Controlling the chemical concentration in droplets is an essential function in numerous applications of biology, chemistry, and engineering. However, it is difficult to perform an automated control of wide-ranging chemical concentrations accurately without any high-precision off-chip controllers. In this study, we present a preprogrammed microdroplet generator that controls the chemical concentration of droplets by more than three orders of magnitude. The device used constant water-head pressure as the sole driving force and regulated the concentration in a periodic and stepwise manner while keeping the droplet size constant. For this process, an autonomous microfluidic circuit controlled the flow rate ratio of two solutions, and an external mechanical capacitor tuned the step size of the control pressure over a wide range of 0.31-237 Pa. The utility of the device was demonstrated using a DNA-mediated homogeneous binding assay.
Keywords: Chemical concentration gradient; Droplet; Electronic–hydraulic analogy; Fluorescence resonance energy transfer; Microfluidic circuit.
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