Wet Spinning of Silk Fibroin-Based CoreâSheath Fibers | ACS

Article pubs.acs.org/journal/abseba

Cite This: ACS Biomater. Sci. Eng. 2019, 5, 3119−3130

Wet Spinning of Silk Fibroin-Based Core−Sheath Fibers Pui Fai Ng,† Ka I Lee,† Shengfei Meng,‡ Jidong Zhang,‡ Yuhong Wang,§ and Bin Fei*,† †

Institute of Textiles and Clothing, Hong Kong Polytechnic University, 11 Yuk Choi Road, Kowloon, Hong Kong, China State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, No. 5625, Ren Min Street, Changchun 130022, China § Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, 11 Yuk Choi Road, Kowloon, Hong Kong, China ‡

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ABSTRACT: In order to improve the water absorbency of natural silk and extend its applications in wider areas, silk fibroin (SF)-based fibers were prepared by coaxial wet spinning. Using a custom-made wet spinning device with coaxial spinneret, continuous core−sheath fibers were finally obtained by adjusting the core dope into iota-carrageenan/polyacrylamide hot solution and sheath dope into SF/polyurethane solution. These core−sheath fibers were characterized with respect to morphology, SF secondary structure, mechanical property, and water absorbency. Fibers fabricated from 17 wt % SF/ polyurethane solution presented the most regular morphology with homogeneous and circular cross-section. Double-layered hollow structure was observed in these fibers. β-Sheet conformation was mainly adopted by the SF in fibers as indicated in XRD analysis and FTIR spectra. The fibers demonstrated higher absorbency than the raw silk and fine incorporation of long-lasting glowing pigment, indicating potential applications in water or thermal management textile and phototherapy. KEYWORDS: coaxial wet spinning, core−sheath fiber, silk fibroin, secondary structure, water absorbance and to fabricate regenerated silk fibers with excellent mechanical properties. However, only a few researchers have put silk fibroin into core−sheath fiber via electrospinning,8,14−17 microfluidic spinning,18−20 or dry spinning,21 which are restricted to lab-scale production. To our knowledge, no wet spinning has been reported to set fibroin solution as sheath dope. Furthermore, the water absorbency of all reported silk fibers remains at a low level, which is expected to be improved for biomedical applications.22 Fibers with core−sheath structure show several advantages over single-component fibers as each material can render a unique function to the composite fibers. This technology has been investigated for the production of hollow fibers23 and drug-delivery fibers,14 among others. Several studies have reported the improvement in tensile strength of core−sheath

1. INTRODUCTION Silk, a natural protein fiber spun by the silkworm, has a core− sheath structure consisting of two fibroin cores and a sericin sheath.1 The predominant silk fibroin is constituted mainly by four types of amino acids (i.e., alanine, glycine, serine, and tyrosine) which drive the formation of antiparallel β-sheet microcrystallite.2 Because of its hierarchical structure, silk fibroin presents extraordinary mechanical properties.3 Additionally, its desirable biological properties, such as good oxygen/vapor permeability, biocompatibility, and minimal inflammatory response, have also attracted great attention.4,5 Therefore, regeneration of silk fibroin has been widely investigated for its potential applications as wound-healing materials6,7 and tissue-engineering scaffolds.8,9 The fibroin fibers are dissolved and further processed into various forms including nanoparticles, films, hydrogels, sponges, and fibers.10 Artificial spinning methods like electro-spinning,11 gel spinning,12 and wet spinning13 have been developed to mimic the spinning process and the structure of native silk © 2019 American Chemical Society

Received: February 24, 2019 Accepted: May 18, 2019 Published: May 22, 2019 3119

DOI: 10.1021/acsbiomaterials.9b00275 ACS Biomater. Sci. Eng. 2019, 5, 3119−3130

Article

ACS Biomaterials Science & Engineering

Figure 1. Schematic diagram of the custom-made wet spinning setup: (A) syringe pumps, (B) syringes with loaded dopes, (C) coaxial spinneret: (i) core dope, (ii) sheath dope, (iii) inner tube, (iv) outer tube, (D) coagulation bath, (E) guide rollers, and (F) take-up roller.

fibers via the combination of composite polymer with different mechanical properties.24 Core−sheath structure improves the fiber mechanical properties and allows the incorporation of nonspinnable components. By the use of coaxial electrospinning, nonspinnable egg-shell protein has been incorporated with polycaprolactone to form a tissue-engineering scaffold.25 Various functionalities, such as biocompatibility,26 conductivity,15 and self-healing property,27 have been achieved using different polymer sheaths. Several solvents like organic solvent (e.g., 1,1,1,3,3,3hexafluoro-2-propanol),28 concentrated salt solution (e.g., LiBr and CaCl2/EtOH/H2O),29 and ionic liquid (e.g., 1butyl-2,3-dimethylimidazolium)30 have been employed to prepare the silk fibroin solutions for fiber spinning. However, except for Zuo’s works,31−33 most of the resultant fibers usually had poor mechanical strength that hinders their practical applications. Zuo et al. dissolved fibroin fibers using CaCl2/ formic acid (CaCl2/FA), producing stronger fibers because of the preservation of nanofibril during dissolution. Considering its mildness and convenient usage, CaCl2/ formic acid is selected to dissolve silk fibroin for wet spinning. To produce core−sheath regenerated silk fibroin (SF) fibers with high water absorbency and strength, iota-carrageenan/ polyacrylamide hydrogel was encapsulated into SF/polyurethane fiber by coaxial wet spinning. Iota-carrageenan (ιC) is a natural sulfated polysaccharide extracted from edible red seaweeds, whose aqueous solution with K+ and Ca2+ is flowable above 65 °C and forms a hydrogel at lower temperature. When it was mixed with polyacrylamide in solution, a highly stretchable hydrogel was obtained when it was cooled.34 The rheological behaviors of the spinning dopes were measured to assess their spinnability. The structural change of SF during spinning was characterized by X-ray diffraction and Fouriertransform infrared spectroscopy. The fiber morphology, mechanical properties, and water absorption properties were evaluated. Phosphorescent pigments were also incorporated in the core dope for luminous fiber fabrication. It is predicted that the regenerated SF-based core−sheath fibers will have great potential in various applications, such as water management textiles and wearable phototherapy devices.

