Spider silk has drawn much attention from engineers in the past 20 years for its toughness and elasticity, properties which may be utilized in applications such as suspension bridge wires, bulletproof vests, and medical adhesives. There remains, however, a mystery behind the production of spider silk. Scientists are intensively studying this process in order for engineers to replicate the silk in synthetic form. One of the first successful reproductions of spider silk was produced from genetically engineered goats.
Such inventive approaches lead us closer to a mass-producible, commercializable material that may potentially be as common as ordinary silk. This paper explores the concept of biomimetics, which encompasses the study and engineering of spider silk, spiders and their production of spider silk, and the ways in which engineers approach this puzzle of spider silk reproduction.
Introduction
Conjure up the image of a spider in your mind, crawling around, cocooning its prey to devour later, creating intricate webs, gracefully repelling off ceilings and causing distress to people who might have arachnophobia. Most of your interactions with spiders might result in a disgusted facial expression and a squashed bug, but what if I told you that your children’s children could be wearing a coat made with the same material that makes a spider’s web, or that bridges could be suspended with that same material? The threads of silk that emerge from a spider are so tough that, on a human scale, a net made out of spider silk could stop a passenger plane in mid-flight (see Fig. 1) [1]. Scientists and engineers have been working together to unlock the secret of this incredible material and reproduce it for use in real-life applications such as bulletproof vests, medical adhesives, and military draglines.
The emerging field that seeks to study nature and its potential applications to engineering is known as biomimetics. Biomimeticists work toward mimicking mechanisms found in nature, such as the self-cleaning nature of the lotus leaf in order to produce a self-cleaning tile. Another goal of biomimetics is the replication of manufacturing methods found in nature, such as the replication of spider silk by artificial synthesis.
What is Spider Silk?
If you flip a spider on its back you can see a group of four to six spinnerets out of which the silk appears. Fig. 1 shows the silk emerging from the spinnerets located in the Yellow Garden Spider’s lower abdomen. Each one of these spinnerets consists of six hundred or more spinning tubes which work like mini glue nozzles functioning as a part of a larger nozzle, as shown in Fig. 2. Inside each of these spinnerets is a viscous solution that sloshes around like syrup [2]. This solution, also called ‘dope’, is composed of globular protein molecules dissolved in water that are later found aligned in the silk fiber of the surrounding sheath. These protein components determine the core and the outer structures of the emerging fiber. How does this liquid solution turn into the solid silk fibers used to make webs and repelling thread? This is the complex process that is at the core of engineers’ focus in reproducing spider silk.
The liquid dope is stored in the spinneret glands and has strongly orientated molecules that act as liquid crystal zones (see Fig. 2). The dope travels to a funneled duct with a narrow tubular region specialized for rapid water recovery. There are several advantages to the crystalline zones within the liquid dope. Near the narrow duct, the crystalline liquid flows but maintains a strict orientation. As the liquid dope nears the narrow end of the tube, the main silk protein molecules enter a compact conformation that permits for processing at high concentrations. The liquid crystalline zones allow this viscous silk protein solution to flow through the storage sac and duct slowly enough and at a constant rate so that proteins do not coagulate [2]. The resulting fiber that emerges is a filamentous structure comparable to that of a bungee-jump cord – a core of bundled filaments surrounded by a sheath.
The proteins found in spider silk have both amorphous and crystalline regions [3], a characteristic of composite materials. An example of a composite material is fiberglass, which has complementary properties: brittle glass is matted with ductile resin, resulting in a tougher and more resilient material than either glass or resin unaccompanied. In spider silk, the crystalline region plays the role of the brittle and tough glass component. The amorphous region, lacking in ordered structure, plays the role of the resin and provides enormous elasticity, which means that kinetic (or impact) energy from flying objects converts to heat so that the web gently recoils and flying insects do not shoot back outwards. Composites have the advantage of being able to withstand forces that produce cracks better than a traditional homogenous substance without composite reinforcement. The resulting exceptional qualities of ductility and strength are useful in suspension bridges and bulletproof vests.
Spiders have separate glands to produce the different types of silk for specific tasks; each type has unique chemical and physical properties. A spider produces different types of silk to encase its eggs, cocoon its prey, and repel from high places, among other regular tasks for spider silk [4]. The silk that allows them to repel, also called dragline silk, has received the most attention due to its high tensile strength and toughness. Engineers are focusing on the dragline silk manufacturing process that takes place in the spider in order to replicate it for mass-production or artificial manufacturing methods.
Reproduction of Spider Silk
While silkworms have been domesticated for mass production and industrialization for several centuries, spiders cannot be domesticated so easily. Spiders have an aggressive and solitary nature that leads them to cannibalism if confined in one space with other spiders — silkworms can simply be fed mulberry leaves and observed (Forbes).
