Natural Silk Fibers Research Articles

Strength of Recluse Spider's Silk Originates from Nanofibrils.

Spider silk exhibits a combination of outstanding tensile strength and extensibility unique among all synthetic and biogenic polymer fibers. It has thus generated great interest to understand protein-based high-toughness materials and inspired the design of similar synthetic materials. The unrivaled properties of silk fibers have been recognized to be intimately related to their hierarchical structure. However, in the absence of unambiguous experimental evidence, competing and incompatible structural models of natural silk fibers have been proposed, some of them including various types of fibrillar components. Here we show that the fibers of the recluse (Loxosceles) spider exhibit the typical tensile properties of a very good spider silk and are entirely composed of 20 nm diameter protein fibrils that are more than 1 μm long. Based on these findings, we developed the most detailed structural model for any silk directly supported by experimental evidence. Our work suggests that all the key properties of a spider silk are implemented within a single nanofibril, and we have isolated and imaged such a nanofibril from a native spider silk fiber. The nanofibril breaking force was estimated to be ≈120 nN. Our work underlines the importance of nanofibrils and furthers the understanding of the structure-property relationships of silk, with wide-ranging implications for silk research and the design of silk-inspired high-performance materials.

Isolation of Silk Mesostructures for Electronic and Environmental Applications

AbstractA ubiquitous feature of natural silk fibers is the presence of well‐organized mesostructures, including microfibrils, nanofibrils, and nanoparticles. These mesoscale building blocks are typically well organized into sophisticated arrangements and contribute robust mechanical performance and functions as part of natural silk fibers. However, it remains a major challenge to directly isolate these mesostructures for engineering applications. Here, an environmentally friendly and scalable “partial dissolution and physical dispersion” strategy is developed to exfoliate silk fibers into different mesostructures, including microfibrils, nanofibrils, nanorods, and nanoparticles. On the basis of the advantages of these mesosilks in tunable sizes, sharp size distributions, high modulus, excellent redispersibility, as well as versatile processability, the applications of these mesosilks in electronic and environmental fields are further explored, including water treatment, recycling organic solvent, paper sensors, and nanofertilizers. These explorations open a new avenue for silk fiber applications while also providing a pathway to help address critical issues in electronic and environmental fields.

The Energy Requirements for Flow-Induced Solidification of Silk.

Natural silk spinning has undergone strong selection for resource efficiency and thus presents a biomimetic ideal for fiber production. Industrial replication of natural silk fibers would enable access to low energy, cost-efficient processing, but is hampered by a lack of understanding surrounding the conversion of liquid feedstock into a solid fiber as a result of flow. Previously, shear stress, shear rate, or time have been presented as criteria for silk fiber formation, but here it is proposed that spinning requires carefully balancing all three, and is a result of controlled energy accumulation in the feedstock. To support this hypothesis, rheology is used to probe the energy required for conversion, compare differences between amorphous solid and ordered fiber production and explain the energetic penalty the latter demands. New definitions of what constitutes an artificial silk fiber are discussed, along with methods to ensure that each spinning criterion is met during biomimetic spinning.

Modeling the universal viscoelastic response of polymer fibers

Polymer fibers, including natural silk and synthetic fibers, exhibit universal viscoelastic response. On stretching below yield, they show logarithmic stress decay. On unloading fibers with a glassy amorphous phase, the stress recovers. A simple phenomenological model accurately describes data from independent mechanical experiments and provides insights into the microstructural origins of the fiber response. Counter to intuition, the model indicates that it is the crystalline regions, rather than the amorphous glass, that deform first on stretching fibers at high strain rates. On holding a stretched fiber, stress decays as a consequence of relaxations in amorphous regions. Finally, unloading the fiber transfers stress from the amorphous to crystalline regions resulting in stress recovery. Model parameters correlate well with the fiber microstructure. Crystal and amorphous moduli from the model match those from x-ray diffraction. Activation energies for the temperature dependence of the peak relaxation time are similar to those reported in the literature. Thus, a simple model that invokes only crystal-amorphous coexistence can successfully model the mechanical response of a wide variety of polymer fibers.

