Direct Solvation of Glycoproteins by Salts in Spider Silk Glues Enhances Adhesion and Helps To Explain the Evolution of Modern Spider Orb Webs

Special Prey, Special Glue: NMR Spectroscopy on Aggregate Glue Components of Moth-Specialist Spiders, Cyrtarachninae

Orb-weaver spiders produce upwards of seven different types of silk, each with unique material properties. We focus on the adhesive within orb-weaving spider webs, aggregate glue silk. These droplets are composed of three main components: water, glycoproteins, and a wide range of low molecular mass compounds (LMMCs). These LMMCs are known to play a crucial role in maintaining the material properties of the glycoproteins, aid in water absorption from the environment, and increase surface adhesion. Orb-weavers within the Cyrtarachninae subfamily are moth specialists and have evolved glue droplets with novel material properties. This study investigated the biochemical composition and diversity of the LMMCs present in the aggregate glue of eight moth-specialist species and compared them with five generalist orb-weavers using nuclear magnetic resonance (NMR) spectroscopy. We hypothesized that the novel drying ability of moth-specialist glue was accompanied by novel LMMCs and lower overall percentages by silk weight of LMMCs. We measured no difference in LMMC weight by the type of prey specialization, but observed novel compositions in the glue of all eight moth-catching species. Further, we quantified the presence of a previously reported but unidentified compound that appears in the glue of all moth specialists. These silks can provide insight into the functions of bioadhesives and inform our own synthetic adhesives.

Bio-Inspired Muco-Adhesive Polymers for Drug Delivery Applications

Muco-adhesive drug delivery systems continue to be one of the most studied for controlled pharmacokinetics and pharmacodynamics. Briefly, muco-adhesive polymers, can be described as bio-polymers that adhere to the mucosal (mucus) surface layer, for an extended residency period of time at the site of application, by the help of interfacial forces resulting in improved drug delivery. When compared to traditional drug delivery systems, muco-adhesive carriers have the potential to enhance therapeutic performance and efficacy, locally and systematically, in oral, rectal, vaginal, amongst other routes. Yet, the achieving successful muco-adhesion in a novel polymeric drug delivery solution is a complex process involving key physico-chemico-mechanical parameters such as adsorption, wettability, polymer chain length, inter-penetration and cross-linking, to list a few. Hence, and in light of accruing progress, evidence and interest, during the last decade, this review aims to provide the reader with an overview of the theories, principles, properties, and underlying mechanisms of muco-adhesive polymers for pharmaceutics; from basics to design to characterization to optimization to evaluation to market. A special focus is devoted to recent advances incorporating bio-inspired polymers for designing controlled muco-adhesive drug delivery systems.

Encapsulated salts in velvet worm slime drive its hardening

Slime expelled by velvet worms entraps prey insects within seconds in a hardened biopolymer network that matches the mechanical strength of industrial polymers. While the mechanic stimuli-responsive nature and building blocks of the polymerization are known, it is still unclear how the velvet worms' slime hardens so fast. Here, we investigated the slime for the first time, not only after, but also before expulsion. Further, we investigated the slime's micro- and nanostructures in-depth. Besides the previously reported protein nanoglobules, carbohydrates, and lipids, we discovered abundant encapsulated phosphate and carbonate salts. We also detected CO₂ bubbles during the hardening of the slime. These findings, along with further observations, suggest that the encapsulated salts in expelled slime rapidly dissolve and neutralize in a baking-powder-like reaction, which seems to accelerate the drying of the slime. The proteins' conformation and aggregation are thus influenced by shear stress and the salts' neutralization reaction, increasing the slime's pH and ionic strength. These insights into the drying process of the velvet worm's slime demonstrate how naturally evolved polymerizations can unwind in seconds, and could inspire new polymers that are stimuli-responsive or fast-drying under ambient conditions.

Change of mechanical characteristics in spider silk capture threads after contact with prey

Most spiders rely on specialized capture threads to subdue prey. Cribellate spiders use capture threads, whose adhesion is based on thousands of nanofibers instead of specialized glue. The nanofibers adhere due to van der Waals and hygroscopic forces, but the adhesion is strengthened by an interaction with the cuticular hydrocarbons (CHCs) covering almost all insects. The interaction between CHCs and cribellate threads becomes visible through migration of the CHCs into the thread even far beyond the point of contact. In this study, we were able to show that the migrated CHCs not only influence adhesion but also change the mechanical characteristics of the thread. While adhesion, extensibility and total energy decreased in threads treated with CHCs from different insects, we observed an increasing force required to break threads. Such mechanical changes could be beneficial for the spider: Upon the first impact of the insect in the web, it is important to absorb all the energy without breaking. Afterwards, a reduction in extensibility could cause the insect to stay closer to the web and thus become additionally entangled in neighboring threads. An increased tensile force would additionally ensure that for insects already in the web, it is even harder to free themselves. Taken together, all these changes make it unlikely that cribellate spiders reuse their capture threads, if not reacting rapidly and removing the prey insect before the CHCs can spread across the thread. STATEMENT OF SIGNIFICANCE: Cribellate spiders use capture threads that, unlike other spiders, consist of nanofibers and do not rely glue. Instead, prey adheres mainly because their surface compounds, so-called cuticular hydrocarbons (CHCs), interact with the thread, this way generating strong adhesion forces. Previous studies on biomechanics and adhesion of cribellate threads only dealt with artificial surfaces, neglecting any interaction with surface compounds. This study examines the dramatical mechanical changes of a cribellate thread after interaction with prey CHCs, showing modifications of the thread's extensibility, tensile force and total energy. Our results highlight the importance of studying mechanical properties of silk not only in an artificial context, but also in real life.

