Magnetic microspheres prepared by redox polymerization used in a cell separation based on gangliosides

Magnetic Separation in Bioprocessing Beyond the Analytical Scale: From Biotechnology to the Food Industry

Downstream processing needs more innovative ideas to advance and overcome current bioprocessing challenges. Chromatography is by far the most prevalent technique used by a conservative industrial sector. Chromatography has many advantages but also often represents the most expensive step in a pharmaceutical production process. Therefore, alternative methods as well as further processing strategies are urgently needed. One promising candidate for new developments on a large scale is magnetic separation, which enables the fast and direct capture of target molecules in fermentation broths. There has been a small revolution in this area in the last 10–20 years and a few papers dealing with the use of magnetic separation in bioprocessing examples beyond the analytical scale have been published. Since each target material is purified with a different magnetic separation approach, the comparison of processes is not trivial but would help to understand and improve magnetic separation and thus making it attractive for the technical scale. To address this issue, we report on the latest achievements in magnetic separation technology and offer an overview of the progress of the capture and separation of biomolecules derived from biotechnology and food technology. Magnetic separation has great potential for high-throughput downstream processing in applied life sciences. At the same time, two major challenges need to be overcome: (1) the development of a platform for suitable and flexible separation devices and (2) additional investigations of advantageous processing conditions, especially during recovery. Concentration and purification factors need to be improved to pave the way for the broader use of magnetic applications. The innovative combination of magnetic gradients and multipurpose separations will set new magnetic-based trends for large scale downstream processing.

Affinity chromatography: A versatile technique for antibody purification

Antibodies continue to be extremely utilized entities in myriad applications including basic research, imaging, targeted delivery, chromatography, diagnostics, and therapeutics. At production stage, antibodies are generally present in complex matrices and most of their intended applications necessitate purification. Antibody purification has always been a major bottleneck in downstream processing of antibodies, due to the need of high quality products and associated high costs. Over the years, extensive research has focused on finding better purification methodologies to overcome this holdup. Among a plethora of different techniques, affinity chromatography is one of the most selective, rapid and easy method for antibody purification. This review aims to provide a detailed overview on affinity chromatography and the components involved in purification. An array of support matrices along with various classes of affinity ligands detailing their underlying working principles, together with the advantages and limitations of each system in purifying different types of antibodies, accompanying recent developments and important practical methodological considerations to optimize purification procedure are discussed.

Design and synthesis of novel magnetic core-shell polymeric particles

A novel synthetic strategy was developed for the preparation of magnetic core-shell (MCS) particles consisting of hydrophobic poly(methyl methacrylate) cores with hydrophilic chitosan shells and gamma-Fe2O3 nanoparticles inside the cores via copolymerization of methyl methacrylate from chitosan in the presence of vinyl-coated gamma-Fe2O3 nanoparticles. The magnetic core-shell particles were characterized with transmission electron microscopy, field-emission scanning electron microscopy, particle size and zeta-potential measurements, vibrating sample magnetometry, and atomic force microscopy, respectively. The MCS particles were less than 200 nm in diameter with a narrow size distribution (polydispersity = 1.09) and had a good colloidal stability (critical coagulation concentration = 1.2 M NaCl at pH 6.0). Magnetization study of the particles indicated that they exhibited superparamagnetism at room temperature and had a saturation magnetization of 2.7 A m2/kg. The MCS particles were able to form a continuous film on a glass substrate, where magnetic nanoparticles could evenly disperse throughout the film. Thus, these new materials should be extremely useful in various applications.

Effects of CD44 antibody– or RGDS peptide–immobilized magnetic beads on cell proliferation and chondrogenesis of mesenchymal stem cells

