Novel Accordion-Inspired Foldable Pneumatic Actuators for Knee Assistive Devices

Despite that various soft bending actuators have been presented in recent years, their widespread application in knee assistive devices is still hindered by their uncomfortable geometry, limited torque output, control complexity, and high cost to enhance the knee joint during various human movements. Inspired by accordion bellows, we proposed a novel design of foldable pneumatic bending actuators (FPBAs) in this study, which were fabricated with the thermoplastic polyurethane fabric materials. FPBAs could produce torque by the inflation or interactive compression of their interconnect air chambers with no airflow restriction occurring at any bending angle. General mechanical equations for FPBAs were derived to characterize their output torque as functions of their geometry and internal air pressure. To evaluate the feasibility of FPBAs, a specific prototype with rectangular chambers was constructed and tested by mechanical experiments. A knee exosuit equipped with the prototype was also designed, and five subjects were recruited to perform static postures in knee rehabilitation training with and without the assistance of the knee exosuit. Subjects' persistence time and knee extensors' surface electromyography signals were recorded and compared, which verified the assisting effects of the knee exosuit. Besides knee assistive devices, the FPBAs could also be applied to other soft robots thanks to their competitive advantages, such as flexibility, large torque output, low-pressure input, simple fabrication process, light weight, and low cost.