Hydrogels are soft materials composed of a threedimensional (3D) hydrophilic polymer network filled with a large amount of water. Different from rigid machines, hydrogel-based robots are encoded with energy conversion mechanisms that allow sensing, adaption, transformation, and response in complex environments. In this perspective, we discuss the advances and challenges of hydrogel materials for soft robotic and sensing applications. In the first part, we introduce stimuli-responsive hydrogel sensors that can receive external energy inputs from different stimuli and translate them into in the geometrical, optical, electrical, and biological output signals. Then, we comprehensively discuss the recent development of hydrogel robot systems that exploit the responsive properties to achieve diverse locomotion models and functions. On the basis of the distinct driving force and locomotion mechanisms, we categorize hydrogel robots into two main kinds: one is the active robot that deforms by stimuli-responsive swelling of soft hydrogels, and the other is the passive motor that is propelled by reactive matters. We compare the advantages and challenges of each strategy and show how to transform biomimetic principles into technological capabilities through material and structural designs. We finally provide a critical perspective on the key challenges in the integration of functionality in hydrogel robotic systems and reasonable directions to push hydrogel robots toward diverse applications.

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