Degradation and restoration of the outer hair surface

An intact hydrophobic outer layer protects hair effectively against extrinsic aging and is furthermore essential for the consumer’s perception of healthy and shiny hair. This outer layer, generally referred to as F-layer, consists mainly of the lipid 18-methyl eicosanoic acid (18-MEA), which is covalently bound to the underlying heavily cross-linked protein matrix, forming the epi-cuticle as composite surface structure. The condition of the outer hair surface can be characterized by physical measurement like the determination of contact angle with water under dynamic conditions according to Wilhemy or under pseudo-static conditions by placing water droplets on hair strands. Optical methods like the confocal laser scanning microscopy and atomic force microscopy providing complementary information regarding the constitution of the hair surface. Effects of aging by multiple daily stress was simulated by an automatic test device including shampooing, blow drying and sun light exposure. The test procedure was carried out by a robot and offers a high reproducibility with respect to the induced hair damage profiles. The induced hair surface damages were matched with naturally aged hair and compared to the effects of oxidative treatments. My means of AFM investigations changes in the nano scaled multi layer structure of the outer hair surface could be observed. Several damage pattern related to different stress profiles could be detected. Profilometry evaluation of confocal laser scanning microscope images shows significant changes in the surface roughness due to different damaging processes. The decrease of contact angles correlates with the obtained microscopic results. AFM studies show that lipophilic proteins (proteolipids) are able to re-establish a nano scaled multi layer structured surface on damaged hair similar to the virgin hair surface. This reconstructed hair surface shows a renewed hydrophobic barrier function.