Table of contents

This issue of Journal of Physics: Condensed Matter is dedicated to results in the field of ferrofluid research.

Ferrofluids—suspensions of magnetic nanoparticles—exhibit as a specific feature the magnetic control of their physical parameters and of flows appearing in such fluids. This magnetic control can be achieved by means of moderate magnetic fields with a strength of the order of 10 mT. This sort of magnetic control also enables the design of a wide variety of technical applications such as the use of the magnetic forces for basic research in fluid dynamics. The overall field of ferrofluid research is already about 40 years old. Starting with the first patent on the synthesis of magnetic nanoparticle suspensions by S Papell in 1964, a vivid field of research activities has been established. Looking at the long time in which ferrofluids have been the focus of scientific interest, one can ask the question, what kind of recent developments justify a special issue of a scientific journal?

New developments in a field, which depends strongly on a certain material class and which opens research possibilities in different scientific fields will nowadays usually require an interdisciplinary approach. This kind of approach starting from the synthesis of magnetic suspensions, including research concerning their basic properties and flow behaviour and focusing on new applications has been the core of a special research programme funded by the Deutsche Forschungsgemeinschaft (DFG) over the past 6 years. Within this programme—entitled `Colloidal Magnetic Fluids: Basics, Synthesis and Applications of New Ferrofluids'—more than 30 different research groups have been coordinated to achieve new results in various fields related to ferrofluid research.

The basic approach of the program has been the assumption that new applications well beyond the typical ferrofluid techniques, for example loud speaker cooling or sealing of rotary shafts, will require tailored magnetic suspensions with properties clearly focused towards the need of the application. While such tailoring of fluids to certain well defined properties sounds like a straightforward approach one has to face the fact that it requires a clear definition of the required properties. This definition itself has to be based on a fundamental physical knowledge of the processes determining certain magnetically controlled phenomena in ferrofluids. To make this point concrete one can look into the detailed aims of the mentioned research program. The application areas identified for the future development of research and application of suspensions of magnetic nanoparticles have been on the one hand the biomedical application—especially with respect to cancer treatment—and on the other hand the use of magnetically controlled rheological properties of ferrofluids for new active technical devices.

Both directions require, as mentioned, as a basis for success the synthesis of new ferrofluids with dedicated properties. While the medical applications have to rely on biocompatibility as well as on stability of the suspensions in a biomedical environment, the use of ferrofluids in technical devices employing their magnetically controlled rheological properties will depend on an enhancement of the changes of the fluid's viscous properties in the presence of moderate magnetic fields. For both requirements ferrofluids with a make up clearly different from the usual magnetite based fluids have to be synthesized. The question of how the detailed microscopic make up of the fluids would have to look has to be answered on the basis of basic research results defining the physics background of the respective phenomena. Taking these aspects together it becomes obvious that the aforementioned research program had goals aiming far beyond the state of the art of classical ferrofluid research.

These goals as well as the basic strategy to achieve them is in a way reflected by the structure of this issue of Journal of Physics: Condensed Matter. The issue contains results emerging from the research programme as well as invited papers from researchers not participating in the programme but working in closely related areas. The issue is subdivided into five main sections dealing with synthesis, basic physical description, rheology, and both the medical and technical applications of ferrofluids. As can be expected from work done within an interdisciplinary context many of the papers would fit into more than one of these sections and catagorization is thus sometimes difficult. We have therefore tried to place them into the section reflecting the main field of research to which the respective results belong.

The first section is on synthesis and characterization of magnetic suspensions. The first paper in this section is dedicated partly to magnetite ferrofluids but with special aspects concerning the particle size tailoring them for applications especially in the field of magnetic hyperthermia. After this, three different types of `new' ferrofluids are presented. Fluids based on pure metal particles exhibiting much stronger magnetic properties than the common magnetite fluids, fluids with a temperature sensitive surfactant shell allowing a change of the particle's hydrodynamic diameter by variation of the fluid's temperature and fluids containing spheres of nonmagnetic material with embedded magnetic particles which are already used in new medical applications.

