POLYETHYLENE GLYCOL-8/SMDI COPOLYMER 139 A. STRATUM CORNEUM 0.032 ,•, 0.024 o 0.016 r.• 0.008 0.000 2 3 4 5 6 7 8 Time (Hours) B. VIABLE SKIN 0.032 0.024 0.016 0.008 0.000 2 3 4 5 6 7 8 9 Time (Hours) Figure 2. Effect of PP-15 on the levels of salicylic acid in the stratum corneum and the viable skin over time. A: ¸, Control O, 3% PP-15. * Indicates a statistically significant difference between the two values at the same time-point. B: ¸, Control O, 3% PP-15. The distribution of LA (5.0% w/w) in the SC and the viable skin over time are presented in Figures 3A and 3B, respectively. The presence of the polymer did not affect signifi- cantly the levels of LA in the SC. The polymer lowered the levels of LA significantly in the viable skin at the 20-hour time point. No statistical difference was noted at the three earlier time points. EFFECT OF PP-15 ON THE PERMEATION OF SA FROM SUBSATURATED AND SATURATED FORMULATIONS THROUGH PIG SKIN Three levels of SA were studied, namely, 0.1%, 0.5%, and 2.0% w/w. Formulations

140 JOURNAL OF COSMETIC SCIENCE A. STRATUM CORNEUM 0.08 0.06 0.04 0.02 0.00 • 0.10 2 4 6 8 10 12 14 16 18 20 22 Time (Hours) B. VIABLE SKIN 0.08 0.06 0.04 0.02 0.00 , , , , , i i i i 2 4 6 8 10 12 14 16 18 20 22 Time (Hours) Figure 3. Effect of PP-15 on the levels of lactic acid in the stratum corneum and the viable skin over time. A: O, Control O, 3% PP-15. B: O, Control O, 3% PP-15. * Indicates a statistically significant difference between the two values at the same time-point. containing SA at a 0.1% level were solutions, whereas the others were suspensions. The results obtained are plotted in Figures 4A and 4B. Linear correlations were obtained between the concentration of SA in the formula and the respective rates of permeation for the PP-15 and control formulations. In most cases, the correlation coefficients calculated had values --0.998. The effect of PP-15 was calculated by dividing the flux value of the control formulation (without PP-15) by the flux value of the test formula (with PP-15) for each concentration of SA. The ratios obtained were 2.4, 1.4, and 1.2 for the 0.1%, 0.5%, and 2.0% (w/w) formulations, respectively. This showed that the PP-15 polymer was most effective in reducing permeation in a subsaturated system.