Magnetophoresis of iron oxide nanoparticles at low field gradient: The role of shape anisotropy

Alignment Control of Ferrite-Decorated Nanocarbon Material for 3D Printing

This paper demonstrates the potential of anisotropic 3D printing for alignable carbon nanomaterials. The ferrite-decorated nanocarbon material was synthesized via a sodium solvation process using epichlorohydrin as the coupling agent. Employing a one-pot synthesis approach, the novel material was incorporated into a 3D photopolymer, manipulated, and printed using a low-cost microscale 3D printer, equipped with digital micromirror lithography, monitoring optics, and magnetic actuators. This technique highlights the ability to control the microstructure of 3D-printed objects with sub-micron precision for applications such as microelectrode sensors and microrobot fabrication.

Numerical modeling and in situ small angle X-ray scattering characterization of ultra-small SPION magnetophoresis in a high field and gradient separator

Magnetic nanoparticles (MNPs) have recently gained significant attention in various fields, including chemical and biomedical applications, due to their exceptional properties. However, separating MNPs from solution via magnetophoresis is challenging when MNPs are smaller than 50 nm as Brownian forces become on the order of the magnetic forces. In this study, we successfully separated small MNPs (5-30 nm) by utilizing high magnetic fields and gradients generated by economical permanent magnets. In situ small angle X-ray scattering (SAXS) was used to investigate the time-dependent concentration changes in the ferrofluid, and the results validated that only the 30 nm particles experienced particle aggregation or agglomeration, indicating that dipole-dipole interactions did not play a discernable role in the separation process for particles smaller than ∼15 nm. However, numerical simulations have provided further validation that in the absence of particle-particle interactions, even MNPs with diameters less than 15 nm exhibited magnetophoresis that effectively counteracted the effects of Brownian motion.

Navigating the microenvironment with flip and turn under quadrupole magnetophoretic steering control: Nanosphere- and nanorod-coated microbead

The spatiotemporal accuracy of microscale magnetophoresis has improved significantly over the course of several decades of development. However, most of the studies so far were using magnetic microbead composed of nanosphere particle for magnetophoretic actuation purpose. Here, we developed an in-house method for magnetic sample analysis called quadrupole magnetic steering control (QMSC). QMSC was used to study the magnetophoretic behavior of polystyrene microbeads decorated with iron oxide nanospheres-coated polystyrene microbeads (IONSs-PS) and iron oxide nanorods-coated polystyrene microbeads (IONRs-PS) under the influence of a quadrupole low field gradient. During a 4-s QMSC experiment, the IONSs-PS and IONRs-PS were navigated to perform 180° flip and 90° turn formations, and their kinematic results (2 s before and 2 s after the flip/turn) were measured and compared. The results showed that the IONRs-PS suffered from significant kinematic disproportion, translating a highly uneven amount of kinetic energy from the same magnitude of magnetic control. Combining the kinematic analysis, transmission electron microscopy micrographs, and vibrating sample magnetometry measurements, it was found that the IONRs-PS experienced higher fluid drag force and had lower consistency than the IONSs-PS due to its extensive open fractal nanorod structure on the bead surface and uneven magnetization, which was attributed to its ferrimagnetic nature.

Kinetic and Parametric Analysis of the Separation of Ultra-Small, Aqueous Superparamagnetic Iron Oxide Nanoparticle Suspensions under Quadrupole Magnetic Fields

Superparamagnetic iron oxide nanoparticles (SPIONs) have gathered tremendous scientific interest, especially in the biomedical field, for multiple applications, including bioseparation, drug delivery, etc. Nevertheless, their manipulation and separation with magnetic fields are challenging due to their small size. We recently reported the coupling of cooperative magnetophoresis and sedimentation using quadrupole magnets as a promising strategy to successfully promote SPION recovery from media. However, previous studies involved SPIONs dispersed in organic solvents (non-biocompatible) at high concentrations, which is detrimental to the process economy. In this work, we investigate, for the first time, the magnetic separation of 20 nm and 30 nm SPIONs dispersed in an aqueous medium at relatively low concentrations (as low as 0.5 g·L-1) using our custom, permanent magnet-based quadrupole magnetic sorter (QMS). By monitoring the SPION concentrations along the vessel within the QMS, we estimated the influence of several variables in the separation and analyzed the kinetics of the process. The results obtained can be used to shed light on the dynamics and interplay of variables that govern the fast separation of SPIONs using inexpensive permanent magnets.

