112 JOURNAL OF COSMETIC SCIENCE intercellular lipid bilayers. The bulk of the bilamellar lipid sheets are proposed to be in crystalline/gel domains bordered by lipids in a fluid crystalline state. In skin exhibiting SC barrier damage, the proportion of lipids in the solid state may be elevated, and subsequent skin exposure to glycerol may help maintain these lipids in a liquid crys­ talline state at low relative humidity, thereby enhancing SC barrier function and de­ creasing SC water permeability (30). A second prevalent view is that glycerol may increase the rate of corneocyte loss from the upper layers of the SC, through a kerato- 1 ytical effect due to enhanced desmosome degradation, thereby reducing the scaliness of dry skin and maintaining the SC barrier (31). A third, more recent view advanced by Fluhr et al. (24) is based on the hygroscopic property of glycerol. Glycerol, by virtue of its high transdermal diffusivity, can penetrate into the SC, and, by virtue of its hygro­ scopic property, is able to bind water and thus reduce water evaporation. Therefore, glycerol, by absorbing water, may modulate water fluxes in the SC, which, in turn, may lead to a stimulus for SC barrier repair. However, it is still not well understood how glycerol may mitigate surfactant-induced SC barrier perturbation induced by a formulation containing aqueous mixtures of glyc­ erol and a surfactant, such as SDS. Most of the studies discussed above (24-31) consid­ ered the application of glycerol to forearm skin in vivoJ either: (i) as dilute aqueous solutions containing 5-15 wt% glycerol or (ii) as cosmetic formulations, such as barrier creams, containing a similar range of glycerol concentrations. With this in mind, using such an aqueous mixture of SDS and 10 wt% glycerol, we will demonstrate in vitro that the addition of glycerol eliminates almost completely the contribution of the SDS micelles to SDS skin penetration. Using dynamic light-scattering (DLS) measurements, we will show that the addition of 10 wt% glycerol to an aqueous SDS contacting solution does not increase the size of the SDS micelles, which if increased, could explain the observed reduced ability of SDS (present in the larger SDS micelles) to penetrate into the skin and induce less skin barrier perturbation in the presence of glycerol. Further­ more, using surface tension measurements, we will show that the addition of 10 wt% glycerol to an aqueous SDS contacting solution does not decrease the CMC, and hence, does not reduce the concentration of the SDS monomers contacting the skin, which if reduced, could explain the observed reduced ability of SDS (present in monomeric form) to penetrate into the skin and induce less skin barrier perturbation in the presence of glycerol. Finally, using in vitro mannitol as well as skin permeability and skin electrical current measurements, in the context of a hindered-transport porous pathway model of the SC (6-9,42), we will show that a plausible explanation of our findings is that the addition of 10 wt% glycerol to an aqueous SDS contacting solution reduces the size and the number density of the aqueous pores in the SC relative to the SDS micelle size, such that the SDS micelles present in the contacting solution are sterically hindered from penetrating into the SC. This, in turn, leads to significantly less SDS-induced skin barrier perturbation upon the addition of 10 wt% glycerol. EXPERIMENTAL MATERIALS Sodium dodecyl sulfate (SDS) was purchased from Sigma Chemicals (St. Louis, MO). Analytical-grade glycerol was purchased from VWR Chemicals (Cambridge, MA). 14 C-

SDS MICELLES IN SKIN BARRIER PERTURBATION 113 radiolabeled SDS and 3 H-radiolabeled mannitol were purchased from American Radio­ labeled Chemicals (St. Louis, MO). All these chemicals were used as received. Water was filtered using a Millipore Academic water filter (Bedford, MA). Phosphate-buffered saline (PBS) was prepared using PBS tablets from Sigma Chemicals (St. Louis, MO) and Millipore filtered water, such that a phosphate concentration of 0.01 M and a NaCl concentration of O .13 7 M were obtained at a pH of 7. 2. PREPARATION OF SKIN SAMPLES Female Yorkshire pigs (40-45 kg) were purchased from local farms, and the skin (back) was harvested within one hour after sacrificing the animal. The subcutaneous fat was trimmed off using a razor blade, and the full-thickness pig skin was cut into small pieces (2 cm x 2 cm) and stored in a -80°C freezer for up to two months. The surfactant penetration experiments were performed using pig full-thickness skin, referred to here­ after as p-FTS. IN VITRO TRANSDERMAL PERMEABILITY MEASUREMENTS Vertical Franz diffusion cells (Permegear Inc., Riegelsville, PA) were used in the in vitro transdermal permeability measurements (see Figure 1). All the experiments were per­ formed at room temperature (25 ° C). Prior to each experiment, a p-FTS sample was mounted in the diffusion cell with the SC facing the donor compartment. Both the donor Ag/AgCl SIGNAL Donor Compartment GENERATOR AMMETER - - G Ions - G Receiver G Compartment G G Sample Port Figure 1. Vertical Franz diffusion cell experimental setup to measure transdermal permeability, skin electrical current, and/or skin radioactivity in vitro.