Differences in magnetically induced motion of diamagnetic, paramagnetic, and superparamagnetic microparticles detected by cell tracking velocimetry

Applications of magnetic and electromagnetic forces in micro-analytical systems

Novel applications of magnetic fields in analytical chemistry have become a remarkable trend in the last two decades. Various magnetic forces have been employed for the migration, orientation, manipulation, and trapping of microparticles, and new analytical platforms for separating and detecting molecules have been proposed. Magnetic materials such as functional magnetic nanoparticles, magnetic nanocomposites, and specially designed magnetic solids and liquids have also been developed for analytical purposes. Numerous attractive applications of magnetic and electromagnetic forces on magnetic and non-magnetic materials have been studied, but fundamental studies to understand the working principles of magnetic forces have been challenging. These studies will form a new field of magneto-analytical science, which should be developed as an interdisciplinary field. In this review, essential pioneering works and recent attractive developments are presented.

Intrinsically magnetic susceptibility in human blood and its potential impact on cell separation: Non-classical and intermediate monocytes have the strongest magnetic behavior in fresh human blood

The presence of iron in circulating monocytes is well known as they play an essential role in iron recycling. It has been demonstrated that the iron content of blood cells can be measured through their magnetic behavior; however, the magnetic properties of different monocyte subtypes remain unknown. In this study we report, for the first time, the magnetic behavior of classical, intermediate and non-classical monocytes, which may be related to their iron storage capacity. The magnetic properties of monocytes were compared with those of other blood cells, such as lymphocytes and red blood cells in the oxyhemoglobin and methemoglobin states, and a cancer cell type. For this analysis, we used an instrument referred to as a Cell Tracking Velocimetry (CTV), which quantitatively characterizes the magnetic behavior of biological entities. Our results revealed that significant fractions of the intermediate and non-classical monocytes (up to 59% and 65% depending on the sample, respectively) have paramagnetic properties, suggesting their higher iron storage capacities. Moreover, our findings have implications for the immunomagnetic separation industry; we propose that negative magnetic isolation techniques for recovering monocytes from blood should be used with caution, as it is possible to lose magnetic monocytes when using this technique.

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.

Single cell magnetometry by magnetophoresis vs. bulk cell suspension magnetometry by SQUID-MPMS - a comparison

Paramagnetic constituents of a cell have strong effect on cell's volume magnetic susceptibility even at low volume fraction because of their high susceptibility relative to that of the diamagnetic cell constituents. The effect can be measured at a single cell level by measuring cell terminal velocity in viscous media using a microscope equipped with a well-defined field and gradient magnet configuration (referred to as magnetophoretic analysis by cell tracking velocimetry, CTV). The sensitivity of such a microscopic-scale magnetometry was compared to that of a reference method of superconducting quantum interference-magnetic properties measurement system (SQUID-MPMS) using a red blood cell (RBC) suspension model. The RBC hemoglobin oxygen saturation determines the hemoglobin molecular magnetic susceptibility (diamagnetic when fully oxygenated, paramagnetic when fully deoxygenated or converted to methemoglobin). The SQUID-MPMS measurements were performed on an average of 5,000 RBCs in 20 μL physiological phosphate buffer at room temperature, those by CTV on a single cell track in a mean magnetic field of 1.6 T and mean gradient of 240 T/m, repeated for an average of 1,000 tracks per sample. This suggests 5,000× higher sensitivity of cell susceptometry by magnetophoretic analysis than by SQUID-MPMS. The magnetophoretic mean RBC magnetic susceptibilities were in the range determined by SQUID-MPMS (lower limit) and theory (upper limit). The ability of magnetophoretic analysis to resolve susceptibility peaks in a mixed cell populations was confirmed for an oxy RBC and met RBC mixture. Magnetophoretic analysis by CTV provides new tool for studies of emergence of paramagnetic reaction products in the cell.

