The hidden link between supercontraction and mechanical behavior of spider silks

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.

Artificial Spider Silk with Buckled Sheath by Nano-Pulley Combing

The axial orientation of molecular chains always results in an increase in fiber strength and a decrease in toughness. Here, taking inspiration from the skin structure, artificial spider silk with a buckled sheath-core structure is developed, with mechanical strength and toughness reaching 1.61 GPa and 466 MJ m-3 , respectively, exceeding those of Caerostris darwini silk. The buckled structure is achieved by nano-pulley combing of polyrotaxane hydrogel fibers through cyclic stretch-release training, which exhibits axial alignment of the polymer chains in the fiber core and buckling in the fiber sheath. The artificial spider silk also exhibits excellent supercontraction behavior, achieving a work capacity of 1.89 kJ kg-1 , and an actuation stroke of 82%. This work provides a new strategy for designing high-performance and intelligent fiber materials.

Humidity-Driven Supercontraction and Twist in Spider Silk

Spider silk is a protein material that exhibits extraordinary and nontrivial properties such as the ability to soften, decrease in length (i.e., supercontract), and twist upon exposure to high humidity. These behaviors stem from a unique microstructure in combination with a transition from glassy to rubbery as a result of humidity-driven diffusion of water. In this Letter we propose four length scales that govern the mechanical response of the silk during this transition. In addition, we develop a model that describes the microstructural evolution of the spider silk thread and explains the response due to the diffusion of water molecules. The merit of the model is demonstrated through an excellent agreement to experimental findings. The insights from this Letter can be used as a microstructural design guide to enable the development of new materials with unique spiderlike properties.

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.

Spider Silk-Inspired Artificial Fibers

Spider silk is a natural polymeric fiber with high tensile strength, toughness, and has distinct thermal, optical, and biocompatible properties. The mechanical properties of spider silk are ascribed to its hierarchical structure, including primary and secondary structures of the spidroins (spider silk proteins), the nanofibril, the "core-shell", and the "nano-fishnet" structures. In addition, spider silk also exhibits remarkable properties regarding humidity/water response, water collection, light transmission, thermal conductance, and shape-memory effect. This motivates researchers to prepare artificial functional fibers mimicking spider silk. In this review, the authors summarize the study of the structure and properties of natural spider silk, and the biomimetic preparation of artificial fibers from different types of molecules and polymers by taking some examples of artificial fibers exhibiting these interesting properties. In conclusion, biomimetic studies have yielded several noteworthy findings in artificial fibers with different functions, and this review aims to provide indications for biomimetic studies of functional fibers that approach and exceed the properties of natural spider silk.

Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine-proline-glycine-glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin's bark spider (Caerostris darwini) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine-proline-glycine-proline motif and may explain C. darwini MA silk's extraordinary toughness. However, no direct correlation has been made between the silk's molecular structure and its mechanical properties for C. darwini. Here, we correlate the relative protein secondary structure composition of MA silk from C. darwini and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that C. darwini MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure-property correlation model.

Size-related increase in inducible mechanical variability of major ampullate silk in a huntsman spider (Araneae: Sparassidae)

Most spiders use major ampullate silk (MAS) to perform many functions across their lifetimes, including prey capture, vibratory signal detection, and safety/dragline. To accommodate their various needs, adult spiders can use inducible variability to tailor MAS with specific mechanical properties. However, it is currently unknown whether this inducible mechanical variability develops gradually or remains consistent across spider size. Supercontraction -a process by which "native-state" silk fibers axially shrink when exposed to water or high humidity-can be used to reveal the extent of inducible variability in MAS. Supercontraction removes some processing effects that occur during the spinning of the solid fiber from its liquid precursor by allowing a native-state MAS fiber to return to a low energy "ground-state". Here, we examined the relative extent of inducible variability of MAS across spider size by assessing supercontraction properties and the difference between ground- and native-state MAS tensile properties using silk from the huntsman spider Heteropoda venatoria (Sparassidae). Stiffness of forcibly pulled native-state silk increased by 200% with spider size. After exposure to 90% RH and subsequent supercontraction, axial shrinkage of native-state silk fibers increased by 15% in larger spiders. Supercontracted, ground-state fibers demonstrated a 200% increase in extensibility across spider size. Our results indicate a gradual increase in inducible variability of MAS mechanical properties across spider size potentially caused by shifts in internal processing or chemical composition. These findings imply both development of inducible variability and changes in use of MAS as a safety line or aiding jumps across a spider's lifetime.

