Biomedical Engineering Reference
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mixture was changed to a self-supporting gel. Scanning electron microscopy
(SEM) examination showed that GP-1 formed a homogeneous interconnecting
fiber network after treatment by ultrasound. When repeated heating and cooling
were applied, precipitation of spherulite reoccurred without ultrasound treatment.
This indicates that ultrasound only exerts a physical impact on the transition of the
networks. The conversion of GP-1 spherulites to interconnecting fiber networks
was also observed when propylene glycol was used as solvent.
While ultrasound-mediated transition of fiber networks is an interesting topic, a
clear mechanismhas not yet been obtained. Since ultrasound treatment was applied
immediately after the formation of spherulites, the possible mechanism we postu-
late is that the micro-segment of the broken fibers serves as seeds for nucleation
(nucleation centers). The formation of seeds lowers the concentration of GP-1 dis-
solved in the solvent, which reduces the thermodynamic driving force (supersatura-
tion of GP-1). This contributes to the one-dimensional growth of fibers into fibrillar
networks, as in the thermally controlled self-seeding process described earlier.
Ultrasound-induced gel formation from suspension or precipitates has also
been reported by other researchers in the last few years [35]. The proposed
mechanism is that ultrasound can break the intramolecular hydrogen bonds of
gelator molecules and facilitate the formation of intermolecular hydrogen bonds
between the gelator molecules. This leads to gel formation by facilitating the
formation of three-dimensionally interconnecting fiber networks. A detailed review
of ultrasound-induced gelation has been given by Cravotto and his colleague [42].
2.4.2.3
Kinetically Controlled Homogenization of Fiber Networks
In most gelling systems, the fiber formation and gelation is a non-isothermal
process due to the insufficient cooling rate/speed. That is to say, the temperature
changes as the fibers crystallize. In other words, for such a system, the nucleation
and growth of fibers take place (at
T
2
) before a hot solution (at
T
1
) is cooled to
a settling temperature
T
3
(for example, environmental temperature as in most
industrial manufacturing process) (
T
3
<
T
2) (Figure 2.12A). Due to the difference
in supersaturation at
T
2
and
T
3
, the fiber networks of different morphologies
normally form at these temperatures [34]. At the higher temperature
T
2
or a lower
supercooling (supercooling
T, T
eq
: equilibrium temperature of the
gelator solution,
T
: actual temperature), less branched fibers normally form. While
at a lower temperature, the correspondingly higher supercooling can lead to the
formation of highly branched fibers (i.e., spherulites) due to enhanced mismatch
nucleation [34]. Consequently, the entire fiber network is often a heterogeneous
(mixed) fiber networks of different types (Figure 2.12A) [24]. This is not desired in
many important applications such as separation, when a homogeneous (pure) fiber
network is desired. Therefore, it is important that the formation of fiber networks
in such a material can be controlled to prevent the formation of a heterogeneous
fiber network. In addition, as the non-isothermal process significantly affects the
morphology, size, and spatial distribution of fibers, as well as the mechanical and
solvent binding capacity of the gels [28, 30], it is practically significant to control
the non-isothermal effects.
T
=
T
eq
−
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