Hyperthermia is an antitumor technique, that consists in a local rise of temperature up to 43°C within a neoplastic region, having the aim of damaging tumour cells. Magnetic fluid hyperthermia (MFH) reaches the goal of tumour treatment by using magnetic nanoparticles (MNPs) made biocompatible thanks to special coatings, and exposed for a time of the order of 0.5/1 hours to an alternating magnetic field. For in vivo applications, superparamagnetic MNPs are preferred because it seems they are safer thanks to their smaller dimensions .
The MNPs magnetization, also called superspin, can point along two directions, parallel and antiparallel to the easy axisdetermined by the anisotropy, switching from one configuration to the other in a characteristic time called Néel relaxation time. Nanoparticles suspended in a solvent can modify the orientation of their superspins also through a rotation of the entire particles in a characteristic Brown relaxation time [2,3]. These two relaxation times, which are influenced mainly by the particle size and magnetic properties, affect the Specific Absorption Rate (SAR), a parameter which quantifies the heat released by a certain quantity of MNPs in an alternating magnetic field [4,5].
Recent studies showed that Brownian rotation cannot take place when performing MFH treatment in vivo or in vitro because most MNPs are immobilized when injected into the cells . Despite this evidence, the majority of works published on MFH studies MNPs only in liquid solutions where also the Brownian relaxation takes place.
In this work we studied systematically three samples of MNPs, made of magnetite (Fe3O4) with core sizes of 10, 14 and 20 nm, evaluating their chemico-physical properties and measuring the SAR at different frequency (110 to 990 kHz) and intensity (up to 20 mT) of the applied alternating magnetic field. We performed measurements both in aqueous solution and in agarose gel: this latter allowed the immobilization of MNPs reproducing an experimental condition similar to the one of the tissues, in which Brownian relaxation is essentially suppressed. Results show that SAR is relevantly reduced by the immobilization of the MNPs in gel in the case of the biggest ones: this outcome permitted us to evaluate the relative weight of the mechanisms of Néel and Brown relaxations by fitting the SAR(H) curves with known models as the Linear Response Theory (LRT).
 S. Dutz and R. Hergt, Nanotechnology 25 (2014) 452001
 R.E. Rosensweig, J. Magn. Magn. Mater. 252 (2002) 370-374
 S. Laurent et al., Chem. Rev. 2008, 108, 2064-2110
 S. Dutz et al., Nanotechnology 22 (2011) 265102
 J. Carrey et al., Journal of Applied Physics 109, 083921 (2011)