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Cell Washing and Solution Exchange in Droplet Microfluidic Systems.

Water-in-oil emulsion droplet microfluidic systems have been extensively developed, and currently, almost all cell handling steps can be conducted in this format. An exception is the cell washing and solution exchange step, which is commonly utilized in many conventional cell assays. This paper presents an in-droplet cell washing and solution exchange technology that utilizes dielectrophoretic (DEP) force to move all cells to one side of a droplet, followed by asymmetrical splitting of the droplet to obtain a small daughter droplet that contains all or most of the cells, and then finally merges this cell-concentrated droplet with a new droplet that contains the desired solution. These sequential droplet manipulation steps were integrated into a single platform, where up to 88% of the original solution in the droplet could be exchanged with the new solution while keeping cell loss to less than 5%. Two application examples were demonstrated using the developed technology. In the first example, green microalga Chlamydomonas reinhardtii cells were manipulated using negative DEP force to exchange the regular culture medium with a nitrogen-limited medium to induce lipid production. In the second example, Salmonella enterica cells were manipulated using positive DEP force to replace fluorescent dye that models fluorescent cell stains that contribute to high background noise in fluorescence-based droplet content detection with fresh buffer solution, significantly improving the droplet content detection sensitivity. Since the cell washing step is one of the most frequently utilized steps in many cell biology assays, we expect that the developed technology can significantly broaden the type of assay that can be conducted in droplet microfluidic format.