Biomedical Engineering Reference
In-Depth Information
Fig. 10 (a) Chemical structure of PEG-
b
-PCL copolymer. (b) CLSM image of PEG-
b
-PCL
polymersomes containing membrane-encapsulated Nile Red (2 mol%) and aqueous entrapped
Calcein dyes. Scale bar: 5
m
m. (c) Cryo-TEM image of PEG-
b
-PCL polymersomes. Scale bar:
100 nm. Reprinted from [
228
] with permission
block and aliphatic polyesters as hydrophobic block. PEG-
b
-PLA [
226
,
227
] and
PEG-
b
-PCL (Fig.
10a
)[
228
] were developed by the groups of Feijen and Hammer,
respectively. As described above, the block copolymers can be synthesized by
ROP of cyclic esters in the presence of mono-hydroxyl-terminated PEG as a
macro-initiator. The molecular weight of polyester blocks can be tuned by control
of the feed molar ratio of cyclic esters to PEG in the polymerization process. Thus,
the volume fraction of block copolymers can be tailored to self-assemble into
polymersomes. In addition to regulation of volume fraction of the copolymers,
the size distributions of the polymersomes can be roughly controlled with standard
techniques such as sonication, freeze/thaw extraction, and extraction at above the
T
g
of polyester blocks to give monodispersed vesicular structure with diameters
ranging from nano- to micrometers (Fig.
10b,c
), which are useful for in vivo
applications [
228
].
Hammer and coworkers prepared PEG-
b
-PCL polymersomes entrapping DXR
(Fig.
11a
). The release of DXR from the polymersomes was in a sustained manner
over 14 days at 37
C in PBS via drug permeation through the PCL membrane,
and hydrolytic degradation of the PCL membrane [
228
]. The release rate of
encapsulated molecules from polymersomes can be tuned by blending with another
type of block copolymer [
229
]. Indeed, the release rate of encapsulated DXR
from polymersomes prepared from mixtures of PEG-
b
-PLA with PEG-
b
-PBD
copolymers increased linearly with the molar ratio of PEG-
b
-PLA in acidic
media (Fig.
11b
). Under acidic conditions, the PLA first underwent hydrolysis
and, hours later, pores formed in the membrane followed by final membrane
