Effects of magnetic field gradients on the aggregation dynamics of colloidal magnetic nanoparticles

Magnetophoresis of paramagnetic metal ions in porous media

We report a numerical investigation of the magnetophoresis of solutions containing paramagnetic metal ions. Using a simulated magnetic field of a superconducting magnet and the convection-diffusion model, we study the transport of transition metal salts through a porous medium domain. In particular, through a detailed comparison of the numerical results of magnetophoretic velocity and ion concentration profiles with prior published experiments, we validate the model. Subsequent to model validation, we perform a systematic analysis of the model parameters on the magnetophoresis of metal ions. Magnetophoresis is quantified with a magnetic Péclet number Pem. Under a non-uniform magnetic field, Pem initially rises, exhibiting a local maximum, and subsequently declines towards a quasi-steady value. Our results show that both the initial and maximum Pem values increase with increasing magnetic susceptibility, initial concentration of metal solutes, and ion cluster size. Conversely, Pem decreases as the porosity of the medium increases. Finally, the adsorption of metal salts onto the porous media surface is modeled through a dimensionless Damkohler number Daad. Our results suggest that the adsorption significantly slows the magnetophoresis and self-diffusion of the paramagnetic metal salts, with a net magnetophoresis velocity dependent on the kinetics and equilibrium adsorption properties of the metal salts. The latter result underscores the crucial role of adsorption in future magnetophoresis research.

Computational methodology for drug delivery to the inner ear using magnetic nanoparticle aggregates

BACKGROUND AND OBJECTIVE

The main goal of the proposed study is to improve the efficiency of the ear treatment via targeted drug delivery to the inner ear, i.e. the cochlea. Although pharmacotherapy has been proposed as a solution to prevent damage or restore functionality to hair cells, the main challenge in such treatments is ensuring adequate drug delivery to the cells. To this end, we present a methodology for the evaluation of the magnetic forces needed to move magnetic particle nanorobots (abbreviated as MNP) and their aggregates through the cochlea round window membrane (RWM).

METHODS

The FEM - Lagrangian-Eulerian approach (Abaqus software) was used to determine the specific parameters of movement of the nanoparticles crossing the RWM. This method results in a high consistency of FEM simulations and in-vivo experimental results in regards to the required magnetic force during the movement of spherical nanoparticles with a given viscosity η_ave. Based on the analysis of the experimental studies found in subject literature, the sizes of the MNPs and their aggregates able to cross RWM with and without the application of magnetic force F_M have been determined.

RESULTS

The present work accounts for both the experimental and theoretical aspects of these investigations. Presented research confirms the definite usability of the Lagrange-Euler method for the precise determination of the required magnetic force value FM to control the accelerated motion of MNP aggregates of complex shapes through RWM. It is possible to determine the predominant parameters with a precision of less than 5% for single-layer aggregates and spatial aggregates crossing the RWM. It can be concluded that the MNPs and their aggregates should not be larger than 500-750 nm to cross the RWM with high velocities of penetration close to 800 nm/s for magnetic forces of hundreds 10^-14 Newtons.

CONCLUSIONS

The proposed Lagrangian-Eulerian approach is capable of accurately predicting the movement parameters of MNP aggregates of irregular shape that are close to the experimental test cases. The presented method can serve as a supplementary tool for the design of drug delivery systems to the inner ear using MNPs.

Magnetically driven in-plane modulation of the 3D orientation of vertical ferromagnetic flakes

External magnetic fields are known to attract and orient magnetically responsive colloidal particles. In the case of 2D microplatelets, rotating magnetic fields are typically used to orient them parallel to each other in a brick-and-mortar fashion. Thanks to this microstructure, the resulting composites achieve enhanced mechanical and functional properties. However, parts with complex geometries require their microstructure to be specifically tuned and controlled locally in 3D. Although the tunability of the microstructure along the vertical direction has already been demonstrated using magnetic orientation combined with sequential or continuous casting, controlling the particle orientation in the horizontal plane in a fast and effective fashion remains challenging. Here, we propose to use rotating magnetic arrays to control the in-plane orientation of ferromagnetic nickel flakes distributed in curable polymeric matrices. We experimentally studied the orientation of the flakes in response to magnets rotating at various frequencies and precessing angles. Then, we used COMSOL to model the magnetic field from rotating magnetic arrays and predicted the resulting in-plane orientations. To validate the approach, we created composites with locally oriented flakes. This work could initiate reverse-engineering methods to design the microstructure in composite materials with intricate geometrical shapes for structural or functional applications.

