Fabrication of Large‐Area, Ultra‐Strong, and Anti‐Swelling Hydrogel Thin Films for Underwater Invisible Electronic Skin

Abstract Thin hydrogel films (THFs) with submillimeter thicknesses are promising for on‐skin electronics due to their exceptional shape adaptability and interfacial adhesion. However, it remains challenging for THFs simultaneously achieving high transparency, conductivity, and mechanical robustness. Moreover, the fabrication of THFs is limited by the rapid dehydration and uncontrollable swelling of hydrogels. Herein, through the design of a crosslinker 3‐cinnamamylphenylboric acid (RBA), a triple‐crosslinking strategy is developed to fabricate large‐area THFs. Anchoring RBA onto polyvinyl alcohol (PVA) chains followed by UV‐crosslinking affords hydrogels (denoted as RBVA) comprising three crosslinks including dynamic boronate ester groups, [2 + 2] photocycloaddition of cinnamamide groups, and hydrophobic aggregation. RBVA THFs with thicknesses tunable from ≈ 50 to 200 µm exhibit light transmittance ≥90% at a wavelength of range 400–800 nm, toughness of 7.2–48.0 MJ m −3 , and conductivity up to 3.63 mS cm −1 . RBVA THFs maintain constant swelling ratios and mechanical properties during long time underwater soaking or cyclic drying‐swelling treatment. These thin, transparent, conductive, strong, and underwater stable RBVA THFs can be processed into invisible electronic skin with highly stable underwater sensing performances. The crosslinking strategy may inspire the design of durable and underwater stable THFs, advancing their potential in flexible electronics and biomimetic materials.