Quantification of magnetic susceptibility in several strains of Bacillus spores: implications for separation and detection

Design of a high-throughput bio-ferrograph for isolation of cancer cells from whole blood

Enumeration and morphological characterization of circulating tumor cells (CTCs) can be useful in diagnosis and prognosis of metastatic cancer patients. The bio-ferrograph (BF) with its five flow channels, which was developed in the late 1990s for magnetic isolation of biological cells and tissue fragments from fluids, is a modification of the analytical ferrograph. Its use for isolation of rare CTCs from human whole blood (HWB) is a novel approach for the detection of cancer at a cellular level. The isolation process is facilitated by the interaction of specifically magnetized cells with a strong external magnetic field, yielding high recovery rates with no morphological alternation of cells that are isolated on a coverslip glass slide, thus allowing complementary microscopic, chemical, biological, and mechanical analyses. Here, a full mechanical and magnetostatic design of a novel high-throughput BF is presented. The system design is based on an optimized procedure for bio-ferrographic isolation of CTCs from HWB. It incorporates a semi-automated CTC separation system consisting of sample preparation, labeling, and staining; magnetic isolation; and system recovery. The design process was optimized based on experimental feasibility tests and finite element analyses. The novel bench-top system consists of 100 flow channels, allowing simultaneous analysis of multiple samples from 20 patients in each run, with the potential to become a decision-making tool for medical doctors when monitoring patients in a hospital setting. It opens a new route for early diagnosis, prognosis, and treatment of cancers, as well as other diseases, such as osteoarthritis.

Microfluidic chip for graduated magnetic separation of circulating tumor cells by their epithelial cell adhesion molecule expression and magnetic nanoparticle binding

The enumeration of circulating tumor cells (CTCs) in the peripheral bloodstream of metastatic cancer patients has contributed to improvements in prognosis and therapeutics. There have been numerous approaches to capture and counting of CTCs. However, CTCs have potential information beyond simple enumeration and hold promise as a liquid biopsy for cancer and a pathway for personalized cancer therapy by detecting the subset of CTCs having the highest metastatic potential. There is evidence that epithelial cell adhesion molecule (EpCAM) expression level distinguishes these highly metastatic CTCs. The few previous approaches to selective CTC capture according to EpCAM expression level are reviewed. A new two-stage microfluidic device for separation, enrichment and release of CTCs into subpopulations sorted by EpCAM expression level is presented here. It relies upon immunospecific magnetic nanoparticle labeling of CTCs followed by their field- and flow-based separation in the first stage and capture as discrete subpopulations in the second stage. To fine tune the separation, the magnetic field profile across the first stage microfluidic channel may be modified by bonding small Vanadium Permendur strips to its outer walls. Mathematical modeling of magnetic fields and fluid flows supports the soundness of the design.

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.

Paramagnetism in Bacillus spores: Opportunities for novel biotechnological applications

Spores of Bacillus megaterium, Bacillus cereus, and Bacillus subtilis were found to exhibit intrinsic paramagnetic properties as a result of the accumulation of manganese ions. All three Bacillus species displayed strong yet distinctive magnetic properties arising from differences in manganese quantity and valency. Manganese ions were found to accumulate both within the spore core as well as being associated with the surface of the spore. Bacillus megaterium spores accumulated up to 1 wt.% manganese (II) within, with a further 0.6 wt.% adsorbed onto the surface. At room temperature, Bacillus spores possess average magnetic susceptibilities in the range of 10-6 to 10-5 . Three spore-related biotechnological applications-magnetic sensing, magnetic separation and metal ion adsorption-were assessed subsequently, with the latter two considered as having the most potential for development.

Fundamentals and application of magnetic particles in cell isolation and enrichment: a review

Magnetic sorting using magnetic beads has become a routine methodology for the separation of key cell populations from biological suspensions. Due to the inherent ability of magnets to provide forces at a distance, magnetic cell manipulation is now a standardized process step in numerous processes in tissue engineering, medicine, and in fundamental biological research. Herein we review the current status of magnetic particles to enable isolation and separation of cells, with a strong focus on the fundamental governing physical phenomena, properties and syntheses of magnetic particles and on current applications of magnet-based cell separation in laboratory and clinical settings. We highlight the contribution of cell separation to biomedical research and medicine and detail modern cell-separation methods (both magnetic and non-magnetic). In addition to a review of the current state-of-the-art in magnet-based cell sorting, we discuss current challenges and available opportunities for further research, development and commercialization of magnetic particle-based cell-separation systems.

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.

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.

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

Magnetic separation in biomedical applications is based on differential magnetophoretic mobility (MM) of microparticulate matter in viscous media. Typically, the difference in MM is obtained by selectively labeling the target cells with superparamagnetic iron oxide nanoparticles (SPIONs). We have measured the MM of monodisperse, polystyrene microspheres (PSMs), with and without attached SPIONs as a model of cell motion induced by nanoparticle magnetization, using variable H field and cell tracking velocimetry (CTV). As a model of paramagnetic microparticle motion, the MM measurements were performed on the same PSMs in paramagnetic gadolinium solutions, and on spores of a prokaryotic organism, Bacillus globigii (shown to contain paramagnetic manganese). The CTV analysis was sensitive to the type of the microparticle magnetization, producing a value of MM independent of the applied H field for the paramagnetic species, and a decreasing MM value with an increasing field for superparamagnetic species, as predicted from theory. The SPION-labeled PSMs exhibited a saturation magnetization above H approximately = 64,000 A m(-1) (or 0.08 tesla). Based on those data, the average saturation magnetizations of the SPIONs was calculated and shown to vary between different commercial sources. The results demonstrate sensitivity of the CTV analysis to different magnetization mechanisms of the microparticles.