Stimuli-Responsive Lipid -Based Self-Assembled Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 268

Spiropyran Merocyanine

NO 2

N +
R

N

O NO 2

h

O

R

Figure 9.6 Isomerization of benzospiropyran to merocyanine on exposure to UV
light, demonstrating its change in state of ionization and molecular shape.

colorless, whereas the open merocyanine structure has a strong absorption in
the visible region and so is a purple color. The photoisomerization of the spi-
ropyran moiety is shown in Figure 9.6. Spiropyrans have been used to alter
the packing of amphiphiles in bilayer membranes (Khairutdinov and Hurst,
2001; Wohl and Kuciauskas, 2005) and micelles (Takumi et al., 2007) and so
modifying the permeation of solutes through the interface.

Cinnamic Acid (CA) On exposure to UV light, cinnamic acid and its deriva-
tives undergo a trans/cis isomerization. These photoisomers are also ionizable
with a change in pH. There have been no light-switchable drug delivery systems
reported; however, Baglioni et al. (2009) have reported that the trans-to-cis
isomerization of o - methoxyCA modifi es the molecular packing of a surfactant
and, as a consequence, induces a reduction in fl uid viscosity.

Irreversible Photoactivated Systems Light - induced polymerization, frag-
mentation, and oxidation are irreversible processes that disrupt the integrity
of the lipid structure. As the reactions of the photosensitive components dis-
cussed in this section are irreversible, the use of these in drug delivery systems
is effective for single use but not pulsatile release.
Photo-induced oxidation utilizes the reaction of a photosensitizer to a spe-
cifi c wavelength of light with oxygen, which generates free radicals and singlet
oxygen ( 1 O 2 ) to mediate their effects. Photodynamic therapy (PDT) uses these
free radicals to cause cell death. These radical oxygen species cause localized
oxidative damage to the surrounding tissue (Dolmans et al., 2003; Zeimer and
Goldberg, 2001). The consequent physiological effect depends on the type of
photosensitizer and where it is applied and activated. Current PDTs are
focused on treating tumors by either targeting the tumor microvasculature,
tumor cells, or the infl ammatory and immune host system. However, PTD is
limited by the need to keep doses of the photosensitizer and light low enough
to avoid collateral damage. The formation of free radicals has also been