Mechanical Properties Of Silk Fibers Research Articles

Influence of experimental methods on the mechanical properties of silk fibers: A systematic literature review and future road map.

Spider silk fibers are of scientific and industrial interest because of their extraordinary mechanical properties. These properties are normally determined by tensile tests, but the values obtained are dependent on the morphology of the fibers, the test conditions, and the methods by which stress and strain are calculated. Because of this, results from many studies are not directly comparable, which has led to widespread misconceptions in the field. Here, we critically review most of the reports from the past 50 years on spider silk mechanical performance and use artificial spider silk and native silks as models to highlight the effect that different experimental setups have on the fibers' mechanical properties. The results clearly illustrate the importance of carefully evaluating the tensile test methods when comparing the results from different studies. Finally, we suggest a protocol for how to perform tensile tests on silk and biobased fibers.

Dynamic Changes and Characterization of the Metal Ions in the Silk Glands and Silk Fibers of Silkworm

Metal ions are involved in the conformational transition of silk fibroin and influence the structure and mechanical properties of silk fibers. However, the dynamic characteristics of metal ions during the formation of silk fibers remain unclear. In this study, we found that the silk glands of silkworms contain various metal elements, with varying levels of the metal elements in different zones of the glands and higher levels in the anterior silk glands. Additionally, the content of various metallic elements in the silk glands varied greatly before and after spinning, similar to their content in different cocoon layers, thus, indicating that the anterior silk glands maintain a certain metal ion environment for the transport and conformational transformation of the silk proteins. Most of the metallic elements located in fibroin were confirmed using degumming experiments. For the first time, a scanning electron microscope energy spectrometry system was used to characterize the metal elements in the cross-section of silk and cocoons. These findings have deepened our understanding of the relationship between the overall metal ion environment and silk fiber formation and help us further conceptualize the utilization of metal ions as targets to improve the mechanical properties of the silk fibers.

Combined CRISPR toolkits reveal the domestication landscape and function of the ultra-long and highly repetitive silk genes.

Highly repetitive sequences play a major structural and function role in the genome. In the present study, we developed Cas9-assisted cloning and SMRT sequencing of long repetitive sequences (CACS) to sequence and manipulate highly repetitive genes from eukaryotic genomes. CACS combined Cas9-mediated cleavage of a target segment from an intact genome, Gibson assembly cloning, and PacBio SMRT sequencing. Applying CACS, we directly cloned and sequenced the complete sequences of fibroin heavy chain (FibH) genes from 17 domesticated (Bombyx mori) and 7 wild (Bombyx mandarina) silkworms. Our analysis revealed the unique fine structure organization, genetic variations, and domestication dynamics of FibH. We also demonstrated that the length of the repetitive regions determined the mechanical properties of silk fiber, which was further confirmed by Cas9 editing of FibH. CACS is a simple, robust, and efficient approach, providing affordable accessibility to highly repetitive regions of a genome. STATEMENT OF SIGNIFICANCE: Silkworm silk is the earliest and most widely used animal fiber, and its excellent performance mainly depends on the fibroin heavy chain (FibH) protein. The FibH gene is the main breakthrough in understanding the formation mechanism and improvement of silk fiber. In the study, we developed a CACS method for characterizing the fine structure and domestication landscape of 24 silkworm FibH genes. We used CRISPR/Cas9 to edit the repetitive sequence of FibH genes, revealing the relationship between FibH genes and mechanical properties of silkworm silk. Our study is helpful in modifying silk genes to manipulate other valuable highly repetitive sequences, and provides insight for silkworm breeding.

Predicting mechanical properties of silk from its amino acid sequences via machine learning

The silk fiber is increasingly being sought for its superior mechanical properties, biocompatibility, and eco-friendliness, making it promising as a base material for various applications. One of the characteristics of protein fibers, such as silk, is that their mechanical properties are significantly dependent on the amino acid sequence. Numerous studies have been conducted to determine the specific relationship between the amino acid sequence of silk and its mechanical properties. Still, the relationship between the amino acid sequence of silk and its mechanical properties is yet to be clarified. Other fields have adopted machine learning (ML) to establish a relationship between the inputs, such as the ratio of different input material compositions and the resulting mechanical properties. We have proposed a method to convert the amino acid sequence into numerical values for input and succeeded in predicting the mechanical properties of silk from its amino acid sequences. Our study sheds light on predicting mechanical properties of silk fiber from respective amino acid sequences.

