Determination of the magnetic susceptibility of labeled particles by video imaging

A novel magnetophoretic-based device for magnetometry and separation of single magnetic particles and magnetized cells

The use of magnetic micro- and nanoparticles in medicine and biology is expanding. One important example is the transport of magnetic microparticles and magnetized cells in lab-on-a-chip systems. The magnetic susceptibility of the particles is a key factor in determining their response to the externally applied magnetic field. Typically, to measure this parameter, their magnetophoretic mobility is studied. However, the particle tracking system for accurately determining the traveled distance in a certain time may be too complicated. Here, we introduce a lithographically fabricated chip composed of an array of thin magnetic micro-disks for evaluating the magnetic susceptibility of numerous individual magnetic particles simultaneously. The proposed novel magnetometer works based on the phase change in the trajectory of microparticles circulating around the disks in a rotating in-plane magnetic field. We explain that the easily detectable transition between the "phase-locked" and the "phase-slipping" regimes and the frequency at which it happens are appropriate parameters for measuring the magnetic susceptibility of the magnetic particles at the single-particle level. We show that this high-throughput (i.e., ∼ten thousand particles on a 1 cm² area) single-particle magnetometry method has various crucial applications, including i) magnetic characterization of magnetic beads as well as magnetically labeled living cells, ii) determining the magnetization rate of the cells taking up magnetic nanoparticles with respect to time, iii) evaluating the rate of degradation of magnetic nanoparticles in cells over time, iv) detecting the number of target cells in a sample, and v) separating particles based on their size and magnetic susceptibility.

Magnetic characterization of paramagnetic reagents by particle tracking velocimetry

Magnetic particle characterization determines the quality of magnetic particles and is of great importance in particle technology, drug delivery, cell separation, in vivo diagnostics, and other biomedical applications. The quality of the sample depends on the particle size, intrinsic magnetic properties of the particles, and the uniformity of these properties. A commercial particle tracking velocimeter was used to record and capture dark field images of particle trajectories in an applied isodynamic magnetic field. The calibrated particle size, magnetophoretic mobility, and additional image data were collected for each magnetic bead imaged. Using twenty-one different de-identified calibration beads and transmission electron microscopy to validate the vendor-reported particle size enabled the estimation of intrinsic magnetic properties, namely, apparent magnetic susceptibility and saturation magnetization, of individual paramagnetic particles. The distributions of volume magnetic susceptibility based on the magnetophoretic mobility and size of the particle for different magnetic beads were determined and displayed as two-parameter distributions. The measured apparent susceptibility and saturation magnetization were found to be directly proportional to the percentage of iron oxide in the reagent particles.

Characterization of Simple and Double Yeast Cells Using Dielectrophoretic Force Measurement

Dielectrophoretic force is an electric force experienced by particles subjected to non-uniform electric fields. In recent years, plenty of dielectrophoretic force (DEP) applications have been developed. Most of these works have been centered on particle positioning and manipulation. DEP particle characterization has been left in the background. Likewise, these characterizations have studied the electric properties of particles from a qualitative point of view. This article focuses on the quantitative measurement of cells’ dielectric force, specifically yeast cells. The measures are obtained as the results of a theoretical model and an instrumental method, both of which are developed and described in the present article, based on a dielectrophoretic chamber made of two V-shaped placed electrodes. In this study, 845 cells were measured. For each one, six speeds were taken at different points in its trajectory. Furthermore, the chamber design is repeatable, and this was the first time that measurements of dielectrophoretic force and cell velocity for double yeast cells were accomplished. To validate the results obtained in the present research, the results have been compared with the dielectric properties of yeast cells collected in the pre-existing literature.

