Nanodiamond‐Infused Microgels for pH‐Tunable Rheology, Lubrication, and UV Protection

Abstract Ultraviolet (UV) radiation exposure poses a potent threat to human health, with skin cancer cases still prevalent. While skin care formulations mitigate the situation, inherent problematic issues, including toxicity and unpredictable rheology, exist in many of these systems. In this study, the functional properties of carboxylated nanodiamond (cND) infused polyacrylic acid (PAA) hydrogels are explored as a potential alternative in terms of their UV protection, rheology control, and lubrication (skin‐feel) properties. cNDs, carbon‐based nanoparticles (≈200 nm), are used because of their innate biocompatible nature, UV filter capacity and tunable surface functional groups allowing for modulating interaction with the PAA matrix. In particular, it is demonstrated these properties can be controlled through varying pH and cND concentration. Dynamic rheology and confocal fluorescence microscopy reveal different degrees of microstructural reinforcement enabled by changing the nature of polymer‐cND and interparticle interactions above (pH 5.5) and below the pKa (pH 4.1) of carboxyl groups. Tribological experiments in contrast, suggest that lubrication properties to be primarily governed by microgel particles at either pH. Most importantly, UV absorption and simulated irradiation studies show these systems to be photostable with broad‐spectrum UV attenuation capacity rivalling that of commercial systems, while preserving gel microstructure after prolonged UV irradiation.