Structural and microfluidic analysis of hollow side-open polymeric microneedles for transdermal drug delivery applications

In this paper, we present a new design of hollow, out-of-plane polymeric microneedle with cylindrical side-open holes for transdermal drug delivery (TDD) applications. A detailed literature review of existing designs and analysis work on microneedles is first presented to provide a comprehensive reference for researchers working on design and development of micro-electromechanical system (MEMS)-based microneedles and a source for those outside the field who wish to select the best available microneedle design for a specific drug delivery or biomedical application. Then, the performance of the proposed new design of microneedles is numerically characterized in terms of microneedle strength and flow rate at applied inlet pressures. All the previous designs of hollow microneedles have side-open holes in the lumen section with no integrated reservoir on the same chip. We have proposed a new design with side-open holes in the conical section to ensure drug delivery on skin insertion. Furthermore, the present design has an integrated drug reservoir on the back side of the microneedles. Since MEMS-based, hollow, side-open polymeric microneedles with integrated reservoir is a new research area, there is a notable lack of applicable mathematical models to analytically predict structural and fluid flow under various boundary conditions. That is why, finite element (FE) and computational fluid dynamic (CFD) analysis using ANSYS rather than analytical systems has been used to facilitate design optimization before fabrication. The analysis has involved simulation of structural and CFD analysis on three-dimensional model of microneedle array. The effect of axial and transverse loading on the microneedle during skin insertion is investigated in the stress analysis. The analysis predicts that the resultant stresses due to applied bending and axial loads are in the safe range below the yield strength of the material for the proposed design of the microneedles. In CFD analysis, fluid flow rate and pressure drop in the microneedles at applied inlet pressures are numerically and theoretically investigated. The CFD analysis predicts uniform flow through the microneedle array for each microneedle. Theoretical and numerical results for the flow rate and pressure drop are in close agreement with each other, thereby validating the CFD analysis. For the proposed design of microneedles, feasible fabrication techniques such as micro-hot embossing and ultraviolet excimer laser methods are proposed. The results of the present theoretical study provide valuable benchmark and prediction data to fabricate optimized designs of the polymeric, hollow microneedles, which can be successfully integrated with other microfluidic devices for TDD applications.