It can be strong like steel, yet stretchy like rubber. It's lightweight but can be phenomenally tough.

Darwin's bark spider on its web Image by Matjaz Gregoric/ABC News
Darwin's bark spider is famous for the extremely tough silk it makes.

ABC Science
By Suzannah Lyons

We're not talking about the latest high-tech material developed in a lab somewhere, but one of nature's supermaterials — spider silk.

Spiders make many different kinds of silks.

Orb-weaver spiders, which spin the wheel-shaped, spirally webs we're so familiar with, can produce seven types of silk that are used in different parts of their webs.

Now a team of US researchers is starting to figure out why silk produced by one orb-weaver species — Darwin's bark spider (Caerostris darwini) — is so tough.

"This spider is famous because it's been shown that one of the silk types that it makes, called dragline silk, is the toughest spider silk that's ever been measured," said evolutionary biologist Jessica Garb of the University of Massachusetts Lowell and lead author of the study.

Her team has discovered a new silk protein that may help explain this phenomenon, they report in the journal Communications Biology.

Toughness doesn't only refer to the strength of the material, but also how much it can be stretched before it breaks.

Darwin's bark spider uses dragline silk to make the spokes and frame of its web. This silk is 10 times tougher than Kevlar and can be stretched over 90 per cent of its length, making it one of the toughest known biological materials in the world.

The species is native to Madagascar and is also famous for spinning the largest-recorded orb webs, suspended on threads up to 25 metres long.

The team suspect the spider's extremely tough dragline silk has evolved in order to support their large webs.

Silk factories

Darwin's bark spiders have seven different kinds of glands dedicated to making the different silk types.

“These glands are basically factories for making those proteins that make up the silk.” -Jessica Garb
Dragline silk is made by the major ampullate gland inside their abdomen.

To figure out what makes this silk so special, the research team sequenced the gene transcripts — the intermediates between genes and proteins that tell us which genes are being turned on in those glands.

As well as two major silk proteins that had already been identified in other spider species, they also found a third protein that looked different from previously sequenced silk proteins.

"What's particularly striking is that is has a lot of proline [in it], which is an amino acid that has previously been associated with stretchiness… in dragline silk," Dr. Garb said.

The researchers also observed that the spinning duct part of the major ampullate gland is quite long in this species, which could be important for transforming the liquid silk solution into the final strong silk fibre.

“Studies like this, looking at the actual biology of animals ... are useful for fields that might not really be thought to be adjacent to them, like material science and manufacturing.” -Andrew Walker
The study has "some really interesting leads" said Andrew Walker, an invertebrate biologist at the University of Queensland, who was not involved in the work.

If you can't beat them, borrow from them

One of the amazing things about spider silk is the way it's produced, said Cameron Brown, a materials engineer at Queensland University of Technology who was not involved in this study.

While synthetic equivalents like Kevlar are made with incredibly harsh chemicals and at high pressures, degradable and recyclable spider silk is produced at room temperature, with the energy cost of an insect dinner.

"We can use this [research] to understand not just how we can make better silks, but how we can make all sorts of better materials," Professor Brown said.

“If we can take the tricks that spider silk uses and apply those to our other materials then that’s an enormous leap forward.” -Cameron Brown
This knowledge could be applied to all kind of high-performance fibres such as bulletproof vests, seatbelts and even human tissue replacements.

"The real impact of these types of studies is to learn how nature works so we can ... make the materials we're already using a lot better so we can use less of them, and make things lighter and stronger," Professor Brown said.

Dr. Garb said using information we've learnt about how spiders make silk at the genetic level to produce silk-like materials is a big activity.

Spider silk genes have been put into bacteria, silkworms, and even goats.

"You can take the sequence as it exists in nature and try to replicate that as closely as possible," Dr. Garb said.

Darwin's bark spider eating a dragonfly Image by Matjaz Gregoric/ABC News
Darwin's bark spider can produce super tough silk for the energy cost of an insect dinner.
"Or you can mix and match from different things in nature to make something that doesn't quite exist in nature, but maybe puts together different functional properties you might be interested in."

But while a lot of start-up companies are now making products like sneakers and jackets with artificial spider silk, Dr. Garb was not aware of anything that's gone to market yet.

And don't expect to see garments made from natural spider silk in the shops anytime soon.

They're so labour intensive to produce it's not practical, said Dr. Garb.

A cape made of spider silk required the silk of 1.2 million spiders.