The hardest part to replace
The human hand has dozens of joints, many muscles and a rich sense of touch. No artificial hand has come close to matching all of that, so the history of the prosthetic hand is a series of compromises. Designers have always had to choose among three goals: a useful grip, a natural appearance, and low weight and cost. Choosing one usually costs something in the other two. The focus here is the hand end of the arm, not the harnesses and elbows.
Early mechanical hands
Renaissance iron hands are the best-known early examples. Their fingers could be bent into position and held there by a ratchet or spring catch, so that the hand could hold a shield, a rein or a pen. The user adjusted the fingers with the other hand. The designs described by the French surgeon Ambroise Paré in the sixteenth century went a step further, with springs and catches that could be released by moving the stump, but they were intricate and costly. Making them involved the same fine metalwork used in automata.
By the nineteenth century, makers were creating hands with fingers that opened and closed by cords connected to the user’s shoulder or chest movements. Early cosmetic hands of wood, leather or rubber were also common. These did not grip anything, but they filled a glove and made the person look less conspicuous.
Two ways to grip
Two basic mechanisms shaped nearly all later designs. In a voluntary-opening device, the user pulls the cable to open the hook or hand, and a spring or rubber band closes it on the object. The grip force is set by the spring, which means the user does not need to keep tension on the cable, but the grip may be stronger or weaker than the task needs. In a voluntary-closing device, the user pulls to close, so the grip is proportional to effort and can be graded, but it demands continuous pull.
Voluntary-opening hooks became dominant largely because they were simple and robust. A split hook, in which two curved fingers meet at the tip, was popularized in the United States in the early twentieth century, and versions of it are still in use. Many users prefer a hook for work because it can pick up small items, reach into tight places and tolerate rough conditions. A hand-shaped terminal device is bulkier, and it often grips less precisely.
Hooks versus hands
The hook-versus-hand debate has lasted over a century. A hook is effective but unmistakably mechanical, and some users report that it attracts attention. A hand looks more familiar but is heavier and often more fragile. In practice, many people own both a hook and a cosmetic or functional hand and switch according to the situation. The preferences differ by person, occupation and culture, and there is no universal answer.
A glove covering also matters. Silicone and PVC cosmetic gloves can be shaped and colored to match skin, and in some cases even include fine detail like fingerprints and freckles. The cover can be difficult to keep clean and may wear out, but it makes a considerable difference to how some users feel about the device.
Powered hands
From the mid-twentieth century, electric motors began to open and close hands in response to muscle signals. Early powered hands usually had a single opening and closing motion, with the thumb and two fingers moving together in a pinch. This is often described as a three-jaw or tripod grip. It was functional for picking up objects but limited for tasks requiring different grips.
Around the 2000s, hands with a separate motor in each finger came on the market, which could form several grip patterns such as a pinch, a power grip and a pointing finger. The first commercially available hand of this kind is generally dated to 2007, although several research hands existed earlier. Some later models can rotate the thumb into different positions, and many use a phone app or muscle signal patterns to switch grips. Reliability, battery life and cost are still challenges, and weight remains a concern; the motors and lithium-ion batteries add up. These hands also depend on integrated circuits small enough to fit in the palm.
Hands for children and low-cost designs
Children outgrow devices quickly, which made hands expensive to replace. In 2011, a carpenter in South Africa who had lost fingers worked with a puppet maker in the United States to build a mechanical hand using 3D printing, and the design was shared openly. Volunteer groups later used the files to make hands for children. Printed hands use simple cables and elastic bands. They are not a replacement for clinically fitted devices for every user, but they showed that low-cost production was possible, and they influenced the 3D printing industry’s interest in assistive devices.
Sensation and the future
A missing feature in nearly all hands is touch. Users generally rely on sight to judge grip force. Research on stimulating nerves or the skin to give feedback is active. Until sensation is practical and routine, designers will continue to weigh the simple strength of a hook against the more complex promise of a multi-fingered hand.