The capability of magnetic nanoparticles (MNPs) to act as effective heating agents for Magnetic Hyperthermia (MHT) has been demonstrated many years ago. However, the mechanisms and true limits for delivering the absorbed power to small intracellular structures are still open questions. The therapeutic approach of MHT consist in remotely raising the temperature of a target tissue through alternating magnetic fields acting on magnetic nanoparticles previously loaded in that tissue. The safeness of MHT as compared with microwave or conduction-based hyperthermia is related to the frequency region (f = 10^2 - 10^3 kHz) of the electromagnetic radiation used by MHT, where the heating effects on living tissues are negligible.
The origin of the energy dissipation when a magnetic colloid is placed on an ac magnetic field has been subject of extensive work, but experimental difficulties for generating ac magnetic fields of tens of mT at different frequencies have hindered accurate models to be developed. Moreover, most of the experimental and theoretical works reported so far have dealt with single-domain particles, and thus the discussion of absorption models has been restricted to Brownian and Néel relaxation mechanisms. The prevailing picture today is that power absorption takes place mainly through Néel relaxation of the magnetic moments, since energy losses from mechanical rotation of the particles, acting against viscous forces of the liquid medium (Brownian losses) cannot contribute at those frequencies used for MHT (10^2 - 10^3 kHz).
The heating efficiency of a magnetic colloid is measured by the heating power P of a given mass mNP of the constituent nanoparticles diluted in a mass mLIQ of liquid carrier, through the Specific Power Absorption (SPA)
SPA = C_liq D_liq/Phi (dT/dt) (1)
with C_liq is the specific heat, D_liq is the density of the liquid, and PHI is the weight concentration of the MNPs in the colloid, respectively. Therefore, we can obtain SPA from derivation of the curve T vs. t for all samples. In fact, we assume the maximum in of dT/dt to determine SPA.
The strong dependence of SPA with particle size suggests that in addition to internal structure of the material, tailored synthesis routes will be necessary to tune the particle size to a given experimental condition.
The heating efficiency of a magnetic colloid is measured by the heating power P of a given mass mNP of the constituent nanoparticles diluted in a mass mLIQ of liquid carrier, through the Specific Power Absorption (SPA)
SPA = C_liq D_liq/Phi (dT/dt) (1)
with C_liq is the specific heat, D_liq is the density of the liquid, and PHI is the weight concentration of the MNPs in the colloid, respectively. Therefore, we can obtain SPA from derivation of the curve T vs. t for all samples. In fact, we assume the maximum in of dT/dt to determine SPA.
The strong dependence of SPA with particle size suggests that in addition to internal structure of the material, tailored synthesis routes will be necessary to tune the particle size to a given experimental condition.
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