Diamagnetic droplet microfluidics applied to single-cell sorting

The heterogeneity of diseased tissue causes major challenges in the detection and treatment of disease. Such challenges have motivated the development of tools for single-cell isolation and analysis. However, many cell isolation methods in microfluidics rely on the use of cell-labeling steps or expose cells to potentially harmful forces. Here, we present a microfluidic method for label-free control of cell-encapsulating biocompatible droplets using negative magnetophoresis. Our system is distinguished from previous microfluidic diamagnetic sorting approaches by the encapsulation of the cells inside droplets, which isolates the cells from the magnetic continuous phase. The droplet phase is comprised of cells suspended in their growth culture medium, and all of the magnetic content is contained in the oil-based continuous phase. At a flow-focusing junction, empty droplets and cell-encapsulating droplets are both generated and surrounded by the magnetic continuous phase. Cell encapsulation produces a size distinction between empty droplets and cell-encapsulated droplets. Through the application of a permanent magnet to the system, diamagnetic size-based sorting of empty droplets from cell-encapsulated droplets is achieved with a purity of ∼84% in a single pass. Additionally, since the encapsulated cells are completely isolated from the magnetic content in the continuous phase, 88% cell-viability is observed after a two-hour incubation period. If combined with a single-cell assay, this system can provide label-free isolation of viable cells at a high purity for subsequent downstream analysis.