Modeling Dynamic Swelling of Polymer-Based Artificial Muscles

Polymer-based artificial muscles could potentially replace traditional motors and actuators in applications where weight and flexibility are important, such as soft robotics, active prosthetics, and microfluidics. Material chemistry and muscle geometry are important parameters that impact device performance, e.g. strain, strain rate, lifetime, achievable work, and efficiency. Modeling the rate and degree of swelling of polymer fibers is an essential part of developing materials and designing muscles that perform as desired. This study is motivated by the possibility of significant actuation from twisted and coiled polymer fibers that rely on radial swelling to produce reversible work. An analytical thermodynamic expression (based on Flory-Huggins Theory) was combined with a numerical transport model in order to simulate transient swelling of a polymeric network driven by diffusion and migration. The numerical model evaluates the impact of polymer swelling on transport in polymers directly by locally accounting for the length increase of discrete elements due to solvent presence, which cannot be done analytically. The combined model of transient radial swelling of polymer fibers can be used for parametric studies or analysis of experimental data. This study will aid efforts to identify the best material candidates for practical use as artificial muscle fibers and will help evaluate the geometry needed to achieve device requirements.