Actuation of 3D Printed Nanocolloidal Hydrogel with Structural Anisotropy

Polymer hydrogels exhibit actuation properties that result in stimulus responsive reversible shape transformations and have promising applications in soft robotics, drug delivery systems, sensors, and microfluidic devices. Actuation occurs due to differential hydrogel swelling and is achieved by modulating site-specific composition of hydrogel sheets, either laterally (in-plane), or via a bilayer approach.<br/> We proposed a different strategy for hydrogel actuation that originates solely from its structural anisotropy. For 3D-printed single-layer hydrogels formed by cellulose nanocrystals (CNCs) and gelatin methacryloyl we showed that shear-induced orientation of CNCs results in anisotropic mechanical and swelling properties of the hydrogel. Upon swelling in water, planar hydrogels acquired multiple complex 3D shapes that are achieved by (i) varying CNC orientation with respect to the shape on the hydrogel sheet and (ii) patterning the hydrogel with the regions of shear-mediated and random CNC orientation. This study shows the capability to generate multiple shapes from the same hydrogel actuator based on the degree of its structural anisotropy. Furthermore, it introduces a biocompatible nanocolloidal ink with shear-thinning and self-healing properties for additive manufacturing of hydrogel actuators.