Stimuli-Responsive Polymersomes - Intracellular Delivery: Fundamentals and Applications - page 313

polymersomes bursting. Figure 15c shows the outward curling rim expected to be
generated by the change of spontaneous curvature in the membrane where the
inner leaflet is light-responsive. This is actually observed in the giant asymmetric
polymersomes as shown in Fig. 14 . The giant asymmetric polymersomes (>few
microns in diameter) were prepared by the method of inverted emulsion. The
polymersome bursting takes place also if the inner leaflet is inert and the external
leaflet is light-responsive, but with inward curling rim during the vesicle opening.
These results highlight a new general strategy to create stimuli-responsive poly-
mersomes based on the fabrication of asymmetric membranes, and driven by a
change in membrane spontaneous curvature. While UV light was the stimulus
used for this study, temperature, electric or magnetic fields could also act as
remote stimuli provided that one of the two leaflets of the membrane is composed
of suitably designed copolymers sensitive to these physical stimuli. This flexibil-
ity, combined with the low permeability of polymer bilayers, ensures a wide
range of potential applications in the fields of drug delivery, cosmetics and mate-
rial chemistry.
Trans-cis isomerization of azobenzene was also used to modify (normally
increase) the LCST of thermosensitive polymers because the cis conformers is
more polar and hydrophilic (Sugiyama and Sono 2001 ; Desponds and Freitag 2003 ;
Ravi et al. 2005 ; Sin et al. 2005 ). These polymers could potentially be used to pro-
duce temperature and light dual-responsive polymersomes.
An interesting system containing triphenylmethane nitrile (Fig. 12b ) was
described by the group of X. Zhang (Jiang et al. 2007b ), as illustrated Fig. 16 ,
which consists of a PEG chain linked to a group of hydrophobic Malachite green.
This PEG- b -bis(4-dimethylaminophenyl)phenyl methyl nitrile is able to self assem-
ble into vesicles in water. Under UV irradiation, the photochromic group ionizes in
its cationic form and the molecules become soluble in water. Consequently vesicles
are disassembled. The molecules can recover their neutral form by heat treatment
and free chains are reassembled again into vesicles.
Photoactive polymers bearing triphenylmethane leucohydroxide (Kono et al.
1995 ) and nitrocinnamate (Yuan et al. 2005 ) have been used to prepare light respon-
sive microcapsules for controlled encapsulation and release of substance.
Spirobenzopyran derivatives (Fig. 12c) have been used to create water-soluble
polymers that associate into aggregates under UV irradiation (Konak et al. 1997 ).
We foresee that these photoactive groups are also potentially useful for light-
responsive polymersome formation providing that proper chemical design is made
for block copolymers.
The last example, described as follows, with photoactive groups incorporated in
their polymer structures is rather different from the systems discussed above. It is
a polymersome susceptible to UV-induced degradation (Katz et al. 2010 ). Photolabile
2-nitrophenylalanine (2NPA) was used to join the hydrophilic PEG block and
hydrophobic PCL block (Fig. 17a ). Exposure of the polymersomes formed by this
2NPA linked PEG- b -PCL to 365 nm light enable the cleavage between PEG and