Biomedical Engineering Reference
In-Depth Information
triblock, graft, hyperbranched, etc.) and molecular weights (ranging from
hundreds to tens of thousands dalton).
The formation of polymersomes is mainly dictated by the fraction of the
hydrophilic block (f
phil
), the molecular weight, and the effective interaction
parameter of the hydrophobic block with water (x). For block copolymers with
a high x, vesicles are favored when f
phil
5 20-40%, which is the same for
natural lipids. At higher f
phil
(.45%), cylindrical or spherical micelles are
predominantly formed. It should be noted that the block copolymer vesicles
show extremely slow chain exchange dynamics and thus exhibit a much lower
critical aggregation concentration (CAC) compared to liposomes. Eisenberg
and co-workers found that block copolymer vesicles are thermodynamically
stabilized with short hydrophilic chains located preferentially at the watery
core and long hydrophilic chains at the outer surface.
4
The sizes of
polymersomes span from nano to micro scales, depending on their macro-
molecular parameters, e.g. structures, compositions, and molecular weights
(thermodynamics), as well as fabrication methods, e.g. direct dissolution,
phase inversion, film rehydration (kinetics).
The membrane thickness (d) of polymersomes was reported to scale up with
the molecular weights of the hydrophobic blocks (M
phobe
)asd
#
M
phobe
1/2
by
Discher and co-workers.
5,6
The bending rigidity of polymersomes increases
with increasing membrane thickness as a result of enhanced interdigitation.
The membrane fluidity, on the other hand, usually decreases with increasing
molecular weight and becomes more pronounced when the hydrophobic chains
are long enough to entangle. The permeability of polymersomes is governed by
the membrane thickness according to the Fick's first law. The diffusion
coefficients of polymersome membranes are one order lower compared with
those of liposomes (D
#
0.1 mm
2
s
21
or less versus 1mm
2
s
21
). The
permeability of polymersomes can be fine tuned by adjusting M
phobe
and the
nature of polymersome membrane, or by incorporating degradable or stimuli-
sensitive hydrophobic blocks into the vesicle membrane.
7,8
The polymersome surface chemistry and topology can be tailored using
block copolymers with complementary hydrophilic chains. For example,
Battaglia and co-workers reported that binary mixtures of poly[2-(methacry-
loyloxy)ethyl-phosphorylcholine]-b-poly[2- (diisopropylamino)ethyl methacry-
late] (PMPC-b-PDPA) and PEG-b-PDPA copolymers produced hybrid
polymersomes with specific PEG or PMPC domains within their exterior
envelopes, which along with polymersome size play a significant role in cellular
uptake.
9
Discher and co-workers reported the formation of spotted polymer-
somes from a mixture of the charged block copolymer PAA-b-PBD and the
neutral block copolymer PEO-b-PBD in the presence of divalent cations such
as calcium and copper that induced domain formation by selective binding
with polyanionic amphiphiles (Figure 6.1).
10
As a result of the significantly higher molecular weights, polymersomes
usually have thicker membranes (in the range of 5-30 nm versus 3-5 nm for
liposomes),
d
n
4
y
3
n
g
|
4
superb
colloidal
stability,
enhanced
mechanical
strength,
and
