Acoustofluidic control of chemical concentration within picoliter droplets in a disposable microfluidic chip

Abstract

We propose an acoustofluidic method to dynamically control the chemical concentration (CC) within picoliter-scale droplets at high-throughput production exceeding 2000 drops per second. Co-flowing laminar streams of two fluids were rapidly mixed via acoustic streaming flow-induced micro-vortices, and the mixing index was precisely controlled by modulating the wave amplitude. The mixed aqueous sample was fed into a T-junction as a dispersed phase with a continuous phase oil to produce droplets. We conducted numerical and experimental analysis on the flow mixing in terms of the flow rate and wave amplitude. We produced the droplets with tunable CCs with higher than 100-fold enhancement and the droplet libraries with varying inter-droplet chemical concentration (CCD) gradients. The temporal stability and dynamic controllability of the CCD were also verified by the experiments. We demonstrated the applicability of the proposed method by generating doxorubicin-loaded droplets with tunable CCs. The proposed device includes a disposable microfluidic chip to enable the reusable transducer on the piezoelectric substrate, leading to improved flexibility and functionality. We expect that the proposed method to produce droplet libraries with desired CCs is promising for various droplet microfluidic applications including high-through drug screening and discovery, combinatorial biochemistry, and synthesis of functionalized hydrogels.