Controlling fluid flow rate to separate leukocytes and cancer cells based on stiffness differences.

Microfluidic techniques for label-free sorting of circulating tumor cells (CTCs) from peripheral blood are generally based on physical characteristic differences between blood cells and cancer cells, especially size differences. However, because the diameter ranges of some leukocytes and CTCs overlap, size-based sorting can certainly be disturbed by leukocytes. This study proposes a microfluidic sorting strategy based on cell stiffness differences, which achieves efficient separation of cells with overlapping sizes by precisely controlling the fluid flow rate. A quantitative relationship model between critical inlet flow rate, cell diameter, and Young's modulus was established based on the cell deformation effect, and its accuracy was verified through fluid dynamics simulation and experiments. The results showed that when the flow rate was between the critical flow rates of leukocytes and CTCs, high-stiffness leukocytes could be captured by the slit, while low-stiffness CTCs passed smoothly, with a separation resolution of 0.3 µm. This method can serve as a supplement to existing size-based sorting techniques, significantly improving the purity of CTCs.