Elucidating penetration pathways into the hair fiber using novel microscopic techniques

Much controversy exists regarding the route of penetration of molecules into hair fibers. In brief, there are two schools of thought. The first argument is that molecules enter the hair fiber via the cell membrane complex (cmc) of the cuticle and then diffuse throughout the cortex via both the intercellular cement and the bulk of individual cortical cells. The second approach concludes that entry to the fiber is via the endocuticle and other non-keratinous parts of the fiber. In the latter case the cmc is definitely not considered to have a role in the penetration of molecules into the fiber. The tools available for studying penetration into the fiber, e.g., light and electron microscopy, mean that it is usually only possible to extract static information from a dynamic process. Similarly, great care is needed in the interpretation of images produced by the various techniques. Where a molecule is seen to end up does not always indicate how it got there! In these studies I have used novel derivations of conventional electron microscopic techniques, combined with early photographic chemistry, to elucidate further the pathways of penetration into the hair fiber. From these studies one can conclude that both arguments describing penetration into the fiber are complementary, valid, and highly relevant. The techniques allow one to visualize material within the cell membrane complex of the cuticle. In addition, these studies show that the high-sulphur proteins of the cuticle, usually considered as highly cross-linked and inaccessible, are easily penetrated. Therefore, all of the structures within a hair fiber should be considered as penetration routes into the hair fiber for the delivery of industrial and cosmetic materials, even though they may not form continuous pathways throughout the hair. The hair should be viewed as a structure composed of a number of compartments of differing capacity, chemistry, and accessibility, rather than as continuous pathways from the surface to the center of the fiber.