Effect of tourmaline superfine powder additive on mechanical properties of silkworm fiber

• Superfine tourmaline powders of low concentration played a positive role in the growth of silkworm and cocoon quality. • Tourmaline powders absorbed into silkworm improved the mechanical properties of silk. • Tourmaline might provide a wide application prospect in the production of functional silk. This study aims at analyzing the effect of adding superfine tourmaline (Tml) powders on the growth of silkworms, the quality of cocoon, and the mechanical properties of silk fiber. Superfine tourmaline powders and TiO 2 nanoparticles (NPs) were fed to silkworms, respectively, and then the survival and proliferation rate, the weight of cocoon and shells, and the mechanic qualities of silk fiber were investigated. Experimental results demonstrated that the superfine tourmaline powders had no adverse impact on the growth and the survival rate of silkworm. The stress and strain of silk fiber, to which 10mg of Tml were added, increased up to 510.6±48 MPa and 21±2.1%, respectively, which improved by 32.3% and 27.4% on average. Through the analysis of Fourier transforms infrared spectra (FTIR), it has been confirmed that such result was attributable to the increase of random coil/α-helix structure in the silk fiber structure. The research will be of certain significance in simplifying the production mode of silk fiber with more functions by using the radiating far-infrared rays and the spontaneous polarization effect of tourmaline.

BmSuc1 Affects Silk Properties by Acting on Sericin1 in Bombyx mori.

BmSuc1, a novel animal-type β-fructofuranosidase (β-FFase, EC 3.2.1.26) encoding gene, was cloned and identified for the first time in the silkworm, Bombyx mori. BmSuc1 was specifically and highly expressed in the midgut and silk gland of Bombyx mori. Until now, the function of BmSuc1 in the silk gland was unclear. In this study, it was found that the expression changes of BmSuc1 in the fifth instar silk gland were consistent with the growth rate of the silk gland. Next, with the aid of the CRISPR/Cas9 system, the BmSuc1 locus was genetically mutated, and homozygous mutant silkworm strains with truncated β-FFase (BmSUC1) proteins were established. BmSuc1 mutant larvae exhibited stunted growth and decreased body weight. Interestingly, the molecular weight of part of Sericin1 (Ser1) in the silk gland of the mutant silkworms was reduced. The knockout of BmSuc1 reduced the sericin content in the silkworm cocoon shell, and the mechanical properties of the mutant line silk fibers were also negatively affected. These results reveal that BmSUC1 is involved in the synthesis of Ser1 protein in silk glands and helps to maintain the homeostasis of silk protein content in silk fibers and the mechanical properties of silk fibers, laying a foundation for the study of BmSUC1 regulation of silk protein synthesis in silk glands.

Fiber Formation and Mechanical Properties of Bombyx mori Silk Are Regulated by Vacuolar-Type ATPase.

The mechanism of silk fiber formation in silkworms, Bombyx mori, is of particular scientific interest because it is closely related to the mechanical properties of silk fibers. However, there are still substantial knowledge gaps in understanding the details of this mechanism. Studies have found a pH gradient in the silk gland of silkworms. A vacuolar-type ATPase (V-ATPase) is thought to be involved in establishing this pH gradient. Although it is reported that the pH gradient plays a role in silk fibrillogenesis, the direct relationship between V-ATPase and silk mechanical properties is unclear. Thus, this study aims to clarify this relationship. We found that V-ATPase is highly and stably expressed in the anterior silk gland (ASG) and maintains the pH gradient and the fine structure of ASG. Inhibition of V-ATPase activity increased the β-sheet content and crystallinity of silk fibers. Tensile testing showed that the mechanical properties of silk fibers improved after inhibiting V-ATPase activity. All the data suggest that V-ATPase is a key factor in regulating silk fibrillogenesis and is related to the final mechanical properties of the silk fibers. V-ATPase is a potential target for silk mechanical property improvement.

