Effect of moisture absorption on the thermal properties of Bombyx mori silk fibroin films

Films of regenerated Bombyx mori silk are strongly affected by absorbed moisture, a phenomenon studied here by differential scanning calorimetry (DSC). Exposure of previously dried films to environments of controlled relative humidity produces test samples of well-defined equilibrium moisture content. Ultimate moisture uptake is as high as 20–23% (by weight) at 75% relative humidity. The glass transition temperature, T_g, drops by 40°C at moisture uptakes as low as 2%, and T_g depressions as large as 140°C are observed at higher relative humidity. The moisture-induced decrease of T_g is completely reversible, as a film remoistened and then redried possesses an unchanged T_g. Trends in T_g with water uptake correspond reasonably well to predictions of a classical thermodynamic theory, indicating that the plasticization effect of moisture on the combined silk-water system can be satisfactorily explained from macroscopic properties of the constituents without any reference to specific interactions. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 401–410, 1997     

Enzymatic degradability and release properties of graphene oxide/silk fibroin nanocomposite films

The structure evolution of silk fibroin (SF) in the nanocomposite films with graphene oxide (GO) was investigated and related to the enzymatic degradability and release property. The interaction with GO was found to induce conformation transition of SF from random coil to β-sheet. However, the surface binding constrained the rearrangement of the silk chains, leading to a decrease of β-sheet when GO content was more than 1.0%. The crystal structure of SF played a key role in the degradation of GO/SF composites. The preferential degradation of the hydrophilic blocks resulted in a faster degradation of SF films with higher β-sheet content. The addition of GO to SF matrix led to a slower release and a reduction of the burst release of RhB, the model compound. The release profile was well fitted to the Rigter–Peppas equation, from which the characteristic constant decreased and the diffusional exponent increased with increasing GO content but quickly leveled off when GO content was more than 1.0%. Degradation of the composites had little influence on the characteristic constant of RhB release, however, led to an increased diffusional exponent, which was more evident for the composites with higher β-sheet content.     

Preparation and characterization of Bombyx mori silk fibroin and wool keratin

Silk and wool are well‐known protein‐based fibers. Their environmental stability, biocompatibility, and unique mechanical properties provide an important basis for using these natural proteins in biomedical applications. To use them as biomaterials in the form of fibers, films, or membranes, it is necessary to characterize these proteins in their solution and solid states because structural characteristics and morphological features have a great influence on the physical and mechanical properties of these new regenerated protein forms. Therefore, in the present study, silk fibroin and wool keratin were dissolved and their solution behaviors and secondary structures are analyzed and compared, using particle size distribution, molecular weight distribution (SDS‐PAGE), Fourier transform infrared, and X‐ray diffraction techniques. It was shown that keratin is more stable in solution and more amorphous in the solid state. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100:4260–4264, 2006     

Structure of silk fibroin fibers made by an electrospinning process from a silk fibroin aqueous solution

In this article, the electrospun silk fibroin (SF) fibers with an average diameter of 700 nm were prepared from a concentrated aqueous solution with an electrospinning technique. The morphology, conformation, and crystalline structure of the SF fibers were characterized by scanning electron microscopy, Raman spectroscopy, and wide‐angle X‐ray diffraction, respectively. The structure and morphology of the fibers were strongly influenced by the solution concentration and the processing voltage. In addition, the fiber formation parameters, including spinning velocity, elongation rate, and draw ratio, were also calculated. A kind of SF fiber with a structure between an amorphous film and a natural silk was found. We suggest that the high draw ratio was not the only factor in the transformation of SF from random‐coil and α‐helix conformations to a β‐sheet conformation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 961–968, 2006     

