Natural Silk Fibers Research Articles

Restructuring the Interface of Silk-Polycaprolactone Biocomposites Using Rigid-Flexible Agents.

Natural fiber-reinforced biocomposites with excellent mechanical and biological properties have attractive prospects for internal medical devices. However, poor interfacial adhesion between natural silk fiber and the polymer matrix has been a disturbing issue for such applications. Herein, rigid-flexible agents, such as polydopamine (PDA) and epoxy soybean oil (ESO), were introduced to enhance the interfacial adhesion between Antheraea pernyi (Ap) silk and a common medical polymer, polycaprolactone (PCL). We compared two strategies of depositing PDA first (Ap-PDA-ESO) and grafting ESO first (Ap-ESO-PDA). The rigid-flexible interfacial agents introduced multiple molecular interactions at the silk-PCL interface. The "Ap-PDA-ESO" strategy exhibited a greater enhancement in interfacial adhesion, and interfacial toughening mechanisms were proposed. This work sheds light on engineering strong and tough silk fiber-based biocomposites for biomedical applications.

Peculiar Tensile and Fracture Behaviors of Natural Silk Fiber in the Presence of an Artificial Notch

Natural silk fibers achieve remarkable toughness by exploiting hierarchical structure organizations over many length scales. These hierarchical architectures are often decorated with defects that usually lead to catastrophic failure of artificial polymer materials. Silk fibers are self-tolerant of intrinsic/artificial defects to give surprising fracture toughness, but how they fulfill this function and the underlying mechanism remain elusive. Here, we unveil this puzzle by experimentally and theoretically investigating the tensile and facture behaviors of Antheraea pernyi (A. pernyi) silk monofilaments containing an artificial notch of controlled depth. The introduced transverse notch can be blunted effectively by ahead developed longitudinal crack through fibrillar separation. The notched silk fibers show ductile failure with tensile behavior dominated by the magnitude of longitudinal crack, which depends not only on the notch size but also on the interface interaction between nanofibrils. We propose that the highly oriented, disentangled nanofibrillar structure of A. pernyi silk offers it distinct tensile properties, path of crack propagation, and fracture mode in comparison with highly oriented synthetic fibers.

The mechanical behavior of silk-fibroin reinforced alginate hydrogel biocomposites - Toward functional tissue biomimetics

Soft tissues are constructed as fiber-reinforced composites consisting of structural mechanisms and unique mechanical behavior. Biomimetics of their mechanical behavior is currently a significant bioengineering challenge, emphasizing the need to replicate structural and mechanical mechanisms into novel biocomposite designs. Here we present a novel silk-based biocomposite laminate constructed from long natural silk and fibroin fibers embedded in an alginate hydrogel matrix. Controlling the mechanical features of these laminates were studied for different fiber volume fractions (VF) and orientations using unidirectional tensile tests. Three material systems were investigated having different fiber orientations: longitudinal (0°), transverse (90°), and cross-plied (0/90°). The general behavior of the biocomposite laminates was anisotropic hyperelastic with large deformations. Longitudinal fibroin laminates have shown a tensile modulus of 178.55 ± 14.46 MPa and tensile strength of 18.47 ± 2.01 MPa for 0.48 VF. With similar VF, cross-plied fibroin laminates demonstrated structural shielding ability, having a tensile modulus and tensile strength of 101.73 ± 8.04 MPa and 8.29 ± 1.63 MPa for only a third of the VF directed in the stretching direction. The stress-strain behavior was in a similar range to highly stiff native human soft tissues such as ligament and meniscus. These findings demonstrate the potential of the fibroin fiber-reinforced biocomposites to mimic the mechanics of tissues with a quantitatively controlled amount of fibers and designed spatial arrangement. This can lead to new solutions for the repair and replacement of damaged functional and highly stiff soft tissues.

Understanding the Interfacial Adhesion between Natural Silk and Polycaprolactone for Fabrication of Continuous Silk Biocomposites.

