Band-stop microfluidics for high-purity, label-free enrichment of viable cancer cells from whole blood.

Liquid biopsy requires the isolation of viable circulating tumour cells (CTCs) from whole blood at high purity and with sufficient quality for functional assays. These requirements are not readily met by either label-based methods or size-only low-pass filters. Antibody labels can be specific yet miss phenotypically diverse CTCs, while size-based approaches may capture more broadly but suffer from leukocyte contamination. Here, we introduce a microfluidic band-stop filter that implements a size-selective transfer function, replacing a single threshold with an engineered peaked size-capture response. This is achieved by coupling hydrodynamic filtering with hydrodynamic trapping in a high-density array. Small cells follow streamlines that pass through trap apertures, intermediate-sized cells are admitted but retained by downstream constrictions, whereas larger cells occupy streamlines displaced from the channel wall and are hydrodynamically transported past the traps. As a result, only cells with diameters within the band-stop window are retained, while both smaller and larger cells pass. Using rigid microspheres, the device exhibits a canonical band-stop profile. Using cultured cell lines, capture efficiency peaks for CTC-like diameters and is suppressed for leukocyte-sized cells. Critically, the behaviour is preserved in undiluted, full haematocrit whole blood, where no leukocyte retention is observed and target-sized cancer cells are selectively enriched without fouling at capture efficiencies comparable to commercial systems. Trapped cells tolerate buffer exchange and on-chip drug exposure, where responses were concordant with matched off-chip culture. Cells are released on demand and expanded off-chip, confirming post-processing viability.