From powder to cloth: Facile fabrication of dense MOF-76(Tb) coating onto natural silk fiber for feasible detection of copper ions

The deposition of powdered MOFs material onto other substrates is essential to avoid inconvenience during its practical applications. In this work, domestic silk fiber was utilized as the skeleton, for successful coating of dense luminescent MOF-76(Tb). Its surface functionality which consist of abundance of intrinsic carboxylic groups, smooth surface structure, and 80% of tensile strength were maintained after being immersed in different thermal solvents (water, ethanol, DMF @ 80 °C) for 24 h, revealing good solvent and thermal resistance. By using hydrothermal, microwave assisted, and layer-by-layer methods, different crystal morphologies (pillar-like, sedimentary-rock-like, and needle-like morphology) and varying degrees of surface coverage rate were obtained, as a result of different levels of anchoring promotion and crystal controlling effect. The MOFs coating can be confirmed by its XRD pattern and fluorescent property. More importantly, the quenching effect of the composite in a condition of Cu2+ was first reported with high selectivity, sensitivity (i.e. a linear detection concentration range of 10−3–10−5 M with a low detection limit up to 0.5 mg/L, KSV of 1192 M−1 at 293 K), and rapid response time (5 min), making the composite a good candidate for colorimetric and fluorescent detection of aquatic Cu2+. The quenching mechanism is proposed to associate with the interaction between Cu2+ and benzene-tricarboxylate (BTC) ligand, which resulted in the decrease of energy transfer efficiency. The selectivity over other common cations depends on the unsaturated electron configuration and the smaller ionic radius of Cu2+.

Combination of Amorphous Silk Fiber Spinning and Postspinning Crystallization for Tough Regenerated Silk Fibers.

An artificial spinning system using regenerated silk fibroin solutions is adopted to produce high-performance silk fibers. In previous studies, alcohol-based agents, such as methanol or ethanol, were used to coagulate silk dope solutions, producing silk fiber with poor mechanical properties compared with those of native silk fibers. The alcohol-based coagulation agents induce rapid β-sheet crystallization of the silk molecules, which inhibits subsequent alignment of the β-sheet crystals. Here, we induce gradual β-sheet formation to afford adequate β-sheet alignment similar to that of native silk fiber. To this aim, we developed an amorphous silk fiber spinning process that prevents fast β-sheet formation in silk molecules by using tetrahydrofuran (THF) as a coagulation solvent. In addition, we apply postdrawing to the predominantly amorphous silk fibers to induce β-sheet formation and orientation. The resultant silk fibers showed a 2.5-fold higher extensibility, resulting in 1.5-fold tougher silk fibers compared with native Bombyx mori silk fiber. The amorphous silk fiber spinning process developed here will pave the way to the production of silk fibers with desired mechanical properties.

Silk nanofibril self-assembly versus electrospinning.

Natural silk fibers represent one of the most advanced blueprints for (bio)polymer scientists, displaying highly optimized mechanical properties due to their hierarchical structures. Biotechnological production of silk proteins and implementation of advanced processing methods enabled harnessing the potential of these biopolymer not just based on the mechanical properties. In addition to fibers, diverse morphologies can be produced, such as nonwoven meshes, films, hydrogels, foams, capsules and particles. Among them, nanoscale fibrils and fibers are particularly interesting concerning medical and technical applications due to their biocompatibility, environmental and mechanical robustness as well as high surface-to-volume ratio. Therefore, we introduce here self-assembly of silk proteins into hierarchically organized structures such as supramolecular nanofibrils and fabricated materials based thereon. As an alternative to self-assembly, we also present electrospinning a technique to produce nanofibers and nanofibrous mats. Accordingly, we introduce a broad range of silk-based dopes, used in self-assembly and electrospinning: natural silk proteins originating from natural spinning glands, natural silk protein solutions reconstituted from fibers, engineered recombinant silk proteins designed from natural blueprints, genetic fusions of recombinant silk proteins with other structural or functional peptides and moieties, as well as hybrids of recombinant silk proteins chemically conjugated with nonproteinaceous biotic or abiotic molecules. We highlight the advantages but also point out drawbacks of each particular production route. The scope includes studies of the natural self-assembly mechanism during natural silk spinning, production of silk fibrils as new nanostructured non-native scaffolds allowing dynamic morphological switches, as well as studying potential applications. This article is categorized under: Biology-Inspired Nanomaterials > Peptide-Based Structures Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Controllable exfoliation of natural silk fibers into nanofibrils by protein denaturant deep eutectic solvent: nanofibrous strategy for multifunctional membranes

A protein denaturant deep eutectic solvent to exfoliate natural silk fibers into nanofibrils for multifunctional membranes with enhanced properties.

