A Self-Strengthening Hydrogel Coating for Long-Term Underwater Adhesion in Seawater Over 900 Days

Reliable underwater adhesion is a prerequisite for the practical application of hydrogels as underwater coatings. Despite extensive efforts to enhance adhesion strength, strong initial adhesion does not ensure long-term adhesion stability. Achieving durable underwater adhesion of hydrogels remains fundamentally challenging due to interfacial hydration and continuous water-induced network degradation, particularly in complex aqueous environments such as seawater. To overcome these limitations, a hydrogel coating that achieves persistent underwater adhesion in natural seawater for over 900 days is developed. Strong initial adhesion is established across diverse substrates through synergistic interfacial interactions, while network integrity is maintained during prolonged immersion through multiple cooperative intermolecular interactions and continuous quinone-mediated oxidative crosslinking. Consequently, the hydrogel exhibits persistently low water uptake (≈162 wt% after 900 days), suppresses swelling-induced deterioration of interfacial adhesion, and maintains a shear adhesion strength of ≈110 kPa after 900 days of seawater immersion. The hydrogel coating also exhibits antibacterial and anti-algal activity for marine antifouling, underwater superoleophobicity, and applicability as a gel electrolyte for flexible supercapacitors. This work establishes a design principle for achieving long-term underwater functional hydrogels beyond previously reported time scales and provides a general strategy for developing continuously curing materials in underwater and marine environments.