Structure of photo damaged white and naturally pigmented human hair

The effect of photo degradation processes on the morphology and the ultrastructure of pigmented and non-pigmented hair were 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 in totally white and totally black strands) and one dark-brown hair exposed to solar irradiation for 75 h or to a mercury lamp for 600 h. SAXS patterns showed that irradiation damaged hair amorphous regions mainly and they also evidenced an ordered structure with ca. 4.5 nm of distance spacing that appeared in some types of hair. 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 the ones present in the cuticle layers, which are the most exposed areas of the hair strands. Also, these results can be an indicative that some types of lipids have a photo protective function in hair structure.