for 90 min (the possible depolymerization was not clarified in this work). HydroMed D6, a water absorbent polyurethane (PU), was provided by AdvanSource Biomaterials Co., USA. Calcium chloride (CaCl2, ≥ 96%), formic acid (FA, ≥ 98%), iota-carrageenan (ιC, with 4.48 wt % K+ and 2.68 wt % Ca2+ tested by ICP-MS),34 polyacrylamide (PAAm, Mw = 5 000 000−6 000 000), and ammonium sulfate ((NH4)2SO4, ≥ 99%) were all purchased from SigmaAldrich Co., Ltd., USA. Blue-emitting persistent phosphor (4SrO· 7Al2O3:Eu, λem = 451 nm) with long afterglow duration of more than 6 h, respectively, were obtained from Yaodesheng Scientific Co., China, for core filling. All chemicals were used as received without further purification. 2.2. Preparation of Spinning Dopes. IC aqueous solution of 2 wt % was prepared by dissolving ιC in deionized water at 80 °C under gentle stirring for at least 1 h. PAAm (1/2 of iC mass) was added to obtain a solution with suitable spinning viscosity. Complete dissolution was confirmed by visual observation as a homogeneous clear solution. The phosphorescent pigment (5 wt % based on iC +PAAm mass) was dispersed into the core dope by sonication at 80 °C for the fabrication of after-glowing fibers. The SF protein and PU (mass ratio 9:1) were dissolved in CaCl2 /FA with a CaCl 2 concentration of 4 wt % overnight at room temperature. SF/PU blend dopes were obtained with a concentration range of 13−19 wt %. The inclusion of PU can offer high stretchability and water absorbency to the sheath layer. The solutions were filtered and degassed in vacuum prior to spinning. 2.3. Coaxial Wet Spinning of SF-Based Core−Sheath Fibers. SF-based core−sheath fibers were spun at well-controlled temperatures using a custom-made wet spinning device as shown in Figure 1. For coaxial wet spinning, ιC/PAAm aqueous solution was transferred to an injection syringe connected to the inner core of spinneret (18gauge; outer diameter (OD) = 1.24 mm, inner diameter (ID) = 0.84 mm), while SF/PU dopes were loaded into the outer spinneret channel (14-gauge; OD = 2.11 mm, ID = 1.60 mm). To ensure a smooth spinning process, the core dope temperature was carefully controlled at 80 °C using a customized circulating device. Two separate syringe pumps (LSP01-1A, Longer Precision Pump Co., Ltd., China) were used to extrude the core and sheath dopes simultaneously. The core and sheath components were extruded at the optimized flow rate of 0.2 mL/min (3.61 cm/min) and 0.4 mL/ min (4.98 cm/min), respectively. Extrudant was then subjected to coagulation treatment, with an effective coagulation bath length of 50 cm, at room temperature using (NH4)2SO4 solution of 0, 2.5, or 5 wt %. The as-spun fibers were collected with take-up speed of 80 cm/min and poststretched at a draw-ratio of 3.0 (without special notification). The draw-ratio was defined as the ratio of the fiber length after drawing to the original length. All resulting fibers were left in the coagulation bath (1.0 L) for various times for further solidification and crystallization. Finally, the fibers were repeatedly washed in deionized water at room temperature for salt removal (1 h × 3 cycles), followed by air-drying, affording ιC/PAAm-SF/PU core− sheath fibers.

2. EXPERIMENTAL SECTION 2.1. Materials. Silk yarn of 20/22D, composed of 14 natural silk filaments, was supplied by Huasheng Industrial Co., Ltd., China. Its sericin content of 28% was measured by the infrared (IR) degumming method.35 To obtain silk fibroin (SF), all silk yarns were degummed by IR heating in deionized water, with a liquor ratio of 40:1, at 120 °C 3120


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Figure 2. Rheological behaviors of spinning dopes showing the dependences of the shear viscosity on the shear rate for (A) ιC/PAAm core dope at 80 °C, and (B) SF and SF/PU sheath dopes at room temperature. Total 32 scans were recorded over the spectral range 4000−650 cm−1 at a resolution of 4 cm−1. The intensity ratio of the peaks at 1262 and 1230 cm−1 was used to investigate the crystalline fraction. Crystallinity index was calculated using the eq 2,36−38

2.4. Characterization. Rheological properties of core and sheath dopes were evaluated using a rheometer (MCR 702 TwinDrive), equipped with 50 mm cone-and-plate geometry with 1° cone angle over a shear rate of 0.10∼300 s−1. During measurement, the polymer solutions were sealed with silicone oil under the cone plate, to prevent evaporation of solvent and change in concentration. The surface morphology, cross-section, and the phase separation of the resultant fibers were observed by scanning electron microscope (SEM, Tescan VEGA3) at 20 kV after gold sputtering. To obtain the cross-section, the fiber was fractured along the direction perpendicular to the fiber axis in liquid nitrogen. The cross-sectional areas of the fibers were calculated using the software provided with the equipment. Average area of each sample was obtained by at least five measurements. To further investigate the phase distribution, an additional etching process was performed to remove PU by immersing the fiber in warm ethanol (70 °C) for 10 min. Optical images of the as-prepared luminous fibers were obtained under and after stimulation by CIE Standard Illuminant D65. Illuminant D65 represents the average midday light with a correlated color temperature of approximately 6500 K over the spectral range from 300 to 830 nm. The fiber phosphorescence was measured with a time-resolved fluorescence spectrometer (Edinburgh FLS920 with 450W Xe lamp). The fiber cross-section was also characterized under SEM (Tescan VEGA3 with EDS) to identify the pigment distribution. X-ray diffraction analysis was carried out by a high-resolution X-ray (XRD, Rigaku SmartLab) with Ni-filtered Cu Kα radiation (λ = 1.54 Å) using 45 kV and 200 mA in the range of 5−40° at 5°/min. XRD was also performed using a six-circle surface X-ray diffractometer (XRD, Huber Type 5021) at the Shanghai Synchrotron Radiation Facility (SSRF). Beamline (BL14B1) at a wavelength of 0.689 Å was obtained from a Si(111) monochromator with a flux of 2 × 1011 photons per second. The X-ray beam with energy of 10 keV was focused to 210 μm × 300 μm at the horizontally aligned fiber samples. Data were recorded on a NaI Scintillation detector with a detector-tosample distance of 263.736 mm. Background measurement was performed under the same condition. The 2D XRD patterns were analyzed using a software program FIT2D (ver. 18 beta). The scattering vector, q, for fiber analysis is denoted here as q = 4π

sinθ λ

crystallinity index (%) =

I1262cm−1 × 100% I1230cm−1

(2)

where I1262 cm−1 and I1230 cm−1 are the intensity of absorbance at 1262 and 1230 cm−1, respectively. Tensile strength was measured using a universal testing machine (Instron 4411) with a 10 N load cell and a gauge length of 20 mm, at a constant extension speed of 10 mm/min. Tensile stress was calculated by Stress (MPa) = F/A, where F is the applied load, and A represents the cross-section area measured under SEM. Usually six measurements were performed for each sample. Prior to the test, all samples were conditioned at 25 ± 2 °C and relative humidity of 65 ± 5% for 24 h. To measure water absorbency (Q), the dried samples were immersed in DI water for various periods sequentially to achieve equilibrium. The Q was calculated by Q (g/g) =

Wt − Wd Wd

(3)

where Wd is the dry sample weight (mg) before immersion, and Wt is the sample weight (mg) at immersion time t.