Magnetic microbead transport during resistive pulse sensing

Tunable resistive pulse sensing (TRPS) experiments have been used to quantitatively study the motion of 1 μm superparamagnetic beads in a variable magnetic field. Closed-form theory has been developed to interpret the experiments, incorporating six particle transport mechanisms which depend on particle position in and near a conical pore. For our experiments, calculations indicate that pressure-driven flow dominates electrophoresis and magnetism by a factor of ∼100 in the narrowest part of the pore, but that magnetic force should dominate further than ∼1 mm from the membrane. As expected, the observed resistive pulse rate falls as the magnet is moved closer to the pore, while the increase in pulse duration suggests that trajectories in the half space adjacent to the pore opening are important. Aggregation was not observed, consistent with the high hydrodynamic shear near the pore constriction and the high magnetization of aggregates. The theoretical approach is also used to calculate the relative importance of transport mechanisms over a range of geometries and experimental conditions extending well beyond our own experiments. TRPS is emerging as a versatile form of resistive pulse sensing, while magnetic beads are widely used in biotechnology and sensing applications.

Resistive pulse sensing of magnetic beads and supraparticle structures using tunable pores

Tunable pores (TPs) have been used for resistive pulse sensing of 1 μm superparamagnetic beads, both dispersed and within a magnetic field. Upon application of this field, magnetic supraparticle structures (SPSs) were observed. Onset of aggregation was most effectively indicated by an increase in the mean event magnitude, with data collected using an automated thresholding method. Simulations enabled discrimination between resistive pulses caused by dimers and individual particles. Distinct but time-correlated peaks were often observed, suggesting that SPSs became separated in pressure-driven flow focused at the pore constriction. The distinct properties of magnetophoretic and pressure-driven transport mechanisms can explain variations in the event rate when particles move through an asymmetric pore in either direction, with or without a magnetic field applied. Use of TPs for resistive pulse sensing holds potential for efficient, versatile analysis and measurement of nano- and microparticles, while magnetic beads and particle aggregation play important roles in many prospective biosensing applications.

Application of magnetic particle tracking velocimetry to quadrupole magnetic sorting of porcine pancreatic islets

Magnetic isolation is a promising method for separating and concentrating pancreatic islets of Langerhans for transplantation in Type 1 diabetes patients. We are developing a continuous magnetic islet sorter to overcome the restrictions of current purification methods that result in limited yield and viability. In Quadrupole Magnetic Sorting (QMS) islets are magnetized by infusing superparamagnetic microbeads into islets' vasculature via arteries that serve the pancreas. The performance of the islet sorter depends on the resulting speed of the islets in an applied magnetic field, a property known as magnetophoretic mobility. Essential to the design and successful operation of the QMS is a method to measure the magnetophoretic mobilities of magnetically infused islets. We have adapted a Magnetic Particle Tracking Velocimeter (MPTV) to measure the magnetophoretic mobility of particles up to 1,000 µm in diameter. Velocity measurements are performed in a well-characterized uniform magnetic energy gradient using video imaging followed by analysis of the video images with a computer algorithm that produces a histogram of absolute mobilities. MPTV was validated using magnetic agarose beads serving as islet surrogates and subjecting them to QMS. Mobility distributions of labeled porcine islets indicated that magnetized islets have sufficient mobility to be captured by the proposed sorting method, with this result confirmed in test isolations of magnetized islets.

Multitarget magnetic activated cell sorter

Magnetic selection allows high-throughput sorting of target cells based on surface markers, and it is extensively used in biotechnology for a wide range of applications from in vitro diagnostics to cell-based therapies. However, existing methods can only perform separation based on a single parameter (i.e., the presence or absence of magnetization), and therefore, the simultaneous sorting of multiple targets at high levels of purity, recovery, and throughput remains a challenge. In this work, we present an alternative system, the multitarget magnetic activated cell sorter (MT-MACS), which makes use of microfluidics technology to achieve simultaneous spatially-addressable sorting of multiple target cell types in a continuous-flow manner. We used the MT-MACS device to purify 2 types of target cells, which had been labeled via target-specific affinity reagents with 2 different magnetic tags with distinct saturation magnetization and size. The device was engineered so that the combined effects of the hydrodynamic force produced from the laminar flow and the magnetophoretic force produced from patterned ferromagnetic structures within the microchannel result in the selective purification of the differentially labeled target cells into multiple independent outlets. We demonstrate here the capability to simultaneously sort multiple magnetic tags with >90% purity and >5,000-fold enrichment and multiple bacterial cell types with >90% purity and >500-fold enrichment at a throughput of 10(9) cells per hour.

