Polymeric microneedles for transdermal drug delivery

Building off previous work using silicon and metal microneedles, we fabricated polymer microneedles out of polyglycolide, polylactide and their copolymer using a micromolding technique that created needles with shaper tips. In this work, fabrication approaches for various tip geometries and tapered column structures with high aspect ratio were presented. Polymer microneedles were strong enough to be inserted into cadaver skin without breaking. Polymer microneedles impregnated with both low- and high-molecular weight model compounds to simulate drug release were fabricated and inserted into full thickness cadaver skin. Quantitative measurement of model compound release as a function of time was obtained. In this study, the force of insertion as a function of tip geometry was measured in human subjects, and the microneedle tip diameter was the critical geometric parameter to determine the force of insertion. The failure force by axial loading and transverse loading of microneedles as a function of their geometry and mechanical properties of polymer were also measured. The failure force by axial loading was found to increase linearly with the base diameter of a microneedle and the Young's modulus of the material, and to decrease with the length of the microneedle and encapsulating drug content in microneedles. The Johnson formula and the Euler formula were used to predict the axial failure forces and both models predicted the tendency as well as the quantitative axial failure force.