Structural effects of glue application in spiders-what can we learn from silk anchors?

Silk is a key innovation in spiders that equally fascinates biologists and material scientists. The anchoring of silken threads is a critical, yet poorly understood aspect of web-building—one that could lead to the development of advanced adhesives. To anchor silk, most spiders produce a structurally unique and rather unexplored bio-adhesive: the two-compound piriform gland secretion. The secretion is spun into elaborate patterns, so called attachment discs that anchor silken threads to substrates. The piriform gland secretion is a glue-coated fibre that combines the high toughness of silk with strong adhesion, even to highly repellent surfaces like Teflon. The glue is used highly economically, dries within less than a second after extrusion and can remain stable for years. Its hierarchical organization, discontinuous contact area and the embedding of compliant fibres may explain the high adhesive performance and flaw tolerance of such a composite product coming from a single, rather simple, type of silk glands. These principles contradict, in many regards, the paradigms of adhesives design and application, like aimed homogeneity of the bonding. Understanding the function of silk anchors may therefore trigger the development of novel industrial adhesives with embedded fibres, and methods of gaining higher bonding strength with less material consumption by smart glue application.