Adhesive contact and protein elastic modulus tune orb weaving spider glue droplet biomechanics to habitat humidity

Tiny glue droplets along the viscous capture threads of spider orb webs prevent insects from escaping. Each droplet is formed of a protein core surrounded by a hygroscopic aqueous layer, which cause the droplet's adhesion to change with humidity. As an insect struggles to escape the web, a thread's viscoelastic core proteins extend, transferring adhesive forces to the thread's support fibers. Maximum adhesive force is achieved when absorbed atmospheric moisture allows a flattened droplet to establish sufficient adhesive contact while maintaining the core protein cohesion necessary for force transfer. We examined the relationship between these droplet properties and adhesive force and the work of extending droplets at five relative humidities in twelve species that occupy habitats which have different humidities. A regression analysis that included both flattened droplet area and core protein elastic modulus described droplet adhesion, but the model was degraded when core protein area was substituted for droplet. Species from low humidity habitats expressed greater adhesion at lower humidities, whereas species from high humidity habitats expressed greater adhesion at high humidities. Our results suggest a general model of droplet adhesion with two adhesion peaks, one for low humidity species, which occurs when increasing droplet area and decreasing protein cohesion intersect, and another for high humidity species, which occurs when area and cohesion have diverged maximally. These dual peaks in adhesive force explain why some species from intermediate and high humidity habitats express high adhesion at several humidities. STATEMENT OF SIGNIFICANCE: We characterized the effect of humidity on the adhesion of twelve orb weaving spider species' glue droplets and showed how humidity-mediated changes in the contact area of a droplet's outer, hygroscopic aqueous layer and the stiffness of its protein core affect droplet performance. This revealed how droplet adhesion has been tuned to the humidity of a species' habitat and allowed us to revise a model that describes the environmental determinants of droplet biomechanics.

Molecular Changes in Spider Viscid Glue As a Function of Relative Humidity Revealed Using Infrared Spectroscopy

Spider aggregate glue can absorb moisture from the atmosphere to reduce its viscosity and become tacky. The viscosity at which glue adhesion is maximized is remarkably similar across spider species, even though that viscosity is achieved at very different relative humidity (RH) values matching their diverse habitats. However, the molecular changes in the protein structure and the bonding state of water (both referred to here as molecular structure) with respect to the changes in RH are not known. We use attenuated total reflectance-infrared (ATR-IR) spectroscopy to probe the changes in the molecular structure of glue as a function of RH for three spider species from different habitats. We find that the glue retains bound water at lower RH and absorbs liquid-like water at higher RH. The absorption of liquid-like water at high RH plasticizes the glue and explains the decrease in glue viscosity. The changes to protein conformations as a function RH are either subtle or not detectable by IR spectroscopy. Importantly, the molecular changes are reversible over multiple cycles of RH change. Further, separation of glue constituents results in a different humidity response as compared to pristine glue, supporting the standing hypothesis that the glue constituents have a synergistic association that makes spider glue a functional adhesive. The results presented in this study provide further insights into the mechanism of the humidity-responsive adhesion of spider glue.

Correlated evolution between orb weaver glue droplets and supporting fibres maintains their distinct biomechanical roles in adhesion

Orb weaving spiders employ a 'silken toolkit' to accomplish a range of tasks, including retaining prey that strike their webs. This is accomplished by a viscous capture spiral thread that features tiny glue droplets, supported by a pair of elastic flagelliform fibres. Each droplet contains a glycoprotein core responsible for adhesion. However, prey retention relies on the integrated performance of multiple glue droplets and their supporting fibres, with previous studies demonstrating that a suspension bridge forms, whose biomechanics sum the adhesive forces of multiple droplets while dissipating the energy of the struggling insect. While the interdependence of the droplet's glycoprotein and flagelliform fibres for functional adhesion is acknowledged, there has been no direct test of this hypothesized linkage between the material properties of each component. Spider mass, which differs greatly across orb weaving species, also has the potential to affect flagelliform fibre and glycoprotein material properties. Previous studies have linked spider mass to capture thread performance but have not examined the relationship between spider mass and thread material properties. We extend earlier studies to examine these relationships in 16 orb weaving species using phylogenetic generalized least squares. This analysis revealed that glycoprotein stiffness (elastic modulus) was correlated with flagelliform fibre stiffness, and that spider mass was related to the glycoprotein volume, flagelliform fibre cross-sectional area and droplets per unit thread length. By shaping the elastic moduli of glycoprotein adhesive and flagelliform fibres, natural selection has maintained the biomechanical integration of this adhesive system.

