Field-free, sheathless cell focusing in exponentially expanding hydrophoretic channels for microflow cytometry

Cell focusing into a narrow stream is an essential step prior to counting and sorting cells in microfluidic devices for flow cytometry and cell sorting applications. Hydrodynamic focusing techniques, however, rely on the need for large volumes of sheath liquid and complex mechanical setup for flow control, preventing miniaturization of the systems. Although microfluidic methods based on active or passive particle control offer sheathless and efficient focusing, they often accompany fabrication complexities or bulky external setups, and operate in a certain range of flow rates. We present here a microfluidic device to focus cells into a narrow stream. The device employs hydrophoresis to guide cells by locally patterned slanted grooves, and channel expansion to improve focusing efficiency and produce a narrow stream of cells. This device principle allows easy improvement of focusing efficiency by adding more expansion steps. Adjusting channel expansion also ensures successful cell focusing without defocusing by inertial effects even at high Reynolds numbers. Using this device, we successfully produced a narrow stream of cells having size variation of >11% in a coefficient of variation (CV), achieving a narrow cell stream with a focusing variation below CV of 3.0%.