j. Soc. Cosmet. Chem., 45, 149-158 (May/June 1994) Effects of the cuticle on the permanent wave set of human hair F.-J. WORTMANN, Deutsches Woll)$rschungsinstitut a.d. TH Aachen e. V. , Veltmanplatz 8, D-52062 Aachen, Germany, and N. KURE, Kao Corporation Tokyo Laboratories, 2-1-3 Bunka Sumida-ku, Tokyo 131, Japan. Received January 6, 1994. Synopsis A significant inhibitory effect of the cuticle on the permanent wave set of human hair was confirmed by measuring the set of fiber loops for both alescaled (cuticle-removed) and normal (cuticle-unremoved) hairs. The analysis of fiber set, based on the theory established in a previous investigation (7), showed that the set obtained for both types of hair is in the same way related to the relaxation of fiber bending stiffness during reduction. The consideration of the differences between the bending relaxation rate of normal and alescaled hairs during reduction suggests that the main role of the cuticle in waving is to reduce the setting ability of human hair by effectively operating as a chemical barrier. It lowers the concentration of the reducing agent through reaction prior to its diffusion into the fiber cortex. A further, minor, and mechanical role of the cuticle for permanent set is discussed. INTRODUCTION The morphological components constituting human hair are generally believed to con- tribute in different ways, depending on their chemical and physical properties, to the mechanisms of cosmetic processes, namely to permanent waving, which is the objective of our investigation. In the case of permanent set, the importance of the hair fiber corex has been emphasized on the basis of its dominance of the overall fiber mechanical properties (1--4), which in water mainly reside in the helical fraction of the intermediate filaments. However, little is known about the mechanical properties of the cuticle (5,6) and even less about its role in permanent waving. In a previous study by the authors (7) it was shown that the permanent wave set of human hair in bending, simulating permanent waving, is quantitatively determined by the bending stiffness of the hair fiber and its change with reduction, thus providing a tool to study the physical and chemical mechanisms of permanent wave set in detail. In the present study the approach is further extended to the comparative analysis of the permanent bending set of descaled (cuticle-removed) and normal (cuticle-unremoved) 149

150 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS human hair fibers from which the influence of the cuticle on the permanent wave set is deduced. METHODS Descaled fibers were prepared by abrading the cuticle layers from a hair using fine grade emery paper. The principle of this method is based on the procedures described by Chamberlain (8) and Snaith (9). An electrical laboratory motor was fixed to a burette stand by a clamp with the motor shaft down. A single hair fiber of approximately 20-cm length was connected to the head of the motor shaft through an adapter. A weight of 500 mg was suspended from the free end of the fiber, which was rotated at a speed of approximately 200 rev/min. A small piece of fine emery paper was bent, carefully contacted with the rotating hair fiber, and by exerting a small normal force slowly moved up and down along the fiber. The progress of the cuticle removal was checked discontinuously and continued until the anisotropic friction of the fiber, detected by rubbing the fiber between thumb and index finger, was removed. The electron microscopic examination of the surface of the treated fibers showed for fibers that had passed this check that the cuticle had largely been removed without any significant damage to the cortex. All experiments were carried out on chemically untreated Japanese hair taken from the head of one of the authors (N.K.) just prior to the experiments. Virtually straight, normal, and descaled hairs, whose tips were clipped off for 2 cm, were rinsed in methylene chloride and then in distilled water. After drying in air, the hairs were cut into halves, each of which was subjected to one of two tests, either for the determination of fiber bending set or for the measurement of bending stiffness. To determine bending set, a hair fiber was wound around a PVC cylinder (12 mm diameter), keeping the fiber perpendicular to the cylinder axis and finally fixing both ends with small drops of nail polish. The cylinder was then suspended in a beaker and treated in sequence with the following reagents at room temperature: 1. Distilled water for 30 min 2. Reduction by thioglycolic acid for 20 min 3. Rinse in distilled water for 20 min 4. Reoxidation by 2.3% hydrogen peroxide for 20 min 5. Rinse in distilled water for 20 min The concentrations of thioglycolic acid (TGA) used in Step 2 were varied from 0.1 to 0.8 M. The initial pH of the TGA solution was adjusted to 9.0 by an appropriate amount of 25 % ammonia solution. After the final rinse (Step 5), the fiber loops were cut and dropped into distilled water, followed by the immediate measurement of their diameters. The set acquired by the fiber loops is given by (3,7): S e = dc/d• (1) where S e is the experimental loop set, d c the cylinder diameter, and d• the mean diameter of the circles defined by the partially opened fiber loops. The bending stiffness of a hair fiber was measured by the balanced fiber method