Acousto-viscoelastic droplet microfluidics enhancing single particle-in-droplet encapsulation unlimited by the Poisson distribution

Droplet microfluidic technologies have enormous advantages for single cell-based multiparametric studies. The capability of single-cell encapsulation in a picoliter droplet with high efficiency (i.e. achieving the generation of droplets with negligible empty droplet rate) is a pivotal factor for addressing cellular heterogeneity, understanding fundamental biological processes, and advancing applications such as early diagnosis, drug screening and cell therapy. However, single-cell encapsulation efficiency is highly affected by the Poisson distribution, a discrete probability distribution, where empty droplets are generated with 57 % probability when aiming to produce droplets containing a single-cell. Thus, the generation of empty droplets becomes unavoidable, ultimately compromising accuracy, efficiency, and cost-effectiveness of cellular heterogeneity analysis. Here, we developed a droplet microfluidic system where particles or cells suspended in viscoelastic medium can be focused in the middle of microchannel and uniformly ordered using acoustophoresis, following by droplet generation, enabling single-particle or cell encapsulation with only 7 % empty droplet generation. The acousto-viscoelastic force-based system was evaluated using polystyrene (PS) particles, red blood cells (RBCs), and pancreatic cancer cells (PANC-02 cells), achieving single-particle/cell encapsulation efficiencies of 90 % for PS particles, 63 % for RBCs, and 79 % for PANC-02 cells. Taken together, Poisson distribution did not apply when particles or cells flowed through the microchannel in our system, enabling high-efficiency single-particle or single-cell encapsulation. The proposed system allows particle ordering in a wider range of flow conditions for the ordering of particles with similar size, compared to the inertial or viscoelastic force-based particle ordering systems. Moreover, this suggests that the developed system holds significant potential for broad applications in the field of single-cell analysis.