Elevating single-particle encapsulation in droplet microfluidics by utilizing surface acoustic wave and flow control

Target particle encapsulation is crucial in droplet microfluidics for high-throughput applications like single-cell sequencing and drug screening. However, it faces limitations, with encapsulation rates of only 10% to 30% due to suspension density and the inherent functionality of the chip being restricted by the Poisson distribution. This leads to reagent waste and reduced effectiveness in applications requiring ultra-high multiplexing or extensive particle analysis, due to the massive empty droplets. Here we propose a droplet microfluidic system integrating surface acoustic wave (SAW) and sheath flow control. Suspensions of varying concentrations within the channel are initially pre-focused by sheath fluid, and then acoustically focused into a linear arrangement by SAW. Spacing between particles can be regulated by modulating the sheath fluid, ensuring sequential encapsulation of cells or beads in individual droplets. Thermal shock generated by the SAW, particle and droplet frequency, and particle encapsulation ratio are all elaborately evaluated. The results demonstrate that our system reaches an exciting single-bead packing efficiency of up to 78%, and achieves a packing rate of more than 60% for both high and low concentrations of solutions for polystyrene microspheres, magnetic beads and H22 cells, 6 times higher than the theoretical upper limit of the conventional method and 1.8 times higher than the Poisson distribution. More importantly, our system is designed to be free of structural and parametric constraints, which is quite important in future practical application. Thus, our on-chip particle focusing control method and droplet microfluidic system provide great potential in biological applications needing a high single-particle encapsulation ratio in limited partitions, such as ultra-high multiplex digital biomolecular detection, single-cell analysis, drug screening, and single exosome detection.