Design and Material Characterization of a Hyperelastic Tubular Soft Composite

Abstract

Research within the field of human motion assistive device development, with the purpose of reducing 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. Designs of novel exoskeletons at the University of Ottawa implement the Pneumatic Artificial Muscle (PAM) as the primary method of nonlinear, passive actuation. The PAM is proven as a superior actuator for these devices when compared to the linear mechanical springs used by other researchers. There are, however, challenges regarding PAM pressure loss and the limitation of PAM elongation that have been identified.

This thesis aims to develop a hyperelastic tubular soft composite that replicates the distinctive mechanical behaviour of the PAM without the need for internal pressurization. The final soft composite solution was achieved by impregnating a prefabricated polyethylene terephthalate braided sleeve, held at a high initial fibre angle, with a silicone prepolymer. A comprehensive experimental evaluation was performed on numerous prototypes for a variety of customizable design parameters including: initial fibre angle, silicone stiffness, and braided sleeve style. Moreover, two separate analytical models were formulated based on incompressible finite elasticity theory using either a structural model of Holzapfel’s type, or a phenomenological model of Fung’s type. Both models were in good agreement with the experimental data that were collected through a modified extension-inflation test.

This research has successfully developed, tested, and validated an innovative soft composite that can achieve specific mechanical properties, such as contraction distance and nonlinear stiffness, for optimal use in human motion assistive devices.