The second section is dedicated to the basic physics of ferrofluids and highlights three different topics. First the question of magnetization dynamics is discussed and different aspects of this fundamental problem, which determines the basic description of ferrofluids, are highlighted. The second topic is the well known surface instability appearing in ferrofluids in a homogeneous magnetic field perpendicular to the fluid surface. This part shows clearly how many undiscovered phenomena can be found, even in an area which is as old as the whole research field, if an appropriate measuring technique is used and fresh ideas help to find unexpected effects. The last part of this section deals with the question of dynamics and structure of ferrofluids and shows the experimental possibilities of scattering techniques in this field.

Within the third section the question of field dependent changes of the rheological behaviour of ferrofluids is discussed. The first three papers provide theoretical approaches for the understanding of the connection between the rheological properties and shear and field induced changes in the fluid's microstructure. The fourth paper provides the related experimental results showing the combination of microstructural and rheological measurements under well defined conditions. The last paper of this section does not directly belong to ferrofluid research but to a closely related field—so called magneto-rheological (MR) suspensions, which differ from ferrofluids mainly by the size of the suspended particles and the strength of the rheological effects. As modern theoretical approaches, like the one discussed by Liu et al in the second section have shown, the relation between the effects in ferrofluids and those in MR fluids is so close that it could probably be described in a common theory.

Sections four and five contain the application orientated results. In the fourth section the medical applications are the focus of interest. The section starts with a paper which could have also been placed in the synthesis section—the growth of magnetotactic bacteria and the extraction of the magnetic particles produced by these bacteria. The paper also contains information about the characterization of the particles especially with respect to their application. The characterization aspect is then continued in two papers outlining new diagnostic techniques with close relation to future biomedical application of magnetic fluids. Next in vitro applications, especially questions of cell separation using magnetic forces, are highlighted before the final papers address the therapeutic aspects of magnetic drug targeting and magnetic hyperthermia. Finally the fifth section describes three different new approaches for the technical use of ferrofluids. Again, the specialized design of the fluids themselves is an important step towards the new application goals.

Altogether the papers within this issue outline the unique potential of magnetically controlled suspensions, the interdisciplinary nature of the related research and the prospects of strongly networked and interdisciplinary activities in the field. I hope that it will give an insight into the fascination of ferrofluid research and a feeling for the advances made in the past years.

Magnetic fluids based on Ba hexaferrite as well as iron oxide particles with enhanced anisotropy barriers show heating effects in ac magnetic fields which may be useful in technical processes as well as medical applications (magnetic hyperthermia). Such particles also allow the detection of biological binding reactions through an enhanced Néel relaxation time above the Brown relaxation. The loss processes and the relaxation times depend strongly on the mean particle size and the size distribution width. To influence and improve the mean size as well as the size distribution, new approaches to the preparation are promising, where nucleation and growth of the particles can be influenced independently or where further growth is possible on small given particles without further nucleation. We used a glass crystallization method for preparation of nanocrystalline Ba hexaferrite as well as magnetic iron oxide and a cyclic growth method based on 'conventional' precipitation for iron oxide preparation. Properties of the powders prepared, as well as water based ferrofluids, were analysed using x-ray diffraction, transmission electron microscopy and magnetic methods. Values of the specific loss power of the order of 100 W/g maghemite may be achieved with the option of further increase by improving the core size distribution.

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.

Magnetic core–shell particles were synthesized by the attachment of a polymeric brush on the surface of magnetite nanoparticles. The hybrid particles are well dispersible in good solvents for the polymeric shell and form thermoreversible magnetic fluids in carrier fluids with a critical solution behaviour. The thermoresponsive effect can be activated by the application of a high frequency electromagnetic field and may be useful for magnetic separation, recoverable catalysts and for drug release purposes.

