Double network gels and the toughness of terrestrial slug glue

Abstract

The terrestrial slug Arion subfuscus produces a defensive secretion that is sticky and tough, despite being a dilute gel. It is unusual in having high stiffness for a gel, yet retaining the high extensibility typical of mucus. In tensile tests, it sustains an average peak stress of 101 kPa, and fails at an average strain of 9.5. This gives the gel toughness; it requires much greater strain energy to fracture than most gels. This toughness may arise from a double-network type mechanism. In this mechanism, two separate, interpenetrating networks of polymers with different properties combine to give toughness that can be several orders of magnitude greater than either network individually. Native gel electrophoresis suggests that A. subfuscus glue consists of two networks: a network of negatively charged proteins ranging in M r from 40×10 3 to 220×10 3 that can be dissociated by hydroxylamine and a network of heparan sulfate-like proteoglycans. The two networks are not tightly linked, though proteins of M r 40×10 3 and 165×10 3 may associate with the carbohydrates. Targeted disruption of either network separately, using enzymatic hydrolysis, disulfide bond breakage or imine bond disruption completely disrupted the glue, resulting in no measurable toughness. Thus, the two networks separately provide little toughness, but together they work synergistically to create a tough material, as predicted in the double-network mechanism.

RNA-Seq reveals a central role for lectin, C1q and von Willebrand factor A domains in the defensive glue of a terrestrial slug

“…The proteins with a relative mobility indicating a molecular mass less than 20 kDa were found in multiple closely spaced bands. In previous work, these proteins were collectively identified as Asmp15 because the bands varied in number and relative intensity, and it was not clear if they were separate proteins or different size variants of the same protein (Pawlicki et al 2004; Werneke et al 2007; Smith et al 2009; Bradshaw et al 2011; Wilks et al 2015). Most commonly, there were two dark bands with a region below containing several less intense bands (Fig.1).…”

“…This material is dilute, containing roughly 97% water (Smith 2016). Unlike most dilute mucous gels, it is stiff, with an average elastic modulus of 30 kPa (Wilks et al 2015). This is stiffer than 7% gelatin and approaches the stiffness of 20% gelatin (Eysturskard et al 2009; Czerner et al 2015).…”

“…This is stiffer than 7% gelatin and approaches the stiffness of 20% gelatin (Eysturskard et al 2009; Czerner et al 2015). Unlike gelatin and other stiff gels, however, slug glue can be stretched over ten times its initial length (Wilks et al 2015). The combination of stiffness and deformability makes it an unusually tough gel.…”

“…They stiffen the glue, providing cohesive strength (Pawlicki et al 2004, Braun et al 2013), and they create an interpenetrating polymer network along with polysaccharides to toughen the glue through a double network mechanism (Wilks et al 2015). Such double networks can be orders of magnitude tougher than simple gels (Gong, 2010).…”

“…Such double networks can be orders of magnitude tougher than simple gels (Gong, 2010). The main polysaccharide in the glue has been tentatively identified as heparan sulfate (Wilks et al 2015). In contrast, little is known of the structure of the proteins.…”