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Magnetic hyperthermia is an experimental treatment for cancer. It is theoretically based on the fact that magnetic nanoparticles can transform electromagnetic energy from an external high-frequency field to heat. As a result, if magnetic nanoparticles are put inside a tumor and the whole patient is placed in an alternating magnetic field, the tumor temperature will rise. The elevation of temperature may enhance tumor oxygenation and radio- and chemosensitivity, hopefully shrinking tumors. This experimental cancer treatment has also been investigated for the aid of other ailments, such as bacterial infections.
Contents
Definition
Many magnetic materials display a magnetic hysteresis when subjected to a magnetic field that alternates direction. This hysteresis cycle represents work which is dissipated in the environment as thermal energy. This heat is undesirable in many industrial applications, though it is the basis of magnetic hyperthermia. This power is often called the "Specific Absorption Rate" (SAR) and it is usually expressed in watts per gram of nanoparticles.
Application
In hyperthermia application, the nanoparticles form a ferrofluid in the blood. They move and rotate randomly in the fluid, exhibiting Brownian motion. When a magnetic field is applied to them, magnetic nanoparticles rotate and align with the magnetic field. The magnetization of nanoparticles can spontaneously change their orientation under the influence of thermal energy, a phenomenon called superparamagnetism. The magnetization of the nanoparticle is also reversed when an applied magnetic field is large enough to suppress the energy barrier between the two equilibrium positions, a phenomenon which is known as the Stoner–Wohlfarth model of magnetization reversal. In the most general case, the reversal of the magnetization is due to a combination of the three mechanisms described above. For instance, for a single domain nanoparticle that is inside a fluid at room temperature and a sweeping magnetic field is suddenly applied in a direction opposite to the one of the nanoparticle magnetization. At the same time, i) the nanoparticle will rotate in the fluid, ii) the barrier between the two equilibrium positions of the magnetization will decrease, iii) when the energy barrier becomes of the order of the thermal energy, the magnetization will switch (if the nanoparticle is not already aligned with the magnetic field due to its physical rotation).