As superparamagnetic nanoparticles capture new applications and markets, the flexibility and modifications of these nanoparticles are increasingly important aspects. Therefore a series of magnetic polystyrene particles encapsulating magnetite nanoparticles (10–12 nm) in a hydrophobic poly(styrene-co-acrylic acid) shell was synthesized by a three-step miniemulsion process. A high amount of iron oxide was incorporated by this process (typically 30–40% (w/w)). As a second reporter, a fluorescent dye was also integrated in order to obtain 'dual reporter particles'. Finally, polymerization of the monomer styrene yielded nanoparticles in the range 45–70 nm. By copolymerization of styrene with the hydrophilic acrylic acid, the amount of carboxyl groups on the surface was varied. The characterization of the latexes included dynamic light scattering, transmission electron microscopy, surface charge and magnetic measurements. For biomedical evaluation, the nanoparticles were incubated with different cell types. The introduction of carboxyl groups on the particle surfaces enabled the uptake of nanoparticles as demonstrated by the detection of the fluorescent signal by fluorescent activated cell sorter (FACS) and laser scanning microscopy. The quantity of iron in the cells that is required for most biomedical applications (like detection by magnetic resonance imaging) has to be significantly higher, as can be achieved by the uptake of magnetite encapsulated nanoparticles functionalized only with carboxyl groups. A further increase of uptake can be accomplished by transfection agents like poly-L-lysine or other positively charged polymers. This functionality was also engrafted into the surface of the nanoparticles by covalently coupling lysine to the carboxyl groups. The amount of iron that can be transfected was even higher than with the nanoparticles with a transfection agent added and this only physically adsorbed. Furthermore, the subcellular localization of these nanoparticles was demonstrated to be clustered in endosomal compartments.

In this paper we present a model which allows numerical studies of ferrofluid dynamics taking into account the internal magnetic degrees of freedom of the ferrofluid particles. In standard ferrofluid models the magnetic moment of a ferrofluid particle is supposed to be fixed with respect to the particle itself, which corresponds to the limit of an infinitely high single-particle magnetic anisotropy. In contrast to this strongly simplifying assumption, we take into account that in real ferrofluids the magnetic moments of ferrofluid particles are allowed to rotate with respect to the particles themselves. Our model results in a system of equations of motion describing both magnetic and mechanical degrees of freedom, where the 'magnetic' equations are coupled with the 'mechanical' equations via (i) the interparticle distances determining the magnetodipolar interaction fields and (ii) orientations of the particle anisotropy axes with respect to their magnetic moments which define the mechanical torque on the particle.

Using our model we have studied the ferrofluid magnetization dynamics for various particle concentrations, i.e., for various magnetodipolar interaction strengths. In particular, we present numerical results (a) the magnetization relaxation of a ferrofluid after the external field is switched off and (b) the frequency dependence of the ferrofluid AC susceptibility. Comparing these results with the corresponding dependences obtained for the rigid dipoles model, we demonstrate that for magnetic anisotropy values typical for commonly used ferrofluid materials (like magnetite) the inclusion of 'magnetic' degrees of freedom is qualitatively essential to obtain a correct description of the ferrofluid dynamics.

The relaxation of magnetization, a well-accepted part of ferrofluid dynamics, is shown to give rise to a broad range of non-Newtonian behaviour in ferrofluids, including shear thinning or shear thickening, normal stress differences, a visco-elastic response and a varying Trouton, or elongational, viscosity.

The influence of polydispersity on the magnetization is analysed in a non-equilibrium situation where a cylindrical ferrofluid column is forced to rotate with constant frequency, like a rigid body in a homogeneous magnetic field that is applied perpendicular to the cylinder axis. Then, the magnetization and the internal magnetic field are no longer parallel to each other and their directions differ from that of the applied magnetic field. Experimental results on the transverse magnetization component perpendicular to the applied field are compared and analysed as functions of rotation frequency and field strength with different polydisperse Debye models that take into account the polydispersity in different ways and to a varying degree.

