Biomedical Engineering Reference
In-Depth Information
3
Polymersomes Responsive to Physical Stimuli
3.1
Temperature Responsive Polymersomes
In the previous section, we mentioned that polymersomes made from PBD- b -PLys
(Gebhardt et al. 2008 ) and from PLys- b -PBGlu- b -PLys (Iatrou et al. 2007 ) are tem-
perature responsive in basic solutions. With a temperature increase, polymersome
size increased and the poly(L-lysine) block underwent a conformational transition
from an a-helix to a b-sheet structure. In the following examples (Li et al. 2006a,
2007 ; Qin et al. 2006 ), a well-known thermo-responsive polymer, poly( N -isopropy-
lacrylamide) (PNIPAM), was used to produce temperature responsive polymer-
somes. In aqueous solution, PNIPAM undergoes a reversible phase transition from
hydrophilic to hydrophobic at a lower critical solution temperature (LCST, typically
around 32°C). Hydrophilic-hydrophilic block copolymers can then be designed
using this kind of thermo-responsive block. When the solution temperature is higher
than the LCST, the thermo-responsive block becomes hydrophobic and the block
copolymer amphiphilic, which can self-assemble to form polymersomes. Below the
LCST, the thermo-responsive block is turned hydrophilic again and the polymer-
somes dissociate to rapidly release of encapsulated substances.
McCormick and co-workers (Li et al. 2006a, 2007 ) have recently prepared
hydrophilic-hydrophilic block copolymers, poly( N -(3-aminopropyl)methacrylamide
hydrochloride)- b -poly( N -isopropylacrylamide) (PAMPA- b -PNIPAM) and poly(2-
(dimethylamino)ethyl methacrylate- b -poly( N -isopropylacrylamide) (PDMAEMA-
b -PNIPAM) (see Fig. 9 ), by reversible addition-fragmentation chain-transfer
(RAFT) polymerization. These researchers reported that these water-soluble block
copolymers could directly from vesicular aggregates in water and that aggregate
*
*
m
n
m
n
O
O
O
O
NH
NH
O
NH
N
NH 3 Cl
PAMPA- b -PNIPAM
PDMAEMA- b -PNIPAM
Fig. 9 Hydrophilic-hydrophilic block copolymers with a temperature responsive block PNIPAM
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