Unveiling the link between surface physicochemistry and nanomechanical behavior in bleached and UV-irradiated human hair fibers

The mechanical integrity and failure mechanisms of human hair fibers are critically dependent on the hierarchical structure of the cuticle, which serves as a durable record of biological and environmental history. In this study, we employed a multi-parametric approach to quantify the nanomechanical evolution of hair degradation, establishing a correlation between surface physicochemistry and local tribological properties. We combined Atomic Force Microscopy in Quantitative Nanomechanical Mapping (QNM) mode with Power Spectral Density (PSD) analysis, Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray Spectroscopy (EDS). QNM revealed distinct nanomechanical signatures: bleached fibers exhibited elevated adhesion domains and markedly increased heterogeneity compared to virgin counterparts. These tribological behaviors were consistent with trends observed in EDS analysis regarding chemically adsorbed sodium and oxidative residues, although morphological changes remained the primary indicator of damage. While natural aging in distal regions induced surface deterioration, the magnitude of damage was significantly more pronounced in fibers subjected to combined bleaching and UV irradiation. This suggests a synergistic degradation pathway where chemical pre-treatment amplifies the susceptibility to UV-induced defects. This study demonstrates that chemical adsorption and weathering can co-occur, yet manifest along statistically distinct chemical and structural dimensions that jointly compromise the cuticle barrier function. These findings highlight the potential utility of nanoscale multiparametric mapping not only for assessing cosmetic treatments, but also for informing forensic and clinical contexts, where such tribological descriptors may support diagnostic profiling of structural and chemical anomalies.