Correlation of simulation/finite element analysis to the separation of intrinsically magnetic spores and red blood cells using a microfluidic magnetic deposition system

Magnetic separation of cells has been, and continues to be, widely used in a variety of applications, ranging from healthcare diagnostics to detection of food contamination. Typically, these technologies require cells labeled with antibody magnetic particle conjugate and a high magnetic energy gradient created in the flow containing the labeled cells (i.e., a column packed with magnetically inducible material), or dense packing of magnetic particles next to the flow cell. Such designs, while creating high magnetic energy gradients, are not amenable to easy, highly detailed, mathematic characterization. Our laboratories have been characterizing and developing analysis and separation technology that can be used on intrinsically magnetic cells or spores which are typically orders of magnitude weaker than typically immunomagnetically labeled cells. One such separation system is magnetic deposition microscopy (MDM) which not only separates cells, but deposits them in specific locations on slides for further microscopic analysis. In this study, the MDM system has been further characterized, using finite element and computational fluid mechanics software, and separation performance predicted, using a model which combines: 1) the distribution of the intrinsic magnetophoretic mobility of the cells (spores); 2) the fluid flow within the separation device; and 3) accurate maps of the values of the magnetic field (max 2.27 T), and magnetic energy gradient (max of 4.41 T2 /mm) within the system. Guided by this model, experimental studies indicated that greater than 95% of the intrinsically magnetic Bacillus spores can be separated with the MDM system. Further, this model allows analysis of cell trajectories which can assist in the design of higher throughput systems.

The larmor frequency shift in magnetically heterogeneous media depends on their mesoscopic structure

PURPOSE

Recent studies have addressed the determination of the NMR precession frequency in biological tissues containing magnetic susceptibility differences between cell types. The purpose of this study is to investigate the dependence of the precession frequency on medium microstructure using a simple physical model.

THEORY

This dependence is governed by diffusion of NMR-visible molecules in magnetically heterogeneous microenvironments. In the limit of fast diffusion, the precession frequency is determined by the average susceptibility-induced magnetic field, whereas in the limit of slow diffusion it is determined by the average local phase factor of precessing spins.

METHODS

The main method used is Monte Carlo simulation of isotropic suspensions of impermeable magnetized spheres. In addition, NMR spectroscopy was performed in aqueous suspensions of polystyrene microbeads.

RESULTS

The precession frequency depends on the structural organization of magnetized objects in the medium. Monte Carlo simulations demonstrated a nonmonotonic transition between the regimes of fast and slow diffusion. NMR experiments confirmed the transition, but were unable to confirm its precise form. Results for a given pattern of structural organization obey a scaling law.

CONCLUSION

The NMR precession frequency exhibits a complex dependence on medium structure. Our results suggest that the commonly assumed limit of fast water diffusion holds for biological tissues with small cells. Magn Reson Med 79:1101-1110, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Femtogram Resolution of Iron Content on a Per Cell Basis: Ex Vivo Storage of Human Red Blood Cells Leads to Loss of Hemoglobin

The magnetic characteristics of hemoglobin (Hb) changes with the binding of dioxygen (O2) to the heme prosthetic groups of the globin chains: from paramagnetic ferrous Hb to diamagnetic ferrous oxyhemoglobin (oxyHb) with reversibly bound O2, or paramagnetic ferric methemoglobin (metHb). When multiplied over the number of Hb molecules in a red blood cell (RBC), the effect is detectable through motion analysis of RBCs in a high magnetic field and gradient. This motion is referred to as magnetophoretic mobility, which can be conveniently expressed as a fraction of the cell sedimentation velocity. In this Article, using a previously developed and reported instrument, cell tracking velocimetry (CTV), we are able to detect difference in Hb concentration in two RBC populations to a resolution of 1 × 107 Hb molecules per cell (4 × 107 atoms of Fe per cell or 4-5 femtograms of Fe). Similar resolution achieved with inductively coupled plasma-mass spectrometry requires on the order of 105-106 cells and provides an average, whereas CTV provides a measurement for each cell. CTV analysis revealed that RBCs lose, on average, 17% of their Hb after 42 days of storage, the maximum FDA-approved length of time for the cold storage of RBCs in additive solution. This difference in Hb concentration was the result of routine RBC storage; clinical implications are discussed.