Effects of a high-gradient magnetic field on the migratory behavior of primary crystal silicon in hypereutectic Al-Si alloy

The migration of primary Si grains during the solidification of Al-18 wt%Si alloy under a high-gradient magnetic field has been investigated experimentally. It was found that under a gradient magnetic field, the primary Si grains migrated toward one end of the specimen, forming a Si-rich layer, and the thickness of the Si-rich layer increased with increasing magnetic flux density. No movement of Si grains was apparent under a magnetic field below 2.3 T. For magnetic fields above 6.6 T, however, the thickness of the Si-rich layer was almost constant. It was shown that the static field also played a role in impeding the movement of the grains. The primary Si grains were refined in the Si layer, even though the primary silicon grains were very dense. The effect of the magnetic flux density on the migratory behavior is discussed.

Cell tracking velocimetry as a tool for defining saturation binding of magnetically conjugated antibodies

BACKGROUND

Continuous flow immunomagnetic separation is an attractive alternative to current batch mode immunomagnetic separation methods because it is capable of high sorting speeds at mild cell conditions, and grants the operator better control of separation process. The control of the separation is dependent on knowledge of the amount of magnetic label attached to the cell (magnetic labeling intensity), however. Determination of the magnetic labeling is accomplished by measuring cell magnetophoretic mobility using a newly developed technique of Cell Tracking Velocimetry (CTV).

METHODS

Flow cytometry was used to define the antibody binding characteristics of a fluorescently tagged primary antibody. Subsequently, CTV was used to measure antibody-binding characteristics of a magnetically tagged secondary antibody.

RESULTS

The results of this study show that CTV is capable of providing valuable information concerning the cell labeling by magnetically tagged antibodies. It was demonstrated that the magnetically conjugated antibody binding curve exhibits the same exponential increase to saturation characteristics as that seen with the fluorescently tagged antibody. Further, it was shown that the intensity of the secondary magnetic labeling is directly proportional to the intensity of the primary fluorescent label.

CONCLUSIONS

CTV is an accurate tool for evaluation of magnetically conjugated antibodies. The ability to determine the intensity of magnetic labeling is necessary for the development of continuous flow immunomagnetic separations based on cell magnetophoresis.

Image analysis of bioparticles accumulation and diamagnetic alignment in high-gradient magnetic field

Magnetic properties of biological particles are measured in high-gradient magnetic separation (HGMS) analysis, revealing the concentrating process of nucleoprotein particles, ferritin, red blood cells, and eggs. A magnetic force acting on micrometer and submicrometer biological particles having diamagnetic or paramagnetic susceptibility with respect to the solution causes their movement and accumulation in gradient magnetic fields dependent on the values of the magnetic moments. The methods developed enable us to obtain the magnetic moments values of single particles and their assembly directly from magnetic separation and image analyses without assuming the detection of sizes. Our precision methods for the measurement of the capture traveling (magnetic diffusion) time and the accumulation (magnetic sedimentation) radius in HGMS show that it is really possible to determine the weak dia- or paramagnetic shifts of magnetic susceptibility up to 0.7x10(-10) (SI units). HGMS analysis of the concentrating process of nucleoprotein granules (microcells, DNA granules, or nucleosome core particles) with polarization microscopy reveals phase transitions for DNA in granules, and separation accumulation of particles enables the determination of the diamagnetic susceptibility and anisotropy properties. Magnetic concentration effects always occur in living systems because micrometer-located gradient magnetic fields inside an organism are strong enough to cause drifts of cellular complexes and organelles of micrometer and submicrometer sizes. We report the appearance of superparamagnetic contamination inside developing shrimp eggs. In the developing shrimps eggs, ferritin aggregates are observed under weak gradient magnetic fields and diaparaferromagnetic changes are detected. A significant interruption of egg development is revealed in such fields.

