Thursday, January 26, 2012

Tech - Stretching spider silk to its high-tech limits

Keep spinning, worms <i>(Image: Yuji Sakai/Digital Vision/Getty)</i> 
Keep spinning, worms (Image: Yuji Sakai/Digital Vision/Getty)
The marvellous stuff that spiders and silkworms make has a big future in technologies from artificial corneas to brain implants

Chris Holland kicks a bucket of rotten fruit. "It smells like death in here," he says cheerfully as a cloud of tiny fruit flies swarms up to its fate. We're inside a glasshouse that would have any arachnophobe quaking. As many as 100 giant spiders are hanging from the ceiling, lurking on branches and lolling about on enormous webs. With their legs outstretched, they are the size of an adult's palm, their bodies the size of your thumb. Littering the webs are remnants of larger meals – the sucked-out husks of bluebottles.
The spiders are golden orb weavers and their bright yellow webs are some of the largest and most impressive of any arachnid. For researchers who study silk, like Holland at the University of Oxford, orb weavers are ideal because of their large webs and the fact that they are easy to handle. "They make great experimental subjects," says Fritz Vollrath, who heads the Oxford group.
Weight for weight, a typical spider silk is 20 times as strong as steel and four times as tough as Kevlar. It is also extremely flexible, stretching up to 50 per cent of its length without breaking. And it's not just the silk's physical properties that are impressive. It elicits no immune reaction in our bodies, it is biodegradable, and it is produced at low temperatures and pressures relative to other polymers. This is surely the kind of material that all of us, and not just our favourite fictional superheroes, could put to good use.

The reason we don't is that spider silk is incredibly labour-intensive to exploit. The only textiles made of spider silk that we know of are a rug and cape on display at the Victoria and Albert Museum in London. The rug is little more than 4 metres square, but its production involved 80 people spending five years collecting and weaving the silk from a million golden orbs.
Yet the promise of silk is too inviting to ignore, which is why several biotech companies and research groups have sprung up recently to develop devices and applications using the stuff. Much of their research has been done with silk from silkworms, which has a structure similar to that of spider silk and is much more plentiful. Even so, to make the most of silk's properties on an industrial scale we need to understand how these creatures make silk and then copy the process wholly or in part, to make new polymers. Here there has been significant progress too.
Humans have been trying to tease out silk's secrets for more than five millennia. The Chinese were the first to cultivate silkworms and enjoyed a monopoly on the resulting, much-sought-after cloth for more than 3000 years, establishing the Silk Road trade routes throughout Asia and Europe. Their selective breeding led to the Bombyx mori silkworm, whose cocoons are far larger and easier to unravel than those of wild counterparts. "These things are essentially the cows of the insect world," says Holland. "They have been inbred to produce huge quantities of silk."

A safe distance from the spider glasshouse, Holland keeps a huge box of silkworm cocoons in his office. They look like fluffy polystyrene packaging chips. "See if you can make a dent in it," he says handing me one. I can't, even though it is only half a millimetre thick. The cocoon offers perfect protection from such dangers as bacteria and hungry birds.
Before they make a cocoon, the silkworms are fattened up on a diet of mashed-up mulberry leaves. Eating almost continuously for a month, they are about 10,000 times heavier by the time they are ready to pupate. The worms then anchor a thread onto a convenient mulberry branch and pull their heads back, reeling the silk from a tiny hole in their mouths. As they move their heads in a figure-of-eight configuration, they reel up to a kilometre of silk for their cocoon, starting with the outermost layer and working inwards until they are locked in their protective home.
Since Bombyx mori silkworms are cultivated purely to satisfy our desire for their shimmering wares, only a few are allowed to metamorphose into moths to lay the next generation of eggs. The rest of the cocoons are collected and plunged into boiling water. This kills the silkworms and removes the sticky protective glue that holds the cocoons together. Workers then scrape each cocoon to find the end of the filament, which is threaded onto a bobbin. It takes several filaments wound together to make yarn.

These fibres are not stored like a fishing reel inside the silkworm's body. Bombyx mori produces its silk from two glands that add up to a third of its body weight. The silk itself is a protein, made of a long chain of repeating amino acids. Inside the glands the silk has the consistency of jelly, but when it is time to make a cocoon, the jelly travels through specially shaped ducts that align the protein chains and strip away water molecules, converting it into solid fibres. These fibres are then covered in sericin, a protein glue that binds the strands coming from the two ducts before they leave the worm through a single "spigot" in its head.
It is the structure of the thread that gives silk its impressive properties, which even let it protect soldiers' genitals (see "Military-grade smalls"). Silk is made up of protein crystals spread throughout a matrix of haphazardly organised protein chains. The strong hydrogen bonding in the tiny crystals gives the silk its strength, while weaker bonds in the protein chains impart toughness.
If silkworms are the industrial powerhouses of the silk world, then spiders are its model citizens, which is why Vollrath's group studies them so keenly. For a start, the dragline silk of a spider – the thread it dangles from – is entirely uniform and round in cross section, whereas silkworm thread varies in thickness with the stages of the worm's figure-of-eight dance. What's more, spider silk is typically four times as strong and can be extracted under carefully controlled conditions. "We bring a spider down from the roof and either reel a silk fibre from it, or extract the silk from a removed gland," Vollrath says. "You can reel long lengths of fibre, controlling for humidity, temperature and silking speeds."
Using laser light and X-rays to study the molecular structure of the fibre, his team can observe how different spinning conditions alter the thread. This is essential if we are one day to create synthetic silk as good as the real thing.

