Stefan Odenbach

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Drug loaded magnetic nanoparticles for cancer therapy

Magnetic nanoparticles have been investigated for biomedical applications for more than 30 years. In medicine they are used for several approaches such as magnetic cell separation or magnetic resonance imaging (MRI). The development of biocompatible nanosized drug delivery systems for specific targeting of therapeutics is the focus of medical research, especially for the treatment of cancer and diseases of the vascular system. In an experimental cancer model, we performed targeted drug delivery and used magnetic iron oxide nanoparticles, bound to a chemotherapeutic agent, which were attracted to an experimental tumour in rabbits by an external magnetic field (magnetic drug targeting). Complete tumour remission could be achieved. An important advantage of these carriers is the possibility for detecting these nanoparticles after treatment with common imaging techniques (i.e. x-ray-tomography, magnetorelaxometry, magnetic resonance imaging), which can be correlated to histology.

Ferrofluids—magnetically controlled suspensions

Abstract Suspensions of magnetic nanoparticles exhibit normal liquid behaviour coupled with superparamagnetic properties. This leads to the possibility to control the properties and the flow of these liquids with moderate magnetic fields. The magnetic control enables the design of various applications as well as basic experiments in hydrodynamics. Ferrofluids and their general properties will be introduced and as an example the control of their viscous properties by means of magnetic fields will be discussed to show the potential of magnetic fluid control.

Shear dependence of field-induced contributions to the viscosity of magnetic fluids at low shear rates

Abstract Viscoelastic properties of ferrofluids are an upcoming field of scientific interest, since the magnetic control of the related fluid behavior would give rise to new applications as well as for new possibilities in basic research concerning viscoelasticity. We have constructed a specialized rheometer for the investigation of fluids under the influence of magnetic fields, to examine such effects in stable suspensions of magnetic particles. In particular we will report the change of field-induced increase of viscosity due to variation of the shear rate applied to the fluid. The results show that the available theoretical approach, namely the concept of rotational viscosity, is not valid for the description of the field-induced increase of viscosity in concentrated fluids at low shear rates.

Investigation of the microscopic reason for the magnetoviscous effect in ferrofluids studied by small angle neutron scattering

Experimental studies made on different ferrofluid samples under shear flow have shown that an increase of magnetic field strength yields an increase of the fluids viscosity, the so-called magnetoviscous effect, while increasing shear rate leads to a decrease of the viscosity. The change of the viscosity with magnetic field strength can be theoretically explained as an effect of chain-like structure formation and therefore can be related to the modification of the microstructure of ferrofluids. Using a specially designed rheometer, ferrofluids having different magnitude of the magnetoviscous effect were investigated by small angle neutron scattering (SANS). Correlated to the structure formation in the fluid, the scattered intensity shows a variation with magnetic field and shear rate only for fluids with a high magnetoviscous effect. The results obtained show a good agreement with the qualitative model elaborated to explain the magnetoviscous effect, indicating a strong connection between the rheological behaviour of ferrofluids and their microstructure.

X-ray micro-tomographic characterization of field-structured magnetorheological elastomers

Anisotropic magnetorheological elastomers (MREs) with four different mass percentages of iron powder were prepared in an external magnetic field. The inner structure of the samples was characterized by using computed tomography. It has been shown that this kind of non-destructive analysis of MRE samples can be efficiently used for a detailed structural investigation. It was found that even small changes in the mass content of the magnetic filler led to the formation of completely different morphologies, which were reproducible for all samples. There were the familiar column formations in patterns with a mass content of ~ 5% iron powder. Increasing the mass fraction to ~ 14% resulted in the formation of tubular structures. Samples with ~ 23 and ~ 33 wt% had a densely packed structure, where the particle formations broke up: meanders without particles penetrate the samples over the entire height like canyons.

Rheological properties of dense ferrofluids. Effect of chain-like aggregates

Rheological properties of dense ferrofluid are studied both experimentally and theoretically. Experimental dependence of the fluid effective viscosity on magnetic field is much more than predicted by known theories. New theoretical model is suggested to explain and describe these results. This model is based on assumption that linear chain-like aggregates appear in the ferrofluid; these chains induce strong magnetoviscous effect. The results of the theoretical calculations are in good agreement with the experiments.

Direct Detection of Domains in Phospholipid Bilayers by Grazing Incidence Diffraction of Neutrons and Atomic Force Microscopy

The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10 nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10 nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.

Development of a lauric acid/albumin hybrid iron oxide nanoparticle system with improved biocompatibility

The promising potential of superparamagnetic iron oxide nanoparticles (SPIONs) in various nanomedical applications has been frequently reported. However, although many different synthesis methods, coatings, and functionalization techniques have been described, not many core-shell SPION drug delivery systems are available for clinicians at the moment. Here, bovine serum albumin was adsorbed onto lauric acid-stabilized SPIONs. The agglomeration behavior, zeta potential, and their dependence on the synthesis conditions were characterized with dynamic light scattering. The existence and composition of the core-shell-matrix structure was investigated by transmission electron microscopy, Fourier transform infrared spectroscopy, and zeta potential measurements. We showed that the iron oxide cores form agglomerates in the range of 80 nm. Moreover, despite their remarkably low tendency to aggregate even in a complex media like whole blood, the SPIONs still maintained their magnetic properties and were well attractable with a magnet. The magnetic properties were quantified by vibrating sample magnetometry and a superconducting quantum interference device. Using flow cytometry, we further investigated the effects of the different types of nanoparticle coating on morphology, viability, and DNA integrity of Jurkat cells. We showed that by addition of bovine serum albumin, the toxicity of nanoparticles is greatly reduced. We also investigated the effect of the particles on the growth of primary human endothelial cells to further demonstrate the biocompatibility of the particles. As proof of principle, we showed that the hybrid-coated particles are able to carry payloads of up to 800 μg/mL of the cytostatic drug mitoxantrone while still staying colloidally stable. The drug-loaded system exhibited excellent therapeutic potential in vitro, exceeding that of free mitoxantrone. In conclusion, we have synthesized a biocompatible ferrofluid that shows great potential for clinical application. The synthesis is straightforward and reproducible and thus easily translatable into a good manufacturing practice environment.

XμCT analysis of magnetic field-induced phase transitions in magnetorheological elastomers

The phase behavior of polymer solutions and composites is a complex issue and is of both technological and fundamental interest. For a better understanding of the microstructure formation in magnetorheological (MR) elastomers, x-ray micro-computed tomography (XμCT) investigations were carried out. Magnetorheological elastomers with 5% mass content of iron powder were prepared under different magnetic field strengths between 1 and 220 kA m−1. Through quantitative analysis, valuable information was obtained regarding the number, size and frequency distribution of column structures in MR elastomers, as well as the magnetic field required to force structure formation.