Spiders have been spinning silk threads for over In Brief 400m years. While their woven webs may look delicate and fragile, weight-for-weight certain silks produced by spiders are around six times stronger than steel and tougher than the high-performing synthetic fibre, Kevlar. So what is stopping us from building the next generation of bridges and aircraft with these natural yarns?
In fact, a spider can produce up to seven different silks, composed of silk proteins or fibroins in seven separate glands in its body. There are 'sticky'/adhesive silks for holding or sticking threads together, highly elastic silks that make up the spirals of the web and dragline silks used for the non-sticky web spokes and for abseiling. It is the dragline silks that have superior strength and toughness compared with synthetic polymers, such as nylon, polyethene and polycarbonates, and which are the focus of research around the world.
‘Because the strength of the spider silk fibre is so enormous when compared with polycarbonate – it makes polycarbonate look like a cheap and cheerful polystyrene – there will be aspects of it from an engineering perspective that we can learn from and hopefully take those lessons into other synthetics, which may or may not be silk,’ says David Porter, a theoretical physicist in the University of Oxford’s Silk Group.
X-ray studies in the early 1940s, mainly by German chemists looking for a replacement for parachute silk in World War II, revealed the nanocrystalline nature of the fibroin proteins – the crystal domains being of the order of 3nm. Later (crystalline) states and glycine-rich disordered (amorphous) states within these domains. The former are responsible for the strength and stiffness of the fibre, the latter for its toughness or elasticity. The folded structure leads to H-bonding between the CO and NH groups, which further strengthens the fibre.
The disordered regions can absorb water; consequently natural silk can show a wide range of properties, ranging from rubbery to stronger stiffer fibres, depending on the amount of hydration. It is this diversity that makes it so attractive to study.
The natural silk polymer is 10 times stronger than the polycarbonates used extensively in the electronics, for example, DVDs, and car and aircraft industries, for example, cockpit canopies and reflector coverings for lights. The actual strength of dragline silk – the force per unit area required to break the fibre – is around 1.5 GPa. On a weight-for-weight basis, this is six times stronger than steel and around half as strong as the highperforming, synthetic aramid fibre, Kevlar.
According to Porter, dragline silk is the toughest material known – it can absorb more energy than any other material before it ruptures and breaks. 'Some can stretch up to double their original length before they rupture,' he explains, 'making them tougher than any synthetic polymer, including Kevlar.' But herein lies its limit in terms of potential applications. Being so elastic currently precludes its use in aircraft and other construction materials, as well as in high-performance military applications, such as bullet-proof vests, without major changes to engineering product design.
Kevlar is around 20 times 'stiffer' than a typical synthetic polymer and 10 times stiffer than natural spider silk. So while spider silk and its synthetic analogues may find use in numerous applications where strength and toughness are paramount, they are unlikely to replace Kevlar in engineering composite materials.
That said, Kevlar is produced like other synthetic polymers, including nylon and rayon, from oil using high temperatures and pressures and, in the case of Kevlar, concentrated sulphuric acid is required in the final spinning stage. In contrast, spiders spin their fibres from green starting materials – leaves, flies and other spiders – in water and at a temperature of ca 30 ºC.
Thomas Scheibel, a biomaterials professor at Universität Bayreuth in Germany, is working on the processing of silk proteins into different structures: fibres, films, beads, capsules, foams, hydrogels and non-woven materials. He uses silk proteins made by genetically engineering E. coli bacteria. While the fibres produced from this process don't have the strength and toughness of natural silk, this is not necessarily a drawback, explains Scheibel. 'We are looking to make fibres for very defined applications. Natural silk is too elastic for most of these.'
Scheibel is currently working on a drug delivery system based on silk beads, which has many advantages, he explains. The carrier is biodegradable and biocompatible – it does not cause inflammation and water-soluble drugs can be released over a long time without any sudden ‘burst’ of the compound. His team is also working on non-woven silk fibres for wound dressings and air/water filter systems.
The Silk Group in Oxford, an interdisciplinary team of scientists and engineers led by biologist Fritz Vollrath, has concentrated on medical applications from both natural and reconstituted or 'synthetic' silks, the latter made from the discarded cocoons of silk worms. Linked to this research, there are several spin-out companies working on scaffold materials for all sorts of biological repair. Bone scaffolds, silk-based cartilage and suture materials are all in development. 'The advantage of using silk over metal replacements, in the knee, for example,' explains Porter, 'is that the protein will encourage healing, and a few years on, new bone will grow. In contrast, metal replacements can cause long-term problems associated with stress on the surrounding bone. And since silk is biocompatible, it is already FDA [US Food and Drug Administration] approved so many of these products could be on the market within the next few years.’
