A microfluidic device may, in an example, include at least one microfluidic channel, a dielectrophoresis separator to separate a plurality of cells passing within the at least one microfluidic channel, and a thermal resistor to eject at least one cell from the microfluidic device. A cassette may, in an example, include a die coupled to a substrate of the cassette, the die including at least one microfluidic channel, a dielectrophoresis separator along the microfluidic channel to separate a plurality of cells passing within the microfluidic channel, and an ejection device to eject at least one of the plurality of cells into an assay well.