J. Soc. Cosmet. Chem. 23 471479 (1972) ¸ 1972 Society of Cosmetic Chemists of Great Britain The effect of saturated salt solutions on the elastic properties of stratum corneum A. C. PARK and C. B. BADDIEL* Synopsis--ELASTIC MODULUS values have been obtained for native and solvent extracted STRATUM CORNEUM in saturated SOLUTIONS of various SALTS. In nearly every case the modulus was lowered significantly relative to its value at the same equivalent water activity in the vapour phase. This effect was attributed to disruption of the protein matrix of the CELL junctions which are believed to be responsible for the mechanical integrity of the corneum. A simple structural model is proposed which accounts for the response of the corneum to various TREATMENTS. INTRODUCTION The protection of the body against environmental hazards is main- tained by a thin layer of dead cells called the stratum corneum. The function of this outermost layer of the skin is to act as a barrier between the environ- ment and the complex system of living tissues which it covers. In the in vivo situation the stratum corneum must be able to respond quickly and without damage to mechanical stresses and strains imposed on it by various perturbations. As a result, studies in this and other laboratories (1-6) have been based on the premises that (i) maintenance of the mechanical integrity of this layer is important if it is to continue to fulfil its proper protective role, and (ii) changes in its mechanical properties can be used as a probe of those factors which influence its function and structure. The principal constituent (60-70•o dry weight), of the stratum corneum is a soft form of the fibrous protein, keratin, which is contained within the * Unilever Research Laboratory, 455 London Road, Isleworth, Middlesex. 471

472 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS corneum's cellular network (7). A different protein (8) constitutes the bulk of the cell membranes (• 65•) and other known components, which are ill-defined with regard to their position in the structure, include lipids (mainly cholesterol and phospholipids), water-soluble hygroscopic materials sometimes referred to as the natural moisturizing factor or NMF (these include simple peptides, organic acid salts and free amino acids) and poly- saccharides (9-11). A dual role has been established for the lipids. These substances reduce the rate of water transport through the corneum (12) and prevent the NMF from being extracted with water (9). The hygroscopics act as emollients by causing the corneum to retain water under dry environ- mental conditions. The role of the polysaccharides is ill-defined but they are probably involved in conjunction with the proteins of the cell junction in intercellular adhesion (11). In this communication, the effects of saturated solutions of salts on the elastic modulus of isolated strips of pigs' ear stratum corneum are described and the results discussed in terms of the crucial role of water in eftcoting changes in the mechanical characteristics of the material. By use of in- organic ions as simple models, for the hygroscopics, insight has been gained into the location of the NMF in the corneum structure and the types of interactions likely to occur between the hygroscopic materials and the corneum proteins. Reason is also given for the permanence of these water- soluble materials in the corneum. A model is proposed for the intercellular junction of the corneum in which, it is believed, the mechanical integrity of this material lies. EXPERIMENTAL The techniques employed for isolating intact sheets of stratum corneum from the skin of pigs' ears and for obtaining accurate elastic modulus values for this material, by means of the Instron extensometer, have been previously described (1). Narrow strips of stratum corneum were equilibrated in saturated solu- tions of the salts shown in Figs. 1 and 2 prior to obtaining their elastic moduli. Since errors due to buoyancy effects arose if the top clamp which is connected to the load cell of the Instron was in contact with the solutions, it was impossible to extend the corneum while it was totally immersed in the salt solutions. However, by use of the device shown in Fig. 3 it was possible to measure the elastic modulus of a corneum strip within 30 s of its removal from the solution exposures of up to 10 min produced no