  We present a method and model for the direct and continuous separation of red
and white blood cells in plasma. The method is implemented at the microscale
using a microfluidic system that consists of an array of integrated
soft-magnetic elements embedded beneath a microfluidic channel. The microsystem
is passive, and is activated via application of a bias field that magnetizes
the elements. Once magnetized, the elements produce a nonuniform magnetic field
distribution in the microchannel, which gives rise to a force on blood cells as
they pass through the microsystem. In whole blood, white blood cells behave as
diamagnetic microparticles while red blood cells exhibit diamagnetic or
paramagnetic behavior depending on the oxygenation of their hemoglobin. We
develop a mathematical model for predicting the motion of blood cells in the
microsystem that takes into account the dominant magnetic, fluidic and buoyant
forces on the cells. We use the model to study red/white blood cell transport,
and our analysis indicates that the microsystem is capable of rapid and
efficient red/white blood cell separation.
