Fig. 1.

Schematic representation of methods of MN application to the skin to achieve enhanced transdermal drug delivery, * stratum corneum, ** epidermis. (A) Solid MN that are applied and removed to create transient micropores, followed by application of the formulation. (B) Solid MN are coated with drug for instant delivery and to remove the two step process associated with solid MNs. (C) Drug is mixed with soluble polymeric/carbohydrate MNs that dissolve in skin interstitial fluid over time. (D) Hollow MNs puncture the skin, after which liquid drug can be actively infused through the needle bores. (E) Hydrogel-forming MNs imbibe skin interstitial fluid upon application to the skin. This induces drug diffusion through the swollen microprojections. Drug is often stored above the microprojections in a lyophilised wafer prior to interstitial fluid uptake