Silk Fibers Extruded Artificially from Aqueous Solutions of Regenerated Bombyx mori Silk Fibroin are Tougher than their Natural Counterparts

2009 WILEY-VCH Verlag Gm The dragline silk of certain spiders has excellent tensile properties that are maintained over a remarkable temperature range. Although the commercial Mulberry Silkworm (Bombyx mori) silk is considerably weaker and less tough than the best spider dragline silks, fibers with comprehensive mechanical properties approaching that of spiders can be obtained by force-reeling directly from silkworms. Remarkably, both spider and silkworm silks are spun naturally from aqueous protein solutions at very low hydraulic pressures and at ambient temperature, not requiring a noxious coagulation bath. These considerations have lead to the search for methods to extrude strong and tough fibers from regenerated silk protein solutions. The processes developed so far depend on extruding silk fibroin dissolved in formic acid, N-methyl morpholine N-oxide (NMMO), 1,1,1,3,3,3-hexafluoro-2propanol (HFIP), trifluoro-acetic acid (TFA), hexafluoroacetone (HFA) or 1-ethyl-3-methylimidazolium chloride, usually into a methanol bath. However, most of these silk solvents either severely degrade the silk fibroin or are too expensive or toxic for use in industrial processes. Moreover, in all cases described, fibers spun from silk fibroin generated fibers weaker than their natural counterparts, with the exception of Ha et al.’s use of fibroin dissolved in TFA. However, the cold methanol coagulation bath used in these examples produced fibers that were much larger than the natural ones, lacked smooth surfaces, and were so stiff in their as-extruded form that their tensile properties could only be brought above those of natural fibers by manual neck drawing. This, and the cost of TFA, render Ha et al.’s process unsuitable for industrial scale up. In this report, we describe and evaluate a novel and environmentfriendly integral extrusion and drawing process that overcomes the limitations of earlier processes. This process produces stronger, tougher, andmore extensible fibers, compared to natural undegummed B. mori silkworm cocoon silk (raw silk). The process is novel in two respects: first, it uses highly concentrated aqueous solutions of regenerated silk fibroin (RSF); and second, it employs a hot ammonium sulphate solution for the coagulation bath. The use of a high-concentration aqueous RSF solution as spinning dope is important to ensure the high packing fraction of the protein chains, required for good mechanical performance. For the coagulation bath, many other organic and inorganic compounds, including methanol, ethanol, isopropanol, n-butanol, glycol, glycerin, and various sodium, potassium, zinc, magnesium, and ammonium salts (chloride, sulphate, nitrate, phosphate and acetate), have been tried. We chose ammonium sulphate after careful consideration of the mechanism and rate of the conformation transition and solidification of silk fibroin during the extrusion and drawing process. The rate at which this occurs is crucial to the behavior of the silk during drawing, and consequently to the mechanical properties of the RSF fibers. If the rate of transition is too slow, either the cross-section of the fibers is irregular, or the fiber cannot be formed at all. On the other hand, if the coagulation rate is too fast, as in previous work using methanol, the resulting fiber is very brittle. We therefore sought a coagulation bath that could provide a moderate coagulation rate. At such a rate, the conformation transition of silk fibroin can take place while allowing the protein chains sufficient freedom of movement to permit further alignment during extrusion and post drawing. We constructed a wet-spinning apparatus that met the design requirements of an industrial process to extrude the RSF fibers artificially (Fig. 1a). A RSF solution was prepared by dissolving degummed silk in 9.3mol L 1 LiBr and concentrating it by reverse dialysis against polyethylene glycol (PEG), to give a range of protein concentrations from 13 to 19% (w/w) in 2% steps. These solutions were extruded into an aqueous 30% (w/v) ammonium sulphate coagulant solution at 60 8C. We found that 15% RSF solution produced fibers with the smoothest surface and most uniform diameter, so this concentration was selected for further investigation. For this, as-extruded fibers (RSF-1 ) were wound out of the coagulation bath using a constant-speed 50 rpm plastic roller across an air gap of 40 cm. The weakly gelled fibers were then continuously wound by additional rollers of same diameter revolving at 100 and 200 rpm, to produce fibers (RSF-2 and RSF-4 ) with draw-down ratios of 2 and 4, respectively. The RSF-4 fiber was highly lustrous (Fig. 1b), and had uniform diameter and circular cross-section (Fig. 1c). The fiber was fairly strong, and more extensible and tougher than raw silk (Table 1). The strength of these fibers could be increased by