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]. Cosmet. Sci., 55, 351-371 Quly/August 2004) Failure of intercellular adhesion in hair fibers with regard to hair condition and strain conditions CLARENCE ROBBINS, HANS-DIETRICH WEIGMANN, SIGRID RUETSCH, and YASH KAMATH, 12425 Lake Ridge Circle, Clermont, FL 34711 (C. R.), and Textile Research Institute, PO Box 625, Princeton, NJ 08540 (H.-D. W., S. R., Y. K.). Accepted for publication March 31, 2004. Synopsis Although adhesion failure in hair fibers can occur inside cells, it occurs more frequently in the cell membrane complex (CMC), often involving the rupture of interlayer bonds. Therefore, a model of the CMC is presented, based on prior research in which we propose interconnecting bonds between the layers to assist in our interpretation of hair-fracturing mechanisms for cuticle chipping, deep transverse cuticle cracks, cracks during heat drying, scale lifting by surfactants, and catastrophic failure. Failure in the wet state generally involves hydrophilic layers, e.g., the contact zone of the CMC or the endocuticle or bonding to these hydrophilic layers, whereas failure in the dry state generally involves bonding between hydrophobic layers, e.g., beta-delta failure. Chemical damage by perms, bleaches, and sunlight, by breaking specific chemical bonds, influences the sites of initial failure and increases the number of routes for crack propa­ gation, leading to more complex fracture patterns. INTRODUCTION Failure of adhesion can occur at different sites in hair fibers, depending on the condition of the hair and the conditions of strain that produce failure. The condition of the hair is determined primarily by prior treatments and fiber twists and/or flaws, including whether or not it contains a medulla and whether it has been chemically treated or exposed to sunlight, and the kind and amount of sorbed materials. The conditions of strain include stretching, bending, twisting, percentage deformation, the employment of heat such as heat drying, and water content or percent relative humidity. The cosmetic importance of adhesion failure resides in the fact that it controls hair breakage and fragmentation, actions that are so important to the look, the feel, and the condition of the hair. Fragmentation of hair is determined primarily by chemical treat­ ments to the fibers, including sunlight exposure, the conditions of deformation, and abrasive actions that are employed in everyday grooming actions (1). It is the combi­ nation of these actions at high water content that results in the most severe hair erosion, especially when combined with repetitive actions, as in extension cycling, fatiguing, and abrasion to break. 351