BUBBLES IN GELS 805 The answer to bubble formation in Carbopol gels due to such a mechanism is quite simple. Since the bubbles result from the undis- solved air produced in the system when alcohol and water are combined, a logical solution is to wait until all the undissolved air is liberated before neutralizing the gel. Using such a technique, one can prepare a bubble-free hydroalcoholic Carbopol gel provided that no air is entrained from the surroundings. It was demonstrated that the amount of air bubbles in the finished gels could be controlled by varying the mixing time prior to neutraliza- tion. Apparently, mixing encourages liberation of the undissolved air in the system. In preparing such gels in production scale, the use of an ultrasonic homogenizer may be advantageous. By a cavitation process, such a machine can remove the dissolved air from an ethanol- water mixture within a short time and help to yield bubble-free gels. Furthermore, such a unit may also help to disperse Carbopol resins more completely and prevent lump formation. ACKNOWLEDGMENT The author gratefully acknowledges the experimental assistance of Tony Lew, Max Factor & Co. (Received February 17, 1969) REFERENCES (1) Martin, A. N., and Banker, G. S., Rlzeology, in Bean, It. S., Beckett, A. H., and Carless, J. E., Advances in Pharmaceutical Sciences, Vol. 1, Academic Press, New York, 1964, p. 58. (2) Lin, T. J., Rheology Fundamentals and Application in Cosmetic Industry, in deNavarre, M. G., Chemistry and Manufacture of Cosmetics, Vol. 1, Van Nostrand, Princeton, N.J., 1962, pp. 329-36. (3) Oldshue, J. ¾., Fluid mixing of cosmetic formulations, J. Soc. Cosmetic Chemists, 10, 332-40 (1959). (4) Lin, T. J., and Donnelly, H. G., Gas bubble entrainment by plunging laminar liquid jets, A.J. Ch.E. Y., 12,563-71 (1966). (5) Adamson, A. W., Physical Chemistry of Surfaces, 2nd ed., Interscience Publishers, New York, N.¾., 1967, pp. 2--4.

J. Soc. Cosmetic Chemists, 20, 807-824 (Dec. 9, 1969) Comparison of in Vitro and Sunscreen Testing Methods* i gi ?y o GUINTER KAHN, M.D.,t and GEORGE WILCOX{ Synopsis---Ten sunscreening agents have been tested by seven methods, six in vitro and one in vivo. Five of the six in vitro methods employ a spectrophotometer while the sixth uses sunshine, photography, and densitometry. The photographic test appears to correlate most closely with the in vivo, but not to a statistically significant degree proposed modifications are suggested to devise an in vitro test for sunscreens that will accurately reflect the protection achieved in vivo. An in vivo test was done on the low back of three people using precise areas, qu antities, and application methods. The study was "blinded" and the sunscreens were placed in different order on each patient. The protection achieved was recorded by multiple random reflectometer readings at 8, 24, and 72 hours through blue and amber filters deviation from mean readings varied by about 5%. Statistical analysis showed the experiment was re- producibly performed and that there is much patient, treatment, and interaction effect when applying sunscreens to skin. The results show that no one sunscreen is always the best, but that a mean ranking and grouping is obtainable even in a small population. INTRODUCTION The ideal method for measuring the ability of a product to shield skin from sunlight has not yet been devised. The variety of testing methods used by industrial and biological researchers only emphasizes the need to find a suitable, reproducible, standard method for testing light absorbers. This paper is concerned with problems encountered while testing the ability of 10 commercial sunscreening products to filter damaging ultraviolet light. Seven methods, six in vitro and one in vivo, are employed to test the agents. A spectrophotometer is used in five of the in vitro methods the sixth utilizes sunshine, photography, and densitometry. The protection achieved by each agent is ranked and in vitro results are compared to in vivo results. * This study was supported in part by the American Cancer Society Grant Photosensitivity IN-SJ #8, by USPHS Research Training Grant 5 T01 AM05527, and by CA 08408. • Division of Dermatology, University of Colorado Medical Center, Denver, Colo. 80220. • Columbia University, New York, N.Y. 8O7