Geometry-tuned inertial microfluidics for label-free enrichment of CTC-sized cells in serpentine microchannels

Circulating tumor cells (CTCs) are important biomarkers for metastatic progression, but their label-free enrichment from blood remains challenging because of their rarity, heterogeneity, and partial size overlap with hematologic cells. In this study, we present a serpentine inertial microfluidic platform for the label-free, outlet-specific enrichment of CTC-sized model cells. Serpentine microchannels with a fixed width of 350 μm, inner radius of 800 μm, and curvature angle of 280° were evaluated using numerical simulations, hemocytometer counting, flow cytometry, and confocal fluorescence imaging. HeLa cells were used as CTC-sized model cells, while Jurkat and K562 cells represented smaller hematologic background populations. The two candidate two-outlet devices were evaluated as complete configurations: a 73 μm-depth device incorporating a 90° terminal outlet branch and a 105 μm-depth device incorporating a 72° terminal outlet branch. Among the tested configurations, the 105 μm/72° device showed the strongest preferential partitioning of outlet-recovered HeLa cells toward the designated collection outlet. At the flowrate of 1000 μL/min, numerical simulations predicted that approximately 92% of the outlet-recovered HeLa cells would be directed to a certain outlet, whereas the corresponding experimental measurements yielded collection-outlet HeLa fractions of 92.0% and 97.5%. At the flowrate of 2000 μL/min, the experimental values showed a greater between-run variability, at 92.5% and 71.1%. Whole-blood experiments with spiked HeLa cells further demonstrated preferential HeLa-associated partitioning toward a certain outlet in a complex biological matrix. Because the channel depth and outlet angle were varied simultaneously, the observed improvement is interpreted as the combined effect of upstream Dean-inertial migration and downstream outlet-stream alignment rather than as an isolated depth effect.