Single Stream Inertial Focusing in Low Aspect-Ratio Triangular Microchannels

A wide range of microfluidic devices for single stream focusing of cells and particles has emerged in recent years, based on both passive and active methods. Inertial microfluidics offers an attractive alternative to these methods, providing efficient and sheathless passive focusing of cells and beads. Nevertheless, in rectangular microchannels, presence of multiple equilibrium positions necessitates complicated solutions involving manipulation of 3D structure in order to achieve single stream flows. Here, we present a new approach to single-stream inertial focusing based on a triangular microchannel geometry. Changing channel cross-sectional shape leads to asymmetry in velocity profile, resulting in a size-dependent single stable equilibrium position near channel apex. We demonstrate that soft lithography masters for such microchannels can be fabricated in PMMA through micromilling, and 15 µm diameter beads can be efficiently focused in a single stream. Confocal microscopy was used to confirm focusing positions in the microchannel cross-section. We further integrate this device with a laser counting system to form a sheathless flow cytometer and demonstrated counting of beads with ~326 s -1 throughput. The use of triangular cross-section offers a number of benefits, including simplicity in fundamental principle and geometry, continence in design, small footprint, ease of integration, as well as high-precision single position focusing.