The poor interfacial adhesion between silk fiber and polyester species remains a critical problem for the optimal mechanical performance of silk-reinforced polyester composites. Here, we investigated in quantitative terms the interfacial properties between natural silk fibers and polycaprolactone (PCL) at nano-, micro-, and macroscales and fabricated continuous silk-PCL composite filaments by melt extrusion and drawing processing of PCL melt at 100, 120, and 140 °C. Bombyx mori (Bm) silk, Antheraea pernyi (Ap) silk, and polyamide6 (PA6) fiber were compared to the composite with PCL. The Ap silk exhibited the highest surface energy, the best wettability, and the largest interfacial shear strength (IFSS) with PCL. The silk-PCL composite from the 120 °C melt processing displayed the highest tensile modulus, implying an optimal temperature for interfacial adhesion. The Raman imaging technique revealed in detail the nature of the physical fusion of the interface phase in these silk- and polyamide-reinforced PCL composites. This work is intended to lay a foundation for the design and processing of robust composites from continuous silk fibers and bioresorbable polyesters for potential structural biomaterials.

Characterization of direct ink write pure silk fibroin based on alcohol post-treatments

Silk fibroin (SF) has been successfully used for medical devices due to its biodegradability with non-toxic end products, high tensile strength and mechanical robustness, but also because of its high flexibility potential. Although natural silk fibers have excellent strength and flexibility, regenerated silk materials generally become brittle in the dry state. For this reason, researchers have studied the effects that manufacturing and post-treatment parameters have on their mechanical properties, but they have also studied the impact on other factors such as biodegradability on the environment. This work presents an optimized process for direct ink write a regenerated SF bioink and its possible post-treatments. A simple method to concentrate the aqueous SF has been reported. The technique was then employed to 3D print test specimens for multiple mechanical analysis to characterize the resulting parameters of the processed silk. The effect of post-treating the material with different processes (no-treatment, immersion in ethanol for 24 h, and subsequently immersion in methanol for 30, 60 or 120 min) was explored. The different post-treatments resulted in distinct effects on the silk properties, suggesting that SF molecular structure could be controlled by the post-treatment process. The results showed a transition to phosphate-buffered saline (PBS)-insoluble silk when the silk was treated with alcohols. This resulted in a more brittle material than the untreated group, with a lower strain at break. Nevertheless, the post-treatments enhanced the stability of silk in water, as they reported greater insolubility in PBS than the untreated group. This study characterizes and discusses the mechanical properties of SF processed with a novel additive manufacturing method intended for customization of medical devices.

Changes in Natural Silk Fibres by Hydration, Tensile Loading and Heating as Studied by 1H NMR: Anisotropy in NMR Relaxation Times.

B. mori silkworm natural silk is a fibrous biopolymer with a block copolymer design containing both hydrophobic and hydrophilic regions. Using 1H NMR relaxation, this work studied B. mori natural silk fibres oriented at 0° and 90° to the static magnetic field B0 to clarify how measured NMR parameters reflect the structure and anisotropic properties of hydrated silk fibres. The FTIR method was applied to monitor the changes in the silk I and β-sheet conformations. Unloaded B. mori silk fibres at different hydration levels (HL), the silk threads before and after tensile loading in water, and fibres after a stepped increase in temperature have been explored. NMR data discovered two components in T1 and T2 relaxations for both orientations of silk fibres (0° and 90°). For the slower T2 component, the results showed an obvious anisotropic effect with higher relaxation times for the silk fibres oriented at 90° to B0. The T1 component (water protons, HL = 0.11) was sequentially decreased over a range of fibres: 0° oriented, randomly oriented, silk B. mori cocoon, 90° oriented. The degree of anisotropy in T2 relaxation was decreasing with increasing HL. The T2 in silk threads oriented at 0° and 90° also showed anisotropy in increased HL (to 0.42 g H2O/g dry matter), at tensile loading, and at an increasing temperature towards 320 K. The changes in NMR parameters and different relaxation mechanisms affecting water molecular interactions and silk properties have been discussed. The findings provide new insights relating to the water anisotropy in hydrated Bombyx mori silk fibres at tensile loading and under a changing HL and temperature.

