Magnetic nanoparticles (MNPs) are really interesting for biomedical applications due to their ability to respond to external magnetic fields which allow their remotely manipulation for targeting, cell separation, drug delivery, and of course also as nanoHeaters to destroy the tumors. The requirements to use MNPs for clinical purposes are their stability in biological environments and their low toxicity, and also their appropriate magnetic properties to be remotely activated by an external magnetic field.
The particles most commonly used in clinical applications are the nanoparticles composed by iron-oxides phases (i.e. Fe3O4 and Fe2O3) and other magnetic ferrites in spinel structures doped with Mn, Co, Ni or Zn. To improve their biocompatibility, nanoparticles designed for biomedicine are coated with both organic and inorganic compounds. In addition, these particles can be functionalized with several ligands improving their biocompatibility and stabilizing them in biological environments, and also providing functionality as specific targeting and controlled drug delivery.
Therefore, MNPs used as nanoHeaters can be activated by an external magnetic field, through the magnetic coupling between the magnetic component of the field and their magnetic moment. Magnetic nanoparticles absorb the energy from this coupling phenomenon, and dissipate it as heat. For these magnetic fields, biological tissues are “transparent” with no significant energy deposition, thus this technique is safe for all living organism.
The heating capacity depends on the magnetic and physicochemical properties of the MNPs. Regarding magnetic properties, the heat is related to dynamic hysteresis losses produced by the relaxation of the single domain nanoparticles magnetic moments. Relaxation process involve two simultaneous mechanisms associated with the relaxation magnetic moment as shown in Image 1.
One of them is related to the physical rotation of the MNPs on the surrounding medium such as liquid carrier and tissue. This process is known as Brownian relaxation, and depends on the medium viscosity and hydrodynamic volume. On the other hand, the Néel relaxation which depends on the rotation of the atomic magnetic moments within the crystal lattice of the MNPs.
Both mechanisms are independent, even though they occur simultaneously, the faster one is is dominant to induce the heating.
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