Highly improved water tolerance of hydrogel fibers with a carbon nanotube sheath for rotational, contractile and elongational actuation

Abstract

Hydrogel fibers drawn from a bulk gel showed high mechanical strength and can be used for artificial spider silk, while their supercontraction in response to water restricts their applications. Moreover, in the field of artificial muscles, although contraction and extension have been realized by different techniques, the selective realization of contraction and extension has not been achieved for a straight, non-coiled fiber by controlling the fiber internal structure. In this paper, the water tolerance of hydrogel fibers was highly improved by coating a carbon nanotube sheath, and hygromorph torsional, contractile, and elongational actuations were realized for twisted, non-coiled hydrogel fibers coated with aligned carbon nanotubes. The wrapping angle between the carbon nanotube and the hydrogel fiber determines the contraction or elongation. The fiber actuator showed hygromorph contractile actuation if the carbon nanotube alignment direction is parallel to the hydrogel fiber direction, and the actuator showed elongational actuation if the wrapping angle is non-zero. The actuation originated from synergistic volume expansion of the hydrogel fiber and anisotropic restriction of aligned carbon nanotubes. This paper provides a new design for improving the mechanical properties of hydrogel fibers and exploring actuation modes using a sheath–core structure with different wrapping angles of a sheath material.