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J, Cosmet. Sci., 52, 265-280 (September/October 2001) ElucidatinD penetration pathways into the hair fiber usinD novel microscopic techniques C. L. GUMMER, Procter & Gamble Technical Centres Ltd., Rusham Park, Whitehall Lane, Egham, S/•rrey, TW20 9NW, United Kingdom. Accepted for publication May 31, 2001. Synopsis 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 tlne 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 tlne 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 tinat 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, ratlner than as continuous pathways from the surface to the center of the fiber. INTRODUCTION Understanding the penetration of molecules into animal fibers is particularly important for the textile dyeing and cosmetic industries. Fiber penetration is an intrinsic compo- nent of dye uptake, dye fastness, cosmetic ingredient penetration, and efficacy. If one knows the route a molecule takes into a fiber and the physicochemical nature of that pathway, then one can better control the penetration of molecules into, and out of, the fiber. Similarly, one can determine the capacity and reactivity of structures in the fiber. Many of the published works consider penetration as the movement of a molecule from a formulation or dye bath into the fiber cortex. This, in turn, implies that the cuticle is 265