Mechanical engineers at Rice University in the US claim to have built an 'extra limb' capable of grasping objects, powered by compressed air derived from walking.

Anoop Rajappan, left, and Daniel Preston of Rice University set up an experiment with their fabric air pump. The lab developed its textile-based energy harvesting shoe able to power assistive devices for people with disabilities
Anoop Rajappan, left, and Daniel Preston of Rice University set up an experiment with their fabric air pump. The lab developed its textile-based energy harvesting shoe able to power assistive devices for people with disabilities - Brandon Martin/Rice University

The proof-of-concept ‘extra limb’ is one of several ideas the team has implemented with a textile-based energy harvesting system. The device was designed and built by Daniel Preston, an assistant professor of mechanical engineering, along with lead authors Rachel Shveda and Anoop Rajappan and their team.

Described in Science Advances, the prototype ‘arm’ is a piece of fabric that hugs the body when not in use, but extends outward when activated and incorporates an elastomer lining on the surface to maintain its grip on slippery objects. 

For demonstrations, Rice alumna Shveda would operate the arm with a switch. Preston said future versions could have sensors that anticipate the wearer’s intent and complete the movements.

In addition to the curling arm that can grip a cup or other small objects while one’s hands are full, the Rice lab built a shirt with a bellows-like actuator attached at the armpit that expands, enabling the wearer to pick up a ten pound object. Testing the apparel on a mannequin showed it could do so without an assist from human muscles.

“Census statistics say there are about 25 million adults in the United States who find it difficult to lift ten pounds with their arms,” said Rajappan, a postdoc supported by the Rice Academy of Fellows.

“That’s something we commonly do in our daily lives, picking up household objects or even a baby.”

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The system requires two components: textile pumps embedded in the soles of walking shoes that harvest air pressure, and pneumatic actuators that make use of that pressure where needed. The pumps are filled with open-cell polyurethane foam that allows them to recover their shape after every footfall.

Preston said the pump is small enough to be comfortable, with the stiffness of the foam ‘about on par’ with a typical shoe insert. All components for a single device cost the lab around $20, he added, and products were simple and robust enough to be cleaned in a washing machine with no degradation in performance.

Tests by the Rice lab showed the devices produce the equivalent of 3W of power with a conversion efficiency of more than 20 per cent, outperforming electromagnetic, piezoelectric and triboelectric strategies for foot-strike energy harvesting including one designed by students at Rice’s Oshman Engineering Design Kitchen.

“The fabrication approach uses techniques that are already employed in the garment industry, things like cutting textile sheets and bonding them with heat and pressure,” said Preston. “We're ready to think about translating our work towards products.”

Rajappan said that along with test units, the lab also developed mathematical models to predict how well an assistive device would perform based on a user’s weight and walking speed, among other parameters. “One way to take this forward will be to use the model to optimise performance for specific user groups,” he said.

“We’re also thinking about devices like pneumatic actuators that apply therapeutic compression for things like deep vein thrombosis, blood clots in the legs. Anything that requires air pressure can be powered by our system.”

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