A Study on Mechanical Properties of Silk Fiber Reinforced Epoxy Resin Bio-Composite With SiC As Filler Addition

The present work describes an attempt to develop hybrid bio-composite material using raw untreated dupion silk fiber reinforced in epoxy resin as matrix in addition with silicon carbide powder of 220 grit number as filler material with varying percentage of 0%,2%,4% and 6% by weight, the procured raw materials were fabricated using the hand lay-up technique. The different mechanical testing namely tensile, flexure, hardness and S.E.M tests were conducted on the standard samples prepared. In this case, by their application in orthopedic as implantable material in the bone fixation has been considered and studied. In this work it is found that tensile strength of 6% silicon carbide filled with dupion silk fiber reinforced epoxy bio-composite material obtained about 41.4 MPa, flexural strength about 53 Mpa and hardness value about 88 RBHN. Hence in this work Tensile strength and flexural strength of 6% silicon carbide filled natural dupion silk fiber reinforcement epoxy bio-composite material matches the femur bone’s tensile and flexural strength while comparing; hence this material can be suitable for the human body bone replacement subjecting to suitable coating fabrication.

Regenerated silk matrix composite materials reinforced by silk fibres: Relationship between processing and mechanical properties

Silk is a biocompatible material with remarkable mechanical properties. A variety of composite materials using silk as a matrix or for reinforcement have been prepared for many biological applications. However, although a range of composites containing silk have been designed, the mechanical properties achieved for the regenerated silk (solubilisation + film/matrix or fibre production) remain relatively weak (strength: 30–50 MPa). In this study, by combining the high ultimate strength (up to 1 GPa) of natural silk fibres and the chemical properties of regenerated silk, we developed a composite based on silk alone. We investigated how to improve the tensile mechanical properties of silk matrices derived from Bombyx mori silkworms by film processing and by incorporating silk fibres, either natural or degummed, that is, without the sericin coating. The structure and orientation modifications of fibroin macromolecules within the matrix were monitored by Raman spectroscopy; unexpectedly it showed that the most amorphous fibroin matrix exhibits the best mechanical behaviour. The mechanical behaviour of regenerated silk-based materials is dependent upon the temperature of evaporation and the filtration of the solution. In summary, a biocompatible composite was obtained here with distinct mechanical properties (strength >100 MPa; Young's modulus ∼2.5 GPa) which can be optimised for focused applications with silk fibres (slow degradation, skeleton of the composite) and silk matrix (fibroin molecules accessible to chemical or biophysical modification).

Comparative Study of Strain-Dependent Structural Changes of Silkworm Silks: Insight into the Structural Origin of Strain-Stiffening.

Structure-property relationships of silk is an intriguing topic for silk-based biomaterials research since these features are related to biomimicking the processing in natural silk fiber formation which results in excellent mechanical properties. Strain-stiffening is common for spider silks and nonmulberry silkworm silks. However, the structural origin of strain-stiffening remains unclear. In this paper, the strain-dependent structural change of Antheraea pernyi silkworm silk is studied by X-ray fiber diffraction and Fourier transform infrared spectroscopy under stretching. Based on a combination of mechanical and structural analysis, the molecular origins of strain-stiffening in A. pernyi silk were determined. The relatively high content of the β-sheets within the amorphous domains in A. pernyi silk is responsible for strain-stiffening, where "molecular spindles" enhance the extensibility and toughness of the fiber.

Natural Flavonoid-Functionalized Silk Fiber Presenting Antibacterial, Antioxidant, and UV Protection Performance

Natural bioactive compounds as promising alternatives to synthetic finishing agents have recently gained increasing attention in the textile industry due to their eco-friendliness, low irritation, and biocompatibility. The present study reports a sustainable approach for preparing antibacterial, antioxidant, and UV-protective silk fiber using two natural flavonoids (quercetin and rutin) by an adsorption technique. The adsorption kinetics and isotherms of the two flavonoids were investigated, and their functionalities and the washing durability of their functionalities were discussed. The equilibrium adsorption isotherms fitted well to the Langmuir and Freundlich adsorption models, demonstrating that ion–ion interactions, hydrogen bonding, and van der Waals forces play major roles in the adsorption of quercetin and rutin on silk. The adsorption isotherm parameters of quercetin and rutin had a decisive effect on their adsorption kinetics, which fit well to the pseudo-second-order kinetic equation. Quercetin...

Self-Assembling Silk-Based Nanofibers with Hierarchical Structures.

