A critical step that remains for the commercial production of synthetic silk fibers is successful spinning of recombinant proteins into materials that resemble natu­ral silks, specifically in their mechanical and thermal properties. One technique in­volves spinning fibers into a liquid coagulation bath, which is referred to as wet spinning. In 1993, it was first described by the DuPont® group with fibroins spun from silkworms62 and then later modified by Nexia Biotechnologies in 2002.63 Suc­cessful reports that integrate arthropod biomimicry to produce synthetic spider silks also have been reported in the scientific literature.64 These procedures have relied on purifying recombinant spidroins, followed by spidroin concentration via lyophi — lization, and then solubilization of spidroins with chaotropic solvents to produce a highly concentrated spinning dope (Fig. 1.5).64b This solution is then pushed through a syringe equipped with a needle into an alcohol bath, allowing a slower solidifica­tion (Fig. 1.5). The flow of the liquid is best controlled by a syringe infusion pump that can move the liquid through the syringe at a constant rate. Isopropanol is of­ten used as coagulation medium during extrusion and provides a dehydration step that removes water from the fiber (Fig. 1.5). The resulting products are referred to “as-spun threads,” which can be subsequently subject to postspin draw, a process that dramatically improves the breaking stress, toughness, and Young’s modulus.4063 This methodology has led to fibers (some biocomposites) with mechanical proper­ties that are lower quality relative to natural spider silks. For example, truncated recombinant fibroins spun into fibers have reported breaking stress values that range from 35-350 MPa (Table 1.2), which are lower than the typical 1000 MPa values reported for natural dragline silk. The strongest fibers produced via this technique have been reported from a fiber spun from a recombinant 96-mer MaSp1 protein construct expressed in E. coli, which lead to fibers that exceeded a breaking stress of 500 MPa, a value about 50% lower relative to natural dragline silk fibers (Table 1.2).41 This recombinant protein was approximately 285 kDa and represents the larg­est molecular weight recombinant protein used for synthetic fiber production.

Although these results are good indicators of progress, several caveats are worth noting for synthetic fiber production. Firstly, the reproducibility of the quality of the synthetic fibers has been a major issue. There is still much variability between the mechanical properties of the fibers generated from different regions of the same spun fiber, which highlights several technical issues that remain to be resolved dur­ing the manufacturing process. Much variability is due to the introduction of hu­man errors during the processing steps and can be circumvented by automation of the process. Secondly, the solvent choice for the majority of the laboratories has been a challenge. Hexafluoro-2-propanol (HFIP) and formic acid are two solvents that are excellent at dissolving the silk proteins to achieve the necessary spinning dope concentrations, but are volatile and toxic compounds that are less than ideal for manufacturing conditions. Additionally, maintaining protein solubility during the spinning process can be difficult. Too high of protein concentration can lead to precipitation or gelation, making the spinning process unmanageable. During fiber curation, HFIP has been reported to evaporate, leaving voids within the interior of the material that impact the mechanical properties of the fibers (Fig. 1.6).40 One laboratory has produced recombinant silk protein constructs that have been demon­strated to form fibers from an aqueous solution, potentially offering an advantage to fiber production without the use of HFIP and formic acid.65