Establishment and implications of a characterization method for magnetic nanoparticle using cell tracking velocimetry and magnetic susceptibility modified solutions

Magnetic micro and nanoparticles conjugated to affinity labels have become a significant, commercial reagent. It has been demonstrated that the performance of cell separation systems using magnetic labels is a function of the magnitude of the magnetic force that can be generated through labeling. This magnetic force is proportional to the number of magnetic particles bound to the cell, the magnetic energy gradient, and the particle-field interaction parameter. This particle-field interaction parameter, which is the product of the relative volumetric, magnetic susceptibility and the volume of the micro or nanoparticle, is a fundamental parameter which can be used to characterize the magnetic particles. An experimental technique is presented which measures the volumetric magnetic susceptibility of particles through the use of susceptibility modified solutions and an experimental instrument, Cell Tracking Velocimetry, CTV. Experimental studies were conducted on polystyrene microspheres alone and those bound to four different magnetic nanoparticles. The experimentally determined values of the magnetic susceptibility of the polystyrene microspheres are consistent with values found from literature. Consequently, magnetic susceptibility measurements of these polystyrene microspheres bound with the magnetic nanoparticles combined with particle size measurements using commercial dynamic light scattering instrument allowed estimates of the particle-field interaction parameter to be made for four commercial, magnetic nanoparticles. The value found for MACS beads is close to what is reported from an independent study. The values for MACS beads and Imag beads are found to agree with what is observed from experiments. Finally, an experimental demonstration of the impact that differences in this field interaction parameter has on the labeling of human lymphocytes is presented.

Control of Magnetophoretic Mobility by Susceptibility-Modified Solutions As Evaluated by Cell Tracking Velocimetry and Continuous Magnetic Sorting

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-microm diameter and nonmagnetic polystyrene microspheres of 6.99-microm diameter suspended in solutions of paramagnetic 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 viscosity agrees well with the theoretical prediction, differing only by approximately 11%. Further, a 26% 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.

Migration Analysis of Micro-Particles in Liquids Using Microscopically Designed External Fields

The recent development of new migration methods of micro-particles in liquids using various external fields is reviewed. The combination of a laser scattering force and a photothermal effect produced photothermal-conversion laser-photophoresis. A dielectric field generated in a planer or a capillary quadrupole electrode realized dielectrophoresis. Using a micrometer-scaled magnetic field gradient, the "Magnetophoretic velocimetry" of micro-particles was invented. Furthermore, the Lorentz force generated by combining an electric field and a magnetic field was utilized for electromagnetophoresis. These new methods were overlooked and the advantages in analytical use were discussed.

Mobility measurements of immunomagnetically labeled cells allow quantitation of secondary antibody binding amplification

Magnetic cell separation methods commonly utilize paramagnetic materials conjugated to antibodies that target specific cell surface molecules. The amount of magnetic material bound to a cell is directly proportional to the magnetophoretic mobility of that cell. A mathematical model has been developed which characterizes the fundamental parameters controlling the amount of magnetic material bound, and thus, the magnetophoretic mobility of an immunomagnetically labeled cell. In characterization of the paramagnetic labeling, one of the parameters of interest is the increase in magnetophoretic mobility due to the secondary antibody binding to multiple epitopes on the primary antibody, referred to as the "secondary antibody binding amplification," Psi. Secondary antibody-binding amplification has been investigated and quantitated by comparing the mobilities of lymphocytes directly labeled with anti-CD4 MACS (Miltenyi Biotec, Auburn, CA) magnetic nanoparticle antibody with the mobilities of lymphocytes from the same sample labeled with two different indirect antibody-labeling schemes. Each indirect labeling scheme incorporated a primary mouse anti-CD4 FITC antibody that provides both FITC and mouse-specific binding sites for two different secondary antibody-magnetic nanoparticle conjugates: either anti-FITC MACS magnetic nanoparticle antibody or anti-mouse MACS magnetic nanoparticle antibody. The magnetophoretic mobilities of the immunomagnetically labeled cells were obtained using Cell Tracking Velocimetry (CTV). The results indicate that an average of 3.4 anti-FITC MACS magnetic nanoparticle antibodies bind to each primary CD4 FITC antibody, Psi(1,2f) = 3.4 +/- 0.33, and that approximately one, Psi(1,2m) = 0.98 +/- 0.081, anti-mouse MACS magnetic nanoparticle antibody binds to each primary mouse CD4 FITC antibody on a CD4 positive lymphocyte. These results have provided a better understanding of the antibody-binding mechanisms used in paramagnetic cell labeling for magnetic cell separation.

