RHEOLOGY OF STRATUM CORNEUM--I sample unwrinkling and that the conditioning procedure described was a valid technique for obtaining reasonably consistent modulus values. A tentative explanation of the fact that the modulus becomes constant after a conditioning extension of 10% will be given below. Graphs of modulus versus rh were constructed for conditioned corneum strips in the range 30--100• rh. Fig. 2 is a composite of several of such graphs in which modulus values are plotted on a log scale for clarity. To reduce the effects of sample heterogeneity further, the graphs have been normalized to the average modulus value of 2 x 109 N m -• at 30•o rh. All points were within experimentally acceptable distances from the average curve (•2•o in rh and +5•o in the elastic modulus). Yield point extensions were difficult to determine with accuracy at low rh values owing to the fact that brittle fracture generally occurred. The data listed in Table H were taken as the point of intersection of tangents to the load extension curve before and after yield (see Fig. 1, point X). Table II Approximate yield point extensions for stratum corneum at various relative humidities RH Extension at (•) yield point (%) 30 1.0 50 1.5 75 2.0 85 3.5 90 8.0 100 20.0 DISCUSSION Before entering upon a discussion of the theological properties of the stratum corneum it is necessary to outline briefly the salient compositional and structural details of this substrate in so far as they are known (10). Stratum corneum is a proteinaceous cellular material, the main component being a soft form of the fibrous protein keratin (60-70•o of the dry weight) which is contained within the cells. A protein of different amino acid com- position to that of the keratin is the principal constituent of the corneum cell membrane (11). Both of these proteins contain the cross-linking amino acid, cystine, which constitutes 2-3• of the keratin and 7-8• of the cell

10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS wall protein. Adjacent corneum cells do not appear to be in contact at all points along their connecting surfaces and the areas where adhesion does occur have been called desmosomes (12). Other important constituents of the corneum are lipids (5) and so-called hygroscopic materials including free amino acids, simple peptides and salts of organic acids (3). The keratin is principally in the a-helical conformation (13) and bundles of these helices (protofibrils) are randomly distributed in the plane of the comeurn surface within the cells. In the following discussion it is assumed that the load applied to the corneum is carried mainly by protein molecules although a role for lipids in this respect cannot be ruled out. Evidence is available (14) which suggests that the cell membrane protein is more important than the keratin in determining the rheological properties of stratum corneum but the term 'protein' when used will be understood to refer to both of these proteins. The types of bonds most likely to occur between and within protein molecules considered here are hydrogen bonds, salt linkages and disulphide bonds. The two most studied keratinous materials in terms of their stress/strain properties are hair and wool (15, 16) and it is informative to compare these substrates with stratum corneum. The stress/strain curve for hair or wool comprises a Hookean region (•2•o extension), a yield region (_•2-25•o extension) followed by a post-yield region (255/0 to break). The Hookean region is believed to arise mainly from the elastic straining of intramolecular hydrogen bonds formed between peptide groups in the keratin helix. In the yield region, as the name suggests, large extensions are produced by very small increases in stress. In this region, hydrogen bonds are broken and the onset of the a-0 conformational transformation can be observed (17). Larger stresses are required to extend the keratin in the post-yield region of the curve and molecular changes are thought to comprise further change with some straining of the extended • chains. At low rh, fracture prevents observation of the stress/strain characteristics of stratum corneum much beyond the Hookean region. A yield region can be observed at high rh values and possibly some semblance of a post-yield region, although the transformation from yield to post-yield is not as distinct as for hair and wool. The comparatively high modulus values observed for stratum corneum at low relative humidities (Fig. 2 and Table II) suggests that a similar mechanism of extension (bond stretching) to that suggested for hair and wool is operating in this region of the curve. The variation in modulus with rh is, however, much more pronounced for corneum (about a factor of 1 000 between 30 and 100•o) than for either hair or wool (about a factor of 4).