Single-pass inertial fractionation of leukocyte subtypes in a straight-channel microfluidic device

Microfluidic fractionation of leukocyte subtypes is important for immunological analysis and cell-based workflows, yet many existing approaches rely on complex channel architectures, multi-stage processing, or active control. Here, we present a straight-channel inertial microfluidic device that achieves single-pass, simultaneous fractionation of leukocyte subtypes using intrinsic hydrodynamic forces augmented by sheath-flow pre-focusing. By constraining cells to a narrow stream at the inlet, size-dependent inertial divergence is realized in a uniform straight channel without curvature, microstructures, or cascaded separation stages. When applied to minimally preprocessed, erythrocyte-lysed human blood samples, the platform produces parallel streams enriched in lymphocytes, granulocytes, and monocytes, as confirmed by microscopy and flow cytometry. While complete subtype isolation is limited by overlapping size distributions and cell deformability, fractionation is achieved without serial processing or recirculation. Flow-cytometric analysis further indicates that inertial transport does not induce additional leukocyte activation beyond that associated with standard preprocessing. These results establish architectural minimalism as a viable strategy for inertial microfluidics and position the device as a simple, scalable, front-end leukocyte-fractionation module compatible with downstream analytical workflows.