Biomedical Engineering Reference
In-Depth Information
than for Feridex, while the T
1
relaxivity was better for Feridex, suggesting a greater
sensitivity of their magnetic nanoparticle in T
2
- weighted imaging.
A variety of processes are available for coating nanoparticles, including
in situ
coating (i.e., coated during the synthesis process) and post-synthesis coating. The
coating can further be crosslinked chemically to increase its stability, and this
crosslinking approach is frequently used to produce dextran-coated SPIO nanopar-
ticles that are both biocompatible and water-miscible. Polymer coating usually
involves post-synthesis coating methods, although recent studies have explored
ways of coating with polymers or copolymers
in situ
[10, 11]. This type of process,
which is sometimes referred to as the “ one - pot synthesis ” method, has several
advantages over other methods, including a reduced aggregation because the
particles are coated immediately and there are fewer processing procedures.
However, the presence of polymers during nanocrystal nucleation and growth can
cause imperfections on the crystal structure and morphology of the magnetic
nanoparticles obtained through these processes, which could in turn signifi cantly
compromise their magnetic properties.
One of the challenges with polymer chemistry is that it needs to be compatible
with the material that it encapsulates, particularly if the nanoparticles are biodrug-
or protein-loaded, and also with the types of ligand that are to be attached to the
surface. This is not simple or straightforward, however. Encapsulation effi ciency
is not great or consistent for all methods, and the ability to control the thickness
of the coating and limit the size of the nanoparticles to just tens of nanometers
may not always be feasible.
One state-of-the-art technology for the synthesis of nanocapsules involves
electrohydrodynamic (EHD) technology [12, 13]. The EHD encapsulation process
involves pumping nanoparticle components as fl uids via coaxially arranged
capillaries/needles across a high-voltage region onto a collector surface. The
action of the electric fi eld and rapid solvent evaporation between the fl uid ejection
nozzle and the collector electrode cause the electrosprayed liquids to form poly-
meric nanocapsules. The advantage of EHD technology over conventional encap-
sulation strategies is that it is possible to control shell thickness and shell
porosity.
The use of biocompatible silica or gold is also widespread in the encapsulation
of magnetic nanoparticles for developing MRI contrast agents. These inert shells
serve to both protect against chemical degradation of the magnetic cores and
prevent the release of potentially toxic components from the core. In addition, the
use of alkoxysilanes such as 3 - aminopropyltriethyoxysilane ( APS ) allows surface -
reactive groups to be easily added to these shell structures for functionalization
with specifi c targeting agents and other ligands. Recently, Ma
et al.
described a
multifunctional magnetic nanoparticle composed of iron oxide cores of approxi-
mately 10 nm, surrounded by a shell of SiO
2
which was 10 - 15 nm thick [14] . In
these studies, an organic dye was doped inside a second silica shell to create a
superparamagnetic nanoparticle with luminescent properties for application in
biomedical imaging.
