Microvalves for precise dosing

Abstract

Precise control of fluid flow becomes increasingly challenging as systems and instruments are scaled down. Smaller dimensions allow smaller flow ranges, but also leave smaller margins for error in performance. Reliable and effective fabrication and assembly procedures are therefore a primary requirement for any microfluidic system, if it is to be successful in real-world applications. This thesis describes the achievements made in ongoing efforts to create miniaturized, proportional control valves for minute gas flows. In this research we focus on MEMS-based fabrication processes, as they allow high precision manufacturing of miniature devices with high paralellism. This enables higher fabrication yields, improved reliability and increased integration. We present four valve designs that can be divided into two main categories, being either based on silicon-on-insulator (SOI) technology or based on surface channel technology (SCT). The first group focuses on obtaining a proportional flow controller using simple, straight-forward fabrication processes with a small number of process steps, for use in an ambulant blood pressure waveform (BPW) measurement system. Two single-wafer valve designs are presented, both offering built-in capacitive sensing of the valve displacement which can be used to correct for actuator hysteresis and improve control precision. Nitrogen gas flows up to 13 g h−1 at a pressure of 500 mbar are demonstrated in good agreement with analytical and numerical models, with leakage below 0.1mgh−1 at 1 bar. Maximum throughput is estimated at 25 g h−1 at 1 bar. A fully functional control valve assembly is demonstrated with integration of a miniaturized piezoelectric bimorph actuator. Time-dynamic characterization demonstrates that the control valve is suitable for high-speed flow control, with a mechanical bandwidth of 8 kHz and a frequency-independent response up to 3 kHz. The second group demonstrates two proportional control valve designs in an already existing (SCT) fabrication process, to allow integration with existing components such as flow sensors. The first valve design allows flow control from a chip inlet or outlet to a fluidic channel embedded in the silicon surface, with a flow range of > 1250mgh-1 at 600 mbar and a leak flow below 0.05 mgh-1 at 1 bar. The second valve design supports smaller flows (>70 mgh-1 at 200 mbar), but allows fully on-chip flow control between any two surface channels. A good fit is obtained between the measured flow profiles and analytical flow models of both valves. The valve designs aimed at the BPW monitoring system are shown to be very well suited for the aimed application, although a smaller bimorph piezo may be required to meet the demanded physical dimensions. The combination of capacitive displacement sensing with bimorph piezoelectric actuation allows for high-speed, high-precision flow control with low power usage. The SCTbased microvalves demonstrate proportional, on-chip flow control, suitable for integration with existing flow sensors. Their designs are tailored to match an existing high-resolution mass flow sensor which has a limited flow range, but the designs of both the valves and the sensor can be scaled up to increase the range.