Applied Cosmetic Science and Technology Applied Cosmetic Science and Technology

In recent years, hair bleaching has gained popularity. While bleaching can increase the brightness of hair compared to normal coloring and can produce pale and highly saturated hair colors that cannot be achieved by coloring alone, it puts a lot of strain on the hair, raising concerns about the damage caused by bleaching. Furthermore, there are limited reports on the relationship between the internal condition of hair and its shape, as well as the mechanism underlying complex damage to both the interior and exterior of hair caused by bleaching. Therefore, in this study, we measured the effect of different bleaching times on the internal structure of hair and the resulting changes in its cross-sectional shape, focusing on the thickness and internal condition of each bleached hair strand. Additionally, we investigated the effect of keratin ingredients in repairing the damage. The results showed that as the number of bleaching cycles increased, the hair became flatter and the cross-sectional shape of the hair approached an ellipse. The number and average size of voids in the hair increased owing to bleaching. It was also confirmed that as the hair flattened, its strength decreased. We found that 0.1% aqueous solutions of the keratin ingredients WK-F and KR-30 had a repairing effect against such damage. The repair effect of these keratins is related to the depth of damage and molecular weight, and each keratin has an appropriate molecular size to act on the internal structure of hair.