We report on different surface patterns on magnetic liquids following the Rosensweig instability. We compare the bifurcation from the flat surface to a hexagonal array of spikes with the transition to squares at higher fields. From a radioscopic mapping of the surface topography we extract amplitudes and wavelengths. For the hexagon–square transition, which is complex because of coexisting domains, we tailor a set of order parameters like peak-to-peak distance, circularity, angular correlation function and pattern specific amplitudes from Fourier space. These measures enable us to quantify the smooth hysteretic transition. Voronoi diagrams indicate a pinning of the domains. Thus the smoothness of the transition is roughness on a small scale.

The numerical treatment of free surface problems in ferrohydrodynamics is considered. Starting from the general model, special attention is paid to field–surface and flow–surface interactions. Since in some situations these feedback interactions can be partly or even fully neglected, simpler models can be derived. The application of such models to the numerical simulation of dissipative systems, rotary shaft seals, equilibrium shapes of ferrofluid drops, and pattern formation in the normal-field instability of ferrofluid layers is given. Our numerical strategy is able to recover solitary surface patterns which were discovered recently in experiments.

We present a nonlinear description of the Rosensweig instability in isotropic magnetic gels based on the energy minimizing method used by Gailitis to describe the Rosensweig instability in typical ferrofluids. We extend his discussion to media with elastic degrees of freedom, assuming the shear modulus as a perturbation to the pure fluid case. We study the relative stability of the regular planforms of stripes, squares and hexagons as a function of the elastic shear modulus.

Experiments and Brownian dynamics simulations have been coupled in order to better understand structural and dynamical properties of ferrofluids, especially the role of the magnetic dipolar interaction. The ferrofluid used is a 'home-made' well defined suspension, the experimental characteristics of which are introduced in the modelled system. In this system, the determination of the structure using simulations and small angle neutron scattering (SANS) experiments shows no sign of chaining in the suspensions, both without and with a magnetic field. Nevertheless, on the scale of the interparticle distance, the structure is strongly anisotropic. This is at contrast with the weak anisotropy of the translational diffusion coefficient under magnetic field on the same scale. Moreover, on the macroscopic scale, both structure and translational dynamics are strongly anisotropic. Also the rotational diffusion is strongly modified if determined without field or after a weak orientation of the particles with a weak field. These results all emphasize the role of the collective phenomena associated with the dipolar interaction.

By coating cobalt ferrite nanoparticles with a silica shell, the polydispersity of the resulting core–shell particles can be reduced. Hereby, opposite to the case for conventional ferrofluids, self-organization to liquid-like and even crystalline structures in aqueous media is enabled. The resulting structures mainly originate from the predominant electrostatic repulsion of colloidal macroions bearing charged groups at the surface of the silica shell. Due to the small magnetic moment of the cobalt ferrite cores, however, these structures can be influenced by external magnetic fields or field gradients. While field gradients act as a magnetic trap for these particles, homogeneous fields induce an aligning of the magnetic momenta. Hereby a decrease of symmetry from spherical to cylindrical symmetry of the structures appearing can be observed. Due to collective phenomena, even interactions significantly smaller than the thermal energy can induce clearly observable structural distortions. Even in the absence of an external field, suspensions of such magnetic particles show an unexpected slow diffusion caused by hydrodynamic interactions.

Polarized small angle neutron scattering (SANSPOL) was used to investigate the microstructure of various ferrofluids (FF) where magnetic materials (Co, Fe magnetite), stabilization mechanisms (electrostatic, monolayers and bilayers of surfactants) and carrier liquids (water, organic solvents) have been systematically varied. Magnetic core–shell particles, non-magnetic micelles and magnetic aggregates were identified and size distributions and density, composition, and magnetization profiles were determined. Partial penetrations of solvent molecules inside the surfactant layer and formation of non-magnetic oxide coatings were established. The magnetic nanostructure in diluted samples consists of non-interacting ferromagnetic single domain particles. In concentrated Co FF a pseudo-crystalline ordering was found to be induced by an external magnetic field where cobalt core–shell particles are arranged in hexagonal planes. The particle ordering and magnetic moment direction followed the direction of the applied field. In addition, segments of uncorrelated dipolar chains were found to be present. The dynamics of the field induced ordering was studied by means of time-resolved SANS. Individual particle moments are stuck by field induced dipolar interactions in domains of local hexagonal ordering which relax by rotational diffusion when the field is switched off, with a characteristic time of a few seconds.

