Evaluation of hydrodynamic focusing in a microfluidic channel for flexible high-throughput analysis in digital holographic imaging flow cytometry

Quantitative Phase Imaging (QPI) combined with microfluidics enables high-throughput label-free imaging flow cytometry for acquisition of physical data from heterogeneous particle suspensions. For a reliable analysis of the sample flow, it is crucial that all particles in the sample fluid flow are monitored within the Field of View (FOV) of the QPI image recording device. We thus evaluated the capabilities of hydrodynamic focusing on a microfluidic system with a rectangular cross section-area and its compatibility with off-axis Digital Holographic Microscopy (DHM), an interferometry-based variant of QPI. To characterize the hydrodynamic focusing effect, the lateral distribution of living pancreatic tumor cells in flow that were used as probe particles in the sample flow was analyzed at different positions along the microfluidic channel from acquired series of DHM QPI images. Moreover, the influence of sample flow velocity and variations in the sample and sheath flow ratio on the sample stream with was determined. Our results demonstrate that the utilized micro fluidics unit is capable for hydrodynamic focusing of the sample fluid in DHM-based QPI and that its operation parameters allow a precise variable tuning of a focused particle stream.