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The heating power of magnetic nanoparticles (MNPs) submitted to high-frequency magnetic fields is generally probed using calorimetric methods, which suppose that the heating power does not evolve with time. Among the several parameters governing MNP heating properties, their organization into chains under the influence of the applied magnetic field is of key importance, though the dynamic of this phenomenon has been rarely studied experimentally. In the present article, time-resolved high-frequency hysteresis loops are used to probe the dynamics of chain formation on a sample of 17.3 ± 2.2 nm FeNi3 MNPs. Chains are formed on a timescale, which strongly depends on the magnetic field amplitude, ranging from several tens of seconds to less than 100 ms, but does not depend on frequency in the range studied here (from 9 to 78 kHz). Both the heating power and hysteresis loop squareness increase with time as chains progressively form. These findings have important methodological consequences when defining protocols or analyzing data issued from calorimetric measurements since, in samples where chains form, the heating power varies on a time scale that can be comparable to typical measurement times. [ABSTRACT FROM AUTHOR]

Studies with transplanted tumors in animals and clinical trials have provided the proof-of-concept of magnetic hyperthermia (MH) therapy of cancers using iron oxide nanoparticles. Interestingly, in several studies, the application of an alternating magnetic field (AMF) to tumor cells having internalized and accumulated magnetic nanoparticles (MNPs) into their lysosomes can induce cell death without detectable temperature increase. To explain these results, among other hypotheses, it was proposed that cell death could be due to the high-frequency translational motion of MNPs under the influence of the AMF gradient generated involuntarily by most inductors. Such mechanical actions of MNPs might cause cellular damages and participate in the induction of cell death under MH conditions. To test this hypothesis, we developed a setup maximizing this effect. It is composed of an anti-Helmholtz coil and two permanent magnets, which produce an AMF gradient and a super-imposed static MF. We have measured the MNP heating power and treated tumor cells by a standard AMF and by an AMF gradient, on which was added or not a static magnetic field. We showed that the presence of a static magnetic field prevents MNP heating and cell death in standard MH conditions. The heating power of MNPs in an AMF gradient is weak, position-dependent, and related to the presence of a non-zero AMF. Under an AMF gradient and a static field, no MNP heating and cell death were measured. Consequently, the hypothesis that translational motions could be involved in cell death during MH experiments is ruled out by our experiments. [ABSTRACT FROM AUTHOR]

We show theoretically that, in an inhomogeneous alternating magnetic field of frequency f, due to the alternating gradient, magnetic nanoparticles oscillate mechanically and generate ultrasound waves. This effect is maximized and better controlled if a static magnetic field is superimposed to an alternating gradient. It makes possible the generation of ultrasounds intracellularly and might also explain recent experiments in magnetic hyperthermia in which cells have been killed without any global temperature increase. Combined to an efficient targeting, it could permit ultrasound therapy with an unprecedented spatial resolution. [ABSTRACT FROM AUTHOR]

In the last 10 years, it has been shown in various types of experiments that it is possible to induce biological effects in cells using the torque generated by magnetic nanoparticles submitted to an alternating or a rotating magnetic field. In biological systems, particles are generally found under the form of assemblies because they accumulate at the cell membrane, are internalized inside lysosomes, or are synthesized under the form of beads containing several particles. The torque undergone by assemblies of single-domain magnetic nanoparticles has not been addressed theoretically so far and is the subject of the present article. The results shown in the present article have been obtained using kinetic Monte Carlo simulations, in which thermal activation is taken into account, so the torque undergone by ferromagnetic and superparamagnetic nanoparticles could both be simulated. The first system under study is a single ferromagnetic particle with its easy axis in the plane of the rotating magnetic field. Then, elements adding complexity to the problem are introduced progressively and the properties of the resulting system presented and analyzed: random anisotropy axes, thermal activation, assemblies, and finally magnetic interactions. The most complex studied systems are particularly relevant for applications and are assemblies of interacting superparamagnetic nanoparticles with randomly oriented anisotropy axes. Whenever it is possible, analytical equations describing the torque properties are provided, as well as their domain of validity. Although the properties of an assembly naturally derive from those of single particles, it is shown here that several of them were unexpected and are particularly interesting with regard to the maximization of torque amplitude in biological applications. In particular, it is shown that, in a given range of parameters, the torque of an assembly increases dramatically in the direction perpendicular to the plane of the rotating magnetic field. This effect results from a breaking of time reversal symmetry when the field is rotated and is comprehensively explained. This strong enhancement occurs only if the magnetic field rotates, not if it oscillates. When this enhancement does not occur, the total torque of an assembly scales with the square root of the number of particles in the assembly. In the enhancement regime, the total torque scales with a power exponent larger than 1/2. It is also found that, in superparamagnetic nanoparticles, this enhancement is induced by the presence of magnetic interactions so that, in a rather large range of parameters, interacting superparamagnetic particles display a much larger torque than otherwise identical ferromagnetic particles. In all cases studied, the conditions required to obtain this enhancement are provided. The concepts presented in this article should help chemists and biologists in synthesizing nano-objects with optimized torque properties. For physicists, it would be interesting to test experimentally the results described in this article. For this purpose, torque measurements on well-characterized assemblies of nanoparticles should be performed and compared with numerical simulations. [ABSTRACT FROM AUTHOR]