Humidity mediated performance and material properties of orb weaving spider adhesive droplets

Capture thread glue droplets retain insects that strike an orb web and are key to the success of over 4,600 described spider species. Each droplet is a self-assembling adhesive system whose emergent biomechanical properties are centered on its viscoelastic, protein core. This bioadhesive is dependent on its surrounding hygroscopic aqueous layer for hydration and chemical conditioning. Consequently, a droplet's water content and adhesive performance track environmental humidity. We tested the hypothesis that natural selection has tuned a droplet's adhesive performance and material properties to a species' foraging humidity. At 55% relative humidity (RH) the adhesive properties of 12 species ranged from that of PEG-based hydrogels to that of silicone rubber, exhibiting a 1088-fold inter-specific difference in stiffness (0.02-21.76 MPa) and a 147-fold difference in toughness (0.14-20.51 MJ/m³). When tested over a 70% RH range, droplet extension lengths per protein core volume peaked at lower humidities in species from exposed, low humidity habitats, and at higher humidities in nocturnal species and those found in humid habitats. However, at the RH's where these species' maximum extension per protein volume indices were observed, the stiffness of most species' adhesive did not differ, documenting that selection has tuned elastic modulus by adjusting droplet hygroscopicity. This inverse relationship between droplet hygroscopicity and a species' foraging humidity ensures optimal adhesive stiffness. By characterizing the humidity responsiveness and properties of orb spider glue droplets, our study also profiles the range of its biomimetic potential. STATEMENT OF SIGNIFICANCE: Over 4,600 described species of orb weaving spider rely on tiny glue droplets in their webs to retain insect that the web intercepts. The aqueous layer that covers each droplet's core allows this adhesive to remain pliable and to stretch as an insect struggles to escape. The aqueous solution also attracts water from the air, causing the glue droplet's performance to change with humidity. By characterizing the droplet extensions and adhesive material properties of twelve species at relative humidities between of 20 and 90%, this study examined how this unique adhesive system responds to its environment and how it is tuned to the humidity of a species' habitat.

Repeat variation resolves a complete aggregate silk sequence of bolas spider Mastophora phrynosoma

Many species of spider use a modified silk adhesive, called aggregate glue, to aid in prey capture. Aggregate spidroins (spider fibroins) are modified members of the spider silk family, however they are not spun into fibers as are their solid silk relatives. The genes that encode for aggregate spidroins are the largest of the known spidroin genes and are similarly highly repetitive. In this study, we used long read sequencing to discover the aggregate spidroin genes of the toad-like bolas spider, Mastophora phrynosoma, which employs the glue in a unique way, using only a single, large droplet to capture moths. While Aggregate Spidroin 1 (AgSp1) remains incomplete, AgSp2 is more than an extraordinary 62 kilobases of coding sequence, 20 kb longer than the longest spidroin on record. The structure of repeats from both aggregate silk proteins follows a similar pattern seen in other species, with the same strict conservation of amino acid residue number for much of the repeats' lengths. Interestingly, AgSp2 lacks the elevated number and groupings of glutamine residues seen in the other reported AgSp2 of a classic orb weaving species. The role of gene length in glue functionality remains a mystery, and thus discovering length differences across species will allow understanding and harnessing of this attribute for the next generation of bio-inspired adhesives.

Robust performance of spider viscid silk on hairy and smooth insect substrates

Spider viscid silk adheres to insects in orb webs and is a "smart-adhesive" that quickly changes droplet size, viscosity, and adhesiveness in response to atmospheric humidity. Different species of spiders "tune" water uptake to match the humidity of their foraging environments, achieving a similar "universal" viscosity that optimizes tradeoffs in spreading versus cohesive bulk energy needed to enhance adhesion. Too much water lowers viscosity so that the glue spreads well, but cohesive failure occurs easily, generating poor adhesion. However, the optimal viscosity model of adhesion is based on experiments using smooth glass. Here we test the hypothesis that a less viscous, "over-lubricated" glue, which shows poor adhesion on smooth glass, will be stickier on hairy insects because of its greater ability to spread across three-dimensional rough surfaces. We ran adhesion tests of the furrow spider (Larinioides cornutus (Clerck, 1757)) viscid silk on honey bee (Apis mellifera) thorax, with and without hairs, in either high or medium humidity. Our results show that "over-lubricated" glue increases adhesion on hairy surfaces, performing equally as well as an optimally viscous glue.

A Spider-Silk-Inspired Wet Adhesive with Supercold Tolerance

Conventional adhesives often encounter interfacial failure in humid conditions due to small droplets of water condensed on surface, but spider silks can capture prey in such environment. Here a robust spider-silk-inspired wet adhesive (SA) composed of core-sheath nanostructured fibers with hygroscopic adhesive nanosheath (poly(vinylpyrrolidone)) and supporting nanocore (polyurethane) is reported. The wet adhesion of the SA is achieved by a unique dissolving-wetting-adhering process of core-sheath nanostructured fibers, revealed by in situ observations at macro- and microscales. Further, the SA maintains reliable adhesion on wet and cold substrates from 4 to -196 °C and even tolerates splashing, violent shaking, and weight loading in liquid nitrogen (-196 °C), showing promising applicability in cryogenic environments. This study will provide an innovative route to design functional wet adhesives.