Another development in the application of 'synthetic silks' has recently emerged from a collaborative project between Swedish and Spanish researchers. In a paper published this year (Advanced Materials, 2011, 23(7), 898), Christian Müller and coworkers explain how they have taken silk fibres from the silk worm Bombyx mori and woven them – basically criss-crossed two fibres – and then deposited a polymer electrolyte at the cross-junction. The research represents possibly the first attempt to make a transistor out of silk fibres and a polymer electrolyte rather than with silicon. Such transistors would have the advantage of being biocompatible and could be used in biomedical monitoring devices.
In a slightly different approach, David Kaplan and his group at Tufts University in Boston, US, have made integrated circuits out of ultra-thin silicon and laid these on silk fibres made from silkworm cocoons. The resulting structure is highly flexible. The researchers implanted their 'flexible circuit' onto the brain of a cat; as the silk dissolves, it is reabsorbed, the latter initiating a spontaneous capillary force which sticks the electronic circuit to the brain tissue (Nature Materials, 2010, 9, 511). They hope to use such a device to monitor brain activity and detect epileptic seizures, for example.
The Holy Grail
But while the quest for industry is to get fibres with the right material properties for applications, the scientific challenge lies in unravelling Nature's secrets and ultimately producing an exact copy of spider silk.
Chris Holland, a biologist in the Oxford Silk Group, explains: 'Since silk is selected for its mechanical properties which perform outside the spider's body, we can use a host of analytical techniques to characterise the material.'
Silk Group researchers not only analyse the chemical structure and mechanical properties of natural and synthetic silks, but also delve into the natural process of silk production – the transformation of the highly concentrated protein solution ('dope') in the silk glands, through a specially shaped spinning duct, into a hard, solid fibre. According to Porter, the spinning duct is basically a water removal system. 'From a combination of computer modelling techniques and TEM (transition electron microscope) studies, we suggest that that the proteins in the dope are stored as folded stacks with an amyloid or cross beta-sheet structure, lying perpendicular to the flow axis. On demand, when the spider needs the silk, the stacks are gradually reoriented along the flow axis, becoming what looks like a string of beads and precipitate out of solution. This comes about as a result of stress or shearing and a change in the pH of the environment.'
Vollrath and his colleagues believe that natural silk cannot be not replicated solely by knowing its origin and material composition, but requires an understanding of the specific denaturing process that occurs as the dope is spun into silk fibres. According to the Oxford researchers, the spinning process and the material co-evolved, so one goes hand in hand with the other.
David Porter sums up the current state of play in terms of reproducing the natural fibre: unfortunately spiders can't be farmed for their silk because, put in close proximity, they eat each other. But silk can be made either by genetic engineering techniques, using goats or bacteria as the German scientists have done, or by reconstituting waste silk as they do in Oxford. The problem with both routes is that they involve dissolving the proteins at some point, which destroys their threedimensional folded structure and nanocrystallinity. The final product has similar properties to silkworm silk – it has around a fifth of the strength and a tenth the toughness of the natural fibre, and is similar to nylon and polyester-type materials in terms of its stiffness. 'Our goal,' says Porter, 'is to figure out how to make the dope with the same folded structure of the natural dope. Only then will we be in a position to process it as the spider does and make a biomimetic silk.'
And here lies a major obstacle. No one understands hydrated proteins – in particular, how a protein, once denatured or unfolded, can be hydrated back to its original form in solution. ‘Could it be,' asks Porter, 'that once you have denatured or processed the protein, it is an irreversible process, a one-way street?' Proteins fold naturally as they are synthesised. It is an instantaneous process determined by the system energetics. As an amino acid jumps onto the protein chain, a hydrated protein forms instantaneously. The water stabilises the molecule by blocking the amide bond and prevents the amide bond from forming H-bonds with its neighbours, thus inhibiting precipitation. It is a very elegant process, suggests Porter, who spends much of his time doing ab initio quantum mechanics simulations of water interacting with proteins in hope of a solution.
Holland adds: 'Silk is an excellent model from which to learn about controlled protein denaturation because it has been designed by Nature to perform at every level of hydration. It is a very powerful tool which should help us understand other proteins.’
Unravelling this conundrum will have other major potential spin-offs. The problem is not confined to silks and spider silks, but is generic to all proteins. The denaturing and misfolding of amyloid proteins, for example, is a major cause of diseases such as Alzheimer's and cancer.
Thomas Scheibel is optimistic about mimicking the dope. By analysing small protein fragments using a combination of computer modelling and IR, MS and NMR, Scheibel and his team have already been able to throw some light on the molecular switching process in the middle region of the protein that occurs as it changes from liquid to solid fibre. By studying the individual parts, and gradually increasing the size of the domains, he hopes to build a full molecular picture of the protein in solution. 'After all,' he notes, 'in the early 1990s people said it was impossible to work out the structure of the large chaperone molecules [which support other proteins during folding] but, by studying the structures of fragments of these molecules and then combining this information, we now know the structures of all of them.'
Kathryn Roberts is a freelance science writer based in London, UK.