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A soft, wearable device that mimics the muscles, tendons, and ligaments of the lower leg could aid in the rehabilitation of patients with foot-ankle disorders such as drop foot, said Yong-Lae Park, an assistant professor of robotics at Carnegie Mellon University, Pittsburgh, PA.

Working with collaborators at Harvard University, the University of Southern California, MIT, and BioSensics, Park developed an active orthotic device using soft plastics and composite materials, instead of a rigid exoskeleton. The soft materials, combined with pneumatic artificial muscles (PAMs), lightweight sensors, and advanced control software, made it possible for the robotic device to achieve natural ankle motion.

The soft orthotic device enabled the researchers to mimic the biological structure of the lower leg. The device's artificial tendons were attached to four PAMs, which correspond with three muscles in the foreleg and one in the back that control ankle motion. The prototype was capable of generating an ankle range of sagittal motion of 27 degrees, sufficient for a normal walking gait.

The tradeoff, however, is that the soft device is more difficult to control than a rigid exoskeleton. It thus required more sophisticated sensing to track the position of the ankle and foot and a more intelligent scheme for controlling foot motion, Park said.

Among the innovations in the device are sensors made of a touch-sensitive artificial skin, thin rubber sheets that contain long microchannels filled with a liquid metal alloy. When these rubber sheets are stretched or pressed, the shapes of the microchannels change, which in turn causes changes in the electrical resistance of the alloy. These sensors were positioned on the top and at the side of the ankle.

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