Investigations of micrometer sample stream profiles in a three-dimensional hydrodynamic focusing device

In this work we describe the design and fabrication of a novel three-dimensional flow focusing device with channel dimensions of 50 μm × 50 μm at the focusing region. The device is suitable for one-by-one cell or particle detection near the channel bottom, in for example a Coulter counter or for optical near field detection. The device comprises a silicon-glass sandwich with SU-8 resist in between. The sample flow is hydrodynamically focused between three sheath flows. At the fourth side the sample flow is moving along the channel bottom. We investigated the focusing limitation of the sample flow and found that the minimum width amounts to 2.5 μm at a channel width of 50 μm. Confocal laser scanning measurements were constituted to obtain the sample flow profile. We show the experimental and numerical images of the focused stream shape from a cross-section perspective. Furthermore, we analyzed the measured fluorescent color intensity of the sample stream and from that, calculated the sample flow height. These results were compared with the corresponding confocal images. Additionally, the influence of the hydrodynamic focusing on the sample profile and its uniformity over a certain distance was investigated.