J. $oc. Cosmet. Chem. 23 3-12 (1972) ¸ 1972 Society of Cosmetic Chemists of Great Britain Rheology of stratum corneum--I: A molecular interpretation of the stress-strain curve A. C. PARK and C. B. BADDIEL* Synopsis--Some of the problems involved in obtaining mechanico-chemical data on STRATUM CORNEUM are outlined. A conditioning process is suggested which allows reasonably con. sistent ELASTIC MODULUS data to be obtained for this complex substrate. The effect of RELATIVE HUMIDITY, over the range 30-100•o, on the elastic modulus of stratum corneum is dramatic, the value changing from2 x 109 N m- o. at 30 •o relative humidity to 3 x 10 a N m- at 100•o relative humidity. An attempt has been made to interpret these changes in modulus at a molecular level by comparing data from other keratinous substrates and using POLYMERS as models. INTRODUCTION During normal body movements, certain areas of the skin are sub- jected to relatively large deformations, and, in this connection, the impor- tance of the intrinsic rheological properties of the material have been recognized by a number of investigators. Medical studies have centred on the mechanical properties of the whole skin (1, 2) but the region of interest to the cosmetic chemist does not extend much below the skin's surface (3-5). Lesions arising from the effects of soaps and detergents on the skin are probably confined to its outer layers. Large extensions which are rapidly reversible can be imparted to healthy skin without any detectable damage. However, over a long period damage does occur, one manifestation of the * Unilever Research, 455 London Road, Isleworth, Middlesex. 3
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ageing process being loose, flaccid skin which undoubtedly has lost its power of elastic recovery. The stratum corneum being the outermost layer of the skin, is the first part of the body to be subjected to environmental insult. It has two prime functions to regulate water loss and in consequence body heat, and to act as a barrier against the penetration of foreign substances. Corneum dis- ruption will result in these functions being impaired to a greater or lesser degree. There is evidence to suggest that the physical integrity of the stratum corneum depends upon effective plasticization of the material (6) and, since water is probably the most efficient plasticizer, upon its water content. The effect of water as a plasticizer should be indicated by a change in the rheological properties of the corneum with its water content. Owing to difficulties involved in obtaining accurate dimensions, par- ticularly thickness, for isolated stratum corneum, no previous attempt has been made to quantify rheological data, and as a result comparisons between different types of corneum and other related substrates such as hair and wool have been impossible. This communication attempts to introduce a quan- titative element into the study of the mechanical properties of stratum corneum. Load/extension curves have been obtained for narrow strips of pigs' ear stratum corneum and elastic (Young's) moduli have been calculated in the rh range 30-100•o. Although some attempt has been made to interpret these data at a molecular level the main purpose of this initial paper is the establishment of techniques. EXPERIMENTAL Stratum corneum was removed from the skin of pigs' ears [in both structure and function there is little difference between animal and human stratum corneum (7)] by trypsin digestion of the underlying tissues (8). Excised skin was incubated at 37 ø overnight with the dermis side in contact with a solution of trypsin (1•o) and urea (2 M), the pH of the solution having been adjusted to 7.2 with sodium bicarbonate. After such treatment the stratum corneum could be teased away from the underlying tissue in a water bath. The corneum was then removed from the bath and dried in the form of a flat sheet. Trypsin was obtained from BDH (reagent grade, activity not less than 0.5 Anson units g-X) and urea and sodium bicarbonate were of Analar grade (BDH). Rheological tests were carried out on narrow strips of stratum corneum (approx. 0.2 x lff a m wide) which were cut with a stainless steel punch. Calculation of the desired parameter, the elastic modulus, required a know-
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