3D printable tough hydrogel actuators

Nature autonomously actuates many of its structures in response to changes in environmental conditions. Inspired by nature, stimuli-responsive hydrogel-based actuators that wirelessly operate without external energy sources have been developed. By exploiting the tunable swelling behavior of hydrogels, these systems can actuate in the form of bending, twisting, folding, and even locomotion. However, for these materials to function effectively across a broad range of applications, their mechanical properties - especially their stiffness and toughness - must be improved to increase their actuation force and operational reliability. Addressing these mechanical performance challenges in hydrogel-based actuators would bring them closer to replicating the remarkable combination of mechanical toughness, resilience, and actuation observed in nature. This review outlines established toughening strategies for hydrogels and highlights advances in their additive manufacturing into actuators with well-defined structures and locally varying compositions. It concludes with a brief outlook on potential opportunities that arise if self-healing or improved fatigue resistance are incorporated into actuating systems.