We've recently been hearing a lot about how exoskeletons can be used in rehabilitation, guiding patients' disabled limbs through a normal range of motion in order to develop muscle memory. The problem is, most exoskeletons are rigid, limiting their degrees of freedom to less than those of the body part they're moving. A team of scientists are looking at changing that, with a partial "soft exoskeleton" that replicates the body's own muscles, tendons and ligaments.

The orthotic device was created through a collaboration between researchers at Carnegie Mellon University, Harvard University, the University of Southern California, MIT and wearable sensors developer BioSensics. It incorporates pneumatic artificial muscles (PAMs), lightweight sensors and control software.

The current prototype is designed to be worn on the lower leg, the biological structure of which is replicated in the device. Three of its cylindrical PAMs correspond to muscles in the front of the leg, while a fourth copies a muscle in the back. Artificial tendons (steel cables) extend from the ends of those PAMs down to the foot, and serve to move the ankle.

Feedback is provided by the hyperelastic strain sensors, located on the top and side of the ankle. Each sensor is made up of a rubber sheet containing microchannels filled with a liquid metal alloy. The shape of those channels changes when the rubber is stretched or compressed, thus changing the electrical resistance of the metal. By registering that change in resistance, the control software is able to ascertain the position of the ankle, and the amount of strain being placed upon it.

In lab tests, the device was able to move test subjects' ankles within a 27-degree range of motion, which was considered "sufficient for a normal walking gait."

While the present version of the device is intended for use by people with disorders of the foot and ankle, subsequent versions could reportedly be designed for use in other areas. In fact, Harvard's Wyss Institute for Biologically Inspired Engineering (one of the project's sponsors) has previously developed a larger "soft exosuit".

Before any of the devices are used on actual patients, though, the scientists want to refine the design, making them less awkward to wear.

A paper on the research was recently published in the journal Bioinspiration & Biomimetics. The prototype can be seen in use in the video below.