On the Origin of Supercontraction in Spider Silk

Spider silk is a protein material that exhibits extraordinary and nontrivial properties such as the ability to soften and decrease its length by up to ∼60% upon exposure to high humidity. This process is commonly called supercontraction and is the result of a transition from a highly oriented glassy phase to a disoriented rubbery phase. In this work, we derive a microscopically motivated and energy-based model that captures the underlying mechanisms that give rise to supercontraction. We propose that the increase in relative humidity and the consequent wetting of a spider silk have two main consequences: (1) the dissociation of hydrogen bonds and (2) the swelling of the fiber. From a mechanical viewpoint, the first consequence leads to the formation of rubbery domains. This process is associated with an entropic gain and a loss of orientation of chains in the silk network, which motivates the contraction of the spider silk. The swelling of the fiber is accompanied by the extension of chains in order to accommodate the influx of water molecules. Supercontraction occurs when the first consequence is more dominant than the second. The model presented in this work allows us to qualitatively track the transition of the chains from glassy to rubbery states and determine the increase in entropy, the loss of orientation, and the swelling as the relative humidity increases. We also derive explicit expressions for the stiffness and the mechanical response of a spider silk under given relative humidity conditions. To illustrate the merit of this model, we show that the model is capable of capturing several experimental findings. The insights from this work can be used as a microstructural design guide to enable the development of new materials with unique spider-like properties.

Silkworm and spider silk electrospinning: a review

Issues of fossil fuel and plastic pollution are shifting public demand toward biopolymer-based textiles. For instance, silk, which has been traditionally used during at least 5 milleniums in China, is re-emerging in research and industry with the development of high-tech spinning methods. Various arthropods, e.g. insects and arachnids, produce silky proteinic fiber of unique properties such as resistance, elasticity, stickiness and toughness, that show huge potential for biomaterial applications. Compared to synthetic analogs, silk presents advantages of low density, degradability and versatility. Electrospinning allows the creation of nonwoven mats whose pore size and structure show unprecedented characteristics at the nanometric scale, versus classical weaving methods or modern techniques such as melt blowing. Electrospinning has recently allowed to produce silk scaffolds, with applications in regenerative medicine, drug delivery, depollution and filtration. Here we review silk production by the spinning apparatus of the silkworm Bombyx mori and the spiders Aranea diadematus and Nephila Clavipes. We present the biotechnological procedures to get silk proteins, and the preparation of a spinning dope for electrospinning. We discuss silk's mechanical properties in mats obtained from pure polymer dope and multi-composites. This review highlights the similarity between two very different yarn spinning techniques: biological and electrospinning processes.

Forcibly spun dragline silk fibers from web-building spider Trichonephila clavata ensure robustness irrespective of spinning speed and humidity

Web-building spiders secrete dragline silk fibers to sustain their body and use them as frameworks during web construction. They spin dragline silk fibers at various spinning speed and humidity conditions depending on their natural habitat. Here, we investigated the effect of spinning speed and humidity on the structural and mechanical properties of dragline silk fibers from web-building spider Trichonephila clavata obtained by the forcibly spinning method. We found that the crystal and morphological structures did not rely on the spinning speed and humidity. Furthermore, the mechanical strength and extensibility of the dragline silk fibers were maintained, demonstrating that dragline silk fibers ensure robustness irrespective of the spinning speed and humidity. The results obtained in the present study are helpful not only to understand the biological basis of the silk fiber formation of spiders but also contribute to consider the spinning conditions for the process of creating synthetic silk fibers.

