A review of physical principles and experimental data on magnetic hyperthermia are presented. The main principles of magnetic hyperthermia are considered. Results of its application in the therapy of oncology diseases are presented.
PHYSICAL PRINCIPLES OF HYPERTHERMIATwo components of ac electromagnetic fields E and H can cause heating of tissues. Based on the Maxwell equations and thermodynamic relations, not only the electrocaloric effect, namely, the heat absorption in substance caused by the electric field component E [6], but also the magnetocaloric effect caused by variations in H can be calculated. These effects are mainly determined by the dielectric (ε and δε/δТ) and magnetic (μ) properties of substances, respectively. Because magnetism of biological objects is negligibly small, biologically compatible nontoxic magnetic nanoparticles (based on magnetite and so on) are used to strengthen the influence of an external magnetic field.Thermodynamic relations for a magnet in a magnetic field are similar to those for a dielectric in an electric field [6]. However, an essential difference is that the magnetic field, unlike the electric field, does no work on charges moving in it, because the Lorentz force is perpendicular to the velocity vector of the moving charge. To calculate a change in the energy of the medium when the magnetic field is switched on, electric fields induced by magnetic field variations should be considered. In their turn, high-frequency electromagnetic fields cause heating due to the electric field component. At low frequencies, this effect is insignificant. At the same time, the role of the magnetic field component in magnet heating is significant only at low frequencies [6]. Therefore, inclusion of electromagnetic induction hyperthermia in a separate class of high-frequency phenomena [1], on the one hand, and of magnetic hyperthermia as a low-frequency influence, on the other hand, though relative, is justified.Low-frequency (less than 100 kHz) electromagnetic fields can cause heating of magnetic nanoparticles at the expense of the magnetocaloric effect [7], magnetic reversal in the presence of a hysteresis loop, and magnetic crystal anisotropy of superparamagnetic particles. Physics of the magnetocaloric effect of this phenomenon is the following: elementary magnetic moments are directed chaotically without magnetic field, and hence the magnetic contribution to the entropy is significant. As the magnetic field increases, the magnetic moments are ordered along the field. As a result, the magnetic entropy component S M decreases. Because the magnetization process is close to adiabatic one, the total entropy S does not change, but the entropy component caused by thermal motion increases. Thus, the ferromagnet temperature increases with the magnetic field. Quantitatively, the temperature change is calculated from the formula From the above formula it follows that the temperature can raise when the magnetic moment М S changes with temperature, since the heat capacity C is always positive. The ...