• Low-frequency rotating magnetic fields are useful for biological applications. • We describe a setup generating rotating fields of variable amplitude and frequency. • It is based on a six-pole electromagnet. • Frequency of 0–10 Hz and amplitude of 1–120 mT are achieved. • Apppying magnetic field kill cancer cells when combined with magnetic nanoparticles. We describe a setup able to generate a rotating or alternating low-frequency (up to 10 Hz) magnetic field (up to 120 mT) for biological applications inside a 6 cm in diameter and 5 cm in height cylindrical space. It is suitable to insert several Petri dishes for cell death experiments and mice for in vivo ones. Since a particular attention was given to the field homogeneity, it is based on a six-pole electromagnet. The characteristics of the field are changed by varying the phase, amplitude or frequency of the electrical currents going through the coils, using a micro-controller-based human–machine interface and a RL chopper, which is described. The results of the magnetic simulations run to optimize the global design are provided. The cooling of the electromagnet is done using a chiller, and the sample temperature regulation using an air blower. All details on the mechanical, electrical and electronic elements, on the mounting, and on the programing are provided, so the setup can be reproduced. Suggestions for a further improvement of the electrical efficiency are given. This setup has already been used to demonstrate the possibility to kill cells using targeted magnetic nanoparticles and low-frequency magnetic fields. [ABSTRACT FROM AUTHOR]

When magnetic nanoparticles (MNPs) are single domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed: the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhances their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduce the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. Our finding can thus explain contradictory results in the literature on the influence of magnetic interactions on magnetic hyperthermia. It also provides an alternate explanation to some experiments where an enhanced specific absorption rate for MNPs in liquids has been found compared to the one of MNPs in gels, usually interpreted with some contribution of the brownian motion. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments. [ABSTRACT FROM AUTHOR]

• Medium viscosity influences high-frequency magnetic properties of nanobeads. • Nanobead's mechanical behaviors depend on their size and medium viscosity. • SAR measurements for biological applications should be done in a viscous medium. Magnetic nanoparticles placed in a high frequency alternating magnetic field release heat. This phenomenon can be used to heat and kill cancerous cells, the so-called magnetic hyperthermia treatment. Our study focuses on the influence of the medium viscosity on the heating power of magnetic nanobeads (MNBs) containing superparamagnetic nanoparticles. The specific absorption rate (SAR) of two types of MNBs differing by their size was determined by measuring the area of their high-frequency hysteresis loops. Three different behaviors are observed as viscosity rises: (i) SAR first drops to a minimum value for both MNB sizes; (ii) interestingly, and only for smaller MNBs, after going through a minimum, SAR increases again towards a steady value; (iii) for larger MNBs, SAR remains constant after reaching its minimum value. The physical origin of these different behaviors is interpreted, the key ingredient to explain them being the magnetic interactions, the possibility of the bead to form chains, and their possibility to physically rotate under the influence of the magnetic field. Finally these results lead to questioning the representativeness of SAR measurements in water for nanoparticles intended for biological applications, in which the medium viscosity is very different from the one of water. [ABSTRACT FROM AUTHOR]