In the present study, 13 magnetic nanoparticles that are various diameters of the same materials Fe3O4 were precisely prepared by the same way and SAR was measured under various AC frequency and AC power. Such summarized data have been firstly reported by us in the present paper. To the best of our knowledge, it was firstly found that SSA is better index for SAR.
From polydespersity index of DLS listed in Table and TEM photographs, it was found that there was not any particle with extremely wide size distribution. Therefore, it is considerable that there are different degree of aggregates and different packing density of aggregate, because large SSA was obtained even in samples with similar DLS diameter. It is considerable that samples with high packing density behave as a multidomain particle and higher saturated magnetization was obtained.
In Fig. , we showed the SARs of the magnetite particles which were measured under variable AMF conditions. There were two local maximum values of the SAR observed when the SARs were plotted against to the SSAs, which were approximately 90 m2/g (a) and 120 m2/g (b) separately in all the experimental intensities of AC magnetic fields.
It has been reported that the SAR of magnetite particles in an external AC magnetic field can be attributed to two kinds of power loss mechanisms; one is hysteresis loss and others is relaxation loss [16
]. The grade of these two power losses depends on the particle sizes. The heating due to hysteresis losses are caused by magnetic domain wall displacements under an AC magnetic field. Therefore, it has been reported that the hysteresis loss induced heating needs larger size of magnetic multidomain particles. On the other hand, heating induced by relaxation loss under an AC magnetic field occur to smaller particles that has not domain wall and consists of the single domain structure [18
The SAR values for the particles below the dividing line which exist around 110 m2/g of SSA seemed more susceptible against to the power of AC magnetic field in all frequencies as well. As shown in Fig. , it was found that heat generation by two processes was found against SSA. It is strongly suggested that these are happened by clearly different mechanism for heat generation. It is likely that those are relaxation loss and hysteresis loss, respectively, although we could not mention the reason why local maximum is 120 m2/g and that was not the smallest SSA in the particles. More profound discussion might be revealed by further experiment. From Table , it was found that saturated magnetization was strongly correlated with SSA. In the hysteresis loss, SAR is defined by the area of hysteresis curve. This might be one of the reasons why SSA is better index for SAR, although the reason why saturated magnetization is correlated with SSA still remains to be elucidated.
Since the heating mechanism of the magnetite nanoparticles of different SSA have different attributions from the intensity of AMF, it is considerably needed to optimize the particle SSA for the treatment of MFH. As shown in Fig. , the diagram of SAR of the sample K, which has larger SSA, was maintained virtually constant against to the power of AMF in all range of its frequency (Slopes are closed to 0). Therefore for the MFH treatment, when the magnetite particles those have more than 110 m2/g of SSA and less than 10 nm of particle diameter are used as the heating mediator, we expect the stable supply of heat could be performed imperviously to the power of AMF.
Comparably, smaller SSA particles generates heat linearly against the strength of the AMF (Slopes in Table are large). That is to say, smaller SSA particles seem to be suited for treating various region of the body part for the MFH treatment because the SAR curve for the smaller SSA particles are adjustable and easily increased linearly by manipulating the power of AMF. In addition the dose of the smaller SSA particles possibly could be hold down when the high-power of AMF are applicable for treatment. It would be also able to heat deep portion of the body part sufficiently by controlling higher dose of the magnetic particles or intensity of the AMF power.
As for the AMF frequency, it should be noted that lower ones within the range of 50 kHz to 100 kHz of AMF are recommended for human therapy depending on the body cross-section and tissue conductivity [7
]. When we use smaller SSA particles, we could overcome the disadvantage of smaller frequency of the AMF by controlling the intensity of power of the AMF.
Our group is now planning to the application to actual cancer patients. SSA and AC frequency was one of the important criteria for magnetite particle preparation. Actual apparatus for cancer patient was already designed and fabricated, that is AC frequency of 110 kHz. Sample C or D was applicable to actual cancer treatment.
In the present study, we observed collateral evidence that the SAR of the magnetite nanoparticles in an external AMF are induced by two heating mechanisms that depends on the SSA of the particles. The critical change of the SAR value was observed at approximately 110 m2/g of the SSA which exists among the 10 nm diameter particles. This is likely due to the structure change of magnetic domain. Additionally, we suggested that heating property of these two mechanisms is defined under the different influences of the frequency and the power of the AC magnetic field.