Effects of excessive bleaching on hair: comparative analysis of external morphology and internal microstructure

This study investigates the impact of excessive bleaching on the external morphology and internal microstructure of hair, compared to untreated hair. Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we observed significant changes in both the surface and internal structures of bleached hair. SEM analysis of normal hair revealed a relatively clean surface with intact cuticle scales, while bleached hair showed brittle, torn scales with a rough appearance. In areas where the cuticle was broken, remnants of endocuticle debris were still attached, contributing to the rough surface. Complete separation of the cuticle layer resulted in numerous longitudinal fissures along the exposed cortical surface of bleached hair. TEM analysis further confirmed distinct differences; in normal hair, the cuticle layer and cortex were well-separated, and a small hole was observed within the endocuticle of the cuticle cells. Conversely, in bleached hair, the cuticle layer was separated from the cortex, with numerous pores formed by the dissolution of melanin granules scattered within the cortex, specifically between the macrofibrils. No melanin granules were detected in the cortex of bleached hair, although the macrofibril structure remained intact. The findings clearly indicate that excessive bleaching leads to the loss of the cuticle layer, exposing the cortex and significantly altering the hair's structural integrity.