J. Soc. Cosmet. Chem. 23 13-21 (1972) ¸ 1972 Society of Cosmetic Chemists of Great Britain Rheology of stratum corneumII: A physico-chemical investigation of factors influencing the water content of the corneum A. C. PARK and C. B. BADDIEL* Synopsis--ELASTIC MODULI have been obtained as a function of relative humidity in the range 30-100•o for STRATUM CORNEUM samples that have been extracted successively with CHLOROFORM and water or with SODIUM DODECYL SULPHATE and water. These moduli differ significantly from the values obtained for untreated corneum and these changes have assisted in elucidating the mechanism of corneum hydration. After extraction, a diminution in the water-retaining ability of the stratum corneum was observed by infra-red and thermogravimetric ANALYSES. The infra-red studies also established that LIPIDS were being removed from the corneum by the extracting media and that the prin- cipal protein component of the corneum was unchanged. INTRODUCTION The factors controlling absorption of water by stratum corneum and its subsequent effects on the mechanical properties of this material have been the subject of several studies (1-9). It has been shown that suc- cessive extractions of the corneum (1) with lipid solvents (e.g. chloroform/ methanol, ether, detergent solutions), and (2) with water, reduces the water binding capacity of this substrate whilst either extraction alone has little effect. In addition, studies concerning changes in the stratum corneum after extraction demonstrated that it became less flexible, particularly at relative humidities between 60 and 90•. On the basis of these findings it has been Unilever Research, 455 London Road, Isleworth, Middlesex. 13

14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS established that the equilibrium water content and corresponding elasticity of stratum corneum depend on its efficiency in holding water-retaining substances. The way in which these hygroscopic materials are fixed in the corneum is not known but it is believed to be controlled by a lipid barrier as evidenced by their easy removal after extraction of the corneum with, for example, chloroform/methanol (2: 1, v/v). Middleton (7) has proposed that this lipid barrier resides in the corneum cell membranes and that the keratinous contents of the cells are plasticized by water bound to the hygro- scopic materials. However, it is difficult to envisage how the available water can hydrate the protein components and also satisfy the requirements of the hygroscopic compounds, and, in fact, Bulgin and Vinson (10) have demon- strated, using DTA methods on the isolated corneum components, that endothermic heat changes at 114 ø and 135 ø, attributable to water in the untreated corneum, were still present in the protein fractions. The above result suggests that bound water in the corneum is largely associated with the proteins. In an attempt to resolve this point and thus further elucidate the mechanism of corneum plasticization, this paper describes the measure- ment of the elastic modulus of extracted stratum corneum over the relative humidity range 30-1005/o . Infra-red spectroscopic and differential thermal analysis techniques provided information on structural and water binding changes in the corneum which helped to clarify the model arrived at by mechanical methods. EXPERIMENTAL Techniques and procedures employed in obtaining elastic moduli for stratum corneum have been previously described (9). A constant tempera- ture of 25 4- 0.5 ø was maintained throughout, and relative humidity values were accurate to 4-2•o. High resolution ir spectra of stratum corneum were recorded in the range 4 000-400 cm -• on a Perkin-Elmer 521 double-beam spectrophoto- meter. The spectrometer was calibrated using water vapour, CO2 and polystyrene. For normal instrumental conditions the scan rate was 1 cm -x 4 s -a, but when expansion was used (ordinate x 10 abscissa x 4) the scan rate was slower, 1 cm -a 8 s -a. All samples were dried for 24 h under vacuum before their spectra were recorded and the relative humidity within the sample cavity of the spectrometer was estimated to be •0•o. Complete deuteration of untreated stratum corneum required 6 months in an atmo- sphere of D20 vapour with a somewhat shorter time being necessary for solvent extracted samples.