Spider Silk

responsibility for wording of article: Akira Takashima (OIST)

Compared to conventional silk from moths, that serves purely protective functions, spider silk comes in a wide variety of types that are used for particular applications. In general, spider silk as very interesting mechanical properties, being highly flexible but also very tough. No synthetic fibers can match this peculiar combination of favorable properties. The possibility of producing materials with spider silk properties is being very actively researched.

The silk secreted by spiders have been known to be extremely flexible and tough. Studies on functions of these spider filaments are not only scientifically interesting but also lead to industrial applications. Here, we will shed light on the mechanical aspects of spider silk. The many spinneret tubes in the abdomen of the spider are connected to 7 abdominal spinneret glands, and each of these secretes proteins with different amino acid compositions. Individual proteins play different roles relevant to different purposes.

The circular net-like web often spun by web-building spiders is composed of various silk. The center of the web is tightly woven together (the “hub”). The “warp” radiates out from the center of the web. The “weft” spirals around in circles with glue drops distributed at approximately equal intervals. The “frame thread silk” surround the web. The “anchor thread silk” connects the frame and the wood. When the spider is catching a prey or encounters danger, it produces “drag line silk” that act as a safety net. Here, we describe the particularly interesting mechanical properties of the drag line silk and the weft. 

Mechanical properties of the drag line silk

The drag line silk of the jorō spider (Nephila clavata) is a structural protein composed of amino acid residues, consisting of glycine (approximately 37%), alanine (approximately 18%), and glutamic acid (approximately 16%). The drag line silk is mainly composed of 2 proteins: MaSp1 and MaSp2. These proteins are composed of 3 segments. MaSp1 is composed of a quasi-crystalline β-sheet segment, a non-crystalline helical segment called GGX, and a terminal nonrepeating segment. The MaSp2 protein has a β-turn spiral structure segment called GPGXX, a quasi-crystalline β-sheet structure segment, and a nonrepeating terminal segment. It is the β-turn spiral structure that gives the drag line silk its elastic properties. The β-sheet structure provides its high strength. Its elasticity is substantially higher than that of synthetic polymers such as nylon. It is thought that while the protein is still within the spider secretory glands, it is in a high-concentration liquid crystal state, with the protein molecules being partly lined up along the longitudinal axis.  When it passes through the thin excretory duct, the proteins line up along the fiber axis and become partially ordered.

The drag line silk that supports the spider's weight is very thin; however, it needs to support the dangling spider without breaking—a matter of life and death. With the human’s naked eye, the drag line silk looks like a single thread; however, under the electron microscope, it is observed to be composed of cylindrical filaments. Studies of the mechanical properties of the drag line silk show that if a small force is applied, the silk extends like a spring in proportion to the force until it nears a limiting point when this proportionality breaks down and the line eventually breaks. This limiting point is known as the elastic limit point. The tension at the limit point is the maximum limit of elasticity. The length to which the line has extended is known as the elasticity extension limit. This maximum elasticity limit is twice the weight of the spider. The line silk is made of 2 filaments so that even if 1 breaks, the other can still support the spider. Thus, the drag line silk is a very effective lifeline with some leeway built in.  

Having 2 filaments on the drag line silk is very important from a crisis management standpoint. The presence of 2 filaments gives some “idle” in that either 1 of them will still function in crisis. The drag line silk is the most effective manner of supporting the spider's weight. From the point of view of safety and energy efficiency, the drag line silk enables both efficiency and an idle for safety and thus matches agile spiders' lifestyles. The function of the drag line silk provides important suggestions for safety and risk management of constructs such as elevators, bridges, airplanes, houses, and tunnels and industrial materials such as cords.

The elastic weft

When catching prey, the weft stretches up to 200%. The weft is made of glycine residues (approximately 45%), proline residues (approximately 20%), and other amino acids with dissociative side chains, including glutamic acid, aspartic acid, serine, and lysine. The drag line silk elasticity is conferred by non-crystalline β-turn spiral structures called GPGXX.  The β-spiral structure is also found in the weft. In the extremely elastic weft, at least 43 consecutive GPGXX structures can be observed in 1 repeating segment.  The extreme elasticity of the weft is thought to be caused by the repeating β-turn spiral structures in the segments.  

Mechanical characteristics of the warp and weft

When an insect collides with the elastic weft, the adhesive properties of the weft bind the insect at a faster rate. The weft can absorb the energy of the moving insect.  If an overly large insect collides with the weft, it breaks off at the junction with the warp, which acts as the backbone to the web. This breaking ensures that the entire web does not fall apart.  Understanding the function of the warp and weft in prey capture is an interesting way of understanding their mechanical properties.  If a force is applied to the weft, it is slowly stretched to 200% or more; however, the breaking strength is relatively small. In contrast, the warp that forms the backbone of the web stretches only slightly, but its breaking strength is high, making the web a durable structure.  These properties endow the whole web with the special features mentioned above. 

Further Reading

昆虫ミメティックス Insdct Mimetics(2008),針山孝彦,下澤楯夫,pp. 616–623

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