Low-Gradient Magnetophoresis of Nanospheres and Nanorods through a Single Layer of Paper

The possible magnetophoretic migration of iron oxide nanoparticles through the cellulosic matrix within a single layer of paper is challenging with its underlying mechanism remained unclear. Even with the recent advancements of theoretical understanding on magnetophoresis, mainly driven by cooperative and hydrodynamics phenomena, the contributions of these two mechanisms on possible penetration of magnetic nanoparticles through cellulosic matrix of paper have yet been proven. Here, by using iron oxide nanoparticles (IONPs), both nanospheres and nanorods, we have investigated the migration kinetics of these nanoparticles through grade 4 Whatman filter paper with a particle retention of 20-25 μm. By performing droplet tracking experiments, the real-time stained area growth of the particle droplet on the filter paper, under the influences of a grade N40 NdFeB magnet, were recorded. Our results show that the spatial and temporal expansion of the IONP stain is biased toward the magnet and such an effect is dependent on (i) particle concentration and (ii) particle shape. The kinetics data were first analyzed by treating it as a radial wicking fluid, and later the IONP distribution within the cellulosic matrix was investigated by optical microscopy. The macroscopic flow front velocities of the stained area ranged from 259 μm/s to 16 040 μm/s. Moreover, the microscopic magnetophoretic velocity of nanorod cluster was also successfully measured as ∼214 μm/s. Findings in this work have indirectly revealed the strong influence of cooperative magnetophoresis and the engineering feasibility of paper-based magnetophoretic technology by taking advantage of magnetoshape anisotropy effect of the particles.

Magnetophoretic Effect on the Nanofluid Flow Over Decelerating Spinning Sphere with the Presence of Induced Magnetic Field

Magnetophoresis is the movement of magnetic nanoparticles under the guidance of magnetic field. Our goal is to perceive an unsteady magneto-nanofluid flow over a decelerating rotating sphere induced by an external magnetic field. In the flow analysis Brownian motion, thermophoresis and magnetopheresis effect are implemented in the model equations. At the boundary situation, zero-mass flux has been thought out. Utilizing similarity conversions we have converted the constitutive equations into ODEs and solve them numerically. The control of various factors on the flow properties has been staged through proper figures and tables. It is remarkably perceived that the magnetophoresis parameter increases the heat transmission ability of the fluid. Also, the magnetic-Prandtl number decreases the Nusselt number significantly. It is also professed that induced magnetic field elevates the heat transferability of the flow system and intensifies the velocity along the x-direction and diminishes the velocity along the z-direction.

Design and operation of magnetophoretic systems at microscale: Device and particle approaches

Combining both device and particle designs are the essential concepts to be considered in magnetophoretic system development. Researcher efforts are often dedicated to only one of these design aspects and neglecting the interplay between them. Herein, to bring out importance of the idea of integration between device and particle, we reviewed the working principle of magnetophoretic system (includes both device and particle design concepts). Since, the magnetophoretic force is influenced by both field gradient and magnetization volume, hence, accurate prediction of the magnetophoretic force is relying on the availability of information on both parameters. In device design, we focus on the different strategies used to create localized high-field gradient. For particle design, we emphasize on the scaling between hydrodynamic size and magnetization volume. Moreover, we also briefly discussed the importance of magnetoshape anisotropy related to particle design aspect of magnetophoretic systems. Next, we illustrated the need for integration between device and particle design using microscale applications of magnetophoretic systems, include magnetic tweezers and microfluidic systems, as our working example. On the basis of our discussion, we highlighted several promising examples of microscale magnetophoretic systems which greatly utilized the interplay between device and particle design. Further, we concluded the review with several factors that possibly resulted in the lack of research efforts related to device and particle design integration.