High mechanical property silk produced by transgenic silkworms expressing the spidroins PySp1 and ASG1

Spider silk is one of the best natural fibers with excellent mechanical properties; however, due to the visual awareness, biting behavior and territory consciousness of spiders, we cannot obtain spider silk by large-scale breeding. Silkworms have a spinning system similar to that of spiders, and the use of transgenic technology in Bombyx mori, which is an ideal reactor for producing spider silk, is routine. In this study, the piggyBac transposon technique was used to achieve specific expression of two putative spider silk genes in the posterior silk glands of silkworms: aggregate spider glue 1 (ASG1) of Trichonephila clavipes (approximately 1.2 kb) and two repetitive units of pyriform spidroin 1 (PySp1) of Argiope argentata (approximately 1.4 kb). Then, two reconstituted spider silk-producing strains, the AG and PA strains, were obtained. Finally, the toughness of the silk fiber was increased by up to 91.5% and the maximum stress was enhanced by 36.9% in PA, and the respective properties in AG were increased by 21.0% and 34.2%. In summary, these two spider genes significantly enhanced the mechanical properties of silk fiber, which can provide a basis for spidroin silk production.

In vitro and in vivo degradation of silk fibers degummed with various sodium carbonate concentrations

Silk fiber-based scaffolds have attracted growing attentions as promising candidate biomaterials for tissue regeneration and repair due to its unusual features. However, the slow degradability of silk fiber endowed by hierarchical structure has restricted its clinical application. Degumming is a routine process to remove sericin which allows a controllable destruction on the structure of silk fibers, thus tunes its degradation. In the present study, an attempt to tune the biodegradation period of silk through degumming with various sodium carbonate concentrations was studied. The results showed that the weight loss, surface erosion and the decrease of mechanical properties of silk fibers depended on the sodium carbonate concentrations. Mostly, the degradation behavior of degummed silk fibers could be tuned through degumming conditions, thus to match the diverse needs of tissue regeneration with specific functional requirements and repair rates. The implantation of degummed silk fibers caused mild inflammatory response, suggesting good compatibility. The information gained in the present study is important for the further development of silk fiber-based scaffolds with microfibrous surface, mechanical properties and biodegradation tuned to specific biomedical applications.

Effect of basalt fiber hybridization on mechanical properties of silk fiber reinforced epoxy composites

Poor mechanical properties and constraints on production presently limit the utilization of bio-based reinforcing agents to non-structural and structural automotive elements. The conjugation of natural fibre with volcanic rock fibre provides a way to improve the mechanical properties of composites over natural fibre alone. In this study, physico-mechanical properties of hybrid fibre (silk and basalt) reinforced epoxy composites were found by experimentation following acceptable ASTM standards. Hybrid composites were produced by combining silk/basalt fibres in the ratio of 50:0, 25:25 and 30:20, whereas overall weight fraction was maintained at 0.5. The experimental results showed that the performance of combined fibres were superior compared to that of silk fibre bolstered epoxy composites. Among the 2 varieties of hybrids, the silk/basalt (25:25 by weight ratio) combination offered the very best hardness, strength, modulus, and toughness to the epoxy matrix owing to the similar modulus and synergistic interaction between the two reinforcing fibres. The results also steered that the morphology and surface adhesion affected the strength of the hybrid composites. These observations give insight into the advantages of various fibre reinforcements to the mechanical performance of epoxy matrix which is considered to be brittle. The failure mechanisms and the adhesion between fibres and matrix were studied by analysing the photomicrographs of broken coupons.

Disruption of the Metal Ion Environment by EDTA for Silk Formation Affects the Mechanical Properties of Silkworm Silk.

Silk fiber has become a research focus because of its comprehensive mechanical properties. Metal ions can influence the conformational transition of silk fibroin. Current research is mainly focused on the role of a single ion, rather than the whole metal ion environment. Here, we report the effects of the overall metal ion environment on the secondary structure and mechanical properties of silk fibers after direct injection and feeding of silkworms with EDTA. The metal composition of the hemolymph, silk gland, and silk fiber changed significantly post EDTA treatment. Synchrotron FTIR analysis indicated that the secondary structure of silk fiber after EDTA treatment changed dramatically; particularly, the β-sheets decreased and the β-turns increased. Post EDTA treatment, the silk fiber had significantly decreased strength, Young’s modulus, and toughness as compared with the control groups, while the strain exhibited no obvious change. These changes can be attributed to the change in the metal ion environment in the silk fibroin and sericin in the silk gland. Our investigation provides a new theoretical basis for the natural silk spinning process, and our findings could help develop a method to modify the mechanical properties of silk fiber using metal ions.

