Hydrogels for Long‐Term Moisture Retention under Ambient Conditions: Inhibiting the Evaporation of Free Water from Macroscopic to Molecular Scales

Abstract Hydrogels are soft‐wet materials with unique properties and multiscale network structures. However, traditional hydrogels suffer from water evaporation under ambient conditions, leading to structural changes and functional decline. This review systematically summarizes recent advances in long‐term moisturizing hydrogels under ambient conditions, focusing on the mechanisms for inhibiting free‐water evaporation within hydrogels and elaborating on moisture‐regulation mechanisms and strategies from the macroscopic to the molecular scale. Based on evaporation thermodynamics theory, the review clarifies the intrinsic relationship between free water/bound water states and evaporation rates. Strategies to enhance water retention include: 1) physical barrier encapsulation, such as hydrophobic polymer membranes or shell structures; 2) network structure optimization, like interpenetrating or dual‐network designs to improve water‐holding capacity and regulate pore distribution; 3) molecular‐level control, incorporating hydrophilic components (e.g., glycerol, ionic liquids) via hydrogen bonding to manipulate water states. Notably, eutectogels using low‐volatility deep eutectic solvents outperform traditional water‐based systems, achieving excellent mechanical performance and long‐term moisturization. The review also discusses potential applications and identifies knowledge gaps, offering future research directions. Ultimately, a multiscale design strategy is emphasized for hydrogels to maintain moisturizing properties under ambient conditions, paving the way for their extensive real‐world applications.