In this paper we investigate a bidisperse model ferrofluid, where the aggregates are treated as flexible chains, under the influence of an arbitrary valued external magnetic field. An extensive comparison of the theoretical predictions to the results of the computer simulations is provided. Both magnetostatic properties and structural observables are investigated with the help of the newly developed theoretical approach and molecular dynamic simulations. It is shown that the results of the cluster analysis are very sensitive to the cluster definition. Here we use two different criteria for the particles to be bound: an energy criterion which is slightly different in the theory and simulations due to technical problems, and an entropy criterion which is the same for the molecular dynamics and theoretical model. This enables us to compare qualitatively and quantitatively theoretical and numerical microstructural observables, as well as the macro properties of the bidisperse ferrofluids. Finally, an answer to the question which chain criterion should actually be used is provided in this paper.

Magnetoviscous and viscoelastic phenomena in ferrofluids are intimately related to their internal structures. The available kinetic models describing the rheological behaviour rely on strong assumptions and simplifications of these structures. Using equilibrium and nonequilibrium computer simulations, here we discuss the validity of the crucial assumption of rigid, chain-like aggregates underlying the chain model. The simulation results support the existence of chain-like aggregates in strongly interacting ferrofluids, at least for sufficiently strong magnetic fields. In addition, shear-induced degradation of the clusters is observed, which apparently is related to strong shear thinning behaviour. For weakly interacting ferrofluids, only slightly anisotropic spatial structures are observed. In this regime, the simulation results of the magnetoviscous effect are in good agreement with the predictions of a dynamical mean-field theory. Further, we explore some first steps towards a unified kinetic model that is applicable in both, the weakly and strongly interacting regimes.

This paper presents results of a theoretical study of the effects of linear chain-like as well as bulk drop-like heterogeneous aggregates on the rheological properties and behaviour of ferrofluids. The results demonstrate that the appearance of both these internal structures leads to a strong (one–two orders of magnitude) increase of the ferrofluid effective viscosity under the action of the magnetic field applied parallel to the gradient of the ferrofluid flow. When the ferrofluid fills a thin channel (gap) placed into a normal magnetic field, the drop-like structures can overlap with the channel. In the case of a rigid connection between the drop-like domains and the channel walls, the appearance of elastic and yield stress effects on the ferrofluid is expected.

Experimental studies made on different ferrofluid samples under shear flow have shown that an increase of magnetic field strength yields an increase of the fluid's 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.

In this work, the magnetorheological properties of suspensions of micron-sized iron particles dispersed in magnetite ferrofluids were studied. With this aim, the flow properties of the suspensions in the steady-state regime were investigated using a commercial magnetorheometer with a parallel-plate measuring cell. The effect of both magnetite and iron concentration on the magnitude of the yield stress was studied for a broad range of magnetic fields. In addition, the experimental values of the yield stress were compared with the predictions from the chain model. With this purpose the values of the yield stress were obtained by means of finite element calculations. Interestingly, it was found that the experimental yield stress increases with the concentration of magnetite nanoparticles in the ferrofluid. Unfortunately, this behaviour is not obtained from calculations based on the chain model, which predict just the opposite trend.

The ability of magnetotactic bacteria (MTB) to navigate along magnetic field lines is based on unique nanosized organelles (magnetosomes), which are membrane-enclosed intracellular crystals of a magnetic iron mineral that assemble into highly ordered chain-like structures. The biomineralization of magnetosomes is a process with genetic control over the accumulation of iron, the deposition of the magnetic crystal within a specific compartment, as well as the assembly, alignment and intracellular organization of particle chains. Magnetite crystals produced by MTB have uniform species-specific morphologies and sizes, which are mostly unknown from inorganic systems. The unusual characteristics of magnetosome particles have attracted a great interdisciplinary interest and inspired numerous ideas for their biotechnological application. In this article, we summarize the current knowledge of magnetosome biomineralization in bacteria. In addition, we will present results on the mass production, as well as the biochemical and physico-chemical analysis and functionalization of bacterial magnetosomes, with emphasis on their characterization as a novel class of magnetic nanoparticles. Finally, we describe the potential of magnetosomes in various biomedical and technological applications.

