Vladimir B. Bregar

Advantages of ferromagnetic nanoparticle composites in microwave absorbers

Composites with nanosize ferromagnetic particles can be useful in microwave absorbers, since nanoparticles exhibit distinct magnetic properties compared to bulk materials. This paper examines theoretically the properties of composites with nanoparticles, the key material parameters, and the characteristics of a single-layer absorber made from a nanoparticle composite. In such an absorber, high magnetic losses over a wide frequency range induce a series of strong and wide absorption peaks at increasing frequencies. By using metallic (iron and cobalt) nanoparticles, absorbers with relatively low volume fraction of metallic inclusions can be made. The paper compares the characteristics of nanocomposite absorbers to those of common dielectric or ferromagnetic absorbers and identifies the potential advantages of nanocomposite absorbers. Because nanocomposite absorbers combine advantages of both dielectric and ferromagnetic absorbers and have significantly better characteristics, they could become essential for production of microwave and millimeter-wave absorbers.

Surface modified magnetic nanoparticles for immuno-gene therapy of murine mammary adenocarcinoma

Cancer immuno-gene therapy is an introduction of nucleic acids encoding immunostimulatory proteins, such as cytokine interleukin 12 (IL-12), into somatic cells to stimulate an immune response against a tumor. Various methods can be used for the introduction of nucleic acids into cells; magnetofection involves binding of nucleic acids to magnetic nanoparticles with subsequent exposure to an external magnetic field. Here we show that surface modified superparamagnetic iron oxide nanoparticles (SPIONs) with a combination of polyacrylic acid (PAA) and polyethylenimine (PEI) (SPIONs-PAA-PEI) proved to be safe and effective for magnetofection of cells and tumors in mice. Magnetofection of cells with plasmid DNA encoding reporter gene using SPIONs-PAA-PEI was superior in transfection efficiency to commercially available SPIONs. Magnetofection of murine mammary adenocarcinoma with plasmid DNA encoding IL-12 using SPIONs-PAA-PEI resulted in significant antitumor effect and could be further refined for cancer immuno-gene therapy.

Visualization of internalization of functionalized cobalt ferrite nanoparticles and their intracellular fate.

In recent years, nanoparticles (NPs) and related applications have become an intensive area of research, especially in the biotechnological and biomedical fields, with magnetic NPs being one of the promising tools for tumor treatment and as MRI-contrast enhancers. Several internalization and cytotoxicity studies have been performed, but there are still many unanswered questions concerning NP interactions with cells and NP stability. In this study, we prepared functionalized magnetic NPs coated with polyacrylic acid, which were stable in physiological conditions and which were also nontoxic short-term. Using fluorescence, scanning, and transmission electron microscopy, we were able to observe and determine the internalization pathways of polyacrylic acid–coated NPs in Chinese hamster ovary cells. With scanning electron microscopy we captured what might be the first step of NPs internalization – an endocytic vesicle in the process of formation enclosing NPs bound to the membrane. With fluorescence microscopy we observed that NP aggregates were rapidly internalized, in a time-dependent manner, via macropinocytosis and clathrin-mediated endocytosis. Inside the cytoplasm, aggregated NPs were found enclosed in acidified vesicles accumulated in the perinuclear region 1 hour after exposure, where they stayed for up to 24 hours. High intracellular loading of NPs in the Chinese hamster ovary cells was obtained after 24 hours, with no observable toxic effects. Thus polyacrylic acid–coated NPs have potential for use in biotechnological and biomedical applications.

Effect of different parameters used for in vitro gene electrotransfer on gene expression efficiency, cell viability and visualization of plasmid DNA at the membrane level

Gene electrotransfer is a nonviral method used for DNA delivery into cells. Several steps are involved. One of them is the interaction of DNA with the cell membrane, which is crucial before DNA can enter the cell. We analysed the level of DNA–membrane interaction in relation to electrotransfer efficiency and the importance of the electrophoretic accumulation of DNA at the cell membrane. Systematic comparison of long‐duration, short‐duration and combinations of electropermeabilizing short (high‐voltage; HV) and electrophoretic long (low‐voltage; LV) pulses were performed. The effect of Mg2+ ion concentrations on electrotransfer and their effect on DNase activity were explored.

Dispersion of Nanoparticles in Different Media Importantly Determines the Composition of Their Protein Corona

Protein corona of nanoparticles (NPs), which forms when these particles come in to contact with protein-containing fluids, is considered as an overlooked factor in nanomedicine. Through numerous studies it has been becoming increasingly evident that it importantly dictates the interaction of NPs with their surroundings. Several factors that determine the compositions of NPs protein corona have been identified in recent years, but one has remained largely ignored—the composition of media used for dispersion of NPs. Here, we determined the effect of dispersion media on the composition of protein corona of polyacrylic acid-coated cobalt ferrite NPs (PAA NPs) and silica NPs. Our results confirmed some of the basic premises such as NPs type-dependent specificity of the protein corona. But more importantly, we demonstrated the effect of the dispersion media on the protein corona composition. The differences between constituents of the media used for dispersion of NPs, such as divalent ions and macromolecules were responsible for the differences in protein corona composition formed in the presence of fetal bovine serum (FBS). Our results suggest that the protein corona composition is a complex function of the constituents present in the media used for dispersion of NPs. Regardless of the dispersion media and FBS concentration, majority of proteins from either PAA NPs or silica NPs coronas were involved in the process of transport and hemostasis. Interestingly, corona of silica NPs contained three complement system related proteins: complement factor H, complement C3 and complement C4 while PAA NPs bound only one immune system related protein, α-2-glycoprotein. Importantly, relative abundance of complement C3 protein in corona of silica NPs was increased when NPs were dispersed in NaCl, which further implies the relevance of dispersion media used to prepare NPs.

