Structural properties of magnetic nanoparticles determine their heating behavior – an estimation of the in vivo heating potential

Magnetically induced heating of magnetic nanoparticles (MNP) in an alternating magnetic field (AMF) is a promising minimal invasive tool for a localized tumor treatment by sensitizing or killing tumor cells with help of thermal stress. Therefore, the selection of nanoparticles exhibiting a sufficient heating capacity (specific absorption rates, SAR) to achieve satisfactory temperatures in vivo is necessary. Up to now, the heating potential of MNP is mainly determined experimentally using ferrofluidic suspensions and may distinctly differ from the heating capacity in vivo due to immobilization of nanoparticles in tissues and cells. The aim of our investigations was to study the correlation between the heat generation and the degree of particle immobilization in dependence on the physicochemical particle characteristics. In this study, clustered and non-clustered MNP in respect of varying physicochemical properties, including core/particle size (5 – 175 nm / 76 – 210 nm) and coating (DMSA, PAA, PEG and starch), were analyzed. SAR values were determined after suspension of MNP in water. Nanoparticle immobilization in vivo was simulated in the presence of 1 % agarose gels and 10 % polyvinyl alcohol (PVA) hydrogels. To allow the comparison of heating potentials independent of the characteristics of the used magnetic field, intrinsic loss power (ILP) was calculated. Highest SAR values were observed in ferrofluidic suspensions, whereas a strong reduction of SAR values after immobilization of nanoparticles with PVA was found. Generally, PVA embedment led to a higher immobilization of MNP compared to immobilization in agarose gels. Furthermore, for here applied magnetic field parameters investigated single core particles exhibited higher SAR/ILP values than the used multi core particles of the same core size. Multi core nanoparticles with differentially clustered cores showed different heating potentials although they exhibited comparable core and hydrodynamic sizes. Additionally, no correlation between ζ-potential and SAR values after immobilization could be observed. Our data show that an immobilization of MNP, independent of their physicochemical properties, can distinctly affect their SAR. Similar processes are supposed to take place in the in vivo situation in tumors, particularly when MNP are immobilized in cells and tissues (e.g. components of the extracellular matrix etc.). Accordingly, this aspect should be adequately considered when determining the heating potential of nanoparticles for magnetic hyperthermia applications.