Disposition of nanoparticles and an associated lipophilic permeant following topical application to the skin

The objective was to determine the disposition of polymer nanoparticles and an associated, lipophilic, model “active” component on and within the skin following topical application. Polystyrene and poly(methyl methacrylate) nanoparticles containing covalently bound fluorescein methacrylate and dispersed Nile Red were prepared by emulsion polymerization. The two fluorophores differentiate the fate of the polymeric vehicle on and within the skin from that of the active. Nanoparticles were characterized by dynamic light scattering, transmission electron microscopy and NMR spectroscopy. In vitro skin permeation experiments were performed using dermatomed porcine skin. Post-treatment with nanoparticle formulations, the skin surface was either cleaned carefully with buffer or simply dried with tissue, and then immediately visualized by confocal microscopy. Average nanoparticle diameters were below 100 nm. Confocal images showed that nanoparticles were located in skin “furrows” and around hair follicles. Surface cleaning removed the former but not all of the latter. At the skin surface, Nile Red remained partly associated with nanoparticles, but was also released to some extent and penetrated into deeper layers of the stratum corneum (SC). In summary, polymeric nanoparticles did not penetrate beyond the superficial SC, showed some affinity for hair follicles, and released an associated “active” into the skin.