Self-assembling fibrous supramolecular assemblies with sophisticated hierarchical structures at the mesoscale are of interest from both fundamental and applied engineering. In this paper, the relatively hydrophilic domains of silk fibroin (HSF) were extracted and used in studies of self-assembly. The HSF fraction spontaneously self-assembled into nanofibers, 10 to 100 μm long and 50 to 250 nm in diameter, within 2 to 8 h in aqueous conditions. Interestingly, these HSF nanofibers consisted of dozens of nanofibrils arranged in a parallel organization with assembled diameters of ∼30 nm, similar to the sophisticated hierarchical structure observed in native silk fibers. Dynamic morphology and conformation studies were carried out to determine the mechanisms underlying the HSF self-assembly process at both the nanoscale and mesoscale. The HSF self-assembled into nanofibers in a bottom-to-up manner, from "sticky" colloid particles to cylindrical globules, to form nanofibrous networks. Because of the enhanced HSF self-assembling kinetics and the hierarchical structure of HSF nanofibers, this hydrophilicity-driven approach provides further insight into silk fibroin (SF) self-assembly in vivo and also offers new tools for the recapitulation of high-performance materials for engineering applications.

Silk Resin with Hydrated Dual Chemical-Physical Cross-Links Achieves High Strength and Toughness.

Native silk fibers are known to demonstrate excellent mechanical properties such as high strength and ductility. However, regenerated silk material has not yet been used as a tough structural material in our everyday life. To recreate the mechanical properties with regenerated silk material, the network structure and hydration state of silk materials are studied and optimized in this study. This is the first to demonstrate the effect of chemical and physical cross-links in hydrated and dehydrated silk materials, namely, silk hydrogels and resins. Mild hydration conditions (relative humidity 20-60%) realizes tough and strong silk materials with chemical and physical cross-links. In the case of relatively high concentrations of silk molecules, contributions to the high strength and toughness of silk-based materials are considered to come not only from β-sheet cross-links and chemical dityrosine links but also from entanglements and assembly via the hydrophobic interactions of silk molecules. In addition, dehydration treatment does not disturb the biodegradability of the silk resins in natural environments. Based on the overall results, the silk resins with controlled network structures and hydration state have successfully achieved the highest toughness possible for a bulk silk material while maintaining favorable biodegradability.

Silk: Optical Properties over 12.6 Octaves THz-IR-Visible-UV Range.

Domestic (Bombyx mori) and wild (Antheraea pernyi) silk fibers were characterised over a wide spectral range from THz 8 cm ( 1.25 mm, 0.24 THz) to deep-UV cm ( 200 nm, 1500 THz) wavelengths or over a 12.6 octave frequency range. Spectral features at -sheet, -coil and amorphous fibroin were analysed at different spectral ranges. Single fiber cross sections at mid-IR were used to determine spatial distribution of different silk constituents and revealed an -coil rich core and more broadly spread -sheets in natural silk fibers obtained from wild Antheraea pernyi moths. Low energy T-ray bands at 243 and 229 cm were observed in crystalline fibers of domestic and wild silk fibers, respectively, and showed no spectral shift down to 78 K temperature. A distinct cm band was observed in the crystalline Antheraea pernyi silk fibers. Systematic analysis and assignment of the observed spectral bands is presented. Water solubility and biodegradability of silk, required for bio-medical and sensor applications, are directly inferred from specific spectral bands.

A host-guest approach to fabricate metallic cobalt nanoparticles embedded in silk-derived N-doped carbon fibers for efficient hydrogen evolution

Hydrogen evolution reaction (HER) plays a key role in generating clean and renewable energy. As the most effective HER electrocatalysts, Pt group catalysts suffer from severe problems such as high price and scarcity. It is highly desirable to design and synthesize sustainable HER electrocatalysts to replace the Pt group catalysts. Due to their low cost, high abundance and high activities, cobalt-incorporated N-doped nanocarbon hybrids are promising candidate electrocatalysts for HER. In this report, we demonstrated a robust and eco-friendly host-guest approach to fabricate metallic cobalt nanoparticles embedded in N-doped carbon fibers derived from natural silk fibers. Benefiting from the one-dimensional nanostructure, the well-dispersed metallic cobalt nanoparticles and the N-doped thin graphitized carbon layer coating, the best Co-based electrocatalyst manifests low overpotential (61 mV@10 mA/cm2) HER activity that is comparable with commercial 20% Pt/C, and good stability in acid. Our findings provide a novel and unique route to explore high-performance noble-metal-free HER electrocatalysts.

Tyrosine Templating in the Self-Assembly and Crystallization of Silk Fibroin.

