Optimization of magnetic fluid hyperthermia with respect to nanoparticle shape-related parameters: case of magnetite ellipsoidal nanoparticles

Issues related to the optimization of heat transfer mechanisms dominated by superparamagnetic relaxation are considered in the case of AC (alternating current) magnetic field hyperthermia procedures. The key role in the conversion of electromagnetic energy to the thermal one via the superparamagnetic relaxation mechanism is played by the magnetic anisotropy of nanoparticles, easily to be controlled via the shape anisotropy component. The optimization process has been discussed in the case of magnetite (Fe₃O₄) ellipsoidal nanoparticles with dominant shape anisotropy dispersed in different media. Nanoparticles of different sizes and aspect ratios have been considered in correlation with those specific parameters of the actuating AC magnetic field which respect an established biological safely criterion. It has been proven that the dissipated power can be maximized for a given set of biological compatible RF (radiofrequency) field parameters (frequency and field amplitude at the sample space) only for specific pairs of particle sizes and aspect ratios. For instance, it has been shown that ellipsoidal magnetite nanoparticles with 10 nm equatorial size and aspect ratio of 2 are optimal for a maximum transferred power under radiofrequency excitations of 250 kHz and field amplitude of 20 kA/m, if high viscosity dispersion media are used. The methodology for deriving the optimal shape (geometrical) parameters of a specific type of nanoparticles in conditions of using available radiofrequency excitations, or vice versa, for deriving the optimal radiofrequency working parameters in the case of ferrofluids with specific nanoparticles (type and geometry) is described and discussed in detail.