A curing method for permanent hair straightening using thioglycolic acid (TGA) has been widely used for decade. On the basis of the chemical reactions occurring during the curing treatment and the conformational changes of -helical chains, the straightening mechanism has been proposed. However, the changes of the hair structure in supermolecular level have not been clarified so far. In this report, the effects of reduction and heat treatments on the morphological changes of hair were studied by transmission electron microscopy (TEM). The ultrathin cross sections of the untreated and cured hair samples stained with uranyl acetate and followed by lead citrate were subjected to TEM observations. It was found that the intact cuticle and cortex morphologies of the untreated hair were confirmed from the fact that the A-layer, exocuticle, and endocuticle with high TEM contrast were clearly observed in each layer of the cuticle and the ill-defined cortex morphology with a low contrast was observed due to weak reactions of the staining reagents to the cortex. On the other hand, it was revealed for the stained cured hair, that (i) the decrease in TEM contrast toward the cortex center reflects the decrease of reduction reaction of the cortex components resulting from the extent of diffusion of TGA into the fiber, (ii) many voids were produced on the cuticle layers due elution of the proteins contained therein, (iii) a fine morphology with a high contrast of the structure of the whorl-like macrofibril is resulted from the reactions of the staining reagents with carboxyl groups produced by the disruption of -crystallites, and (iv) deformation, disruption and melting of the -crystallites occur on the arrangements of microfibril + matrix structural units near the cell membrane complex (CMC).
