Lignin‐Based Ultrathin Hydrogel Coatings with Strong Substrate Adhesion Enabled by Hydrophobic Association

Abstract Hydrogel coatings are crucial strategies for integrating substrates with functional surfaces. Hydrogel coating with a facile coating process, high toughness, and ultrathin thickness is highly desired. This study utilizes the self‐assembly of hydrophobic lignin into chain‐dense regions during solvent evaporation and constructed a hydrogel coating with Janus structure from hydrophilic polymer and hydrophobic lignin through a facile coating‐drying‐swelling process at room temperature (25 °C). The Janus structure formed as the hydrophobic lignin‐aggregated regions remained at the bottom while the hydrophilic polymer swell at the top. The lignin‐aggregated regions constrained the swelling of the hydrogel coating, allowing for an ultrathin coating layer of ≈8 µm. Additionally, the lignin‐aggregated region increases the stiffness and toughness of the thin hydrogel coating film by establishing strong interfacial bonding with the substrate via dense Van der Waals forces, thereby increasing overall adhesion energy with thin thickness and to various substrates such as glass, polytetrafluoroethylene (PTFE), metal, and rubber. The hydration layer is formed by the hydrophilic polymer swollen on top, which improves lubrication and antifouling properties. Furthermore, the hydrogel coating exhibits high stability in water and saline solutions, ultraviolet shielding properties, and the ability to be removed and recycled after use.