Multistage magnetic and electrophoretic extraction of cells, particles and macromolecules

Improved techniques for separating cells, particles, and macromolecules (proteins) are increasingly important to biotechnology because separation is frequently the limiting factor for many biological processes. Manufacturers of new enzymes and pharmaceutical products require improved methods for recovering intact cells and intracellular products. Similarly isolation, purification, and concentration of many biomolecules produced in fermentation processes is extremely important. Often such downstream processing contributes a large portion of the product cost. In conventional methods like centrifugation and even modern methods like chromatography, scale-up problems are enormous, making them uneconomical and prohibitively expensive unless the product is of very high value. Therefore there has been a need for efficient and economical alternative approaches to bioseparation processes to eliminate, reduce, or facilitate solids handling. Magnetic and electric field assisted separations may hold considerable potential for providing a future major improvement in bioseparation technology. In the present review the merits and demerits of the existing methods are discussed. We present mainly our own research on the development of unified multistage extraction processes that are versatile enough to handle cells and particles as well as macromolecules as described below. We describe multistage methods, namely ADSEP (Advanced Separator), MAGSEP (Magnetic Separator), and ELECSEP (Electrophoretic Separator), for quantitatively separating cells, particles, and solutes by using magnetically and electrophoretically assisted extraction processes. To the best of our knowledge, multistage magnetic and electrophoretic separations have not been reported in the earlier literature. The theoretical underpinnings of these separations are crucial to their success and to the identification of their advantages over other separation processes in particular applications. Hence mathematical modeling is stressed here, presenting our own models while also reviewing models reported in the literature. We also present suggestions for future work while analyzing the scale-up and economic aspects of these extraction processes. Commercial uses of the magnetic and electrophoretic processes, having both ground- and space-based research elements, also are presented in this review.

Magnetophoretic mobilities correlate to antibody binding capacities

METHODS

A methodology and a mathematical theory have been developed, which allow quantitation of the expression levels of cellular surface antigens using immunomagnetic labels and cell tracking velocimetry (CTV) technology.

RESULTS

Quantum Simply Cellular (QSC) microbeads were immunomagnetically labeled with anti-CD2 fluorescein isothiocyanate (FITC) antibodies and anti-FITC MACS paramagnetic nanoparticles. Magnetophoretic mobility has been defined as the magnetically induced velocity of the labeled cell or microbead divided by the magnetophoretic driving force, proportional to the magnetic energy density gradient.

DISCUSSION

Using computer imaging and processing technology, the mobility measurements were accomplished by microscopically recording and calculating the velocity of immunomagnetically labeled QSC microbeads in a nearly constant magnetic energy gradient. A calibration curve correlating the measured magnetophoretic mobility of the immunomagnetically labeled microbeads to their antibody binding capacities (ABC) has been obtained.

CONCLUSION

The results, in agreement with theory, indicate a linear relationship between magnetophoretic mobility and ABC for microbeads with less than 30,000 ABC. The mathematical relationships and QSC standardization curve obtained allow determination of the number of surface antigens on similarly immunomagnetically labeled cells.

