Hair photoaging

Abstract

Hair photoaging is shown to consist of a number of concurrent processes that result in chemical and physical changes in fiber properties. Lipid oxidation, disulfide bond cleavage, tryptophan degradation, and cysteic acid formation lead to an increase in fiber porosity, loss of mechanical strength, and an increase in surface roughness. Hair exposed to sunlight is claimed to be more brittle, stiffer, and drier than before irradiation and exhibits a reduced water absorption capacity. Photochemical alteration includes the breakdown of disulfide bridges within the structural units of hair and the establishment of new intra and intermolecular cross links via reaction of carbonyl groups with protein amino groups within and between structural units, thereby decreasing structural integrity. These reactions most likely lead to a gradual increase in brittleness and a gradual loss of structural differentiation. Photodegradation of cystine occurs through the C-S fission pathway and the highest level of photodegradation occurs in the cuticular region, where cystine is present at its highest concentration. For hair damaged by sunlight, in most cases, the amino acids of the cuticle are altered to a greater extent than the amino acids of the cortex, because the outer layers of the fiber receive higher intensities of radiation. Proteins of the cuticle are degraded by ultraviolet A and ultraviolet B, but much less by visible (VIS) light. Hair pigments function to provide some photochemical protection to hair proteins. Hair pigments accomplish this protection by absorbing and filtering the impinging radiation and subsequently dissipating this energy as heat. However, in the process of protecting the hair proteins from light, the pigments are degraded or bleached. Dark hair is more resistant to photodegradation than light hair, because of the higher photostability of eumelanin when compared with pheomelanin. But, hair damages caused by ultraviolet (UV) exposure are related not only to the melanin type of each hair but also to the total amount of melanin. Pheomelanin is far more sensitive to UV light than eumelanin, though these two types of melanin are similarly sensitive to VIS light. UVA irradiation can penetrate deeply into the cortex, so photochemical changes, including cuticles and cortex together, may appear greater after UVA irradiation. On the other hand, UVB causes severe morphological damages, especially confined to the hair cuticles because of its restricted depth of penetration. Integral lipids (ILs) of hair fibers are degraded by UV light as well as by VIS light, helping to explain the weakening of the cell membrane complex (CMC) exposed to light radiation. © 2010 Springer-Verlag Berlin Heidelberg.