Microneedle geometry modulates drug transport in transdermal delivery systems

Microneedles have emerged as a promising strategy for transdermal drug delivery, owing to their ability to bypass the stratum corneum in a minimally invasive manner and enable controlled drug administration. In practice, microneedles have been developed with a wide range of geometries to meet diverse requirements in delivery efficiency, targeting, and release kinetics. However, the role of microneedle geometry in governing delivery performance remains incompletely understood. In this study, a modelling framework is developed to systematically investigate the effects of microneedle geometry on drug transport across multilayered skin and into the systemic circulation. A set of quantitative metrics is employed to characterise both the magnitude and temporal dynamics of delivery, including peak concentration, time to peak, cumulative exposure, temporal dispersion, asymmetry, and residence time. To enable generalised comparison across different geometries, a geometric transport scaling index is introduced, which largely removes dependence on shape class and captures the overall influence of geometric variation. The results demonstrate that microneedle geometry exerts a strong and compartment-dependent effect on delivery behaviour, with distinct and often non-monotonic trends observed across the viable epidermis, papillary dermis, reticular dermis, and systemic circulation. Geometry-dependent trade-offs are identified between delivery efficiency, transport timescale, and temporal dispersion, reflecting competing transport mechanisms across skin layers and systemic circulation. In several cases, optimal geometric regimes are identified depending on the target compartment and therapeutic objective. Overall, this study provides mechanistic insight into microneedle-mediated drug delivery and establishes a quantitative framework for the rational design and optimisation of microneedle-based systems.