Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 153

Hysteresis in a time varying or alternating magnetic field leads to the generation of heat in ferromagnetic and ferrimagnetic materials. The amount of heat P generated per unit volume is given by

(4.44)

where, μ_o is the permeability of free space, f is the frequency of the alternating applied field, and the circular integral sign represents integration over the closed hysteresis loop, yielding the area of the loop.

The generation of heat in ferromagnetic and ferrimagnetic nanoparticles when they are subjected to an alternating magnetic field forms the basis of hyperthermia treatment of tumors. Nanoparticles of the ferrimagnetic oxide Fe₃O₄ or γ‐Fe₂O₃ are most commonly used because of their favorable biocompatibility. The procedure involves dispersing nanoparticles in the target tumor and applying an alternating magnetic field of sufficient strength and frequency to generate heat in the particles. This heat is transferred to the surrounding tumor and if the temperature is maintained above ~42 °C for a sufficient period (~30 minutes or more), the tumor can be destroyed. This process is attractive because of the possibility of eradicating the intended tumor without causing significantly adverse effects to healthy tissue. However, there are constraints on the magnitude of H and f that healthy tissues can tolerate and questions about whether a sufficient number of nanoparticles can be delivered to the tumor to ensure the generation of sufficient heat over the required time to eradicate the tumor.