Silk-based 2D nanocomposites for superior oily wastewater remediation

Oil spills hazard has increased along with the increase of industrial production, transportation and refining of oil. Therefore, it is becoming urgent to develop natural and efficient adsorbents for oily wastewater remediation. Herein, we report the development of superhydrophobic silk-based nanocomposites via plasma treatment followed by controlled surface decoration. The oxygen-plasma process was optimized to improve the surface properties and enhancing the adhesion of 2D nanomaterials on the natural silk fibers (SF). The SF was decorated with reduced graphene oxide (rGO) or molybdenum disulfide (MoS2) nanosheets to obtain robust silk-based nanocomposites ([email protected] and MoS2@PSF) with high oil selectivity. Detailed characterizations confirmed the presence of the composite structure with ultrathin coating of rGO or MoS2 nanosheets over the entire surface of SF. The water uptake of the silk-based nanocomposites was substantially suppressed after the decoration with nanosheets. The silk-based nanocomposites showed outstanding chemical stability and durability after the exposure to several corrosive solutions and organic solvents. On the contrary, the pristine SF was chemically unstable hindering their applicability under harsh conditions. The [email protected] and MoS2@PSF nanocomposites showed excellent oil adsorption capacity up to ∼120 g/g and ∼90 g/g for oil/water mixtures, respectively. In case of high concentration (up to 30 g/l) and highly stable oil-in-water emulsions, the composites demonstrated high adsorption capacity for crude, pump, and shell oil-in-water emulsions. [email protected] was more efficient than MoS2@PSF in crude oil-in-water emulsion separation with an adsorption capacity up to 8203 and 8018 mg/g for [email protected] and MoS2@PSF, respectively. Whereas MoS2@PSF demonstrated a higher performance for pump oil-in-water emulsions separation with an adsorption capacity up to 3079 and 3529 mg/g for [email protected] and MoS2@PSF, respectively. These differences in oil-in-water emulsions adsorption for both nanocomposites could be attributed to the materials flake size in relation to oil droplet size. Both silk-based nanocomposites exhibited excellent recyclability after ten adsorption cycles. Detailed separation mechanisms were introduced for the developed nanocomposites.

ε-Poly-l-lysine-modified natural silk fiber membrane wound dressings with improved antimicrobial properties

Many complex diseases, such as bacterial infections, frequently accompany cutaneous wound healing, adding to the difficulty of clinical wound management. Consequently, in addition to displaying strong biocompatibility and actively promoting wound healing, an optimal wound dressing should also possess antimicrobial qualities to address issues with bacterial infection. This paper developed natural silk fiber (SF) membranes (also known as a flat silk cocoon (FSC)) with antimicrobial properties as a dressing for skin wounds. By changing the spinning tools and environment of silkworm larvae, a novel natural SF membrane with a cocoon structure and controllable size was prepared. The functional SF membranes were obtained via a hot press process and grafted with ε-Poly-l-lysine (EPL). The results showed that the SF membrane dressing was adjustable in size with a similar structure to the extracellular matrix (ECM), displaying inherent mechanical properties, excellent antimicrobial qualities, and biocompatibility. In vivo experiments using a full-thickness skin defect model indicated that EPL-modified SF membranes significantly promoted the rate of wound healing, exhibiting thicker granulation tissue and higher collagen disposition than commercial dressings (Tegaderm™ film). Therefore, the excellent mechanical qualities and cytocompatibility of the antimicrobial EPL-modified SF membranes substantially promote their potential application as a chronic wound dressing.

Tiny NaOH Assisted Facile Preparation of Silk Nanofibers and Their Nanotube-Compositing Strong, Flexible, and Conductive Films.

Natural silk nanofibers (SNFs) can not only be used as good building blocks for two- or three-dimensional biomaterials but also provide a clue for understanding the theory of structure-function relationships. Nevertheless, it is still difficult to directly extract SNFs from natural silk fibers due to their high crystallinity and recalcitrant complex structures. In the present study, a dilute alkali-assisted separation of high-yield SNFs is proposed. The degummed silk was first treated with a tiny amount of alkali at a mild temperature, followed by high-pressure homogenization. Under the optimized conditions (2% sodium hydroxide, 0 °C, 48 h), SNFs with diameters of 8-42 nm and lengths of 0.9 ± 0.3 μm were prepared with yields higher than 75%, which retained the natural structures at the nanoscale and some inherent properties of silk fibers. Interestingly, SNFs can be used as a stabilizing matrix to assist carbon nanotubes (CNTs) to disperse, aiming to form a uniform and stable CNT/SNF dispersion. Thereafter, a strong and flexible conductive composite film was fabricated with good mechanical properties. The composite film showed good piezoelectric properties and electric thermal response, which has promising application prospects for SNFs, such as in optical devices, nanoelectronics, and biosensors.

