354 JOURNAL OF COSMETIC SCIENCE � Fatty acid--�--�� j I c 1 i 1 � r ...,..__INNER BETA LAYER Hydrophilic protei � f GI b I t 0 O OnOQo Peptide, Hydrogen DELTA o uar proe. � 0 OUos-,....- b d &Salt l· -d.J on s iks LAYER Hy�rophobic f brous t . 0 0009 0u /Peptide & van der Waals l 1 pro e1� □- 0 o-o.,:._...r Fatty aci � ____✓E_ .._ _e_ _ lin _ _a_g_ _ ______ _ 18-MEA----- I LJ +---OUTER BETA LAYER Hydrophobic } !�van der Waals keratinprotein----........._� } =.,..,,,hioester bonds Keratin protein �-Disul f ide & Peptide bonds Figure 3. The bilayer model for the cuticle-cuticle CMC, with principle bonding proposed between layers. bilayer model). Some of these hydrophobic amino-acid side chains in the monolayer model are also branched, making the outer beta layer even more susceptible to failure at the delta layer, where it has been shown to occur (7). • The monolayer model contains fewer layers of ester/thioester linkages thus it will be more specific in its reactions to nucleophilic agents like mercaptans, hydroxide, and hydroperoxide anions-a point addressed later in this paper. Another point of contention concerning the CMC is whether or not the delta layer contains globular proteins or glycoproteins (2). We believe the current evidence favors globular protein layers in the delta layer for the following reasons: • No further evidence for glycoprotein in the CMC has been presented since 1991 (8) however, Bryson et al. in 1995 (5) isolated lipid-soluble lipoproteins from the delta layer of cortex-cortex CMC and not glycoproteins, favoring the globular protein model. • The delta layer resists solubilization by reducing or oxidizing agents and by acids and alkalies (5). If the CMC contains globular proteins similar to those in many other membranes containing large domains of hydrophobic amino acids on their surfaces (9), then these should be resistant to aqueous reagents as Bryson found. Thus the globular protein model is consistent with its known reactivity. • The delta layer swells only about 10-15% (10) therefore, much of it is hydrophobic. Such a relatively hydrophobic delta layer is more consistent with a globular protein model than with a more hydrophilic glycoprotein.

ADHESION FAILURE IN HAIR 355 • The delta layer stains with PTA. This is either a reaction of hydroxyl groups of a polysaccharide or of a primary amine function. Swift (2) has explained that this reaction is blocked with FDNB therefore, it is more likely a reaction involving primary amine groups, consistent with a globular protein. • The delta layer reacts with periodic acid/silver methenamine (2), a method for poly­ saccharides however, since cystine interferes with this reaction, it is still consistent with a globular protein in the delta layer. We believe that these models, with their proposed interconnecting bonds, help to explain where failure can occur and how it occurs, and we will use these models in that manner in the following discussion. SOME GENERAL RULES OF ADHESION FAIL URE Strain (the amount of deformation) more than stress determines how failure occurs in hair fibers. Stress-strain curves in the wet versus the dry state show much greater strain (deformation) in the wet state prior to entering into each successive region of the curve or even breakage (11). Faster rates of extension tend to inhibit the diversion of cracks in the axial direction and thus to promote smooth fractures (12). This is why fibers that can be extended to very high strains (particularly in the wet state) and are in good condition, i.e., undamaged with no flaws, tend to fracture rapidly at high strains, providing smooth fractures (Figure 4A). Undamaged hair with no flaws, including no medulla when extended to break, is more likely to provide smooth fractures under most conditions, but especially at high hu­ midities or when wet. Most Caucasian hair fibers in good condition and relatively straight with no twists, when extended to break in water or at very high humidities, provide mostly smooth fractures with crack initiation near the periphery of the fiber, often at the cuticle-cortex CMC. At lower humidities, extension provides mainly step fractures (see Figure 5), although under very dry conditions, smooth fractures are also observed with crack initiation in the cortex (Figure 4B). Figure 4. A. Extension to break when wet generally gives smooth fractures, high RH (100%). SEM by S. Ruetsch, TRI Princeton. B. Extension to break when very dry can give smooth fractures (12). Reprinted with permission of the Journal of Applied Polymer Science.