Adhesion of spider cribellate silk enhanced in high humidity by mechanical plasticization of the underlying fiber

The disruptive nature of water presents a significant challenge when designing synthetic adhesives that maintain functionality in wet conditions. However, many animal adhesives can withstand high humidity or underwater conditions, and some are even enhanced by them. An understudied mechanism in such systems is the influence of material plasticization by water to induce adhesive work through deformation. Cribellate silk is a dry adhesive used by particular spiders to capture moving prey. It presents as a candidate for testing the water plasticization model as it can remain functional at high humidity despite lacking an aqueous component. We performed herein tensile and adhesion tests on cribellate threads from the spider, Hickmania troglodytes; a spider that lives within wet cave environments. We found that the work of adhesion of its cribellate threads increased as the axial fibre deformed during pull-off experiments. This effect was enhanced when the silk was wetted and as spider body size increased. Dry threads on the other hand were stiff with low adhesion. We rationalized our experiments by a series of scaling law models. We concluded that these cribellate threads operate best when the nanofibrils and axial fibers both contribute to adhesion. Design of future synthetic materials could draw inspiration from how water facilitates, rather than diminishes, cribellate silk adhesion.

Bäcklund F, Barth A, Johansson J, M. Pugno N, Rising A. Properties of Biomimetic Artificial Spider Silk Fibers Tuned by PostSpin Bath Incubation

Efficient production of artificial spider silk fibers with properties that match its natural counterpart has still not been achieved. Recently, a biomimetic process for spinning recombinant spider silk proteins (spidroins) was presented, in which important molecular mechanisms involved in native spider silk spinning were recapitulated. However, drawbacks of these fibers included inferior mechanical properties and problems with low resistance to aqueous environments. In this work, we show that ≥5 h incubation of the fibers, in a collection bath of 500 mM NaAc and 200 mM NaCl, at pH 5 results in fibers that do not dissolve in water or phosphate buffered saline, which implies that the fibers can be used for applications that involve wet/humid conditions. Furthermore, incubation in the collection bath improved the strain at break and was associated with increased β-sheet content, but did not affect the fiber morphology. In summary, we present a simple way to improve artificial spider silk fiber strain at break and resistance to aqueous solvents.

Effect of stress on the molecular structure and mechanical properties of supercontracted spider dragline silks

Supercontraction is one of the most interesting properties of spider dragline silks. In this study, changes in the secondary structures of the Nephila edulis spider dragline silk after it was subjected to different supercontraction processes were investigated by integrating synchrotron Fourier transform infrared (S-FTIR) microspectroscopy and mechanical characterization. The results showed that after free supercontraction, the β-sheet lost most of its orientation, while the helix and random coils were almost totally disordered. Interestingly, by conducting different types of supercontractions (i.e., stretching of the free supercontracted spider dragline silk to its original length or performing constrained supercontraction), it was found that although the molecular structures all changed after supercontraction, the mechanical properties almost remained unchanged when the length of the spider dragline silk did not change significantly. The other interesting conclusion obtained is that the manual stretching of a poorly oriented spider dragline silk cannot selectively improve the orientation degree of the β-sheet in the spider silk, but increase the orientation degree of all conformations (β-sheet, helix, and random). These experimental findings not only help to unveil the structure-property-function relationship of natural spider silks, but also provide a useful guideline for the design of biomimetic spider fiber materials.

Artificial spider silk from ion-doped and twisted core-sheath hydrogel fibres

Spider silks show unique combinations of strength, toughness, extensibility, and energy absorption. To date, it has been difficult to obtain spider silk-like mechanical properties using non-protein approaches. Here, we report on an artificial spider silk produced by the water-evaporation-induced self-assembly of hydrogel fibre made from polyacrylic acid and silica nanoparticles. The artificial spider silk consists of hierarchical core-sheath structured hydrogel fibres, which are reinforced by ion doping and twist insertion. The fibre exhibits a tensile strength of 895 MPa and a stretchability of 44.3%, achieving mechanical properties comparable to spider silk. The material also presents a high toughness of 370 MJ m-3 and a damping capacity of 95%. The hydrogel fibre shows only ~1/9 of the impact force of cotton yarn with negligible rebound when used for impact reduction applications. This work opens an avenue towards the fabrication of artificial spider silk with applications in kinetic energy buffering and shock-absorbing.

