INVESTIGATION OF THE INTERNAL STRUCTURE OF HUMAN HAIR 125 EFFECT OF BLEACHING ON HAIR MORPHOLOGY Hair bleaching is a common cosmetic procedure that is used to introduce highlights to the hair. A typical bleaching formulation contains a two-part system the fi rst is the bleaching powder, which contains persulfate salts that accelerate the bleaching process, whereas the second is typically a developer and contains H2O2. The ultimate goal of this procedure is the destruction of melanin granules however, it results in a number of other secondary effects resulting in damage to the hair fi ber. Such damage is manifested in changes to the surface properties of hair, as monitored by surface techniques such as combing force and surface tension measurements, as well as damage to the internal components of the fi ber, which can be followed by tensile strength and differential scanning calorimetry measure- ments of hair (54,55). At the chemical level, one of the hallmarks of hair bleaching is the breakdown of disulfi de bonds, resulting in the formation of cysteic acid residues (56,57). In addition, the lipid structures in hair are also vulnerable to the bleaching process (58). Figure 6 contains an AFM image of a cross section of the cuticle region of bleached hair. Three cuticle cells can be discerned in the micrograph. A comparison should be made Figure 5 . AFM micrograph of the medulla region of virgin hair illustrating its rough and porous nature. Figure 6. AFM micrograph of the cuticle of bleached hair. In the image of the cuticle, the white arrows indicate regions of the cuticle where severe damage has been induced in the cell membrane complex and endocuticle.
JOURNAL OF COSMETIC SCIENCE 126 with the corresponding image of the cuticle of virgin hair in Figure 2. Structurally, the most noticeable difference between the virgin and bleached hair is the appearance of dark voids that are especially evident in the endocuticle region. Because the endocuticle has the lowest density of crosslinking of the sublamina layers, it seems likely that this would be the region most susceptible to damage from the bleaching procedure. There are also voids in the cuticle of the virgin fi ber (Figure 2) however, they are not nearly as large or deep as those found in bleached hair. These fi ndings are in agreement with TEM studies where it was shown that the electron density of the endocuticle of bleached hair—fi xed with osmium tetroxide and stained with uranium acetate and lead acetate—was greater than that of virgin hair, more so than the other lamellar structures (59). An AFM micrograph of the cortex of bleached hair is provided in Figure 7. Immediately apparent in the image are the large amounts of cracks, crevices, and other asperities in the bleached hair cross section (compare with virgin hair shown in Figure 3). Focusing in on the cortex at greater resolution allows us to discern the macrofi brillar structures and to make a comparison between virgin and bleached hair (Figure 8). In the bleached sample, there appears to be an erosion of biomaterial at the outer edges of the macrofi brils. This could suggest the presence of a low-sulfur–containing protein more susceptible to degra- dation by bleaching than the inner core of the macrofi bril, or the possibility that a cell membrane-like material could be present on the exterior of the macrofi bril and then removed by the bleaching procedure. It should also be pointed out that, regardless of the material, erosion would be expected to proceed from the exposed edges inward. We did not observe noticeable differences in the medulla region of bleached hair as com- pared with virgin hair. This is surprising because previous studies demonstrated greater quantities of lipids in the medulla than other regions of the fi ber (52). Removal of lipids by bleaching and other chemical processes could result in a change in the morphological appearance of the medulla, although this requires further investigation. INVESTIGATION OF DELIPIDATED HAIR Hair contains a variety of lipids that consist of free fatty acids, cholesterol, ceramides, cholesterol esters, and cholesterol sulphate (60). Typically, hair lipids are categorized as Figure 7. A FM micrograph of the cortex of bleached hair. The arrows in the image of the cortex point to major cracks and crevices within the cortical structure caused by the bleaching procedure.
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