UC Irvine

Droplet microfluidics has become an indispensable technology to encapsulate cells of interest in a monodispersed aqueous compartment for single-cell analysis. In addition, the confinement of cells in picoliter droplets offers high-throughput, single-cell resolution, and high signal-to-noise ratio for various cellular assays that unmasks cellular heterogeneity from a bulk population. Particularly, co-encapsulation of two distinct cells in a droplet is critically important for studying cell–cell interaction, transcriptomics, genomics, antibody, and drug screening. However, the co-encapsulation of one type A cell and one type B cell per single droplet, termed 1–1–1 encapsulation, has been dictated by double Poisson distribution due to the intrinsic random dispersion of cells, which yields mostly empty droplets and only up to 13.5% of droplets under optimal conditions. Such low 1–1–1 encapsulation efficiency makes it impractical for biological analyses at scale involving low cell concentrations or a large number of variables. Here, we demonstrate a passive co-encapsulation microfluidic device that leverages close packing of cells by hydrodynamic draining to overcome the double Poisson limitation. The results suggest a significant improvement of the 1–1–1 encapsulation efficiency by over two-fold compared to the double Poisson model. The enhanced encapsulation efficiency of this platform demonstrates great potential for a high-throughput, versatile, and simple platform for cell–cell interaction studies within a confined microenvironment.

In addition, we present a microfluidic droplet trapping array capable of modulating the translational and rotational dynamics of trapped cells inside the droplets by means of shear-induced microvortices driven by external fluidic control. Using this platform, we demonstrate that droplet microvortices are sensitive to particle/cell size to droplet ratios, cell compliance, and the external phase fluid velocity. We further analyzed the hydrodynamic forces experienced by particles through theoretical and numerical simulations to better understand the effects of different physical variables on droplet recirculation dynamics. Our experimental and theoretical results provide insights into the fluidic conditions that lead to translational or rotational dynamics inside trapped droplets, and could enable future uses for the characterization of single-cell biophysical properties.