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Inertial microfluidics functions solely based on the fluid dynamics at relatively high flow speed. Thus, channel geometry is the critical design parameter that contributes to the performance of the device. Four basic channel geometries (i.e., straight, expansion-contraction, spiral and serpentine) have been proposed and extensively studied. To further enhance the performance, innovative channel design through combining two or more geometries is promising. This work explores embedding periodic concave and convex obstacle microstructures in sinusoidal channels and investigates their influence on particle inertial focusing and separation. The concave obstacles could significantly enhance the Dean flow and tune the flow range for particle inertial focusing and separation. Based on this finding, we propose a cascaded device by connecting two sinusoidal channels consecutively for rare cell separation. The concave obstacles are embedded in the second channel to adapt its operational flow rates and enable the functional operation of both channels. Polystyrene beads and breast cancer cells (T47D) spiking in the blood were respectively processed by the proposed device. The results indicate an outstanding separation performance, with 3 to 4 orders of magnitude enhancement in purity for samples with a primary cancer cells ratio of 0.01% and 0.001%, respectively. Embedding microstructures as obstacles brings more flexibility to the design of inertial microfluidic devices, offering a feasible new way to combine two or more serial processing units for high-performance separation. [ABSTRACT FROM AUTHOR]

• This work studied the effects of obstacle in curvilinear channels on particle inertial focusing and separation. • A numerical model was developed to predict the migration trajectories of particles in periodic curvilinear channel. • The concave obstacles were more effective in tuning particle inertial focusing and separation than convex obstacles. • The separation resolution can be as high as 2 µm in the channels with square concave obstacles. • The high-resolution separation technique was applied for isolating rare cancer cells from blood samples. Inertial microfluidics manipulates and separates microparticles based on the finite inertia of the fluid at high flow speed. In inertial microfluidics, modifying the geometry by embedding periodic micro-obstacles into curvilinear channels is emerging as a promising strategy to improve inertial focusing and separation. This work systematically investigated the influence of micro-obstacles on inertial focusing and developed a high-resolution microfluidic device for particle and cell separation. First, we developed numerical modelling to simulate the migration trajectories of particles. Then, we studied the effects of various obstacles on the inertial focusing in the sinusoidal channels. The concave obstacles were more effective in tuning particle inertial focusing and separation than convex obstacles. Furthermore, the square concave obstacle channel could offer the highest separation resolution. Finally, we developed a microfluidic device based on square concave obstacle channel, and applied it for the high-efficiency separation of polystyrene beads and U87MG cancer cells from the blood. [ABSTRACT FROM AUTHOR]