Scalable Spider-Silk-Like Supertough Fibers using a Pseudoprotein Polymer

Spider silks are tougher than almost all other materials in the world and thus are considered ideal materials by scientists and the industry. Although there have been tremendous attempts to prepare fibers from genetically engineered spider-silk proteins, it is still a very large challenge to artificially produce materials with a very high fracture energy, not to mention the high scaling-up requirements because of the extremely low productivity and high cost levels. Here, a facile spider-silk-mimicking strategy is first reported for preparing scalable supertough fibers using the chemical synthesis route. Supertoughness (≈387 MJ m^-3 ), more than twice the reported value of common spider dragline silk and comparable to the value of the toughest spider silk, the aciniform silk of Argiope trifasciata, is achieved by introducing β-sheet crystals and α-helical peptides simultaneously in a pseudoprotein polymer. The process opens up a very promising avenue for obtaining excellent spider fibers.

Emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers

The conditions required for the emergence of supercontraction in regenerated silkworm (Bombyx mori) silk fibers are assessed through an experimental approach that combines the spinning of regenerated fibers with controlled properties and their characterization by ¹³C solid-state nuclear magnetic resonance (NMR). Both supercontracting and non-supercontracting regenerated fibers are produced using the straining flow spinning (SFS) technique from ¹³C labeled cocoons. The short-range microstructure of the fibers is assessed through ¹³C CP/MAS in air and ¹³C DD/MAS in water, and the main microstructural features are identified and quantified. The mechanical properties of the regenerated fibers and their microstructures are compared with those of natural silkworm silk. The combined analysis highlights two possible key elements as responsible for the emergence of supercontraction: (1) the existence of an upper and a lower limit of the amorphous phase compatible with supercontraction, and (2) the existence of two ordered phases, β-sheet A and B, which correspond to different packing arrangements of the protein chains.

Biomimetic Supramolecular Fibers Exhibit Water-Induced Supercontraction

Spider silk is a fascinating material, combining high strength and elasticity that outperforms most synthetic fibers. Another intriguing feature of spider silk is its ability to "supercontract," shrinking up to 50% when exposed to water. This is likely on account of the entropy-driven recoiling of secondary structured proteins when water penetrates the spider silk. In contrast, humidity-driven contraction in synthetic fibers is difficult to achieve. Here, inspired by the spider silk model, a supercontractile fiber (SCF), which contracts up to 50% of its original length at high humidity, comparable to spider silk, is reported. The fiber exhibits up to 300% uptake of water by volume, confirmed via environmental scanning electron microscopy. Interestingly, the SCF exhibits tunable mechanical properties by varying humidity, which is reflected by the prolonged failure strain and the reversible damping capacity. This smart supramolecular fiber material provides a new opportunity of fabricating biomimetic muscle for diverse applications.

Unraveling the Molecular Requirements for Macroscopic Silk Supercontraction

Spider dragline silk is a protein material that has evolved over millions of years to achieve finely tuned mechanical properties. A less known feature of some dragline silk fibers is that they shrink along the main axis by up to 50% when exposed to high humidity, a phenomenon called supercontraction. This contrasts the typical behavior of many other materials that swell when exposed to humidity. Molecular level details and mechanisms of the supercontraction effect are heavily debated. Here we report a molecular dynamics analysis of supercontraction in Nephila clavipes silk combined with in situ mechanical testing and Raman spectroscopy linking the reorganization of the nanostructure to the polar and charged amino acids in the sequence. We further show in our in silico approach that point mutations of these groups not only suppress the supercontraction effect, but even reverse it, while maintaining the exceptional mechanical properties of the silk material. This work has imminent impact on the design of biomimetic equivalents and recombinant silks for which supercontraction may or may not be a desirable feature. The approach applied is appropriate to explore the effect of point mutations on the overall physical properties of protein based materials.

Predicted networks of protein-protein interactions in Stegodyphus mimosarum by cross-species comparisons

BACKGROUND

Stegodyphus mimosarum is a candidate model organism belonging to the class Arachnida in the phylum Arthropoda. Studies on the biology of S. mimosarum over the past several decades have consisted of behavioral research and comparison of gene sequences based on the assembled genome sequence. Given the lack of systematic protein analyses and the rich source of information in the genome, we predicted the relationships of proteins in S. mimosarum by bioinformatics comparison with genome-wide proteins from select model organisms using gene mapping.

