Structure of photo-damaged white and naturally pigmented human hair

The effect of photo-degradation processes on the morphology and ultrastructure of pigmented and non-pigmented hair was evaluated in this work by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Samples consisted of three types of grey hair (separated out into subsamples of totally white and totally black strands) and one type of dark-brown hair exposed to solar irradiation for 75 h or to a mercury lamp for 600 h. SAXS patterns showed that irradiation mainly damaged the amorphous regions of hair, and in some types of hair, an ordered structure appeared, with spaces of ca. 4.5 nm. This structure occurred independently of the presence of pigments and its scattering intensity increased after irradiation. Layers of lipids present in the cuticle may be responsible for this scattering around 4.5 nm, as indicated by TEM images. TEM also showed morphological changes in the outermost cuticle layers, such as the degradation of endocuticle and the detachment of the cell membrane complex. Therefore, the results presented herewith showed that irradiation caused different damages to the hair amorphous fractions, especially in the cuticle layers, which are the most exposed areas of the hair strands.