IMPEDANCE-BASED FEEDBACK CONTROL OF COMBINATORIAL DROPLET INJECTIONS ON A VALVE-ACTUATED MICROFLUIDIC DEVICE
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Droplet microfluidics has found use in many biological assay applications as a means of high-throughput sample processing. One of the challenges of the technology, however, is the ability to control and merge droplets on-demand as they flow through the microdevices. It is in the interest of developing lab-on-chip devices to be able to program additive mixing steps for more complex multistep and multiplex assays. Existing technologies to merge droplets are either passive in nature or rely on open-loop control systems, making them vulnerable to errors during high throughput or long duration experimentation. Herein is described and demonstrated a microfluidic valve-based device for the purpose of combinatorial droplet injection at any stage in a multistep assay. Microfluidic valves are used to control fluid flow, generate droplets, and inject droplets on-demand, while on-chip impedance measurements taken in real time are used as feedback to accurately time the droplet injections. The presented system is compared to open-loop control and its reliability is demonstrated over long time durations. Additionally, the system is shown to be able to differentiate between phosphate-buffered saline and deionized water droplets, and programmatically inject an arbitrary pattern based on droplet content on-demand.