The aggregation behaviour of magnetic nanoparticles (MNP) is a decisive factor for their application in medicine and biotechnology. We extended the moment superposition model developed earlier for describing the Néel relaxation of an ensemble of immobilized particles with a given size distribution by including the Brownian relaxation mechanism. The resulting cluster moment superposition model is used to characterize the aggregation of magnetic nanoparticles in various suspensions in terms of mean cluster size, aggregate fraction, and size dispersion. We found that in stable ferrofluids 50%–80% of larger magnetic nanoparticles are organized in dimers and trimers. The scaling of the relaxation curves with respect to MNP concentration is found to be a sensitive indicator of the tendency of a MNP suspension to form large aggregates, which may limit the biocompatibility of the preparation. Scaling violation was observed in aged water based ferrofluids, and may originate from damaged MNP shells. In biological media such as foetal calf serum, bovine serum albumin, and human serum we observed an aggregation behaviour which reaches a maximum at a specific MNP concentration. We relate this to agglutination of the particles by macromolecular bridges between the nanoparticle shells. Analysis of the scaling behaviour helps to identify the bridging component of the suspension medium that causes agglutination.

Measurements of the magneto-optical relaxation of ferrofluids (MORFF) were applied as a novel homogeneous immunoassay for the investigation of biomolecular interactions. The technique is based on magnetic nanoparticles (MNP) functionalized with antibodies. The relaxation time of the optical birefringence that occurs when a pulsed magnetic field is applied to the nanoparticle suspension depends on the particle size. This enables the detection of particle aggregates formed after the addition of the antigen coupling partner. MORFF size measurements on the original ferrofluid and its fractions obtained by magnetic fractionation are comparable with results from other methods such as atomic force microscopy and photon correlation spectroscopy. In kinetic studies, the binding properties of five antigens and their polyclonal antibodies were investigated: human immunoglobulin G (hIgG), human immunoglobulin M (hIgM), human Eotaxin (hEotaxin), human carcinoembryonic antigen (hCEA), and human insulin (hInsulin). The enlargement of the relaxation time observed during the coupling experiments is expressed in terms of a size distribution function, which includes MNP monomers as well as aggregates. The kinetic process can be described by a model of stepwise polymerization. The kinetic parameters obtained are compared to results of surface plasmon resonance measurements.

Circulating tumour cells are a key challenge in tumour therapy. Numerous approaches are on the way to achieving the elimination of these potential sources of metastasis formation. Antibody-directed magnetic cell sorting is supposed to enrich tumour cells with high selectivity, but low efficiency. The short term application of carboxymethyl dextran (CMD) coated magnetit/maghemit nanoparticles allows the discrimination of tumour cells from leukocytes. In the present work we show that the interaction of CMD nanoparticles is cell-type specific and time dependent. The breast cancer cell line MCF-7 and the CML cell line K-562 are characterized by a rapid and high interaction rate, whereas leukocytes exhibit a decelerated behaviour. The addition of carboxymethyl dextran or glucose stimulated the magnetic labelling of leukocytes. The variation of the degree of substitution of dextran with carboxymethyl groups did not affect the labelling profile of leukocytes and MCF-7 cells. In order to verify the in vitro results, whole blood samples from 13 cancer patients were analysed ex vivo. Incubation of the purified leukocyte fraction with CMD nanoparticles in the presence of low amounts of plasma reduced the overall cell content in the positive fraction. In contrast, the absolute number of residual tumour cells in the positive fraction was 90% of the initial amount.

