Biomedical Engineering Reference
In-Depth Information
The surface - initiated ROP of L - lactide ( LLA ) on the surface of Fe
3
O
4
MNPs has
been used to prepare magnetic core- polylactide ( PLLA ) shell nanoparticles [68] .
Fe
3
O
4
MNPs synthesized by the coprecipitation method were treated with 0.1
M
glycolic acid under ultrasonic treatment for 30 min, and then maintained for 12 h
at room temperature so as to impart the MNPs with hydroxyl groups. The glycolic
acid- modifi ed MNPs were dispersed in dry toluene and the suspension was heated
to 130 °C to remove any remnant water by azeotropic dehydration. The residual
water was removed in order to prevent the formation of free PLLA. When the
suspension had cooled to room temperature, LLA was slowly added and the polym-
erization carried out with 0.2 wt% Sn(Oct)
2
as catalyst, at 130 °C under nitrogen
for 20 h. TEM images revealed a shell of
3 nm thickness around the MNP, while
a TGA indicated the amount of grafted PLLA polymer to be approximately 13.3%
in weight. The
M
s
for the glycolic acid-modifi ed MNPs and PLLA-modifi ed MNPs
were 53.3 and 52.1 emu g
− 1
, respectively. As the polymer shell was expected to
weaken the magnetism of the MNPs, this similarity in values obtained was attrib-
uted to an improved crystallinity of the magnetic core resulting from exposure to
high temperatures during the coating process. The PLLA- modifi ed MNPs with
high magnetization may potentially be useful in the fi eld of biomedicine, since
PLLA has already been used in many such applications.
∼
10.4
Encapsulation of Magnetic Nanoparticles in a Polymeric Matrix
In Section 10.3, attention was focused on the modifi cation of single MNPs with a
polymeric shell to achieve biocompatibility and possible biomedical applications,
especially as MRI contrast agents. Such MNPs can be used for passive tracking or
imaging of biological systems via naturally directed physiological processes, and
active targeting of specifi c receptors on cell surfaces via the use of targeting
ligands. The most commonly used targeting ligand is FA or its toxic equivalent,
MTX. However, whilst other cancer-targeting ligands, such as antibodies, may be
as large as 20 nm, conjugation to single MNPs would be diffi cult and might also
result in a loss of activity due to steric hindrance [4]. Similarly, it would be diffi cult
to conjugate both drugs and targeting ligands to single MNPs. Hence, an alterna-
tive approach would be to encapsulate multiple MNPs in a polymeric nanosphere.
In this section, emphasis will be placed on these larger polymer nanospheres (of
a few hundred nm diameter), and their applications.
These larger polymer nanospheres do have some
disadvantages
, at least poten-
tially. In general, the
in vivo
half-life of nanoparticles depends inversely on the
particle size [69]; particles with diameters
40 nm would have the longest residence
times (of the order of hours), and can accumulate in the lymph nodes and be
excreted via the urine and feces. Particles in the range of 40-200 nm can be removed
by the liver and spleen in a shorter time, while particles larger than 200 nm would
have the shortest half-life as a result of opsonization and clearance by the MPS.
Furthermore, receptor-mediated targeting using larger nanoparticles may be less
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