http://orion.bme.columbia.edu/lulab/pdf/Altman_Silk-Based-Biomaterials_2003.pdf

Silk-based biomaterials

Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the 'wet-rebuilt' fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Directional water collection on wetted spider silk

Jets, ie collimated streams of matter, occur from the microscale up to the large-scale structure of the universe Our focus will be mostly on surface tension effects, which result from the cohesive properties of liquids Paradoxically, cohesive forces promote the breakup of jets, widely encountered in nature, technology and basic science, for example in nuclear fission, DNA sampling, medical diagnostics, sprays, agricultural irrigation and jet engine technology Liquid jets thus serve as a paradigm for free-surface motion, hydrodynamic instability and singularity formation leading to drop breakup In addition to their practical usefulness, jets are an ideal probe for liquid properties, such as surface tension, viscosity or non-Newtonian rheology They also arise from the last but one topology change of liquid masses bursting into sprays Jet dynamics are sensitive to the turbulent or thermal excitation of the fluid, as well as to the surrounding gas or fluid medium The aim of this review is to provide a unified description of the fundamental and the technological aspects of these subjects

Physics of liquid jets

Spider silk has outstanding mechanical properties despite being spun at close to ambient temperatures and pressures using water as the solvent. The spider achieves this feat of benign fibre processing by judiciously controlling the folding and crystallization of the main protein constituents, and by adding auxiliary compounds, to create a composite material of defined hierarchical structure. Because the 'spinning dope' (the material from which silk is spun) is liquid crystalline, spiders can draw it during extrusion into a hardened fibre using minimal forces. This process involves an unusual internal drawdown within the spider's spinneret that is not seen in industrial fibre processing, followed by a conventional external drawdown after the dope has left the spinneret. Successful copying of the spider's internal processing and precise control over protein folding, combined with knowledge of the gene sequences of its spinning dopes, could permit industrial production of silk-based fibres with unique properties under benign conditions.

/pdf/liquid-crystalline-spinning-of-spider-silk-1li41hra2c.pdf

Liquid crystalline spinning of spider silk.

Elastomeric biomaterials for tissue engineering

THE orb web of the garden spider Araneus diadematus, like those of other ecribellate orb spiders, relies on the remarkable extensibility of its sticky capture spiral to intercept and entrap prey1,2. The spiral strands consist of core fibres and an all-enveloping aqueous coat, which forms into a pattern of droplets3. The core fibres are paired and probably made of typical spider silk4,5. The aqueous solution droplets are more or less evenly spaced6 and are required to allow the special mode of extension of the fibres3,7. Knowledge of the chemical composition and general physical properties of this liquid phase is necessary to understand its function in web mechanics and prey capture8. We have now investigated the chemical composition of this aqueous solution and found the droplets to be a concentrated solution of hygroscopic substances related to neurotransmitters. We offer an explanation as to the function and origin of this solution.

Compounds in the droplets of the orb spider's viscid spiral

The garden spider's web filters from the air insects that arrive with considerable kinetic energy. Critical for the absorption of this energy are the tensile strength and extensibility of the silk [1, 2]. To channel this energy, the fine threads must stick firmly to the prey rather than slide off. Thus, adhesiveness may be as important to a web's success as the mechanics of its silk [3]. Although thread mechanics has recently attracted much attention [ 4 8 ] , next to nothing is known about the glue. We have now identified the glue in the web of araneid orb spiders as microscopic nodules made of glycoprotein. The nodules sit astride the capture threads beneath the visible droplets. The finding puts to rest the old notion that the viscous coat itself is the principal adhesive, whereas in reality it is a water reservoir that maintains thread elasticity [6, 8] and also supports glue formation. Orb webs and their builders fall into two categories classified by the silk that renders the web sticky. Cribellate spiders use very fine silk of nm diameter which they comb, in hackled bands of several hundred threads at a time, onto support threads of/~m diameter [9]. We do not understand how this hackled silk functions nor do we understand the mechanism of its adhesiveness [9]. Ecribellate spiders (e.g., the garden cross spider Araneus diadematus) lack cribellure silk. Instead they catch insects using threads of #m diameter coated with an aqueous solution that forms droplets. Coat and droplets are crucial for the function of the capture spiral [8]. The high water content of the coat (80 %) plasticizes the threads and renders them elastic [6]. The droplets, powered by surface tension, keep the threads tight by reeling them in when slackened and unraveling them again when stretched [8]. Chemical analysis of the coat shows high concentrations of five major water-soluble organic compounds [10] and low concentrations of inorganic salts [11, 12] as well as one or more glycoproteins [13 16]. These glycoproteins are the only identified compounds that have sufficiently long side chains to act as glue sensu strictu rather than adhere by virtue of being a highly viscous solution. We now report that the glycoprotein may be concentrated inside distinct structures, nodules that sit astride the capture threads. The nodules, "droplets within droplets" [17], have so far been ignored by research and textbooks on spider orb webs. We have studied nodule morphology, development, function, and chemistry. Nodules, along with the droplets of the viscous coat, form within minutes after the sticky silk has been laid down (Fig. l a c ) . In Araneus, like all ecribellate orb weavers, sticky silk is confined to the capture spiral and consists of a pair of core threads each paid out f rom the central spigot of a triad, the other two spigots providing the aqueous solution of the coat [18]. The two threads immediately merge and the coat flows into distinct droplets connected by a fine layer of liquid [19]. We observed that nodules and droplets correlated in spacing as well as size

