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

Abstract

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...

Insights into the production and characterization of electrospun fibers from regenerated silk fibroin

“…The estimated values (see above) of ultimate true stress (o-u ; MPa) and Young modulus (£; GPa) obtained after ethanol immersion of both as-spun threads (SI and S2), but especially those of S2 (266 MPa and 30 GPa for Sl-E and 404 MPa and 32 GPa for S2-E), compare well not only with the values reported in the literature for regenerated silk fibroin fibers [53,[76][77][78], but also with those of natural B. mori silk [47].…”

“…Ultimate stress (a u ) values for as-spun threads (SI and S2) range from 196 to 360 MPa, depending on the particular batch tested ( Table 2). These values are comparable with the best bio-inspired fibers [53,[76][77][78] and remarkably close to those found in degummed natural silk fibers (300-600 MPa) [47], Similar outstanding results were obtained here for the Young moduli (£, GPa) of as-spun fibers (14-20 GPa; Table 2), which resemble those of natural silk fibroin (9-17 GPa) [47] However, it should be noted that the actual densities of the threads made from aligned fibers in our study can be significantly lower than that of natural silk, due to the lower crystallinity of the electrospun fibers and the voids between them. Therefore, both the ultimate stress (a u ) and the initial elastic moduli (£, GPa) -calculated from the assumption made aboveshould be taken with caution.…”

Enhancement of osseointegration of polyethylene terephthalate artificial ligament by coating of silk fibroin and depositing of hydroxyapatite

“…[17][18][19] However, it still has problematic characteristics for further clinical applications and use in industrial processes, such as low stiffness and high production cost. 20,21 For this reason, several attempts have been made to improve the properties of silk biomaterials, and this field has become a rapidly growing area of biomaterial research. [22][23][24] Research has demonstrated that the main inorganic component of human bone tissues, HAP, enhances the compressive strength of silk biomaterials and also induces new bone formation.…”

Development of New Smart Materials and Spinning Systems Inspired by Natural Silks and Their Applications

“…This drawing process orients the silk proteins along the longitudinal axis of the fibers and improves the fibers' mechanical properties. The post-spin drawing process used by silkworms and spiders has been replicated in conventional processes to improve the mechanical properties of the artificial showed mechanical properties comparable to those of the natural silkworm silk fibers ( Figure 3C) (Zhou et al, 2009;Yan et al, 2010). The same spinning process was applied by Chen's group to a spinning dope composed of a silk fibroin-CNT composite to prepare strong artificial fibers with breaking energies comparable to that of the spider dragline silk fibers (Fang et al, 2015).…”