Low-frequency sonophoresis: ultrastructural basis for stratum corneum permeability assessed using quantum dots

Low-frequency sonophoresis (LFS) has been well documented to enhance the permeability of skin to macromolecular drugs via induction of localized transport regions. However, the organizational details of epidermis, specifically stratum corneum (SC), during sonophoresis are beyond the resolution limit of common histo-optical microscopy tools, which fail to reveal any notable structural alterations in these regions at a submicroscopic scale. Here we report, using quantum dots (QDs) as a tracer and confocal microscopy and transmission electron microscopy (TEM) (with OsO(4) and RuO(4) post-fixation) as visualization methods, on LFS-induced permeation pathways in the SC. QDs (20 nm diameter) penetrated well beyond the SC. TEM revealed that ultrasound significantly increased the frequency of occurrence of the otherwise scattered and separated lacunar spaces in the SC. A significant increase in lacunar dimensions was observed when 1% w/v sodium lauryl sulfate was added to the coupling medium. These studies show that LFS induces dilatation and higher connectivity of voids in the SC, possibly leading to formation of a three-dimensional porous network, which is capable of transporting QDs as well as macromolecules across the SC. This contention is consistent with previously conceived theoretical mechanistic understanding of LFS-induced enhanced transport across the skin.