3D Printing of Polymer Hydrogels—From Basic Techniques to Programmable Actuation

This review discusses the currently available 3D printing approaches, design concepts, and materials that are used to obtain programmable hydrogel actuators. These polymer materials can undergo complex, predetermined types of motion and thereby imitate adaptive natural actuators with anisotropic, hierarchical substructures. 3D printing techniques allow replicating these complex shapes with immense design flexibility. While 3D printing of thermoplastic polymers has become a mainstream technique in rapid prototyping, additive manufacturing of softer polymers including polymer hydrogels is still challenging. To avoid deliquescence of printed hydrogel structures, the polymer inks used for hydrogel manufacture need to be sheer‐thinning and thixotropic, with fast recovery rates of the high viscosity state. This is achieved by adding polymer or particle‐based viscosity modifiers. Further stabilization of the interfaces of the printed voxels, e.g., by UV cross‐linking, is often also required to obtain materials with useful mechanical properties. Here state‐of‐the‐art techniques used to 3D print stimulus responsive, programmable polymer hydrogels, and hydrogel actuators, as well as ink formulation and post‐printing strategies used to obtain materials with structural integrity are reviewed.