Straining Flow Spinning of Artificial Silk Fibers: A Review

Abstract

This work summarizes the main principles and some of the most significant results of straining flow spinning (SFS), a technology developed originally by the authors of this work. The principles on which the technology is based, inspired by the natural spinning system of silkworms and spiders, are presented, as well as some of the main achievements of the technique. Among these achievements, spinning under environmentally friendly conditions, obtaining high-performance fibers, and imparting the fibers with emerging properties such as supercontraction are discussed. Consequently, SFS appears as an efficient process that may represent one of the first realizations of a biomimetic technology with a significant impact at the production level.

Keywords: fibroin; regenerated fibers; silk; spidroin.

Figure 1

Schematic of a straining flow spinning process with its main elements. The inset shows a detail of the capillary–nozzle system in which the flow of the dope (Q_d) and of the focusing fluid (Q_f) are indicated. V_R1: Velocity of the take-up roller; V_R2: Velocity of the post-spinning roller.

Figure 2

Schematic of the basic processes that result from the interaction between the dope jet and the focusing fluid. (a) Diffusion processes include the exchange of ions, including protons and the removal of water molecules from the dope to the focusing fluid. (b) The hydrodynamic interaction of both fluids results in the deformation of the dope jet.

Figure 3

Morphology and fractographic analysis of regenerated silk fibers spun from dopes with different compositions. (a) Morphology of the fibers observed using optical microscopy. (b) Scanning electron microscopy (SEM) migrographs of the fracture surfaces of tensile tested samples. The percentage indicates the fibroin concentration in the dope and Ca indicates that the dope contains CaCl₂ at 1 M concentration. The 4% sample shows the presence of voids in the fracture surface. The 8% sample shows the considerable necking undergone by the fiber prior to fracture. The 16% sample shows an essentially flat fracture surface. The scale bar in the SEM micrographs corresponds to 1 μm.

Figure 4

Elementary analysis of fibers produced by straining flow spinning. (a) Scanning electron micoscopy micrograph of regenerated silk fibers spun from the Ca-16% dope and its elementary energy dispersive X-ray spectroscopy (EDS) mapping for (b) carbon, (c) oxygen, (d) nitrogen, and (e) calcium. In spite of the high concentration of Ca²⁺ ions in the dope, no calcium remains in the fibers upon spinning to the resolution limit of the technique.

Figure 5

Morphology and Fourier-transform infrared spectroscopy (FTIR) spectra of regenerated silk fibers spun using different focusing fluid chemistries. (a) Morphology of the fibers observed using optical microscopy. (b) FTIR of the silk fibers in (a) shows amide I peaks between 1580 and 1720 cm⁻¹. The FTIR spectrum of a degummed natural silkworm silk fiber (black line) is shown for comparison.

Figure 6

Chart of the spinning processes as determined by hydrodynamic parameters. The compositions of the dope and of the focusing fluid are kept fixed for all spinning processes in the chart. Three regions are defined and referred to as non-spinnable, spinnable, and low V_R1. In the non-spinnable region, the fiber either does not form or breaks upon collection on the take-up roller. In the spinnable region, the fiber can be continuously spun and collected on the take-up roller. Finally, in the low V_R1 region the fiber is formed, but accumulates in the coagulating bath. The morphologies of three fibers produced under different hydrodynamic conditions are shown, and the set of hydrodynamic conditions for spinning each fiber is indicated by a yellow circle in the chart. Q_d: Flow rate of the dope; Q_f: Flow rate of the focusing fluid; V_R1: Speed of the take-up roller.

Figure 7

True stress–true strain curves of regenerated silk fibers produced by straining flow spinning and tested in air. Coagulating bath and focusing fluid for Et-A and Et-B samples: mixture of 80% ethanol and 20% acetic acid solution. Coagulating bath and focusing fluid for polyethyleneglycol (PEG) sample: 30% PEG in water. (a) As-spun fibers. The inset shows the true stress–true strain curves at low values of strain. (b) Wet-stretched fibers after drying. The spinning conditions are indicated in the text and the work to fracture (W_f) of each fiber is shown here.

Figure 8

Recovery test of an Et-A regenerated silk fiber. Coagulating bath and focusing fluid for Et-A: mixture of 80% ethanol and 20% acetic acid solution. The fiber is stretched in air in a first cycle up to a strain of 0.1 (black curve), immersed in water and allowed to supercontract. Subsequently, the fiber is stretched up to a strain of 0.18 (red curve) and allowed to supercontract again. The contraction steps in water between each cycle of stretching in air are indicated by the light blue arrows on the true strain axis. Finally, the fiber is tensile tested up to breaking (blue curve). The concurrence of all the curves proves the supercontraction ability of this fiber.