Biomedical Engineering Reference
In-Depth Information
1.3
Polymersome Preparation
Polymersome preparation requires the mutual diffusion of water into the bulk block
copolymer and vice versa (Battaglia and Ryan
2006
). In general, all reported meth-
ods for liposome preparation can also be used for polymersome preparation (Szoka
and Papahadjopoulos
1978
; Gregoriadis
1992
; Angelova and Dimitrov
1986
;
Pautot et al.
2003a
). In preparation protocols, the contact between water and poly-
mer can be achieved directly or with the aid of organic solvent if the hydrophobic
block is glassy at the preparation temperature. For block copolymers with hydro-
phobic blocks with a low Tg, such as PEO-
b
-PBD (T
g,PBD
~ −90°C to −8°C accord-
ing to their relative 1,4- and 1,2-content), vesicles can be formed by direct hydration
techniques assisted by sonication or electrical field (Dimova et al.
2002
). In con-
trast, block copolymers with a glassy hydrophobic block, such as PAA-
b
-PS
(T
g,PS
~ 100°C), often require an organic co-solvent to fluidize the polymer layers.
Typically, a polymer solution is first prepared in an organic solvent and the solvent
is then gradually exchanged with water (Zhang and Eisenberg
1995b
). It belongs to
a more general nanoprecipitation method based on the interfacial deposition due to
the displacement of a solvent with the non-solvent. Recently, microfluidic and
micro-patterning technology have opened some fascinating ways to prepare poly-
mersomes with controlled size and efficient encapsulation (Hauschild et al.
2005
;
Shum et al.
2008
; Stachowiak et al.
2009
; Howse et al.
2009
).
Polymersome preparation methods discussed above naturally lead to a symmet-
ric copolymer distribution between both leaflets of bilayer membrane. One special
method, called inverted emulsion which is first developed by Weitz' group for lipo-
somes (Pautot et al.
2003c
), permits to obtain asymmetric vesicles by independent
assembly of the inner and outer leaflets of the vesicle (Pautot et al.
2003b
). Briefly,
the inner monolayer is first formed via the emulsification of water droplets in oil
containing the first amphiphile of interest. The outer monolayer is then formed by
the centrifugation of the water droplets stabilized by the first amphiphile through
the monolayer of the second amphiphile at the interface between a second oil solution
(containing the second amphiphile of interest) and a water solution.
1.4
Strategies for Controlled release
The major advantage of polymersomes is that they can be used as carriers of
hydrophilic substances (in the interior compartment) as well as hydrophobic sub-
stances (in the membrane) offering a cocktail-treatment or a diagnostic-therapy
combination in biomedical applications. A crucial question here is how to release
the active substances when and where they are needed. In general, the continuous
loss of encapsulated substance via diffusion is very slow as a result of the consider-
able thickness of the polymer membrane. In some cases, more selective permeabil-
ity has been achieved through the use of special chemical structures for the block
copolymer (Battaglia et al.
2006
), (Vriezema et al.
2007
), or by incorporating
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