Three-Dimensional Hydrodynamic Focusing With a Circular Microchannel

Abstract

We propose a simple method to implement three-dimensional (3D) hydrodynamic focusing with a circular micro-channel for the sample flow. We utilized soft lithography technique and a cylindrical object such as an optical fiber to construct a 3D hydrodynamic focusing master mold made of photoresist and the fiber. The 3D microfluidic focusing device can be duplicated by polydimethylsiloxance (PDMS) casting. When peeling the PDMS cast from the master, there is a crack along the circular channel. However, this crack can be recovered from the thermal bonding of the PDMS cast and a PDMS substrate. We applied this technique to fabricate 3D flow-focusing devices for a microfluidic fluorescence detection chip and a micro-emulsion chip. In the case of microfluidic fluorescence detection, the sample flow can form a small and stable cylindrical flow by controlling the flow rates of sheath flow and sample flow. This allows fluorescent particles to be focused at the center of the channel and to be excited uniformly by laser. Experimental results show that the coefficients of variation for 3D hydrodynamic focusing is 4 to 10 times lower than those of 2D hydrodynamic focusing. As a consequence, the 3D focusing chip provides advantages of stable sample flow and highly uniform detection signals. The micro emulsion chip with 3D hydrodynamic focusing can avoid the wetting problem of two immiscible fluids and successfully produce emulsion droplets.