Design and characterization of a hyperelastic tubular soft composite

Research in the field of human mobility assistive devices, aiming to reduce the metabolic cost of daily activities, is seeing the benefits of the exclusive use of passive actuators to store and release energy during the gait cycle. Current devices commonly employ either mechanical springs or Pneumatic Artificial Muscles as the primary method of passive actuation. The Pneumatic Artificial Muscle has proven to be a superior actuation choice for these devices, when compared to its alternatives. However, challenges regarding muscle pressure loss and limited elongation potential have been identified. This paper presents a hyperelastic tubular Soft Composite that replicates the distinctive mechanical behaviour of the Pneumatic Artificial Muscle without the need for internal pressurization. The proposed Soft Composite solution is achieved by impregnating a prefabricated polyethylene terephthalate braided sleeve, held at a high initial fibre angle, with a silicone prepolymer. A comprehensive experimental evaluation is achieved on numerous prototypes for a variety of customizable design parameters including: the initial fibre angle, the silicone stiffness, and the braided sleeve style. This research has successfully developed, tested, and validated a novel Soft Composite that can achieve the desired nonlinear stiffness and elongation potential for optimal use as passive actuation in human mobility assistive devices.