Molecular-level insight into how spiders make glue that works in humid conditions could provide clues for better performance in commercially made adhesives.
In a paper published in the journal Nature Communications, a team led by polymer scientist Saranshu Singla from the University of Akron in Ohio, US, describes the glue-manufacturing process of the common orb spider (Larinioides cornutus) and uncovers how it overcomes the primary obstacle to achieving adhesion in the wet – water between the glue and the target surface.
It is the same challenge faced – and largely so far unmet – by glue manufacturers. Interfacial water, as it’s known, forms a slippery and non-adhesive layer between the glue and the surface to which it is meant to stick, with the result that a bond fails to manifest.
Many species of spider, however, live in humid conditions, where pretty much everything is permanently wet, yet succeed in producing a glue that sticks firmly to anything and stays stuck, allowing the silk to which it is attached to be used for nesting and prey capture.
Key to this success, Singla and colleagues note, is the way in which it overcomes the issue of interfacial water. Glue from spiders that live in dry, desert environments, the authors note, still performs well in humid conditions – and fails only because stress causes it to fracture, rather than because moisture makes it slip.
To investigate the processes involved, the scientists took orb spider glue, set it on sapphire, then examined it using a combination of interface-sensitive spectroscopy and infrared spectroscopy.
They discovered that the glue comprises three elements – two specialised glycoproteins (dubbed, imaginatively, aggregate spider glue 1 and 2) and a collection of low molecular mass organic and inorganic compounds (LMMCs).
The first glycoprotein contains a high proportion of charged amino acids, while the second has structures similar to those that comprise spider silk, making the glue highly elastic. The LMMCs are a mixed bag, which variously function to combine with external water, sequester it, and to make the glue soft and tacky.
Singla and colleagues determined that it is the interaction of all three elements that governs the adhesive quality of the glue produced, with the respective proportions varying by species, thus optimising adhesive strength to match the relative humidity of each habitat.
The ability of the glue to remove the problem of interfacial water by effectively absorbing it is the key finding of the research, and the one with perhaps the strongest prospect for commercial development.
The concept of using hygroscopic compounds – as water-absorbers are known – say the authors, “provides a novel design principle for developing water-resistant synthetic adhesives”.
