STATEMENT OF PROBLEM AND CRITICAL REVIEW OF PAST METHODS 67 cutaneous surface and the blood concentration of this substance is deter- mined at stated intervals, the amount of material which has entered the e determined from the calibration curve blood through the skin can b . . This method is most accurate when the rate at which the materrol reaches the blood stream is the same for the two methods of administration. Av. alysis of a single body tissue does not, of course, give information concerning total penetration it only shows the amount which has become localized in the tissue being analyzed (23). At times, this may be a very important weasurement if one is studying a specific, toxic reaction which mav follow percutaneous absorption. CONCLUDING REMARKS Loss of Penetrating Substance from the Surface If a known amount of a nonvolatile material which has been allowed to remain on the cutaneous surface for a definite period of time is quantita- tively removed from the surface and analysed, the difference between the amount applied and the amount removed may be thought to have pene- trated the skin (24). There are several pitfalls in this method. Quantita- tive removal is difficult. Some materials combine chemically with the constituents of the cornified epithelium and then cannot be easily separated from it although they actually have not penetrated into the skin any further than the stratum comeurn. The amount of most substances which penetrates is often so small that errors in analysis may be of the same order of magnitude as the amount absorbed, and therefore, one can never be sure that the difference between "before" and "after" determinations truly represents the amount of material which has penetrated. Now that radioactive substances can be prepared, it is possible to determine loss from the surface simply by determining the decrease in radioactivity (25). When some of the substance penetrates into the skin and is carried away by the blood stream, the radioactivity, as measured on the surface, decreases. Although other investigators have used this method successfully, it has not proved very satisfactory when used in our laboratory. Small movements of the experimental animal cause changes in radioactivity as measured by a Geiger tube held above the site of application. The amount of material which penetrates may be so small that the decrease in radioactivity is difficult to determine accurately. According to the title of this paper, I am supposed to have discussed "older" methods. There can be no clean cut distinction between "older" methods and "newer" methods. I have chosen to discuss some methods which have been in use over the years and have tried to point out some of their limitations. As knowledge has advanced, methods have been refined, and completely new techniques have become available. Ainsworth will

68 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS discuss new methods and refinements of older techinques with which it is now possible to obtain more accurate data concerning the number of molecules of a substance which leave the surface of a unit a•ea of skin in a unit time and how much of the penetrating material reaches the blood stream. He will show a correlation of the results obtained by different methods. No one can yet say where more accurate determinations of percutaneous absorption will lead us. They certainly should be useful to the cosmetic chemists. With such methods available, the initial step in the study of mechanisms of action can be taken. When we understand how a cosmetic acts, present products can be evaluated and perhaps improved and new products can be formulated. REFERENCES (1) Rothman, S., "Physiology and Biochemistry of the Skin," Chicago, University of Chicago Press (1954), pp. 26-59. (2) Guillot, M., and Valette, G., 5 e. physiol. (Paris), 46, 31 (1954). (3) Calvery, H. O., Draize, J. H., and Laug, E. P., Physiol. Review, 26, 495 (1946). (4) Lockie, L. D., and Sprowls, J. B., y. •lm. Pharm. •lssoc., $ci. Ed., 40, 72 (1951). (5) Strakosch, E. A., •lrch. Dermatol. and $yphilol., 47, 16 (1943). (6) Goldzieher, J. W., Roberts, I. S., RaMs, W. B. and Goldzieher, M. A., Ibid., 66, 304 (1952). (7) Mackee, G. M., Sulzberger, M. B., Herrmann, F., and Baer, R. L., y. Invest. Dermatol., 6, 43 (1945). (8) Eller, J. J., and Wolff, S., •lrch. Dermatol. and $yphilol., 40, 900 (1939). (9) Montagna, W., Proc. Soc. Exptl. Biol. Med., 86, 668 (1954). (10) Riska, E. B.,/lcta Pathol. Microbiol. Scand., Suppl. No. 114, 46 (1956). (11) Malkinson, F. D., y. $oc. Cosmetic Chemists, 7, 109 (1956). (12) Butcher, E. O., y. Invest. Dermatol., 21,243 (1953). (13) Blank, I. H., Griesemer, R. D., and Gould, E., Ibid., 29, 299 (1957). (14) Blank, I. H., Gould, E., Ibid., 33, 327 (1959). (15) Boyd, G. A., "Autoradiography in Biology and Medicine," New York, Academic Press, Inc. (1955). (16) Shelley, W. B., and Melton, F. M., y. Invest. Dermatol., 13, 61 (1949). (17) Weiss, W., •lm. y. Med. Sci., 231, 13 (1956). (18) Mack, W. L., and Nelson, J. W., y. •lm. Pharm. •lssoc., $ci. Ed., 42, 101 (1953). (19) Treherne, J. E., y. Physiol. (London), 133, 171 (1956). (20) Flesch, P., Satanore, A., and Brown, C. S., y. Invest. Dermatol., 25, 289 (1955). (21) Nyiri, W., and Jannitti, M., y. Pharm. Exptl. Therap., 45, 85 (1932). (22) Griesemer, R. D., Blank, I. H: and Gould, E., y. Invest. Dermatol., 31,255 (1958). (23) Laug, E. P., Vos, E. A., Urnberger, E. J., and Kunze, F. M., y. Pharm. Exptl. Therap., 89, 42 (1947). (24) Nagy, S. M., Golumbic, C., Stein, W. H., Fruton, J. S., and Bergmann, M., y. Gen. Physiol., 29, 441 (1946). (25) Malkinson, F. D., y. Invest. Dermatol., 31, 19 (1958).