688 JOURNAL OF COSMETIC SCIENCE analysis of moisture in the cuticle sheath. This paper reports preliminary observations on this subject. However, before describing the experimental details of this report, it seems appropriate to review one of the most common patterns of hair damage that appears in most individuals, cuticle cell decementation, lifting, and buckling. These patterns are relevant because they are a form of cuticle cell delamination, and, as will be shown later, they are strongly dependent on moisture. Cuticle cell decementation and buckling usually appears in hair fibers at distances larger than 10 or 20 cm from the scalp, depending on the type of care the hair has received. In fact, this type of damage is so prevalent that it can be considered the initial indication of hair damage. It appears way before hair breakage or before the appearance of any other type of hair cortex physical trauma. Its ubiquitous nature and its relation to moisture demands a clear understanding, first, of how grooming practices lead to its formation, and, second, of its effects on hair sensorial properties. In this paper, experimental results associated to the former will show that cuticle cell decementation, delamination, and buckling are strongly related to the state of moisture in the cuticle cell layers. It will also be shown that this type of damage is usually associated to conditions involving any of the following repetitive processes: (1) cyclic tensile stresses involving elongations higher than 12% and (2) applications of thermal stresses accompanied by elongations lower than 5%. The first process will be frequently encountered by hair fibers during combing and tangling, especially when the hair is totally or partially wet and when poorly conditioned. The second process will result from blow drying or combing or when using hot irons both involve rapid dehydration of cortex and cuticle sheath. In this respect, it is worth mentioning the well-known fact that moisture loss in hair has a strong impact on its mechanical modulus. Water loss in hair usually is associated with dehydration of proteins in the matrix since the intermediate filaments (IFs) do not absorb water (2). Water loss from the cuticle cells also occurs, although it has not been studied as much and most certainly stems from the endocuticle, the cell membrane complex (CMC), and to a lesser extent from the exocuticle (2,3). As will be shown later, as the exocuticle, endocuticle, and CMC lose moisture at high temperatures, they become stiffer and change their mechanical moduli, compromising their response to mechanical stresses. METHODOLOGY The hair used in the experiments was Virgin Premium Grade Brown Caucasian from International Hair Importers. Two sets of hair fiber samples were subjected each to different thermal stress conditions. In the first set, a tensile deformation of 5% was applied to the hair fibers while they were simultaneously subjected to blow drying within a temperature range between 40 and 80°C a Diastron Tensile Tester was used to deform the fibers. In the second set, the hair fibers were hot ironed using temperatures between 160 and 220°C. Before thermal stress application, the hair fibers were equilibrated at different relative humidity conditions for 24 hours. Moisture equilibration was made in a glass bowl with a tray containing water saturated with various salts, depending on the moisture value required. When needed, cuticle cell lifting was produced by applying cyclic elongations using the method described elsewhere (3). The presence of delaminated, lifted, and buckled cuticle cells was readily detected by SEM and optical microscopy. A Hi-Scope KH-3000 from Hirox was employed to detect and analyze buckled cuticle cells via the colorful patterns

689 MOISTURE IN THE CUTICLE SHEATH of light interference that they produce. In the literature, it has already been shown that colorful patterns of light interference are produced when cuticle cells delaminate and buckle, creating micron-size air gaps that lead to the conditions for light interference (4–6). RESULTS AND DISCUSSION CUTICLE CELL LIFTING OR BUCKLING AT HIGH ELONGATIONS In the past, it has been shown that cyclical elongations with deformation values higher than 12% and in the moisture range between 40% and 65% relative humidity (RH) lead to patterns of cuticle cell buckling, forming parabolic shapes (3). Fig. 1a shows a hair fiber displaying cuticle cell buckling after being subjected to similar conditions. Unlike other types of cuticle cell buckling, which will be reviewed later, this type of damage is not caused by transient losses of moisture but rather by high elongations. Its formation, however, depends strongly on the equilibrium moisture content of the cuticle cells. The mechanism is as follows: when hair fibers are equilibrated at moisture conditions between 30% and 65% RH, and then subjected to elongations ranging between 5% and 17%, mechanical energy appears across the hair fiber in the form of tensile, shear, and Poisson stresses. In this range of deformation, the keratin intermediate filaments in the cortex undergo an alpha to beta transformation (7). This protein phase transition enables the cortex to unfold and relax, thereby allowing for higher elongations while partially dissipating high concentrations of mechanical stresses. In contrast, for similar elongations and humidity conditions, the cuticle sheath does not have such a phase transition mechanism to dissipate stresses. Thus, while the cortex can undergo high extensions without building up high levels of stresses, the cuticle sheath elongates in parallel but develops stress concentrations across its layers. The result is lifting and buckling of cuticle cells, as shown in Fig. 1a. Furthermore, Fig. 1b shows a magnified version of the fiber shown in Fig. 1a. In this picture, it can be seen that when cuticle cells lift under these conditions, there are no endocuticular remnants left on the cuticle cell Figure 1. SEM pictures of a hair fiber showing cuticle cell lifting and buckling at different magnifications Fig. 1a (×500), Fig. 2 (×3000). Lifting was produced by applying 10 cycles of 15% elongation at 50% RH.