Spiders Tune Glue Viscosity to Maximize Adhesion

Abstract

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.

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

“…Differences in diet and prey type cause striking differences in both a web's architecture and the properties of its threads (Blamires et al, 2016;Herberstein and Tso, 2011;Scharf et al, 2011;Townley et al, 2006;Tso et al, 2007), as can environmental factors such as wind (Wu et al, 2013). After a web has been spun, its frame and axial threads and its capture threads respond to daily changes in environmental conditions, with humidity having the greatest impact (Agnarsson et al, 2009;Sahni et al, 2011;Opell et al, 2011Opell et al, , 2013Stellwagen et al, 2014Stellwagen et al, , 2015Amarpuri et al, 2015).…”

“…As explained more fully in the following paragraphs, studies of the effect of humidity on the adhesion of capture threads have concluded that they evolved to perform optimally in the humidity regime typical of a species' habitat Amarpuri et al, 2015). In characterizing this performance, these studies have used flat glass and steel contact surfaces and constant velocity extensions to ensure uniform and precise comparisons.…”

“…Over the course of a day, the volumes of these hygroscopic droplets change as they respond to changes in environmental humidity (Townley et al, 1991;Opell et al, 2013). Although the allocation of absorbed atmospheric water has not been fully studied, some of it solvates the glycoprotein, ensuring its interaction with a surface, some of it hydrates the glycoprotein core, reducing its viscosity, and some of it probably increases flagelliform fiber extensibility (Sahni et al, 2011(Sahni et al, , 2014Opell et al, 2011Opell et al, , 2013Amarpuri et al, 2015).…”

“…Optimal glycoprotein viscosity is crucial for droplet adhesion (Amarpuri et al, 2015). To resist adhesive failure, glycoprotein must be pliable enough to establish contact with an insect surface (Fig.…”

“…But the glycoprotein must also be stiff enough to resist cohesive failure when it extends as an insect struggles to escape from a web ( Fig. 1C; Amarpuri et al, 2015). When glycoprotein absorbs too much water, it becomes over lubricated and its extensibility is reduced (Sahni et al, 2011;Opell et al, 2013).…”