Smart hydrogel surface patterning: From static structures to programmable interfaces.

Hydrogels, as three-dimensional hydrophilic polymeric networks, present outstanding biocompatibility, tunable mechanics, and sensitive stimuli responsiveness, but conventional homogeneous hydrogels fail to meet the demand for multifunctional integration in advanced applications. Surface patterning provides a powerful approach to precisely engineer interfacial structures, chemical distributions, and dynamic responsive behaviors. This review systematically classifies hydrogel surface patterning into top-down techniques (photolithography, templating, printing, stimulus-response writing) and bottom-up strategies (self-assembly, field-induced directed assembly), along with hybrid strategies that synergistically combine multiple fabrication routes. A brief cross-comparison with surface patterning of other soft materials, including PDMS elastomers, organogels, and cryogels, is also provided to clarify the technical advantages and limitations of hydrogels. Furthermore, the review summarizes multiscale characterization methodologies covering morphology, surface chemistry, and functional responsiveness, and highlights frontier applications in anti-counterfeiting, tissue engineering, biosensing, flexible electronics, and environmental remediation. Key challenges including fabrication scalability, dynamic reversibility, and biocompatibility are identified, and future directions toward programmable intelligent interfaces, multi-material integrated manufacturing, and sustainable green processing are prospected.