Fabrication of Silicon Out-of-Plane Microneedles for Potential Drug Delivery and Interstitial Fluid Extraction

Abstract

Microneedles represent a successful application of MEMS technology, forming minimally invasive platforms for transdermal drug delivery, body fluid sampling, and diagnostics. Silicon microneedles, in particular, are favored due to their exceptional mechanical strength and biocompatibility. This thesis focuses on the three different fabrication methods of silicon microneedles using MEMS techniques. Initially, we fabricated silicon out-of-plane cone-shaped hollow microneedles with sharp apexes and off-axis pores. This process involved backside hole etching and frontside pillar etching via the Bosch process, followed by pillar sharpening using a HF-HNO3 mixed solution. The resulting microneedles were 160 μm high. However, to penetrate the epidermis and access abundant body fluids for health monitoring systems, taller microneedles longer than 500 μm are required. Fabricating these higher microneedles proved challenging due to difficulties in achieving uniform sharpening through wet etching. To address this, we developed a novel method for fabricating silicon out-of-plane hollow microneedles with beveled tips. This method included frontside slope etching, backside hole etching, and frontside pillar etching, combining anisotropic wet etching and dry etching (Bosch process). The resulting microneedles were approximately 600 μm tall with beveled sharp tips. We tested various fundamental functions of these microneedles by connecting the chip to a syringe using a 3D-printed applicator, successfully demonstrating liquid extraction, liquid injection, and simulated drug delivery process. To minimize the impact of inevitable lateral etching during frontside pillar etching in the Bosch process, we proposed sacrificial structures surrounding the pillars to shield them from lateral etching. Testing two types of sacrificial structures, we found both structures could effectively reduce lateral etching, enabling the fabrication of 370 μm high ring pillars with vertical sidewalls. Additionally, grayscale lithography combined with subsequent Bosch processing presents an effective and flexible method for fabricating complex 3D structures like bevels. We first acquired contrast curve for the photoresist before grayscale lithography. Then we used this technique integrating frontside slope etching and frontside pillar etching into a single step, resulting in the fabrication of hollow microneedles measuring 325 μm in height.