Hydrodynamic-flow-enhanced rapid mixer for isothermal DNA hybridization kinetics analysis on digital microfluidics platform

DNA hybridization kinetics has been playing a critical role in molecular diagnostics for binding discrimination, but its study on digital microfluidic (DMF) systems is ultimately restrained by the laminar flow condition. The kinetic mixing technique is widely employed to ensure a fast reaction rate, but poses intrinsic risk in cross contamination and exhibits instable fluorescence intensity during the droplet transportation. While the electrothermal technique can provide stationary droplet mixing through the established thermal gradient within the hybridization solution, the significant increase in the droplet temperature will inevitably undermine the hybridization equilibrium and jeopardize the binding discrimination. To enhance the hybridization efficiency while ensuring a stable droplet temperature (within +/- 0.1 degrees C), this paper presents a DMF platform that can perform isothermal hydrodynamic-flow-enhanced droplet mixing. Specifically, with a single electrode, droplet-boundary oscillation under a slow AC actuation is studied for improving the reaction rate. The dependencies between the mixing efficiency and the actuation voltage, actuation frequency and the spacer thickness are also systematically studied. Reliable mixing efficiency improvement is further validated over a wide range of solute concentrations. The results from real-time on-chip DNA hybridization kinetics with stationary droplets using the complete sandwiched DMF system shows that the proposed rapid mixer can achieve the same hybridization equilibrium with > 13 times faster reaction rate when compared to the reference one through pure diffusion, while preventing biased hybridization kinetics as demonstrated in the electrothermal technique.