Open Gradient Magnetic Red Blood Cell Sorter Evaluation on Model Cell Mixtures

The emerging applications of biological cell separation to rare circulating tumor cell (CTC) detection and separation from blood rely on efficient methods of red blood cell (RBC) debulking. The two most widely used methods of centrifugation and RBC lysis have been associated with the concomitant significant losses of the cells of interest (such as progenitor cells or circulating tumor cells). Moreover, RBC centrifugation and lysis are not well adapted to the emerging diagnostic applications, relying on microfluidics and micro-scale total analytical systems. Therefore, magnetic RBC separation appears a logical alternative considering the high iron content of the RBC (normal mean 105 fg) as compared to the white blood cell iron content (normal mean 1.6 fg). The typical magnetic forces acting on a RBC are small, however, as compared to typical forces associated with centrifugation or the forces acting on synthetic magnetic nanoparticles used in current magnetic cell separations. This requires a significant effort in designing and fabricating a practical magnetic RBC separator. Applying advanced designs to the low cost, high power permanent magnets currently available, and building on the accumulated knowledge of the immunomagnetic cell separation methods and devices, an open gradient magnetic red blood cell (RBC) sorter was designed, fabricated and tested on label-free cell mixtures, with potential applications to RBC debulking from whole blood samples intended for diagnostic tests.

Continuous separation principles using external microaction forces

During the past decade, methods for the continuous separation of microparticles with microaction forces have rapidly advanced. Various action forces have been used in designs of both microchannel and capillary continuous separation systems, which depend on properties such as conductivity, permittivity, absorptivity, refractive index, magnetic susceptibility, and compressibility. Particle migration velocity has been used to characterize the particles. Biological cells have been the most interesting targets of these continuous separation methods.

Simultaneous, single particle, magnetization and size measurements of micron sized, magnetic particles

Single particle magnetization and size measurements of micron and nano sized, magnetic particles were made using a previously described device referred to as Cell Tracking Velocimetry, CTV. Three types of commercially available, and commonly used, magnetic particles were studied in this report. While the CTV instrument provides individual particles measurements, the average magnetization and size measurements were found to have reasonable agreements with reported values from instruments which measure bulk values. In addition, the CTV instrument, using electromagnets, can also determine magnetization curves, which also proved to have reasonable agreement with other published studies. Given that magnetic separation and analysis technology is dependent on the quality of the magnetic particles used, studies such as this one using CTV provide not only average data, but also provides information with respect to the distribution of the properties such as magnetization and size. For example, the spread of the data in magnetic and settling velocities were found to be predominately due to the size distribution of the analyzed particles.

Erythrocyte enrichment in hematopoietic progenitor cell cultures based on magnetic susceptibility of the hemoglobin

Using novel media formulations, it has been demonstrated that human placenta and umbilical cord blood-derived CD34+ cells can be expanded and differentiated into erythroid cells with high efficiency. However, obtaining mature and functional erythrocytes from the immature cell cultures with high purity and in an efficient manner remains a significant challenge. A distinguishing feature of a reticulocyte and maturing erythrocyte is the increasing concentration of hemoglobin and decreasing cell volume that results in increased cell magnetophoretic mobility (MM) when exposed to high magnetic fields and gradients, under anoxic conditions. Taking advantage of these initial observations, we studied a noninvasive (label-free) magnetic separation and analysis process to enrich and identify cultured functional erythrocytes. In addition to the magnetic cell separation and cell motion analysis in the magnetic field, the cell cultures were characterized for cell sedimentation rate, cell volume distributions using differential interference microscopy, immunophenotyping (glycophorin A), hemoglobin concentration and shear-induced deformability (elongation index, EI, by ektacytometry) to test for mature erythrocyte attributes. A commercial, packed column high-gradient magnetic separator (HGMS) was used for magnetic separation. The magnetically enriched fraction comprised 80% of the maturing cells (predominantly reticulocytes) that showed near 70% overlap of EI with the reference cord blood-derived RBC and over 50% overlap with the adult donor RBCs. The results demonstrate feasibility of label-free magnetic enrichment of erythrocyte fraction of CD34+ progenitor-derived cultures based on the presence of paramagnetic hemoglobin in the maturing erythrocytes.

