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W. Qiu and X.-Y. Liu
fibers to promote SF crystallization and, consequently, to improve the crystallization as well as the mechanical performance. Finally, an additional steam-annealing
treatment was applied to enhance the crystallinity and mechanical properties of the
fibers. The final artificial RSF fibers were obtained by the take-up device.
Chen et al. [19] introduced PS NPs into an RSF solution and then ultilized a
homemade apparatus to synthesize RSF composite fibers. By simply varying the PS
NPs content added, the macroscopic performance of the final obtained composite
RSF fibers was altered due to their strong interactions with SF molecules. Consequently, the addition of PS NPs can greatly improve mechanical performance [19]
Within the framework of the continuous nucleation mechanism (cf. Fig. 6.23d, e),
this affinity interaction between PS NPs and SF molecules (specifically referred to
as the intermolecular hydrogen bonds between the carboxyl groups on the surface
of PS NPs and amino groups within SF peptide chains) can significantly decrease
surface tension and lower the nucleation barrier. Thus, the heterogeneous nucleation
of SF molecules is greatly promoted on/near the surface of PS NPs, which results
in the generation of more nucleation sites (corresponding to a larger crystal density
within the spun RSF fibers), and alteration of crystal networks, and the structures of
nanofibrils. This gives rise to the reconstruction of mesoscopic hierarchical structures of SF materials (cf. Fig. 6.23). As discussed, the crystallization in both natural
silk fibers and RSF fibers exhibit strong structural interactions. Consequently, the
increase in density will result in stronger fibers.
Apart from the aforementioned nanoparticles, other functionalized composite RSF
fibers (or mats), such as those containing hydroxyapatite (HAP) nanoparticles [72]
and cellulose [73] have also been successfully fabricated using the electrospinning
technique.
However, it is notable that not all additives are beneficial for the heterogeneous
nucleation of SF molecules [74]. In fact, some additives have a high tendency for selfassembly. When such additives are mixed with SF dope solutions, large clusters (or
aggregates) of additives will form spontaneously, which leads to defects and results in
the generation of fibers that are inferior to composite RSF fibers. Hence, a uniformly
dispersed mixture solution is essential. Normally, small ratios of surfactants can be
incorporated into the mixture to increase the dispersion of additives. Upon performing
this procedure, the obtained composite RSF fibers generally display a bead-free
nanofiber morphology that does not exhibit any aggregation of additives [72] and the
mechanical properties are improved. Further structural analyses have clearly shown
that such composite RSF fibers have a larger β-sheet and β-crystallite content than
the neat RSF fibers [74].
Another notable finding is that the addition other functionalized additives into SF
materials during the formation is an effective method for endowing RSF fibers with
some new and unique performance. Such added performance can range from the
biological, optical to electronic aspects [72–74]. For instance, Ma et al. successfully
functionalized SF fibers by carbon nanotubes based on the CNT-templated nucleation
of SF networks. This leads to electrically conductive biocompatible RSF fibers. In
this context, both the mechanical properties and the electronic conductivity of the
reconstructed mesoscopic functional fibers can be tuned by varying the density of the
W. Qiu and X.-Y. Liu
fibers to promote SF crystallization and, consequently, to improve the crystallization as well as the mechanical performance. Finally, an additional steam-annealing
treatment was applied to enhance the crystallinity and mechanical properties of the
fibers. The final artificial RSF fibers were obtained by the take-up device.
Chen et al. [19] introduced PS NPs into an RSF solution and then ultilized a
homemade apparatus to synthesize RSF composite fibers. By simply varying the PS
NPs content added, the macroscopic performance of the final obtained composite
RSF fibers was altered due to their strong interactions with SF molecules. Consequently, the addition of PS NPs can greatly improve mechanical performance [19]
Within the framework of the continuous nucleation mechanism (cf. Fig. 6.23d, e),
this affinity interaction between PS NPs and SF molecules (specifically referred to
as the intermolecular hydrogen bonds between the carboxyl groups on the surface
of PS NPs and amino groups within SF peptide chains) can significantly decrease
surface tension and lower the nucleation barrier. Thus, the heterogeneous nucleation
of SF molecules is greatly promoted on/near the surface of PS NPs, which results
in the generation of more nucleation sites (corresponding to a larger crystal density
within the spun RSF fibers), and alteration of crystal networks, and the structures of
nanofibrils. This gives rise to the reconstruction of mesoscopic hierarchical structures of SF materials (cf. Fig. 6.23). As discussed, the crystallization in both natural
silk fibers and RSF fibers exhibit strong structural interactions. Consequently, the
increase in density will result in stronger fibers.
Apart from the aforementioned nanoparticles, other functionalized composite RSF
fibers (or mats), such as those containing hydroxyapatite (HAP) nanoparticles [72]
and cellulose [73] have also been successfully fabricated using the electrospinning
technique.
However, it is notable that not all additives are beneficial for the heterogeneous
nucleation of SF molecules [74]. In fact, some additives have a high tendency for selfassembly. When such additives are mixed with SF dope solutions, large clusters (or
aggregates) of additives will form spontaneously, which leads to defects and results in
the generation of fibers that are inferior to composite RSF fibers. Hence, a uniformly
dispersed mixture solution is essential. Normally, small ratios of surfactants can be
incorporated into the mixture to increase the dispersion of additives. Upon performing
this procedure, the obtained composite RSF fibers generally display a bead-free
nanofiber morphology that does not exhibit any aggregation of additives [72] and the
mechanical properties are improved. Further structural analyses have clearly shown
that such composite RSF fibers have a larger β-sheet and β-crystallite content than
the neat RSF fibers [74].
Another notable finding is that the addition other functionalized additives into SF
materials during the formation is an effective method for endowing RSF fibers with
some new and unique performance. Such added performance can range from the
biological, optical to electronic aspects [72–74]. For instance, Ma et al. successfully
functionalized SF fibers by carbon nanotubes based on the CNT-templated nucleation
of SF networks. This leads to electrically conductive biocompatible RSF fibers. In
this context, both the mechanical properties and the electronic conductivity of the
reconstructed mesoscopic functional fibers can be tuned by varying the density of the