We evaluated the efficacy of a novel mesenchymal stem cell (MSC) delivery system using an external magnetic field for cartilage repair in vitro. MSCs were isolated from the bone marrow of Sprague Drawley rats and expanded in a monolayer. To use the MSC delivery system, two types of MSC-magnetic bead complexes were designed and compared. Expanded MSCs were combined with small-sized (diameter: 310 nm) carboxyl group-combined (0.01-0.04 micromol/mg) magnetic beads, Ferri Sphere 100C, through either anti-rat CD44 mouse monoclonal antibodies or a synthetic cell adhesion factor, arginine (R)-glycine (G)-aspartic acid (D)-serine (S) (RGDS) peptide. Both cell complexes were successfully created, and were able to proliferate in monolayer culture up to at least day 7 after separation of magnetic beads from the cell surface, although the proliferation of the complexes was slower in the early period of culture than that of non-labeled rat MSCs (after 7 days of culture: proliferation of CD44 antibody-bead complexes, approximately 50%; RGDS peptide-bead complexes, 70% versus non-labeled rat MSCs, respectively). These complexes were seeded onto culture plates with or without an external magnetic force (magnetic flux density was 0.20 Tesla at a distance of 2 mm from plate base) generated by a neodymium magnet, and supplemented with chondrogenic differentiation medium. Both complexes could be attached and gathered effectively under the influence of the external magnet, and CD44-bead complexes could effectively generate chondrogenic matrix in monolayer culture. In a three-dimensional culture system, the production of a dense chondrogenic matrix and the expression of type II collagen and aggrecan mRNA were detected in both complexes, and the chondrogenic potential of these complexes was only a little less than that of rat MSCs alone. Thus, we conclude that due to the fact that MSC-RGDS peptide-bead complexes are composed using a biodegradable material, RGDS peptide, as a mediator, the RGDS peptide-bead complex is more useful for minimally invasive clinical applications using our design of magnetic MSC delivery system than CD44 antibody-beads.

Preparation and colloidal stability of monodisperse magnetic polymer particles

A previously proposed method was examined for producing monodisperse, submicrometer-sized magnetic polymer particles. The method applies soap-free emulsion polymerization during which Fe3O4 magnetic nanoparticles are heterocoagulated onto precipitated polymer nuclei. To chemically fix the magnetic particles to the polymer nuclei, vinyl groups were introduced on the Fe3O4 particles in a preliminary surface modification reaction with methacryloxypropyltrimethoxysilane, and methacryloxypropyldimethoxysilane (MPDMS) was added to reaction systems of the soap-free emulsion polymerization. The colloidal dispersion stability of magnetic polymer particles was improved by the addition of an ionic monomer, sodium p-styrenesulfonate (NaSS), during the polymerization. The polymerizations were carried out with styrene monomer and potassium persulfate initiator in ranges of NaSS concentrations (0-2.4 x 10(-3) M), NaSS addition times (60-80 min), and monomer concentrations (0.3-0.6 M) at fixed concentrations of 1.6 x 10(-2) M initiator and 1.3 x 10(-2) M MPDMS for pH 4.5 adjusted with a buffer system of [CH3COOH]/[NaOH]. The addition of NaSS during the polymerization could maintain the dispersion stability of magnetic polymer particles during the polymerization. Selection of the reaction conditions enabled the preparation of colloidally stable, submicrometer-sized magnetic polymer particles that had coefficients of variation of distribution smaller than the standard criterion for monodispersity, 10%.

Nanoscale size effect of magnetic nanocrystals and their utilization for cancer diagnosis via magnetic resonance imaging

Since the use of magnetic nanocrystals as probes for biomedical system is attractive, it is important to develop optimal synthetic protocols for high-quality magnetic nanocrystals and to have the systematic understanding of their nanoscale properties. Here we present the development of a synthetically controlled magnetic nanocrystal model system that correlates the nanoscale tunabilities in terms of size, magnetism, and induced nuclear spin relaxation processes. This system further led to the development of high-performance nanocrystal-antibody probe systems for the diagnosis of breast cancer cells via magnetic resonance imaging.

Magnetically modulated therapeutic systems

Magnetically targeted drug delivery by particulate carriers is an efficient method of delivering drugs to localized disease sites, such as tumors. High concentrations of chemotherapeutic or radiological agents can be achieved near the target site without any toxic effects to normal surrounding tissue. Non-targeted applications of magnetic microspheres and nanospheres include their use as contrast agents (MRI) and as drug reservoirs that can be activated by a magnet applied outside the body. Historic and current applications of magnetic microspheres will be discussed, as well as future directions and problems to be overcome for the efficient and beneficial use of magnetic carriers in clinical practice. More information about the field and an extensive bibliography is available at "."

