SDS MICELLES IN SKIN BARRIER PERTURBATION 111 pores in the SC (11-23). The MPM does not consider the possibility that surfactant micelles may also contribute to surfactant skin penetration, and consequently, to sur­ factant skin barrier disruption, since it considers the micelles to be too large to penetrate through the aqueous pores that exist in the SC. In her comprehensive review of surfac­ tant-skin interactions, Rhein (14) stated that the observed dose dependence of surfac­ tant-induced skin irritation beyond the CMC cannot be explained solely by the contri­ bution of the monomeric surfactant. Indeed, Agner and Serup (13) had earlier observed that the severity of the transepidermal water loss (TEWL) induced by SDS increased as the SDS concentration increased beyond the CMC of SDS (8.7 mM) (13). In separate studies, Ananthapadmanabhan et al. (15) and Faucher and Goddard (16) observed that as the SDS concentration increased beyond the CMC, the extent of SDS skin penetration also increased. Through in vitro SDS skin penetration studies, Moore et al. (11) provided substantial evidence that indicates that the amount of SDS that can penetrate into the skin barrier is dose-dependent, and furthermore, that the SDS surfactant in micellar form also contributes to SDS skin penetration. In addition, Moore et al. demonstrated conclusively that the contribution of the SDS micelles to SDS skin penetration dominates that of the SDS monomers at concentrations above the CMC, which are typically encountered in skin care formulations (11). In this paper, we have further investigated, from a mechanistic perspective, how SDS micelles may contribute to SDS skin penetration, thereby leading to the previously observed dose dependence of SDS-induced skin barrier perturbation (11-23). Specifi­ cally, we will provide new evidence, through in vitro transdermal permeability and skin electrical current measurements, in the context of a hindered-transport porous pathway model of the SC (6-9,42), that the aqueous pores in the SC increase both in size and in number density when skin is exposed to an aqueous SDS contacting solution, such that the average pore radius is larger than the SDS micelle radius. As a result, SDS micelles, contrary to the view put forward by the MPM, are not sterically hindered from pen­ etrating into the skin barrier through these pores. Inspired by our proposed mechanistic understanding of how SDS micelles may contrib­ ute to SDS-induced skin barrier perturbation, we have also investigated in vitro whether the addition of glycerol, a well-known humectant and skin beneficial agent, to the aqueous SDS contacting solution can minimize the observed contribution of the SDS micelles to SDS skin penetration. Although not within the scope of this paper, if shown to be valid in vivo1 such a strategy can also significantly reduce the amount of SDS that can penetrate into the skin barrier and induce skin barrier perturbation in vivo. Our approach considers exposing skin in vitro to aqueous mixtures of glycerol and SDS. The importance of glycerol (or glycerin) in skin care products is well established, and glycerol is widely used in cosmetic and pharmaceutical formulations (24-31). To explain its in vivo benefits, studies have focused on its humectant and smoothing effects (25) and on its protective functions in emulsion systems against skin irritation (26). Researchers have shown that glycerol diffuses into the SC, increases skin hydration, and relieves clinical signs of dryness (27-29). One of the views regarding the effect of glycerol on the skin held by researchers today is that it may influence the crystalline arrangement of the

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-