J. Cosmet. Sci., 60, 85–95 (March/April 2009) 85 The effects of lipid penetration and removal from subsurface microcavities and cracks at the human cuticle sheath MANUEL GAMEZ-GARCIA, Ciba Specialty Chemicals, Polymer Effect Research, 540 White Plains Road, Tarrytown, NY 10591-9005. Synopsis An analysis of the light interference patterns produced by the penetration and removal of lipids from the cuticle sheath has shown that cuticle cells in their virgin state have a few intrinsic imperfections in the form of voids and cracks. The experiments also showed that when the cuticle sheath undergoes friction, extension, torsion, and thermal stresses additional microvoids, cavities, and gaps are created at the cuticle cells subsurface. Because of the activity of the sebaceous glands it is quite normal to fi nd these cavities fi lled with exogenous lipids. Cuticle sheath dehydration, lipid addition, and lipid removal indicate that the viscoelastic deformations giving rise to microcavities can be reversible or irreversible. The presence of ex- ogenous lipids in these cavities was found to be critical in maintaining the mechanical integrity of the cuticle cells. Regions presenting microcavities and cracks produced by reversible deformations were seen to fully recover and heal with the onset of a plasticization effect produced by the synergy of lipids and water. In contrast, microcavities produced by irreversible deformations were always fi lled with lipids. In both cases the lipids acted as weak adhesives, in particular, in those cavities and gaps opened in the cuticle cell interfaces. INTRODUCTION The cuticle sheath of human hair is a composite structure formed by hundreds of cuticle cells that overlap continuously in stacks of 5 to 10 units like shingles on a roof (1–2). This part of the human hair is considered to be essential for maintaining the physical integrity of the whole fi ber as it acts as a protective shield against environmental stresses that may otherwise damage the cortical cells. Unfortunately, modern grooming practices impose harsh mechanical and thermal stresses to the hair fi bers (3). Consequently, early signs of damage in relation to the life span of a hair fi ber appear in the cuticle sheath compromis- ing its protective role. One type of damage that is prone to appear within the cuticle sheath due to its multilayer character is the formation of gaps and cavities at the junctions of the various internal lay- ers. Once these defects form they coalesce under the application of further stresses causing decementation and delamination of large portions of cuticle cells. As a result, breakage of these cuticle cell portions rapidly ensues even under the action of slight friction (1–4). This paper discusses the formation of gaps and cavities at the cuticle sheath subsurface and analyzes how water and lipid penetration affects their development.

JOURNAL OF COSMETIC SCIENCE 86 EXPERIMENTAL PROTOCOLS The method used to study the formation of gaps and cavities within the cuticle sheath consisted in analyzing the light interference patterns (LIPs) produced by cuticle cells when seen under a powerful light by optical microscopy (4). As it has already been shown (4) cuticle cell decementation causes disruptions in the continuity of the cuticle sheath producing the inclusion of air in the formed cavities. Light interference patterns are then produced by a mismatch in the indexes of refraction between the cuticle cell layers and the air cavities as the light passes through. The presence of these defects within the cu- ticle sheath is thus revealed by the appearance of colorful patterns appearing at the hair surface. A similar phenomenon has been reported to occur with pores within the medulla of hair (5). MATERIALS AND INSTRUMENTS The light interference patterns were analyzed by microscopic analysis using a Hi-Scope KH-3000 equipped with a metal halide lamp. The hair used for testing thermal and mechanical damage was European virgin hair from International Hair Importers. For experiments requiring analysis of hair close to the root, virgin hair from 4 female donors was obtained from areas close the scalp. Hair from the same source was used for lipid extraction. Solvents used for oil and lipid extraction from hair were IPA and Hexane GC grade. Extraction of lipids from hair was made by immersing 1 g bundles of hair fi bers in 100 ml of solvent at room temperature conditions for periods of time ranging from min- utes to months. Analysis of the lipids extracted from hair was made by GC-MS after methylation. During solvent immersion single hair fi bers were selected and taken for microscopic anal- ysis at various time intervals. When needed, cycles of mechanical and thermal stresses were applied to single hair fi bers using a blow dryer or curling iron according to methods described elsewhere (6–7). The number of gaps and cavities appearing at the cuticle sheath of hair fi bers, either treated with solvents or damaged mechanically or thermally, were counted and plotted as a function of hair length. All defects producing patterns of light interference ranging in size from 0.1 to 5 microns were considered as gaps or micro- cavities. The oil used for deposition into these cavities was mainly Jojoba oil. The deposi- tion method consisted in immersing the hair fi bers in a solution of oil at 0.1% in IPA at RTC for three minutes. RESULTS GAPS AND CAVITIES FORMED BY MECHANICAL AND THERMAL STRESSES Microscopic analysis of hair fi bers taken from the various donors showed that the presence of gaps and cavities in people’s hair was pervasive. Figure 1 shows, for instance, that the average count of gaps and microcavities increases in regions of the fi ber distant from the root, while in areas close to the root the count is nil. Also, it was observed that the average