Planar micro-devices capable of continuously separating large volumes of dilute suspensions were designed and modeled using a commercial CFD package. The devices consist of a single high aspect ratio spiral micro channel with a bifurcation at the exit. The device exploits small inertial and hydrodynamic differences between particles of dissimilar size, which arise as a result of the curvature of the flow through the device. The channel length and location of the bifurcation were found to affect the separation achievable by the devices. Devices of varying geometries were fabricated using conventional silicon micro fabrication processes and were tested by flowing dilute aqueous suspensions of polystyrene particles (diameters of 1μm, 8μm and 10μm) through the devices at various flow rates. A 3.5 fold concentration enhancement of 10μm particles was achieved in the longer devices at flow rates of 2 ml/min, whereas the 1μm particles showed negligible concentration increases at similar flow rates. The devices may be used as a sample preparation stage in a complex μTAS, where rapid, continuous concentration of dilute suspensions is often required.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.