Biomedical Engineering Reference
In-Depth Information
formation could be reversed by changing the solution temperature with respect to the
LCST of PNIPAM (32°C or higher, depending on the molecular weight of both
blocks). The aggregates could also be trapped by ionic cross-linking of the hydro-
philic PAMPA block, either through the addition of an oppositely charged polyelec-
trolyte that causes inter-polyelectrolyte complexation or by forming gold nanoparticles
from the reduction of NaAuCl
4
complexed to amine groups in the PDMAEMA.
Qin et al. described polymersome formation using a well-defined and monodis-
perse (M
w
/M
n
< 1.2) block copolymer PEO-
b
-PNIPAM (Qin et al.
2006
). The block
copolymer was synthesized by RAFT polymerization and polymersomes were
grown in an aqueous solution above the LCST of PNIPAM. This family of biocom-
patible block copolymers is a promising system for drug delivery because the LCST
can be adjusted to a value near or slightly above physiological temperatures.
Our group developed a new class of polymersomes in which the hydrophobic
part is a liquid crystal (LC) polymer (Yang et al.
2005, 2006
). It is well known that
liquid crystal systems excel as responsive systems and can respond to multiple
stimuli including temperature, light, electric and magnetic fields. If this responsive-
ness could be retained in the liquid crystal membrane, we speculated that liquid
crystal polymersomes would have potential as multi-responsive, smart polymer-
somes. Recently, we studied the structural changes in liquid crystal polymersomes
triggered by changes in temperature using small angle neutron scattering (SANS),
cryo-TEM, SEM and high sensitivity DSC (Hocine et al.
2011
). PEG-
b
-PA444 and
PEG-
b
-PMAazo444 (Fig.
10
), two block copolymers with side-on nematic hydro-
phobic blocks, were used to form vesicles with a bilayer membrane thickness of
10-15 nm at room temperature (Fig.
11a
) . Upon heating the membrane thickness,
d, started to increase dramatically from a temperature (~55°C) above T
g
but below
T
NI
of the LC polymer block, and reached up to 120 nm at T > T
NI
(T
NI
~ 80-85°C)
(Fig.
11b
). The vesicles transformed into thick-walled capsules. The thickness of
the membrane was inconsistent with a bilayer structure and surprisingly the cap-
sules were stable even for temperatures above T
NI
. As the PEG chains should par-
tially dehydrate with temperature, we propose that the membrane reorganized into
a structure consisting of microphase separated LC and PEG domains. Analysis of
changes in structural parameters such as the internal aqueous volume and the poly-
mer membrane volume suggest that capsule scission and fusion also occurred dur-
ing this transition. Substance release would be accompanied by capsule scission.
O
H
O
CH
3
CH
3
n
CH
3
(OCH
2
CH
2
) OCC
CH
2
CH
CO
O
(CH
2
)
4
O
m
n
CH
3
(OCH
2
CH
2
) OCC
CH
2
C
CO
O
(CH
2
)
4
O
CH
3
m
CH
3
C
O
O
O
O
C
4
H
9
O
N
N
O
C
OC
4
H
9
C
4
H
9
O
C
O
O C
OC
4
H
9
PEG-
b
-PA444
PEG-
b
-PMAazo444
Fig. 10
Molecular structures of liquid crystal copolymers PEG-
b
-PA444 and PEG-
b
-PMAazo444
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