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.

Properties of orb weaving spider glycoprotein glue change during Argiope trifasciata web construction

An orb web’s prey capture thread relies on its glue droplets to retain insects until a spider can subdue them. Each droplet’s viscoelastic glycoprotein adhesive core extends to dissipate the forces of prey struggle as it transfers force to stiffer, support line flagelliform fibers. In large orb webs, switchback capture thread turns are placed at the bottom of the web before a continuous capture spiral progresses from the web’s periphery to its interior. To determine if the properties of capture thread droplets change during web spinning, we characterized droplet and glycoprotein volumes and material properties from the bottom, top, middle, and inner regions of webs. Both droplet and glycoprotein volume decreased during web construction, but there was a progressive increase in the glycoprotein’s Young’s modulus and toughness. Increases in the percentage of droplet aqueous material indicated that these increases in material properties are not due to reduced glycoprotein viscosity resulting from lower droplet hygroscopicity. Instead, they may result from changes in aqueous layer compounds that condition the glycoprotein. A 6-fold difference in glycoprotein toughness and a 70-fold difference in Young’s modulus across a web documents the phenotypic plasticity of this natural adhesive and its potential to inspire new materials.

Linking properties of an orb-weaving spider's capture thread glycoprotein adhesive and flagelliform fiber components to prey retention time

An orb web's adhesive capture spiral is responsible for prey retention. This thread is formed of regularly spaced glue droplets supported by two flagelliform axial lines. Each glue droplet features a glycoprotein adhesive core covered by a hygroscopic aqueous layer, which also covers axial lines between the droplets, making the entire thread responsive to environmental humidity.We characterized the effect of relative humidity (RH) on ability of Argiope aurantia and Argiope trifasciata thread arrays to retain houseflies and characterize the effect of humidity on their droplet properties. Using these data and those of Araneus marmoreus from a previous study, we then develop a regression model that correlated glycoprotein and flagelliform fiber properties with prey retention time. The model selection process included newly determined, humidity-specific Young's modulus and toughness values for the three species' glycoproteins.Argiope aurantia droplets are more hygroscopic than A. trifasciata droplets, causing the glycoprotein within A. aurantia droplets to become oversaturated at RH greater than 55% RH and their extension to decrease, whereas A. trifasciata droplet performance increases to 72% RH. This difference is reflected in species' prey retention times, with that of A. aurantia peaking at 55% RH and that of A. trifasciata at 72% RH.Fly retention time was explained by a regression model of five variables: glue droplet distribution, flagelliform fiber work of extension, glycoprotein volume, glycoprotein thickness, and glycoprotein Young's modulus.The material properties of both glycoprotein and flagelliform fibers appear to be phylogenetically constrained, whereas natural selection can more freely act on the amount of each material invested in a thread and on components of the thread's aqueous layer. Thus, it becomes easier to understand how natural selection can tune the performance of viscous capture threads by directing small changes in these components.

Synergistic adhesion mechanisms of spider capture silk

It is well known that capture silk, the main sticky component of the orb web of a spider, plays an important role in the spider's ability to capture prey via adhesion. However, the detailed mechanism with which the spider achieves its unparalleled high-adhesion performance remains elusive. In this work, we combine experiments and theoretical analysis to investigate the adhesion mechanisms of spider silk. In addition to the widely recognized adhesion effect of the sticky glue, we reveal a synergistic enhancement mechanism due to the elasticity of silk fibres. A balance between silk stiffness, strength and glue stickiness is crucial to endow the silk with superior adhesion, as well as outstanding energy absorption capacity and structural robustness. The revealed mechanisms deepen our understanding of the working principles of spider silk and suggest guidelines for biomimetic designs of spider-inspired adhesion and capture devices.

Loss of Phosphate Determines the Versatility of a Spider Orb-web Glue Ball

Spiders capture their prey by weaving an "invisible" orb-web that has both adhesive and fixed properties. Different types of silk in the orb-web have different functions, wherein the key to capturing a prey is the ball-like glue (glue ball), which coats the silk strands. This glue ball has highly versatile properties, but the mechanisms leading to its versatility remain unclear. The salts found in the web have been previously suggested to play an important role in terms of viscosity, not water. However, the distribution of salt and water in the glue ball has not yet been directly observed. Here, we mapped the salts in different states using a homemade time-of-flight secondary ion mass spectrometer (TOF-SIMS) with a high lateral resolution. To our surprise, the glue ball was found to contain little water. The functional transformation of the glue ball from a viscous glycoprotein (capturing prey) to a hardened protein (retaining prey) relies solely on the stimulation of mechanical forces. The phosphate is a key factor for its versatility.

