Simulation-based optimization of out-of-plane, variable-height, convoluted quartz micro needle arrays via single-step anisotropic wet etching

Abstract

Variable-height, out-of-plane, convoluted microneedle arrays with high aspect ratio, tapered shafts, sharp tips, and smooth surfaces are required for applications where the microneedle heights must conform to a specific target geometry. Here, we show that it is possible to achieve such convoluted microneedle arrays on a Z-cut quartz substrate by a single wet etching step in saturated NH4HF2 at 80 °C. Due to the low etch rate of quartz, the procedure is based on carefully controlling the dimensions of every needle-like structure by properly dimensioning the masked area on top. In particular, we show that the mask dimensions for all the microneedles can be optimized via a combination of Level Set (LS) simulations of wet etching and an Evolutionary Algorithm (EA) to search the optimal dimensions. As an example, we fabricate two different microneedle arrays, where the needle heights follow two different spherical surfaces. Our experiments confirm that (i) the LS method correctly predicts the evolution of the etch front for microneedles etched on AT, BT and Z quartz cuts; and (ii) the LS + EA approach provides a suitable procedure to determine an optimal collection of mask sizes for the fabrication of convoluted microneedle arrays on the Z cut.