Fibers based on cellulose–silk fibroin blend

Fibers made of cellulose (CE) and silk fibroin (SF) are wet spun from solutions in N,N‐dimethylacetamide containing 7% LiCl (w/w). Different coagulation baths (water and ethanol) and spinning conditions are used. By using water as the coagulant, a partial dissolution of SF occurs and negligible variation of the mechanical properties of the CE–SF fibers with respect to the CE fibers is found. Fibers coagulated in ethanol are dimensionally homogeneous and show better properties. A modulus of about 13 GPa and elongation to break of 16% for the blend containing 30% (w/w) SF are obtained with a 20‐mm air gap. The fibers are characterized by FTIR micro‐Raman, scanning electron microscopy, and wide‐ and small‐angle X‐ray analyses. In particular, the X‐ray results show that CE–SF fibers are amorphous with a homogeneous dispersion of small SF domains (1.3 nm) in the CE matrix. These results confirm the good compatibility between the two natural polymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2187–2196, 2007     

从丝素水溶液中再生的丝纤维的结构与性能

通过使用表面皿直接拉伸、毛细管重力纺丝和人工拉伸3种不同的成丝方法,从高浓度再生丝素水溶液中制得了丝纤维。用偏光显微镜观察了丝纤维的取向,用拉曼光谱仪和Instron拉力仪表征了丝纤维的结构和力学性能。结果发现,经毛细管剪切流动后再拉伸有利于再生丝性能的提高,所得的丝有较好的取向和较多的β折叠结构,力学性能也相对较好。剪切在丝纤维的成形过程中起重要的作用。     

Glycerin and ethanol as additives on silk fibroin films: Insoluble and malleable films

Silk fibroin (SF) films have been largely studied as biomaterials due to their biocompatibility and biodegradability. Casting a SF aqueous solution at room temperature is a common technique to produce SF films at relative low cost and processing time; however, their brittleness and solubility in water make them unsuitable for certain biomedical applications. In this study, the incorporation of additives, ethanol and glycerin, are presented as an alternative to both improve mechanical properties of SF films and decrease their solubility in water. SF films with additives were further characterized using scanning electron microscopy, X‐ray diffraction, Fourier transformed infrared spectroscopy with attenuated total reflection, analysis of water solubility, mechanical test of traction, and in vitro cytotoxicity experiments. The results show that SF films containing additives are stable in water due to the effect of glycerin and ethanol, and do not require post‐treatments. Furthermore, great improvements on elongation of the films were achieved, mainly in the presence of both additives. In addition, all films were not toxic to cells, which is a first indication of their biocompatibility. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Acrylic polymer–silk fibroin blend fibers

Blends of acrylic polymer (containing acrylonitrile 91.7%, methyl acrylate 7%, and sodium methyl propenyl sulfonate 1.3% [wt %], denoted as PAC) with silk fibroin (SF) were studied in the form of drawn fibers with varied compositions. The strength, elongation, and specific work of rupture of the blend fibers decrease with increase of the SF content, whereas the modulus has a slight increase up to 20% (wt) SF and then decreases. With the addition of up to 30% (wt %) SF in the PAC matrix, the moisture absorption increases from 2.06 to 6.2% in comparison with the PAC. Scanning electron microscopy studies show that the blend fibers have a sheath–core structure, with SF mainly in the sheath and PAC in the core. FTIR, ATR, and X-ray diffraction results of the blend fibers are also presented. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:959–966, 1997     

Biocomposite membranes of sodium alginate and silk fibroin fibers for biomedical applications

Biocomposite membranes from biodegradable and biocompatible natural polymers were prepared from sodium alginate solution reinforced with silk fibroin fibers in several fiber content by casting and solvent evaporation. The properties of these biocomposites were investigated by scanning electron microscopy, swelling test, water vapor transmission, mechanical and thermal analyses, and cytotoxicity test. A biocomposite with uniform fiber dispersion and good fiber–matrix interaction was obtained through the incorporation of fibroin fibers in the alginate membrane, even though the fibers were used without any surface treatment to enhance the interfacial adhesion. The incorporation of fibroin fibers improved the tensile strength and also provided a new property to the alginate, that is, the resistance to tear. Moreover, the use of silk fibroin fibers in polymeric composites can result in a material with adequate characteristics for application in the biomaterial field, especially as wound dressings, because of its nontoxic effect to cells, flexibility, and resistance to tear. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3451–3457, 2013     