Secondary structure transformation and mechanical properties of silk fibers by ultraviolet irradiation and water

Probing the photo-degradation of silk fibers is helpful to protect silk cultural relics. Studying the interactions between sericin and fibroin is essential for understanding the ageing degradation ...

A strategy for improving the mechanical properties of silk fiber by directly injection of ferric ions into silkworm

Silk fibers produced by the silkworm, Bombyx mori, are widely used bio-materials due to their superior toughness and elongation. However, the stress and stiffness of silkworm silk are not comparable to the synthetic fiber or other natural silk such as the spider silk. How to improve the mechanical properties of silk fibers has been of great interest. In this paper, we developed a new strategy for improving the mechanical properties of silk fibers by directly injecting ferric ions (Fe3+) into silkworm. The Fe3+-injected silkworms could produce robust silk fibers with improved stress, stiffness and toughness. Secondary structural analysis indicated that Fe3+ promoted the conformation transition of silk from random coil or helical structure to β-sheet structure. These evidences might explain why these fibers became stronger, stiffer and tougher. Trying to unravel the molecular foundation behind this interesting phenomenon, turbidity assays and fluorescence spectroscopy were introduced. The results indicated that Fe3+ was able to interact with tyrosine and tryptophan residues within the silk. The crosslinking might act as the “bridge” to form the β-sheet structure, thus increasing the mechanical properties of silk fiber. These findings suggested that Fe3+ could be a promising target to modify the mechanical properties of silk fibers.

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.

In situ biomineralization by silkworm feeding with ion precursors for the improved mechanical properties of silk fiber

Possessing excellent biocompatibility, biodegradability and good reactive activity, silk fiber has been attracting great attention in biomedicine including surgical suture, drug delivery and tissue engineering. So far, several protocols have been developed to further improve the mechanical properties of the silk fiber. In current study, a novel in suit biomineralization strategy was developed to produce nano-hydroxyapatite (HA) strengthened silk fiber based on the natural alkaline condition in the body of silkworm by feeding the silkworm with ion precursors of Ca2+ and PO43− ions. Our observation proved that nanocomposite silk fiber contained more α-helix and random coil structures and fewer β-sheets. Tensile test showed that such obtained silk fiber has superior mechanical properties compared to normal silk fiber. To the best of our knowledge, no attempts have been made to fabricate the nanocomposite SF by in situ biomineralization and such protocol established in the current study may shed light on the investigation of nanoparticles reinforced silk fiber organisms of Bombyx mori.

Micromechanics of fresh and 30-year-old Nephila inaurata madagascariensis dragline silk

We report tensile tests for the dragline silk of Nephila madagascariensis golden silk spider strained at 7% min−1. The average Young’s modulus (8.2 GPa), work of fracture (250 MJ m−3) and ultimate strain (28%) fall in the same range as for most previously tested spider silks. The average strength is rather low yet (645 MPa in the engineering convention), even in comparison with other Nephila spiders having similar secondary structures (as controlled by polarized Raman microscopy). The scattering of stress/strain failure values is often seen as an obstacle for the unequivocal characterization of silks, but our study evidences a great shape similarity between normalized tensile curves. The so-called strain-hardening region may be divided into a nonlinear regime (nLHR) and a linear one (LHR). The nLHR tends to disappear on averaged spectra, suggesting that it does not correspond to any specific mechanism. Instead, we propose that the plastic deformation to linear hardening transition threshold changes from one point of each fiber to the other, as a result of local heterogeneities. No major aging effect was found comparing the average properties of fresh and 30-year-old fibers, but the lower mechanical variability of the latter reveals a structural consolidation over time, to the detriment of the plastic deformation plateau. The relative extension of this plateau and both hardening regimes, the deviation to linearity in the nLHR and a newly defined e T strain may be used to compare the mechanical properties of silk fibers as a function of age, origin or testing conditions.

