High-efficiency inertial focusing based on enhanced secondary flow generated by ring-inner obstacle combined channels

Inertial-focusing microfluidics enables extensive applications such as particle manipulation, single-cell analysis, and flow cytometry due to its various advantages, including high throughput, simplicity of devices, ease of operation, and freedom from external fields. Generally, only one type of secondary flow, such as Dean or geometry-induced secondary flow, is used in inertial focusing, leading to a low focusing efficiency. Combining channels with two or more geometries can enhance the secondary flows and thus improve the focusing performance. This study investigated the inertial focusing mechanism of a combination of four channel types. First, we constructed an annular channel, a contraction-expansion array channel, and an annular channel with obstacles distributed along the inner and outer walls. Then, theoretical modeling and focusing experiments for the four channels were carried out using four kinds of fluorescent particles as well as breast cancer cells. The results demonstrated that the annular channel combined with obstacles along the inner wall (ring-inner obstacle combined channel) generated an enhanced secondary flow and exhibited a particle-focusing efficiency of > 99% and a cell-focusing efficiency of > 95%. Furthermore, we summarized the design considerations of the combined channels for promoting cell focusing and separation. The inertial focusing devices with combined channels could offer an efficient means for continuous cell manipulation, high-throughput cytometry, and high-precision single cell analysis.