The dragline silk of certain spiders has excellent tensile properties [1] that are maintained over a remarkable temperature range.[2] 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.[3] 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.[4] 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, [5] N-methyl morpholine N-oxide (NMMO), [6] 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), [7] trifluoro-acetic acid (TFA), [5a] hexafluoroacetone (HFA) [8] or 1-ethyl-3-methylimidazolium chloride, [9] 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, [10] with the exception of Ha et al.'s use of fibroin dissolved in TFA.[5a] 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 [5a] 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, and more 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...

Regenerated silk fibroin (RSF) fibers were obtained by extruding a concentrated aqueous silk fibroin solution into an ammonium sulfate coagulation bath. A custom-made simplified industrial-type wet-spinning device with continuous mechanical postdraw was used. The effect of dope concentration, coagulation bath, extrusion rate, and postdraw treatment on the morphology of RSF fiber was examined. The results showed that although RSF fiber could be formed with dope concentration between 13 and 19% (w/w), the ones spun from 15% RSF solution showed the most regular morphology being dense and homogeneous in cross-section with a smooth surface and a uniform cylindrical shape. Though it had little effect on morphology, postdraw treatment especially under steam, significantly improved the mechanical properties of the RSF fibers.