A spiral microchannel particle separator includes an inlet for receiving a solution containing particles, at least two outlets, and a microchannel arranged in a plurality of loops. Particles within a solution flowing through the spiral microchannel experience a lift force FL and a Dean drag force FD. The spiral radius of curvature R and the hydraulic diameter Dh of the spiral microchannel are such that for a flow rate U of the solution, the lift force FL and a Dean drag force FD are approximately equal and act in opposite directions for particles of a first size. The particles of the first size are focused in a single stream located at an equilibrium position near an inner wall of the microchannel. In another embodiment, a straight microchannel particle separator separates particles by modulating shear rates via high aspect ratios that focuses particles of a first size along two first walls.