Toward Spider Glue: Long Read Scaffolding for Extreme Length and Repetitious Silk Family Genes AgSp1 and AgSp2 with Insights into Functional Adaptation

An individual orb weaving spider can spin up to seven different types of silk, each with unique functions and material properties. The capture spiral silk of classic two-dimensional aerial orb webs is coated with an amorphous glue that functions to retain prey that get caught in a web. This unique modified silk is partially comprised of spidroins (spider fibroins) encoded by two members of the silk gene family. The glue differs from solid silk fibers as it is a viscoelastic, amorphic, wet material that is responsive to environmental conditions. Most spidroins are encoded by extremely large, highly repetitive genes that cannot be sequenced using short read technology alone, as the repetitive regions are longer than read length. We sequenced for the first time the complete genomic Aggregate Spidroin 1 (AgSp1) and Aggregate Spidroin 2 (AgSp2) glue genes of orb weaving spider Argiope trifasciata using error-prone long reads to scaffold for high accuracy short reads. The massive coding sequences are 42,270 bp (AgSp1) and 20,526 bp (AgSp2) in length, the largest silk genes currently described. The majority of the predicted amino acid sequence of AgSp1 consists of two similar but distinct motifs that are repeated ∼40 times each, while AgSp2 contains ∼48 repetitions of an AgSp1-similar motif, interspersed by regions high in glutamine. Comparisons of AgSp repetitive motifs from orb web and cobweb spiders show regions of strict conservation followed by striking diversification. Glues from these two spider families have evolved contrasting material properties in adhesion (stickiness), extensibility (stretchiness), and elasticity (the ability of the material to resume its native shape), which we link to mechanisms established for related silk genes in the same family. Full-length aggregate spidroin sequences from diverse species with differing material characteristics will provide insights for designing tunable bio-inspired adhesives for a variety of unique purposes.

DNP NMR spectroscopy reveals new structures, residues and interactions in wild spider silks

DNP solid state NMR spectroscopy allows non-targeted analysis of wild spider silk in unprecedented detail at natural abundance, revealing hitherto unreported features across several species. A >50-fold signal enhancement for each silk, enables the detection of novel H-bonding networks and arginine conformations, and the post-translational modified amino acid, hydroxyproline.

Supersaturation with water explains the unusual adhesion of aggregate glue in the webs of the moth-specialist spider, Cyrtarachne akirai

Orb webs produced by araneoid spiders depend upon aggregate glue-coated capture threads to retain their prey. Moths are challenging prey for most spiders because their scales detach and contaminate the glue droplets, significantly decreasing adhesion. Cyrtarachne are moth-specialist orb-weaving spiders whose capture threads adhere well to moths. We compare the adhesive properties and chemistry of Cyrtarachne aggregate glue to other orb-weaving spiders to test hypotheses about their structure, chemistry and performance that could explain the strength of Cyrtarachne glue. We show that the unusually large glue droplets on Cyrtarachne capture threads make them approximately 8 times more adhesive on glass substrate than capture threads from typical orb-weaving species, but Cyrtarachne adhesion is similar to that of other species after normalization by glue volume. Glue viscosity reversibly changes over 1000-fold in response to atmospheric humidity, and the adhesive strength of many species of orb spiders is maximized at a viscosity of approximately 10⁵-10⁶ cst where the contributions of spreading and bulk cohesion are optimized. By contrast, viscosity of Cyrtarachne aggregate glue droplets is approximately 1000 times lower at maximum adhesive humidity, likely facilitating rapid spreading across moth scales. Water uptake by glue droplets is controlled, in part, by hygroscopic low molecular weight compounds. NMR showed evidence that Cyrtarachne glue contains a variety of unknown low molecular weight compounds. These compounds may help explain how Cyrtarachne produces such exceptionally large and low viscosity glue droplets, and also why these glue droplets rapidly lose water volume after brief ageing or exposure to even slightly dry (e.g. < 80% RH) conditions, permanently reducing their adhesion. We hypothesize that the combination of large glue droplet size and low viscosity helps Cyrtarachne glue to penetrate the gaps between moth scales.

Elastic modulus and toughness of orb spider glycoprotein glue

An orb web's prey capture thread features tiny glue droplets, each formed of an adhesive glycoprotein core surrounded by an aqueous layer. Small molecules in the aqueous layer confer droplet hygroscopicity and maintain glycoprotein viscoelasticity, causing droplet volume and glycoprotein performance to track changes in environmental humidity. Droplet extension combines with that of a thread's supporting flagelliform fibers to sum the adhesive forces of multiple droplets, creating an effective adhesive system. We combined measurements of the force on an extending droplet, as gauged by the deflection of its support line, with measurements of glycoprotein volume and droplet extension to determine the Young's modulus (E) and toughness of three species' glycoproteins. We did this at five relative humidities between 20-90% to assess the effect of humidity on these properties. When droplets of a thread span extend, their extensions are constrained and their glycoprotein filaments remain covered by aqueous material. This was also the case during the first extension phase of the individual droplets that we examined. However, as extension progressed, the aqueous layer was progresses disrupted, exposing the glycoprotein. During the first extension phase E ranged from 0.00003 GPa, a value similar to that of fibronectin, a glycoprotein that anchors cells in the extracellular matrix, to 0.00292 GPa, a value similar to that of resilin in insect ligaments. Second phase E increased 4.7-19.4-fold. When compared at the same humidity the E of each species' glycoprotein was less than 5% of the value reported for its flagelliform fibers. This difference may facilitate the coordinated extension of these two capture thread components that is responsible for summing the thread's adhesive forces.