Surface modification of textile material through deposition of regenerated silk fibroin

Regeneration of silk fibroin from calcium chloride/ethanol/water solution is of high interest to shape biomaterial based products for medical and technical application. In this study a continuous process for surface modification of cellulose or polyamide fibers by regenerated fibroin deposits has been investigated. The decomposition of the fibroin‐calcium complex was initiated by addition of K₂CO₃ followed by a methanol rinse. Reactive Blue 19 labeled fibroin was used to monitor the deposition of fibroin on the substrates by colour measurement. The fibroin deposits on the fabric were characterized by microscopy, N‐content, calcium, and protein content. Stiffness and surface resistivity of modified fabrics were determined as representative physical parameters. The reduced mobility of fibers increased fabric stiffness. Surface resistivity of treated samples was reduced by a factor of 10 to 100, which gives an example for the potential of the technique as coating for man‐made fiber textiles. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45098.     

Structure and dyeability of Bombyx mori silk fibers with different filament sizes

This study deals with the analysis of structure, physical properties, and dyeing behavior of silk fibers with different filament sizes. Fine and coarse silk fibers were obtained from Akebono and Ariake cocoon varieties, respectively. Both samples exhibited a fairly similar x-ray crystallinity, while the degree of molecular orientation increased with decreasing the fiber size. Tensile strength and energy of fine silk fibers were significantly higher, while elongation at break did not change in relation to the fiber size. Fine silk fibers exhibited a slightly higher thermal stability, as shown by the upward shift of both the DSC decomposition temperature and the TMA final extension step at above 300°C. The TGA and DMA (E″) patterns remained unchanged regardless of fiber size. The amino acid analysis confirmed the absence of any difference of chemical structure between fine and coarse silk fibers, the content of acidic, basic, and other characteristic amino acid residues being exactly the same. Accordingly, both samples adsorbed the same amount of hydrochloric acid. Ex-haustion dyeing tests with various direct, acid, and reactive dyes showed the occurrence of appreciable differences in the perceived color between fine and coarse silk fibers. The size of color difference was quantitatively evaluated by reflectance measurements and discussed in relation to silk fiber morphology, structure, as well as dyeing conditions. The kinetics of diffusion of two model dyes was not significantly affected by the different fiber size and structure. © 1996 John Wiley & Sons, Inc.     

Preparation of regenerated Antheraea yamamai silk fibroin film and controlled‐molecular conformation changes by aqueous ethanol treatment

The methods of dissolving Antheraea yamamai (A. yamamai) silk fibroin fibers and preparing the regenerated silk fibroin were studied. The molecular conformations of A. yamamai silk fibroin film treated by different concentration ethanol solution were investigated by XRD, FTIR, and Raman spectra. The results indicated that A. yamamai silk fibroin fibers could be completely dissolved in dense lithium thiocyanate solution at about 35°C for 1 h. The initial regenerated film consisted of α‐helix and random coil components. The aqueous ethanol treatment of the film resulted in significant increase in β‐sheet component and improvement of water resistance of the film. This effect was strongly dependent on ethanol concentration, and 60–80% (w/w) ethanol was most effective. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structure and physical properties of silk fibroin/polyacrylamide blend films

This article deals with the characterization of blend films obtained by mixing silk fibroin (SF) and polyacrylamide (PAAm). The DSC curves of SF/PAAm blend films showed overlapping of the main thermal transitions characteristic of the individual polymers. The exothermic peak at 218°C, assigned to the β‐sheet crystallization of silk fibroin, slightly shifted to a lower temperature by blending. The weight‐retention properties (TG) of the blend films were intermediate between those of the two constituents. The TMA response was indicative of a higher thermal stability of the blend films, even at low PAAm content (≤25%), the final breaking occurring at about 300°C (100°C higher than pure SF film). The peak of dynamic loss modulus of silk fibroin at 193°C gradually shifted to lower temperature in the blend films, suggesting an enhancement of the molecular motion of the fibroin chains induced by the presence of PAAm. Changes in the NH stretching region of silk fibroin were detected by FTIR analysis of blend films. These are attributable to disturbance of the hydrogen bond pattern of silk fibroin and formation of new hydrogen bonds with PAAm. The values of strength and elongation at break of blend films slightly improved at 20–25% PAAm content. A sea–island structure was observed by examining the air surface of the blend films by scanning electron microscopy. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1563–1571, 1999     

