Magnetic Hyperthermia Based on Hysteresis Heating Alone

In this work we describe a new approach to magnetic hyperthermia based on hysteresis alone. This is achieved by encapsulating magnetite nanoparticles in polymer spheres to remove dynamic effects which cause unpredictable and uncontrollable heating. This also prevents aggregation of the nanoparticles. To generate significant hysteresis and hence heat, we have used nanoparticles grown by the standard coprecipitation process which are used as seeds and grown to a larger size by the addition of further Fe2+ salts in an alkaline solution and heating. We define this as a Controlled Growth Process (CGP). The CGP particles are found to have a much higher saturation magnetisation than particles produced by coprecipitation enhancing their magnetic shape anisotropy. Via magnetic measurements and complementary studies of the heat generated when the polymer spheres are exposed to an ac field, we show that the heat generated is due to hysteresis losses with potentially residual contributions from dynamic effects. At 111 kHz and a maximum applied field of 180 Oe, a specific absorption rate (SAR) value of ∼35 W/gFe was determined from calorimetry measurements. This value agrees within error with the losses calculated from the hysteresis loop. When the same particles are not encapsulated, significant viscous heating is present.