j. Cosmet. Sci., 49, 213-222 (July/August 1998) Cuticle decementation and cuticle buckling produced by Poisson contraction on the cuticular envelope of human hair MANUEL GAMEZ-GARCIA, Croda North American Technical Center, 180 Northfield Avenue, Edison, NJ 08832. Accepted for publication July 15, 1998. Synopsis Cyclical extension stresses in dry hair at strain levels between 7% and 15% were seen to produce localized cuticle decementation and buckle formation at the cuticle edges. The experiments show that this type of damage results from circumferential compression stresses on the dry cuticular envelope that acts as a rigid thin-wall pipe during the process of Poisson contraction as the fiber is being elongated. Higher cyclical extensions (25%) were seen to result instead in cuticle lifting with no buckle formation. This latter type of cuticle damage is similar to the one observed by Reutsch et al. and was already ascribed to the action of longitudinal shear stresses. When the hair was moistened or solvent-swollen, no damage was observed at each individual cuticle instead, long deep transversal cracks and decementation of the whole cuticular envelope from the cortex were observed. The experiments indicate that the lack of individual cuticle damage under swollen conditions results from a strengthened hydrogen-bonding crosslinked endocuticle and from a lesser fiber Poisson contraction when the fiber is swollen. Cuticle buckling and decementation at the cuticle edges like those described above were also frequently found in the hair of a panel of 100 individuals. The prevention and repair of this type of damage is also discussed. INTRODUCTION The cuticular envelope of human hair is constituted by six to ten cuticle cells cemented and overlapping each other like shingles on a roof. The role of the cuticle in maintaining the integrity of the whole hair fiber is of paramount importance and has already been discussed elsewhere (1-4). Damage to the cuticle can be produced by a variety of factors such as combing abrasion, UVB light, and chemical treatment (5-9). Therefore, pro- tection of the cuticle from any of these damaging factors is one of the main goals of a hair care formulator. A great variety of conditioners and shampoos have already been formulated with actives that can help to prevent or reduce overall hair damage. Success in fully protecting the cuticle from deterioration depends, however, on understanding the mechanisms involved in the damaging process. A great deal of progress in this direction has already been made. For instance, changes in cuticle morphology due to combing abrasion have been studied by Swift (1) and by Swift and Brown (2), while several other researchers have analyzed the effects of chemical 213

214 JOURNAL OF COSMETIC SCIENCE treatments on the cuticle (5,6). Cuticle lifting and decementation resulting from the imposition of high extension strains (30%) to hair have also been reported by Reutsch et aL (10). These workers have shown that depending on the hair conditions, cuticle lifting may occur by two main mechanisms: 1) cuticle decementation or failure of the cement layer at the cell membrane complex and 2) mechanical failure of the endocuticle. Since an analysis of hair cuticles from a panel of 100 individuals showed that cuticle lifting and buckling at the cuticle edges is a common phenomenon, we decided to investigate the conditions under which this type of damage occurs. In this paper it is shown that localized cuticle decementation leading to lifting and buckling of the cu- ticles starts to take place when dry hair fibers are cyclically extended at strains higher than 7%. It is also shown that this type of damage can be prevented and even repaired with the use of adequate cosmetic actives. EXPERIMENTAL METHODOLOGY Cyclical extensions ranging from 7% to 30% were applied to hair with a Diastron tensile tester in the automatic mode. The number of cycles applied varied between 50 and 200. The extension speed was 120 mm/min, and there was an interval of about two seconds between each cycle. The hair used for all these experiments was from a subject whose hair was washed only with a 10% aqueous SLS solution for a period of one year. The hair fibers used were snippets 2.5 inches long, cut close to the root. Hair from International Hair Importers was also used when needed. Some hair fibers were also allowed to swell for 36 hours in DMSO, IPA, and ethanol, and were then stress-cycled to investigate the effect of swelling on cuticle damage. Hair from a panel of 100 individuals was also analyzed by SEM for cuticle lifting at least ten fibers per each individual were tested. Also, in order to test the effect of some cosmetic chemicals on cuticle damage, a different set of fibers was treated with each one of the following aqueous solutions: glycerin, propylene glycol, a cationically modified gluconamide compound (11), hydrolyzed wheat protein polysiloxane copolymer (12), cystine polysiloxane (13), polyacrylate poly- mer, and polyethylenimide, each of these at a level of 3% w/w. After treatment the fibers were dried at 65 % relative humidity (RH) and stressed cyclically. The experiments were carried out in a controlled humidity chamber at 10, 65, 80, or 100% RH. After stressing, the hair fibers were immediately prepared for SEM analysis. RESULTS AND DISCUSSION Figures la and lb show typical patterns of cuticle lifting found in the hair of people from the panel. The analysis showed that this type of cuticle damage is present, although to different extents in about 80% of the hair population from the panel. In an attempt to reproduce these patterns of cuticle lifting in the laboratory, first a set of 2.5-inch hair snippets was subjected systematically to tension, torsion, and thermal cyclical stresses. The experiments showed that only the tensile cyclical stresses lead to such patterns of cuticle lifting. The other stresses produced very distinct forms of cuticle damage, and they will be described elsewhere (14). Most of the patterns of cuticle lifting found in this investigation were seen to occur by cuticle decementation or failure of the cuticular cement. For instance, in Figures 1-3 it can be seen that the sections of lifted cuticles do