The performance of transdermal drug delivery systems are affected by the presence of stratum corneum (SC). Carriers such as microemulsions (MEs) have been developed as skin penetration enhancers. Although the main permeation route for MEs appears to be the pathway through the lipid layers between corneocytes, the interaction mechanism between MEs and lipid layers has not been clarified. In the lipid layers in SC, there are two types of lamellar structures, the long lamellar (L-La) and the short lamellar (S-La). Additionally, the alkyl chains in L-La and S-La are packed in a hexagonal array (Hex) and in an orthorhombic array (OR), respectively. Four types of MEs with different structures containing a deep eutectic solvent (DES) or H2O in the inner phase were prepared using two surfactants, Tween80 and Span20 (T3S1DES, T3S1H2O, T1S3DES, and T1S3H2O as indicated in Table1). Then, we evaluated which ME easily disturbed the lipid packing structures in SC by X-ray diffraction. The result showed that MEs at all compositions disturbed both Hex (d=0.41nm) and OR (d=0.37nm and 0.41nm) (Fig.1a, b), and T3S1H2O disturbed the lipid packing the most. We found that only T3S1H2O disturbed Hex more strongly because of a decrease in the ratio of the peak area of Hex to OR at 50 min after applying samples. Taking into account the result of skin penetration (Fig.1c), T3S1H2O appeared to mainly remain in Hex. For T3S1H2O, the hydrocarbon chains of the surfactants may easily interact with the lipid hydrocarbon chains in SC because the surfactants of ME are not packed densely (Table 1). The smallest ME (T1S3H2O) easily penetrates S-La because it appeared to easily pass through the narrow spaces of the intercellular lipid matrix. Hence, we concluded that the penetration mechanism depends on not only the sizes of MEs but also their aggregation numbers.

Penetration mechanism through the stratum corneum depending on the structure of microemulsions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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