Quantification of Cellular Properties from External Fields and Resulting Induced Velocity: Magnetic Susceptibility Jeffrey J. Chalmers, 1 Seungjoo Haam, 1, * Yang Zhao, 1 Kara McCloskey, 1 Lee Moore, 2 Maciej Zborowski, 2 P. Stephen Williams 3 1 Department of Chemical Engineering, The Ohio State University, 140 WEST 19th Avenue, Columbus, Ohio 43210; telephone: +1 (614) 292-2727; fax: +1 (614) 292-3769; e-mail: Chalmers.1@osu.edu 2 Department of Biomedical Engineering, The Cleveland Clinic Foundation, 1900 Euclid Avenue, Cleveland, Ohio 44195 3 Department of Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401 Received 17 May 1998; accepted 11 January 1999 Abstract: An experimental technique is discussed in which the magnetic susceptibility of immunomagneti- cally labeled cells can be determined on a cell-by-cell basis. This technique is based on determining the mag- netically induced velocity that an immunomagnetically labeled cell has in a well-defined magnetic energy gradi- ent. This velocity is determined through the use of video recordings of microscopic images of cells moving in the magnetic energy gradient. These video images are then computer digitized and processed using a computer al- gorithm, cell tracking velocimetry, which allows larger numbers (>10 3 ) of cells to be analyzed. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 519–526, 1999. Keywords: magnetic susceptibility measurements; com- puter imaging; immunomagnetic labeling; magnetic cell separation INTRODUCTION The ability to analyze and/or separate cells based on the presence of specific molecules, either on the surface or within the cell, is a significant tool with applications ranging from fundamental biological studies to clinical practice. The development of the technology that allows these analyses and separations to be conducted has occurred in conjunction with the significant advances that have been made in the development of specific cellular probes. The sophistication of these probes led to the increasing knowledge of cellular processes. In many ways, it is the joint development of the technology and the fundamental understanding that have contributed to the current state of knowledge of the field. The list of properties/characteristics that can be analyzed exceeds the objective of this paper; however, examples in- clude total protein content, cellular pigment content, intra- cellular pH, membrane organization, DNA content, and probably the most important characteristic from a clinical point of view, the presence of specific surface receptors or clusters of differentiation (CD) such as CD 4, CD 8, CD 34, etc. on lymphocytes (Shapiro, 1995). The fundamental bases of these analytical and separation instruments are 2-fold: the ability to label the specific cell of interest with the probe, and the ability to either detect or differentiate the cell with the bound probe from the rest of the cells in the suspension. In the case of cell separation, this bound probe is used as a “handle” to separate the labeled cell from the other cells in the suspension. Specific Cellular Probes A variety of labels are used in a number of different tech- nologies. The most specific labels are those that bind only to a narrow class of cell associated molecules. While a number of these highly specific labels exist, i.e., streptavidin, the most commonly used labels are antibodies. These antibody labels are then usually covalently linked to a molecule, a particle, or a support matrix. Analytical Instruments Analytical use of the cell labels range from qualitative fluo- rescence microscopic studies to the much more highly quan- titative fluorescence activated cell scanning (FACS) sys- tems. A second and much simpler analytical device is one which uses paramagnetic particles covalently bound to an- tibodies (Winto-Morbach et al., 1994, 1995, 1996). A fur- ther discussion of this device will be given below. Separation Instruments A number of commercial separation systems exist based on immunological interactions. These can be grouped into im- munofluorescent, immunomagnetic, and immunosolid/ immunomatrix categories. The most common immunofluo- rescent cell separation system is the sorting version of the FACS system mentioned previously. A number of commer- cial, immunomagnetic separation technologies exist (MPC * Present address: Chemical Engineering Department, Yonsei Univer- sity, 134 Shinchon-Dong, Seodaemoon-Ku, Seoul 120-752, Korea Correspondence to: J. J. Chalmers Contract grant sponsors: National Science Foundation; National Insti- tutes of Health; Whitaker Foundation Contract grant numbers: BCS 9258004; CA 62349; 20010308 © 1999 John Wiley & Sons, Inc. CCC 0006-3592/99/050519-08