Transdermal drug delivery by coated microneedles: Geometry effects on effective skin thickness and drug permeability

Although transdermal drug delivery has been used for about three decades, the range of therapeutics that are administered this way is limited by the barrier function of the stratum corneum (the top layer of skin). Microneedle arrays have been shown to increase the drug permeability in skin by several orders of magnitude by bypassing the stratum corneum. This can potentially allow the transdermal delivery of many medicaments including large macromolecules that typically cannot diffuse through the skin. This paper addresses the use of microneedles coated with a drug solution film. In particular, we identify how the geometries of various microneedles affect the drug permeability in skin. Effective skin permeability is calculated for a range of microneedle shapes and dimensions in order to identify the most efficient geometry. To calculate effective permeability (Peff), the effective skin thickness (Heff) is calculated. These are then plotted for insulin as a model drug to see how various microneedle parameters affect the profiles of both Heff and Peff. It is found that the depth of penetration from the microneedle array is the most important factor in determining Peff, followed by the microneedle spacings. Other parameters such as microneedle diameter and coating depth are less significant.