feels like one to the user.
Dr Gu and his colleagues compared the efficacy of their invention with that of existing models using tests borrowed from research into strokes and spinal-cord injuries. These included writing, grasping and lifting objects, lifting food to the mouth, and stacking draughtsmen. Normally, they found, it worked better—particularly for delicate tasks like handling fragile objects, petting a cat and shaking hands.
The other advantage of Dr Gu’s invention is that it is cheap. The components it is made from cost about $500. Existing models may sell for $10,000 or more. If it, or something similar, goes into production, that will permit the transformation of many more of the lives of the 5m people who have lost a hand, or were born without one, than is possible at the moment.