In vivo effects of metal ions on conformation and mechanical performance of silkworm silks

BackgroundThe mechanism of silk fiber formation is of particular interest. Although in vitro evidence has shown that metal ions affect conformational transitions of silks, the in vivo effects of metal ions on silk conformations and mechanical performance are still unclear. MethodsThis study explored the effects of metal ions on silk conformations and mechanical properties of silk fibers by adding K+ and Cu2+ into the silk fibroin solutions or injecting them into the silkworms. Aimed by CD analysis, FTIR analysis, and mechanical testing, the conformational and mechanical changes of the silks were estimated. By using BION Web Server, the interactions of K+ and N-terminal of silk fibroin were also simulated. ResultsWe presented that K+ and Cu2+ induced the conformational transitions of silk fibroin by forming β-sheet structures. Moreover, the mechanical parameters of silk fibers, such as strength, toughness and Young's modulus, were also improved after K+ or Cu2+ injection. Using BION Web Server, we found that potassium ions may have strong electrostatic interactions with the negatively charged residues. ConclusionWe suggest that K+ and Cu2+ play crucial roles in the conformation and mechanical performances of silks and they are involved in the silk fiber formation in vivo. General significanceOur results are helpful for clarifying the mechanism of silk fiber formation, and provide insights for modifying the mechanical properties of silk fibers.

Modifying the Mechanical Properties of Silk Fiber by Genetically Disrupting the Ionic Environment for Silk Formation.

Silks are widely used biomaterials, but there are still weaknesses in their mechanical properties. Here we report a method for improving the silk fiber mechanical properties by genetic disruption of the ionic environment for silk fiber formation. An anterior silk gland (ASG) specific promoter was identified and used for overexpressing ion-transporting protein in the ASG of silkworm. After isolation of the transgenic silkworms, we found that the metal ion content, conformation and mechanical properties of transgenic silk fibers changed accordingly. Notably, overexpressing endoplasmic reticulum Ca2+-ATPase in ASG decreased the calcium content of silks. As a consequence, silk fibers had more α-helix and β-sheet conformations, and their tenacity and extension increased significantly. These findings represent the in vivo demonstration of a correlation between metal ion content in the spinning duct and the mechanical properties of silk fibers, thus providing a novel method for modifying silk fiber properties.

Ca2+ and endoplasmic reticulum Ca2+-ATPase regulate the formation of silk fibers with favorable mechanical properties

Calcium ions (Ca2+) are crucial for the conformational transition of silk fibroin in vitro, and silk fibroin conformations correlate with the mechanical properties of silk fibers. To investigate the relationship between Ca2+ and mechanical properties of silk fibers, CaCl2 was injected into silkworms (Bombyx mori). Fourier-transform infrared spectroscopy (FTIR) analysis and mechanical testing revealed that injection of CaCl2 solution (7.5mg/g body weight) significantly increased the levels of α-helix and random coil structures of silk proteins. In addition, extension of silk fibers increased after CaCl2 injection. In mammals, sarcoplasmic reticulum Ca2+-ATPase in muscle and endoplasmic reticulum Ca2+-ATPase in other tissues (together denoted by SERCA) are responsible for calcium balance. Therefore, we analyzed the expression pattern of silkworm SERCA (BmSERCA) in silk glands and found that BmSERCA was abundant in the anterior silk gland (ASG). After injection of thapsigargin (TG) to block SERCA activity, silkworms showed a silk-spinning deficiency and their cocoons had higher calcium content compared to that of controls. Moreover, FTIR analysis revealed that the levels of α-helix and β-sheet structures increased in silk fibers from TG-injected silkworms compared to controls. The results provide evidence that BmSERCA has a key function in calcium transportation in ASG that is related to maintaining a suitable ionic environment. This ionic environment with a proper Ca2+ concentration is crucial for the formation of silk fibers with favorable mechanical performances.

Origin of the variability of the mechanical properties of silk fibers: 4. Order/crystallinity along silkworm and spider fibers

The poor crystallinity of proteic fibers has fuelled an ongoing debate over their exact organization. We present a full‐range Raman comparison of Nephila madagascariensis spider and Bombyx mori silkworm silks that sheds some new light on that matter. On the one hand, a large variability is observed along the fibers in the −200 to 200 cm−1 spectral window, which is sensitive to the long‐range order signature of polyamide chains. This questions the validity of previous literature data considering silk fiber as a homogeneous material. On the other hand, the ‘amide I’ band is almost independent of the targeted point, which sets a limit to this widely used structure probe. In‐line mapping of the fibers showed that the extension of the ordered zones ranges between 1 and 3 µm. The correlation between the macromechanical behavior (the stress–strain curves) and the nanomechanics (Raman low wavenumber signatures) under controlled tensile strain demonstrates a Prevorsek's type microstructure: the macromolecular chains belong to both ordered and amorphous ‘regions’. Copyright © 2014 John Wiley & Sons, Ltd.