128 JOURNAL OF COSMETIC SCIENCE HUMAN HAIR'S REMARKABLE CUTICLE- PRESENTING OPPORTUNITIES FOR FUTURE HAIR TOILETRY PRODUCTS? J. Alan Swift, Ph.D. Swift Hair Consultancy, 29 Moorland Park, Wirral CH60 8Q J, UK Human hair, which is almost entirely proteinaceous in composition and contains a multitude of different proteins associated with its various internal structures, can best be described as a multi-component hierarchical composite. The major part of the fibre is occupied by a central cortex, which largely determines the fibre's elastic behaviour. The focus of attention of the present paper is the series of overlapping thin cellular sheets, the cuticle, which sheath the fibre and the contribution these layers might be making to the fibre's bending behaviour. The cuticle occupies a key position within the hair's structure. It not only controls the diffusion of materials into the fibre but, by dint of the overlapping scales presented at the outer surface, it imbues the fibre with important directionally-dependent fr/ctional properties. Such differential fr/ction, aided by the presence of a lubricating film covalently attached to the outer surface, is the reason why the hairs on the head normally maintain the maximum of disentangled alignment and helps in the rejection of irritant detritus from the scalp surface. At most 25% of the fibre bulk is occupied by the cuticle but in theory it has the potential for contributing a much greater proportion than this to the hair's bending resistance. It offers the prospect for being a controlling factor in styling hair to a changed level of curliness. The hair cuticle consists of large but relatively thin sheets (typically 50 gm across by 0.5 gm thick) wrapped around the cortex. They overlap each other from the root to the tip of the fibre to produce a series of scale edges on the outer surface at inter•als of approximately 5 I.tm along the length of each hair. A convenient working analogy is of stack of bottomless disposable plastic cups, with approximately 10 cup- layers being present in transverse section at the perimeter of the undamaged fibre. Each cuticle sheet, which contains mainly protein, is internally laminated and is separated from its neighbours by a very thin (25 nm) cell membrane complex of which, in turn, covalently-bound saturated lipids form an important part. Cuticle cell 1 Fibre surface Outer •3-1ayer ( ,',-layer [j:..:: }: 3 • • .:.:.) ! ?. ' Exocuticle Inner [3-layer layer •-Iayer Cell membrane complex Outer [3-layer Cuticle cell 2 • -- Epicuticle Figure 1. Epicuticle Schematic diagram of cuticle sub- structure The nomenclatures for the laminated sub-components of the cuticle are shown in figure 1, which illustrates a small transverse section close to the hair surface. In terms of chemical composition and physical behaviour it is convenient to consider each cell sheet as being divided into two main components. One is a highly crosslinked layer (epicuticle + A-layer + exocuticle) on the outer-facing aspect which possesses a high modulus of elasticity. The other is the endocuticle, 1/2 to 3/4s the thickness of the fast layer, which contains few chemical crosslinks, is thereby relatively soft and is susceptible to massive swelling (ca. 100%) when immersed in water. The fineness/coarseness of hair are best considered in terms of the cantilever bending of the fibre. There can be little doubt that the force to bend is a major factor by which the consumer judges the diametric thickness of their hairs ( 1 ) and is of import for hair setting processes. Taking a simplistic approach for the cantilever bending of a homogeneous cylindrical rod as a model for the hair fibre, then the force to bend

2001 ANNUAL SCIENTIFIC MEETING 129 varies as the fourth power of the hair's diameter (1). Furthermore, in a fibre of diameter 70 gm, an outer shell of the same thickness as that of the cuticle would contribute almost 50% of the bending resistance. The elastic moduli of various components of the hair have recently been determined. Using this information in the simplistic model one f'mds that the exocuticle could be contributing as much as 66% to the total bending stiffness and the endocuticle only a matter of 8% (2). The model, however, is NOT a good one because it fails to take account of the geometric arrangement of the cuticle cells in the fibre. Neither does it take account of the transfer of stress between the different parts of the structure as the fibre is bent the latter being something highlighted by Feughelman (3) in relation to setting processes involving the cortex. On the other hand the model's simplistic outcome provides a goal to be striven for and highlights the opportunities that might be presented by modifying the behaviour of the cuticle to cosmetic advantage. Hair outer surface Angle I ...... •'•-• , ,, •_•,, ,•o :,'' • ,,, Figure 2. Schematic of the :Y'•'••'' '•••' ', '-"• • through the cuticle. . •/•,,-., ,',, - •'•,, •. 3._• • • ,,,• ',,/• • •,,,r• .......... ..... ..•' A_layer • ufi-•ele //to fibre Cell membrane oe long axis complex Exoeu tiele A better model for predicting the influence of the cuticle upon bending processes in hair is to consider longitudinal bending about a section of the cuticle as shown in figure 2 where we now take account of the geometry of the layers, their internal chemistry and physical properties. A key event is of shear between the different layers. Bending stress in the stiffer outer layers (exocuticle + A-layer) will be transferred in the assembly to the softer endocuticle, leading its distortion. Such shear distortion of the endocuticle will occur particularly in the wet state and under extreme stress it will fail mechanically, as indeed has been frequently seen in the scanning electron microscope (4). The outcome predictably is for the cuticle to make little contribution to the hair's overall bending stiffness, On the other hand cosmetic processes aimed at increasing the shear strength of the endocuticle (and it is very accessible to reagents penetrating from the fibre surface!) ought to significantly increase the cuticle's influence upon hair bending stiffness. Such potential processes will be discussed both in terms of increasing the hair's apparent thickness and in terms of setting processes. References (1) J. A, Swift. Internat. ,/. Costnet. Sci. 17, 245-253 (1995) (2) J. A. Swift, J. Costnet. Sci. 51, 37-38 (2000) (3) M. Feughelman, J. Soc. Cosmet. Chern. 42, 129-131 (1991) (4) M. Gamez-Garcia, J. Cosmet. Sci. 49,213-222 (1998)