Hydrogel actuators: from building blocks selection, anisotropic structures design to multifunctional applications

Flexible smart actuators that produce mechanical output in response to external stimuli have numerous applications in robotics, electronics, and biomedical engineering. With high similarity to biological tissues, high water content, and good tissue adhesion, hydrogels present unique advantages in preparing soft actuators. The development of integrated building blocks further enhances the structural controllability and performance tunability of hydrogel-based actuators. While current research predominantly focuses on the types of environmental responses and multi-field applications for hydrogel actuators, the critical role of integrated building blocks remains relatively underexplored. This review summarizes anisotropic strategies and multifunctional applications for hydrogel actuators based on the selection of stimuli-responsive building blocks (SRBs). Firstly, the deformation mechanism of hydrogel actuators with different polymer matrices is briefly introduced, followed by an overview of the anisotropic strategies employed in hydrogel actuators based on SRBs. The preparation strategies and high actuation performance of hydrogel actuators are discussed from the perspectives of building block properties and anisotropic structures. Moreover, the multifunctional application prospects of SRBs-based hydrogel actuators in information encryption, self-sensing, and intelligent grasping are discussed. Finally, the design principles and current challenges of stimuli-responsive hydrogel actuators are emphasized, and an outlook on the future development of novel hydrogel actuators is presented.