Monosize microbeads based on polystyrene and their modified forms for some selected medical and biological applications

Polymeric particles are produced by different polymerization techniques. Phase inversion (dispersion) polymerization is one of the recent techniques to obtain monosize polymeric microbeads in the size range of 1-50 microns. The size and monodispersity of these microbeads can be adjusted by using several solvent systems (e.g., alcohol-water mixtures) with different polarities and by changing the type and amount of monomer, initiator and stabilizer. Surfaces of these microbeads can be further modified by different techniques including coating with different copolymers. Monosize polymeric microbeads are widely used in medical and biological applications as carriers, such as in immunoassays and cell separation, in site-specific drug delivery systems, in nuclear medicine for diagnostic imaging, in studying the phagocytic process, in affinity separation of biological entities, etc. Here, some important aspects of the production of monosize microbeads based on polystyrene and their modified forms are briefly discussed, and some selected medical and biological applications are summarized.

Application of magnetic beads in bioassays

This review outlines the possibilities of magnetic separation techniques and the application of magnetic beads in bioassays. By linking monoclonal antibodies or DNA to magnetic beads, or by using magnetic beads coated with streptavidin, a specific interaction with the corresponding target is ensured. By means of an external magnet, the recovery of material for further studies is greatly simplified. Magnetic beads have proven valuable in cell separations, for example, removal of tumor cells from bone marrow and isolation of lymphoid cells from peripheral blood, and for the isolation, identification and genetic analysis of specific nucleic acid sequences (DNA or RNA) and for isolation of DNA binding proteins. In addition, some of these techniques have also proven to be useful in the detection of specific nucleic acids from viruses or bacteria.

Magnetic fields, brain and immunity: effect on humoral and cell-mediated immune responses

Magnetic fields (MF) can influence biological systems in a wide range of animal species and humans. We report here on the influence of static MF, locally applied to the brain area, on immune system performances in the rat. In the first series of experiments two AKMA micromagnets (M) with the influx density of 600 Gauss were bilaterally implanted (with "N" polarity facing the cranial bones) and fixed to the skull posterior to the fronto-parietal suture (parietal brain exposure). Rats implanted with iron beads (I) and sham-operated (SO) rats served as controls. Animals were exposed to MF or I during different periods of time before and after immunization with several soluble or cellular antigens. We report here on the in vivo immunoregulating effects of centrally applied MF on plaque-forming cell (PFC) response, local hypersensitivity skin reactions and experimental allergic encephalomyelitis. The selective influence of MF applied to different brain regions on PFC response was evaluated, as well. For this purpose, two M were bilaterally implanted in the area of (a) frontal, (b) parietal and (c) occipital brain regions. Rats were under the influence of MF for 20 days before and 4 days after immunization with sheep red blood cells. Groups of nonimmunized rats were exposed for 14, 24 and 34 days to parietally implanted M or I, and the number of peripheral blood CD4+ and CD8+ cells determined by mouse anti-rat W3/25 and MRC OX 8 monoclonal antibodies. The results show an overall in vivo immunopotentiation of humoral and cell-mediated immune responses in rats exposed to MF. Furthermore, these immunomodulating effects of centrally applied MF depend on at least two basic parameters, time of exposure and brain region exposed. The highest immune performance was obtained after exposure of the occipital brain region for a total period of 24 days. The results provide further evidence of the complex interrelationship between the environment, the central nervous system and the immune system.

Iron oxide nanoparticles for use as an MRI contrast agent: pharmacokinetics and metabolism

The pharmacokinetics and metabolism of a new preparation of superparamagnetic iron oxide nanoparticles were evaluated by 59Fe radiotracer studies and histologic examination of mice liver and spleen tissues (light and transmission electron microscopy). In the first 30 min following IV injection of the product half of the dose injected remains in the blood, the other part being sequestered mainly by the mononuclear phagocyte system (MPS). In the first five days following IV administration of the nanoparticles, early metabolization of the iron oxide cores occurs, revealed by modification of their aspect in the lysosomes of Kupffer cells and macrophages of the splenic red pulp. The incorporation of 59Fe is then observed in RBC of the mice. These results are discussed in relation with the physicochemical properties of this new preparation of nanoparticles, and compared with current pharmacokinetic data concerning injectable particle systems.

