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.

POLYETHYLENE GLYCOL-8/SMDI COPOLYMER 141 A. SUBSATURATED 0.016 0.012 0.008 0.004 0.000 1.2 i i i i 2 4 6 8 Time (Hours) B. SATURATED 10 • 0.6- 0.4- "• 0.2- 0.0 0 2 4 6 8 Time (Hours) lO Figure 4. Effect of PP-15 on the permeation of salicylic acid from subsaturated and saturated solutions through pig skin. A: O, Control-0.1% SA O, 3% PP-15-0.1% SA. B: O, Control-0.5% SA [•, control- 2.0% SA O, 3% PP-15-0.5% SA l, 3% PP-15-2.0% SA. EFFECT OF THE IONIC STRENGTH OF THE FORMULATION ON THE EFFICACY OF PP-15 The results obtained for the four formulations tested are plotted in Figure 5. The permeation of SA from all formulations was linear with time after approximately a two-hour lag time. The rates of permeation were 2.37 x 10 -3, 1.13 x 10 -3, 1.14 x 10 -3, and 1.03 x 10 -3 mg/cm2/hour for the control, PP~15 without salts, PP-15 with 0.1 M sodium chloride, and PP-15 with 0.1 M calcium chloride, respectively. The similarity in flux values indicated that the ionic strength of the solution did not influence the effect of PP-15.