Functionalization of Colored/Fluorescent Silkworm Silk Fibrous Materials - Soft Fibrillar Materials: Fabrication and Applications - page 212

was inserted into female moths to give fluorescence by an insect virus. The
female moths then mated with normal male moths. Glow-in-the-dark silk fibers
were subsequently produced by around 3% of larvae. The fluorescent gene was
able to be passed down after two more generations of larvae were bred from
these.
In a study to produce foreign proteins using an insect system, an engineered
baculovirus was developed to fuse one of the chains of the silkworm fibroin gene
coding to that of green fluorescent protein (GFP) [49]. The genetically engineered
gene replaces the endogenous fibroin gene through a homologous recombination
process when silkworms were infected with the virus. The silkworm silk glands
and fiber containing the fusion gene were reported to glow green. The advantage
of this approach is that the insertion of foreign DNA is site specific at the fibroin
gene, thus keeping it under the control of endogenous promoters. This is in
contrast to other approaches which use transposons to integrate foreign genes into
non-specific sites in the genome. They suggest that silkworms may be used as an
insect factory for the production of various useful proteins, since 400mg of silk
protein can be produced per silkworm. One example is production of spider-like
silk by replacing the silkworm fibroin gene with a spider spidroin gene. Spider silk
has a range of uses from bullet-proof vests to parachutes.
In another study to produce fluorescent silk, fibroinheavy chain gene replacement
in silkworm through site-directed homologous recombination was carried out [48].
The DNA fragment consists of GFP gene as a reporter driven by an IE promoter.
The GFP gene is flanked by 5 and 3 sequences of the fibroin heavy chain gene
of silkworm at both sides and transferred into silkworm eggs via electroporation.
However, only 3 out of 5000 fifth-instar silkworms (the larvae advance through
five stages of growth, called instars ) had green fluorescent flecks under UV light,
which shows that this approach is highly inefficient. In addition, the transgenic
silkworms could not spin silk.
Recently red, green, and red fluorescence silkworm silks have been reported in
the literature [52, 53]. The downsides of the transgenic methods include high cost
and low production efficiency [48, 51] as well as the complexity of the process. In
most cases, the transgenic silkworms are not able to spin silks [48]. In addition, the
narrow excitation spectra of GFP and other fluorescent proteins limit the choice of
excitation light. Furthermore, the transgenic fluorescence silkworm silk fibers turn
out to be mechanically weaker than the control. The advantage of such a technique
is that one can utilize silkworms as bioreactors to produce silk similar to spider
silk [49], which is scarce in nature. A more stable transfer of gene to the future
generation remains a challenge in this work [48, 54]. Only when these difficulties
are overcome will genetic engineering be a promising technique for the production
of recombinant proteins for pharmaceutical and biomedical interest.
7.4.1.2 Nanoparticles
Unlike those silks with organic dyes produced by the traditional transgenicmethods,
fluorescent silkworm silks were obtained by the electrostatic adsorption of a cationic
polyelectrolyte and CdTe quantum dots (QDs) on the surface of silk fibers [15].