Polyphosphazene Microparticles with High Free Radical Scavenging Activity for Skin Photoprotection

Ultraviolet (UV) filters are the core ingredients in sunscreens and protect against UV-induced skin damage. Nevertheless, their safety and effectiveness have been questioned in terms of their poor photostability, skin penetration, and UV-induced generation of deleterious reactive oxygen species (ROS). Herein, an organic UV filter self-framed microparticle sunblock was exploited, in which quercetin (QC) and hexachlorocyclotriphosphazene (HCCP) were self-constructed into microparticles (HCCP-QC MPs) by facile precipitation polymerization without any carriers. HCCP-QC MPs could not only significantly extend the UV shielding range to the whole UV region but also remarkably reduce UV-induced ROS while avoiding direct skin contact and the resulting epidermal penetration of small-molecule QC. Meanwhile, HCCP-QC MPs possess a high QC-loading ability (697 mg g^-1) by QC itself as the microparticles' building blocks. In addition, there is no leakage issue with small molecules due to its covalently cross-linked structure. In vitro and vivo experiments also demonstrated that the HCCP-QC MPs have excellent UV protection properties and effective ROS scavenging ability without toxicity. In summary, effective UV-shielding and ROS scavenging ability coupled with excellent biocompatibility and nonpenetration of small molecules make it a broad prospect in skin protection.