4D printing of stimuli-responsive hydrogel actuators for intelligent biomedical soft robotics.

4D printing, or additive manufacturing of stimuli-responsive materials, is transforming soft robotics by enabling the fabrication of complex, adaptive systems that evolve in shape and function over time. Due to their exceptional responsiveness and biocompatibility, hydrogels are well suited for 4D printing because they mimic the physicochemical and biochemical properties of natural tissues while generating programmable mechanical work that bridges stimuli-responsive chemistry and biomimetic motion. This article reviews the state of the art in 4D-printed hydrogel actuators across emerging biomedical applications, focusing on the intersection of functional soft materials and biomedical robotics. While existing reviews cover advancements in 4D-printed hydrogels, a focused review on hydrogel actuators specifically for biomedical soft robotics is still needed. Recent innovations, such as soft robotic grippers, shape-morphing surgical tools, actuated implants, and self-sensing soft interfaces, are discussed here to provide a roadmap for translating complex material architectures into next-generation intelligent biomedical soft robotics.