EVALUATION OF SKIN SOFTENING 181 CONCLUSIONS A simple new method For evaluating the skin-soFtening eFFects oF cosmetic ingredients was developed and demonstrated: (1) The stratum corneum was plasticized rapidly by treatment with water and the elastic modulus gradually recovered to the non-treated level concomitant with the evaporation oF water. (2) Both polar and non-polar oils which are widely used in cosmetic products did not soften the stratum corneum. (3) The eFFects oF aqueous surFactant solutions on the stratum corneum were charac- terized by an increase oF elasticity after the evaporation oF water. (4) DMSO and lactic acid with their high capacities For swelling and unfolding several soluble proteins markedly plasticized the stratum corneum. On the other hand, Formic acid had a markedly astringent eFFect on the stratum corneum. (5) The plasticizing abilities oF humectants depended on their water-holding capacities, i.e., the higher the water-holding capacity, the higher the plasticizing effect. REFERENCES (1) I. H. Blank, Factors which influenced the water content of the stratum corneum, J. Invest. Dermatol., 18, 433-440 (1952). (2) I. H. Blank, Further observations on factors which influence the water content of the stratum corneum, J. Invest. Dermatol., 21, 259-271 (1953). (3) J. D. Middleton, The mechanism of water binding in stratum corneum, Brit. J. Dermatol., 80, 437- 450 (1968). (4) A. C. Park and C. B. Baddiel, Rheology of stratum corneum. I. A molecular interpretation of the stress-strain curve, J. Soc. Cosmet. Chem., 23, 3-12 (1972). (5) A. C. Park and C. B. Baddiel, Rheology of stratum corneum. II. A physicochemical investigation of factors influencing the water content of stratum corneum, J. Soc. Cosmet. Chem., 23, 13-21 (1972). (6) B. F. Van Duzee, The influence of water content, chemical treatment and temperature on the rheological properties of stratum corneum, J. Invest. Dermatol., 71, 140-144 (1978). (7) A.M. Kligman and E. Christophers, Preparation of isolated sheets of stratum corneum, Arch. Dermatol., 88, 702-705 (1963). (8) G. L. Wilkes and R. H. Wildnauer, Structure-property relationships of the stratum corneum of human and neonatal rat. II. Dynamic mechanical studies, Biochim. Biophys. Acta, 304, 276-289 (1973). (9) G. J. Putterman, N. F. Wolejsza, M. A. Wolfram and K. Laden, The effect ofdetergents on swelling of stratum corneum, J. Soc. Cosmet. Chem., 28, 521-532 (1977). (10) C. R. Robbins and K. M. Fernee, Some observations on the swelling of human epidermal membrane, J. Soc. Cosmet. Chem., 34, 21-34 (1983). (11) S. G. Elfbaum and K •. Laden, The effect of dimethyl sulfoxide on percutaneous absorption: A mech- anistic study. Part II, J. Soc. Cosmet. Chem., 19, 163-172 (1968). (12) E. Menefee, A mechanical model for wool, Textile Res. J., 38, 1149-1163 (1968). (13) J. H. Bradbury and G. V. Chapman, An investigation by light microscopy of the swelling of wool fibers, Textile Res. J., 33, 666-668 (1963). (14) J. H. Gradburg, G. V. Charpman and N. L. R. King, The chemical composition of wool. III. Analysis of cuticle, skin flakes and cell membrane materials, IWTRC, Section I, 359-364 (1965). (15) M. Takahashi, K. Kawasaki, M. Tanaka, S. Ohta and Y. Tsuda, "The Mechanism of Stratum Corneum Plasticization With Water," in Bioengineering and the Skin, R. Marks and P. A. Pine, Eds. (MTP Press, 1981), pp 67-73.