Incorporating fluid-structure interactions for modelling of pyramidal hollow microneedles for transdermal drug delivery

Hollow microneedles (HMNs) have gained significant attention as a potential alternative to traditional hypodermic needles for delivering drugs through the skin. The rational selection of the HMN geometry and materials is essential for balancing the MN's mechanical stability and efficient drug delivery. In addressing this, the current study aims to develop a numerical model for fluid-structure interactions (FSI) in hollow pyramidal-shaped MNs where HMNs made from two polymers (polylactic acid (PLA) and polyglycolic acid (PGA)) and one metal (stainless steel (SS)) are considered. Finite element (FE) simulations have been performed with COMSOL 6.2 Multiphysics to determine the effect of MN design parameters such as wall thickness, pitch, channel diameter, and dual-zone MN structure (different MN lengths) on fluid flow and von Mises stress distribution in the HMNs. The FSI analysis has been conducted for a laminar flow of water-fentanyl mixture as a model fluid. The findings revealed that raising the inlet pressure from 10 kPa to 30 kPa at the HMN entrance increases the flow rate to 0.005 μl/s, velocity 0.003 m/s, and total drug flux by 248.63%. As expected, SS demonstrated the lowest von Mises stress (13213 N/m 2 ) while PLA and PGA exhibited a decrease in the stress level as the HMN wall thickness increased. Increasing the MN pitch from 400 μm to 1200 μm reduced skin pore pressure by 72% and enhanced drug concentration by 11.4%. The dual-zone MN arrangement, combining shorter and longer needles, improved the HMN penetration and led to a 90.8% increase in overall flux. These findings provide a foundation for optimising HMN designs, ensuring a balance between mechanical stability and improved transdermal drug diffusion for clinical applications.