67. Silk-silk Conduits Filled with Native Spider Silk Fibers Successfully Promoted Nerve Regeneration in a 10 Mm Sciatic Nerve Defect in Rats

67. Silk-silk Conduits Filled with Native Spider Silk Fibers Successfully Promoted Nerve Regeneration in a 10 Mm Sciatic Nerve Defect in Rats

Bio-Inspired 4D Printing of Dynamic Spider Silks.

Spider silks exhibit excellent mechanical properties and have promising application prospects in engineering fields. Because natural spider silk fibers cannot be manufactured on a large scale, researchers have attempted to fabricate bio-inspired spider silks. However, the fabrication of bio-inspired spider silks with dynamically tunable mechanical properties and stimulation–response characteristics remains a challenge. Herein, the 4D printing of shape memory polyurethane is employed to produce dynamic bio-inspired spider silks. The bio-inspired spider silks have two types of energy-absorbing units that can be adjusted, one by means of 4D printing with predefined nodes, and the other through different stimulation methods to make the bio-inspired spider silks contract and undergo spiral deformation. The shape morphing behaviors of bio-inspired spider silks are programmed via pre-stress assemblies enabled by 4D printing. The energy-absorbing units of bio-inspired spider silks can be dynamically adjusted owing to stress release generated with the stimuli of temperature or humidity. Therefore, the mechanical properties of bio-inspired spider silks can be controlled to change dynamically. This can further help in developing applications of bio-inspired spider silks in engineering fields with dynamic changes of environment.

Biomimetic Silk Macroporous Materials for Drug Delivery Obtained via Ice-Templating.

Silk from Bombyx mori is one of the most exciting materials in nature. The apparently simple arrangement of its two major components─two parallel filaments of silk fibroin (SF) coated by a common sericin (SS) sheath─provides a combination of mechanical and surface properties that can protect the moth during its most vulnerable phase, the pupal stage. Here, we recapitulate the topology of native silk fibers but shape them into three-dimensional porous constructs using an unprecedented design strategy. We demonstrate, for the first time, the potential of these macroporous silk foams as dermal patches for wound protection and for the controlled delivery of Rifamycin (Rif), a model antibiotic. The method implies (i) removing SS from silk fibers; (ii) shaping SF solutions into macroporous foams via ice-templating; (iii) stabilizing the SF macroporous foam in a methanolic solution of Rif; and (iv) coating Rif-loaded SF foams with a SS sheath. The resulting SS@SF foams exhibit water wicking capacity and accommodate up to ∼20% deformation without detaching from a skin model. The antibacterial behavior of Rif-loaded SS@SF foams against Staphylococcus aureus on agar plates outperforms that of SF foams (>1 week and 4 days, respectively). The reassembly of natural materials as macroporous foams─illustrated here for the reconstruction of silk-based materials─can be extended to other multicomponent natural materials and may play an important role in applications where controlled release of molecules and fluid transport are pivotal.

Electrospinning of spider silk‐based nanofibers

AbstractSpider silk has garnered attention as a structural material and a medical resource for next generation because it is tough, lightweight, and biodegradable. Nonwoven silk fabrics are emerging in research and industries owing to advanced electrospinning techniques. Several studies have reported nonwoven fabrics made of spider silk‐like proteins, which are originated from recombinant expression systems, with lower molecular weights than the natural counterpart. The lower molecular weight causes a lack of mechanical strength and durability, preventing the evaluation of industrial potential of the spider silk‐based nonwoven fabric to use as a structural and medical material. In this study, an electrospun nonwoven fabric is produced using native spider silk fibers forcibly reeled from live spiders. The spider silk‐based nonwoven fabrics demonstrate cell compatibility, and the mechanical properties are superior to the silkworm silk‐based nonwoven fabrics. The availability of spider silk‐based nonwoven fabrics introduces an unexplored set of potential uses, along with the context of sustainable development goals.

