Biomedical Engineering Reference
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of aggregates which are structurally similar to the amyloid fibrils associated with
Alzheimer's, Huntington's, and Parkinson's disease [127]. This observation leads
to a fascinating question: Is the process of ultrasound-induced fibrous aggregation
in proteins related to that in molecular gels? Results from several other studies
suggest that the answer is ''Yes!'' [128]. Further studies relating these areas would
undoubtedly be beneficial.
1.3.3
Radiation-Induced Gelation and Degelation
Ultraviolet and visible radiation can be a powerful tool to induce shape or charge-
distribution changes in
s and to cause isothermal phase transitions, either
from a sol to a gel or a gel to a sol. Only a few of the many examples now in
the literature will be highlighted here. A recent example employs the
trans
LMOG
⇒
cis
isomerization of
trans-o
-methoxycinnamic acid (
) which binds to the zwitterionic
surfactant, erucyl dimethylamidopropyl betaine (
70
69
), forming short cylindrical
micelles in aqueous media [129]. As
is isomerized to its
cis
isomer by UV
radiation,itisexpelledfromthemicellesof
70
, which grow into long, worm-like
micelles and entangle to form a gel-like network (Figure 1.44).
In another example, it was shown that reversible sol
69
gel transformations
can be induced by irradiation of a cholesteric-azobenzene ether (
⇔
Me) [54].
Irradiation in the UV region to induce
trans
⇒
cis
isomerization of the azobenzene
moiety destroyed the fibrillar network of the organogel, leading to a sol phase.
Subsequent irradiation of the sol in the visible region allowed reformation of the
trans
isomer and gelation. A similar approach has been demonstrated with gels
consisting of aromatic liquids and
N
-glycosylazobenzenes as the
LMOG
s [130]. An
71
;R
=
H
3
C
CH
3
69
O
−
NH
N
+
O
O
CH
3
+
+
O
UV
Light
O
OH
HO
O
O
trans-
70
cis-
70
Low viscosity
(short micelles)
High viscosity
(lond micelles)
Figure 1.44
Radiation-induced gelation via isomerization of a cinnamic acid derivative.
Reproduced from Ref. [129] with permission of The Royal Society of Chemistry.
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