Until then, researchers have to make do with various types of artificial silk, the most common of which is known as "reconstituted silk". It is made by boiling silkworm cocoons to remove the sericin coating and then soaking them in chemicals to break down the protein chains into their original units. This is essentially reverse-engineering the fibres back to their original state, says Cedric Dicko at Lund University in Sweden.
The silk solution can then be spun using a variety of techniques depending on the type of thread required, or poured into moulds, cast into transparent films, or freeze-dried to make a sponge or foam. The advantage of reconstituted silk is that you can effectively reprogram the silk, rather like uncooking a cooked egg, allowing you to make it from scratch exactly as you want it, Dicko says.
Unfortunately, reconstituted silk is far more energy-intensive to produce, and can't match the real thing. No one understood why until Dicko and his colleagues compared the structure of the jelly-like proteins from reconstituted silk and a silkworm gland by bombarding them with neutrons at the Institute Laue-Langevin in Grenoble, France. The team showed that the processing involved in making reconstituted silk destroys the structure and bonding of the protein chains that gives natural silk its strength and toughness (Soft Matter, vol 6, p 4389). The result is "like chicken wire", says physicist David Porter at the University of Oxford, and has only one-fifth the strength of natural silk.
An alternative method is to genetically engineer other organisms to produce the silk protein. In recent years, goats, potato and tobacco plants, and Escherichia coli bacteria have all been enlisted, having had the silk gene implanted into their genomes. But again, the nanostructure of the protein is not preserved.
For researchers who are interested in silk for reasons other than its mechanical properties, this isn't too much of a problem. "If you need high-strength, high-performance fibres then you need to be as close to the native process as possible," says David Kaplan, a bioengineer at Tufts University in Medford, Massachusetts, who has been researching silk for more than 20 years. "For most applications we are working on, you don't need to be near that. Most of what is needed is covered by what can be made today." For Kaplan and his colleague Fiorenzo Omenetto, also at Tufts, the most enticing aspects of silk are its optical and biological properties.

The perfect scaffold

Omenetto, an optical physicist, got involved five years ago when Kaplan asked him to laser-cut a film of artificial silk to create synthetic corneas. The material was like nothing Omenetto had seen before. "It looked unusually, amazingly transparent," he recalls. And it was something of a eureka moment for Omenetto. He has since developed ways to pattern silk films and has made holograms, lenses, sensors and diffraction gratings from the stuff (Nature Photonics, vol 2, p 641).
Silk's failure to produce an immune response, plus the fact that it biodegrades, mean it has a huge range of possible applications in the body. In 2004, Kaplan demonstrated that stem cells are happy to grow around the stuff, so a silk sponge could in theory be used as a scaffold to help mend broken bones or torn muscles (Biomaterials, vol 23, p 4131).
Omenetto and Kaplan have also shown that enzymes and proteins continue to function when embedded in silk, and that the material can be engineered to release its payload after a delay lasting anything from a few seconds to a year, just by tweaking how fast it will dissolve. That can be done during the production process, making silk a great candidate for novel drug delivery systems, says Omenetto. "You could imagine having a cup that stores antibiotics. You could pour water into it and dissolve the first layer of the cup to take your medicine." Other colleagues at Tufts are looking into silk implants doped with drugs for people who are on long-term medication (Expert Opinion on Drug Delivery, vol 8, p 797).
Together with John Rogers, a materials scientist at the University of Illinois at Urbana-Champaign, and others, the Tufts researchers are also developing "meltable electronics" designed to become part of the fabric of living tissue. Last year, they demonstrated that silk could be used to deliver ultra-thin electronics directly onto the surface of the brain, a capability which could one day be used to diagnose epilepsy or improve brain-computer interfaces. Silk films offer a much more useful surface on which to embed electronics than traditional silicon wafers as they can conform to the contours of the brain without damaging tissue. The idea is that once in place, the silk is dissolved with salt water and broken down by the surrounding tissue. Capillary forces between the silk and brain tissue help the electronics to wrap around the brain (Nature Materials, vol 9, p 511).
For Kaplan, the most interesting discovery of the last two years is that silk can become sticky at the flip of a switch. "It's an unusual property you don't find in other polymers. You can turn silk into a sticky gel that adheres to tissue just by switching on an electric field. Reverse the field and you can turn it back into an unsticky solution," he explains. This property was totally unexpected. "After all these years, silk is still surprising and impressing me."
The team is still working out exactly what drives this reversible stickiness but they can already see an application. "A situation that springs to mind is that the gel could be pumped into someone who is bleeding en route to hospital. When they arrive, it could be reversed so the surgeon can open the person up and get to the site of the injury," says Kaplan.
So in years to come, we may be grateful to silkworms for something a lot more precious than our silk shirts and underwear. We may be thanking them for saving our lives.

Military-grade smalls

To you, silk underwear might be a little touch of luxury. Not so for the British, American and Australian troops in Afghanistan. For them, they are an essential piece of kit. "The first thing a soldier tends to ask when they have been injured is 'are my bits OK?'," says armour scientist Simon Holden. "Damage to their groin area can really affect their quality of life, both physically and psychologically."
Of all the fabrics to fashion underpants from, it seems a traditional favourite is best. "Silk is lightweight so it doesn't impede a soldier's movement, and it can stretch a lot before it yields," says Holden, who is at the Defence Science and Technology Laboratory in Porton Down, UK. Because it can absorb an impressive amount of energy, silk helps reduce the severity of injuries from bomb blasts, especially the small-scale devices often used against troops in Afghanistan. Small pieces of shrapnel are unlikely to pierce the silk, and if they do embed themselves in the groin, the silk is pulled in with them. This means fragments can be extracted simply by pulling on the surrounding material.

Jessica Griggs is the careers editor at New Scientist

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