Acoustofluidic Droplet Sorter Based on Single Phase Focused Transducers

Abstract

Droplet microfluidics has revolutionized the biomedical and drug development fields by allowing for independent microenvironments to conduct drug screening at the single cell level. However, current microfluidic sorting devices suffer from drawbacks such as high voltage requirements (e.g., >200 Vpp), low biocompatibility, and/or low throughput. In this article, a single-phase focused transducer (SPFT)-based acoustofluidic chip is introduced, which outperforms many microfluidic droplet sorting devices through high energy transmission efficiency, high accuracy, and high biocompatibility. The SPFT-based sorter can be driven with an input power lower than 20 Vpp and maintain a postsorting cell viability of 93.5%. The SPFT sorter can achieve a throughput over 1000 events per second and a sorting purity up to 99.2%. The SPFT sorter is utilized here for the screening of doxorubicin cytotoxicity on cancer and noncancer cells, proving its drug screening capability. Overall, the SPFT droplet sorting device shows great potential for fast, precise, and biocompatible drug screening.

Keywords: acoustofluidics; droplet sorting; high throughput sorting; single phase focused transducer.

Figure 1.

(A) Schematic of the SPFT based droplet sorter. Droplets containing the single green ball: single-fluorescence-cell-laden droplets; Droplets containing the single orange ball: non-fluorescence/dead cell laden droplets; bare droplets: empty droplets. The oil inlet injects HFE-7500, which functions as the spacing adjustment oil. (B) An optical image of the SPFT droplet sorter (optical fibers are not plugged in).

Figure 2.

The geometry schematic of (A) the interdigital transducer pair and (B) the SPFT pair (only paint 3 pairs as a display in (A) and 1 pair in (B)). Simulation results for energy distribution rate of (C) the interdigital transducer pair and (D) SPFT pair, respectively.

Figure 3.

Snapshots of the droplets flow track at the SPFT based droplet sorter when standing SAW was turned (A) off and (B) on. Input voltage: 32 Vpp. In comparison, snapshots of the droplets flow track at the regular interdigital transducer based sorter when standing SAW was turned (C) off and (D) on with the same experiments were conducted. Scale bar: 200 μm. (E) Comparison of droplet-shifting distances perpendicular to the flow, driven by the SPFT pair (red dots) and the regular interdigital transducer pair (blue dots). The results show that when the input voltage increases from 0 to 24 Vpp, droplet-shifting distance increases much more rapidly by SPFT than by interdigital transducers, indicating that the SPFT design possesses a higher energy utilization capability. (F) comparison between the SPFT design and the interdigital transducer design at 32 Vpp. After standing SAW were turned on at 0 s, droplets were continuously pushed into the sorted outlet by the SPFT pair (red line), while four out of ten droplets were missed in the regular interdigital transducer based sorter (blue line). The detection region was selected at the entrance of sorted outlet.

Figure 4.

(A)~(F) Snapshots of five droplets flowing through the SPFT device within 3 ms. Scale bar: 200 μm. The periodic pulse signal applied on the SPFT pair: resonant frequency: 9.696 MHz; interval time between each pulse: 5 ms; and wave numbers of each pulse: 1000. Droplet #3 was moved by standing SAW and flowed into the sorted outlet while droplets #1, 2, 4, and 5 remained in the original trajectory.

Figure 5.

(A) Image of single-cell-laden droplets right after droplet generation, before acoustic sorting. (B) Image of droplets from the sorted outlet, after acoustic sorting. Scale bar: 200 μm. Both two images are composed of the green fluorescence picture and bright-field picture. (C) The purity of fluorescence-cell-laden droplets from the sorted outlet after sorting at different throughputs. (D) The ratio of the distribution of different numbers of cells per droplet after sorting.

Figure 6.

(A) A plot of the states of MCF-7 cells in droplets after the doxorubicin assay. Live cell dye is green fluorescent and dead cell reagent is red fluorescent. Scale bar: 50 μm. (B, C) The comparison of MCF-7 cell and HEK-293T cell viability after the doxorubicin assay. The results indicate that 5 μM doxorubicin is more toxic to cancer cells (MCF-7 cells) than to non-cancer cells (HEK-293T cells).