Complex coacervate-derived hydrogel with asymmetric and reversible wet bioadhesion for preventing UV light-induced morbidities

Protecting the skin from UV light irradiation in wet and underwater environments is challenging due to the weak adhesion of existing sunscreen materials but highly desired. Herein we report a polyethyleneimine/thioctic acid/titanium dioxide (PEI/TA/TiO2) coacervate-derived hydrogel with robust, asymmetric, and reversible wet bioadhesion and effective UV-light-shielding ability. The PEI/TA/TiO2 complex coacervate can be easily obtained by mixing a PEI solution and TA/TiO2 powder. The fluid PEI/TA/TiO2 coacervate deposited on wet skin can spread into surface irregularities and subsequently transform into a hydrogel with increased cohesion, thereby establishing interdigitated contact and adhesion between the bottom surface and skin. Meanwhile, the functional groups between the skin and hydrogel can form physical interactions to further enhance bioadhesion, whereas the limited movement of amine and carboxyl groups on the top hydrogel surface leads to low adhesion. Therefore, the coacervate-derived hydrogel exhibits asymmetric adhesiveness on the bottom and top surfaces. Moreover, the PEI/TA/TiO2 hydrogel formed on the skin could be easily removed using a NaHCO3 aqueous solution without inflicting damage. More importantly, the PEI/TA/TiO2 hydrogel can function as an effective sunscreen to block UV light and prevent UV-induced MMP-9 overexpression, inflammation, and DNA damage in animal skin. The advantages of PEI/TA/TiO2 coacervate-derived hydrogels include robust, asymmetric, and reversible wet bioadhesion, effective UV light-shielding ability, excellent biocompatibility, and easy preparation and usage, making them a promising bioadhesive to protect the skin from UV light-associated damage in wet and underwater environments.