In Situ Assembly of Bilayer Hydrogels by Templated Enzymatic Polymerization for Ultrahigh UV Absorption and Photothermal Transdermal Therapy

Ultraviolet radiation from sunlight is a principal cause of skin photodamage, leading to sunburn, photoaging, and skin cancers. Although numerous sunscreen materials have been developed, their performance remains limited by insufficient ultraviolet protection, low biosafety due to skin penetration of ultraviolet filters, poor skin adhesion, and a lack of therapeutic capacity for damaged tissues. Here a polyvinyl alcohol (PVA)-based bilayer hydrogel fabricated by in situ assembly is reported. During gelation, interpenetration and cross-linking of PVA chains at the interface fuse the two layers into an integrated structure. The resulting hydrogel exhibits exceptional ultraviolet absorption (SPF 1997.6 PA++++)-twentyfold greater than previously reported values-robust skin adhesion (317 J m^-2), photothermally controlled transdermal drug delivery (over 12 h), and excellent biocompatibility. The top layer employs template-directed enzymatic polymerization of dopamine to produce polydopamine (PDA) nanoparticles that are firmly anchored within the PVA network. Extensive hydrogen bonding between PVA and PDA reduces the bandgap of PDA and enhances electron delocalization, markedly enhancing ultraviolet absorption. The bottom layer incorporates polysaccharides that confer robust skin adhesion and regulate drug release. This layered design enables the integration of skin protection and therapy, providing a generalizable approach for engineering complex soft materials.