Inflatable robotic hand offers cheap prosthetics for people on low incomes

There is a growing number of commercial neuroprosthetics designed for amputees that use highly articulated bionic limbs and are engineered to sense a user’s residual muscle signals and robotically mimic their intended motions.

However, these devices can cost tens of thousands of dollars and are built around metal skeletons with electrical motors that can be heavy and rigid.

Engineers at MIT and Shanghai Jiao Tong University have tested their inflatable version on amputees who were able to perform daily activities - such as zipping a suitcase, pouring a carton of juice and petting a cat - just as well or better than those with more rigid neuroprosthetics.

The prosthetic was designed with a system for tactile feedback that even restored some primitive sensation in a volunteer’s residual limb. The new design was also proven to be durable and survived being struck with a hammer and run over with a car.

The total cost of its components is only around $500 compared to up to $100,000 for those with 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,” said MIT professor Xuanhe Zhao. “There’s huge potential to make this soft prosthetic very low cost, for low-income families who have suffered from amputation.”

The team’s artificial hand is made from a soft, stretchy material called EcoFlex and comprises five balloon-like fingers, each embedded with segments of fibre, similar to articulated bones in actual fingers. The bendy digits are then connected to a 3D-printed 'palm' shaped like a human hand.

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. This system, including a small pump and valves, can be worn at the waist to reduce the prosthetic’s weight.

A computer model was developed to relate a finger’s desired position to the corresponding pressure a pump would have to apply to achieve that position. Using this 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.

Sensors are fitted at the prosthetic’s opening, where it attaches to a user’s limb, which can pick up signals from a residual limb, such as when an amputee imagines making a fist.

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.

A patent has been filed on the design and work is now underway to improve its sensing and range of motion.

“We now have four grasp types. There can be more,” Zhao said. “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.”