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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