The lipid lamellae form the only continuous path across the SC and are important for the barrier properties of SC (Boddé et al., 1989 and Potts and Guy, 1992). However, depending on the diffusional transport path taken by the substance, one might also need to consider the barrier properties of the

protein components, which indeed constitute the main fraction of the total SC material. It is clear that structural changes in the lipid or protein components in response to interactions with molecules present in the formulation in contact with the skin membrane can have important implications for the SC barrier properties. The SAXD and WAXD results (Fig. 2A and B, respectively) show that pretreatment of SC in formulations that contain either glycerol or urea (water activities around 0.93–0.94) has a similar effect on the organization of the lipid lamellae Hydroxychloroquine chemical structure and the soft keratin proteins as pretreatment in neat PBS solution (water activity of 0.996). Considering these results it may selleckchem be expected that the skin permeability is similar for these formulations, as observed in the present results (Fig. 1A). Thus, the steady state flux results in Fig. 1A may be related to that glycerol and urea penetrate into the SC and retain the structure of a fully hydrated SC membrane, which leads to similar transport characteristics of Mz across the skin membrane at reduced water activities. The effect of glycerol and urea is in contrast to the relatively larger polymer molecules,

which do not partition into the skin membrane (Albèr et al., unpublished results, Tsai et al., 2001 and Tsai et al., 2003) and thus only affect the skin membrane by dehydration irrespective of the presence of glycerol or urea. The abrupt decrease in permeability upon dehydration

in Fig. 1B can thus be attributed to a larger fraction of less permeable solid SC components (lipids and proteins) (Alonso et al., 1996, Björklund et al., 2013a and Björklund et al., 2013b). In relation to the present diffraction data it has previously been demonstrated from SAXD and FTIR measurements that pretreatment of human SC in glycerol solution (35% w/v) for 24 h at 32 °C does not alter the organization of the lipid lamellar structures as however compared to pretreatment in pure water (Caussin et al., 2008). Likewise, previous EPR spectroscopy studies, using spin labels to probe lipid dynamics, showed that treatment of SC with 8 M urea (approx. 43 wt%) only has a minor effect on the fluidity of the SC lipids (do Couto et al., 2005). These findings are in line with the present results (Fig. 2A and B). The position of the diffraction peak from soft keratin is slightly affected by the type of pretreatment as it is shifted from around 1.00 nm in the pure SC sample to approx. 0.95 nm when glycerol or urea are present in SC sample (Fig. 2B). We also note that the diffraction from this peak is weaker for the SC sample pretreated in urea formulation, which makes the determination of the peak position less certain.