Superparamagnetic iron oxide nanoparticles-based detection of neuronal activity

Precise detection of brain regions harboring heightened electrical activity plays a central role in the understanding and treatment of diseases such as epilepsy. Superparamagnetic iron oxide nanoparticles (SPIONs) react to magnetic fields by aggregating and represent interesting candidates as new sensors for neuronal magnetic activity. We hypothesized that SPIONs in aqueous solution close to active brain tissue would aggregate proportionally to neuronal activity. We tested this hypothesis using an in vitro model of rat brain slice with different levels of activity. Aggregation was assessed with dynamic light scattering (DLS) and magnetic resonance imaging (MRI). We found that increasing brain slice activity was associated with higher levels of aggregation as measured by DLS and MRI, suggesting that the magnetic fields from neuronal tissue could induce aggregation in nearby SPIONs in solution. MRI signal change induced by SPIONs aggregation could serve as a powerful new tool for detection of brain electrical activity.

Influence of size polydispersity on magnetic field tunable structures in magnetic nanofluids containing superparamagnetic nanoparticles

We probe the influence of particle size polydispersity on field-induced structures and structural transitions in magnetic fluids (ferrofluids) using phase contrast optical microscopy, light scattering and Brownian dynamics simulations. Three different ferrofluids containing superparamagnetic nanoparticles of different polydispersity indices (PDIs) are used. In a ferrofluid with a high PDI (∼0.79), thin chains, thick chains, and sheets are formed on increasing the in-plane magnetic field, whereas isotropic bubbles, and hexagonal and lamellar/stripe structures are formed on increasing the out-of-plane magnetic field over the same range. In contrast, no field-induced aggregates are seen in the sample with low polydispersity under the above conditions. In a polydisperse sample, bubbles are formed at a very low magnetic field strength of 30 G. Insights into the structural evolution with increasing magnetic field strength are obtained by carrying out Brownian dynamics simulations. The crossovers from isotropic, through hexagonal columnar, to lamellar/stripe structures observed with increasing field strength in the high-polydispersity sample indicate the prominent roles of large, more strongly interacting particles in structural transitions in ferrofluids. Based on the observed microstructures, a phase diagram is constructed. Our work opens up new opportunities to develop optical devices and access diverse structures by tuning size polydispersity.

Feasibility of implantable iron oxide nanoparticles in detecting brain activity-proof of concept in a rat model

BACKGROUND

Precise detection of zones of increased brain activity is a crucial aspect in the delineation of the cortical region responsible for epilepsy (epileptic focus). When possible, removal of this area can lead to improved control of epilepsy or even its cure. This study explores a new method of detection of electrical brain activity based on the surgical implantation of iron oxide superparamagnetic nanoparticles (SPIONs). By their magnetic nature, SPIONs tend to aggregate in the presence of magnetic fields. This study aims to demonstrate if brain's magnetic fields could change the aggregation status of SPIONs in a rat model.

METHODS

Plastic containers (capsules) containing SPIONs in aqueous suspension were implanted over the cortex of either rats rendered epileptic or naive rats (sham). A model of focal epilepsy using cortical penicillin injection was used for the epileptic rats. Capsules not implanted in rats served as control. Using magnetic resonance imaging (MRI), the aggregation status of SPIONs contained in the capsules was assessed via measurement of the T₂ relaxivity time of the solutions.

RESULTS

Eight Rats were used for the experiments, with 4 rats in each group (epileptic and sham). One Rat in the sham group died immediately after surgery and 3 rats failed to demonstrate the expected behavior after intervention (2 rats in epileptic group with limited observable seizures and 1 rat in the sham group having repeated seizures). T₂ of the control capsules were significantly lower than those implanted in rats (146 ms vs 7.6 ms, p < 0.001), suggesting a higher degree of SPIONs aggregation in the implanted capsules. No significant difference in T₂ could be demonstrated between epileptic and sham rats.

