Low-cost soft prosthetic hand offers real-time tactile control
Engineers from MIT and Shanghai Jiao Tong University have developed a prosthetic hand that precisely inflates individual fingers to take hold of objects, while also providing the user with tactile feedback.
MIT states that more than five million upper-arm amputations are performed around the world every year, and though many amputees may have to make do with static prostheses, recent advances in articulating "robot" solutions and sensory feedback have made devices commercially available that can take residual muscle signals and convert them into hand motions. But such technology can be heavy and costly.
The soft prosthetic hand from the engineering team at MIT and Shanghai Jiao Tong University is said to weigh about half a pound (~225 g), and has a total component cost of about US$500 – making it potentially more affordable, as well as easier to use, if or when it goes into production.
The fingers in the new system are made of a commercially available soft and stretchy elastomer called EcoFlex with bone-like fiber segments embedded within. These are attached to a 3D-printed support shaped like a human palm.
Instead of mounting motors to control each finger module, the MIT setup makes use of a small pump and valves at the user's waist (to keep the weight of the prosthesis itself down) that precisely inflate the fingers to form specific shapes. And this pneumatic system is controlled via EMG sensors attached to the user's limb.
Using computer modeling, the researchers designed a controller to inflate the fingers into something resembling five common grasping actions – including pinching, making a fist and cupping the palm.
An algorithm that converts muscle signals into such grasp types was then used to program the controller for common tasks, such as holding a wine glass – where muscle signals picked up by sensors are converted to suitable pressures by the controller and each finger is then inflated by the pump to create the required grasp shape.
The team also incorporated a pressure sensor into each fingertip, and then wired them all up to specific regions on the user's residual limb to provide real-time tactile feedback.
Two volunteers were trained to contract muscles in their arm while picturing the five common grasps in their minds, and then tasked with performing manual strength and dexterity tests using the soft prosthetic hand, such as stacking checkers, writing with a pen, picking up fragile objects and lifting heavy balls. These tests were repeated using a commercially available prosthesis, with the volunteers reporting that the MIT system "was as good, or even better, at most tasks, compared to its rigid counterpart."
The researchers also noted that one of the volunteers effectively demonstrated the device in daily use – able to get to grips with such things as hammer and pliers, use the soft prosthetic hand for eating foods like cake and apples, and even shake someone's hand. When blindfolded, the volunteer was also able to determine which finger had been touched by the researchers, and reported being able to register different-sized bottles when placed in the hand.
A patent has been filed for the design, and work continues to improve the sensing capabilities and range of motion ahead of potential commercialization.
"This is not a product yet, but the performance is already similar or superior to existing neuroprosthetics, which we’re excited about," says MIT's Prof. Xuanhe Zhao. "There’s huge potential to make this soft prosthetic very low cost, for low-income families who have suffered from amputation."
A paper on the development has been published in the journal Nature Biomedical Engineering. The video below shows the prosthetic in use.
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