Fabrication of sharp silicon hollow microneedles by deep-reactive ion etching towards minimally invasive diagnostics

Microneedle technologies have the potential for expanding the capabilities of wearable health monitoring from physiology to biochemistry. This paper presents the fabrication of silicon hollow microneedles by a deep-reactive ion etching (DRIE) process, with the aim of exploring the feasibility of microneedle-based in-vivo monitoring of biomarkers in skin fluid. Such devices shall have the ability to allow the sensing elements to be integrated either within the needle borehole or on the backside of the device, relying on capillary filling of the borehole with dermal interstitial fluid (ISF) for transporting clinically relevant biomarkers to the sensor sites. The modified DRIE process was utilized for the anisotropic etching of circular holes with diameters as small as 30 μm to a depth of >300 μm by enhancing ion bombardment to efficiently remove the fluorocarbon passivation polymer. Afterward, isotropic wet and/or dry etching was utilized to sharpen the needle due to faster etching at the pillar top, achieving tip radii as small as 5 μm. Such sharp microneedles have been demonstrated to be sufficiently robust to penetrate porcine skin without needing any aids such as an impact-insertion applicator, with the needles remaining mechanically intact after repetitive penetrations. The capillary filling of DRIE-etched through-wafer holes with water has also been demonstrated, showing the feasibility of use to transport the analyte to the target sites. Microneedles are fabricated with a diameter as small as 30 µm and a tip radius of 5 µm, enabling skin penetration for in-vivo sampling of interstitial fluid. A key aspect to wearable health monitoring is the availability of devices that can sample key biomarkers in-vivo; microneedles are particularly promising for minimally invasive sampling of interstitial fluid. However, careful control of the microneedle size, geometry and quality are needed for effective penetration and sampling. Now, a team led by Bo Cui from University of Waterloo, and Eric Blondeel from ExVivo Labs Inc., demonstrates a modified deep reactive ion etching process for creating cone-shaped silicon microneedles with a hole opening for fluid extraction. Capillary filling of the holes is demonstrated, as well as repeatable penetration of porcine skin.