Spider silk aging: initial improvement in a high performance material followed by slow degradation

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.

Strain softening and stiffening responses of spider silk fibers probed using a Micro-Extension Rheometer

Spider silk possesses unique mechanical properties like large extensibility, high tensile strength, super-contractility, etc. Understanding these mechanical responses requires characterization of the rheological properties of silk beyond the simple force-extension relations which are widely reported. Here we study the linear and non-linear viscoelastic properties of dragline silk obtained from social spider Stegodyphus sarasinorum using a Micro-Extension Rheometer that we have developed. Unlike continuous extension data, our technique allows for the probing of the viscoelastic response by applying small perturbations about sequentially increasing steady-state strain values. In addition, we extend our analysis to obtain the characteristic stress relaxation times and the frequency responses of the viscous and elastic moduli. Using these methods, we show that in a small strain regime (0-4%) dragline silk of social spiders shows a strain softening response followed by a strain stiffening response at higher strains (>4%). The stress relaxation time, on the other hand, increases monotonically with increasing strain for the entire range. We also show that the silk stiffens while ageing within the typical lifetime of a web. Our results demand the inclusion of the kinetics of domain unfolding and refolding in the existing models to account for the relaxation time behavior.

Structural properties and their influence on the prey retention in the spider web

Orb webs absorb the impact energy of prey and transmit vibratory information to the spider with minimal structural damage. The structural properties of the web and the arrangement of threads within the web affect transmission time during the prey impact. The objective of the present study is to determine damping, stiffness, and transmissibility of healthy and damaged spider webs. Experimental results show that stiffness and transmissibility diminish from the inner to outer spiral threads and gradient variation in the structural properties of spiral threads enhances signal transmission capability toward the centre regardless of the position of prey impact within the healthy web. Spiral threads exhibit excellent prey retention properties due to their stretching capability. Kinetic energy produced by prey is absorbed in the threads, which help the spider to analyse the prey retention properties and also determine the response time. The minor damage (up to 25%) does not alter the basic characteristics of the web due to self-adjustment of tension within the web. Damping, natural frequency, stiffness and transmissibility decrease with the increase in the percentage of damaged web. The present study addresses the structural sustainability of the spider web irrespective of minor damages and also provides guidance in designing the structures under impact. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology'.

Imaging and analysis of a three-dimensional spider web architecture

Spiders are abundantly found in nature and most ecosystems, making up more than 47 000 species. This ecological success is in part due to the exceptional mechanics of the spider web, with its strength, toughness, elasticity and robustness, which originate from its hierarchical structures all the way from sequence design to web architecture. It is a unique example in nature of high-performance material design. In particular, to survive in different environments, spiders have optimized and adapted their web architecture by providing housing, protection, and an efficient tool for catching prey. The most studied web in literature is the two-dimensional (2D) orb web, which is composed of radial and spiral threads. However, only 10% of spider species are orb-web weavers, and three-dimensional (3D) webs, such as funnel, sheet or cobwebs, are much more abundant in nature. The complex spatial network and microscale size of silk fibres are significant challenges towards determining the topology of 3D webs, and only a limited number of previous studies have attempted to quantify their structure and properties. Here, we focus on developing an innovative experimental method to directly capture the complete digital 3D spider web architecture with micron scale resolution. We built an automatic segmentation and scanning platform to obtain high-resolution 2D images of individual cross-sections of the web that were illuminated by a sheet laser. We then developed image processing algorithms to reconstruct the digital 3D fibrous network by analysing the 2D images. This digital network provides a model that contains all of the structural and topological features of the porous regions of a 3D web with high fidelity, and when combined with a mechanical model of silk materials, will allow us to directly simulate and predict the mechanical response of a realistic 3D web under mechanical loads. Our work provides a practical tool to capture the architecture of sophisticated 3D webs, and could lead to studies of the relation between architecture, material and biological functions for numerous 3D spider web applications.

