Micromachined pre-focused M x N flow switches for continuous multi-sample injection

In this paper we present a novel microfluidic chip capable of continuous multi-sample switching and injection for bio-analytical applications. The innovative device integrates two important microfluidic phenomena, including hydrodynamic focusing and valveless flow switching inside multi-ported microchannels. The multiple samples can be pre-focused to narrow streams and can then be continuously injected into desired outlet ports. In this study, a theoretical model based on the 'flow-rate-ratio' method is first proposed to predict the performance of the microfluidic device. Then, a simple but reliable one-mask micromachining process is developed to fabricate the pre-focused M x N flow switch on a quartz substrate. The multi-sample switching and injection is then verified experimentally with the use of microscopic visualization of water sheath flows and dye-containing sample flows. The experimental data indicate that the multi-sample flows can be hydrodynamically pre-focused and then guided into the desired outlet ports precisely based on relative sheath and sample flow rates. The data predicted by the proposed theoretical model are highly consistent with the experimental results. It is also noted that the 'pre-focusing' function added prior to multi-sample flow switching is crucial for precise sample injection. The novel microfluidic chip has great potential for high-throughput chemical analysis, cell fusion, fraction collection, fast sample mixing and many other applications in the field of micro-total-analysis systems.