In 1855, however, silkworm production was threatened by disease, so Count Hilaire de Chardonnet, an early French pioneer in biomimetics, succeeded in creating an artificial silk that utilizes the cellulose of digested mulberry leaves and a spinning process similar to that of the silkworm’s spinnerets. This artificial silk is what we now know as rayon. The next breakthrough took place in 1937 with the invention of nylon, synthesized from small chemical units linked together to form long-chain molecules. This was followed in 1963 by an early generation of bulletproof vests made of Kevlar, a tougher variant of nylon. Finally, with the invention of genetic modification (GM) in the late 1970’s, scientists and engineers were able to devote much more effort to the field of spider silk reproduction, which could yield much more powerful materials [5].
Genetic modification involves inserting a gene into a foreign organism to produce a certain protein — without affecting the organism’s normal processes. This major development in the field of genetics really opened the door to potential production of spider silk. However, due to the complexity of the spider silk genes, it was not until 2002 that Nexia, a Canadian biotechnology company, had reported the production of industrial quantities of spider silk using milk from genetically engineered goats [1]. Unfortunately, Nexia ran into a problem common in the field of biomimetics: the difficulty of producing in bulk to meet commercial demand [5].
With this difficulty in mind, the goal of spider silk production was headed in a new direction – mimicking the natural spinning process. David Knight, a pioneering researcher in spider silk, and his colleague Fritz Vollrath received a grant in 2003 from the UK Government’s Department of Trade and Industry to perfect their patented spinning process. Knight and Vollrath founded Spinox, a company focused on understanding and reproducing the mechanisms of the spider’s spinnerets. They have patented a spinning nozzle that mimics some of the processes that occur within the spider’s spinneret and produces tough fibers from a range of solubilized silks, recycled protein from other silks. What happens in this spinning machine? Within the nozzle, water is extracted from the liquid dope and, as this solution is passing through the nozzle, an acid bath forces the proteins into a structured alignment [2].
At the same time in Boston, Tuft University’s David Kaplan was also working on recycling silkworm silk to produce research-grade, reconstituted silk resembling spider silk. Since ordinary silk is only intended to make cocoons, it is not as strong as spider silk, which is intended to make webs that can withstand heavy impact. Thus, the spun silkworm silk must be modified and is done so by reconstitution, whereby harsh solvents are used to redissolve the silk in water until it is a liquid solution again. Kaplan’s process of reconstituting silks resulted in a sheet of crystalline silk proteins that can be stretched up to 300% into a water-insoluble film to be used like a sponge [3].
In 2001, the US Defense Department awarded a grant to Kaplan’s Massachusetts-based hi-tech firm Foster-Miller “in an effort to produce films from silk that possess unique and tailorable properties for emerging Air Force applications [that] is likely to be well suited for highly optimized large space structures such as solar sails or space telescopes.” Two years later Foster-Miller received a follow-up grant for ‘Large-scale Production of Spider Silk by Immortalized Spider Cells.’ In this context, ‘immortalized spider cells’ refers to stem cells which retain the ability to develop into a specialized cell. Kaplan’s goal is to produce a mass of silk-producing cells from stem cells differentiated into spider cells that produce the protein found in the liquid dope. These harvested stem cells could potentially be used in a replicated spinneret environment to process the liquid dope [5].
The Future of Spider Silk
The future of spider silk will largely be determined by how it will be used in different practical applications. For example, if we would like to make a bulletproof vest out of spider silk, the tendency for spider silk to stretch when hit with a flying object would not be a good fit; for this application, we would need a material that mimics the strength of spider silk to stop the bullet but has properties to improve its strength. Lee et al. (2009) found that depositing layers of metal onto spider silk fibers yields a material that is significantly stronger and more break-resistant [6]. They used a method called atomic layer deposition (ALD) to coat the fibers with different metals and tested its toughness. They found that titanium deposition increases toughness tenfold, aluminum deposition by ninefold, and zinc deposition by fivefold. This breakthrough in improving the mechanical properties of spider silk paves a new potential application for this material in artificial fibers and tendons that is much stronger than conventional artificial ligaments.
Kaplan and researchers at Tufts’ school of Engineering and Medicine have developed a method of bioengineering artificial anterior cruciate ligaments (ACLs) using spider silk as a scaffold for cell growth (Spiders). As a tougher material with greater break-resistance, spider silk holds the potential to become a common scaffold for tissue [7].
The European Science Foundation (ESF) hosted a workshop that revived the possibility of using genetic modification for spider silk synthesis by engineering bacteria to produce silk similar in physical properties to spider silk [8]. Genetic engineering of bacteria entails insertion of genes to produce proteins similar to spider silk proteins. These synthesized proteins are then fabricated into silk using a microfluidic approach that utilizes the microscale behavior of fluids, a process similar to inkjet printing.
Conclusion
The effort to reproduce spider silk has encountered many creative and innovative methods and materials, from genetic modification to recycled silk to stem cells, each with unique successes and drawbacks. While these innovations have only been moderately successful in producing spider silk that can be used in commercial applications, the talented and creative minds of biomimeticists have provided a new way of looking at inventions and technology, with an eye on nature for inspiration.
References
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