RESULTS

The protein-protein interactions (PPIs) of 11 organisms were integrated from four databases (BioGrid, InAct, MINT, and DIP). Here, we present comprehensive prediction and analysis of 3810 proteins in S. mimosarum with regard to interactions between proteins using PPI data of organisms. Interestingly, a portion of the protein interactions conserved among Saccharomyces cerevisiae, Homo sapiens, Arabidopsis thaliana, and Drosophila melanogaster were found to be associated with RNA splicing. In addition, overlap of predicted PPIs in reference organisms, Gene Ontology, and topology models in S. mimosarum are also reported.

CONCLUSIONS

Addition of Stegodyphus, a spider representative of interactomic research, provides the possibility of obtaining deeper insights into the evolution of PPI networks among different animal species. This work largely supports the utility of the "stratus clouds" model for predicted PPIs, providing a roadmap for integrative systems biology in S. mimosarum.

Physicochemical Property Variation in Spider Silk: Ecology, Evolution, and Synthetic Production

The unique combination of great stiffness, strength, and extensibility makes spider major ampullate (MA) silk desirable for various biomimetic and synthetic applications. Intensive research on the genetics, biochemistry, and biomechanics of this material has facilitated a thorough understanding of its properties at various levels. Nevertheless, methods such as cloning, recombination, and electrospinning have not successfully produced materials with properties as impressive as those of spider silk. It is nevertheless becoming clear that silk properties are a consequence of whole-organism interactions with the environment in addition to genetic expression, gland biochemistry, and spinning processes. Here we assimilate the research done and assess the techniques used to determine distinct forms of spider silk chemical and physical property variability. We suggest that more research should focus on testing hypotheses that explain spider silk property variations in ecological and evolutionary contexts.

Biomechanical characterization of spider webs

In light of recent focus on the behaviour of the natural structures for revolutionary technological growth, spider web seems to have seized considerable attention of product designer due to its amazing behaviour. In present work, mechanism behind the structural integrity of the spider web along with the materialistic analysis of its constituent silk threads has been extensively investigated. The nanoindentation tool both in static and dynamic mode has been utilized for complete analysis of the mechanical behaviour of the spiral and radial threads separately. Both the average elastic modulus and hardness of the radial silk thread is higher than the spiral silk thread which reveals the radial silk thread is the major structural component of the web. The sustainability of spider webs under storm, windy conditions and during the impact of pray has been investigated under dynamic conditions. The radial silk thread exhibits elastic like response and the spiral silk thread exhibits viscous like response in a wide frequency range (1-200Hz). The damping characteristic of the radial and spiral silk threads, an important parameter to investigate the energy dissipation properties of the materials has also been investigated in windy conditions.

Evidence of Decoupling Protein Structure from Spidroin Expression in Spider Dragline Silks

The exceptional strength and extensibility of spider dragline silk have been thought to be facilitated by two spidroins, major ampullate spidroin 1 (MaSp1) and major ampullate spidroin 2 (MaSp2), under the assumption that protein secondary structures are coupled with the expressed spidroins. We tested this assumption for the dragline silk of three co-existing Australian spiders, Argiope keyserlingi, Latrodectus hasselti and Nephila plumipes. We found that silk amino acid compositions did not differ among spiders collected in May. We extended these analyses temporally and found the amino acid compositions of A. keyserlingi silks to differ when collected in May compared to November, while those of L. hasselti did not. To ascertain whether their secondary structures were decoupled from spidroin expression, we performed solid-state nuclear magnetic resonance spectroscopy (NMR) analysis on the silks of all spiders collected in May. We found the distribution of alanine toward β-sheet and 3,10helix/random coil conformations differed between species, as did their relative crystallinities, with A. keyserlingi having the greatest 3,10helix/random coil composition and N. plumipes the greatest crystallinity. The protein secondary structures correlated with the mechanical properties for each of the silks better than the amino acid compositions. Our findings suggested that a differential distribution of alanine during spinning could decouple secondary structures from spidroin expression ensuring that silks of desirable mechanical properties are consistently produced. Alternative explanations include the possibility that other spidroins were incorporated into some silks.

