Three-Dimensional Shape Transformation of Bilayer Polymers Regulated by Anisotropic Dehydration-Mediated Hygroscopic Expansion of Metallohydrogels

Abstract

Shape-morphing polymer hydrogels that mimic humidity-change-triggered shape transformations are of interest for developing soft robotics and actuators. Yet, covalent cross-links limit accessible curvature for three-dimensional shape-morphing, especially when generating Gaussian curvatures on two-dimensional (2D) thin plates. Herein, we demonstrate a bilayered shape-morphing polymer material composed of a thermoplastic polymer passive layer and a poly(acrylic acid) hydrogel active layer cross-linked only by Fe3+-COOH coordination bonds (PAA/Fe), which exhibited remarkable Gaussian curvature shape deformation on a 2D thin polymer film. The PAA/Fe hydrogel first underwent anisotropic dehydration and then swelled in water to achieve shape-morphing. During the entire shape-morphing process, the well-maintained stiffness and elasticity of the PAA/Fe hydrogel provide sufficient internal stress to drive deformation. This anisotropic dehydration-mediated hygroscopic expansion-triggered shape-morphing can be applied for printable and rewritable patterning, soft gripping, and impact resistance. The alternating assembly of the PAA/Fe hydrogel and covalent hydrogel sheets on a thermoplastic film enables various wavy-shaped hygroscopic shape transformations with potential actuation and impact resistance applications. Our work demonstrates a design strategy to fabricate shape-morphing materials with potential applications in multifunctional soft robotics and actuators.