Magnetophoresis-Assisted Capillary Assembly: A Versatile Approach for Fabricating Tailored 3D Magnetic Supercrystals

The fabrication and integration of sub-millimeter magnetic materials into predefined circuits is of major importance for the realization of portable devices designed for telecommunications, automotive, biomedical, and space applications but remains highly challenging. We report here a versatile approach for the fabrication and direct integration of nanostructured magnetic materials of controlled shaped at specific locations onto silicon substrates. The magnetophoresis-assisted capillary assembly of magnetic nanoparticles, either spherical or anisotropic, leads to the fabrication of high-performance Co-based permanent magnets and Fe-based supercrystals. Integrated sub-millimeter magnets as well as millimeter self-standing magnets exhibiting magnetic properties competing with NdFeB-based composites were obtained through this cost- and time-efficient process. The proof-of-concept of electromagnetic actuation of a micro-electromechanical system cantilever by means of these supercrystals highlights their potentiality as efficient integrated magnetic materials within nomadic devices.

Magnetic separation of iron oxide nanoparticles to improve their application for magnetic particle imaging

Magnetic particle imaging (MPI) is a promising medical imaging technique for visualizing the three-dimensional distribution of tracer materials, specifically iron oxide nanoparticles (IONP). The optimization of magnetic nanoparticles (MNP) plays an essential role to improve the image resolution and sensitivity of imaging techniques.

OBJECTIVE

In this work, the optimization of commercial IONP (EMG 700, Ferrotec) coated with anionic surfactants was carried out using magnetic separation (MS) technique, by a low gradient magnetic separation (LGMS) (<15 T m^-1) method, to improve their performance as MPI tracers.

APPROACH

The magnetophoretical behavior of the samples in different concentrations ranging from 2 to 120 mmol l^-1 was investigated over 24 h of separation. The samples were characterized by dynamic light scattering (DLS), AC susceptibility (ACS), magnetic particle spectroscopy (MPS) and they were imaged in a preclinical MPI scanner, before and after MS.

MAIN RESULTS

DLS results showed that by increasing the concentration from 2 to 120 mmol l^-1 the hydrodynamic diameter of MNP decrease from 75 to 47 nm and size distribution decrease from 0.19 to 0.11 after 4 min MS. In addition, the MPS results demonstrated the third harmonic amplitude normalized to the iron amount [Formula: see text] and harmonic ratio [Formula: see text] of signal increase from 8.38 to 10.59 Am² kg^-1 (Fe) and 24.21-26.60, respectively. Furthermore, the MPI images of the samples after separation showed higher MPI resolution.

SIGNIFICANCE

Therefore, LGMS can be considered as a valuable method to narrow and control the size distribution of MNP for MPI.

The Effects of a Varied Gold Shell Thickness on Iron Oxide Nanoparticle Cores in Magnetic Manipulation, T1 and T2 MRI Contrasting, and Magnetic Hyperthermia

