Control of Magnetophoretic Mobility by Susceptibility-Modified Solutions As Evaluated by Cell Tracking Velocimetry and Continuous Magnetic Sorting Lee R. Moore, † Sarah Milliron, ‡ P. Stephen Williams, † Jeffrey J. Chalmers, § Shlomo Margel, | and Maciej Zborowski* † Department of Biomedical Engineering, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195, Department of Bioengineering, University of Toledo, Toledo, Ohio 43606, Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210, and Department of Chemistry, Bar-Ilan University, Ramat Gan, Israel With the analytical expression for the magnetophoretic mobility of an ideal, linearly polarizable sphere undergoing creeping motion in viscous medium, we have shown that both attractive and repulsive motions are possible in the magnetic field. We have validated theoretical predictions using magnetic monodisperse microspheres of 5.2-μm diameter and nonmagnetic polystyrene microspheres of 6.99-μm diameter suspended in solutions of paramag- netic ions. The microsphere magnetophoretic mobility was measured using a modified particle tracking velocimetry system, developed in-house and called a cell tracking velocimeter. The product of measured mobility and vis- cosity agrees well with the theoretical prediction, differing only by ∼1 1 %. Further, a 2 6 % increase in resolution between magnetic and nonmagnetic particle distributions was evaluated when paramagnetic ion carrier was used instead of water. Continuous particle sorting based on differences in magnetophoretic mobility was performed with another device developed by us, the quadrupole magnetic flow sorter (QMS). In the QMS, the introduction of paramagnetic ions into the carrier was effective in suppressing nonspecific crossover (i.e., the transport of low-mobility particles into the magnetic particle fraction) in particles and in biologically relevant red blood cells and thus showed promise as a means of increasing the purity of the magnetic separation. The term “magnetophoretic mobility” has been proposed to describe the behavior of a magnetic particle moving through a viscous medium under the influence of an external magnetic field. 1-10 Its origin can be traced to a similar term used to describe particle motion in a viscous medium under the influence of an electric field, the “electrophoretic mobility”. 11-14 The concept of magnetophoretic mobility has not been as widely applied due to peculiarities of the magnetic dipole-field interaction, such as its strong dependence on position, 15 and due to applications relying on equilibrium rather than transport separations. The former are exemplified by magnetic particle capture inside a high-gradient magnetic separator column and the latter by a continuous separation in a laminar flow exposed to an open field gradient. 6,16 An examination of the expression for the magnetophoretic mobility reveals the potential for a wider range of magnetic separation applications than currently practiced in biological and clinical laboratories. 10,17,18 In particular, it suggests the possibility of not only attractive, but repulsive particle motion in a magnetic field, by modifying the magnetic susceptibility of the solution. This could * Corresponding author. Tel.: 1-216-445-9330. Fax: 1-216-444-9198. E-mail: zborow@ bme.ri.ccf.org. † The Cleveland Clinic Foundation. ‡ University of Toledo. § The Ohio State University. | Bar-Ilan University. (1) Hartig, R.; Hausmann, M.; Schmitt, J.; Herrmann, D. B. J.; Riedmiller, M.; Cremer, C. Electrophoresis 1992 , 13, 674-676. 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