Hygroscopic compounds in spider aggregate glue remove interfacial water to maintain adhesion in humid conditions

Adhesion in humid environments is fundamentally challenging because of the presence of interfacial bound water. Spiders often hunt in wet habitats and overcome this challenge using sticky aggregate glue droplets whose adhesion is resistant to interfacial failure under humid conditions. The mechanism by which spider aggregate glue avoids interfacial failure in humid environments is still unknown. Here, we investigate the mechanism of aggregate glue adhesion by using interface-sensitive spectroscopy in conjunction with infrared spectroscopy. We demonstrate that glycoproteins act as primary binding agents at the interface. As humidity increases, we observe reversible changes in the interfacial secondary structure of glycoproteins. Surprisingly, we do not observe liquid-like water at the interface, even though liquid-like water increases inside the bulk with increasing humidity. We hypothesize that the hygroscopic compounds in aggregate glue sequester interfacial water. Using hygroscopic compounds to sequester interfacial water provides a novel design principle for developing water-resistant synthetic adhesives.

Spider aggregate glue avoids failure in humid environments but the fundamental mechanism behind it is still unknown. Here, the authors demonstrate that humidity-dependent structural changes of glycoproteins and sequestering of liquid water by low molecular mass compounds prevents adhesion failure of the glue in humid environments.

Tuning orb spider glycoprotein glue performance to habitat humidity

Orb-weaving spiders use adhesive threads to delay the escape of insects from their webs until the spiders can locate and subdue the insects. These viscous threads are spun as paired flagelliform axial fibers coated by a cylinder of solution derived from the aggregate glands. As low molecular mass compounds (LMMCs) in the aggregate solution attract atmospheric moisture, the enlarging cylinder becomes unstable and divides into droplets. Within each droplet an adhesive glycoprotein core condenses. The plasticity and axial line extensibility of the glycoproteins are maintained by hygroscopic LMMCs. These compounds cause droplet volume to track changes in humidity and glycoprotein viscosity to vary approximately 1000-fold over the course of a day. Natural selection has tuned the performance of glycoprotein cores to the humidity of a species' foraging environment by altering the composition of its LMMCs. Thus, species from low-humidity habits have more hygroscopic threads than those from humid forests. However, at their respective foraging humidities, these species' glycoproteins have remarkably similar viscosities, ensuring optimal droplet adhesion by balancing glycoprotein adhesion and cohesion. Optimal viscosity is also essential for integrating the adhesion force of multiple droplets. As force is transferred to a thread's support line, extending droplets draw it into a parabolic configuration, implementing a suspension bridge mechanism that sums the adhesive force generated over the thread span. Thus, viscous capture threads extend an orb spider's phenotype as a highly integrated complex of large proteins and small molecules that function as a self-assembling, highly tuned, environmentally responsive, adhesive biomaterial. Understanding the synergistic role of chemistry and design in spider adhesives, particularly the ability to stick in wet conditions, provides insight in designing synthetic adhesives for biomedical applications.

Fitness consequences of plasticity in an extended phenotype

Like regular phenotypes, extended phenotypes have demonstrable fitness advantages and their properties may vary plastically across environments. However, the fitness advantages of plasticity are only known for a select few extended phenotypes. It is known that the form and functions of spider orb webs can be manipulated by laboratory experiments. For instance, the physical and chemical properties of the spiral and gluey silks vary in property as protein intake varies. Orb web spiders thus represent good models for extended phenotypic plasticity studies. We performed experiments manipulating the protein intake of two vertically aligned orb web building spiders to determine whether variations in the chemical and physical properties of their spiral and gluey silk affect prey retention in their webs. We found in both spider species that individuals deprived of protein had a greater gluey silk glycoprotein core volume, and this correlated strongly with spiral thread stickiness and increased prey retention by the webs. Moreover, we found strong positive correlations between glue droplet volume and glycoprotein core volume for spiders in the protein-deprived treatment, but weaker correlations for protein-fed spiders. We interpreted these findings as the spiders investing more in glycoprotein when nutrient deprived. We attribute the associated increase in prey retention capacity as a fitness consequence of plasticity in the spiral properties.

Adhesion modulation using glue droplet spreading in spider capture silk

Orb web spiders use sticky capture spiral silk to retain prey in webs. Capture spiral silk is composed of an axial fibre of flagelliform silk covered with glue droplets that are arranged in a beads-on-a-string morphology that allows multiple droplets to simultaneously extend and resist pull off. Previous studies showed that the adhesion of capture silk is responsive to environmental humidity, increasing up to an optimum humidity that varied among different spider species. The maximum adhesion was hypothesized to occur when the viscoelasticity of the glue optimized contributions from glue spreading and bulk cohesion. In this study, we show how glue droplet shape during peeling contributes significantly to capture silk adhesion. Both overspreading and underspreading of glue droplets reduces adhesion through changes in crack propagation and failure regime. Understanding the mechanism of stimuli-responsive adhesion of spider capture silk will lead to new designs for smarter adhesives.

