Anisotropic Hydrogel Fibers for Soft Robotics: From Structural Engineering to Multi-Responsive Actuation.

Hydrogel fibers provide a one-dimensional platform for constructing soft robotic materials that combine tissue-like compliance, high water content, structural anisotropy, and stimulus responsiveness. Compared with bulk hydrogels, their reduced radial dimensions shorten mass-transport pathways, while programmable fiber architectures convert otherwise isotropic swelling or contraction into directional deformation. This review summarizes the recent progress in anisotropic hydrogel fibers for soft robotics, with emphasis on the relationships among fabrication strategies, fiber architectures, actuation mechanisms, and robotic functions. Representative architectures, including Janus, bilayer, core-sheath, hollow, helically twisted, gradient, axially patterned, woven, and printed systems, are discussed in terms of their strain-conversion mechanisms, structural advantages, limitations, and suitable applications. Major fabrication approaches and stimulus-responsive mechanisms are further compared with respect to structural programmability, response kinetics, mechanical output, cyclic stability, scalability, and device integration. Particular attention is given to architecture selection, long-term environmental stability, interference from secondary stimuli, and the transition from laboratory demonstrations to practical soft robotic systems. Finally, key design principles and future directions are outlined for developing faster, more durable, manufacturable, and autonomous hydrogel-fiber-based soft robots.