120 JOURNAL OF COSMETIC SCIENCE 150 135 1 120 .... 105 90 iii 75 .g 60 45 30 15 8.7 (CMC) 35 50 100 200 Total SDS Concentration in the Aqueous Contacting Solution (mM) Figure 2. Comparison of the in vitro skin electrical currents induced by SDS aqueous contacting solutions (striped bars) and by SDS + 10 wt% glycerol aqueous contacting solutions (filled bars). The error bars represent standard errors based on 6-10 p-FTS samples. penetration model (MPM) adopted by many researchers in the past, only the surfactant monomers are able to penetrate into the skin barrier and induce skin barrier perturba­ tion, while the micelles, due to their larger size relative to that of the monomers, are not able to do so. Hence, according to the MPM, the skin barrier perturbation induced by a surfactant contacting solution should not increase significantly upon increasing the total surfactant concentration above the CMC. 4 However, our skin electrical current results clearly show that an increase in the SDS concentration in the contacting solution above the CMC induces a significant increase in the skin electrical current (see Figure 2). This observation is consistent with the results reported by other researchers in previous studies (11-19). For example, Moore et al. (11) found that SDS micelles contribute to SDS skin penetration. Therefore, it is natural that SDS micelles should also contribute to skin barrier perturbation, as we have demonstrated experimentally through these skin electrical current measurements. Indeed, these measurements indicate unequivocally that SDS micelles contribute to skin barrier perturbation, as reflected in the observed increase in the skin electrical current above the CMC. 5 Next, we measured skin electrical currents upon exposing p-FTS to aqueous contacting solutions of SDS (1-200 mM) + 10 wt% glycerol. The results of these measurements are shown as filled bars in Figure 2, which clearly shows that the filled bars (corresponding to the skin electrical currents induced by the SDS + 10 wt% glycerol aqueous contacting solutions) are much shorter than the striped bars (corresponding to the skin electrical currents induced by the SDS aqueous contacting solutions). This important finding clearly shows that the addition of 10 wt% glycerol to an SDS aqueous contacting solution significantly reduces SDS­ induced skin barrier perturbation, as quantified by the skin electrical currents. 4 This statement implies that a surfactant monomer, or a micelle, has to first penetrate into the skin barrier in order to induce skin barrier perturbation. Consequently, if one can minimize, or prevent altogether, penetration of surfactant into the skin, one should be able to minimize skin barrier perturbation induced by the surfactant monomers or by the micelles. 5 It is noteworthy that the skin electrical current induced by PBS (phosphate-buffered saline), which served as the control for these experiments, was 11 ± 4 µA, which is comparable to that induced by a 1 mM SDS solution (see Figure 2).

SDS MICELLES IN SKIN BARRIER PERTURBATION 121 Finally, we measured in vitro mannitol skin permeabilities upon exposing p-FTS samples to aqueous contacting solutions of SDS (1-200 mM) and of SDS (1-200 mM) + 10 wt% glycerol. The results of these measurements are shown in Figure 3, in which the dia­ monds correspond to the permeability values resulting from exposure to the SDS aque­ ous contacting solutions, and the triangles correspond to the permeability values result­ ing from exposure to the SDS + 10 wt% glycerol aqueous contacting solutions. These measurements seem to indicate that: (i) the SDS micelles, in general, do contribute to skin barrier perturbation, as reflected in the increasing P values with increasing SDS concentration above the CMC of SDS (8.7 mM), and (ii) the addition of glycerol minimizes SDS micelle-induced skin barrier perturbation, as reflected in the triangles lying below the diamonds in Figure 3. EFFECT OF GLYCEROL ON SDS SKIN PENETRATION We developed the skin radioactivity assay discussed above to directly quantify the amount of SDS that can penetrate into the skin barrier from an SDS aqueous contacting solution in the absence and in the presence of 10 wt% glycerol. Use of this assay allowed us to directly measure the contribution of the SDS micelles, in the absence and in the presence of 10 wt% glycerol, to SDS skin penetration. The results of our measurements are shown in Figure 4. The concentrations of SDS in the skin barrier (in wt%) resulting from the exposure of p-FTS to aqueous contacting solutions of SDS (1-200 mM) correspond to the diamonds in Figure 4. One can clearly see that upon increasing the total SDS concentration in the contacting solution above the CMC (8.7 mM), the concentration of SDS in the skin E � 1.E-03 1.E-03 � 8.E-04 :E cu a, 6.E-04 C :s 4.E-04 ·c C cu 2.E-04 0.E+00 0 CMC of SDS = 8.7 mM I i f I I 40 80 120 160 200 Total SDS Concentration in the Aqueous Contacting Solution {mM) Figure 3. Comparison of the in vitro mannitol skin permeability induced by SDS aqueous contacting solutions (diamonds) and by SDS 1 10 ',Vt% glycerol aqueous contacting solutions (triangles). The dotted vertical line at an SDS concentration of 8.7 mM denotes the CMC of SDS. The error bars represent standard errors based on 6-10 p-FTS samples.