RHEOLOGY OF STRATUM CORNEUM--I 11 Another significant difference between these two types of 'keratinous' substrate is the variation in yield and break point although these are difficult to measure accurately. The extension at which turnover from the Hookean to the yield region occurs is sensibly constant at about 2•o for animal fibres in the rh range under discussion, while for stratum corneum an increase from about 1-20•o is observed in extension at yield on going from 30 to 100•o rh. These observations suggest that a different molecular mechanism of extension is operating for stratum corneum at high rh values. Water is probably the most efficient plasticizer of these three proteins and it is known that stratum corneum is capable of inbibing five or six times its dry weight of water at 100•o rh (18). Hair and wool on the other hand, absorb only about 35•o of their dry weights. It has been shown that the substantial water-absorbing power of stratum corneum is due to the presence of water-soluble hygroscopic materials which are trapped within the substrate by lipids (3). A possible mechanism of water absorption by corneum will be suggested in a following paper. Whatever the mechanism, there is no doubt that excessive hydration of the corneum occurs and that this is manifested in its rheological properties. The simplest model which adequately explains the change in elastic properties of stratum corneum with rh is provided by the transition of a polymer from a glassy to a rubbery state. At low rh the corneum shows the characteristics of a polymeric glass in which large chain movements are restricted and extension takes place by the stretching of bonds. In the wet state where hydrogen bonds and salt linkages are hydrated but the disulphide bonds remain intact, the corneum protein chains form a lightly cross-linked entanglement network similar to that of a rubber. Some explanation of the lack of change in modulus with conditioning above 10•o extension can now be offered. After the corneum is unwrinkled, further extension at high rh results in chain disentanglement. A stress build-up in the corneum is observed during the drying out process but this can be alleviated by slightly shortening (1-2•o) the corneum strip. Upon returning to 30•o rh the protein chains are frozen in a new 'configuration', and a different set of hydrogen and ionic bonds are formed. There is no reason to suppose that the numbers of such bonds will be grossly different from the number in the unwrinkled state after 10•o extension, and in con- sequence the Hookean modulus is similar. There are two possible explana- tions for the absence of ir dichroism which the process of chain straighten- ing outlined above should have produced either the effect is too small to be observed, or the protein of the cell membrane rather than the keratin is

12 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS being extended (the ir spectrum of stratum corneum is mainly due to the keratin which is the largest component of this substrate). In the conditioning process after the extended corneum had been dried out it showed no tendency to contract over a period of several months. This has obvious implications with regard to the wrinkling of skin with ageing. If over a period of time the water content of the corneum is reduced, the ability to return to its original configuration after extension will be less. That is to say it will gradually lose its rubber-like qualities. (Received: 27th May 1971) (1) (2) (3) (4) (5) (6) (7) (8) (9) (lO) (11) (12) (13) (14) (15) (16) (17) (18) REFERENCES Tomlinson, M., Daly, C.H., Odland, G.F. and Short, J.M. In vivo measurements of skin elasticity--a clinical evaluation. 3rd Annu. Symp., Bio-Engineering Program, University of Washington (October, 1969). Finlay, B. Dynamic mechanical testing of human skin 'in vivo'. J. Biomechanics, 3, 557 (1970). Middleton, J.D. The mechanism of action of surfactants on the water binding properties of isolated stratum corneum. J. $oc. Cosmet. Chem. 20, 399 (1969). Blank, I.H. and Shappirio, E.B. The water content of the stratum corneum. III. Effect of previous contact with aqueous solutions of soaps and detergents. J. Invest. Dermatol. 25, 391 (December 1955). Flesch, P. Chemical basis of emollient function in horny layers. Proc. $ci. Sect. Toilet Goods Ass. 40, 12 (December, 1963). Blank, I.H. Factors which influence the water content of the stratum corneum. J. Invest. Dermatol. 18, 433 (1952). Vinson, L.J., Singer, E.J., Kochlet, W.R., Lehman, M.D. and Masurat, T. The nature of the epidermal barrier and some factors influencing skin permeability. Toxicol. Appl. Pharmacol. 7 ($uppl. 2) 7 (October, 1965). Kligman, A.M. and Christophers, E. Preparation of isolated sheets of human stratum corneum. Arch. Dermatol. 88, 702 (1963). Bendit, E.G. Infra-red absorption spectrum of keratin 1. Spectra of a-, 13-, and super- contracted keratin. Biopolymers, 4, 539 (June, 1966). Matoltsy, A.G. In: The biology of hair growth 135 (1958) (Academic Press, New York). Matoltsy, A.G. and Matoltsy, M.N. The membrane protein of horny cells. J. Invest. Dermatol. 46, 127 (January, 1966). Selby, C.C. An electron microscope study of thin sections of human skin. II. Superficial cell layers of foot-pad epidermis. J. Invest. Dermatol. 29, 131 (August, 1957). Giroud, A. and Champcrier, G. R6ntgenograms of keratins. Bull. Soc. Chim. Biol. 18, 656 (1936). To be published. Speakman, J.B. Mechano-chemical methods for use with animal fibres. J. Text. Inst. Trans. 38, 102 (1947). Wall, R.A., Morgan, D.A. and Dasher, G.F. Multiple mechanical relaxation phenomena in human hair. J. Polym. Sci. Part C, 14, 299 (1966). Bendit, E.G. A quantitative X-ray diffraction study of the alpha-beta transformation in wool keratin. Text. Res. J. 30, 547 (August, 1960). Scheuplein, R.J. and Morgan, L.J. 'Bound water' in keratin membranes measured by a microbalance technique. Nature (London), 214, 456 (April, 1967).