Vasav Sahni

Viscoelastic solids explain spider web stickiness

Modern orb-weaving spiders have evolved well-designed adhesives to capture preys. This adhesive is laid on a pair of axial silk fibres as micron-sized glue droplets that are composed of an aqueous coat of salts surrounding nodules made of glycoproteins. In this study, we measure the adhesive forces required to separate a small microscopic probe after bringing it in contact with a single glue droplet. These forces are highly rate-dependent and are two orders of magnitude higher than the capillary forces. The glycoproteins in the glue droplets behave as a viscoelastic solid and the elasticity is critical in enhancing adhesion caused by specific adhesive ligands. These results have important implications in mimicking bioadhesives.

Spider silk as a novel high performance biomimetic muscle driven by humidity

SUMMARY The abrupt halt of a bumble bees flight when it impacts the almost invisible threads of an orb web provides an elegant example of the amazing strength and toughness of spider silk. Spiders depend upon these properties for survival, yet the impressive performance of silk is not limited solely to tensile mechanics. Here, we show that silk also exhibits powerful cyclic contractions, allowing it to act as a high performance mimic of biological muscles. These contractions are actuated by changes in humidity alone and repeatedly generate work 50 times greater than the equivalent mass of human muscle. Although we demonstrate that this response is general and occurs weakly in diverse hydrophilic materials, the high modulus of spider silk is such that it generates exceptional force. Furthermore, because this effect already operates at the level of single silk fibers, only 5 μm in diameter, it can easily be scaled across the entire size range at which biological muscles operate. By contrast, the most successful synthetic muscles developed so far are driven by electric voltage, such that they cannot scale easily across large ranges in cross-sectional areas. The potential applicability of silk muscles is further enhanced by our finding that silkworm fibers also exhibit cyclic contraction because they are already available in commercial quantities. The simplicity of using wet or dry air to drive the biomimetic silk muscle fibers and the incredible power generated by silk offer unique possibilities in designing lightweight and compact actuators for robots and micro-machines, new sensors, and green energy production.

A Review on Spider Silk Adhesion

Spiders employ clever behavioral strategies combined with almost invisible custom-made adhesive silk fibers to spin prey capture webs. The adhesives used in these webs evolved over millions of years into a class of natural materials with outstanding properties. Here, we review how spiders use different adhesives to capture prey. We show how spiders take advantage of the elasticity of both the capture silk and the glue to enhance adhesive forces, thereby providing important insights in designing new synthetic adhesives.

Spider Silk Inspired Functional Microthreads

We employ the adhesive web building strategy used by modern orb-weaving spiders to produce functional microthreads that are similar in structure (beads-on-a-string (BOAS) morphology) and adhesive properties to the capture-silk threads of the spider web. The diameter and spacing of droplets (beads) are controlled by varying the viscosity, velocity, and surface tension of the coating fluid. Using these functional threads, we also describe the behavior of the BOAS morphology during contact (mimicking the collision of an insect with the web) and during separation (mimicking insect rescue from the web). Our results show that the BOAS structure performs better than a cylindrical structure for adhesion, which may explain why this morphology is so prevalent in spider webs despite the cost of increasing the visibility of the web.

MECHANICAL PROPERTIES OF THE CHITIN-CALCIUM-PHOSPHATE "CLAM SHRIMP" CARAPACE (BRANCHIOPODA: SPINICAUDATA): IMPLICATIONS FOR TAPHONOMY AND FOSSILIZATION

Spinicaudata (colloquially ‘the clam shrimp’) are freshwater branchiopod crustaceans that occur worldwide in lakes and temporary pools. The spinicaudatans are easily recognizable by their bivalved carapace which is unusual among arthropods in that it is subject to only partial molting. During ecdysis (molting), the outer surface of the carapace is not shed, resulting in the retention of the ontogenetic record of an individual through distinct growth-rings representing each molt. When this unusual feature is considered alongside the interesting chemical properties of the carapace, “clam shrimp” present an interesting biological material not seen anywhere else: a multi-laminar calcium-phosphate-chitin composite. In addition, the carapace survives numerous destructive taphonomic processes (including transport, decay, compaction, and desiccation) to become the dominant body component of Spinicaudata preserved in their 380 million year fossil record. Understanding the mechanical properties and chemical composition of this structure may not only aid in a better understanding of the evolutionary history of this group but also facilitate efforts to develop novel materials that retain functional material properties even in harsh aquatic conditions. Therefore, this study aims to provide quantitative information about the composition and mechanics of this unique and interesting biological material and help predict possible biases in the fossilization of different species of Spinicaudata to aid future palaeontological research.

Prey Capture Adhesives Produced by Orb-Weaving Spiders

Spiders spin a variety of silk fibers and integrate them into webs with a wide range of architectures. Combined with clever behavioral strategies, these webs serve as effective prey capture devices. One of the most stereotypical and familiar web forms is the orb web, characterized by radiating lines of dry silk that support a sticky capture spiral. Dragline silk forms the attachment lines, perimeter frame lines, and radial scaffolding. These dragline threads are the most investigated spider silk fibers due to their strength and toughness. By comparison, the orb web’s adhesive capture threads have been largely ignored, which is rather surprising, as they hold insects in the web until a spider can subdue them. Here, we discuss two of the most prominent adhesive fibers produced by orb-weaving spiders – cribellar silk and viscid silk. We review the structure, chemistry, mechanics, and adhesive mechanisms of both these systems, to help in understanding how spider webs function in prey capture and to provide insights into designing novel adhesives.