3. RESULTS AND DISCUSSION 3.1. Fiber Spinnability and Morphology. 3.1.1. Effect of Sheath Dope Concentration. The spinning behavior of dope solution is highly affected by its viscoelasticity, which depends on the interaction between polymer chains with physical entanglements or secondary bonds. Any change in the degree of interaction results in a viscosity change. Therefore, the effect of shear flow at the spinneret orifice during fiber spinning should be considered. Figure 2 showed the shear viscosity of the core ιC/PAAm and sheath SF-based solutions as a function of shear rate. The core dope exhibited nonNewtonian shear-thinning behavior at 80 °C, which was consistent with the previous observation:39 The polysaccharide-PAAm chain entanglement promoted the formation of network structure, contributing to the high initial viscosity at low shear rate (Figure 2A). With increasing shear rates, the mechanical force in shearing disrupted the aggregates, hence reducing the viscosity. In comparison of 3% PAAm and 3% ιC/ PAAm, the PAAm contributes evidently higher viscosity than

(1)

where λ is the X-ray wavelength, and θ is the Bragg angle of the desired diffraction peak. Crystallite orientation degree of the regenerated SF fibers was further determined from the XRD images as a function of radius integrated azimuthally over a sector along the breadth of the desired diffraction. To further investigate crystalline structure and conformational change of the wet-spun fibers, they were analyzed by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR, PerkinElmer Spectrum 100) equipped with a beam condenser. 3121


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Figure 3. Surface (i) and cross-section (ii) morphology of as-spun ιC/PAAm-SF/PU core−sheath fibers from different sheath dope concentrations: (A) 15%, (B) 17%, (C) 19% (coagulated in 2.5% (NH4)2SO4 solution for 8 h).

the ιC, because the PAAm used here has an ultrahigh molecular weight. Over the tested range, all regenerated SF-based solutions exhibited non-Newtonian fluid behavior at room temperature. This viscosity−shear rate profile consisted of a shear-thinning region at low rates and a plateau region at high shear rates (Figure 2B). In this work, the use of CaCl2/FA as solvent for SF demonstrated a dissolution behavior with nanofibril preservation.31,32 The entanglement of nanofibril began to dominate in the low shear rate region, leading to a high apparent viscosity. At higher polymer concentration, the amount of entanglements obviously increased, contributing to a higher viscosity. The occurrence of shear-thinning phenomenon was attributable to the flow-induced nanofibril orientation.40,41 This quick pseudoplastic behavior is similar to the earlier report on hydroxyapatite/FA solutions of fibroin.42 In contrast, the natural silkworm dopes displayed a constant viscosity at low shear rate and a shear-thinning only at high shear rate.43,44 This shear-thinning behavior can aid fiber spinning as the dope fluidity increases because of the induced shear in the fiber extrusion process. Further increase in shear rate, especially at a concentration below 13 wt %, resulted in a plateau viscosity region, of which the fibroin chains and nanofibrils have been oriented in the maximum degree. In contrast, the use of highly concentrated or mixed salt aqueous solutions, such as LiBr and CaCl2-EtOH-H2O,29 for silk fibroin dissolution results in completely different rheological properties.37,38 Those SF solutions exhibited Newtonian behavior, displaying independent shear viscosity against shear rate. Instead of disassembling into nanofibril, silk fibroin is dissolved into individual molecules in salt aqueous solution, which shows less chance of stiff aggregation during shearing.45

For biomimetic spinning of artificial silks, Holland et al. have suggested that the rheology of the regenerated and native dopes should be matched before actual operation.43 In this work, the effect of dope viscosity on fiber formation was evaluated at a shear rate of 10 s−1,38 as shown in Figure S1. The shear viscosity increased with increasing polymer concentration. According to our trial, when the sheath dope viscosity exceeded 1.0 Pa·s, continuous spinning can be obtained. A more viscous core component was utilized to maintain a stable core−sheath spinning stream during the elongation flow in takeoff process between the spinneret and the first driven roller. By matching the viscosities of the core and sheath dopes, the variation of interfacial shear stress can be minimized, which was critical to avoid flow buckling and the subsequent fiber deformation.46 For this reason, the suitable sheath dope viscosity was identified in the range of 1.0∼6.7 Pa· s. To meet this requirement, 17 wt % SF dope solution was used for the spinning, which resulted in a fiber with the expected core−sheath structure (Figure S2). However, this asspun fiber was still easy to break in postdrawing. When the water absorbent PU was added to replace partial SF (10%), the spinning fluid became more stretchable in taking off process, producing fibers ready for higher postdrawing ratio. Therefore, the rheological property of SF/PU dope solutions in the same concentration range was also measured, giving the similar trend with increasing shear rate (Figure 2B). At the operational spinning shear rate 10 s−1, the SF/PU solution presented slightly higher viscosity than the SF solution (Figure S1) because of the contribution of additional PU. All the following works are derived from the SF/PU sheath dopes. The effect of the sheath dope concentration on the spinning viability and resulting fiber morphology was systematically investigated. At the concentration below 15 wt %, no fiber was obtained, but rather, the morphology was entirely in the form 3122


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Figure 4. Cross-sectional morphology of ιC/PAAm-SF/PU core−sheath fibers solidified using (NH4)2SO4 with different concentrations for 8 h: (A) 0, (B) 2.5 wt %, (C) 5.0 wt %.

Figure 5. Cross-section morphology of ιC/PAAm-SF/PU core−sheath fibers (sheath dope concentration: 15 wt %) with different draw-ratios in air: (A) 0×, (B) 1.5×, (C) 2.5×, (D) 3.5×.

solidification in the water bath. In order to improve the fiber morphology, ammonium sulfate was added into the bath to accelerate coagulation of sheath dope. The fibers became more regular in cross-section with the aid of 2.5 wt % (NH4)2SO4 to improve the rate of solvent extraction. At a higher (NH4)2SO4 concentration (5.0 wt %), faster solidification was observed during the coagulation and fiber collection while the fiber surface morphology did not change obviously. The internal stress of the as-extruded fiber had no way to release, resulting in an expansion of polymer network and hence a circular fiber structure.47 During the coagulation and fiber collection, fibroin conformation transition and chain orientation occurred under the elongation flow. During a longer time coagulant immersion after the fiber spinning (usually 8 h), the sheath dope was allowed to fully solidified through the complete solvent extraction, during which the salt (NH4)2SO4 may also enter and stay in the fiber. The salt crystal was noticed in the SEM image of fiber cross-section (Figure S3A), where a clear boundary (marked by dash line) was also observed between core layer and sheath layer. To obtain clean fibers, the fully coagulated fiber was further rinsed in DI H2O for 1 h by 3 times. The resultant fiber was confirmed free from salt crystal in sheath layer by the SEM observation of cross-section (Figure S3B). Without special notation, all the following coagulation and immersion processes were carried out in 2.5 wt % (NH4)2SO4 solution for 8 h. 3.1.3. Effect of Postdrawing. Figure 5 illustrated the effect of postdrawing on the fiber morphology in a total draw-ratio of 3.5× under constant condition (i.e., sheath dope concentration: 15 wt %; coagulant: 2.5 wt % (NH4)2SO4; coagulation time: 8 h). The as-spun fiber showed a rough surface and irregular cross-section. At increasing draw-ratio, the fiber surface became smoother, showing a more uniform surface morphology. This improvement was attributed to uniform molecular chain alignment during the postdrawing process.

of dispersed floccules. Fiber formation occurred at the increased sheath dope concentrations of 15−19 wt %. Fibers obtained from these concentrations were observed under SEM as shown in Figure 3. Weak fibers with irregular cross-section and longitudinal grooves were obtained at the concentration of 15 wt %. This serious deformation was due to the low packing density of fibroin in coagulation and subsequent shrinkage during drying. At the dope concentration of 17 wt %, the resultant fiber was improved obviously in morphology: the asspun fibers showed smooth surface profile and homogeneous circular cross-section. A higher packing fraction of the fibroin may explain the improvement on fiber morphology. However, when the sheath concentration was further increased to 19 wt %, the obtained fibers were irregular in surface and appeared highly folded in the cross-sectional view. The fibers were too brittle for postdrawing, probably because of rapid coagulation of the dope, which may lead to a premature β-sheet formation and hence a reduction of fiber stretchability.13 Without special notation, all the following fibers are spun from the SF/PU sheath dope of optimal concentration 17 wt %. 3.1.2. Effect of Coagulation Bath. In wet spinning, the coagulation process allows solvent extraction and thus fiber solidification. It is crucial to fiber drawing behavior and subsequent mechanical properties. A suitable coagulation bath may guarantee proper rate of solvent extraction and sheath solidification. If the sheath solidification is too slow, the crosssection of the fiber is always irregular; sometimes no fiber can be formed. On the contrary, if the sheath solidification is too fast, the resultant fiber is very brittle and difficult to be drawn for chain orientation and crystallization. The effect of coagulation bath on the fiber morphology was revealed in Figure 4. The SF/PU sheath dope was weakly gelled as it extruded into the water bath and its cross-section collapsed from round into ribbon-like shape. This fiber deformation could be attributed to the low rate of sheath 3123