We investigate planar periodically magnetized structures for use in targeting or controlling the delivery of therapeutic agents attached to small magnetic particles, and derive simple analytic expressions for the relevant magnetostatic forces. We show that improved particle trapping or confinement characteristics are possible relative to those that can be obtained with more conventional (i.e. uniformly magnetized) structures. These improvements include forces that are larger at close range (for equivalent magnetization densities) and that are both unidirectional and uniform over arbitrarily large areas parallel to the magnet surface. Expressions for the magnetostatic forces exerted on point-like magnetic particles in the vicinity of long rods (with circular and ellipsoidal cross sections) uniformly magnetized perpendicular to their axes are summarized in an appendix.

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.

In biomedical applications of ferrofluids, the resulting distribution of the magnetic nanoparticles is a crucial parameter for the effect of the therapeutic approach. In order to increase the efficacy of local cancer treatments incorporating ferrofluids like magnetic drug targeting and hyperthermia, the bio-distribution of theses fluids in the respective tissue has to be optimized. Usually, the distribution of particles is determined by histological cuts of the investigated specimen, a technique which provides only local information about the overall distribution of the magnetic material, e.g. in a tumour. Radioscopic techniques based on gamma or x-rays are well established, suitable for in vivo examination and non-destructive, but only provide two-dimensional integral information in the direction of the beam. Here we have used micro-tomography—incorporating a conventional x-ray tube as well as monochromatic synchrotron radiation—as a tool for a three-dimensional analysis of the distribution of magnetic nanoparticles in biological applications. Compared to biological matter, the iron-based magnetic nanoparticles provide sufficiently high absorption for x-rays and thus serve as an intrinsic contrast agent for the examinations. The results show the principle feasibility of the method for a quantitative determination of the agglomeration behaviour of the nanoparticles within carcinogenic tissue after intravascular or intratumoural injection.

Loss processes in magnetic nanoparticles are discussed with respect to optimization of the specific loss power (SLP) for application in tumour hyperthermia. Several types of magnetic iron oxide nanoparticles representative for different preparation methods (wet chemical precipitation, grinding, bacterial synthesis, magnetic size fractionation) are the subject of a comparative study of structural and magnetic properties. Since the specific loss power useful for hyperthermia is restricted by serious limitations of the alternating field amplitude and frequency, the effects of the latter are investigated experimentally in detail. The dependence of the SLP on the mean particle size is studied over a broad size range from superparamagnetic up to multidomain particles, and guidelines for achieving large SLP under the constraints valid for the field parameters are derived. Particles with the mean size of 18 nm having a narrow size distribution proved particularly useful. In particular, very high heating power may be delivered by bacterial magnetosomes, the best sample of which showed nearly 1 kW g⁻¹ at 410 kHz and 10 kA m⁻¹. This value may even be exceeded by metallic magnetic particles, as indicated by measurements on cobalt particles.

Magnetic nanoparticles (MNP) are intended for utilization in cancer therapy as they produce damaging heat in the presence of AC magnetic fields. In order to reach the required temperature with minimum particle concentration in tissue the specific heating power (SHP) of MNP should be as high as possible. The aim was to clarify the influence of magnetic field parameters and nanoparticle properties on the SHP. As usual ferrofluids exhibit broad size distributions, a magnetic fractionation of a commercial iron oxide nanoparticle suspension was performed in order to obtain particles with varying properties. The fractions obtained were characterized by means of atomic force microscopy and magnetometry, among other techniques. Frequency spectra of the susceptibility show clear peaks at low frequencies related to the Brown relaxation. This effect vanishes after particle immobilization. Theoretical spectra considering experimentally determined size distributions are in agreement with experimental data. The SHP derived from AC susceptometry is in accordance with that directly determined by calorimetry. A maximum SHP of 160 W g⁻¹ (400 kHz, 8 kA m⁻¹) was detected for the largest particles, showing a behaviour in the transitional regime between superparamagnetic and stable ferromagnetic.

