Development of SU-8 hollow microneedles on a silicon substrate with microfluidic interconnects for transdermal drug delivery

Abstract

Hollow microneedle (MN) arrays offer versatility and control to transdermal drug delivery systems where a variety of drugs and their continuous supply are concerned. They may be used as a standalone device or be integrated with the drug reservoir or micropump. An important aspect in this regard is the effective fluidic communication between the reservoir and the MNs on a robust substrate. In a novel attempt of its kind, we present the development hollow SU-8 MNs on a pre-etched silicon wafer having through holes. SU-8 MNs are fabricated by direct laser writing by aligning them on the silicon substrate with microfluidic ports pre-etched by wet chemical etching. Each process step was optimized after a parametric study. The optimized MNs (500–600 µm length, 100 µm outer diameter and 40 µm inner diameter) have an aspect ratio of 5. The MNs have been characterized for mechanical strength and biological insertion tests for their effectiveness in puncturing the skin without breaking. The maximum compressive force and bending forces for the MNs are 0.27 N and 0.022 N per needle, which are higher than the resistive skin penetrating forces. The microfluidic characterizations show the development of hollow MN lumen with a flowrate of around 0.93 µl s−1 at 2 KPa pressure difference at the inlet. The array of 10 × 10 MNs with 500 µm spacing was able to successfully penetrate mice and rat skin without any breakage.