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obtained in the postdrawn fibers (Figure 6D), indicating the PU extension and orientation together with SF matrix during postdrawing. 3.2. Secondary Structure and Crystallinity of SF in Core−Sheath Fibers. The secondary structure of fibroin is one of the main factors determining its physical properties. In order to gain a deeper insight into the postdrawing and coagulation process of SF-based fibers, the conformation change of fibroin was revealed using XRD and FTIR analyses. 3.2.1. XRD Analysis. The crystalline structure and crystallinity of the fibers were evaluated using XRD as shown in Figure 7A. All curves were normalized at 2θ = 19.2°. For better comparison, the spinning dope was plasticized by water before assessment, presenting an amorphous state with a broad peak at 28.3°. For the as-spun fibers without further coagulation treatment, multiple peaks corresponded to ammonium sulfate were found over the selected measurement region due to the presence of coagulant residue, and they were completely removed after wash by water. Diffraction peak at 19.2° appeared in all fiber samples, corresponding to a lattice spacing of 4.6 Å and the diffraction from crystal face (020) of fibroin β-sheet.48 During fiber spinning, the induced shear flow and elongation flow promoted the alignment of fibroin chains, thus enhancing the transition of amorphous random coil to βsheet structure along the crystal intersheet direction (Figure S4), characterized by the peak shift from 28.3° to 19.2°. Upon coagulant immersion, formic acid was eliminated from the asspun fibers, which facilitated the transformation of random coil to Silk II structure along the crystal interchain direction (Figure S4).49 The peptide bonds of the adjacent chains were further stabilized by H-bonding which plays a key role to the remarkable performance of native silk.50 With increasing immersion time, the diffraction peak moved to 2θ = 20.6° with a lattice spacing of 4.4 Å, corresponding to the diffraction from lattice plane (200) of fibroin β-sheet. Furthermore, new peak at 2θ = 23.4° was observed after the postdrawing treatment. It confirmed the ordering of protein chains along the fiber axis under stretching, corresponding to a lattice spacing of 3.8 Å and the diffraction from lattice plane (201) of fibroin β-sheet (Figure S4).49 To further investigate the crystalline structure of the regenerated SF fibers, their 2D XRD patterns were collected as shown in Figure 8A−C. No discrete reflection was observed in the diffraction pattern obtained for 0 h fiber. This diffuse amorphous halo indicated the unoriented random coil

However, the irregular cross-section of the as-spun fiber was hardly improved and even worsened, when the fiber became thinner after higher stretch. Postdrawing of as-spun fibers at 3.5× was possible in air without breakage for most fibers (∼70%). No fiber breakage was observed at 3.0× stretching. 3.1.4. Phase Separation of SF/PU Sheath Layer. Figure 6 shows the cross sections of the pure SF, SF/PU, and ethanol-

Figure 6. Cross-section morphology of (A) pure SF, (B) SF/PU, ethanol-etched SF/PU sheath layer (C) without and (D) with 3× postdrawing (sheath dope concentration: 17 wt %; coagulant: 2.5% (NH4)2SO4 for 8 h).

etched SF/PU sheath layers. The fracture surface of pure SF was sharp and smooth, which was associated with a brittle failure, exhibiting no sign of plastic deformation (Figure 6A). After the incorporation of PU into SF, the modified sheath layer had a fluctuant and coarse fracture surface with no large aggregates (Figure 6B), indicating a fine dispersion of PU throughout the SF matrix. When the PU content is further removed by ethanol-etching, an island-in-sea phase structure is confirmed by the resultant pores (Figure 6C). Consistent with most cases, the major SF component formed the continuous phase, while the minor PU component formed the dispersed phase of micron scale. Smaller voids less than 1 μm were

Figure 7. XRD patterns (A) and FTIR spectra (B) of 17% SF/PU spinning dope, as-spun (dotted lines) and postdrawn (solid lines) ιC/PAAm17% SF/PU core−sheath fibers with different coagulant immersion times (coagulant: 2.5% (NH4)2SO4). All curves were normalized at 19.2° (“020” face) or 3275 cm−1 (N−H stretching). 3124


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Figure 8. 2D XRD patterns of as-spun (i) and postdrawn (ii) ιC/PAAm-17% SF/PU core−sheath fibers with different coagulant immersion times (0−8 h) (coagulant: 2.5% (NH4)2SO4).

segments of SF protein. The induced formation of β-sheet structure during coagulant immersion can be characterized by two concentric diffraction rings of (020) and (201) lattice planes embedded in the amorphous matrix (Figure 8B,C). The presence of these crystalline reflections agreed with the aforementioned XRD results. As expected, the regenerated SF fibers showed a strong trace of molecular orientation after postdrawing: the molecular chain segments in the crystalline phase orientated along the fiber axis. Intense (020) and (200) reflections were observed along the equator, respectively attributed to the interchain and intersheet arrangements of the β-sheet structure.51 A layer line (201) reflection was detected at a higher order parallel to the equator. The lattice spacings of these reflections were identical to those of characteristic crystalline peaks at 2θ = 19.2°, 20.1°, and 23.4°, respectively, from the above XRD analysis. Regardless of the immersion time, the highly orientated diffraction pattern with characteristic arcs depicted a semicrystalline structure containing both crystalline-Bragg reflections and amorphous fractions. This result resembled those natural and regenerated silk fibers reported in literatures.51−53 With increased immersion time, the fiber sample exhibited a similar pattern with greater diffraction intensity due to a higher proportion of β-sheet conformation after the elimination of formic acid. The orientation degree of molecular chain segments in the regenerated SF fiber was further determined as a function of radius integrated azimuthally over a sector along the breath of diffraction patterns corresponded to (020) and/or (200) reflections. The integration of the amorphous halo was subtracted from the equatorial integration to yield the crystal orientation degree of 84%, 87%, and 89% for fiber samples 0, 1,

and 8 h, respectively. The structural conformation reached its optimum after 8 h coagulant immersion. This result gave further confirmation that coagulant immersion process can facilitate the preferential orientation of crystalline chain into a higher extent. Furthermore, analysis of the equatorial (020) reflection and the layer line (201) reflection gave the detail of unit cell in the crystal lattice. The average lattice spacing of the intense (020) reflection corresponded to the intersheet distance of β-sheet (or b/2), while that of (200) reflection might give the interchain distance along the H-bonding direction (or a/2) (Figure S4). However, the (200) reflection of SF was found very wide and diffuse. This result did not support a precise calculation of a/2 due to the low regularity of H-bonding chain arrangement in a direction. For this reason, the sharp (201) reflection was employed, of which its radial and axial projections gave the values of a/2 and c (distance along the chain axis). These values were calculated as a/2 = 4.13 Å and c = 9.42 Å according to Bragg’s law. From the above (020) and (201) reflections, the rectangular unit cell dimensions can then be indexed as a = 8.26 Å, b = 9.2 Å, and c = 9.42 Å (Figure S4). Here, because of the high draw-ratio of the regenerated SF fibers, the resultant c dimension was obviously greater than that reported for natural silk in literature.54 The exact chain arrangement in unit cell is still subject to adjustment by further theoretical simulation. 3.2.2. FTIR Analysis. FTIR spectra in Figure 7B also provided information about the structural changes of ιC/ PAAm-SF/PU core−sheath fibers after different coagulant immersion times. All spectra were normalized at 3275 cm−1 (N−H stretching) for better visualization. The presence of amide groups in SF protein can be confirmed by the 3125


