6 Cocoon Silk: From Mesoscopic Materials Design …
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also altered. It is reported that genetic modification techniques have been successfully applied for producing fluorescent colored silks [65]. According to previous
studies, the gene for the fluorescent colored protein (e.g. green fluorescent protein,
GFP) can be inserted into the silkworm genome and then the SF proteins and fluorescent proteins are co-expressed simultaneously. Consequently, transgenic, fluorescent
colored silk fibers are obtained. As the primary structure of silk fibers is incorporated
within the sequence of fluorescent proteins, the mechanical strength of the hybrid
silk fibers is slightly improved (or decreased) [65].
Although fluorescent protein sequences can be incorporated into the primary structure of silk fibers and consequently change the mechanical properties, the modulation
ability is not satisfying. Because the mechanical properties of spider silks (especially
the dragline silk fiber) are superior to those of silkworm silk, numerous efforts have
recently been devoted to the use of genetically modified silkworms as the host for the
production of transgenic spider silk fibers [66, 67]. Previously, most relevant attempts
focus on transposon-mediated transgenic silkworms. These silkworms can successfully produce reinforced silk fibers. However, the amount of spider silk proteins in
these transgenic silkworms is very low (<5%); this may be attributable to the variable
promoter activities and endogenous SF protein expression [67]. Recently, Xu et al.
[66] put forward a system for the production of massive amounts of spider silk in
silkworms using a transcription activator-like effector nuclease (TALENs)-mediated
technique, to replace the fibroin H-chain gene of silkworms with the MA spidroin-1
gene. According to their experimental results, the achieved yield of chimeric spider
protein within the obtained hybrid silk fibers is up to 35.2% of the total amount.
More importantly, the relative abundance of the primary structure of spidroin significantly improves the toughness of the transgenic fibers, and especially increase the
extensibility [66].
The feeding technique is another in vivo method for producing intrinsically functionalized silk fibers. In comparison to the gene modification method, the feeding
method is much more versatile and effective. Besides, this method can also be well
understood within the framework of SF molecule crystallization theory. It is reported
that fluorescent silk cocoons and fibers can be directly obtained by simply feeding
silkworms various small fluorescent molecules (e.g., rhodamine B, rhodamine 101,
and rhodamine 110) [9]. It follows that all these small fluorescent molecules can
directly conjugate with the SF molecules via molecular recognition [68]. Thus, these
small fluorescent dyes are trapped into the silk fibers along with the SF molecule crystals during the natural spinning process. Further structural characterization confirms
that the secondary structure of silk fibers is modified via feeding [68]. Specifically,
micro-FTIR shows that there is no apparent structural homogeneity within silk fibers;
such structural fluctuations might be the reason why these modified fibers display
unsatisfactory mechanical properties (e.g., tensile strength) [68]. Apart from the small
fluorescent dye moleculesIt was recently reported that some specific nanoscale materials, such as functionalized TiO 2 nanoparticles [69], single-walled carbon nanotubes
(SWNTs), and graphene [70]. can also be directly conjugated into silk fibers by
feeding silkworms artificial diets. Moreover, these additives can greatly improve the
macroscopic performance of the composite silk fibers. For instance, it was determined
289
also altered. It is reported that genetic modification techniques have been successfully applied for producing fluorescent colored silks [65]. According to previous
studies, the gene for the fluorescent colored protein (e.g. green fluorescent protein,
GFP) can be inserted into the silkworm genome and then the SF proteins and fluorescent proteins are co-expressed simultaneously. Consequently, transgenic, fluorescent
colored silk fibers are obtained. As the primary structure of silk fibers is incorporated
within the sequence of fluorescent proteins, the mechanical strength of the hybrid
silk fibers is slightly improved (or decreased) [65].
Although fluorescent protein sequences can be incorporated into the primary structure of silk fibers and consequently change the mechanical properties, the modulation
ability is not satisfying. Because the mechanical properties of spider silks (especially
the dragline silk fiber) are superior to those of silkworm silk, numerous efforts have
recently been devoted to the use of genetically modified silkworms as the host for the
production of transgenic spider silk fibers [66, 67]. Previously, most relevant attempts
focus on transposon-mediated transgenic silkworms. These silkworms can successfully produce reinforced silk fibers. However, the amount of spider silk proteins in
these transgenic silkworms is very low (<5%); this may be attributable to the variable
promoter activities and endogenous SF protein expression [67]. Recently, Xu et al.
[66] put forward a system for the production of massive amounts of spider silk in
silkworms using a transcription activator-like effector nuclease (TALENs)-mediated
technique, to replace the fibroin H-chain gene of silkworms with the MA spidroin-1
gene. According to their experimental results, the achieved yield of chimeric spider
protein within the obtained hybrid silk fibers is up to 35.2% of the total amount.
More importantly, the relative abundance of the primary structure of spidroin significantly improves the toughness of the transgenic fibers, and especially increase the
extensibility [66].
The feeding technique is another in vivo method for producing intrinsically functionalized silk fibers. In comparison to the gene modification method, the feeding
method is much more versatile and effective. Besides, this method can also be well
understood within the framework of SF molecule crystallization theory. It is reported
that fluorescent silk cocoons and fibers can be directly obtained by simply feeding
silkworms various small fluorescent molecules (e.g., rhodamine B, rhodamine 101,
and rhodamine 110) [9]. It follows that all these small fluorescent molecules can
directly conjugate with the SF molecules via molecular recognition [68]. Thus, these
small fluorescent dyes are trapped into the silk fibers along with the SF molecule crystals during the natural spinning process. Further structural characterization confirms
that the secondary structure of silk fibers is modified via feeding [68]. Specifically,
micro-FTIR shows that there is no apparent structural homogeneity within silk fibers;
such structural fluctuations might be the reason why these modified fibers display
unsatisfactory mechanical properties (e.g., tensile strength) [68]. Apart from the small
fluorescent dye moleculesIt was recently reported that some specific nanoscale materials, such as functionalized TiO 2 nanoparticles [69], single-walled carbon nanotubes
(SWNTs), and graphene [70]. can also be directly conjugated into silk fibers by
feeding silkworms artificial diets. Moreover, these additives can greatly improve the
macroscopic performance of the composite silk fibers. For instance, it was determined