Controlled self-assembly of glycoprotein complex in snail mucus from lubricating liquid to elastic fiber

Terrestrial snail could intelligently employ elastic fibers clinging itself onto the inverted smooth plate.

Humidity-mediated changes in an orb spider's glycoprotein adhesive impact prey retention time

Properties of the viscous prey capture threads of araneoid orb spiders change in response to their environment. Relative humidity (RH) affects the performance of the thread's hygroscopic droplets by altering the viscoelasticity of each droplet's adhesive glycoprotein core. Studies that have characterized this performance used smooth glass and steel surfaces and uniform forces. In this study, we tested the hypothesis that these changes in performance translate into differences in prey retention times. We first characterized the glycoprotein contact surface areas and maximum extension lengths of Araneus marmoreus droplets at 20%, 37%, 55%, 72% and 90% RH and then modeled the relative work required to initiate pull-off of a 4 mm thread span, concluding that this species' droplets and threads performed optimally at 72% RH. Next, we evaluated the ability of three equally spaced capture thread strands to retain a house fly at 37%, 55% and 72% RH. Each fly's struggle was captured in a video and bouts of active escape behavior were summed. House flies were retained 11 s longer at 72% RH than at 37% and 55% RH. This difference is ecologically significant because the short time after an insect strikes a web and before a spider begins wrapping it is an insect's only opportunity to escape from the web. Moreover, these results validate the mechanism by which natural selection can tune the performance of an orb spider's capture threads to the humidity of its habitat.

Scaling down for a broader understanding of underwater adhesives - a case for the Caulobacter crescentus holdfast

The adhesion of two materials in the presence of water is greatly impeded by a boundary layer of water between the adhesive and the adherend, resulting in adhesive failure of most synthetic adhesives; however, life evolved first in water and there are many aquatic organisms that have to overcome this impediment to underwater adhesion. For example, multicellular aquatic organisms like the mussel, sandcastle worm and the caddisfly larva employ well-studied adhesive mechanisms for sticking in the presence of water. Unicellular organisms such as bacteria also make use of various means for attaching to surfaces, within similar environmental conditions. Prominent among them is the aquatic bacteria, Caulobacter crescentus which utilizes a unique adhesive secretion, the holdfast, to adhere strongly in the presence of water. Here we review the attachment mechanisms of some multicellular aquatic organisms and compare the similarities and differences in the composition and structure of the C. crescentus holdfast, which holds promise as a potential source for bio-inspired synthetic underwater adhesives with prospective applications in medicine, engineering and biomimetics.

Evidence from Multiple Species that Spider Silk Glue Component ASG2 is a Spidroin

Spiders in the superfamily Araneoidea produce viscous glue from aggregate silk glands. Aggregate glue coats prey-capture threads and hampers the escape of prey from webs, thereby increasing the foraging success of spiders. cDNAs for Aggregate Spider Glue 1 (ASG1) and 2 (ASG2) have been previously described from the golden orb-weaver, Nephila clavipes, and Western black widow, Latrodectus hesperus. To further investigate aggregate glues, we assembled ASG1 and ASG2 from genomic target capture libraries constructed from three species of cob-web weavers and three species of orb-web weavers, all araneoids. We show that ASG1 is unlikely to be a glue, but rather is part of a widespread arthropod gene family, the peritrophic matrix proteins. For ASG2, we demonstrate its remarkable architectural and sequence similarities to spider silk fibroins, indicating that ASG2 is a member of the spidroin gene family. Thus, spidroins have diversified into glues in addition to task-specific, high performance fibers.

Diet-induced covariation between architectural and physicochemical plasticity in an extended phenotype

The adaptive benefits of extended phenotypic plasticity are imprecisely defined due to a paucity of experiments examining traits that are manipulable and measurable across environments. Spider webs are often used as models to explore the adaptive benefits of variations in extended phenotypes across environments. Nonetheless, our understanding of the adaptive nature of the plastic responses of spider webs is impeded when web architectures and silk physicochemical properties appear to co-vary. An opportunity to examine this co-variation is presented by modifying prey items while measuring web architectures and silk physiochemical properties. Here we performed two experiments to assess the nature of the association between web architectures and gluey silk properties when the orb web spider Argiope keyserlingi was fed a diet that varied in either mass and energy or prey size and feeding frequency. We found web architectures and gluey silk physicochemical properties to co-vary across treatments in both experiments. Specifically, web capture area co-varied with gluey droplet morphometrics, thread stickiness and salt concentrations when prey mass and energy were manipulated, and spiral spacing co-varied with gluey silk salt concentrations when prey size and feeding frequency were manipulated. We explained our results as A. keyserlingi plastically shifting its foraging strategy as multiple prey parameters simultaneously varied. We confirmed and extended previous work by showing that spiders use a variety of prey cues to concurrently adjust web and silk traits across different feeding regimes.

