Transdermal delivery of 40-nm silk fibroin nanoparticles

[Display omitted] •Silk fibroin nanoparticles with a diameter of 40 nm were prepared.•The nanoparticles were stable at 4, 32, and 37 °C for a week.•The nanoparticles penetrated the stratum corneum.•The nanoparticles were delivered to deep hair follicles, the epidermis, and the dermis. Transdermal administration of drugs improves their bioavailability and is capable of systemic and local treatment. To improve the skin permeability of drugs, nano-sized systems have attracted attention as drug carriers for transdermal drug delivery system. We considered that silk fibroin composed of a crystalline region with many hydrophobic amino acids and an amorphous region with many hydrophilic amino acids was useful as a carrier for transdermal administration of a drug because of the balance between hydrophilicity and hydrophobicity. In this study, silk fibroin nanoparticles with mean volume diameters of 42.3 nm were successfully prepared, and storage stability was confirmed by storing the nanoparticle suspension at 4, 32, and 37 °C for a week. At any storage temperature, the mean volume diameter and standard deviation were stable. The polydispersity indexes were 0.19–0.23, and no specific trends were observed. Then, to investigate the transdermal delivery route of the silk fibroin nanoparticles, skin permeability in vivo was evaluated using mice. Six hours after administration, fluorescent substances were observed in the dermis in addition to the stratum corneum, hair follicles and the epidermis around them. This result indicated that fibroin nanoparticles with the mean volume diameter of 40-nm penetrated the stratum corneum and was delivered deep into the skin. Therefore, it was suggested that small nanoparticles prepared using silk fibroin are useful for drug delivery to the dermis.