Engineers at the Massachusetts Institute of Technology (MIT) and Shanghai Jiao Tong University, China, have designed a soft, lightweight, and potentially low-cost neuroprosthetic hand. Two people with amputations tested the device and were able to perform daily activities, such as zipping a suitcase, pouring a carton of juice, and petting a cat, just as well as—and in some cases better than—those with more rigid neuroprostheses.
The researchers found the prosthesis, designed with a system for tactile feedback, restored some primitive sensation in a volunteer’s residual limb. They say the new design is also surprisingly durable, quickly recovering after being struck with a hammer or run over with a car.
The smart hand is soft and elastic, and weighs about half a pound. Its components total around $500.
“This is not a product yet, but the performance is already similar or superior to existing neuroprosthetics, which we’re excited about,” said Xuanhe Zhao, PhD, a professor of mechanical engineering and 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 hand is made from soft, stretchy material, the commercial elastomer EcoFlex, and has five balloon-like fingers, each embedded with segments of fiber, similar to articulated bones in actual fingers. The bendy digits are connected to a 3D-printed palm.
Rather than controlling each finger using mounted electrical motors, as most neuroprosthetics do, the researchers used a pneumatic system to precisely inflate fingers and bend them in specific positions. The system, including a small pump and valves, can be worn at the waist, significantly reducing the weight of the device.
One of the researchers developed a computer model to relate a finger’s desired position to the corresponding pressure a pump would have to apply to achieve that position. Using that model, the team developed a controller that directs the pneumatic system to inflate the fingers, in positions that mimic five common grasps, including pinching two and three fingers together, making a balled-up fist, and cupping the palm.
The pneumatic system receives signals from EMG sensors on the residual limb.
The team then used an existing algorithm that decodes muscle signals and relates them to common grasp types. They used this algorithm to program the controller for their pneumatic system. When an amputee imagines, for instance, holding a wine glass, the sensors pick up the residual muscle signals, which the controller then translates into corresponding pressures. The pump then applies those pressures to inflate each finger and produce the amputee’s intended grasp.
Going a step further in their design, the researchers looked to enable tactile feedback by stitching a pressure sensor to each fingertip, 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 prosthesis’ thumb is pressed, for example, versus the forefinger.
The participants learned to use the device by repeatedly contracting the muscles in their residual limb while imagining making five common grasps during a 15-minute training. .
After completing training, the volunteers were asked to perform standardized tests to demonstrate manual strength and dexterity, such as stacking checkers, turning pages, writing with a pen, lifting heavy balls, and picking up fragile objects like strawberries and bread. They repeated the same tests using a commercially available bionic hand and found that the inflatable prosthesis performed the tasks at least as well as compared to its rigid counterpart.
One volunteer was able to intuitively use the soft prosthesis in daily activities, such as handling food, laptops, bottles, hammers, and pliers. The researchers also 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 that were placed in the prosthetic hand, and lifted them in response.
The team has filed a patent on the design, through MIT, and is 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 customize the design for mass production, so we can translate soft robotic technology to benefit society.”
Editor’s note: This story was adapted from materials provided by MIT.
The study, “A soft neuroprosthetic hand providing simultaneous myoelectric control and tactile feedback,” was published in Nature.
Photograph: The smart hand is soft and elastic, weighs about half a pound, and costs a fraction of comparable prosthetics.Photograph courtesy of the research team.