Scalable Lithiophilic/Sodiophilic Porous Buffer Layer Fabrication Enables Uniform Nucleation and Growth for Lithium/Sodium Metal Batteries

AbstractMetallic lithium/sodium (Li/Na) is considered an attractive anode for future high‐energy‐density batteries. The root causes of preventing their applications come from uneven Li/Na nucleation and subsequent dendrite formation. Here, a cost‐efficient and scalable solid‐to‐solid transfer method for dense buffer layer construction on Li/Na anodes is proposed, and thin lithiophilic/sodiophilic buffer layers based on natural silk fibers derived carbon (SFC) and carbon nanotubes (CNTs) composites (denoted as SFC/CNTs) are adopted, which facilitate uniform Li/Na nucleation and dendrite‐free, lateral growth behavior upon recurring Li/Na plating/stripping processes. Lithiopilic/sodiophilic buffer layers enable long‐term cycling stability (>250 cycles) with high Coulombic efficiency (99.2% for Li and 98.8% for Na), low polarization, and flat voltage profiles. More importantly, the cycling performance of LiFePO4|Li pouch cells is largely enhanced with a lifespan of 390 cycles. Further, using ultra‐thin Li anodes (25 μm) also achieves stable LiNi1/3Mn1/3Co1/3O2|Li cells with 200 cycles under a low negative/positive ratio (1.67). Similar achievement is also realized in Na‐metal batteries with negligible capacity fading for over 600 cycles in Na3V2(PO4)3|Na cells, further demonstrating that SFC/CNT buffer layer is technically viable in practical batteries. This study provides a facile strategy for fabricating dense and uniform lithiophilic/sodiophilic buffer layers for low‐cost and scale‐up energy storage devices.

Modeling and Analysis of Surface Roughness Parameters in Drilling of Silk-glass/epoxy Composite

In the recent past, the demand for multifunctional and lightweight materials have increased steadily creating an increase in demand for Hybrid polymer matrix composite which consists multiple fibers in conventional resins. In this study, a hybrid composite comprising of two reinforcements - natural silk fiber and E-Glass fiber - in an Epoxy resin matrix which is a partially eco-friendly composite has been fabricated and the effect of drilling, by using an 8 facet solid carbide drill, on the surface roughness has been studied. Taguchi�s L27 Orthogonal array was used for experimentation by modifying three parameters - feed rate, spindle speed and drill diameter - on three levels (low, medium and high) and thereby studying the effects. From the results of experimentation it has been observed that increase in spindle speed and drill diameter reduces surface roughness however it increases with increase in feed rate. Further, regression analysis and Fuzzy modeling are used in order to determine optimum parameter values to get the desired surface finish. Good agreement between the experimental, regression and fuzzy model is observed with the correlation coefficient of 0.9814 and 0.9677 respectively.

Complexity of Spider Dragline Silk.

The tiny spider makes dragline silk fibers with unbeatable toughness, all under the most innocuous conditions. Scientists have persistently tried to emulate its natural silk spinning process using recombinant proteins with a view toward creating a new wave of smart materials, yet most efforts have fallen short of attaining the native fiber’s excellent mechanical properties. One reason for these shortcomings may be that artificial spider silk systems tend to be overly simplified and may not sufficiently take into account the true complexity of the underlying protein sequences and of the multidimensional aspects of the natural self-assembly process that give rise to the hierarchically structured fibers. Here, we discuss recent findings regarding the material constituents of spider dragline silk, including novel spidroin subtypes, nonspidroin proteins, and possible involvement of post-translational modifications, which together suggest a complexity that transcends the two-component MaSp1/MaSp2 system. We subsequently consider insights into the spidroin domain functions, structures, and overall mechanisms for the rapid transition from disordered soluble protein into a highly organized fiber, including the possibility of viewing spider silk self-assembly through a framework relevant to biomolecular condensates. Finally, we consider the concept of “biomimetics” as it applies to artificial spider silk production with a focus on key practical aspects of design and evaluation that may hopefully inform efforts to more closely reproduce the remarkable structure and function of the native silk fiber using artificial methods.

