Poly (vinyl alcohol) based gradient cross-linked and reprogrammable humidity-responsive actuators

Abstract

Actuators are a kind of energy-conversion devices which can perceive external stimulus and undergo reversible deformation or motion to produce a mechanical response. Developing actuators with advanced structures rather than typical bilayer configuration is essential for stable actuations, and installing reprogrammability into actuators for on-demand complex shape transformation within the same piece of material is more convenient and cost-effective for new actuation modes. However, actuators that satisfy these requirements have rarely been reported yet. Herein, a facile and low-cost design strategy for a transparent, gradient cross-linked, patternable and reprogrammable humidity-responsive actuator based on commercial available poly (vinyl alcohol) (PVA) is presented. The actuator is fabricated by grafting PVA with 9-anthracenecarboxylic acid (9-AN) via esterification and then blending with a UV-screening agent 2,2′,4,4′-tetrahydroxybenzophenone (THBP) followed by photo-crosslinking. In such design, the abundant hydroxyl groups in grafted PVA endow the actuator with humidity-responsive ability, the THBP enables the gradient cross-linking of actuator due to the attenuated UV intensity through the film thickness, and the photo-controlled reversible cross-linking of anthracene groups allows the patterning and reprogramming of the actuator. The chemical structure, gradient morphology, and humidity-responsiveness of actuators are thoroughly investigated. The fast actuation speed and large actuation amplitude, the realization of complex shape deformation by photo-controlled programming/reprogramming, and the biomimetic applications such as the soft walking robot are also demonstrated. This study is expected to provide new opportunity for facilely developing high-performance, stable and reprogrammable actuators.