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Figure 9. Schematic illustration of the conformational transition mechanism in SF during coaxial wet spinning of ιC/PAAm-SF/PU fiber.

characteristic bands at 1700−1600 cm−1, 1600−1500 cm−1, and 1300−1200 cm−1, which are mainly attributed to amide I (CO stretching), amide II (N−H in-plane bending), and amide III (C−N stretching and N−H bending) of peptide backbone, respectively. All these characteristic absorbance peaks have been used for studying the secondary structure of silk.55 The SF/PU spinning dope exhibited characteristic peak at 1656 cm−1, which was assigned to the CO stretching mode of formic acid and the amide I band of silk fibroin.31,56 This peak identified the amorphous state of fibroin in dope.31 In contrast, most ιC/PAAm-SF/PU fibers presented the absorption bands at 1621 and 1513 cm−1, as well as a shoulder peak at 1262 cm−1 indicating the typical silk II (β-sheet) conformation.57 A weak absorption peak at 1621 cm−1 was observed in the FTIR spectra of as-spun fiber with the immersion time of 0 h. N−H flexural vibration in 1397 cm−1 and SO42− stretching vibration in 1063 cm−1 were also presented in the spectrum due to the residue of ammonium sulfate, in agreement with the XRD analysis.58,59 These salt peaks completely disappeared after rinsing by water, indicating the successful removal of ammonium sulfate (evidenced by Figure S3B). When the as-spun fiber was stretched, the peak intensity of silk II conformation was increased, which was attributed to the formation of β-sheet conformation. New peaks were also observed, indicating the occurrence of conformational transition during the postdrawing process. According to the previous research, the elimination of formic acid from SF during fiber spinning was accompanied by the conformational transition of random coil to β-sheet crystal.60 As the immersion time increased, the peaks became narrower,

and the intensity of the silk II absorption bands gradually increased, being attributed to the β-sheet growth. All FTIR spectra of postdrawn fibers were similar in peak position. The difference of FTIR spectra between postdrawn core− sheath fibers with various coagulant immersion times was difficult to recognize. However, the relative crystallinity of each sample can be estimated by comparing the absorbance peaks at 1262 and 1230 cm−1.38 Crystallinity index was calculated to determine the effect of coagulant immersion time on the fiber crystallinity, as shown in Figure S5. This index increased to the maximum value at 8 h immersion. Its little drop after 8 h immersion is ignorable, since this index derived from weak IR peaks (Figure 7B) is not precise enough to confirm such a small change. 3.2.3. Conformational Transition Mechanism inside SFBased Core−Sheath Fiber. The XRD and FTIR analyses confirmed the conformational transition of SF from random coil structure to β-sheet conformation. Figure 9 illustrates the possible mechanism of conformational transition inside the ιC/PAAm-SF/PU fiber. During coaxial wet spinning, the polymer chains are slightly aligned along the spinning jet direction under the shear stress and extensional flow inside the tapered spinneret (Figure 9i). Two spinning dopes thereafter come together at the outlet of the spinneret to form a core− sheath interface. Simultaneously, the ιC/PAAm warm solution is cooled and forms soft gel that is still movable. After extruding the spinning dopes into the coagulation bath, a weakly coagulated sheath was formed through a mutualdiffusion of different solvents. Extraction of CaCl2/FA from the sheath solution starts at the core/sheath interface and the sheath/bath interface, facilitating the solidification of sheath 3126


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Figure 10. (A) Typical stress−strain curves of as-spun (dotted line) and 3.5× postdrawn (solid line) ιC/PAAm-SF/PU core−sheath fibers with different coagulant immersion times; (B) stress at break of postdrawnιC/PAAm-SF/PU core−sheath fibers, the numbers represent concentrations of sheath dope and coagulant while the error bars represent the standard deviation of mean (n = 6).

and to further align along the fiber axis, which gave rise to the further growth of crystallite and the enhancement in breaking stress of fiber.62 This observation was consistent with the results reported in previous research that postdrawing could improve the fiber mechanical properties.63 In comparison to the solid fibers spun from 17% SF/PU solution (Figure S6), these core−sheath fibers’ tensile properties are still relatively low because of the additional defects derived from hollow cavity and core−sheath interface. In addition, the increase of immersion time is effective in improving the tensile strength to a certain extent (Figure 10B). The maximum stress at break of 130.4 MPa and strain at break of 22.4% were achieved after 8 h of immersion, becoming less brittle and more stretchable after the immersion. Because this process allowed complete solidification of fibroin and enhanced the integrity of the adjacent polymer chains. The induced growth of β-sheet structure and the subsequent realignment of crystallites (evidenced by Figure 8) resulted in a large improvement in tensile strength. Beyond the optimal immersion time, the tensile strength showed a little drop, which is in the normal error range and thus ignorable. The breaking stress of fiber samples showed a similar trend to that of the above-noted crystallinity index. The crystallinity of fiber was recognized as a main factor affecting the tensile performance. Fibers extruded at the sheath dope concentration of 15% showed similar tensile behavior, attaining the maximum result (126 MPa) after coagulant immersion treatment of 8 h (Figure 10B). The improvement on mechanical properties of these fibers by postdrawing was also revealed in Table 1. They could be stretched mostly up to 3.5× without breaking. Their stress− strain behavior changed from the initial brittle manner to a tougher behavior with the increasing draw-ratio. Porter et al.62