CONCLUSIONS

SPIONs implanted over the cortex of active brain showed an increased aggregation status, confirming their potential as a new marker for brain activity. One of the main advantages of SPIONs is that their aggregation status can be measured at a distance with MRI, taking advantage of its high spatial resolution and imaging capacities. The current model was suboptimal to confirm if epileptic activity can be differentiated from normal brain activity using SPIONs.

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.

Printing 1D Assembly Array of Single Particle Resolution for Magnetosensing

Magnetosensing is a ubiquitous ability for many organism species in nature. 1D assembly, especially that arranged in single-particle-resolution regulation, is able to sense the direction of magnetic field depending on the enhanced dipolar interaction in the linear orientation. Inspired by the magnetosome structure in magnetotactic bacteria, a 1D assembly array of single particle resolution with controlled length and well-behaved configuration is prepared via inkjet printing method assisted with magnetic guiding. In the fabrication process, chains in a "tip-to-tip" regulation with the desired number of particles are prepared in a confined tiny inkjet-printed droplet. By adjusting the receding angle of the substrate, the assembled 1D morphology is kept/deteriorated depending on the pinning/depinning behavior during ink evaporation, which leads to the formation of well-behaved 1D assembly/aggregated dot assembly. Owing to the high-aspect-ratio characteristic of the assembled structure, the as-prepared 1D arrays can be used for magnetic field sensing with anisotropic magnetization M_// /M_⊥ up to 6.03.

A numerical model for aggregations formation and magnetic driving of spherical particles based on OpenFOAM®

BACKGROUND AND OBJECTIVE

This work presents a numerical model for the formation of particle aggregations under the influence of a permanent constant magnetic field and their driving process under a gradient magnetic field, suitably created by a Magnetic Resonance Imaging (MRI) device.

METHODS

The model is developed in the OpenFOAM platform and it is successfully compared to the existing experimental and numerical results in terms of aggregates size and their motion in water solutions. Furthermore, several series of simulations are performed for two common types of particles of different diameter in order to verify their aggregation and flow behaviour, under various constant and gradient magnetic fields in the usual MRI working range. Moreover, the numerical model is used to measure the mean length of aggregations, the total time needed to form and their mean velocity under different permanent and gradient magnetic fields.

RESULTS

The present model is found to predict successfully the size, velocity and distribution of aggregates. In addition, our simulations showed that the mean length of aggregations is proportional to the permanent magnetic field magnitude and particle diameter according to the relation : l¯_a=7.5B₀d_i^3/2. The mean velocity of the aggregations is proportional to the magnetic gradient, according to : u¯_a=6.63G˜B₀ and seems to reach a steady condition after a certain period of time. The mean time needed for particles to aggregate is proportional to permanent magnetic field magnitude, scaled by the relationship : t¯_a∝7B₀.

CONCLUSIONS

A numerical model to predict the motion of magnetic particles for medical application is developed. This model is found suitable to predict the formation of aggregations and their motion under the influence of permanent and gradient magnetic fields, respectively, that are produced by an MRI device. The magnitude of the external constant magnetic field is the most important parameter for the aggregations formation and their driving.

Antimicrobial Lemongrass Essential Oil-Copper Ferrite Cellulose Acetate Nanocapsules

Cellulose acetate (CA) nanoparticles were combined with two antimicrobial agents, namely lemongrass (LG) essential oil and Cu-ferrite nanoparticles. The preparation method of CA nanocapsules (NCs), with the two antimicrobial agents, was based on the nanoprecipitation method using the solvent/anti-solvent technique. Several physical and chemical analyses were performed to characterize the resulting NCs and to study their formation mechanism. The size of the combined antimicrobial NCs was found to be ca. 220 nm. The presence of Cu-ferrites enhanced the attachment of LG essential oil into the CA matrix. The magnetic properties of the combined construct were weak, due to the shielding of Cu-ferrites from the polymeric matrix, making them available for drug delivery applications where spontaneous magnetization effects should be avoided. The antimicrobial properties of the NCs were significantly enhanced with respect to CA/LG only. This work opens novel routes for the development of organic/inorganic nanoparticles with exceptional antimicrobial activities.

Magnetic field induced push–pull motility of liquibots

Self-propelling liquibots as transport and delivery vehicles.