3D printing hydrogels for actuators: A review

Soft and wet actuator systems have attracted great attention in some applications, such as assistive technologies for rehabilitation, training and regenerative biomedicines. Three-dimensional (3D) printing methods have realized the rapid fabrication of complex structures without the need for expensive dies or post processing. In this review, a comprehensive description is presented on stimuli-responsive hydrogels fabricated by light-responsive and extrusion-based 3D printing technologies. Mechanisms of actuations have been introduced based on stimuli types. As the most common method for 3D printed hydrogel actuators, direct-ink-writing has been discussed, including the two printing parameters of resolution and rheology. In addition, applications of 3D printed hydrogel actuators are presented followed by introductions of recent contributions on enhancing the toughness of 3D printed hydrogel and robust design tools, such as finite element analysis and artificial intelligence.