Iron transport in cancer cell culture suspensions measured by cell magnetophoresis

Cell motion in a magnetic field reveals the presence of intracellular paramagnetic elements, such as iron or manganese. Under controlled field and liquid media composition, such motion previously allowed us to compare the paramagnetic contribution to cell magnetic susceptibility in erythrocytes differing in the spin state of heme associated with hemoglobin. The method is now tested on cells with less obvious paramagnetic properties: cell cultures derived from human cancers to determine if the magnetophoretic mobility (MM) measurement is sufficiently sensitive to the dysregulation of the intracellular iron metabolism as suggested by reports on loss of iron homeostasis in cancer. The cell lines included hepatocellular carcinoma (Hep 3B 2.1-7 and Hep G2), promyelocytic (HL-60) and chronic myelogenous (K-562) leukemias, histiocytic lymphoma (U-937), tongue (CAL 27) and pharyngeal (Detroit 562) carcinomas, and epitheloid carcinoma (HeLa), whose MM was measured in complete media with standard and elevated soluble iron (ferric nitrate and ferric ammonium citrate), against oxy- and met-hemoglobin erythrocytes used as controls. Different cell lines responded differently to the magnetic field and the soluble iron concentrations in culture media establishing the possibility of single cell elemental analysis by magnetophoresis and magnetic cell separation based upon differences in intracellular iron concentration.

Emerging technologies for CTC detection based on depletion of normal cells

Properly conducted, an enrichment step can improve selectivity, sensitivity, yield, and most importantly, significantly reduce the time needed to isolate rare circulating tumor cells (CTCs). The enrichment process can be broadly categorized as positive selection versus negative depletion, or in some cases, a combination of both. We have developed a negative depletion CTC enrichment strategy that relies on the removal of normal cells using immunomagnetic separation in the blood of cancer patients. This method is based on the combination of magnetic and fluid forces in an axial, laminar flow in long cylinders placed in quadrupole magnets. Using this technology, we have successfully isolated CTCs from patients with breast carcinoma and squamous cell carcinoma of the head and neck. In contrast to a positive selection methodology, this approach provides an unbiased characterization of these cells, including markers associated with epithelial mesenchymal transition.

Quantification of changes in oxygen release from red blood cells as a function of age based on magnetic susceptibility measurements

This study extends the in vitro understanding of the RBC storage lesion by serially analyzing the RBC's magneophoretic mobility, a property dependent on the content and oxygenation or oxidation state of hemoglobin (Hb) iron, during storage. Four prestorage leukoreduced, AS-5 preserved RBC units were stored between 1 and 6 °C for 42 days. Weekly starting on storage day 7, each unit was sampled, divided into 3 aliquots, each subjected to different reactions: one aliquot was exposed to room air to produce oxyhemoglobin (oxyHb), another aliquot was mixed with sodium nitrite to produce methemoglobin (metHb), while the third aliquot was desaturated of oxygen (deoxyhemoglobin, deoxyHb) using nitrogen gas. These aliquots were placed into a cell tracking velocimetry (CTV) apparatus which measured both the settling velocity (u(s)) of the RBCs as well as their magnetically induced velocity (u(m)). The u(m)/u(s) ratio depends on the oxygenation state of the hemoglobin and the quantity of iron within the RBC. The RBC density was measured by percoll centrifugation. There was a significant reduction in the u(m)/u(s) ratio for the deoxyHb RBC aliquot as storage time elapsed, with a smaller but still significant reduction in the u(m)/u(s) ratio for the metHb aliquot. The average RBC density decreased very slightly during storage, as determined by the percoll centrifugation technique, although the average settling velocity (another measure of cell density) seemed to fluctuate during storage. The decrease in magnetophoretic mobility of the deoxyHb portion is explicable either by Hb's increased affinity for oxygen during storage, or else a loss of iron from the cells.