Water sorption,membrane potentials,and ion permeability of styrene-grafted Bombyx mori silk fibroin membrane

Characterization of styrene-grafted Bombyx mori silk fibroin membrance was investigated. It was revealed from the water sorption and ¹H nuclear magnetic resonance (NMR) measurements that the amounts of water adsorbed on the silk fibroin membranes decreased by the styrene grafting and the states of water adsorbed on the styrene-grafted silk fibroin membranes were not homogeneous; the presence of two components of water adsorbed on the membranes at 60% relative humidity was observed. In addition, the fraction of the fast component decreased with increasing styrene grafting. The membrane potentials increased with increasing of the grafting. The KCl permeability of the membrane strongly depends on the degree of styrene grafting.     

Rapid formation of flexible silk fibroin gel‐like films

Flexible silk fibroin gel‐like films with microporous morphology were prepared from B. mori silk fibroin fibers directly solubilized in formic acid/CaCl₂ solvent. These films were characterized by several analysis techniques to determine the structure and properties of films. The pore size of gel‐like films can be adjusted through SF concentration and Ca ions concentration. The controllable pore size in gel‐like films was grew from 3–5 μm to 100 μm under the increase of fibroin concentration from 1.0 wt % to 8.0 wt %. At the same time, the water content of silk fibroin gel‐like film decreased from 83.5 ± 3.4% to 68.2 ± 2.6%. With increasing Ca ions contents from 2.0 wt % to 10.0 wt % in dissolution process, the pore size and water content of silk fibroin gel‐like films grew larger, especially its water content values reached 86.2 ± 4.0% at 10.0 wt % Ca ions concentration. At wet condition, the gel‐like film with β‐sheet structure showed higher breaking stress (4.26 ± 0.31 MPa) and elongation (45.45 ± 15.79%) at 8.0 wt % concentration. With the preparation method, the membrane is hydrophilic and the pore size is adjustable, which contributes to high toughness and favorable cell growth environment, suggesting that these silk fibroin gel‐like films can be a potential candidate scaffold for biomedical applications, such as wound dressing, facial mask, contact lenses, etc. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41842.     

Recovery in spider silk fibers

The extreme toughness of spider silk is dependent on the silk's ability to dissipate most of the mechanical energy imparted to the fiber during loading processes through irreversible deformations. This basic property makes the tensile behavior of spider silk fibers depend on the silk's previous deformation history in a largely unpredictable way. The resulting variability often represents an insurmountable difficulty for both the characterization of the material and its practical usage. In this study, it was shown that spider silk is endowed with a property that allows to circumvent these problems: supercontraction, the large shrinkage of the longitudinal dimension of spider silk fibers in wet environments, recovers the tensile properties of deformed spider silk fibers in a repetitive and reproducible way. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3537–3541, 2004     

Mechanical properties of silk fibroin–microcrystalline cellulose composite films

Silk fibroin–microcrystalline cellulose (cellulose whisker) composite films with varied compositions were prepared by casting mixed aqueous solution/suspensions of the two components. Silk fibroin was dissolved in 10M LiSCN followed by dialysis; a cellulose whisker suspension was prepared by sulfuric acid hydrolysis of tunicate cellulose. Macroscopically homogeneous films were obtained at all mixing ratios. While the Young's modulus of the composite films showed a linear, additive dependence on the mixing ratio, the tensile strength and ultimate strain showed a maximum at a 70–80% cellulose content, reaching five times those of fibroin‐alone or cellulose‐alone films. At the same mixing ratio, infrared spectra of the composite films showed a shift of the amide I peak from 1654 to 1625 cm^?1, indicating the conformational change of fibroin from a random coil to a β structure (silk II) at the whisker–matrix interface. This change seems to be induced by contact of fibroin molecules with a highly ordered surface of cellulose whisker. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3425–3429, 2002     