Glycoprotein glue beneath a spider web's aqueous coat

Orb web hygroscopicity was examined in three species of araneid spiders, Argiope aurantia, Argiope trifasciata, and Araneus cavaticus. Both whole webs and fractions generated from them (water-soluble and -insoluble fractions; ethanol-soluble and -insoluble subfractions [Etsol and -insol]) were examined at various relative humidities. In general, the water-soluble fraction, consisting of low molecular weight components (LMW) of the adhesive spiral cover, was more hygroscopic than the water-insoluble (protein) fraction. As the Et-sol of the water extract was typically more hygroscopic than the Et-insol, it appears that organic LMW are more responsible for water adsorption by the adhesive spiral cover than inorgnic LMW. Hygroscopicity measurements made on individual synthetic or commercially obtained LMW compounds known to be in the web (both organic and inorganic) agree with this interpretation.



As a first attempt toward correlating web hygroscopicity with composition, the water-soluble web fractions from the three species were examined by proton nuclear magnetic resonance (1H-NMR) spectroscopy and the molar percentages of the organic LMW were estimated. To accomplish this, it was necessary to identify the principal organic LMW in the web of A. trifasciata (since, unlike the situation in other araneids which have been studied, GABamide is not the principal organic LMW in A. trifasciata). Using 1H-NMR and thin layer chromatography this compound was identified as N-acetylputrescine. While considerable interspecific variation in the molar percentages of GABamide, N-acetylputrescine, isethionic acid, and N-acetyltaurine was observed, interspecific differences in Et-sol hygroscopicities were not apparent.

Comparative study of orb web hygroscopicity and adhesive spiral composition in three araneid spiders

The sticky spiral of araneoid spider orb webs consists of silk fibers coated with adhesive droplets. The droplets contain a variety of low-molecular-mass compounds (LMM). Within a species, a fairly consistent ratio of LMM is often observed, but substantial variability can exist. To gain insight into factors influencing LMM composition, spiders of three araneid species were starved and LMM from their webs were analyzed for changes in composition. To determine if these changes were consistent with the spider's ability to synthesize the different organic LMM, synthetic capacities were estimated following the feeding of radiolabeled metabolites. Some changes in droplet composition were broadly consistent with differing synthetic capacities: molar percentages of less readily synthesized compounds (e.g. choline, isethionate, N-acetyltaurine) typically declined with starvation, at least during a portion of the imposed fast, while more readily synthesized compounds (e.g. GABamide, glycine) tended to increase. Most striking was the apparent partial substitution of N-acetylputrescine by the more readily synthesized GABamide in fasting Argiope trifasciata. However, departures from expected compositional shifts demonstrated that synthetic capacity alone does not adequately predict sticky droplet compositional shifts with starvation. Moreover, feeding controls exhibited some changes in composition similar to starving spiders. As the webs of both feeding and starving spiders were removed for chemical analysis and could not be recycled, the loss of LMM contained in these webs likely contributed to similarities between treatments. In addition, feeding spiders molted, oviposited and/or built heavier webs. The added metabolic demands of these activities may have contributed to changes in composition similar to those resulting from starvation.

/pdf/changes-in-composition-of-spider-orb-web-sticky-droplets-4bvu75zx51.pdf

Edward K. Tillinghast

Papers

Compounds in the droplets of the orb spider's viscid spiral

Glycoprotein glue beneath a spider web's aqueous coat

Comparative study of orb web hygroscopicity and adhesive spiral composition in three araneid spiders

Changes in composition of spider orb web sticky droplets with starvation and web removal, and synthesis of sticky droplet compounds

Water extraction by the major ampullate duct during silk formation in the spider, Argiope aurantia Lucas