The use of magnetite-doped polymeric microspheres in calibrating cell tracking velocimetry

Continuous magnetic separation, in which there is no accumulation of mass in the system, is an inherently dynamic process, requiring advanced knowledge of the separable species for optimal instrument operation. By determining cell magnetization in a well-defined field, we may predict the cell trajectory behavior in the well-characterized field environments of our continuous separators. Magnetization is determined by tracking the migration of particles with a technique known as cell tracking velocimetry (CTV). The validation of CTV requires calibration against an external standard. Furthermore, such a standard, devoid of the variations and instabilities of biological systems, is needed to reference the method against day-to-day shifts or trends. To this end, a method of synthesizing monodisperse, magnetite-doped polymeric microspheres has been developed. Five sets of microspheres differing in their content of magnetite, and each of approximately 2.7 microm diameter, are investigated. An average gradient of 0.18 T/mm induces magnetic microsphere velocities ranging from 0.45 to 420 microns/s in the CTV device. The velocities enable calculation of the microsphere magnetization. Magnetometer measurements permit the determination of magnetization at a flux density comparable to that of the CTV magnet's analysis region, 1.57 T. A comparison of the results of the CTV and magnetometer measurements shows good agreement.

Quantification of cellular properties from external fields and resulting induced velocity: cellular hydrodynamic diameter

An experimental technique is discussed in which the size distribution of a population of cells is determined by calculating each cell's settling velocity. The settling velocity is determined from microscopically obtained images which were recorded on SVHS tape. These images are then computer imaged and processed, and the cell's location and velocity are determined using a computer algorithm referred to as cell tracking velocimetry (CTV). Experimental data is presented comparing the distribution of human lymphocytes and a human breast cancer cell line, MCF-7, determined using a Coulter counter and the CTV approach.

Biocompatible magnetic polymer carriers for in vivo radionuclide delivery

The magnetic guidance of antiplastic and antibacterial agents as well as x-ray and MRI contrast substances in vivo by means of magnetic particles has been attempted repeatedly during the last 2 decades with more or less success. When using microparticles, the circulation time in the blood, the biodistribution, and to a greater or lesser extent, the specific targeting are determined by the uniformity of size, chemical composition, surface modification, and the electric surface charge. The electrophoretic mobility is an important parameter for the prediction of the usefulness of the prepared particle, modified by chemical and biological molecules. For its success, radionuclide therapy depends on the critical relationship between the amount of radioactive isotopes in the target tissue and in critical normal tissue. Because the implementation of radioimmunotherapy for the treatment of cancer has proven to be considerably more difficult than initially anticipated, we propose the use of magnetic nanospheres for the well directed delivery of radionuclides to a tumor after the intravenous administration of the biodegradable colloidal suspension.

Lymphocyte fractionation using immunomagnetic colloid and a dipole magnet flow cell sorter

The relationship between cell function and surface marker expression is a subject of active investigation in biology and medicine. These investigations require separating cells of a homogeneous subset into multiple fractions of varying marker expression. We have developed a novel cell sorter, the dipole magnet flow sorter (DMFS), which separates selected T lymphocyte subpopulations, targeted by immunomagnetic colloid, into multiple fractions according to cell surface marker expression, as determined by flow cytometry. A narrow stream of cells is introduced into a sheath of carrier fluid in a rectangular channel while subjected to a perpendicular magnetic force. The special design of the pole pieces ensures a constant magnetic force acting on the magnetically labeled cells in the separation area. Cells are spread across the flow in relation to their magnetophoretic mobility. Separation is achieved by control of the positions of the effluent stream boundaries, which separate fluid volumes with cells of different magnetophoretic mobility. CD4 and CD8 T lymphocytes labeled with primary antibody-fluorescein isothiocyanate (FITC) conjugate and anti-FITC-magnetic colloid are the chosen cell systems. Flow cytometry analysis shows that, for CD4 cells, a three-fold increase in total marker number per cell is observed when comparing the highest to the lowest fluorescence fractions. Similarly, a four-fold increase in total marker number is observed for CD8 cells. We also observed the separation of two dissimilar cell types that differed in expression of the CD4 marker, monocytes and T helper lymphocytes. We believe that this type of separation is applicable to any cells in suspension for which a suitable antibody exists and, due to the comparatively gentle nature of the process, is particularly suitable for the sorting of fragile cells.