Biomedical Engineering Reference
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thermodynamically less stable and therefore it isomerises slowly back to
trans
form
(typically over several hours). However, irradiation (usually by visible light)
reduces the back reaction time to minutes. The photoisomerization of azobenzene
chromophores has no side reactions and the wavelength to induce the transforma-
tion can be tuned by incorporating substituents in the chromophores.
The polarity, shape and size changes that azobenzene undergoes after isomeriza-
tion modify significantly the structures and properties of azobenzene-containing
polymer blocks.
Zhao's group (Wang et al.
2004
), (Tong et al.
2005
) prepared amphiphilic
diblock copolymers composed of hydrophilic poly(acrylic acid) and hydrophobic
polymethacylate with azobenzene side groups. They showed that these copolymers
self-assembled in dioxane/water mixtures to form photoresponsive polymer vesi-
cles of 100-200 nm. Upon UV irradiation, the majority of the vesicular aggregates
disappeared. The disruption of these morphological structures appeared to be fully
reversible as the aggregates reformed following subsequent illumination with visi-
ble light. Solubilization of the hydrophobic block in dioxane/water mixtures caused
by the polarity change of azobenzene after
trans
-to-
cis
isomerization is claimed to
be responsible for vesicle disruption.
We have recently reported photo-responsive polymersomes (del Barrio et al.
2010
) base on amphiphilic PEG-
b
-azodendron block copolymer with a hydrophobic
block based on the fourth generation of aliphatic polyester dendrons functionalized
at the periphery with cyanoazobenzene mesogens (
PEG45-AZO16)
. The polymer-
somes in water were prepared by nanoprecipitation with dioxane/water co-solvents
followed by extensive dialyse against water. Wrinkles, and even rupture, in the mem-
brane of the polymersome were detected upon UV irradiation of polymersome
dispersion in water (Fig.
13
).
Fig. 13
Cryo-transmission electron micrograph of the polymersomes formed in water by the
PEG-
b
-azodendron block copolymer
PEG45-AZO16
(
a
) before and (
b
) after illumination with
360 nm unpolarized light (150 mW/cm
2
) for 35 min
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