Thermosensitive Ferromagnetic Implant for Hyperthermia Using a Mixture of Magnetic Micro-/Nanoparticles

Abstract

Self-controlled heating mediators for magnetic hyperthermia are widely studied. In previous studies, we succeeded in developing a microsize thermosensitive ferromagnetic implant with low Curie temperature (FILCT). The FILCT was then coated with gold (Au-FILCT) to improve its heating efficiency for treating the human body. However, part of the magnetic field was shielded due to the conductive gold coating, thereby decreasing the possibility of our orientable pickup coil system for contactless temperature sensing based on the linearity between magnetic induction and implant temperature around the Curie point. As an alternative approach to the gold coating of FILCT, this paper examined a mixture of FILCT and a high heating-efficient magnetic nanofluid (Resovist) subjected to a magnetic field ( $f = 500$ kHz, $H = 4.95$ kA/m). As a result, the change in magnetic induction detected caused by the mixture is enhanced compared to that of Au-FILCT (1.9 times) and FILCT (1.3 times). Furthermore, the temperature rising rate of the mixture is 4.3 times faster than that of FILCT. The results obtained also suggest that for hyperthermia implant, the preferred volume fraction of magnetic nanoparticles is approximately 0.5%. We hypothesized that the magnetic nanoparticles (median core diameter $d_{0}= 3.6$ nm) in Resovist fill the gaps between the magnetic microparticles ( $d_{0}= 83.6\,\,\mu \text{m}$ ) in FILCT, thereby reducing the demagnetizing field of microparticles and causing its permeability to be improved.