Lamellar subcomponents of the cuticular cell membrane complex of mammalian keratin fibres show friction and hardness contrast by AFM

There is a substantial body of information indicating that 18-methyleicosanoic acid (18-MEA) is covalently linked to the outer surface of all mammalian keratin fibres and also forms the outer β-layer of the cuticular cell membrane complex (CCMC) which separates the cuticle cells from each other. Low cohesive forces are expected between the lipid-containing outer β-layer and the δ-layer of the CCMC, thus providing a weak point for cuticular delamination and presenting a fresh layer of 18-MEA to the newly exposed surface. We have used lateral force microscopy and force modulation atomic force microscopy (AFM) to examine human hair fibres in which the non-covalently linked fatty acids have been removed. Examination of the lateral force images of new cuticle surfaces revealed by the attrition of overlying cuticle layers showed three separate zones of clearly defined frictional contrast. These are thought to correspond with the δ-layer, the proteinaceous epicuticle and outer β-layers of the CCMC. The δ-layer was found to have a thickness of 16 nm (SD = 1 nm, n = 25), comparable to the 18.0 nm thickness measured from transverse crosssections of fibres with transmission electron microscopy. Force modulation AFM showed that the outer β-layer was softer than the epicuticle and the δ-layer. The frictional contrast was removed following treatment with methanolic KOH (0.1 mol dm−3) at 25 °C for 30 min, suggesting the hydrolysis of the thioester linkage and removal of 18-MEA from the surface.