Superparamagnetic iron oxide nanoparticles as a liver MRI contrast agent: contribution of microencapsulation to improved biodistribution

We have developed a new method of synthetizing superparamagnetic iron oxide nanoparticles, consisting in the modifications of Molday's method, which ensures high relaxivity (2.4 10(5) s-1.M-1.L), good chemical stability, singular biodistribution and a considerable safety margin. The ED (Efficace Dose) to LD50 ratio is 1/2400 instead of 1/50 for Gd-DTPA. In order to develop a magnetite-delivery system to the liver we have incorporated the nanoparticles into biodegradable synthetic microcapsules. Encapsulated 59Fe oxide nanoparticles are injected into rats; in these conditions the sequestration is 9-fold greater in liver and 6 and 5 times lower in blood and carcase, respectively. This modification of the biodistribution enables the use of magnetite containing microcapsules at only 0.3 mg/kg iron to obtain an improved contrast in liver.

Paramagnetic microspheres as immunological markers for light and electron microscopy

M-450 Dynabeads are magnetizable polystyrene microspheres 4.5 micron in diameter to which antibodies of IgM isotype can be physically adsorbed. Antibody-coated Dynabeads can be used to label cell surfaces and then to separate the rosetted cells by application of an external magnetic field. We demonstrate here, using cell lines K562 and U937 and the previously undescribed monoclonal antibodies CH-F42 and CH-E25, that Dynabeads can also be used to label cells at the ultrastructural level. Dynabeads can therefore provide a useful bridge between light and electron microscopy. The preservation of specific rosettes at the ultrastructural level without the formation of artifactual aggregates requires rapid but gentle fixation in dilute suspension. We have achieved this by fixing briefly in a large volume of buffered glutaraldehyde followed by neutralization of excess glutaraldehyde with ethanolamine.

Cell sedimentation with gravity activation

Murine monoclonal antibody T101 has been coupled to thinly polymer-coated heavy alloy particles (LaMn2Ge2). These conjugates are coupled to cultured cells of the human T-cell leukemia line RPMI 8402 (T8402). The sedimentation velocities of cells, of particles, and of cells with particles attached are measured. After determining the mean radii of cells, of particles, and of cells with particles attached, one may compute a mean number of 33 particles attached to a cell. Independently one may compute a mean number of 144 particles/cell for surface saturation. The Appendix handles the underlying theory in three parts: number of particles/cell, saturation number of particles/cell, and resolution for gravity activation. Regarding the latter, cell radii from 4 to 10 microns and particle radii from 0.01 to 1 micron are considered.

Purification of transiently transfected cells by magnetic affinity cell sorting

A method was developed to purify transiently transfected HeLa cells or African green monkey kidney CV-1 cells by magnetic affinity cell sorting. Monolayer cultures were transfected with mammalian expression vectors coding for either of two novel cell surface antigens, the Tac subunit of the human IL-2 receptor or vesicular stomatitis virus G protein. During the transient expression phase, cell populations were placed in suspension and mixed with monoclonal-antibody-coated magnetic particles in the presence of a sorting solution designed to minimize nonspecific cell/cell and cell/particle interactions. Transfected cells expressing the vector-encoded cell surface antigen were then isolated by application of a magnetic field. Reconstruction experiments indicated that IL-2 receptor-positive cells were bound about 100-fold more efficiently than receptor-negative cells. In transient transfection experiments, populations of greater than 90% antigen-positive cells were reproducibly obtained.

Gravitational and magnetic activation methods applied to cultured cells (LK 35.2)

Monoclonal antibody 10.2-16 is directed toward the mouse class II major histocompatibility complex gene product 1-Ak expressed on the cell line LK35.2. Instead of activating cells by fluorophor we used (acrylamide-coated) heavy and magnetic microspheres of 0.6 micron in radius. These microspheres are chemically coupled (carbodiimide method) with the antibody toward the surface antigen. The cells are observed through a microscope with horizontal alignment, as they sediment in a (temperature controlled) tube with square cross-section. Stokes Law allows the determination of the density of cells (first alone) using viscosity and density of Dulbecco's modified Eagle's Medium together with the observed mean sedimentation velocity (66 microns/min) and a mean diameter of 10 microns. We found a density of 1.0558 +/- 0.0028 g/cm3 at 10 degrees C. Independently, thinly coated, heavy (and magnetizable) microspheres with the cited antibody are attached to cells and observed likewise. The increased sedimentation velocity permits us to show that the cells were fully covered with microspheres (290 per cell). A magnetic field gradient opposing gravity moved these cells against gravity with two different mean velocities, 340 microns/min and 850 microns/min. The higher velocity resulted in 290 particles per cell, the lower one in 130 particles per cell. The limits for the expansion of this method to smaller particle sizes (down to 10 nm) are evaluated.

