Inertial co-focusing of heterogeneous particles in hybrid microfluidic channels with constantly variable cross-sections†

Heterogeneous particles co-focusing to a single stream is a vital prerequisite for cell counting and enumeration, playing an essential role in flow cytometry and single-cell analysis. Microfluidics-based inertial focusing holds great research prospects due to its simplicity of devices, ease of operation, high throughput, and freedom from external fields. Combining microfluidic channels with two or more different geometries has become a powerful tool for high-efficiency particle focusing. Here, we explored hybrid microfluidic channels for heterogeneous particle co-focusing. Four different annular channels with obstacles distributed on the inner wall were constructed and simulated, obtaining constantly variable secondary flows. Then we used four different fluorescent particles with the size of 10 μm, 12 μm 15 μm, and 20 μm as well as their mixture to perform the inertial focusing experiments of multi-sized particles. Theoretical simulation and experimental results demonstrated a focusing efficiency of >99%. Finally, we further utilized human white blood cells to estimate the co-focusing performance of our hybrid microfluidic channel, resulting in a high focusing efficiency of >92% and a high throughput of ≈8000 cell s⁻¹. The hybrid microfluidic channels, capable of high-precision heterogeneous particle co-focusing, could pave a broad avenue for microfluidic flow cytometry and single-cell analysis.