where SF remains amorphous mostly (Figure 7B 0 and 0 h w/ o rinsing) (Figure 9ii). The diffusion of CaCl2/FA into the core gel makes the ιC/PAAm gel most densely cross-linked (ionically) and strongest at the interface. When the immersion treatment period is extended to 1 h or longer time, the β-sheet conformation of SF increases obviously (Figure 7B 1 and 8 h). In this process, phase separation occurs in the SF/PU sheath layer due to the SF crystallization, leading to an island-in-sea phase structure (Figure 6C) (Figure 9iii). Here the elongation flow in takeoff process between the spinneret and the first driven roller mainly contributed to the fluid thinning instead of fibroin crystallization, since the resultant SF crystallinity is still very low in this stage. When the as-spun fiber is stretched, conformation transition from random coil to β-sheet also increases greatly (Figure 7B 0 and 0 h w/o rinsing).61 This postdrawing process also causes obvious orientation of both SF chains and SF crystals, even together with the simultaneously formed PU island phase (Figure 9iv). In comparison, the postdrawing contributes more than the prolonged coagulant immersion treatment to the SF crystallinity, as evidenced by the Figure S5. After washing out the coagulant ((NH4)2SO4) diffused into sheath, a clean core−sheath fiber is obtained. During drying, the already solidified sheath layer shrinks very little while the gel core shrinks a lot due to its very low polymer content of only 3 wt %. Because the gel is denser and stronger at the site closer to the interface, it shrinks to the interface and adheres to the sheath layer tightly, leaving a hollow channel in the core. Therefore, a hollow core−sheath fiber is obtained after drying (Figures 3−5). 3.3. Mechanical Property of SF-Based Core−Sheath Fibers. As fiber mechanical properties are the important parameters for textile applications, the tensile performance of ιC/PAAm-SF/PU core−sheath fibers have been measured after various immersion times. The typical tensile curves and average stress at break of as-spun and postdrawn specimens are shown in Figure 10. The as-spun fibers exhibited brittle behavior with the lowest apparent breaking stress of 39.4 MPa and breaking strain of 3.1%, as it had little chain orientation (Figure 10A). With the increment of immersion time, only little improvement was noticed in the tensile stress due to the β-sheet crystallite formation during early solidification. In contrast to the poor mechanical property of the as-spun fiber, evident improvement was observed for postdrawn samples owing to the fine alignment of β-sheet crystallites as shown in previous XRD results (Figure 7, 8). During the drawing of wet fibers, the fibroin chains were forced to slide past each other

Table 1. Effect of Draw-Ratio on the Mechanical Properties of ιC/PAAm-15% SF/PU Core−Sheath Fibersa drawratio 1.0× 1.5× 2.5× 3.5×

breaking stress (MPa) 59.4 69.6 96.3 126.0

± ± ± ±

3.8 6.7 7.0 6.0

breaking strain (%) 3.7 5.2 13.3 16.9

± ± ± ±

0.5 1.5 2.7 9.8

Young’s modulus (MPa) 28.8 40.6 44.5 45.0

± ± ± ±

2.1 2.3 2.6 7.3

Values shown are means ± standard deviation for six specimens. Coagulation: 2.5% (NH4)2SO4 (pH ∼ 5.0) for 8 h. a

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Figure 11. (A) Water absorption dynamic of the as-spun fiber from 17% SF/PU sheath dope and (B) the maximum water absorption capacity of fibers from different sheath dope concentrations. Error bars represent the standard deviation of mean (n = 6).

Figure 12. Photos of the fibers with/without pigment under the illuminant D65 (A,B), and their fluorescence spectra with photos in darkness (C,D).

section. The presence of sheath SF layer was able to retain the gel core in fiber form during water absorption. Therefore, these regenerated silk fibers are promising for applications as absorptive fibers. Further modification will be designed to achieve controlled release on the fibers. 3.5. Luminous SF-Based Core−Sheath Fibers. In order to fabricate luminous fibers from regenerated silk fibroin, phosphorescent particles were added into the ιC/PAAm core solution during the spinning process. The optical images of asprepared core−sheath fibers with/without phosphor pigment were presented in Figure 12A,B. Long-lasting blue luminescence was easily observed on the fiber sample with pigment (inset of Figure 12C), after visible light stimulation by the Illuminant D65. Its emission wavelength was identified around 431−484 nm (blue light) in the fluorescent spectrum (excited at 365 nm), while no emission recognized for the blank fiber (Figure 12C,D). When the luminescent fiber was observed

have confirmed the correlation between postdrawing and tensile strength of silk. Higher postdrawing leads to stronger fibers. In this case, after postdrawing to a draw-ratio of 3.5×, its stress and strain at break significantly increased to 126 MPa and 16.9%, respectively. 3.4. Water Absorbency of SF-Based Core−Sheath Fibers. Water absorbency (Q) of ιC/PAAm-SF/PU fibers were evaluated as shown in Figure 11. The core−sheath fiber achieved its absorption equilibrium in ∼100 min (Figure 11A). According to our previous result, raw silk yarn (with sericin reserved) is capable of absorbing 0.85 g/g in deionized water.22 In comparison, our core−sheath fibers achieved a higher average Q value of 1.25∼1.51 g/g at the selected SF concentrations (Figure 11B). It was double that of solid SF/ PU fiber and 4 times that of degummed silk fiber (Figure S7) because of the presence of hydrophilic PU in sheath and ιC/ PAAm gel in core, which helped retaining water in its core 3128


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(4) Wang, Y.; Rudym, D. D.; Walsh, A.; Abrahamsen, L.; Kim, H. J.; Kim, H. S.; Kirker-Head, C.; Kaplan, D. L. In vivo degradation of three-dimensional silk fibroin scaffolds. Biomaterials 2008, 29, 3415− 3428. (5) Zhang, W.; Chen, L.; Chen, J.; Wang, L.; Gui, X.; Ran, J.; Xu, G.; Zhao, H.; Zeng, M.; Ji, J.; Qian, L.; Zhou, J.; Ouyang, H.; Zou, X. Silk fibroin biomaterial shows safe and effective wound healing in animal models and a randomized controlled clinical trial. Adv. Healthcare Mater. 2017, 6, 1700121. (6) Choudhury, A. J.; Gogoi, D.; Chutia, J.; Kandimalla, R.; Kalita, S.; Kotoky, J.; Chaudhari, Y. B.; Khan, M. R.; Kalita, K. Controlled antibiotic-releasing Antheraea assama silk fibroin suture for infection prevention and fast wound healing. Surgery 2016, 159 (2), 539−547. (7) Shan, Y. H.; Peng, L. H.; Liu, X.; Chen, X.; Xiong, J.; Gao, J. Q. Silk fibroin/gelatin electrospun nanofibrous dressing functionalized with astraloside IV induced healing and anti-scar effects on burn wound. Int. J. Pharm. 2015, 479 (2), 291−301. (8) Li, Z.; Song, L.; Huang, X.; Wang, H.; Shao, H.; Xie, M.; Xu, Y.; Zhang, Y. Tough and VEGF-releasing scaffolds composed of artificial silk fibroin mats and a natural acellular matrix. RSC Adv. 2015, 5, 16748−16758. (9) Johari, N.; Madaah Hosseini, H. R.; Samadikuchaksaraei, A. Optimized composition of nanocomposite scaffolds formed from silk fibroin and nano-TiO2 for bone tissue engineering. Mater. Sci. Eng., C 2017, 79, 783−792. (10) Rockwood, D. N.; Preda, R. C.; Yucel, T.; Wang, X.; Lovett, M. L.; Kaplan, D. L. Materials fabrication from Bombyx mori silk fibroin. Nat. Protoc. 2011, 6 (10), 1612−1631. (11) Yen, K. C.; Chen, C. Y.; Huang, J. Y.; Kuo, W. T.; Lin, F. H. Fabrication of keratin/fibroin membrances by electrospinning for vascular tissue engineering. J. Mater. Chem. B 2016, 4, 237−244. (12) Lovett, M. L.; Cannizzaro, C. M.; Vunjak-Novakovic, G.; Kaplan, D. L. Gel spinning of silk tubes for tissue engineering. Biomaterials 2008, 29 (35), 4650−4657. (13) Yan, J.; Zhou, G.; Knight, D. P.; Shao, Z.; Chen, X. Wetspinning of regenerated silk fiber from aqueous silk fibroin solution: Discussion of spinning parameters. Biomacromolecules 2010, 11, 1−5. (14) Li, L.; Li, H.; Qian, Y.; Li, X.; Singh, G. K.; Zhong, L.; Liu, W.; Lv, Y.; Cai, K.; Yang, L. Electrospun poly(ε-caprolactone)/silk fibroin core-sheath nanofibers and their potential applications in tissue engineering and drug release. Int. J. Biol. Macromol. 2011, 49, 223− 232. (15) Zhang, J.; Qiu, K.; Sun, B.; Fang, J.; Zhang, K.; EI-Hamshary, H.; Al-Deyab, S. S.; Mo, X. The aligned core-sheath nanofibers with electrical conductivity for neural tissue engineering. J. Mater. Chem. B 2014, 2, 7945−7954. (16) Hang, Y.; Zhang, Y.; Jin, Y.; Shao, H.; Hu, X. Preparation of regenerated silk fibroin/silk sericin fibers by coaxial electrospinning. Int. J. Biol. Macromol. 2012, 51, 980−986. (17) Zhang, Y.; Huang, J.; Huang, L.; Liu, Q.; Shao, H.; Hu, X.; Song, L. Silk fibroin-based scaffolds with controlled delivery order of VEGF and BDNF for cavernous nerve regeneration. ACS Biomater. Sci. Eng. 2016, 2, 2018−2025. (18) Chae, S. K.; Kang, E.; Khademhosseini, A.; Lee, S. H. Micro/ Nanometer-scale fiber with highly ordered structures by mimicking the spinning process of silkworm. Adv. Mater. 2013, 25, 3071−3078. (19) Renberg, B.; Andersson-Svahn, H.; Hedhammar, M. Mimicking silk spinning in a microchip. Sens. Actuators, B 2014, 195, 404−408. (20) Peng, Q.; Shao, H.; Hu, X.; Zhang, Y. The development of fibers that mimic the core-sheath and spindle-knot morphology of artificial silk using microfluidic devices. Macromol. Mater. Eng. 2017, 302, 1700102. (21) Zhang, Y.; Pan, H.; Luo, J.; Zhang, L.; Li, Z.; Huang, X.; Jin, Y.; Fan, S.; Hang, Y.; Shao, H.; Hu, X. Artificial silk materials with enhanced mechanical properties and controllable structures. Int. J. Soc. Mater. Eng. Resour. 2014, 20, 1−5. (22) Lee, K. I.; Wang, X.; Guo, X.; Yung, Y.; Fei, B. Highly waterabsorbing silk yarn with interpenetrating network via in situ polymerization. Int. J. Biol. Macromol. 2017, 95, 826−832.