Native Silk Feedstock as a Model Biopolymer: A Rheological Perspective

Variability in silk's rheology is often regarded as an impediment to understanding or successfully copying the natural spinning process. We have previously reported such variability in unspun native silk extracted straight from the gland of the domesticated silkworm Bombyx mori and discounted classical explanations such as differences in molecular weight and concentration. We now report that variability in oscillatory measurements can be reduced onto a simple master-curve through normalizing with respect to the crossover. This remarkable result suggests that differences between silk feedstocks are rheologically simple and not as complex as originally thought. By comparison, solutions of poly(ethylene-oxide) and hydroxypropyl-methyl-cellulose showed similar normalization behavior; however, the resulting curves were broader than for silk, suggesting greater polydispersity in the (semi)synthetic materials. Thus, we conclude Nature may in fact produce polymer feedstocks that are more consistent than typical man-made counterparts as a model for future rheological investigations.

Silk-Its Mysteries, How It Is Made, and How It Is Used

This article reviews fundamental and applied aspects of silk-one of Nature's most intriguing materials in terms of its strength, toughness, and biological role-in its various forms, from protein molecules to webs and cocoons, in the context of mechanical and biological properties. A central question that will be explored is how the bridging of scales and the emergence of hierarchical structures are critical elements in achieving novel material properties, and how this knowledge can be explored in the design of synthetic materials. We review how the function of a material system at the macroscale can be derived from the interplay of fundamental molecular building blocks. Moreover, guidelines and approaches to current experimental and computational designs in the field of synthetic silklike materials are provided to assist the materials science community in engineering customized finetuned biomaterials for biomedical applications.

Compliant threads maximize spider silk connection strength and toughness

Millions of years of evolution have adapted spider webs to achieve a range of functionalities, including the well-known capture of prey, with efficient use of material. One feature that has escaped extensive investigation is the silk-on-silk connection joints within spider webs, particularly from a structural mechanics perspective. We report a joint theoretical and computational analysis of an idealized silk-on-silk fibre junction. By modifying the theory of multiple peeling, we quantitatively compare the performance of the system while systematically increasing the rigidity of the anchor thread, by both scaling the stress-strain response and the introduction of an applied pre-strain. The results of our study indicate that compliance is a virtue-the more extensible the anchorage, the tougher and stronger the connection becomes. In consideration of the theoretical model, in comparison with rigid substrates, a compliant anchorage enormously increases the effective adhesion strength (work required to detach), independent of the adhered thread itself, attributed to a nonlinear alignment between thread and anchor (contact peeling angle). The results can direct novel engineering design principles to achieve possible load transfer from compliant fibre-to-fibre anchorages, be they silk-on-silk or another, as-yet undeveloped, system.

Structural hysteresis in dragline spider silks induced by supercontraction: An x-ray fiber micro-diffraction study

Interaction with water causes shrinkage and significant changes in the structure of spider dragline silks, which has been referred to as supercontraction in the literature. Preferred orientation or alignment of protein chains with respect to the fiber axis is extensively changed during this supercontraction process. Synchrotron x-ray micro-fiber diffraction experiments have been performed on Nephila clavipes and Argiope aurantia major and minor ampullate dragline spider fibers in the native dry, contracted (by immersion in water) and restretched (from contracted) states. Changes in the orientation of β-sheet nanocrystallites and the oriented component of the amorphous network have been determined from wide-angle x-ray diffraction patterns. While both the crystalline and amorphous components lose preferred orientation on wetting with water, the nano-crystallites regain their orientation on wet-restretching, whereas the oriented amorphous components only partially regain their orientation. Dragline major ampullate silks in both the species contract more than their minor ampullate silks.

E-spun composite fibers of collagen and dragline silk protein: fiber mechanics, biocompatibility, and application in stem cell differentiation