Isothermal crystallization kinetics of silk fibroin fiber‐reinforced poly(ε‐caprolactone) biocomposites

BACKGROUND: Poly(ε‐caprolactone) (PCL) has attracted great attention due to its wide applications for pharmaceutical controlled released systems and implanted polymer devices. In this study, silk fibroin fiber (SF) obtained from degumming treatment of silk was used to prepare novel reinforced PCL biocomposites. The isothermal crystallization behavior of these composites was investigated using differential scanning calorimetry measurements. RESULTS: With a decrease of isothermal crystallization temperature (T_c) and an increase of fiber filler, the crystallization time of the SF/PCL composites becomes shorter, the crystallization rate constant (K) increases and the Avrami exponent (n) gradually decreases (being between 1 and 2). The crystallization of PCL and SF/PCL composites occurs in the same regime. With the gradual addition of fiber, lateral surface free energy (σ) is nearly unchanged, but fold surface free energy (σ_e) decreases. CONCLUSION: Heterogeneous nucleation is dominant and different growth morphologies coexist during the isothermal crystallization process of the SF/PCL hybrid systems. Although the introduction of SF obviously increases the overall crystallization rate of PCL, the growth rate constant and nucleation constant of PCL are reduced because of the confinement effect of fiber network structures on the molecular mobility of polymer molecular chains. Copyright © 2009 Society of Chemical Industry     

Analyzing the interactions and miscibility of silk fibroin/mucin blends

Mucin, a glycoprotein with viscoelastic properties, and silk fibroin, a protein excreted from silkworms with properties of thermal and mechanical resistance, have been probed as building blocks in the design of biomaterials. Aiming to evaluate the interaction and miscibility between mucin and fibroin, we synthesized silk fibroin and mucin (SF/MU) blends for biomedical applications. The morphological analysis of the SF/MU blends showed the presence of two phases, suggesting a partial miscibility between the polymers. The degradation temperature of the SF/MU blends increased with an increase in the silk fibroin content, indicating that silk fibroin contributed to the thermal stability of the blends. The glass transition temperature of the SF/MU blends lay between the values of the pure polymers. The Fourier-transform infrared spectroscopy results pointed out that the interaction between fibroin and mucin occurred between the amine group of silk fibroin and mucin carboxyl and hydroxyl groups. The outcomes of this work provided essential information on the miscibility of the SF/MU blends. These findings will be critical for further studies with fibroin and mucin-based biomaterials, especially in mucoadhesive systems and wound healing applications.     

Effect of hot drawing on properties of wet‐spun poly(L,D‐lactide) copolymer multifilament fibers

Polylactide stereocopolymer multifilament fibers were prepared by wet spinning and subsequent hot drawing. The stereocopolymers were poly‐(L,D ‐lactide) [P(L,D )LA], L/D ratio 96/4, and poly‐(L,DL ‐lactide) [P(L,DL )LA], L/DL ratio 70/30. They were dissolved in dichloromethane and coagulated in a spin bath containing ethanol. The hot‐drawing temperature was 65°C. The draw ratios (DR) were upto 4.5 to the P(L,D )LA 96/4 filaments and upto 3 to the P(L,DL )LA 70/30 filaments. Wet spinning decreased crystallinities of both copolymers. Hot drawing increased the crystallinity of the P(L,D )LA 96/4 filament but not to the level of the original copolymer, whereas the as‐spun and the hot‐drawn P(L,DL )LA 70/30 filaments were amorphous. The filament diameter, tenacity, Young's modulus, and elongation at break were dependent on the DR. The maximum tenacity (285 MPa) and Young's modulus (2.0 GPa) were achieved with the P(L,D )LA 96/4 filament at the DR of 4.5. Respectively, the maximum tenacity of the hot‐drawn P(L,DL )LA 70/30 filament was 175 MPa and Young's modulus 1.3 GPa at the DR of 3. Hot drawing slowed down in vitro degradation rate of both stereocopolymer filaments. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010