Variable Stiffness Honeycomb Metamaterials for Adaptive Ankle Brace Design

Abstract

Lateral ankle sprains cost billions of dollars in medical expenses annually and frequently result in long-term functional decline and a diminished health-related quality of life. While ankle braces have been shown to be effective in prophylaxis of subsequent ankle sprains, current braces are either too stiff and affect normal gait or too flexible and provide insufficient support during high-intensity activities. In this study, we proposed an adaptive ankle brace design that employs dynamically variable stiffness components to provide minimum support under normal gait movements and maximum rigidity under large ranges of motion. To achieve these unique properties, a honeycomb geometry was designed and three dimensionally printed with thermoplastic polyurethane to exhibit nonlinear, strain-stiffening, elastic behavior. We conducted a series of tensile load tests on different honeycomb unit cell configurations. First, the influence of unit cell designs on their mechanical strength and force-strain profiles was characterized. Second, experimentally calibrated finite element models of individual components simulated the mechanical response of the geometry, which were then used to optimize the geometrical parameters of the honeycomb shape (i.e., ring size, length of lateral elements, and thickness). The results identified promising design parameters for these honeycomb geometries that could be used to realize next-generation adaptive ankle braces.