Magnetic carbohydrate nanoparticles for affinity cell separation

Magnetically responsive nanoparticles were prepared from enzymatically hydrolysed starch and magnetite. Two different monoclonal antibodies were covalently coupled to the particles. The antibody-coupled particles were in the size range of 100-300 nm and had an iron content of about 60%. Using 100 micrograms of magnetic particles (coupled with monoclonal mouse anti-rat Ig kappa light chain antibody) a very high depletion of surface Ig positive cells (mostly B-cells) from one million rat peripheral blood mononuclear cells could be achieved. The separation efficiency was evaluated by flow cytofluorometric analysis. This technique permits the detection of a small number of surface Ig positive cells among 10,000 negative cells.

Rapid, high-yield purification of cell surface membrane using colloidal magnetite coated with polyvinylamine: sedimentation versus magnetic isolation

A new technique for the magnetic isolation of external plasma membrane from Dictyostelium discoideum is described and compared to a previously published procedure employing sedimentation of silica-coated plasma membrane. The magnetic isolation technique involves coating intact cells with a polyvinylamine-magnetite colloid and overcoating with polyacrylate to form a dense pellicle. The magnetite pellicle totally coated the cells and was not internalized. Coated cells were lysed and membrane fragments retrieved from the cell homogenate using a diverging field electromagnet. The membrane obtained in such a manner was analyzed for marker enzyme activity and cell surface label. The plasma membrane was obtained in high yield (42%) with an average purification of 8-fold. The polyvinylamine-magnetite pellicle shielded the external plasma membrane face to proteolysis by papain and pronase. It also acted as a barrier to alpha-methylmannoside in concanavalin A-carbohydrate competition studies.

Magnetic separation techniques: their application to medicine

Whilst separation techniques relying on gravitational forces have become relatively sophisticated in their application to biology the same is not true for magnetic separation procedures. The use of the latter has been limited to the few cells which contain paramagnetic iron. However with the development of several different types of magnetic particles and selective delivery system (e.g. monoclonal antibodies) the use of magnetic separation techniques is growing rapidly. This review describes the different types of particles currently available, the magnetic separation technique applied to the different magnetic compounds and illustrates major uses to which magnetic separation procedures are currently applied in the area of biology and medicine.

Ferritin conjugates as specific magnetic labels. Implications for cell separation

Concanavalin A coupled to the naturally occurring iron storage protein ferritin is used to label rat erythrocytes and increase the cells' magnetic susceptibility. Labeled cells are introduced into a chamber containing spherical iron particles and the chamber is placed in a uniform 5.2 kG (gauss) magnetic field. The trajectory of cells in the inhomogeneous magnetic field around the iron particles and the polar distributions of cells bound to the iron particles compare well with the theoretical predictions for high gradient magnetic systems. On the basis of these findings we suggest that ferritin conjugated ligands can be used for selective magnetic separation of labeled cells.

Separation of cells labeled with immunospecific iron dextran microspheres using high gradient magnetic chromatography

Immunospecific magnetic microspheres, consisting of ferromagnetic iron dextran conjugated to Protein A, were used to specifically label red blood cells (RBC) for cell separation studies using high gradient magnetic chromatography ( HGMC ). When 10(7)-10(8) RBC labeled with Protein A-iron dextran microspheres were applied to a column containing 30 mg stainless steel wire placed in a 7.5 kilogauss magnetic field, 96 +/- 2% of the cells were retained in the column. These cells could be eluted by removing the magnetic field and mechanically agitating the column. The retention of labeled cells by HGMC was shown to be dependent on the applied magnetic field and the amount of wire packed into the column. HGMC in conjunction with cell labeling with immunospecific iron dextran microspheres have useful applications for the separation of specific cell types.

Cell separation: a review

This review summarizes currently available techniques for cell separation. Techniques that exploit differences in physical properties of cells are widely used but have a number of limitations. Those that are based on differences in surface properties may more readily permit reproducible separation of a functionally homogeneous population of cells. Unfortunately very few techniques achieve separation of cells on the basis of differences in their functional characteristics.