Fe₃O₄–Au core–shell magnetic-plasmonic nanoparticles are expected to combine both magnetic and light responsivity into a single nanosystem, facilitating combined optical and magnetic-based nanotheranostic (therapeutic and diagnostic) applications, for example, photothermal therapy in conjunction with magnetic resonance imaging (MRI) imaging. To date, the effects of a plasmonic gold shell on an iron oxide nanoparticle core in magnetic-based applications remains largely unexplored. For this study, we quantified the efficacy of magnetic iron oxide cores with various gold shell thicknesses in a number of popular magnetic-based nanotheranostic applications; these included magnetic sorting and targeting (quantifying magnetic manipulability and magnetophoresis), MRI contrasting (quantifying benchtop nuclear magnetic resonance (NMR)-based T₁ and T₂ relaxivity), and magnetic hyperthermia therapy (quantifying alternating magnetic-field heating). We observed a general decrease in magnetic response and efficacy with an increase of the gold shell thickness, and herein we discuss possible reasons for this reduction. The magnetophoresis speed of iron oxide nanoparticles coated with the thickest gold shell tested here (ca. 42 nm) was only ca. 1% of the non-coated bare magnetic nanoparticle, demonstrating reduced magnetic manipulability. The T₁ relaxivity, r₁, of the thick gold-shelled magnetic particle was ca. 22% of the purely magnetic counterpart, whereas the T₂ relaxivity, r₂, was 42%, indicating a reduced MRI contrasting. Lastly, the magnetic hyperthermia heating efficiency (intrinsic loss power parameter) was reduced to ca. 14% for the thickest gold shell. For all applications, the efficiency decayed exponentially with increased gold shell thickness; therefore, if the primary application of the nanostructure is magnetic-based, this work suggests that it is preferable to use a thinner gold shell or higher levels of stimuli to compensate for losses associated with the addition of the gold shell. Moreover, as thinner gold shells have better magnetic properties, have previously demonstrated superior optical properties, and are more economical than thick gold shells, it can be said that “less is more”.

Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids

Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.

Colloidal Stability of Aqueous Ferrofluids at 10 T

Magnetic density separation is an emerging recycling technology by which several different waste materials-from plastic products, electronics, or other-can be sorted in a single continuous processing step. Larger-scale installations will require ferrofluids that remain stable at several teslas, high magnetic fields at which colloidal stability was not investigated before. Here we optically monitor the concentration profile of iron oxide nanoparticles in aqueous ferrofluids at a field of 10 T and a gradient of 100 T/m. The sedimentation velocities and equilibrium concentration profiles inform on maintenance or breakdown of colloidal stability, which depends on the concentration and magnetic coupling energy of the nanoparticles. Comparison with results obtained with a small neodymium magnet indicate that stability at moderate fields is predictive of stability at much higher fields, which facilitates the development of new ferrofluids dedicated to magnetic density separation.

Unified View of Magnetic Nanoparticle Separation under Magnetophoresis

The migration process of magnetic nanoparticles and colloids in solution under the influence of magnetic field gradients, which is also known as magnetophoresis, is an essential step in the separation technology used in various biomedical and engineering applications. Many works have demonstrated that in specific situations, separation can be performed easily with the weak magnetic field gradients created by permanent magnets, a process known as low-gradient magnetic separation (LGMS). Due to the level of complexity involved, it is not possible to understand the observed kinetics of LGMS within the classical view of magnetophoresis. Our experimental and theoretical investigations in the last years unravelled the existence of two novel physical effects that speed up the magnetophoresis kinetics and explain the observed feasibility of LGMS. Those two effects are (i) cooperative magnetophoresis (due to the cooperative motion of strongly interacting particles) and (ii) magnetophoresis-induced convection (fluid dynamics instability originating from inhomogeneous magnetic gradients). In this feature article, we present a unified view of magnetophoresis based on the extensive research done on these effects. We present the physical basis of each effect and also propose a classification of magnetophoresis into four distinct regimes. This classification is based on the range of values of two dimensionless quantities, namely, aggregation parameter N* and magnetic Grashof number Gr_m, which include all of the dependency of LGMS on various physical parameters (such as particle properties, thermodynamic parameters, fluid properties, and magnetic field properties). This analysis provides a holistic view of the classification of transport mechanisms in LGMS, which could be particularly useful in the design of magnetic separators for engineering applications.

Continuous-Flow Separation of Magnetic Particles from Biofluids: How Does the Microdevice Geometry Determine the Separation Performance?