Silk structure rather than tensile mechanics explains web performance in the moth-specialized spider, Cyrtarachne

Orb webs intercept and retain prey so spiders may subdue them. Orb webs are composed of sticky, compliant spirals of capture silk spun across strong, stiff major ampullate silk threads. Interplay between differences in the mechanical properties of these silks is crucial for prey capture. Most orb webs depend upon insects contacting several radial and capture threads for successful retention. Moths, however, escape quickly from most orb webs due to the sacrificial scales covering their bodies. Cyrtarachne orb webs are unusual as they contain a reduced number of capture threads and moths stick unusually well to single threads. We aimed to determine how the tensile properties of the capture spiral and radial threads spun by Cyrtarachne operate in retention of moth prey. A NanoBionix UTM was used to quantify the material properties of flagelliform and major ampullate threads to test if Cyrtarachne's reduced web architecture is accompanied by improvements in tensile performance of its silk. Silk threads showed tensile properties typical of less-specialized orb-weavers, with the exception of high extensibility in radial threads. Radial thread diameters were 62.5% smaller than flagelliform threads, where commonly the two are roughly similar. We utilized our tensile data to create a finite element model of Cyrtarachne's web to investigate energy dissipation during prey impact. Large cross-sectional area of the flagelliform threads played a key role in enabling single capture threads to withstand prey impact. Rather than extraordinary silk, Cyrtarachne utilizes structural changes in the size and attachment of silk threads to facilitate web function.

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.

The impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread

Many spider orb-webs are exposed to sunlight and the potentially damaging effects of ultraviolet B (UVB) radiation. We examined the effect of UVB on the viscoelastic glycoprotein core of glue droplets deposited on the prey capture threads of these webs, hypothesizing that webs built by species that occupy sunny habitats are less susceptible to UVB damage than are webs built by species that prefer shaded forest habitats or by nocturnal species. Threads were tested shortly after being collected in the early morning and after being exposed to UVB energy equivalent to a day of summer sun and three times this amount. Droplets kept in a dark chamber allowed us to evaluate post-production changes. Droplet volume was unaffected by treatments, indicating that UVB did not damage the hygroscopic compounds in the aqueous layer that covers droplets. UVB exposure did not affect energies of droplet extension for species from exposed and partially to mostly shaded habitats (Argiope aurantia, Leucauge venusta and Verrucosa arenata). However, UVB exposure reduced the energy of droplet extension in Micrathena gracilis from shaded forests and Neoscona crucifera, which forages at night. Only in L. venusta did the energy of droplet extension increase after the dark treatment, suggesting endogenous molecular alignment. This study adds UVB irradiation to the list of factors (humidity, temperature and strain rate) known to affect the performance of spider glycoprotein glue, factors that must be more fully understood if adhesives that mimic spider glycoprotein glue are to be produced.

The effect of ageing on the mechanical properties of the silk of the bridge spider Larinioides cornutus (Clerck, 1757)

Spider silk is regarded as one of the best natural polymer fibers especially in terms of low density, high tensile strength and high elongation until breaking. Since only a few bio-engineering studies have been focused on spider silk ageing, we conducted nano-tensile tests on the vertical naturally spun silk fibers of the bridge spider Larinioides cornutus (Clerck, 1757) (Arachnida, Araneae) to evaluate changes in the mechanical properties of the silk (ultimate stress and strain, Young’s modulus, toughness) over time. We studied the natural process of silk ageing at different time intervals from spinning (20 seconds up to one month), comparing silk fibers spun from adult spiders collected in the field. Data were analyzed using Linear Mixed Models. We detected a positive trend versus time for the Young’s modulus, indicating that aged silks are stiffer and possibly less effective in catching prey. Moreover, we observed a negative trend for the ultimate strain versus time, attesting a general decrement of the resistance force. These trends are interpreted as being due to the drying of the silk protein chains and the reorientation among the fibers.

Unravelling the biodiversity of nanoscale signatures of spider silk fibres

Living organisms are masters at designing outstanding self-assembled nanostructures through a hierarchical organization of modular proteins. Protein-based biopolymers improved and selected by the driving forces of molecular evolution are among the most impressive archetypes of nanomaterials. One of these biomacromolecules is the myriad of compound fibroins of spider silks, which combine surprisingly high tensile strength with great elasticity. However, no consensus on the nano-organization of spider silk fibres has been reached. Here we explore the biodiversity of spider silk fibres, focusing on nanoscale characterization with high-resolution atomic force microscopy. Our results reveal an evolution of the nanoroughness, nanostiffness, nanoviscoelastic, nanotribological and nanoelectric organization of microfibres, even when they share similar sizes and shapes. These features are related to unique aspects of their molecular structures. The results show that combined nanoscale analyses of spider silks may enable the screening of appropriate motifs for bioengineering synthetic fibres from recombinant proteins.

