Ridge-assisted micro positioning of cells in a channel.

The ability to deterministically and precisely control the lateral position of focused cell streams within a microchannel enables a wide range of biomedical applications, including cell concentration, sorting, and sensing. 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 streamlines. Unlike inertial focusing, ridge-assisted micro positioning (RAMP) is independent of particle size and flow rate, enabling polydisperse particles to be focused to the same streamline across a broad range of flow rates. The focusing position can be precisely adjusted by altering the ridge placement and geometry, allowing for micron-level lateral shifts in a controlled manner. Applying the RAMP concept, we demonstrate clog-free microfluidic cell concentrators that can enrich single cells and clusters to desired concentration factors and show 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.