A component-resolved in vitro skin barrier model for assessing nanoplastic retention and barrier susceptibility

Human-relevant in vitro models are needed to assess dermal nanoplastic hazards, yet current skin penetration approaches often treat the stratum corneum as a compositionally uniform barrier. This limits mechanistic understanding of how barrier biochemistry relates to nanoplastic retention, particularly in skin states with altered lipid or protein organization. Here, we developed a component-resolved in vitro stratum corneum model to quantify interactions between 50 nm polystyrene nanoplastics and six major skin barrier constituents: ceramide, cholesterol, keratin, palmitic acid, proline, and phenylalanine. Component-specific operational retention indices (RAI) were determined using standardized gravity-driven flow experiments and high-resolution transmission electron microscopy, then integrated into a multi-phase penetration model. Retention differed markedly among components. Palmitic acid, representing free fatty acids, showed the highest operational retention, retaining approximately 26-fold more particles than keratin. Protein-rich and sterol-associated components displayed lower initial retention but greater time-dependent accumulation, an apparent trend that with only three time points cannot be assigned to a defined kinetic regime. Ex vivo two-photon imaging of porcine skin showed an apparent detectable fluorescence depth of 12.3 ± 2.1 μm; this was not used to calibrate or validate the dimensionless component-weighted score. The framework is exploratory and mechanistic, characterizing how stratum corneum constituents differ in operational retention of nanoplastics, providing a hazard-relevant basis for, rather than a validated prediction of, dermal penetration.