Biomechanics and biocompatibility of woven spider silk meshes during remodeling in a rodent fascia replacement model

OBJECTIVE

The aim of this study was to investigate biomechanical and immunogenic properties of spider silk meshes implanted as fascia replacement in a rat in vivo model.

BACKGROUND

Meshes for hernia repair require optimal characteristics with regard to strength, elasticity, and cytocompatibility. Spider silk as a biomaterial with outstanding mechanical properties is potentially suitable for this application.

METHODS

Commercially available meshes used for hernia repair (Surgisis and Ultrapro) were compared with handwoven meshes manufactured from native dragline silk of Nephila spp. All meshes were tied onto the paravertebral fascia, whereas sham-operated rats were sutured without mesh implantation. After 4 or 14 days, 4 weeks, and 4 or 8 months, tissue samples were analyzed concerning inflammation and biointegration both by histological and biochemical methods and by biomechanical stability tests.

RESULTS

Histological sections revealed rapid cell migration into the spider silk meshes with increased numbers of giant cells compared with controls with initial decomposition of silk fibers after 4 weeks. Four months postoperatively, spider silk was completely degraded with the formation of a stable scar verified by constant tensile strength values. Surgisis elicited excessive stability loss from day 4 to day 14 (P < 0.001), with distinct inflammatory reaction demonstrated by lymphocyte and neutrophil invasion. Ultrapro also showed decreasing strength and poor elongation behavior, whereas spider silk samples had the highest relative elongation (P < 0.05).

CONCLUSIONS

Hand-manufactured spider silk meshes with good biocompatibility and beneficial mechanical properties seem superior to standard biological and synthetic meshes, implying an innovative alternative to currently used meshes for hernia repair.

Sample selection, preparation methods, and the apparent tensile properties of silkworm (B. mori) cocoon silk

Reported literature values of the tensile properties of natural silk cover a wide range. While much of this inconsistency is the result of variability that is intrinsic to silk, some is also a consequence of differences in the way that silk is prepared for tensile tests. Here we explore how measured mechanical properties of Bombyx mori cocoon silk are affected by two intrinsic factors (the location from which the silk is collected within the cocoon, and the color of the silk), and two extrinsic factors (the storage conditions prior to testing, and different styles of reeling the fiber). We find that extrinsic and therefore controllable factors can affect the properties more than the intrinsic ones studied. Our results suggest that enhanced inter-laboratory collaborations, that lead to standardized sample collection, handling, and storage protocols prior to mechanical testing, would help to decrease unnecessary (and complicating) variation in reported tensile properties.

Artificial skin--culturing of different skin cell lines for generating an artificial skin substitute on cross-weaved spider silk fibres

Background

In the field of Plastic Reconstructive Surgery the development of new innovative matrices for skin repair is in urgent need. The ideal biomaterial should promote attachment, proliferation and growth of cells. Additionally, it should degrade in an appropriate time period without releasing harmful substances, but not exert a pathological immune response. Spider dragline silk from Nephila spp meets these demands to a large extent.

Methodology/Principal Findings

Native spider dragline silk, harvested directly out of Nephila spp spiders, was woven on steel frames. Constructs were sterilized and seeded with fibroblasts. After two weeks of cultivating single fibroblasts, keratinocytes were added to generate a bilayered skin model, consisting of dermis and epidermis equivalents. For the next three weeks, constructs in co-culture were lifted on an originally designed setup for air/liquid interface cultivation. After the culturing period, constructs were embedded in paraffin with an especially developed program for spidersilk to avoid supercontraction. Paraffin cross- sections were stained in Haematoxylin & Eosin (H&E) for microscopic analyses.

Conclusion/Significance

Native spider dragline silk woven on steel frames provides a suitable matrix for 3 dimensional skin cell culturing. Both fibroblasts and keratinocytes cell lines adhere to the spider silk fibres and proliferate. Guided by the spider silk fibres, they sprout into the meshes and reach confluence in at most one week. A well-balanced, bilayered cocultivation in two continuously separated strata can be achieved by serum reduction, changing the medium conditions and the cultivation period at the air/liquid interphase. Therefore spider silk appears to be a promising biomaterial for the enhancement of skin regeneration.

