DIMETHYL SULFOXIDE AND SKIN 139 CONCLUSIONS After examining the effect of DMSO and several other chemical re- agents on the mechanical responses of guinea pig stratum corneum the fol- lowing conclusions can be drawn: 1. The action of DMSO within 24 hours depended very much on the concentration of the solvent. Below 50% DMSO, there was no sig- nificant change in the mechanical properties. At solvent concentra- tions greater than 50%, there was a marked reversible increase in the work index. A skin hardening and wrinkling effect due to DMSO had been qualitatively observed with the skin of humans and on animal skin membranes exposed to DMSO, and it is tempting to speculate that these effects are related to the mechanical responses of the stratum cor- neum strips to ElMSO reported here (1, 8, 9). The physical picture emerging from these studies is one in which the fibrous protein com- ponents of the skin barrier are caused to swell within a restraining en- vironment. The action of DMSO upon the keratin in hair compared to the effect in water seenis to be similar in that a swelling effect is ac- companied by a strengthening phenomenon (6). 2. The nature of the proteinaceous components of guinea pig stratum corneum which underwent the experimental mechanical stress described here is such that a cystine bond reducing agent (ammonium thioglyco- late) weakened it in a partially reversible manner. Thus, a structural role for disulfide bonds in stratum corneum has been demonstrated. Trichloroacetic acid, phenol, and a cross-linking reagent (formalde- hyde) have been shown to increase the work index or impart strength to skin strips. With 5% TCA, the increase is reversible with 2 and 5% solutions of phenol, it is only partly reversible. TCA and phenol may be exerting their strengthening effects partly through the formation, re- spectively, of salt-like linkages and hydrophobic-like linkages and partly through the precipitation of soluble protein components of the stratum corneum. Ten per cent formaldehyde increases the work index of stratum corneum strips in an irreversible manner, thus demonstrating its cross-linking ability. Protein denaturating reagents (6M urea and 97% formic acid) weakened the skin strips considerably. The action of 6M urea was only partly reversible. Guinea pig stratum corneum strips immersed in for- mic acid were irreversibly damaged. Thus, a role for salt linkages, hy- drogen bonds, and hydrophobic bonds has been established as structural components of the stratum comeurn. (Received May 21, 1969)

140 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS REFERENCES (1) Elfbaum, S. G., and Laden, K., The effect of dimethyl sulfoxide on percutaneous absorption: a mechanistic study, Part II, J. Soc. Cosmetic Chemists, 19, 163 (1968). (2) Bradbury, J. H., and Chapman, G. V., The chemical composition of wool. I. The separation and microscopic characterization of components produced by ultrasonic disintegration, Australian J. Biol. Sci., 17, 960 (1964). (3) Abbott, N.J., Temin, S.C., and Park, C., The stress-strain behavior of wool in various swelling media, Textile Res. J., 88, 1026 (1968). (4) Elfbaum, S. G., and Laden, K., The effect of dimethyl sulfoxide on percutaneous absorption: a mechanistic study, Part III, J. Soc. Cosmetic Chemists, 19, 841 (1968). (5) Price, V. H., and Menefee, E., On the effect of dimethyl sulfoxide on hair keratin, J. Invest. Dermatol., 49, 297 (1967). (6) Wolfram, L., and Elfbaum, S. G., paper in preparation. (7) Stoughton, R. B., and Fritsch, W., Influence of dimethyl sulfoxide (DMSO) on human percutaneous absorption, Arch Dermatol., 90, 512, (1964). (8) Rosenbaum, E. E., et al., Dimethyl sulfoxide in musculoskeletal disorders, J. Am. Med. Assoc., 192, 109 (1965). (9) Kligman, A.M., Topical pharmacology and toxicology of dimethyl sulfoxide, Part 1, Ibid., 198, 796 (1965). (10) Kligman, A.M., Dimethyl sulfoxide, Part 2, Ibid., 198, 923 (1965). (11) Stone, O. J., Thiabendazole in dimethyl sulfoxide for Tinea Nigra Palmaris, Arch. Dermatol, 98, 241 (1966). (12) Leake, C. D., Dimethyl sulfoxide, Science, 152, 1646 (1966). (1.8) Tillman, W. J., Mechanoelastic behavior of the cornified epithelium of human skin, Ph.D. Dissertation, University of Wisconsin, 1960, p. 29. (14) Vinson, L. J., et al., Basic studies in percutaneous absorption, Semi-annual Rept. No. 7, Contract No. DA 18-108-CML 6573, Lever Brothers Co., Sec. 3, p. 37 (January- June, 1964). (15) Meredith, R., The Mechanical Properties of Textile Fibers, Interscience Publishers, New York, 1956, p. 192. (16) Alexander, P., Wool, Its Chemistry and Physics, Chapman and Hall, London, 1954, p. 58. (17) Skoog, D. A., and West, D. M., Fundamentals of Analytical Chemistry, Holt, Rinehart, Winston, New York, 1963, p. 53. (18) Matoltsy, A. G., Downes, A.M., and Sweeney, T. M., Studies of the epidermal water barrier. Part II. Investigation of the chemical nature of the water barrier, J. Invest. Dermatol., 50, 19 (1968). (19) Alexander, P., op. cit., p 64. (20) Breuer, M. M., The binding of phenols by hair. III. Volume changes accompanying binding of phenols by hair, J. Phys. Chem., 68, 2081 (1964). t21) Grimblcby, F. H., and Ntailianas, H. A., Binding of trichloroacetic acid by protein, Nature, 189, 835 (1961). (22) Klotz, I. M., and Russo, S. F., Studies of Peptide Hydrogen Bonding by Variou• Electromagnetic Methods from Protides of the Biological Fluids, Vol. 14, Elsevier Publishing Co., Amsterdam, 1966, p. 427. (23) Nemethy, G., Hydrophobic interactions, Angew. Chem., 6, 195 (1967).