Applications such as single cell measurements of protein content (e.g., antibodies, surface markers), low analyte concentration (e.g., biomarkers), and implantable and external pumps, require reliable and accurate measurement of low flow rates. In the case of implantable and external pumps used for continuous injection of drugs, inaccurate measurements can be fatal. We are developing surface acoustic wave (SAW) sensors, which can be designed to measure a broad range of flow rates. Other flow-sensing modalities require expensive external equipment (e.g., optical flow meters) or the addition of tracers to the fluid (e.g., doppler shift, time of flight, or optical). In contrast, SAW sensors are compact and do not require the addition of a tracer to measure flow. Our goal is to develop label-free, reliable, and accurate SAW flow sensors to produce SI traceable measurements of microfluidic flow.
As part of our NIST-on-a-Chip efforts, we are developing strategies to measure local flow in microfluidic systems. This project will develop label-free flow sensors using surface acoustic waves embedded in microfluidic devices. In this approach, an electromechanical transducer is placed on a piezoelectric substrate, and a surface acoustic wave is generated, propagating from the emitter to the receiver. The interaction between the fluid and acoustic waves yields a decrease in magnitude (damping) and phase-shift of the surface acoustic wave. Thus, the measured acoustic power and phase at the receiver are directly correlated to the flow rate. This technology covers a broad range of flow rates from 1 μL/min to 10 mL/min. In addition to exhibiting a wide dynamic range, the SAW-based flow meter is easier to fabricate and integrate into microfluidic devices. We are currently developing finite element models to understand the observed responses in these devices. And, we are developing new designs to improve outcomes.