The use of functionalized magnetic particles for the detection or separation of multiple chemicals and biomolecules from biofluids continues to attract significant attention. After their incubation with the targeted substances, the beads can be magnetically recovered to perform analysis or diagnostic tests. Particle recovery with permanent magnets in continuous-flow microdevices has gathered great attention in the last decade due to the multiple advantages of microfluidics. As such, great efforts have been made to determine the magnetic and fluidic conditions for achieving complete particle capture; however, less attention has been paid to the effect of the channel geometry on the system performance, although it is key for designing systems that simultaneously provide high particle recovery and flow rates. Herein, we address the optimization of Y-Y-shaped microchannels, where magnetic beads are separated from blood and collected into a buffer stream by applying an external magnetic field. The influence of several geometrical features (namely cross section shape, thickness, length, and volume) on both bead recovery and system throughput is studied. For that purpose, we employ an experimentally validated Computational Fluid Dynamics (CFD) numerical model that considers the dominant forces acting on the beads during separation. Our results indicate that rectangular, long devices display the best performance as they deliver high particle recovery and high throughput. Thus, this methodology could be applied to the rational design of lab-on-a-chip devices for any magnetically driven purification, enrichment or isolation.

Sedimentation Kinetics of Magnetic Nanoparticle Clusters: Iron Oxide Nanospheres vs Nanorods

A detailed study of the sedimentation kinetics of iron oxide nanoparticle (IONP) clusters composed of nanospheres and nanorods is presented. Measurements were performed to determine the absorbance of an IONP suspension undergoing sedimentation over time by using a UV-vis spectrophotometer with simultaneous monitoring of the hydrodynamic diameter of the clusters formed with dynamic light scattering (DLS). Mathematical analysis based on Happel's spherical and cylindrical models was conducted to reveal the relationship between the settling velocity of the IONP clusters and their packing density. For the case of IONP clusters composed of rodlike particles, two distinctive phases of sedimentation were recorded, with the occurrence of rapid sedimentation at the beginning of the process (phase I) followed by a slower settling rate (phase II). In sedimentation phase II, even though the nanorod clusters had a hydrodynamic size of >500 nm, which was much larger than that of the nanosphere clusters (∼200 nm), their settling velocity of 0.0038 mm/min was still slower than that of the nanosphere clusters. Such observations were mainly a result of the packing density differences between the formed clusters; due to the end-to-end particle interactions of nanorods, the nanorod clusters were less tightly packed and more permeable. In addition to the mathematical analysis, quartz crystal microbalance with dissipation (QCM-D) was employed to measure the "softness" of the IONP clusters formed, and this physical property can be further related to their packing density. This study illustrated that for a rapidly aggregating system, such as magnetic IONPs, not only do the particle shape and size uniformity contribute to the physical properties of the particle clusters formed but also the nature of the aggregation, either end-to-end and/or side-to-side, should be carefully considered when designing a colloidally stable IONP suspension.

Self-Assembly and sedimentation of 5 nm SPIONs using horizontal, high magnetic fields and gradients

Superparamagnetic iron oxide nanoparticles (SPIONs) are employed in multiple applications, especially within medical and chemical engineering fields. However, their magnetic separation is very challenging as the magnetophoretic motion is hindered by thermal energy and viscous drag. Recent studies have addressed the recovery of SPIONs by a combination of cooperative magnetophoresis and sedimentation. Nevertheless, the effect of horizontal, high fields and gradients on the vertical sedimentation of SPIONs has not been described. In this work, we report, for the first time, the magnetically facilitated sedimentation of 5 nm particles by applying fields and gradients perpendicular to gravity. The magnetic field was generated by quadrupole magnetic sorters and the process was measured with time by tracking the concentration along the length of a channel contacting the 5 nm SPIONs within the quadrupole field. Our experimental data suggest that aggregates of 60-90 particles are formed in the system; thus, particle agglomeration by dipole-dipole interactions was promoted, and these clusters settled down as a result of gravitational forces. Multiple variables and parameters were evaluated, including the initial SPION concentration, the temperature, the magnetic field and gradient and operation time. It was found that the process was improved by decreasing the initial concentration and the temperature, but the magnitude of the magnetic field and gradient did not significantly affect the sedimentation. Finally, the separation process was rapid, with the systems reaching the equilibrium in approximately 20 minutes, which is a significant advantage in comparison to other systems that require longer times and larger particle sizes.