under SEM, the pigment was identified only in the core area with EDS analysis results (Figure S8). This regenerated silk fiber is ready to be woven or knitted into fabrics, having potential applications in portable and wearable phototherapy devices (e.g., for the neonatal jaundice treatment).

4. CONCLUSIONS SF-based core−sheath fibers have successfully been fabricated through coaxial wet spinning. The effects of SF concentration on the rheological property and wet spinnability of regenerated SF dopes were examined. A correlation between dope viscosity and fiber formation was observed. The sheath dope viscosity in 1.0∼6.7 Pa·s was found to be essential for continuous fiber spinning, among which the 17% SF/PU sheath dope solution showed optimal spinnability. On the basis of the morphology analysis of the as-prepared fibers, the effects of several processing parameters on resultant fiber properties were evaluated. The average diameter of the core−sheath fibers decreased with increased postdrawing ratio, while improved fiber appearance was obtained with increased coagulant concentration because of the faster solidification of fibroin. XRD and FTIR results suggested that the postdrawing and coagulant immersion processes are in favor of high crystallinity through conformational changes from amorphous to β-sheet structure. The poor mechanical properties of as-spun fibers could be significantly improved by three times drawing, followed by different immersion times. These regenerated SF core−sheath fibers with acceptable mechanical properties are promising in functional textiles such as water management dressing and wearable phototherapy device.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsbiomaterials.9b00275.

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Supporting Figures S1−S8 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +852-2766 4795. ORCID

Pui Fai Ng: 0000-0002-8208-824X Bin Fei: 0000-0002-4274-1873 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the funding support of Hong Kong General Research Fund (GRF) project 15204614. REFERENCES

(1) Altman, G. H.; Diaz, F.; Jakuba, C.; Calabro, T.; Horan, R. L.; Chen, J.; Lu, H.; Richmond, J.; Kaplan, D. L. Silk-based biomaterials. Biomaterials 2003, 24 (3), 401−416. (2) Oyama, F.; Mizuno, S.; Shimura, K. Studies on immunological properties of fibroin heavy and light chains. J. Biochem. 1984, 96, 1689−1694. (3) Lefevre, T.; Rousseau, M.; Pezolet, M. Protein secondary structure and orientation in silk as revealed by raman spectromicroscopy. Biophys. J. 2007, 92, 2885−2895. 3129


