Engineers at Vanderbilt University in Nashville, Tennessee, have devised an unpowered, low-cost ankle exoskeleton that can be worn under clothing, enhancing lower leg strength discretely.

Weighing in under half a kilogram, the device consists of a friction clutch that sits under the sole of the foot, connected to an assistive spring and a soft shank sleeve that wraps around the calf. According to the Vanderbilt team, the ankle exoskeleton costs less than $100 to manufacture and does not require additional components such as batteries or actuators carried on other parts of the body to power the device. It is described in the journal IEEE Transactions on Neural Systems & Rehabilitation Engineering.

“Our design is lightweight, low profile, quiet, uses no motor or batteries, it is low cost to manufacture, and naturally adapts to different walking speeds to assist the ankle muscles,” said senior author Karl Zelik, assistant professor of mechanical engineering at Vanderbilt.

“It could also help reduce fatigue in occupations that involve lots of walking, such as postal and warehouse workers, and soldiers in the field.”

The researchers claim it is the first ankle exoskeleton of its kind that is designed to be worn under regular clothing without restricting movement. In addition, the friction clutch is no thicker than a shoe insole, meaning the entire device can be worn discretely. This feature makes it particularly suitable for elderly people who may not wish others to know they are using an assistive device, or for recreational walkers looking for something that can be worn comfortably and unobtrusively with regular outdoor clothing and boots.

“We’ve shown how an unpowered ankle exoskeleton could be redesigned to fit under clothing and inside/under shoes so it more seamlessly integrates into daily life,” said lead author Matt Yandell, a mechanical engineering PhD student at Vanderbilt University.

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