Pushing of Magnetic Microdroplet Using Electromagnetic Actuation System

Treatment of certain diseases requires the administration of drugs at specific areas of tissues and/or organs to increase therapy effectiveness and avoid side effects that may harm the rest of the body. Drug targeting is a research field that uses various techniques to administrate therapies at specific areas of the body, including magnetic systems able to drive nano “vehicles”, as well as magnetically labeled molecules, in human body fluids and tissues. Most available actuation systems can only attract magnetic elements in a relatively small workspace, limiting drug target applications to superficial tissues, and leaving no alternative cases where deep targeting is necessary. In this paper, we propose an electromagnetic actuation system able to push and deflect magnetic particles at distance of ~10 cm, enabling the manipulation of magnetic nano- and microparticles, as well as administration of drugs in tissues, which are not eligible for localized drug targeting with state-of-the-art systems. Laboratory experiments and modeling were conducted to prove the effectiveness of the proposed system. By further implementing our device, areas of the human body that previously were impossible to treat with magnetically labeled materials such as drugs, cells, and small molecules can now be accessible using the described system.

Working principle and application of magnetic separation for biomedical diagnostic at high- and low-field gradients

Magnetic separation is a versatile technique used in sample preparation for diagnostic purpose. For such application, an external magnetic field is applied to drive the separation of target entity (e.g. bacteria, viruses, parasites and cancer cells) from a complex raw sample in order to ease the subsequent task(s) for disease diagnosis. This separation process not only can be achieved via the utilization of high magnetic field gradient, but also, in most cases, low magnetic field gradient with magnitude less than 100 T m^-1 is equally feasible. It is the aim of this review paper to summarize the usage of both high gradient magnetic separation and low gradient magnetic separation (LGMS) techniques in this area of research. It is noteworthy that effectiveness of the magnetic separation process not only determines the outcome of a diagnosis but also directly influences its accuracy as well as sensing time involved. Therefore, understanding the factors that simultaneously influence the efficiency of both magnetic separation process and target detection is necessary. Moreover, for LGMS, there are several important considerations that should be taken into account in order to ensure its successful implementation. Hence, this review paper aims to provide an overview to relate all this crucial information by linking the magnetic separation theory to biomedical diagnostic applications.

Controlled manipulation of Fe₃O₄ nanoparticles in an oscillating magnetic field for fast ablation of microchannel occlusion

Fe3O4 nanoparticles were controlled by an oscillating magnetic field to enable fast and non-contact ablation of microchannel occlusion. Scalable behaviour of their translational and rotational velocities was experimentally verified. Rotational flows created by such motions are fundamental for ablation as demonstrated by the removal of thrombi in occluded microchannels.

Magnetic Field-Driven Manipulation System and its Applications in Micromixing and Microablation

This study showcases two independent magnetic manipulation systems to remotely control the movement of Fe3O4 nanomaterial in microfluidic chips. One system utilizes a homogeneous rotating magnetic field to carry out magnetic stirring in 100 μm and 300 μm flow channels. The mixing results of this system revealed that adding Fe3O4 nanoparticles to the solution enhances the efficiency of the micromixer by twice as much that of a device without the nanomaterial. The second manipulation system utilizes oscillating magnetic field for rapid microablation of thrombus in a microchannel. A customizable magnetic platform using 3D-printed material is also constructed. This is proposed as a feasible low-cost and portable magnetic manipulation device that can implement both applications.