Spiders' webs are coated with microscopic droplets of glue, but the properties of this adhesive were unclear. It has now been found that the glue's stretchiness underpins its role in catching flies.
Man-made glues are mono-functional — their material properties are designed to stick one thing to another, and that's it. But in Nature Communications, Sahni et al.1 report that the 'glue' droplets that coat spiders' webs are multi-functional. Depending on the rate at which they are extended, the droplets act either as a viscous adhesive or as a rubber-like elastic solid.
Adhesive coatings are found everywhere. Think of the paint that covers walls, cars and ships' hulls; metals, such as gold and silver, plated on jewellery; and dyes that change the colour of fabrics and hair. These coatings have a variety of functions — paint prevents barnacles and mussels from attaching themselves to ships' hulls, reducing drag on the ship and so improving fuel efficiency, whereas gold- and silver-plated surfaces are anti-corrosive.
Yet adhesive materials in nature do much more. Mussels, for example, secrete a substance that forms into 'byssus threads', by which they attach themselves to rocks, shells and even to feathers and fish skin (Fig. 1a). The threads must have a high degree of elasticity, or they would snap under the physical impact of lashing tides. But they need the additional property of adhesiveness, provided by a coating of a glue-like material, to anchor the mussels in place. What's more, the threads must maintain their grip under water, an environment in which most adhesives function poorly.
Sahni and colleagues' study1 of glue droplets on spiders' webs suggests that the coupling of adhesion with extension is a common design principle of natural adhesives. The droplets consist of a complex mixture of glycoproteins along with a variety of viscous small molecules and salts. The role of the components of the droplets has been difficult to prove, not least because it is difficult to separate their properties from those of the underlying spider silk.
The authors overcame this problem by immobilizing silk threads on a glass surface, and touching glue droplets to the threads with a tiny glass probe. They then pulled the probe away at a constant speed, and measured the force exerted on the probe's tip as a function of distance from the droplet, until contact between the droplet and the probe was broken. They found that the force–distance response depended on the speed with which the probe was pulled away: rapid extensions of the glue caused it to become highly viscous, whereas slow extensions turned the glue into an elastic-solid-like material.
This behaviour correlates well with the two functions that the droplets perform in nature: capturing prey and then retaining it. When prey — typically a flying insect — is captured in a web, the silk rapidly extends on impact. Sahni and colleagues' study reveals that the glue droplets would become highly viscous under these conditions, providing maximum adhesion to effectively capture the spider's meal.
But after the prey has been captured, the movement produced by its attempts to escape causes a slow extension of the web. In this scenario, the glue droplets turn into a rubber-band-like material to prevent the unfortunate prey from escaping (Fig. 1b). The glue droplets secreted by spiders therefore constitute an 'intelligent' adhesive whose properties change significantly depending on the extension rate of the underlying silk. Needless to say, human technology has not been successful in producing such a smart adhesive material.
So what is the molecular basis of the adhesion–extension properties of naturally occurring glues? In the case of the adhesive used by mussels, these properties derive from the presence in the glue molecules of a repeating pair of amino acids: lysine, one of the common amino acids, and 3,4-dihydroxy-L-phenylalanine (L-DOPA)2, which is more unusual. Molecules that contain extensively repeating L-DOPA–lysine motifs stick to a wide range of surfaces by forming various kinds of covalent and non-covalent bonds3. Meanwhile, metal ions in sea water infiltrate the adhesive threads, interconnecting scaffold proteins covalently and/or by coordination4 (a process in which molecules bind non-covalently to metal ions). This intermolecular crosslinking confers extensibility on the adhesive threads.
But in-depth knowledge of the molecular structure of the glue droplets on spiders' webs is lacking. It has been reported that the droplets contain small molecules such as neurotransmitters, amino acids and peptides5, as well as macromolecules such as glycoproteins6. There is evidence to suggest that the sugars found in glycoproteins are responsible for the stickiness of glues secreted by other organisms. For example, the sugar N-acetyl-D-glucosamine is important for the adhesion of the holdfast of the bacterium Caulobacter crescentus7. The molecular components that cause spider-glue droplets to form a rubbery solid remain unknown. A series of studies investigating the molecular content and supramolecular assembly of glue droplets is therefore required.
Sahni and colleagues' studies1 are just the tip of an enormous iceberg, as there are many other interesting biomaterials that could be investigated by curious scientists — such as the adhesive flavonoid compounds found in plants, or the viscoelastic biofilms produced by microbes. Future investigations will no doubt reveal their secrets, and provide inspiration for the design of advanced synthetic materials.
Sahni, V., Blackledge, T. A. & Dhinojwala, A. Nature Commun. 1, doi:10.1038/ncomms1019 (2010).
Waite, J. H. & Tanzer, M. L. Science 212, 1038–1040 (1981).
Lee, H. et al. Science 318, 426–430 (2007).
Waite, J. H., Vaccaro, E., Sun, C. & Lucas, J. M. Phil. Trans. R. Soc. Lond. B 357, 143–153 (2002).
Vollrath, F. et al. Nature 345, 526–527 (1990).
Choresh, O., Bayarmagnai, B. & Lewis R. V. Biomacromolecules 10, 2852–2856 (2009).
Tsang, P. H., Li, G., Brun, Y. V., Freund, L. B. & Tang, J. X. Proc. Natl Acad. Sci. USA 103, 5764–5768 (2006).
About this article
Advanced Materials (2019)
Direct Ordering of Anchoring Events at the Surface of Iron Oxide Nanoparticles Enabled by A Stepwise Phase-Transfer Strategy
3,4-Dihydroxy-L-Phenylalanine as a Novel Covalent Linker of Extracellular Matrix Proteins to Polyacrylamide Hydrogels with a Tunable Stiffness
Tissue Engineering Part C: Methods (2016)
The Potential of Silk and Silk-Like Proteins as Natural Mucoadhesive Biopolymers for Controlled Drug Delivery
Frontiers in Chemistry (2015)