Biocomposite matrices with high mechanical strength, high stability, and the ability to direct matrix-specific stem cell differentiation are essential for the reconstruction of lesioned tissues in tissue engineering and cell therapeutics. Toward this end, we used the electrospinning technique to fabricate well-aligned composite fibers from collagen and spider dragline silk protein, obtained from the milk of transgenic goats, mimicking the native extracellular matrix (ECM) on a similar scale. Collagen and the dragline silk proteins were found to mix homogeneously at all ratios in the electrospun (E-spun) fibers. As a result, the ultimate tensile strength and elasticity of the fibers increased monotonically with silk percentage, whereas the stretchability was slightly reduced. Strikingly, we found that the incorporation of silk proteins to collagen dramatically increased the matrix stability against excessive fiber swelling and shape deformation in cell culture medium. When human decidua parietalis placental stem cells (hdpPSCs) were seeded on the collagen–silk matrices, the matrices were found to support cell proliferation at a similar rate as that of the pure collagen matrix, but they provided cell adhesion with reduced strengths and induced cell polarization at varied levels. Matrices containing 15 and 30 wt % silk in collagen (CS15, CS30) were found to induce a level of neural differentiation comparable to that of pure collagen. In particular, CS15 matrix induced the highest extent of cell polarization and promoted the development of extended 1D neural filaments strictly in-line with the aligned fibers. Taking the increased mechanical strength and fiber stability into consideration, CS15 and CS30 E-spun fibers offer better alternatives to pure collagen fibers as scaffolds that can be potentially utilized in neural tissue repair and the development of future nanobiodevices.

Coatings and films made of silk proteins

Silks are a class of proteinaceous materials produced by arthropods for various purposes. Spider dragline silk is known for its outstanding mechanical properties, and it shows high biocompatibility, good biodegradability, and a lack of immunogenicity and allergenicity. The silk produced by the mulberry silkworm B. mori has been used as a textile fiber and in medical devices for a long time. Here, recent progress in the processing of different silk materials into highly tailored isotropic and anisotropic coatings for biomedical applications such as tissue engineering, cell adhesion, and implant coatings as well as for optics and biosensors is reviewed.

Spider genomes provide insight into composition and evolution of venom and silk

Spiders are ecologically important predators with complex venom and extraordinarily tough silk that enables capture of large prey. Here we present the assembled genome of the social velvet spider and a draft assembly of the tarantula genome that represent two major taxonomic groups of spiders. The spider genomes are large with short exons and long introns, reminiscent of mammalian genomes. Phylogenetic analyses place spiders and ticks as sister groups supporting polyphyly of the Acari. Complex sets of venom and silk genes/proteins are identified. We find that venom genes evolved by sequential duplication, and that the toxic effect of venom is most likely activated by proteases present in the venom. The set of silk genes reveals a highly dynamic gene evolution, new types of silk genes and proteins, and a novel use of aciniform silk. These insights create new opportunities for pharmacological applications of venom and biomaterial applications of silk.

Spiders use self-produced venom and silk for their daily survival. Here, the authors report the assembled genome of the social velvet spider and a draft assembly of the tarantula genome and, together with proteomic data, provide insights into the evolution of genes that affect venom and silk production.

Self-tensioning aquatic caddisfly silk: Ca2+-dependent structure, strength, and load cycle hysteresis

Caddisflies are aquatic relatives of silk-spinning terrestrial moths and butterflies. Casemaker larvae spin adhesive silk fibers for underwater construction of protective composite cases. The central region of Hesperophylax sp. H-fibroin contains a repeating pattern of three conserved subrepeats, all of which contain one or more (SX)n motifs with extensively phosphorylated serines. Native silk fibers were highly extensible and displayed a distinct yield point, force plateau, and load cycle hysteresis. FTIR spectroscopy of native silk showed a conformational mix of random coil, β-sheet, and turns. Exchanging multivalent ions with Na(+) EDTA disrupted fiber mechanics, shifted the secondary structure ratios from antiparallel β-sheet toward random coil and turns, and caused the fibers to shorten, swell in diameter, and disrupted fiber birefringence. The EDTA effects were reversed by restoring Ca(2+). Molecular dynamic simulations provided theoretical support for a hypothetical structure in which the (pSX)n motifs may assemble into two- and three-stranded, Ca(2+)-stabilized β-sheets.

