Design and Scalable Fabrication of Hollow SU-8 Microneedles for Transdermal Drug Delivery

Hollow microneedle arrays have been sustained since last two decades and hold lot of promise as effective, painless, and controlled way of transdermal drug delivery because of the immense variety of drugs they can deliver. Design and simulation are quintessential prior to proceeding with fabrication of microneedles in order to successfully overcome the skin barrier. In this paper, optimization of the design of hollow microneedles has been carried out by theoretical and simulation based studies. Subsequently, microneedles based on the optimized design are fabricated by direct laser writing. The SU-8 microneedles with simple cylindrical structure are able to sustain the skin resistive forces since their yield strength under different loading conditions is higher than the skin forces. The flow in microneedle lumen is laminar with Reynold's number 3.9. The choice of cylindrical microneedle simplifies the fabrication steps greatly. SU-8 microneedles which are 500 μm tall, 100 μm outer diameter, and 40 μm inner diameter were fabricated by direct laser writing. Aspect ratio of 5 has been achieved for these microneedles. The hollow SU-8 microneedles have clearly defined lumen. Their hardness, Young's Modulus and maximum compressive and bending forces are 0.3 GPa, 4.94 GPa, 0.27 N, and 0.022 N, respectively. The microfluidic tests indicate the development of fully hollow microneedle lumen with a flowrate of 0.93 μL/s at 2 KPa inlet pressure drop. The experimental results are in good agreement with the theoretical and simulation based studies. These MNs can puncture mice skin effectively and are not broken after multiple insertions.