High performance and tunable artificial muscle based on two-way shape memory polymer

Abstract

Artificial muscles, a class of bio-inspired actuators, have been investigated for decades. Recently, a remarkable breakthrough in artificial muscles was achieved by twisting then coiling polymeric fishing lines or sewing threads. Driven by the negative coefficient of thermal expansion (NCTE), the tensile actuation strain, which was about 4% in the precursor nylon 6,6 fiber, was magnified to about 34% in the muscle. However, the muscle is limited by its higher actuation temperature (up to 160 °C). Also, some applications such as soft robots require even larger actuation strain to maintain locomotion. In this study, chemically cross-linked poly(ethylene-co-vinyl acetate) (cPEVA) two-way shape memory polymer (2W-SMP) was synthesized, characterized, and processed into precursor fibers based on a solid solution approach. Artificial muscles were manufactured through twist insertion in the precursor fibers. It was found that the 2W-SMP fibers contract upon heating and expand upon cooling, similar to polymeric fibers with NCTE, but with lower actuation temperature and higher actuation strain. The cPEVA fiber, which has about 18% contraction when the temperature is increased from 20 °C to 67 °C, exhibits an actuation strain up to 68% in the muscle. It was also found that the muscle actuation is tunable, i.e., independent of its actuation history.