In-droplet cell concentration using dielectrophoresis

Concentrating cells or adjusting the concentration of cells are one of the most fundamental steps in cell biology experiments, and are typically achieved through centrifugation. However this step is challenging to implement in droplet microfluidics. Here we present an in-droplet cell concentrator that operates by first gradually focusing cells inside a droplet to one side of the droplet using negative dielectrophoresis (nDEP), followed by asymmetric droplet splitting using a Y-shaped junction, resulting in two daughter droplets, one of which containing all or most of the cells. The developed platform was first characterized using droplets containing different number of polystyrene (PS) particles and by varying the applied voltages, flow rates, and the width ratios of the droplet splitting microchannels. Using this platform, the volume of one daughter droplet could be reduced up to 84% compared to that of the mother droplet, which resulted in the PS particle concentration to increase by 5.6-fold, with an average recovery rate of 90%. When testing with cells (Chlamydomonas reinhardtii), recovery rates as high as 98% could be achieved while increasing the cell concentration by 5-fold. This technology adds a new capability to droplet microfluidics operation, and can be used for adjusting concentrations of cells in droplets, exchanging solutions in which cells are suspended in droplets (including cell washing steps), and separating cells of different dielectric properties inside droplets, all of which are common steps in conventional cell assays but have been so far difficult to achieve in droplet format.