The Potential of Silk and Silk-Like Proteins as Natural Mucoadhesive Biopolymers for Controlled Drug Delivery

Drug delivery across mucus membranes is a particularly effective route of administration due to the large surface area. However, the unique environment present at the mucosa necessitates altered drug formulations designed to (1) deliver sensitive biologic molecules, (2) promote intimate contact between the mucosa and the drug, and (3) prolong the drug's local residence time. Thus, the pharmaceutical industry has an interest in drug delivery systems formulated around the use of mucoadhesive polymers. Mucoadhesive polymers, both synthetic and biological, have a history of use in local drug delivery. Prominently featured in the literature are chitosan, alginate, and cellulose derivatives. More recently, silk and silk-like derivatives have been explored for their potential as mucoadhesive polymers. Both silkworms and spiders produce sticky silk-like glue substances, sericin and aggregate silk respectively, that may prove an effective, natural matrix for drug delivery to the mucosa. This mini review will explore the potential of silk and silk-like derivatives as a biocompatible mucoadhesive polymer matrix for local controlled drug delivery.

Spiders Tune Glue Viscosity to Maximize Adhesion

Adhesion in humid conditions is a fundamental challenge to both natural and synthetic adhesives. Yet, glue from most spider species becomes stickier as humidity increases. We find the adhesion of spider glue, from five diverse spider species, maximizes at very different humidities that matches their foraging habitats. By using high-speed imaging and spreading power law, we find that the glue viscosity varies over 5 orders of magnitude with humidity for each species, yet the viscosity at maximal adhesion for each species is nearly identical, 10(5)-10(6) cP. Many natural systems take advantage of viscosity to improve functional response, but spider glue's humidity responsiveness is a novel adaptation that makes the glue stickiest in each species' preferred habitat. This tuning is achieved by a combination of proteins and hygroscopic organic salts that determines water uptake in the glue. We therefore anticipate that manipulation of polymer-salts interaction to control viscosity can provide a simple mechanism to design humidity responsive smart adhesives.

Ubiquitous distribution of salts and proteins in spider glue enhances spider silk adhesion

Modern orb-weaving spiders use micron-sized glue droplets on their viscid silk to retain prey in webs. A combination of low molecular weight salts and proteins makes the glue viscoelastic and humidity responsive in a way not easily achieved by synthetic adhesives. Optically, the glue droplet shows a heterogeneous structure, but the spatial arrangement of its chemical components is poorly understood. Here, we use optical and confocal Raman microscopy to show that salts and proteins are present ubiquitously throughout the droplet. The distribution of adhesive proteins in the peripheral region explains the superior prey capture performance of orb webs as it enables the entire surface area of the glue droplet to act as a site for prey capture. The presence of salts throughout the droplet explains the recent Solid-State NMR results that show salts directly facilitate protein mobility. Understanding the function of individual glue components and the role of the droplet's macro-structure can help in designing better synthetic adhesives for humid environments.

Can differential nutrient extraction explain property variations in a predatory trap?

Predators exhibit flexible foraging to facilitate taking prey that offer important nutrients. Because trap-building predators have limited control over the prey they encounter, differential nutrient extraction and trap architectural flexibility may be used as a means of prey selection. Here, we tested whether differential nutrient extraction induces flexibility in architecture and stickiness of a spider's web by feeding Nephila pilipes live crickets (CC), live flies (FF), dead crickets with the web stimulated by flies (CD) or dead flies with the web stimulated by crickets (FD). Spiders in the CD group consumed less protein per mass of lipid or carbohydrate, and spiders in the FF group consumed less carbohydrates per mass of protein. Spiders from the CD group built stickier webs that used less silk, whereas spiders in the FF group built webs with more radii, greater catching areas and more silk, compared with other treatments. Our results suggest that differential nutrient extraction is a likely explanation for prey-induced spider web architecture and stickiness variations.

Experimental strategies for the identification and characterization of adhesive proteins in animals: a review

Adhesive secretions occur in both aquatic and terrestrial animals, in which they perform diverse functions. Biological adhesives can therefore be remarkably complex and involve a large range of components with different functions and interactions. However, being mainly protein based, biological adhesives can be characterized by classical molecular methods. This review compiles experimental strategies that were successfully used to identify, characterize and obtain the full-length sequence of adhesive proteins from nine biological models: echinoderms, barnacles, tubeworms, mussels, sticklebacks, slugs, velvet worms, spiders and ticks. A brief description and practical examples are given for a variety of tools used to study adhesive molecules at different levels from genes to secreted proteins. In most studies, proteins, extracted from secreted materials or from adhesive organs, are analysed for the presence of post-translational modifications and submitted to peptide sequencing. The peptide sequences are then used directly for a BLAST search in genomic or transcriptomic databases, or to design degenerate primers to perform RT-PCR, both allowing the recovery of the sequence of the cDNA coding for the investigated protein. These sequences can then be used for functional validation and recombinant production. In recent years, the dual proteomic and transcriptomic approach has emerged as the best way leading to the identification of novel adhesive proteins and retrieval of their complete sequences.