Fibroin Spinning

Raw silk was dissolved in hexafluoro-iso-propanol (HFIP) [17, 33]. A typical working concentration for spinning was 2.5% (w/v) silk fibroin in HFIP. The spinning solution was pressed through a small needle (0 80-250 pm) into a precipitation bath (methanol for Bombyx mori silk proteins and acetone for Nephila clavipes silk proteins) and the silk solution immediately precipitated as a fiber. The best performing fibers approached the maximum strength measured for native fibers of Bombyx mori, but did not achieve the mechanical properties of natural spider silk. 

Asakura, T., Kuzuhara, A., Tabeta, R., and Saito, H. (1985). Conformation characteriz-tion of Bombyx mod silk fibroin in the solid state by high-frequency 13C cross polarization-magic angle spinning NMR, x-ray diffraction, and infrared spectroscopy. Macromolecules 18, 1841-1845. 

Asakura, T., Ashida, J., Yamane, T., Kameda, T., Nakazawa, Y., Ohgo, K., and Komatsu, K. (2001). A repeated beta-turn structure in poly(Ala-Gly) as a model for silk I of Bombyx mod silk fibroin studied with two-dimensional spin-diffusion NMR under off magic angle spinning and rotational echo double resonance. / Mol. Biol. 306, 291-305. 

Asakura, T., Ohgo, K., Ishida, T., Taddei, P., Monti, P., and Kishore, R. (2005). Possible implications of serine and tyrosine residues and intermolecular interactions on the appearance of silk I structure of Bombyx mod silk fibroin-derived synthetic peptides High-resolution 13C cross-polarization/magic-angle spinning NMR study. Biomacromolecules 6, 468-474. 

From the viewpoint of zootaxa, the silkworm and the spider belong to insect and arachnid of arthropod, respectively. Their silk proteins (fibroin for silkworm silk and spidroin for spider major ampullate silk) do not have any genetic heritage in common and their amino acids sequence compositions are different too. However, the silkworm and spider employ a similar spinning process to produce silk. Furthermore, the silkworm silk and the major ampullate silk have a number of similar structural characteristics, both at the level of the secondary protein structure and the condensed silk morphology. Therefore, for the sake of convenience, they are discussed together in some parts of this text. 

Super-contraction, the chaperonage of the special structure of spidroin, is indeed an obstacle to the use of native spider dragline silk, especially in bioapplication (usually wet condition). Recently, it was found that the intrinsic properties of silk fibroin are much better than the data listed in Table 2. The inferior properties are generated by the spinning habit of... 

As to fibers, it was reported that the inferior mechanical properties of silk from cocoons compared to spider silk result from the silkworm spinning process. If silkworm silk is processed at a constant pulling speed rather than constant force pulling, it possesses excellent properties, approaching the spider dragline silk (Shao and Vollrath, 2002). This suggests that the silkworm silk has the potential to produce better fibers, and the regenerated fibroin, which is easy to harvest, has the possibility to be fabricated into a reconstituted super-fiber. 

The overall performance difference between the artificial fibroin silk and natural silk is induced by many factors. Composition of the spinning dope is critical but not the only factor. Important to understand is that the spinning process which determines the condensed structure of silk is crucial. It suggest that knowing the spinning process details it should be feasible to produce high-performance silk artificially and "design" silk. 

Regenerated spidroin and fibroin dissolved in various solvents are used as spinning dope while the coagulation baths are mainly alcohol (Table 4). In addition certain fiber post-treatments, such as drawing, are used as well. 

Table 4 Examples of wet spinning processes using regenerated fibroin, spidroin, and recombinant spidroin, with different solvents and coagulation baths (Zhou et al., 2006a)...

Pioneering work in fibroin wet spinning can be traced back to 1930s. After that, little work has been done until the late 1980s, when more research was done to investigate the spinning dope systems, and structure and properties of the artificial fibroin silk. The composition of the dope is very important to the properties of the final fiber. Several kinds of solvents, such as LiBr—EtOH, Ca(NOo,)2—MeOH, formic acid, HFIP, hexafluoro acetone (HFA), and so on, are used to prepare the spinning dope (Table 4). Very recently, an ionic liquid was used as dope solvent (Phillips et al., 2005). 

From a scientific perspective, the artificial silk experiments have provided insight into the morphology of reconstituted silk. In the spinning dope, fibroin molecules adapt a random coil or other less extended conformations. 

Ha, S.W., Park, Y.H., and Hudson, S.M. "Dissolution of Bombyx mori silk fibroin in the calcium nitrate tetrahydrate-methanol system and aspects of wet spinning of fibroin solution". 

Marsano, E., Corsini, P., Arosio, C., Boschi, A., Mormino, M., and Freddi, G. "Wet spinning of Bombyx mori silk fibroin dissolved in N-methyl morpholine N-oxide and properties of regenerated fibres". Int. J. Biol. Macromol. 37(4), 179-188 (2005). 

Matsumoto, K., Uejima, H., Iwasaki, T., Sano, Y., and Sumino, H. "Studies on regenerated protein fibers 3. Production of regenerated silk fibroin fiber by the self-dialyzing wet spinning method". J. Appl. Polym. Sci. 60(4), 503-511 (1996). 

Yao, J.M., Masuda, H., Zhao, C.H., and Asakura, T. "Artificial spinning and characterization of silk fiber from Bombyx mori silk fibroin in hexafluoroacetone hydrate". Macromolecules 35(1), 6-9 (2002). 

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