Quantification of both the presence, and oxidation state, of Mn in Bacillus atrophaeus spores and its imparting of magnetic susceptibility to the spores

Bacillus atrophaeus spores were previously reported to have significant magnetic susceptibility in a magnetic field due to the presence of Mn. However, relatively little is known about the total amount and distribution of the oxidation state of Mn associated with this specific strain's spores. Using the instrument, cell tracking velocimetry (CTV) both magnetically induced velocity and settling velocity was quantitatively measured. Visual observations, and calculated diameter using previously reported densities, indicate that the spores are present in the form of clusters of approximately 3-6 µm. Treatment of these clusters with EDTA or pH of 2.0 or below resulted in not only the disruption of the spore clusters, but also a significant decrease in magnetic susceptibility, in some cases by almost two orders of magnitude. Since the magnetic susceptibility of Mn varies significantly between the three typically reported valance states of Mn, Mn(II), Mn(III), and Mn(IV); X-Ray Photoelectron Spectroscopy, XPS, was used to determined the valance states of Mn in the spores. This XPS analysis, which penetrates up to 10 nm into the spore, returned the following fractions: 0.41, 0.38, and 0.21 for the valance states: Mn(II), Mn(III), and Mn(IV), respectively. The total mass of Mn associated with each spore cluster was determined by ICP-MS. A second, completely independent estimate of Mn mass associated with each spore cluster was made, by mathematically solving for the amount of Mn per spore cluster using the experimentally measured magnetophoretic mobility and the magnetic susceptibility of each of the three valence states from the XPS analysis. IPC-MS returned a value of 3.28 × 10(-11) g of Mn per spore cluster while the calculated estimation from mobility and XPS analysis retuned a value of 1.16 × 10(-11) g, which given the complexity of the two techniques, is a reasonable agreement. Finally, a discussion of potential applications of the magnetic properties of these spores is presented.

Quantification of non-specific binding of magnetic micro- and nanoparticles using cell tracking velocimetry: Implication for magnetic cell separation and detection

The maturation of magnetic cell separation technology places increasing demands on magnetic cell separation performance. While a number of factors can cause sub-optimal performance, one of the major challenges can be non-specific binding of magnetic nano- or microparticles to non-targeted cells. Depending on the type of separation, this non-specific binding can have a negative effect on the final purity, the recovery of the targeted cells, or both. In this work, we quantitatively demonstrate that non-specific binding of magnetic nanoparticles can impart a magnetization to cells such that these cells can be retained in a separation column and thus negatively impact the purity of the final product and the recovery of the desired cells. Through experimental data and theoretical arguments, we demonstrate that the number of MACS magnetic particles needed to impart a magnetization that is sufficient to cause non-targeted cells to be retained in the column to be on the order of 500-1,000 nanoparticles. This number of non-specifically bound particles was demonstrated experimentally with an instrument, cell tracking velocimeter, CTV, and it is demonstrated that the sensitivity of the CTV instrument for Fe atoms contained in magnetic nanoparticles on the order of 1 x 10(-15) g/mL of Fe.

Trajectory control of PbSe-gamma-Fe2O3 nanoplatforms under viscous flow and an external magnetic field

The flow behavior of nanostructure clusters, consisting of chemically bonded PbSe quantum dots and magnetic gamma-Fe(2)O(3) nanoparticles, has been investigated. The clusters are regarded as model nanoplatforms with multiple functionalities, where the gamma-Fe(2)O(3) magnets serve as transport vehicles, manipulated by an external magnetic field gradient, and the quantum dots act as fluorescence tags within an optical window in the near-infrared regime. The clusters' flow was characterized by visualizing their trajectories within a viscous fluid (mimicking a blood stream), using an optical imaging method, while the trajectory pictures were analyzed by a specially developed processing package. The trajectories were examined under various flow rates, viscosities and applied magnetic field strengths. The results revealed a control of the trajectories even at low magnetic fields (<1 T), validating the use of similar nanoplatforms as active targeting constituents in personalized medicine.