Non-monotonicity in the influence of nanoparticle concentration on SAR in magnetic nanoparticle hyperthermia

Magnetic nanoparticle hyperthermia (MNH) has shown substantial promise as an alternative therapy for deep-tissue tumors or resistant bacterial films. However, while great progress has been made in understanding the physical mechanisms of thermal energy dissipation in MNH, the influence of interparticle magnetic interactions in concentrated solutions remains a substantial challenge limiting progress toward clinical adoption. We have developed a well-dispersed suspension of magnetite nanoparticles which can be varied smoothly with nanoparticle concentrations ranging from 0 to 900 mg/mL. This material serves as a well-controlled experimental model for exploring the role of nanoparticle concentration on heating. We have measured the specific absorption rate (SAR) of the nanoparticles as a function of nanoparticle concentrations up to 320 mg/mL (10% v/v) in applied field frequencies from 86 to 460 kHz. Our results clearly indicate that SAR is non-monotonic with a maximum at concentrations of approximately 1% v/v, in strong agreement with recent theory. In addition, they suggest that optimal concentration is dependent on applied field frequency, and that the exponential dependence of SAR on frequency varies smoothly with concentration. These results provide experimental support for current theoretical efforts and lend insight into new investigations, ultimately leading to better prediction and control of heating in MNH.