Biomedical Engineering Reference
In-Depth Information
When the external magnetic fi eld is turned off, however, superparamagnetic
nanoparticles revert to their normally random orientations due to the thermal
activity involving free rotation of the particles. This results in a loss of net magne-
tization when the external fi eld is removed, and the superparamagnetic property
is marked by this lack of remnant magnetization. When superparamagnetic
nanoparticles are bound, for example to cells, they become hindered in their ability
to rotate and are unable to disorient very rapidly via the Brownian mechanism.
Hence, there is a strong difference in the persistence of the induced magnetic fi eld
(i.e., remnant magnetization) between the bound and unbound superparamag-
netic nanoparticles, and this property is exploited in molecular imaging applica-
tions to produce a better contrast between nanoparticles that have attached to
cancer cells compared to those that are circulating freely in the body. The magnetic
characteristics of a particle can alternate between ferromagnetic and superpara-
magnetic behavior for a given fi eld and, as already noted, would be dependent on
the particle size. This means that it would be necessary to limit the size distribu-
tion of the particles in nanoparticle production to ensure a homogeneous magnetic
response to a given fi eld.
Although superparamagnetism is a desirable property of nanoparticles, the
reduction in particle size is not without some challenges. As the particle size
decreases, the surface-to-volume ratio will increases, and this results in pro-
nounced surface effects, such as noncollinear spins, spin canting, and spin-
glass-like behavior, which can signifi cantly impact upon the magnetic property of
the material [1] . Furthermore, signifi cant differences in magnetic property can be
observed with magnetic nanoparticles generated through different chemical syn-
thesis processes, due to the incorporation of impurities that may disrupt the crystal
structure, as well as different degrees of order in the crystal structure resulting
from different surface curvature (e.g., a higher surface curvature causes a more
disordered crystal structure).
5.3
Surface Coating for Improved Biocompatibility and Bioavailability
Surface coating is an integral component of all magnetic nanoparticles for bio-
medical applications. Since magnetic nanoparticles have a tendency to aggregate
as a result of their high surface energy, surface coating becomes necessary to
inhibit nanoparticle aggregation. Surface coating is also needed to render the
nanoparticles water-soluble, as well as enable their functionalization - that is, their
conjugation with biomolecules, ligands, or probes. Finally, most nanoparticles,
including magnetic nanoparticles, are subject to opsonization (the adsorption of
plasma proteins onto the nanoparticle surface) as the fi rst step in their clearance
by the reticuloendothelial system (RES); this not only compromises their effi cacy
but also raises a major challenge for their intravenous administration.
In order to help nanoparticles evade uptake by the RES, and thus maintain a
long plasma half-life so as to increase their likelihood of reaching the target tissues,
