GLYCOLIC ACID AND SKIN PENETRATION 27 Table III Percentages of Applied Dose of Vitamin A Palmitate in the Skin of Guinea Pigs Exposed to the Various Formulations (IU/0.5 cm 2 Guinea Pig Skin) Time (h) Formulation 1 2 4 Gel Without glycolic acid With glycolic acid Gel cream Without glycolic acid With glycolic acid Cream Without glycolic acid With glycolic acid 3.93 5.79 5.02 7.83 5.30 9.12 4.55 6.72 6.59 2.26 6.68 8.37 10.07 5.46 2.88 8.83 6.83 9.39 9.34 9.44 3.30 4.96 8.59 7.92 6.21 7.72 9.55 4.16 7.13 4.77 4.09 5.76 9.91 13.26 4.82 8.07 8.16 10.75 6.82 14.90 4.67 6.83 6.08 4.03 13.23 6.38 6.60 6.12 4.44 5.08 7.41 12.09 6.41 5.74 3.20 4.04 7.66 5.28 11.07 7.83 3.30 4.18 3.24 8.39 5.91 10.11 3.45 5.25 7.78 6.62 4.39 10.01 6.89 8.20 5.81 8.78 7.08 4.95 4.19 5.28 7.53 6.62 5.26 9.22 8.67 4.92 8.25 6.48 8.60 5.72 3.35 3.48 7.57 11.74 5.40 3.41 2.95 7.55 5.51 6.09 7.66 5.69 3.61 6.44 3.23 3.55 4.74 5.58 4.86 3.22 4.72 4.09 3.57 5.26 3.18 3.09 5.64 3.18 5.46 4.09 4.77 4.40 4.43 2.49 7.13 3.39 4.17 2.55 4.03 3.27 6.37 5.47 3.71 2.55 6.91 4.54 5.27 5.13 3.50 3.34 5.62 2.25 6.49 3.38 It should be pointed out that, due to the depilation, there probably was a change in the thickness of the corneal layer of the guinea pig skin. An attempt was made to reduce this error to a minimum by depilating the skin 24 hours before the experiment. Formulation factor. Statistical analysis showed a highly significant difference at the 1% level between formulations in terms of vitamin A palmitate penetration into the skin of Table IV Tukey Test Applied to the Time Factor Data Time (h) Means Critical Tukey value (5%) 1 5.20# 2 6.47* 3 6.39* 1,0524 Means followed by identical symbols are not significantly different.

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.