Ridge-assisted Micro Positioning of Cells and Particles in a Microchannel

The capability to deterministically and precisely control the lateral position of focused cell streams in a microchannel provides diverse opportunities for biomedical applications, including cell concentration, imaging flow cytometry, and cell-based liquid biopsy platforms. Inertial microfluidics introduced various high-throughput strategies for ordering particles and cells within microchannels. However, existing approaches do not permit precise control over the lateral focusing position of cells. By leveraging engineered microvortices generated by channel ridges and fluidic lift forces, we present a microfluidic method for deterministically positioning focused cell streams to desired lateral streamlines. Unlike inertial focusing, the ridge-assisted micro positioning (RAMP) is independent of particle size and flow rate, enabling polydisperse particles (10 to 30 μm) to be focused to the same streamline across a broad range of flow rates (250 to 1000 μL/min). The focusing position can be precisely adjusted by altering ridge placement and geometry, achieving both small (5 μm) and large (≥15 μm) lateral shifts in a controlled manner. Applying the RAMP concept, we developed clog-free microfluidic cell concentrators that can enrich single cells and clusters of cells to desired concentration factors and demonstrate the ability to focus particles in undiluted whole blood. Together, these results establish RAMP as a versatile platform for precise cell positioning in complex biological fluids.