Purpose. Among the different minimally invasive methods for the treatment of tumours under investigation, the accumulation of magnetic material at the target region and the exposure to an alternating magnetic field comprises a highly selective approach. In the present study, we assessed if magnetic heating of tumour cells in vitro is feasible after binding of high-affinity magnetic nanoparticles to the tumour specific protein Her-2/neu, which is known to be expressed in 30% of breast cancers.

Material and methods. Antibodies against the Her-2/neu protein (high-affinity probe) or non-specific gamma immunoglobulins (non-affinity probe, control) were coupled to the dextran shell of magnetic nanoparticles (mean total particle diameter, 150 nm). After incubation of Her-2/neu overexpressing SK-BR-3 tumour cells with the high-affinity probe, non-affinity probe or buffer, cell labelling was verified by electron microscopy. The iron content in cells was determined by atomic absorption spectroscopy. Moreover, cells were exposed to an alternating magnetic field (amplitude, 11 kA m⁻¹; frequency, 410 kHz) for 2.8 min. Temperatures were measured using thermocouples.

Results. A distinct cell labelling was observed by electron microscopy after incubation of cells with the high-affinity probe as compared to controls. Magnetic nanoparticles were found to be localized at the cell surface as well as in granules inside the cytoplasm. The iron content of high-affinity labelled cells (e.g. 76 µg/5 × 10⁷ cells) was distinctly higher than in control cells (e.g. up to 25 µg/5 × 10⁷ cells). During magnetic heating, temperature increases of up to approximately 8 °C were observed in relation to high-affinity labelled cells as compared to 1–2 °C in controls.

Conclusion. Our results show that targeted magnetic heating of tumour cells seems to be feasible. Further investigations should focus on the corresponding relationships in the in vivo situation.

This paper presents two different kinds of magnetically controllable fluid bearings and a new magnetorheological fluid damper based upon open porous metallic foams. For the bearings, it will distinguish between a magnetohydrostatic bearing and a hydrostatic bearing with a magnetically controllable fluid. The magnetohydrostatic bearings get their load bearing capacity from the magnetohydrostatic pressure that is generated by the gradient of the magnetic field along a fluid surface. With such magnetohydrostatic bearings a specific load up to 1.6 N cm² can be reached. To support heavier loads hydrostatic bearings with magnetically controllable fluids can be used. This bearing concept makes it possible to achieve a constant bearing gap even if the load of the bearing changes. For this purpose the fluids are used as a hydraulic medium. Due to the magnetically controlled rheological behaviour of the fluid the bearing gap remains constant. The great advantage of this closed loop system compared to that of common hydrostatic bearings using valves is the quicker response to payload changes. The reason for that is that the active element (i.e. the fluid) acts directly inside the bearing gap and not outside like in the case of valves. The foam damper developed uses the fluid to produce controllable damping forces. The open porous foam is directly placed in the active volume of the damper. By moving the foam piston the magnetically controllable fluid is pressed through the pores. The flow in the pores can be controlled by changing the fluid viscosity by applying a magnetic field. With this damper structure it is possible to reach higher damping forces whilst featuring a small design space.

This paper deals with the modelling and the realization of active and passive locomotion systems using the effects of the deformation of a magnetizable elastic material and the deformation of the surface of a membrane filled with a ferrofluid under the influence of a magnetic field. Prototypes implementing these principles have been constructed and proved positive. Theoretically (analytically and numerically) calculated results of the velocity of the mobile system are compared with the experimental data. Artificial worms based on these principles could be autonomous systems, and could be useful in medicine and in inspection technology.

Ferrofluids have a growing importance in technical and medical applications since stable suspensions of magnetic nanoparticles in carrier fluids can be produced. The application of these strong paramagnetic fluids in electric machines by filling the air-gap between stator and rotor to increase the force in linear and the momentum in rotating machines offers an interesting possibility to improve efficiency and thus to save energy. Some calculations of this effect are presented here, including the method of magnetic circuits and field theory. Also, peripheral aspects such as fluid friction and thermal implications are discussed. Experimental verification by testing linear and rotating machines will confirm the positive results.