Article

ACS Biomaterials Science & Engineering (23) Li, D.; Xia, Y. Direct fabrication of composite and ceramic hollow nanofibers by electrospinning. Nano Lett. 2004, 4 (5), 933− 938. (24) Nam, J.; Johnson, J.; Lannutti, J. J.; Agarwal, S. Modulation of embryonic mesenchymal progenitor cell differentiation via control over pure mechanical modulus in electrospun nanofibers. Acta Biomater. 2011, 7, 1516−1524. (25) Kim, G. H.; Min, T.; Park, S. A.; Kim, W. D. Coaxially electrospun micro/nanofibrous poly(ε-caprolactone)/eggshell-protein scaffold. Bioinspiration Biomimetics 2008, 3, No. 016006. (26) Gluck, J. M.; Rahgozar, P.; Ingle, N. P.; Rofail, F.; Petrosian, A.; Cline, M. G.; Jordan, M. C.; Roos, K. P.; MacLellan, W. R.; Shemin, R. J.; Heydarkhan-Hagvall, S. Hybrid coaxial electrospun nanofibrous scaffolds with limited immunological response created for tissue engineering. J. Biomed. Mater. Res., Part B 2011, 99B (1), 180−190. (27) Park, J. H.; Braun, P. V. Coaxial electrospinning of self-healing coatings. Adv. Mater. 2010, 22 (4), 496−499. (28) Zhao, C.; Yao, J.; Masuda, H.; Kishore, R.; Asakura, T. Structural characterization and artificial fiber formation of Bombyx mori silk fibroin in hexafluoro-iso-propanol solvent system. Biopolymers 2003, 69 (2), 253−259. (29) Lee, K. H.; Baek, D. H.; Ki, C. S.; Park, Y. H. Preparation and characterization of wet spun silk fibroin/poly(vinyl alcohol) blend filaments. Int. J. Biol. Macromol. 2007, 41, 168−172. (30) Phillips, D. M.; Drummy, L. F.; Conrady, D. G.; Fox, D. M.; Naik, R. R.; Stone, M. O.; Trulove, P. C.; De Long, H. C.; Mantz, R. A. Dissolution and regeneration of Bombyx mori silk fibroin using ionic liquids. J. Am. Chem. Soc. 2004, 126, 14350−14351. (31) Zhang, F.; Lu, Q.; Yue, X.; Zuo, B.; Qin, M.; Li, F.; Kaplan, D. L.; Zhang, X. Regeneration of high-quality silk fibroin fiber by wet spinning from CaCl2-formic acid solvent. Acta Biomater. 2015, 12, 139−145. (32) Yue, X.; Zhang, F.; Wu, H.; Ming, J.; Fan, Z.; Zuo, B. A novel route to prepare dry-spun silk fibers from CaCl2-formic acid solution. Mater. Lett. 2014, 128, 175−178. (33) Liu, Z.; Zhang, F.; Ming, J.; Bie, S.; Li, J.; Zuo, B. Preparation of electrospun silk fibroin nanofibers from solutions containing native silk fibrils. J. Appl. Polym. Sci. 2015, 132, 41236. (34) Lu, X.; Chan, C. Y.; Lee, K. I.; Ng, P. F.; Fei, B.; Xin, J. H.; Fu, J. Super-tough and thermo-healable hydrogel − promising for shapememory absorbent fiber. J. Mater. Chem. B 2014, 2, 7631−7638. (35) Gupta, D.; Agrawal, A.; Chaudhary, H.; Gulrajani, M.; Gupta, C. Cleaner process for extraction of sericin using infrared. J. Cleaner Prod. 2013, 52, 488−494. (36) Bhat, N. V.; Nadiger, G. S. Crystallinity in silk fibers: Partial acid hydrolysis and related studies. J. Appl. Polym. Sci. 1980, 25, 921− 932. (37) Um, I. C.; Kweon, H. Y.; Park, Y. H.; Hudson, S. Structural characteristics and properties of the regenerated silk fibroin prepared from formic acid. Int. J. Biol. Macromol. 2001, 29, 91−97. (38) Chung, D. E.; Um, I. C. Effect of molecular weight and concentration on crystallinity and post drawing of wet spun silk fibroin fiber. Fibers Polym. 2014, 15 (1), 153−160. (39) Iglauer, S.; Wu, Y.; Shuler, P.; Tang, Y.; Goddard, W. A., III Dilute iota- and kappa-carrageenan solutions with high viscosities in high salinity brines. J. Pet. Sci. Eng. 2011, 75, 304−311. (40) Terry, A. E.; Knight, D. P.; Porter, D.; Vollrath, F. pH induced changes in the rheology of silk fibroin solution from the middle division of Bombyx mori silkworm. Biomacromolecules 2004, 5, 768− 772. (41) Boulet-Audet, M.; Terry, A. E.; Vollrath, F.; Holland, C. Silk protein aggregation kinetics revealed by Rheo-IR. Acta Biomater. 2014, 10, 776−784. (42) Ming, J.; Liu, Z.; Bie, S.; Zhang, F.; Zuo, B. Novel silk fibroin films prepared by formic acid/hydroxyapatite dissolution method. Mater. Sci. Eng., C 2014, 37, 48−53. (43) Holland, C.; Terry, A. E.; Porter, D.; Vollrath, F. Natural and unnatural silks. Polymer 2007, 48, 3388−3392.

(44) Holland, C.; Porter, D.; Vollrath, F. Comparing the rheology of mulberry and “wild” silkworm spinning dopes. Biopolymers 2012, 97, 362−367. (45) Lu, Q.; Zhu, H.; Zhang, C.; Zhang, F.; Zhang, B.; Kaplan, D. L. Silk self-assembly mechanisms and control from thermodynamics to kinetics. Biomacromolecules 2012, 13, 826−832. (46) Southern, J. H.; Ballman, R. L. Stratified bicomponent flow of polymer melts in a tube. Appl. Polym. Sci. 1973, 20, 175−189. (47) Wang, Y. X.; Wang, C. G.; Yu, M. J. Effects of different coagulation conditions on polyacrylonitrile fibers wet spun in a system of dimethylsulfoxide and water. J. Appl. Polym. Sci. 2007, 104, 3723− 3729. (48) Kumar, S.; Singh, S. K. Fabrication and characterization of fibroin solution and nanoparticle from silk fibers of Bombyx mori. Part. Sci. Technol. 2017, 35, 304−313. (49) Kim, U. J.; Park, J.; Li, C.; Jin, H. J.; Valluzzi, R.; Kaplan, D. L. Structure and properties of silk hydrogels. Biomacromolecules 2004, 5, 786−792. (50) Bratzel, G.; Buehler, M. J. Molecular mechanics of silk nanostructures under varied mechanical loading. Biopolymers 2012, 97, 408−417. (51) Sun, M.; Zhang, Y.; Zhao, Y.; Shao, H.; Hu, X. The structureproperty relationships of artificial silk fabricated by dry-spinning process. J. Mater. Chem. 2012, 22, 18372−18379. (52) Shen, Y.; Johnson, M. A.; Martin, D. C. Microstructural characterization of Bombyx mori silk fibers. Macromolecules 1998, 31, 8857−8864. (53) Takahashi, Y.; Gehoh, M.; Yuzuriha, K. Structure refinement and diffuse streak scattering of silk (Bombyx mori). Int. J. Biol. Macromol. 1999, 24, 127−138. (54) Takahashi, Y.; Gehoh, M.; Yuzuriha, K. Crystal structure of silk (Bombyx mori). J. Polym. Sci., Part B: Polym. Phys. 1991, 29, 889− 891. (55) Koperska, M.; Pawcenis, D.; Bagniuk, J.; Zaitz, M.; Missori, M.; Łojewski, T.; Łojewska, J. Degradation markers of fibroin in silk through infrared spectroscopy. Polym. Degrad. Stab. 2014, 105, 185− 196. (56) Tomlinson, G. E.; Curnutte, B.; Hathaway, C. E. Temperature dependence of the raman and infrared spectrum of liquid formic acid. J. Mol. Spectrosc. 1970, 36, 26−33. (57) Jin, H. J.; Park, J.; Karageorgiou, V.; Kim, U. J.; Valluzzi, R.; Cebe, P.; Kaplan, D. L. Water-stable silk films with reduced β-sheet content. Adv. Funct. Mater. 2005, 15, 1241−1247. (58) Qian, J.; Cui, G.; Jing, M.; Wang, Y.; Zhang, M.; Yang, J. Hydrothermal synthesis of nitrogen-doped titanium dioxide and evaluation of its visible light photocatalytic activity. Int. J. Photoenergy 2012, 2012, 198497. (59) Qin, H.; Zhang, X.; Liu, H.; Sang, Y.; Wang, J. Mechanism of ammonium sulfate regulation effect on microstructure of Y2O nanopowders via urea precipitation method. CrystEngComm 2013, 15, 5076−5081. (60) Um, I. C.; Kweon, H. Y.; Lee, K. G.; Park, Y. H. The role of formic acid in solution stability and crystallization of silk proptein polymer. Int. J. Biol. Macromol. 2003, 33, 203−213. (61) Asakura, T.; Sato, Y.; Aoki, A. Stretching-induced conformational transition of the crystalline and noncrystalline domains of 13Clabelled Bombyx mori silk fibroin monitored by solid state NMR. Macromolecules 2015, 48, 5761−5769. (62) Porter, D.; Guan, J.; Vollrath, F. Spider silk: Super material or thin fiber? Adv. Mater. 2013, 25, 1275−1279. (63) Plaza, G. R.; Corsini, P.; Perez-Rigueiro, J.; Marsano, E.; Guinea, G. V.; Elices, M. Effect of water on Bombyx mori regenerated silk fibers and its application in modifying their mechanical properties. J. Appl. Polym. Sci. 2008, 109, 1793−1801.

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