High-performance spider webs: integrating biomechanics, ecology and behaviour

Spider silks exhibit remarkable properties, surpassing most natural and synthetic materials in both strength and toughness. Orb-web spider dragline silk is the focus of intense research by material scientists attempting to mimic these naturally produced fibres. However, biomechanical research on spider silks is often removed from the context of web ecology and spider foraging behaviour. Similarly, evolutionary and ecological research on spiders rarely considers the significance of silk properties. Here, we highlight the critical need to integrate biomechanical and ecological perspectives on spider silks to generate a better understanding of (i) how silk biomechanics and web architectures interacted to influence spider web evolution along different structural pathways, and (ii) how silks function in an ecological context, which may identify novel silk applications. An integrative, mechanistic approach to understanding silk and web function, as well as the selective pressures driving their evolution, will help uncover the potential impacts of environmental change and species invasions (of both spiders and prey) on spider success. Integrating these fields will also allow us to take advantage of the remarkable properties of spider silks, expanding the range of possible silk applications from single threads to two- and three-dimensional thread networks.

Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider

Background

Combining high strength and elasticity, spider silks are exceptionally tough, i.e., able to absorb massive kinetic energy before breaking. Spider silk is therefore a model polymer for development of high performance biomimetic fibers. There are over 41.000 described species of spiders, most spinning multiple types of silk. Thus we have available some 200.000+ unique silks that may cover an amazing breadth of material properties. To date, however, silks from only a few tens of species have been characterized, most chosen haphazardly as model organisms (Nephila) or simply from researchers' backyards. Are we limited to ‘blindly fishing’ in efforts to discover extraordinary silks? Or, could scientists use ecology to predict which species are likely to spin silks exhibiting exceptional performance properties?

Methodology

We examined the biomechanical properties of silk produced by the remarkable Malagasy ‘Darwin's bark spider’ (Caerostris darwini), which we predicted would produce exceptional silk based upon its amazing web. The spider constructs its giant orb web (up to 2.8 m²) suspended above streams, rivers, and lakes. It attaches the web to substrates on each riverbank by anchor threads as long as 25 meters. Dragline silk from both Caerostris webs and forcibly pulled silk, exhibits an extraordinary combination of high tensile strength and elasticity previously unknown for spider silk. The toughness of forcibly silked fibers averages 350 MJ/m³, with some samples reaching 520 MJ/m³. Thus, C. darwini silk is more than twice tougher than any previously described silk, and over 10 times better than Kevlar®. Caerostris capture spiral silk is similarly exceptionally tough.

Conclusions

Caerostris darwini produces the toughest known biomaterial. We hypothesize that this extraordinary toughness coevolved with the unusual ecology and web architecture of these spiders, decreasing the likelihood of bridgelines breaking and collapsing the web into the river. This hypothesis predicts that rapid change in material properties of silk co-occurred with ecological shifts within the genus, and can thus be tested by combining material science, behavioral observations, and phylogenetics. Our findings highlight the potential benefits of natural history–informed bioprospecting to discover silks, as well as other materials, with novel and exceptional properties to serve as models in biomimicry.

The adhesive delivery system of viscous capture threads spun by orb-weaving spiders

The sticky viscous capture threads in araneoid orb-webs are responsible for retaining insects that strike these webs. We used features of 16 species'threads and the stickiness that they expressed on contact plates of four widths to model their adhesive delivery systems. Our results confirm that droplets at the edges of thread contact contribute the greatest adhesion, with each successively interior droplet contributing only 0.70 as much adhesion. Thus, regardless of the size and spacing of a thread's large primary droplets,little adhesion accrues beyond a span of 20 droplets. From this pattern we computed effective droplet number (EDN), an index that describes the total droplet equivalents that contribute to the stickiness of thread spans. EDN makes the greatest positive contribution to thread stickiness, followed by an index of the shape and size of primary droplets, and the volume of small secondary droplets. The proportion of water in droplets makes the single greatest negative contribution to thread stickiness, followed by a thread's extensibility, and the area of flattened droplets. Although highly significant, this six-variable model failed to convincingly describe the stickiness of six species, a problem resolved when species were assigned to three groups and a separate model was constructed for each. These models place different weights on the variables and, in some cases, reverse or exclude the contribution of a variable. Differences in threads may adapt them to particular habitats, web architectures or prey types, or they may be shaped by a species' phylogeny or metabolic capabilities.