What are Spider Silk?
Spider silk is a natural protein fiber and has better mechanical properties than silkworm silk. This is mainly due to the fact that the silkworm uses the silk for protection during metamorphosis, while spider silk uses their silk to catch the prey. Unlike caterpillars, spiders do not produce silk threads with paired salivary glands, but with several different spinning glands in their abdomen. The thread is pulled out with the legs or the whole body from several spinnerets, the number of which is family-specific, after the spider has anchored one end to the substrate. Spider silk has been recognized as fiber with unique combination of high strength and rupture elongation. In 1950s, spider silk in specific dragline silk attracted the focus of material scientist owing to their outstanding mechanical properties which can outperform any natural or synthetic fibres.
Spider silk refers to the protein filaments secreted by spiders, such as are used in the construction of webs. The proteins are produced in special abdominal glands and then spun through a series of ducts to so-called spinnerets, located in the rear of the abdomen. It is interesting to note here that there are similarities between this process and those used to produce synthetic high-performance aramid fibers.
The golden orb spider and the spinneret in it are shown in Figure 1. Spider silk proteins are synthesized from specialized abdominal glands that function as biofactories to produce large quantities of silk fibroins which are spun into silk with different properties, composition, and morphologies.
Types of Spider Silk
There are estimated to be more than 30,000 species of spider. Almost all spiders produce silk threads that perform different tasks: For example, there are stable threads used as security threads and for the basic frame work of the web. Furthermore, there are threads for the non-adhesive spirals of a web as well as adhesive threads with which the spider can attach its web to a base. Other adhesive threads are used to catch prey, while other silk threads are used to spin in prey and build the egg cocoon. Furthermore, there are elastic threads for the axis threads of the catching spiral.
There are 23 tropical and subtropical species of silk spiders (Nephila) that weave particularly stable wheel webs. In Madagascar, for example, the wheel-shaped catching webs, holding threads and cocoon-like protective covers produced by the large Madagascar spider (Nephila madagascariensis) are used for textile and also for technical purposes. The threads, each 200–600 m long, are manually unreeled from a dozen spiders. Within its lifetime of 1 month, the Madagascar spider can produce this thread length 4–6 times. The tensile strength is 30 cN/tex, which is a typical value for protein fibers.
Properties of Spider Silk
The density of spider silk is 1.3 g/cm3. The diameter varies between 0.5 and 5 μm depending on the species. The main components of spider silks are the amino acids glycine (37%) and alanine (21%). Their arrangement in the protein chain has a significant influence on the tensile strength of the thread.
Some spider silk threads are so elastic and resilient that they can be stretched by around 40% of their total length. This gives them an extremely high energy absorption capacity, which is even many times greater than that of aramid.
Spider silk possesses outstanding mechanical properties, and it is thought that successful copying of the secretion mechanism that a spider employs could lead to industrial production of silk fibers of high mechanical performance. Spider silks possess very high work to rupture. Such a combination of mechanical properties would be useful for bulletproof vests and airbags, for example. Besides, spider silks possess an attractive combination of high fiber tenacity and extensibility. Fiber tenacities can be comparable with those of synthetic high-performance fibers. Elongation to break in some silk filaments is as high as 200%, and so greatly exceeds that of many high-performance fibers.
Production of Spider Silk
The spiders have different glands to produce the different types of silk. Spiders produce silk by a process called “pull-trusion.” Unlike “extrusion” in which fiber is squeezed out of a reservoir, the spider pulls the finished thread directly from the silkmaking gland. Large scale of production of spider silk is difficult which is hindering the growth of using spider silk in many applications. Spider silk is still in the very early stages, and it may be decades before it can be put into actual applications. However, there are lots of research groups who are focusing on development of technique for mass productions of spider silk. The efforts to reproduce spider silks have encountered many innovative methods and materials ranging from genetic modification to recycled silk to stem cells with many success and limitations.
Traditional Methods: Spiders are highly territorial and aggressive creatures; hence, it is not possible to raise spiders together in the same environment. In order to collect spider silk directly from spiders, these would have to be captured from the wild and housed individually. Nephilaclavipes, a golden orb-weaving spider, has been studied extensively by numerous groups worldwide because it is a larger spider, which makes routine operations and handling a little bit easier.
Artificial Biosynthesis: Researchers have developed methods to artificially produce the liquid silk precursor using other organisms. Some of these methods are discussed below.
- Chimeric silkworms: Normally, the post production spinning technologies has to be used like extrusion in order to convert the liquid monomers into silk fibres; however, these techniques are not yet reliable or effective. One requirement of this method of production is that silkworms still produce endogenous silk proteins, thus the resulting product is actually a combination of both silkworm and spider silk fibers.
- Transgenic goats: Mammal cells have also been used as a host to produce spider silks monomers.
- Metabolically engineered Escherichia coli: Using E. coli as an expression system to produce spider silks proteins of similar molecular weight and mechanical properties as native spider silk was reported.
Applications of Spider Silk
If the production of spider silks ever becomes industrially viable, it could replace Kevlar and can be used in diverse range of applications such as:
- Bullet-proof clothing
- Wear–resistant lightweight clothing
- Ropes, nets, seat belts, parachutes
- Rust-free panels on motor vehicles or boats
- Biodegradable bottles
- Bandages, surgical threads
- Artificial tendons or ligaments, supports for weak blood vessels
In earlier times, spider silks were used for fishing nets and bird traps or woven into small bags. Spider webs were used as hemostatic dressing material and even served painters as canvas when stretched and folded over. Spider silk, e.g. from cross spiders or the black widow, was also used as material for reticles for optical instruments, e.g. surveying instruments, microscopes and riflescopes. The extremely small diameters of spider silks threads are now technically achievable with various polymers, such as polyamide. Therefore, spider silk threads have now been replaced by synthetic materials in optical applications.
Conclusion
Spider silk is an extraordinary natural fiber known for its exceptional strength, elasticity, and lightweight properties. Its unique characteristics make it promising for diverse applications, including high-performance textiles, medical sutures, tissue engineering, and biodegradable materials. Advances in synthetic production aim to unlock its full potential for sustainable innovation.
References
[1] Karthik, T., Rathinamoorthy, R. (2019). Sustainable Biopolymers in Textiles: An Overview. In: Martínez, L., Kharissova, O., Kharisov, B. (eds) Handbook of Ecomaterials. Springer, Cham. https://doi.org/10.1007/978-3-319-68255-6_53
[2] Veit, D. (2023). Fibers: History, Production, Properties, Market. Springer Nature.
[3] Babu, K. M. (2018). Silk: Processing, Properties and Applications. Woodhead Publishing.
[4] Reddy, N. (2019). Silk: Materials, processes, and applications. Woodhead Publishing.
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