An orb spider, glue-maker extraordinaire, at work on a web. Credit: The University of Akron
The ability of the spider to trap its prey with its water-based silk despite humid conditions, could lead to better commercial adhesives.
Many commercial adhesives fail due to interfacial water that forms a slippery and non-adhesive layer between the glue and the surface, which interferes with the formation of adhesive bonds.
To solve this problem, a team from the University of Akron is working to better understand the properties of the sticky glue that coats the silk threads made by the common orb spider.
The hydrogel that coats the silk in spider webs contains water, which normally would make it difficult for the insect to catch its prey, particularly in humid conditions.
To learn more about this phenomenon, the researchers examined orb spider glue and then set it on a sapphire substrate. They then examined the silk using both interface-sensitive spectroscopy and infrared spectroscopy.
Spider glue is comprised of two specialized glycoproteins, a collection of low molecular mass organic and inorganic compounds (LMMCs) that are hygroscopic and keep the glue soft and tacky to stick and water.
The glycoproteins and LMMCs act synergistically to generate humidity-responsive adhesion behavior of aggregate glue with adhesion maximized at humidity close to each spider species’ foraging habitat humidity.
The researchers found that the glycoproteins are primarily the binding agents to the surface. While glycoprotein-based glues have been identified in several other biological glues like fungi, algae, diatoms, sea stars, sticklebacks and English ivy, it remained unknown why the water in the spider glue does not interfere with the adhesive contact the way it does with most synthetic adhesives.
However, the researchers found that the LMMCs actually performed the previously unknown function of sequestering interfacial water to prevent the adhesive to fail by governing the adhesive quality of the glue produced with the respective proportions varying across species. This optimizes the adhesive strength to match the humidity of the spider habitat.
“The hygroscopic compounds—known as water-absorbers—in spider glue play a previously unknown role in moving water away from the boundary, thereby preventing failure of spider glue at high humidity,” UA graduate student Saranshu Singla said in a statement. “Imagine a paint that is guaranteed for life, come rain or shine.”
According to the study, as humidity increases, the researchers observed 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,” the study states. “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.”
The study was published in Nature Communications.