Cell type-specific response to high intracellular loading of polyacrylic acid-coated magnetic nanoparticles

Magnetic nanoparticles (NPs) are a special type of NP with a ferromagnetic, electron-dense core that enables several applications such as cell tracking, hyperthermia, and magnetic separation, as well as multimodality. So far, superparamagnetic iron oxide NPs (SPIONs) are the only clinically approved type of metal oxide NPs, but cobalt ferrite NPs have properties suitable for biomedical applications as well. In this study, we analyzed the cellular responses to magnetic cobalt ferrite NPs coated with polyacrylic acid (PAA) in three cell types: Chinese Hamster Ovary (CHO), mouse melanoma (B16) cell line, and primary human myoblasts (MYO). We compared the internalization pathway, intracellular trafficking, and intracellular fate of our NPs using fluorescence and transmission electron microscopy (TEM) as well as quantified NP uptake and analyzed uptake dynamics. We determined cell viability after 24 or 96 hours’ exposure to increasing concentrations of NPs, and quantified the generation of reactive oxygen species (ROS) upon 24 and 48 hours’ exposure. Our NPs have been shown to readily enter and accumulate in cells in high quantities using the same two endocytic pathways; mostly by macropinocytosis and partially by clathrin-mediated endocytosis. The cell types differed in their uptake rate, the dynamics of intracellular trafficking, and the uptake capacity, as well as in their response to higher concentrations of internalized NPs. The observed differences in cell responses stress the importance of evaluation of NP–cell interactions on several different cell types for better prediction of possible toxic effects on different cell and tissue types in vivo.

The effect of complex permeability and agglomeration on composite magnetic systems: A three-dimensional numerical analysis and comparison with analytical models

Magnetic permeability of a composite is investigated using three-dimensional numerical models for static and quasistatic case. To analyze the effect of agglomeration and imaginary part of intrinsic permeability on the effective permeability, inclusions with complex permeability and having random distribution or random distribution with agglomeration were used. Significant deviation of analytical effective medium and Maxwell–Garnett models from numerical results for imaginary part of permeability was observed. We obtained that agglomeration increases the real part of the effective permeability for higher volume fractions and higer intrinsic permeabilities. The effect of agglomeration is even more pronounced for the imaginary part of the effective permeability. Our results thus show that agglomeration and complex intrinsic permeability could explain experimentally observed effective permeability.

Optimization, design, and modeling of ferrite core geometry for inductive wireless power transfer

This paper presents a new design of ferrite core made of ferrite bars positioned in radial geometry. New core design is used in inductive wireless power transfer where the optimal design of ferrite bars was analyzed. Numerical simulations of seven ferrite bars geometries were performed in 3D models using Comsol Multiphysics in terms of the coupling coefficient. Two optimal designs of three and nine ferrite bars geometries were used for parameterization of geometric parameters. Both simu- lations of three and nine ferrite bars geometries were verified by measurements. Optimal design of nine ferrite bars geometry is proposed for use in wireless power transfer because of low consumption of ferrite material. Ferrite bars geometry was also used as a magnetic shield to reduce magnetic fields which may interfere with the electronics nearby. Magnetic flux density in ferrite bars is low enough that prevents ferrite bars from the saturation. The angular alignment of optimal design of nine ferrite bars on the transmitter and receiver side was negligible as the difference in the coupling coef- ficient was 0.13%. Advantages of using optimal design of nine ferrite bars geometry are: high value of the coupling coefficient, low consumption of ferrite material, and reduced weight.

Study of Coupling on Parallel Microstrip Lines Due to Magnetic-Loaded Absorber Sheet

Coupling effect of magnetic-loaded absorbing materials on parallel microstrip lines was investigated numerically and experimentally. A microstrip circuits with two 50 Omega parallel lines with different distances between the lines were used to experimentally evaluate coupling effect and the same configuration was modeled and evaluated with finite-element method numerical program. Results showed that absorber sheets, placed over microstrip lines significantly increase coupling at frequencies up to few GHz, depending on the absorber geometry. At higher frequencies, absorber sheets decrease coupling mostly due to dielectric and/or magnetic losses. Both numerical and experimental study show that permittivity has greater effect on coupling than permeability, which suggests that coupling is primarily due to electric field and not magnetic field. Further, we analyzed the geometry effects on the coupling, where we studied influence of length, thickness and different positions of absorber sheet on microstrip and also examined the effect of air gap between microstrip lines and absorber sheet.

Numerical study of effective permeability of soft-magnetic composites with conductive inclusions

We used three-dimensional finite-element modeling in order to study the effect of the microstructure (random, agglomerated, and percolated distributions of inclusions) on the effective permeability of a magnetic composite with incorporation of complex intrinsic properties in a frequency domain. The study includes the effect of the inclusions’ conductivity and eddy currents on the difference in the permeability. The results show a significant effect of inclusions distribution on the effective permeability of the composites for both nonconductive and conductive particles. We further show that in larger agglomerates with well connected conductive particles the eddy currents shield inner particles and significantly increase the imaginary effective permeability. Comparison with our experimental data from a composite with carbonyl iron inclusions showed that the particle agglomeration at small volume fractions gives similar frequency dependence for the effective permeability as in the case of composites with we...