A tapered hollow metallic microneedle array using backside exposure of SU-8

TLDR

The study introduces a new backside‑exposure SU‑8 process for fabricating tapered hollow metallic microneedle arrays and derives analytic buckling solutions for such structures. The method builds SU‑8 mesas on Pyrex, exposes the backside to form tapered pillars, then conformally electrodeposits metal, polishes, and removes organics to yield a hollow microneedle array with a backside fluidic reservoir. Arrays of 200–400 µm tall, 10 × 10 microneedles with 33.6–101 µm tip diameters and 10–20 µm walls were fabricated, and a 400 µm Ni needle with a 3.08° taper exhibited a 1.8 N critical buckling force, validating the analytic model.

Abstract

This paper presents a novel fabrication process for a tapered hollow metallic microneedle array using backside exposure of SU-8, and analytic solutions of critical buckling of a tapered hollow microneedle. An SU-8 mesa was formed on a Pyrex glass substrate and another SU-8 layer, which was spun on top of the SU-8 mesa, was exposed through the backside of the glass substrate. An array of SU-8 tapered pillar structures, with angles in the range of 3.1°–5°, was formed on top of the SU-8 mesa. Conformal electrodeposition of metal was carried out followed by a mechanical polishing using a planarizing polymeric layer. All organic layers were then removed to create a metallic hollow microneedle array with a fluidic reservoir on the backside. Both 200 µm and 400 µm tall, 10 by 10 arrays of metallic microneedles with inner diameters of the tip in the range of 33.6–101 µm and wall thickness of 10–20 µm were fabricated. Analytic solutions of the critical buckling of arbitrary-angled truncated cone-shaped columns are also presented. It was found that a single 400 µm tall hollow cylindrical microneedle made of electroplated nickel with a wall thickness of 20 µm, a tapered angle of 3.08° and a tip inner diameter of 33.6 µm has a critical buckling force of 1.8 N. This analytic solution can be used for square or rectangular cross-sectioned column structures with proper modifications.