Bio-inspired shape-morphing actuator with a large stroke at low temperatures

Abstract

The shape-morphing actuator that structurally and functionally biomimics natural muscle is an active research field. We present a novel twisting-bending coupled self-helix (TBSH) structure driven by reversible shape-morphing between a compact and extended helix at a human-friendly temperature. The following two independent deformations were induced on each perpendicular axis of a hydrogel fiber to obtain a hydrogel helix by mimicking the vorticella: 1) twisting in cross sections induced by pretwisted nylon spring and nontwisted poly(N-isopropylacrylamide) (PNIPAM) hydrogel, called torsional strain mismatch, and 2) bending in the longitudinal direction induced by a nonexpandable nylon spring and expandable PNIPAM hydrogel in a noncoaxial structure, called tensile strain mismatch. The TBSH was formed by the force balance, resulting in reversible shape-morphing with a change in the mechanical properties of PNIPAM (lower critical solution temperature = 33ºC). The elastic modulus increased with increasing temperatures, resulting in a shape change from a compact helix to an extended helix. The TBSH has three remarkably advanced characteristics: 1) a high tensile stroke (165%) with shape-morphing (11 times higher than that with only the PNIPAM fiber (−15%)), 2) extension with increasing temperature, the opposite the contraction of the previous thermally responsive actuator, and 3) fast extension within 3 s under an immediate temperature increment. The novel actuator can be applied in various industries, such as smart textiles and automatic thermostat systems.