Prototype of a novel micro-machined cytometer and its 3D hydrodynamic focusing properties

This paper presents a micro-machined cytometric device which can achieve a three-dimensional (3D) hydrodynamic focusing only through a novel but simple microfluidic structure, with the uniqueness that the depth of the microchannels is non-uniform. By using a SU-8 soft lithography containing two exposures, the PDMS device prototype is fabricated, and tested for its performance through fluorescent optical experiments. At the same time, a two-fluid model to describe the micro-flow transport and interaction behaviors is also established, based on volume of fluid (VOF) method for multi-phase flow. It is found that the experiment and the simulation results have good consistencies. Based on this, the influences of a few geometry parameters on device 3D focusing performance, which is evaluated by the focused width as well as the corresponding height, are further explored by numerical simulations. The results indicate that good 3D focusing could be obtained at relatively not high sheath-sample velocity ratios, mainly due to the introducing of the unique depth difference. The work of this paper, not only validates the design conception of the proposed novel structure convincingly, but also enhances our understanding of 3D hydrodynamic focusing in the design of cytometers, as well as similar microfluidic devices.