Investigation of Thermal and Structure Properties of Silk Dyed with a Natural Dye

The present work aimed to dye silk fabric with natural dye extracted from the Artemisia Herba Alba plant from its leaves on pure silk dyeing fabrics with different dyeing conditions such as time, temperature, and pH value in the presence of Alum as a mordant, which had given different colors and shades to promote the use of natural silk fiber. Optimum results were obtained. Silk fabric was dyed using pH 5 for 45 minutes at temperature 80oC and 1:40 liquor ratio. Finally, the dyed mordant samples were thoroughly washed and dried at ambient conditions. The results obtained us successfully characterized dyed silk fabric by studying with a Fourier Transform Infrared thermogravimetric (TGA) properties, (FTIR) analysis technique, and X-ray diffraction (XRD). Improvement in the dyeing process may have been attributed to the improvement in the molecular structure of the chemical bonds in silk fabrics. This work gives an opportunity to use a new traditional natural dye that meets the future environmental technology that exhibited fantastic dyed patterns of high quality.

Fabrication of silk fibroin film enhanced by acid hydrolyzed silk fibroin nanowhiskers to improve bacterial inhibition and biocompatibility efficacy

In this study, silk fibroin nanowhiskers (SNWs) were extracted from natural silk fiber by sulfuric acid hydrolysis with the assistance of ultrasonic wave treatment. The obtained SNWs were mixed with regenerated silk fibroin (RSF) solution to fabricate the SNWs/RSF films. The fabricating SNWs were systematically characterized by using SEM, FTIR, and the SNWs/RSF films were observed by digital camera, PM, etc. The results show that the monodisperse SNWs are evenly distributed in the RSF film. The presence of SNWs in RSF film significantly improves the performances of the film, including the swelling ability, mechanical properties, hydrophilicity, antibacterial efficacy, cytocompatibility. Meanwhile, the SNWs/RSF film can endorse the wound healing efficiency in vivo mice wound site. The proposed techniques for extracting SNWs and fabricating silk fibroin composite film may provide a valuable method for creating an ideal silk-based material for biomedical applications.

Artificial and natural silk materials have high mechanical property variability regardless of sample size

Silk fibres attract great interest in materials science for their biological and mechanical properties. Hitherto, the mechanical properties of the silk fibres have been explored mainly by tensile tests, which provide information on their strength, Young’s modulus, strain at break and toughness modulus. Several hypotheses have been based on these data, but the intrinsic and often overlooked variability of natural and artificial silk fibres makes it challenging to identify trends and correlations. In this work, we determined the mechanical properties of Bombyx mori cocoon and degummed silk, native spider silk, and artificial spider silk, and compared them with classical commercial carbon fibres using large sample sizes (from 10 to 100 fibres, in total 200 specimens per fibre type). The results confirm a substantial variability of the mechanical properties of silk fibres compared to commercial carbon fibres, as the relative standard deviation for strength and strain at break is 10–50%. Moreover, the variability does not decrease significantly when the number of tested fibres is increased, which was surprising considering the low variability frequently reported for silk fibres in the literature. Based on this, we prove that tensile testing of 10 fibres per type is representative of a silk fibre population. Finally, we show that the ideal shape of the stress–strain curve for spider silk, characterized by a pronounced exponential stiffening regime, occurs in only 25% of all tested spider silk fibres.

Construction and Biocompatibility Evaluation of Fibroin/Sericin-Based Scaffolds.

Because tissue responses to implants determine the success or failure of tissue engineering products, fibroin/sericin-based scaffolds including bionic silk scaffolds, native silk fibers, fibroin fibers, and regenerated fibroin have been fabricated, and their biocompatibility was investigated. Fibroin/sericin-based scaffolds were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Bionic silk scaffolds were beneficial to silk fiber formation through self-assembly. Histological and immunofluorescent staining analysis demonstrated that bionic silk scaffolds did not show significant inflammatory responses. Immunization analysis showed that soluble fibroin and sericin did not show obvious immunogenicity. This work supplied an effective approach to design fibroin/sericin-based scaffolds for tissue engineering and drug delivery.