Most of your exposure to silk probably comes in the form of uncomfortably sensual linens or cobwebs in a dusty old closet. In reality, though, silk is an incredible and overlooked material. While it may have roots in the ancient past, it could also form the building blocks of the future.
You probably know that insects like spiders and silkworms, among others, produce silk, but you might not know exactly how. Composed primarily of a protein called fibroin and bound together by a sticky protein called sericin, silk is produced on-demand: the creature in question has a gland which secretes the two proteins, initially in liquid form, which in turn combine to form the fine strands that you can see in cobwebs.
Crucially, the silk keeps coming as the strand is drawn away from the body. A spider uses its body weight to extrude the stuff (strictly pulltrusion, yes, pedants); it can be drawn by human hand from silkworms.
Regardless of the method, the result is a long, fine fiber, which can be crafted into webs or cocoons. Either one just happens to have amazing mechanical properties. A strand of spider silk has a tensile strength of around 1.3GPa, which is comparable to that of high-grade alloy steel. It's also extremely ductile, which means it can stretch to up to five times its original length without breaking. Those two qualities combine to give it an insanely high toughness, too, which is a measure of how much energy is required to break something. In fact, it's at least four times tougher than either steel or kevlar.
It's the structure of silk that grants it such amazing properties: strong hydrogen bonds within tiny crystals of its structures provide strength; weaker bonds in the protein chains lead to ductility. The real kicker, though? It is super light; with a density of 1.31 g/cm3, it's six times lighter than steel. Add to that the fact that it's also biodegradable, and you have somewhat of a supermaterial on your hands.
As impressive as those properties are, they're associated with long, thin fibers, that are mostly good for weaving into questionably sexy bedding. There's certainly demand for silk—worms are cultivated and harvested for their wares to feed the trade—but truth is, it's not really being used in a way that truly lets it shine. There are, however, some researchers—chief amongst them Fiorenzo Omenetto from Tufts University and Fritz Vollrath at the University of Oxford—who have been thinking way, way outside the boxers.
In fact, they've developed techniques to transform silk from the humble wisps which end up in your tie into something quite amazing. By washing, treating, and remoulding the silk taken from silkworms, scientists can mold it into all manner of complex shapes, and even tweak its material properties to make it more hardwearing, for instance, or even more readily biodegradable. Perhaps understandably, they're a little secretive about what's actually involved in that process, but what is clear is that, once the silk is purified, they can use a combination of natural glues and resins to turns those fibrous clumps into something far more versatile. The result looks a little like plastic, but retains those insane properties that silk can offer—and that really shows when it comes to the host of possible applications on offer. The height of silk's utility in the past might have been the kimono, but in its new guise the possibilities seem limitless.
Silk is clearly well-suited to situations that demand materials with high strength and light weight, sure, but it might come as a surprise that it's enough of both of those things to be used in the next generation of body armor. Researchers have already developed a material which uses a thin sheet of silk as its base, onto which genetically engineered skin cells can be grown. The resulting material is relatively thin and light, completely flexible, and can stop a .22 calibre long rifle bullet traveling at 360mph. That's impressive, and means that soliders could someday not even have to wear extra body armor. Their clothing would be tough enough by itself.
If a bullet did sneak its way through? Silk has that covered, as well. A team of Oxford researchers has set up a company called Orthox that specializes in casting silk into shapes suitable for use in replacement joints. The units provide ultra-tough, long-lasting joint replacements, which is great—but they're also highly biocompaitble, which means they encourage regrowth of replacement tissue, and over time becoming a living part of the body. They can even be doped with drugs to make the transition as smooth as possible. Another, company, Neurotex, uses fine filaments of silk to bridge gaps in damaged nerves. Their experiments show that severed nerves grow along biocompatible silk filaments like plants along a trellis—and they can even make new, functioning connections which carry the electronic signals sent to and from the brain.
That's not silk's only electrifying capability, either. Researchers at the University of Illinois at Urbana-Champaign, for instance, have been using silk to develop a new breed of meltable electronics. Designed specifically to become one with living tissue, the researchers have been using silk films as an amazingly flexible base for electronics because, unlike silicon wafers, they can bend and twist without breaking. The long-term goal is to deliver ultra-thin electronics directly onto the surface of the brain—where the silk will be dissolved and broken down by the surrounding tissue. Hopefully what's left behind is an embedded electronic system, which can be used to monitor and treat conditions like epilepsy.
And there are other exciting possibilities still to explore. The fact that silk can become sticky via a little electricity, for instance; turn on an appropriate electric field nearby, and you can tweak the properties of the polymers that make up the silk so they suddenly switch between being sticky and firm. It's not clear exactly how that can be reliably exploited yet, but it's an exciting and unheard of material property just begging for an application. There's even the hope that, if silk can be mass-produced in large enough quantities, it could be used anywhere that srength and low weight are required. That means everything from car panels to bike tires.
But with all these opportunities open, you have to wonder how on earth those poor silkworms and spiders will keep up with demand. Fortunately for them, the reality is they might not have to—thanks to a farm full of goats, and the clever genetic engineering of a Utah State researcher named Randy Lewis.
When Randy milks his goats, he doesn't get something you'd like to make cheese out of. Instead, he gets a white liquid that actually contains the same proteins as spiders use to cast their silken webs. He's engineered these goats specially to make silk; all he needs to do is treat the proteins—clean them, dry them, and add a few solvents—and he has a silk nearly almost identical to that created by spiders. Dubbed BioSteel, the farm is now home to 30 goats that create the stuff, and it's hoped that over time they'll be able to create enough silk to meet up with demand (and to save a few silkworms some discomfort0.
So, there looks set to be no shortage of the wonder material, nor applications for it. The future isn't made of silk just yet—but it soon could be.
Image by Eric Kilby under Creative Commons license