Out-of-plane, high strength, polymer microneedles for transdermal drug delivery

This paper reports on the high strength of high-aspect ratio (> 50) hollow, polymer microneedles fabricated out-of-plane using a fairly repeatable fabrication process. Further, these microneedle tips were sharpened by a molding principle, with a simple anisotropic etch of silicon wafer. Also, an enhanced elegant process was explored to incorporate the mounting of the microneedle onto a platform without using any additional material, such that the bore of the microneedle is continuous with the bore of the platform in order to facilitate microfluidic delivery through the hollow needles. The high aspect ratio microneedles undergo failure at the critical load of around 4 N, while the insertion force for such a needle into agar gel, which is a fairly good equivalent of the human skin due to its inherent visco-elastic properties, is 7 mN, which translates into a safety factor (ratio of critical loading force to the maximum applied force) of greater than 500 thus, making it adequately strong for skin penetration.