Magnetic separation is a technique that uses magnetism to purify cells, cell organelles, and biological molecules such as nucleic acids, proteins, and xenobiotics directly from crude samples. Compared with other techniques used for cell separation, magnetic cell separation has the advantages of speed, simplicity, selectivity, high throughput and high-cost performance. There are usually two magnetic separation methods used to separate cells. In the first method, the cells to be separated exhibit sufficient intrinsic magnetic moments so that magnetic separations can be performed without any modification. There are only two types of such cells found in nature, namely red blood cells that contain a high concentration of paramagnetic hemoglobin, and magnetotactic bacteria that contain small magnetic particles. In the second method, one or more non-magnetic components of the mixture must be labeled with magnetic labels. Attachment of magnetic labels is usually mediated by affinity ligands that can interact with target structures on the cell surface. The most common approach is to use magnetic beads with high-affinity antibodies against specific cell surface epitopes. The magnetic cell complexes formed can be isolated by a suitable magnetic separator. In some cases, isolated cells with magnetic labels can be used directly, for example in culture experiments. The content of isolated cells can be analyzed by various methods such as chromatography, electrophoresis, and PCR. For specific applications, magnetic labels should be removed from the isolated cells and the free cells are prepared for downstream application and analysis.
Magnetic separation permits the target cells to be isolated directly from samples such as blood, bone marrow, tissue homogenates, stool, cultivation media, food, water, soil, etc. It enables routine purification to achieve high sample throughput and automation without the need of complex protocols, expensive equipment or costly consumables. Static magnetic fields do not interfere with the movement of ions and charged solutes in aqueous solutions as electric fields do. In addition, the large difference between magnetic permeabilities of magnetic and non-magnetic materials can be used to develop highly selective separation methods. This high selectivity can also be achieved by other separation methods, but these rely on expensive instruments. In general, the magnetic separation process is gentle, facilitating the rapid handling of delicate cells in an unfriendly environment. It also simplifies procedures such as change of buffer conditions and repeated washing steps. The shear forces associated with binding and elution are minimal compared to centrifugation or filtration, which is beneficial for increasing the production of viable cells. Compared with traditional flow cytometry methods, magnetic cell separation allows defined cell populations to be easily scaled up if a large number of living cells are required. The cells isolated by magnetic separation are usually pure, active, and unaltered. The whole separation process allows multiple samples to be run simultaneously in the same tube, saving time.
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