Page 185 - Computational Modeling in Biomedical Engineering and Medical Physics

174 Computational Modeling in Biomedical Engineering and Medical Physics

Figure 6.1 Magnetic materials and properties.

(Fig. 6.1). In ferromagnets, a residual magnetization state remains even if an external
magnetic field does not exist. That value is called the remanent magnetization, M r . The
magnetic field strength that cancels M r is called coercive field, H c (Mocanu, 1981).
The MNPs used for medical purposes must be biocompatible, nontoxic to cells,
and biodegradable. They need to preserve their physical properties after surface treat-
ments, may not affect the characteristics of normal cells, have to be effective for thera-
peutic doses, and must pose no menace to neighboring tissues (Markides et al., 2012).
The modular approach in adding components to MNPs may facilitate the incorpo-
ration of particular features and the interchange or combination of functional groups
of molecules. Along this line, ligands (therapeutic agents and targeting agents, optical
dyes, and permeation enhancers) can be conjoined on the surface or inside these
nanostructures (Fig. 6.2; Sun et al., 2008).

Superparamagnetic iron oxide nanoparticles
MNPs for biomedical applications have superparamagnetic properties. In this category, the
superpparamagnetic iron oxide nanoparticles (SPIONs) play a key role. SPIONs have magnetite
(Fe3O4) or maghemite (γ-Fe 2 O 3 ) ferromagnetic kernel, and they are coated with an either
organic or inorganic biocompatible polymer, or precipitated into a porous biocompatible
polymer. When the radius of their ferromagnetic cores fall below 30 nm the MNPs lose
their permanent magnetism and become superparamagnetic (Pankhurst et al., 2003;
Neubergeretal.,2005). SPIONs are, so far, the only clinically approved metal oxide
MNPs. They may be driven to a certain tissue or organ by an external magnetic field, or
they may be usedascontrastagents for MRI, fortargeted drug deliveryorinmagnetic
hyperthermia (Chapter 8: Hyperthermia and Ablation (Thermotherapy methods)).
The hydrodynamic radii of SPIONs, assumed as elastic spheres of fixed size, are in
the range 1 5000 nm (Nacev et al., 2011), their density is 4800 5100 kg/m 3