Native silk fibers exhibit strength and toughness that rival those of the best synthetic fibers. Despite significant research, further insight is still needed to understand the mechanisms by which silkworms are capable of spinning such tough fibers. Here we propose that π-π and π-OH group interactions of tyrosine side chains provide templating effects, such that the crystal-forming domains are in registration, thereby fostering the self-assembly of the spinning dope. Intrinsic fluorescence measurements, in conjunction with circular dichroism, showed that during self-assembly of regenerated silk solutions, the tyrosine residues were localized in a more hydrophobic local environment, suggesting preferential assembly. In situ Fourier transform infrared spectroscopy indicated that cross-linking of the tyrosine residues resulted in the development of extended β-sheet structure. Additionally, control of cross-link density directly influenced the degree of crystallinity upon drying. Molecular dynamics simulations were performed on silk mimetic peptides in order to more thoroughly understand the role of tyrosines. The results indicated that tyrosine residues tended to transiently colocate in solution due to π-π interactions and hydrogen bonds with adjacent residues and with the peptide backbone. These more stable tyrosine interactions resulted in reduced lateral chain fluctuations and increased incidence of coordinated intrachain association, while introduction of a dityrosine bond directly promoted the formation of β-sheet structures. In total, the experimental and modeling data support a critical role for tyrosine-tyrosine interactions as a key early feature in the self-assembly of regenerated silk protein chains and therefore are important in the robust and unusual mechanical properties ultimately achieved in the process.

Regenerated silk materials for functionalized silk orthopedic devices by mimicking natural processing

Silk fibers spun by silkworms and spiders exhibit exceptional mechanical properties with a unique combination of strength, extensibility and toughness. In contrast, the mechanical properties of regenerated silk materials can be tuned through control of the fabrication process. Here we introduce a biomimetic, all-aqueous process, to obtain bulk regenerated silk-based materials for the fabrication of functionalized orthopedic devices. The silk materials generated in the process replicate the nano-scale structure of natural silk fibers and possess excellent mechanical properties. The biomimetic materials demonstrate excellent machinability, providing a path towards the fabrication of a new family of resorbable orthopedic devices where organic solvents are avoided, thus allowing functionalization with bioactive molecules to promote bone remodeling and integration.

Spider Silk-CBD-Cellulose Nanocrystal Composites: Mechanism of Assembly.

The fabrication of cellulose-spider silk bio-nanocomposites comprised of cellulose nanocrystals (CNCs) and recombinant spider silk protein fused to a cellulose binding domain (CBD) is described. Silk-CBD successfully binds cellulose, and unlike recombinant silk alone, silk-CBD self-assembles into microfibrils even in the absence of CNCs. Silk-CBD-CNC composite sponges and films show changes in internal structure and CNC alignment related to the addition of silk-CBD. The silk-CBD sponges exhibit improved thermal and structural characteristics in comparison to control recombinant spider silk sponges. The glass transition temperature (Tg) of the silk-CBD sponge was higher than the control silk sponge and similar to native dragline spider silk fibers. Gel filtration analysis, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (TEM) indicated that silk-CBD, but not the recombinant silk control, formed a nematic liquid crystalline phase similar to that observed in native spider silk during the silk spinning process. Silk-CBD microfibrils spontaneously formed in solution upon ultrasonication. We suggest a model for silk-CBD assembly that implicates CBD in the central role of driving the dimerization of spider silk monomers, a process essential to the molecular assembly of spider-silk nanofibers and silk-CNC composites.

Conferring biological activity to native spider silk: A biofunctionalized protein-based microfiber.

Spider silk is an extraordinary material with physical properties comparable to the best scaffolding/structural materials, and as a fiber it can be manipulated with ease into a variety of configurations. Our work here demonstrates that natural spider silk fibers can also be used to organize biological components on and in devices through rapid and simple means. Micron scale spider silk fibers (5-10 μm in diameter) were surface modified with a variety of biological entities engineered with pentaglutamine tags via microbial transglutaminase (mTG). Enzymes, enzyme pathways, antibodies, and fluorescent proteins were all assembled onto spider silk fibers using this biomolecular engineering/biofabrication process. Additionally, arrangement of biofunctionalized fiber should in of itself generate a secondary level of biomolecular organization. Toward this end, as proofs of principle, spatially defined arrangement of biofunctionalized spider silk fiber was shown to generate effects specific to silk position in two cases. In one instance, arrangement perpendicular to a flow produced selective head and neck carcinoma cell capture on silk with antibodies complexed to conjugated protein G. In a second scenario, asymmetric bacterial chemotaxis arose from asymmetric conjugation of enzymes to arranged silk. Overall, the biofabrication processes used here were rapid, required no complex chemistries, were biologically benign, and also the resulting engineered silk microfibers were flexible, readily manipulated and functionally active. Deployed here in microfluidic environments, biofunctional spider silk fiber provides a means to convey complex biological functions over a range of scales, further extending its potential as a biomaterial in biotechnological settings. Biotechnol. Bioeng. 2017;114: 83-95. © 2016 Wiley Periodicals, Inc.