Engineers have designed a soft, lightweight and potentially low-cost neuroprosthetic hand that has the potential to change the lives of amputees.
A combined research effort between MIT and Shanghai Jiao Tong University found that amputees who tested the artificial limb performed daily activities equally as well, if not better, than those with more rigid neuroprosthetics.
Professor Xuanhe Zhao from MIT’s Department of Mechanical Engineering told create that the research team developed a computer model linked to a pressure pump to robotically mimic a person’s intended motions.
“We developed models that can predict the deformation of the hand under applied pneumatic pressures,” he said.
An algorithm then “decodes” muscle signals and relates them to common ways of grasping the hand.
“The models can guide the design of the hand to achieve desired grasps under programmed pressures,” he said.
Neuroprosthetics is an exciting field of biomedical engineering concerned with developing highly articulated bionic limbs to substitute for impaired motor, sensory or cognitive functions, engineered to sense a user’s residual muscle signals.
Zhao said the key advance in their research is enabling tactile feedback by stitching a pressure sensor to each fingertip, which when touched or squeezed produces an electrical signal proportional to the sensed pressure.
“One challenge is how to inform the amputee of different levels of touch pressures,” he said. “We tuned the frequency of the electrical stimulation — a higher frequency corresponds to a higher touch pressure.”
How it works
Most neuroprosthetics control individual fingers using mounted electrical motors, but the researchers instead developed a simple pneumatic system that precisely inflates and bends fingers in specific positions.
Critically, this system, including a small pump and valves, can be worn at the waist, significantly reducing the prosthetic’s weight.
The pneumatic system receives signals from electromyography (EMG) sensors that measure electrical signals generated by motor neurons to control muscles. The sensors are fitted where the prosthetic attaches to a user’s limb, meaning the sensors can pick up signals from a residual limb, such as when an amputee imagines making a fist.
The key to the breakthrough developed by Zhao and his colleagues is a controller that directs the pneumatic system to inflate the fingers in positions that mimic five common grasps, such as pinching two and three fingers together, making a fist into a ball, and cupping the palm of the prosthetic hand.
“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,” he said.
Zhao believes incorporating different grasp types — which allow for activities such as zipping a suitcase, pouring juice and petting a cat — is only the beginning of what they can achieve.
“We want to customise the design for mass production, so we can translate soft robotic technology to benefit society,” he said.
Lightweight and low cost
The smart hand is soft and elastic, and is only a fraction of the weight and material cost associated with more rigid smart limbs. It weighs about 250 grams and is made up of components costing about US$500.
Zhao cautioned that the prototype is not yet a product, but the performance is already similar or superior to existing neuroprosthetics.
“We are improving the speed, force and dexterity of the soft neuroprosthetic hand, while maintaining its long-term durability and reliability,” he said.
“We are also improving the interfaces between the soft neuroprosthetic hand and the human body, including myoelectric control and tactile feedback.
“Overall, we hope the work will eventually help thousands to millions upper-limb amputees with lightweight, low-cost soft neuroprosthetics.”
Brave new world
While neuroprosthetics research undoubtedly has huge potential, there is some concern about some of the commercial interest in the technology.
Facebook has part-funded research by scientists from the University of California at San Francisco and the University of California at Berkeley that produced a device that could connect the brain of a stroke victim directly to a computer with software to interpret their thoughts.
This form of neuroprosthetics replaces broken connections in the brain, spinal cord and nerves, allowing people to walk, hold objects and even speak again — but at the same time is effectively reading their mind.
While the system has so far only been trialled on one patient, and is only about 50 per cent accurate, the funding from Facebook has raised some ethical concerns.
The tech giant is continuing to develop neuroprosthetics that will connect computers to other parts of the nervous system, for example by detecting electrical signals from muscle twitches.
Another Silicon Valley billionaire, Elon Musk, has formed Neuralink, part-funded by Google Ventures.
Neuralink is said to have experimented with putting a chip into a monkey’s skull and used “tiny wires” to connect it to its brain and play video games with its mind.
According to Musk, Neuralink wants to develop these chips for quadriplegics with brain or spinal injuries to “control a computer mouse, or their phone, or really any device just by thinking”.
