Janus Hydrogels: Design, Properties, and Applications.

Janus hydrogels have attracted significant attention in materials science and biomedicine owing to their anisotropic dual-faced architecture. Unlike conventional homogeneous hydrogels, these heterogeneous systems exhibit structural and functional asymmetry, endowing them with remarkable adaptability to dynamic environmental stimuli. Their inherent biocompatibility, biodegradability, and unique "adhesion-antiadhesion" duality have demonstrated exceptional potential in biomedical applications ranging from advanced wound healing and internal tissue adhesion prevention to cardiac tissue regeneration. Furthermore, "hydrophilic-hydrophobic" Janus configurations, synergistically integrated with tunable conductivity and stimuli-responsiveness, showcase the great potential in emerging domains, including wearable biosensing, high-efficiency desalination, and humidity regulation systems. This review systematically examines contemporary synthesis strategies for Janus hydrogels using various technologies, including layer-by-layer, self-assembly, and one-pot methods. We elucidate the properties and applications of Janus hydrogels in biomedicine, environmental engineering, and soft robotics, and we emphasize recent developments in this field while projecting future trajectories and challenges.