28 JOURNAL OF COSMETIC SCIENCE Table V Tukey Test Applied to the Formulation Data Formulations Means Critical Tukey value (5 %) Gel 7.06' Gel cream 6.46* Cream 4.54# 0.9257 Means followed by identical symbols are not significantly different. the guinea pigs studied. Analysis of variance, however, although stating that this dif- ference exists, does not specify which means differ from one another, so that a comple- mentary Tukey test is needed to define identical and different means. The Tukey test for the formulation factor is presented in Table V. Glycolic acid x time interaction. The interaction between these two factors was also sta- tistically significant at the 5 % level, with glycolic acid having a certain effect after two hours of contact with the skin. The Tukey test for this interaction is presented in Ta- ble VI. Time x g/yco/ic add x formulation interaction. The interaction between these three factors was also significant at the 5% level. Graphs were constructed with the results obtained to better determine the variation of formulations containing or not containing glycolic acid as a function of time. The Tukey test for this interaction is shown in Table VII. Figures 2 through 4 graphically present the data concerning the percentage of vitamin A palmitate penetration into the guinea pig skin as a function of these interactions. The interactions were analyzed separately since the interpretation of a triple interaction was inadequate. DISCUSSION Studies of skin penetration have been gaining importance in the scientific community. The combination of glycolic acid at low concentrations with vitamin A palmitate has been extensively discussed. On the basis of clinical observations, dermatologists have concluded that this combination provides excellent results in the treatment of skin aging. Table VI Tukey Test Applied for Comparision of the Mean Data Obtained for the Glycolic Acid x Time Interaction Interaction Means Critical Tukey value (5%) Without 5.13' Without 5.67* Without 6.90* With 5.28' With 7.27# With 5.87' 1.8292 Means followed by identical symbols are not significant different.

GLYCOLIC ACID AND SKIN PENETRATION 29 Table VII Tukey Test Applied to the Time x Glycolic Acid x Formulation Interaction Interaction Means Critical Tukey value (5%) Gel x without x 1 h 6.91 Gel x without x 2 h 5.21 Gel x without x 4 h 7.77 Gel x with x 1 h 6.59 Gel x with x 2 h 9.78 Gel x with x 4 h 6.13 Gel cream x without x 1 h 4.12 Gel cream x without x 2 h 7.31 Gel cream x without x 4 h 7.68 Gel cream x with x 1 h 5.62 Gel cream x with x 2 h 7.85 Gel cream x with x 4 h 6.20 Cream x without x 1 h 4.36 Cream x without x 2 h 4.50 Cream x without x 4 h 5.26 Cream x with x 1 h 3.62 Cream x with x 2 h 4.19 Cream x with x 4 h 5.33 3.4214 The cornified layer of the human skin is the most important barrier against penetration by several substances. Disarrangement of this barrier using chemical substances is the most rapid method to reach the dermis (15). It has also been observed that glycolic acid promotes an increase in epidermal thickness as well as an increased epidermal hydration (16). These factors indicate that vitamin A palmitate, when combined with glycolic acid, may tend to reach deeper layers of the epidermis or even to remain longer in it since some of the factors that may increase skin absorption may also increase the amount of substance retained in the skin (15). In view of the many empirical conclusions reached with the use of vitamin A palmitate in combination with glycolic acid, in the present study we evaluated this combination on the basis of a parameter judged to be of importance, i.e., skin penetration. The study was based on the use of three different formulations containing vitamin A palmitate combined or not combined with glycolic acid. The experimental data obtained were analyzed statistically to determine differences in percentages of skin penetration (17). The statistical study conducted to determine the effect of glycolic acid and vehicle on the skin penetration of vitamin A palmitate showed that the presence of glycolic acid in the formulation alters the penetration of vitamin A palmitate into the guinea pig skin. The penetrating behavior of vitamin A palmitate in formulations with and without glycolic acid along time differed at the 5% level of probability. The penetration of vitamin A palmitate in formulations containing glycolic acid was accelerated between one and two hours but rapidly fell between two and four hours. In contrast, the formulation without glycolic acid maintained a more or less uniform rate of penetration throughout the experimental period (one to four hours). In summary, the amount of vitamin A palmitate that penetrated the skin in the absence of the acid took four hours to do so, whereas with the addition of glycolic acid it took only two hours.