Increasing silk fibre strength through heterogeneity of bundled fibrils

Can naturally arising disorder in biological materials be beneficial? Materials scientists are continuously attempting to replicate the exemplary performance of materials such as spider silk, with detailed techniques and assembly procedures. At the same time, a spider does not precisely machine silk-imaging indicates that its fibrils are heterogeneous and irregular in cross section. While past investigations either focused on the building material (e.g. the molecular scale protein sequence and behaviour) or on the ultimate structural component (e.g. silk threads and spider webs), the bundled structure of fibrils that compose spider threads has been frequently overlooked. Herein, I exploit a molecular dynamics-based coarse-grain model to construct a fully three-dimensional fibril bundle, with a length on the order of micrometres. I probe the mechanical behaviour of bundled silk fibrils with variable density of heterogenic protrusions or globules, ranging from ideally homogeneous to a saturated distribution. Subject to stretching, the model indicates that cooperativity is enhanced by contact through low-force deformation and shear 'locking' between globules, increasing shear stress transfer by up to 200 per cent. In effect, introduction of a random and disordered structure can serve to improve mechanical performance. Moreover, addition of globules allows a tuning of free volume, and thus the wettability of silk (with implications for supercontraction). These findings support the ability of silk to maintain near-molecular-level strength at the scale of silk threads, and the mechanism could be easily adopted as a strategy for synthetic fibres.

Sequential origin in the high performance properties of orb spider dragline silk

Major ampullate (MA) dragline silk supports spider orb webs, combining strength and extensibility in the toughest biomaterial. MA silk evolved ~376 MYA and identifying how evolutionary changes in proteins influenced silk mechanics is crucial for biomimetics, but is hindered by high spinning plasticity. We use supercontraction to remove that variation and characterize MA silk across the spider phylogeny. We show that mechanical performance is conserved within, but divergent among, major lineages, evolving in correlation with discrete changes in proteins. Early MA silk tensile strength improved rapidly with the origin of GGX amino acid motifs and increased repetitiveness. Tensile strength then maximized in basal entelegyne spiders, ~230 MYA. Toughness subsequently improved through increased extensibility within orb spiders, coupled with the origin of a novel protein (MaSp2). Key changes in MA silk proteins therefore correlate with the sequential evolution high performance orb spider silk and could aid design of biomimetic fibers.

Minor ampullate silks from Nephila and Argiope spiders: tensile properties and microstructural characterization

The mechanical behavior and microstructure of minor ampullate gland silk (miS) of two orb-web spinning species, Argiope trifasciata and Nephila inaurata, were extensively characterized, enabling detailed comparison with other silks. The similarities and differences exhibited by miS when compared with the intensively studied major ampullate gland silk (MAS) and silkworm (Bombyx mori) silk offer a genuine opportunity for testing some of the hypotheses proposed to correlate microstructure and tensile properties in silk. In this work, we show that miSs of different species show similar properties, even when fibers spun by spiders that diverged over 100 million years are compared. The tensile properties of miS are comparable to those of MAS when tested in air, significantly in terms of work to fracture, but differ considerably when tested in water. In particular, miS does not show a supercontraction effect and an associated ground state. In this regard, the behavior of miS in water is similar to that of B. mori silk, and it is shown that the initial elastic modulus of both fibers can be explained using a common model. Intriguingly, the microstructural parameters measured in miS are comparable to those of MAS and considerably different from those found in B. mori. This fact suggests that some critical microstructural information is still missing in our description of silks, and our results suggest that the hydrophilicity of the lateral groups or the large scale organization of the sequences might be routes worth exploring.

Post-secretion processing influences spider silk performance

Phenotypic variation facilitates adaptations to novel environments. Silk is an example of a highly variable biomaterial. The two-spidroin (MaSp) model suggests that spider major ampullate (MA) silk is composed of two proteins-MaSp1 predominately contains alanine and glycine and forms strength enhancing β-sheet crystals, while MaSp2 contains proline and forms elastic spirals. Nonetheless, mechanical properties can vary in spider silks without congruent amino acid compositional changes. We predicted that post-secretion processing causes variation in the mechanical performance of wild MA silk independent of protein composition or spinning speed across 10 species of spider. We used supercontraction to remove post-secretion effects and compared the mechanics of silk in this 'ground state' with wild native silks. Native silk mechanics varied less among species compared with 'ground state' silks. Variability in the mechanics of 'ground state' silks was associated with proline composition. However, variability in native silks did not. We attribute interspecific similarities in the mechanical properties of native silks, regardless of amino acid compositions, to glandular processes altering molecular alignment of the proteins prior to extrusion. Such post-secretion processing may enable MA silk to maintain functionality across environments, facilitating its function as a component of an insect-catching web.