Page 160 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

158 CHAPTER 7 Application of magnetic and electric fields for cancer therapy

Magnetization of diamagnetic materials is negative and very low. The resultant mag-
netic dipole moment in these materials is zero [1]. Pankhurst et al. discussed the
laws of the magnetic field, the various uses of MNPs in medicine and identified the
permitted magnetic fields for different applications [2]. Single domain ferromagnetic
nanoparticles and superparamagnetic nanoparticles are the two important MNPs fea-
tures which are elaborated future.
Single domain ferromagnetic nanoparticles: A magnetic particle that stays in
a single domain state for all magnetic fields is called a single-domain ferromagnetic
particle. Usually nanoparticles with a diameter of less than 100 nm are single-domain
nanoparticles [3,4]. The internal magnetic moment of the single-domain ferromag-
netic nanoparticles is not zero.
Superparamagnetic nanoparticles: The effective time constant for nanopar-
ticles smaller than the 10–15 nm is very small. This causes rapid fluctuations, the
microsecond period, in the magnetic field. Therefore, in the absence of external
magnetic field, the magnetic moment of the nanoparticle and the internal magnetic
moment is zero during this very short period. This property is called superpara-
magnetism [5]. The superparamagnetism expression arises from the coupling of
many atomic spins and is used to indicate an analogy between the behavior of the
small magnetic moment of a single paramagnetic atom and that of the much larger
magnetic moment of a MNPs [5]. In the presence of the external magnetic field,
superparamagnetism nanoparticles are rapidly aligned with the field. The maximum
diameter of the nanoparticles with superparamagnetic properties (D SPM ) is obtained
from the following equation [4,6]:

6 kT
D = 2 3 B (7.1)
DSPM=26kBTK3 SPM K
MNPs that used in medical applications belong to the paramagnetic materials [1].
Fig. 7.1 depicts several types of drug carriers with MNPs.
The coating of magnetic nanocarriers is designed in a manner that it can accom-
plish different mechanisms. Fig. 7.2 schematically shows a core-shell nanocarrier
with several factors on its surface.

FIGURE 7.1 Schematic structure of magnetic nanoparticles and their coatings.
(A) End-grafted polymer-coated MNP, (B) liposome-encapsulated MNP, (C) fully
encapsulated in polymer coating MNP, and (D) core-shell MN.