Reversible actuation of fibrous artificial muscle under external compression load

Herein, we report hybrid fibrous artificial muscles with reversible actuation, i.e., expansion upon cooling and contraction upon heating, under external compression. Although many fibrous polymeric artificial muscles by twist insertion in precursor fibers have been developed, most of them cannot reversibly actuate without an external tensile load. While heterochiral Nylon muscles can reversibly actuate under external compressive load, the compressive stress applied is low (0.078 MPa). In this study, we inserted pre-tensioned polymeric fibers with reversible actuation into pre-compressed helical metallic spring and obtained hybrid fibrous artificial muscles. We employed two types of two-way shape memory polymers, one type of fishing line artificial muscle, and seven types of helical springs in preparing seven types of hybrid muscles. A structural mechanics model was developed, and numerical simulation was conducted to evaluate the effect of the design parameters on the actuation strain. It is found that all the hybrid muscles were free-standing (reversibly actuate without external load) and beyond free-standing (reversibly actuate under external compression load). As an example, one hybrid muscle actuated reversibly under 24 MPa compressive stress without buckling. We expect that this study will open new opportunities for the use of fibrous artificial muscles as linear actuators in soft robotics or other applications that need reversible actuation under external compression.