Varying-Length Polymer Microneedle Arrays Fabricated by Droplet Backside Exposure

This paper presents a single-step backside ultraviolet lithography method, namely droplet backside exposure (DBE), for making slanted microneedle structures monolithically on a flexible polymer substrate. To demonstrate the feasibility of the DBE approach, SU-8 microneedle arrays were fabricated on polydimethylsiloxane substrates. The length of the microneedles was controlled by tuning the volume of the SU-8 droplet, utilizing the wetting barrier phenomenon at a liquid-vapor-hydrophilic surface-hydrophobic surface interface. The experimental results show excellent repeatability and controllability of the DBE method for microneedle fabrication. Analytical models and the finite element method were studied to predict the dimensions of the microneedles, which agreed with the experimental data. To verify the versatility of the proposed DBE method, different microneedle structures were implemented, including a slanted hollow microneedle array and a slanted microscale waveguide (μ-waveguide) array. Furthermore, an as-fabricated μ-waveguide array was coupled with a microlight-emitting diode array, which can potentially be utilized as an optical neural interface for applications in optogenetics-based neural stimulation.