30 JOURNAL OF COSMETIC SCIENCE The interaction among the three sources of variation (times x glycolic acid x formula- tions) was also significant at the 5% level. These interactions were analyzed separately in view of the inadequate interpretation of a triple interaction. Thus, the behavior of formulations along time was studied individually as a function of the presence or absence of glycolic acid. The study of skin penetration by vitamin A palmirate along time in the different formulations and in the absence of glycolic acid showed that the gel cream and the cream virtually presented the same behavior after one hour, whereas the gel favored a much higher vitamin A palmirate penetration. After two hours, however, the pen- etration of vitamin A palmirate in the gel formulation fell abruptly, whereas it increased abruptly in the gel cream formulation, with the two formulations thus presenting opposite behaviors. In the cream formulation, penetration of vitamin A palmirate was practically constant. In contrast, skin penetration by vitamin A palmirate along time in the different for- mulations and in the presence of glycolic acid was higher in the gel formulation within one hour, and this difference was more marked compared to the cream formulation. After two hours, all formulations presented some increase in penetration compared to the previous time, but the gel and gel cream formulations showed identical behavior, i.e., they favored a marked increase in vitamin A palmirate penetration, whereas the cream formulation showed a very small increase in vitamin A palmirate penetration during this time interval. After four hours, vitamin A palmirate penetration was similar for all formulations, although the curves showed a fall in penetration for the gel and gel cream formulations along time, and a continuous increase in vitamin A palmitate penetration for the cream formulation was observed. The study of the effect of glycolic acid on skin penetration by vitamin A palmirate along time in the gel formulation showed that the glycolic acid factor did not alter vitamin A palmirate penetration during the first hours. After two hours, the gel formulation containing glycolic acid favored a much higher vitamin A palmirate penetration com- pared to the gel formulation without glycolic acid. Between two and four hours, the formulation with glycolic acid caused a fall in vitamin A palmirate penetration probably due to the metabolism of vitamin A palmirate into the skin (18), whereas the formu- lation without glycolic acid favored increased penetration. Thus, the gel formulation containing glycolic acid promoted a faster vitamin A palmirate penetration than the formulation without glycolic acid (Figure 2). The gel cream formulation containing glycolic acid promoted a slight increase in vita- min A palmirate penetration during the first hour. After two hours, the formula with glycolic acid and the formula with no glycolic acid behaved in a similar manner, both of them favoring increased vitamin A palmirate penetration. In contrast, after four hours the formula with glycolic acid caused a fall in vitamin A palmirate penetration, whereas the gel cream formula without glycolic acid caused increased penetration (Figure 3). Thus, the behavior of the gel cream was generally similar to that of the gel formula since in the gel cream glycolic acid also promoted a more rapid penetration of vitamin A palmirate along time. However, the gel cream formula without glycolic acid did not show a fall in vitamin A palmirate penetration after two hours, as was the case for the gel formula. The study of skin penetration by vitamin A palmirate along time in the cream formu-

GLYCOLIC ACID AND SKIN PENETRATION 31 10 glycolic acid •. 2 + gel with glycolic acid 0 0 1 2 3 4 time (hours) Figure 2. Skin penetration by vitamin A palmitate along time in a gel formulation. 10 0 1 2 3 4 -.-e,-- gel cream without glycolic acid gel cream with glycolic acid time (hours) Figure 3. Skin penetration by vitamin A palmirate along time in a gel cream formulation. 10 t I 0 1 2 3 4 + cream without glycolic acid --lB--cream with glycolic acid time (hours) Figure 4. Skin penetration by vitamin A palmirate along time in a cream formulation. lation showed that the behavior of the latter did not differ significantly in the presence or absence of glycolic acid (Figure 4). CONCLUSION Skin penetration tests showed that, under the present experimental conditions, the gel