MACS MicroBeads are superparamagnetic particles made from an iron oxide core and a dextran coating (Fig. 4.39). They are nano-sized, ranging from 20 to 150 nm in diameter, and form colloidal solutions - that is, they remain dispersed (Kantor et al., 1997; Miltenyi et al., 1990). Superparamagnetism means that in a magnetic field the iron oxide cores magnetize strongly like ferromagnetic material, but when
these are about 50 nm in diameter, form colloidal solutions, and are biodegradable. Their small size enables fast kinetics of the MicroBead-cell reaction and minimizes unspecific binding. Thus, enrichment of cells of more than 10000-fold is possible from frequencies below one cell in 108 cells.
removed from the magnetic field the particles do not retain any residual magnetism. The dextran coating of the MicroBeads permits chemical conjugation of bio-molecules. Numerous highly specific mAb, fluorochromes, oligonucleotides and various other moieties have all been covalently linked to MicroBeads, thereby transferring additional biochemical and physical properties to them (Miltenyi et al., 1990; Kato et al., 1993).
The nano-sized iron-dextran particles confer several unique features on MACS Technology. The MicroBeads are biodegradable, and do not alter cell function. Effects on the functional status of cells by magnetic labeling with the MicroBeads are primarily dependent on the target cell-surface antigen and on the degree of crosslinking by mAb or ligands conjugated to the MicroBeads, but not on the MicroBeads themselves. Cells labeled with MicroBeads have been used for numerous functional in-vitro assays, experimental transfers into animals, and therapeutic transplantations in humans.
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