SKIN PENETRATION 629 tant (sodium tetrapropylenebenzenesulfonate), a cationic (dodecyltri- methylammonium chloride), and soap probably means that the barrier to these surfactants is being altered by the surfactants themselves as they diffuse into and through the skin. Decrease in the barrier properties al- lows an increased diffusion, which in turn results in a greater alteration of the barrier. Removal of the surfactant causes the process to stop at the stage of barrier alteration to which it had progTessed. CONCLUSION While conflicting ideas and contrary evidence exist in elucidating skin barrier properties and mechanisms of skin penetration, it is becoming increasingly evident that main channels for understanding rest in well- defined studies of the physical-chemical properties of the agents and cor- relation with skin variables. Hopefully, chemical structures may be ma- nipulated to effect selective permeability. The role of vehicles or formu- lation is likely to remain of subsidiary importance. However, there is need to pursue studies to resolve much conflicting evidence and elucidate further the mechanism o1• action of vehicles which enhance penetration ot• varied materials. (Received March 8, 1971) REFERENCES (1) Malkinson, F. D., and Roth•an, S., Handbuch der Haut und Ge6chlechtskrankheiten, J. Jadasohn, Springer, Berlin, 1963. (2) Malkinson, F. D., in Montagna, W., and Lobitz, W. C. Jr., The Epidermis, Academic Prcss, New York, 1964, Chap. 21. (3) Blank, I. H., Penetration of low molecular weight alcohols into skin. I. Effect of con- centration of alcohol and type of vehicle, J. Invest. Dermatol., 43, 415 (1964). (4) fcott, A., The bchavior of radioactive mercury and zinc after application to normal and abnormal skin, Brit J. Dermatol., 71, 181 (1959). (5) Felsher, Z., and Rothman, S., Insensible perspiration of skin in hyperkeratotic condi- tions, ]. Inve6t. Derrnatol., 6, 271 (1945). (6) Elliott, J. A., Jr., and Odel, H. M., Percutaneous absorption of sodium in cases of exfoliative dermatitis, Ibid., 15, 389 (1950). (7) Malkinson, F. D., •tudies on thc percutaneous absorption of C TM labeled stcroids by use of the gas-flow cell, Ibid., 31• 19 (1958). (8) Malkinson, F. D., and Ferguson, E. H., Percutaneous absorption of hydrocortisone-4-C •* in two human subjects, Ibid., 25, 281 (1955). (9) Liddle, J. w., 9-a-Fluorohydrocortisone: A new investigative tool in adrenal physiol- ogy, ]. Clin. Endocrinol. Metab., 16• 557 (1954). (10) LoefIter, R. K., and Thomas, V., A quantitative study of percutaneous absorption. I. Absorption of radiostrontium chloride in minute quantities through intact and me- chanically damaged rat skin, U.S. Atomic Energy Comm. Rept. AD-225, Nucl. Sci. Abstr., 5, 48 (1951).

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