Bionic hand offers real-time tactile control
For the world’s five-million-plus upper-limb amputees, prosthetics have come a long way. Beyond traditional mannequin-like appendages, there are a growing number of commercial neuroprosthetics that are designed to sense a user’s residual muscle signals and robotically mimic their intended motions.
But they’re expensive: neuroprosthetics can cost tens of thousands of dollars and are built around metal skeletons, with electrical motors that can be heavy and rigid.
Now, engineers at the Massachusetts Institute of Technology (MIT) and Shanghai Jiao Tong University have developed a soft, lightweight, and potentially low-cost neuroprosthetic hand. Amputees who tested the artificial limb were able to perform daily activities, such as zipping a suitcase, pouring a carton of juice, and patting a cat, just as well as — and in some cases better than — those with more rigid neuroprosthetics. And, this prosthetic, the engineers say, can restore some primitive sensation in a residual limb.
The new design is also surprisingly durable, quickly recovering after being struck with a hammer or run over by a car. It’s soft and elastic, weighs a little over 200g and costs around US$500 — a fraction of the weight and material cost of more rigid smart limbs.
“This is not a product yet, but the performance is already similar or superior to existing neuroprosthetics, which we’re excited about,” says Xuanhe Zhao, professor of mechanical engineering and of civil and environmental engineering at MIT. “There’s huge potential to make this soft prosthetic very low cost for low-income families who have suffered from amputation.”
The design bears an uncanny resemblance to a certain inflatable robot in the animated movie Big Hero 6. Like the squishy android, the team’s artificial hand is made from a stretchy material called EcoFlex. The hand has five balloon-like fingers, each embedded with segments of fibre, similar to articulated bones in actual fingers. The bendy digits are connected to a 3D-printed “palm”, shaped like a human hand.
A controller directs a simple pneumatic system, which precisely inflates the fingers and bends them in specific positions that mimic five common grasps, including pinching two and three fingers together, making a balled-up fist, and cupping the palm. The sensors pick up signals from a residual limb, so when an amputee imagines — for instance, holding a wine glass — the controller translates those signals into corresponding pressures. A small pump then applies those pressures to inflate each finger and produce the amputee’s intended grasp. The pump and valves, by the way, can be worn at the waist, significantly reducing the prosthetic’s weight.
Going a step further in their design, the engineers have added tactile feedback — a feature not available in most commercial neuroprosthetics. To do this, they stitched to each fingertip a pressure sensor, which when touched or squeezed produces an electrical signal proportional to the sensed pressure. Each sensor is wired to a specific location on an amputee’s residual limb, so the user can “feel” when the prosthetic’s thumb is pressed, for example, rather than the forefinger.
To test the inflatable hand, the engineers enlisted two volunteers, each with upper-limb amputations. Once outfitted with the hand, the volunteers learned to use it by repeatedly contracting the muscles in their arms while imagining making five common grasps. After 15 minutes’ training, the volunteers were asked to perform a number of tests to demonstrate manual strength and dexterity. Tasks included stacking checkers, turning pages, writing with a pen, lifting heavy balls, and picking up fragile objects such as strawberries and bread. They did the same tests using a more rigid, commercially available bionic hand and found the inflatable prosthetic was as good — if not better — at most tasks. One volunteer was also able to intuitively use the inflatable in daily activities, for instance, to eat food such as crackers, cake and apples; to handle objects and tools, such as laptops, bottles, hammers and pliers; and to shake someone’s hand, touch a flower, and pat a cat.
In one particular exercise, the researchers blindfolded the volunteer and found he could discern which prosthetic finger they poked and brushed. He was also able to “feel” bottles of different sizes placed in the inflatable, and could lift them in response.
Zhao and his colleagues see these experiments as a promising sign that amputees can regain a form of sensation and real-time control with the inflatable hand. They’ve filed a patent on the design through MIT, and are working to improve its sensing and range of motion.
“We now have four grasp types. There can be more,” Zhao says. “This design can be improved, with better decoding technology, higher-density myoelectric arrays, and a more compact pump that could be worn on the wrist. We also want to customise the design for mass production, so we can translate soft robotic technology to benefit society.”
PHOTO Courtesy the research team