Agarose-polyaldehyde microsphere beads: synthesis and biomedical applications. Cell labeling, cell separation, affinity chromatography, and hemoperfusion

Polyaldehyde microspheres, polyglutaraldehyde (PGL), and polyacrolein (PA) were synthesized by polymerizing glutaraldehyde and acrolein in the presence of an appropriate surfactant. The microspheres with average diameter of 0.2 micron were used for the specific labeling of human red blood cells (RBC) and mouse lymphocytes. The "naked" microspheres were encapsulated with agarose and formed agarose-polyaldehyde microsphere beads in sizes ranging from 50 microns up to 1 cm. The encapsulated beads, with diameters ranging from 50 to 150 microns were used as insoluble adsorbents for affinity purification of antibodies. Beads with diameters varied from 150 to 250 microns were used for cell fractionation purposes (mouse B splenocytes from T splenocytes). Uniform beads of 1 mm diameter were designed for hemoperfusion purposes. As a model, the removal in vitro of anti-BSA from immunized goat whole blood was studied.

Ferritin as a label for high-gradient magnetic separation

In three model systems, particles the size of cells or smaller have been surface labeled with ferritin to make them slightly paramagnetic, by virtue of the iron in the ferritin. In each case it was possible to show that labeled particles could be magnetically removed from a flowing suspension by the high-gradient magnetic separation (HGMS) technique. The first system of particles consisted of small (1 micron) carboxylate-modified latex spheres to which ferritin was covalently bound to create stable paramagnetic particles analogous to a ferritin-labeled subcellular membrane preparation. In the second system polyacrylamide beads that more closely approximated whole cells in size (5-50 microns) were labeled with immunoferritin. The third system was a biomembrane preparation: erythrocyte ghosts labeled with a ferritin-lectin conjugate. A field of 7 T (tesla) (70 kG) was used in each case, along with buffer flow rates through the HGMS column in the range 0.1-1.0 ml/min.

Immunospecific ferromagnetic iron-dextran reagents for the labeling and magnetic separation of cells

Ferromagnetic iron dextran particles were prepared by reacting a mixture of ferrous chloride and ferric chloride with dextran polymers under alkaline conditions. Particles purified by gel filtration chromatography were in the size range of 30-40 nm, had an electron dense core of about 15 nm, were stable against aggregation in physiological buffer, showed little non-specific binding to cells and had a magnetic moment. Protein A from Staphylococcus aureus was covalently coupled to periodate-oxidized ferromagnetic iron-dextran particles. These conjugates were used to indirectly label antigen sites on human red blood cells and thymocytes for visualization by scanning and transmission electron microscopy. Cells labeled with these immunospecific ferromagnetic particles are were quantitatively retained by a simple permanent magnet and could be separated from unlabeled cells. Applications of these novel reagents in the separation of cells, cell membranes and receptors in drug targeting studies are discussed.

Immunoselection of oligodendrocytes by magnetic beads. I. Determination of antibody coupling parameters and cell binding conditions

Oligodendrocytes from early postnatal mouse cerebellum were isolated using polyacrylamide-coated magnetic beads carrying monoclonal antibody to 04 cell surface antigen. Oligodendrocytes were enriched to a purity of 91 +/- 4% starting from a mixed cell population containing approximately 1.5% antigen-positive oligodendrocytes. Viability of 04 antigen-positive oligodendrocytes was approximately 90% as judged by exclusion of trypan blue. Oligodendrocytes were recovered after detachment from the beads with a yield of 19 +/- 6% and after collection by centrifugation onto glass coverslips with yields of approximately 6% of all 04 antigen-positive cells. The final cell yield of oligodendrocytes is approximately 8 x 10(5) cells/gram fresh cerebellar tissue.

Specific cell binding using staphylococcal protein A magnetic microspheres

Protein A, a protein derived from Staphylococcus aureus, was incorporated into the matrix of magnetic albumin microspheres. Because staphylococcal protein A binds most subclasses of immunoglobulin G through their Fc portions, immunoglobulin may be rapidly bound to microspheres without chemical coupling agents or a diamino-heptane spacer group. Microspheres so prepared bind specifically to a given cell type when incubated in vitro with a heterogeneous cell population. The use of these microspheres as a drug carrier capable of cellular specificity as well as their ability to isolate homogeneous cell populations rapidly is discussed.