Analysis of the Mechanical Failure of Polymer Microneedles by Axial Force

A polymeric microneedle has been developed for drug delivery applications. The ultimate goal of the polymeric microneedle is insertion into the specified region without failure for effective transdermal drug delivery. The mechanical failure of various geometries of microneedles by axial load was modeled using the Euler formula and the :Johnson formula to predict the failure force of tapered-column microneedles. These formulas were compared with measured data to identify the mechanical behavior of microneedles by determining the critical factors, including the actual length and the end-fixed factor. The comparison of the two formulas with the data showed good agreement at an end-fixity (K) of 0.7. This value means that a microneedle column has one fixed end and one pinned end and that; part of the microneedle is overloaded by an axial load. When the aspect ratio of length-to-equivalent diameter is 12:1 at Young's modulus of 3 GPa., there is a transition from the Euler region to the Johnson region clue to the decreased length and the increased base diameter of the microneedle. A polymer having a stiffness of less than 3 GPa would follow the Euler formula. A 12:1 aspect ratio of length-to-equivalent diameter of the microneedle is a mechanical indicator determining the failure mode between elastic buckling and inelastic buckling at; Young's modulus of less than 3 GPa for polymer. Microneedles with an aspect ratio of length-to-equivalent diameter below 12:1 and Young's modulus of more than 3 GPa are recommended for reducing sudden failure by buckling and for successfully inserting a microneedle into the skin.