Page 236 - Handbook of Properties of Textile and Technical Fibres

Engineering properties of spider silk 211

6.5.1 Microfibers

Studies have been carried out on regenerated spider silks by collecting natural spider
silks, redissolving the silks in solvents and spinning the solution through conventional
wet spinning or simply hand-drawing. These methods face the same problem on the
scalability as that exists in collecting natural spider silks.
With recombinant DNA technique the DNA sequence can be manipulated to pro-
duce tailor-made proteins. These advances have allowed scientists to introduce the
fiber-forming genes from spider into variety of bacterial (Escherichia coli), fungal
(Pichia pastoris), plants and animal cells to produce recombinant silk (6, 7, 8, 9, 10,
11, 12). A Canadian company, Nexia Biotechnologies Inc., in 1999 introduced an inno-
vative technology of using a transgenic approach for production of recombinant spider
silk, BIOSTEEL in BELE (Breed Early Lactate Early) (18). The milk produced by
transgenic goats contained MaSp1 and MaSp2 proteins, which can be isolated and
purified to homogeneity. The Nexia scientists successfully demonstrated, by wet spin-
ning, that 10e40 mmsilk monofilaments can be spun from water soluble recombinant
protein produced from dragline silk genes of the spider A. diadematus (ADF-3)
expressed in mammalian cells (Lazaris et al., 2002). N. clavipes spidroins (MaSp1
and MaSp2) recombined in mammalian cells that contain different motifs of sequence
were wet spun into fibers. The resultant fibers exhibit variations in their microstructure
in terms of crystallinity and chain alignment but the tensile properties are similar (Elices
et al., 2011). Whilst the biotechnology pathway to large-scale manufacturing of spider
silk is promising, the strength of the synthetic silk is far from satisfactory in spite of its
high level of elongation at break. Table 6.5 summarizes the mechanical properties of the
regenerated and recombinant spider silks generated by Nexia.

6.5.2 Nanofibers
In order to improve the strength of the recombinant spider silk the spider silk polymer
were electrospun to form nanofibers. Recombinant MaSp1, MaSp2 of transgenic spi-
der silk (Biosteel), and the mixture of MaSp1 and MaSp2 were found by the Ko group
at the University of British Colombia (UBC) to be all electrospinnable while MaSp1
showed a better spinnability than MaSp2 (Gandhi, 2006; Ko and Gandhi, 2007). FTIR
investigation showed that the nanofibers had overall higher b sheet contents than the
recombinant spidroins, proving higher order orientation of crystals, and nanofibers
from MaSp1 had more b sheets than those from MaSp2. As shown in Fig. 6.15,
MaSp1-aligned fibers had modulus, strength, and elasticity of 123.29 25.3 MPa,
9.59 1.98 MPa, and 14.33 0.21%, respectively, which are far below the values
of natural spider silk (Gandhi, 2006). Katz’s study (Katz, 2006) found that the relax-
ation time for the alanine backbone of MaSp1 and MaSp2 powder samples are similar,
while the relaxation time for glycine is significantly faster than that of the spider silk,
suggesting the glycines are in a less rigid structure than that of the silk. The results
found demonstrate that the spider’s spinning process to silk is more crucial for the
orientation and ordering of the polyglycine than it is to the crystallization of the poly-
alanine motifs. The electrospun MaSp1 was found to have very little crystallization