CELL MEMBRANE COMPLEX 457 CHEMICAL AND PHYSICAL ACTIONS ON THE CMC OF HAIR The covalently bound lipids of the CMC of the cuticle are sensitive to oxidation, reduction, and alcoholic alkalinity, while the lipid beta layers of the cortex are affected more by lipid solvents and free radical chemistry. The beta layers of the cuticle are more sensitive to nucleophilic attack by species such as the hydroperoxide anion and mercaptans, but the beta layers of the cortex, with their multiplicity of double bonds (oleic plus palmitoleic acids, plus cholesterol and cholesterol sulfate) and tertiary hydrogen atoms (cholesterol and cholesterol sulfate) are more sensitive to free-radical chemistry. On the other hand, the membranes of the CMC are resistant to oxidizing and reducing agents (78). Several of these chemical actions make the CMC more vulnerable to fracture, to cuticle fragmentation, and to the propagation of cracks through the cortex, as will be described in this section. There is evidence that a signifi cant amount of free lipid (not covalently bound to hair proteins) is in the beta layers of the cuticle and likely in all lipid layers of keratin fi bers (58). Since about 50% of free lipid in human hair is fatty acid, free lipid provides acidic groups to the hair surface and decreases the isoelectric point, as shown by Capablanca and Watt (63). As hair is exposed to repeated shampooing, blow drying, and rubbing, and to sunlight, changes occur on and in the surface layers. These changes involve removal of some free lipids by shampoos and photo-degradation of 18-MEA, disulfi de, and other functional groups, and consequently, fractures form in or between layers from bending, stretching, and abrasive actions. These actions expose “new” protein material and sulfur acids, primarily sulfonate groups, with an accompanying decrease in the free lipid content of the hair surface, thereby con- verting the virgin hair surface from a hydrophobic entity with little surface charge to a hydrophilic, polar, and negatively charged surface. The more the exposure of the hair to chemical and abrasive actions, the further from the root ends the more hydrophilic the hair becomes and the more polar and more negatively charged the surface becomes. DAMAGE BY SHAMPOOS AND CONDITIONERS The dissolution or the removal of structural lipids or proteinaceous matter from hair, primarily from the CMC or endocuticle, by shaking keratin fi bers either in surfactant solutions, shampoo solutions, or even water has been demonstrated by several different scientists. For example, Marshall and Ley (79) demonstrated the extraction of proteina- ceous components from the cuticle of wool fi ber by shaking wool fi ber in surfactant solu- tions of sodium dodecyl sulfate, a common surfactant in many shampoos. Gould and Sneath (80) examined root and tip end sections of scalp hair by TEM. This hair had never been chemically treated. Gould and Sneath observed holes or vacancies in the thin cross sections, and these holes were more frequent and larger in tip ends than in root ends. These scientists attributed these holes to damaging effects by shampooing involving the breakdown and removal of the non-keratin portions (CMC and endocuticle) of the hair, leaving the intercellular regions more susceptible to fracturing. One of the most common types of fractures in hair fi bers forms in the dry state, and it occurs in the cuticle–cuticle CMC between the upper beta layer and the adjoining delta layer (see Figure 2) and is called beta-delta failure (39). Gamez-Garcia (32) noted that the

JOURNAL OF COSMETIC SCIENCE 458 lower the relative humidity or the moisture content of hair, the lower the strain level at which beta-delta failure occurs. Beta-delta failure was observed by Negri et al. (39) on wool fi ber, and they noted disruption of the cuticle–cuticle CMC along the upper beta layer in TEM sections. With this type of fracture, the delta layer and the lower beta layer are both retained on the underside of the “old” outermost cuticle cell, leaving 18-MEA as the “new” hair surface once the “old” outermost cuticle cell is abraded away. This type of fragmentation has been described in detail by Feughelman and Willis (33), who proposed that the failure of adhesion between overlapping scales involves 18-MEA, and that be- cause of its branching, it provides mobility and a reduction in adhesion between scales, leading to beta-delta failure. Therefore, the degradation of 18-MEA between scales as occurs in chemical (81) or photochemical bleaching (81) or even permanent waving treat- ments (82) leads to further weakening of this structure and more rapid beta-delta failure, leading to faster cuticle fragmentation and cuticle loss. WET STATE VS DRY STATE FAILURE Deformations such as stretching (4,83) (including extension cycling (32), bending, or twisting in the wet state) are very different from deformations in the dry state. This is because failure in the wet state generally involves fractures or breaking bonds in hydro- philic layers, e.g., the endocuticle or the central contact zone of the CMC, whereas failure in the dry state generally involves fractures in or between hydrophobic layers, e.g., beta- delta failure (4,33). Failure in the wet state generally involves hydrophilic regions be- cause when a layer or region is completely swollen, less mechanical stress is required to distort that layer and to produce a fracture. On the other hand, the lower the relative humidity or swelling condition of a hydrophilic layer, the more mechanical stress and strain required to distort hydrophilic vs hydrophobic layers, and therefore fractures are generally produced in hydrophobic layers. Extension of undamaged hair-to-break generally produces smooth fractures (32). How- ever, as the hair becomes more damaged or as the relative humidity is decreased, and es- pecially at lower humidities, more step fractures are produced (32), and step fractures involve extensive fracturing in the cortex–cortex CMC, most likely in the beta layers. Kamath and Weigmann (83) have shown, for human hair at low moisture content, that crack initiation occurs most often in the cortex, whereas at high moisture content, frac- tures almost always initiate at or near the surface of the fi ber because of the high pressure of the swollen cortex against the cuticle. Step fractures involve the axial propagation of cracks either through the cortex–cortex CMC or the medulla (83) and therefore occur more frequently in the dry state than when hair fi bers are wet (83). Kamath and Weigmann (83) also concluded that the CMC seems to “play an important role in stress transfer and axial splitting” of human hair fi bers. The abrasion resistance of human hair is decreased by most chemical treatments and photo-oxidation, as shown by the “protein loss” test of Sandhu and Robbins (84) or by the release of labile and eluted proteins as described by Inoue et al. (85). These tests are both wet-state methods. The inside of cuticle and cortical cells is degraded by alkaline peroxide, thereby weakening the cuticle and cortex cells internally, and the cuticle–cuticle CMC is degraded, weakening the cellular cohesion or the resistance of scales to break apart. Cuticle fragmentation in the dry state is caused primarily by the rupture between cuticle cells