Both components of electromagnetic fields, electric and magnetic, can be used to control the motion of biological cells in fluids. In electric manipulation, electric fields interact directly with the electrical properties of biological cells as well as surrounding media to generate force on the cells. For example, in cell electrophoresis DC electric fields interact with the static charges of cells and media ; cell dielectrophoresis exploits the difference of polarizabilities between cells and media under AC electric fields . In magnetic manipulation, however, neither biological cells nor media directly interact with magnetic fields: because of their intrinsically low magnetic susceptibilities, both biological cells and media are transparent to magnetic fields . To manipulate cells using magnetic fields, extrinsic magnetic moments should be imparted to cells, which is achieved by labeling cells with magnetically responsive microspheres, magnetic beads.
In this section, we review the magnetic bead technology. Magnetic properties of magnetic beads are described, followed by the discussion on the motion of magnetic beads under external magnetic fields generated by micro-electromagnets. Then, we describe how magnetic beads can be tagged to biological cells to give magnetic moments to the cells.
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