Why Spider Silk is Stronger Than Steel

This vibrant cape and shawl are the largest pieces of cloth to ever be made from spider silk where over the course of three years it took a team of about 100 people in the highlands of Madagascar to collect the silk spin the threads weave the fibers and embroider the details where in the end 1.2 million golden orb weaver spiders each producing 30 to 50 meters of yellow thread were used to create this masterpiece before being released back into the wild demonstrating the extraordinary labor intensity required to produce spider silk textiles at scale.

The earliest known proto-spider lived about 380 million years ago during the Devonian period and were among the first creatures to live on land where this spider had the classic thin waist and abdominal segmentation of the modern spider where these early spiders also had spinnerets which is the organ that produces silk however their spinnerets were located in the middle of the abdomen instead of at the end of the abdomen like in modern spiders demonstrating how ancient silk production preceded the evolution of web-weaving morphology.

It wasn’t until about 250 million years ago that spiders with spinnerets at the end of their abdomen appeared allowing for more elaborate silk creations where this means that instead of weaving webs they most likely used their silk in sheets to cover their eggs or line their burrows where the middle abdomen position limited functionality demonstrating how a major morphological innovation enabled the diversification of silk-based behaviors and web architecture.

Of the 45,000 or so species of spiders about 12% are orb weavers the builders of the spiral wheel-shaped webs often found in gardens fields and forests where not all spiders are able to make intricately patterned webs like these but the ones that can do so masterfully demonstrating how a specialized subset of spider diversity has evolved exceptional architectural capabilities for prey capture through geometric web construction.

The creation of the silk begins in the spider’s silk glands and the spinnerets located on the posterior end of their abdomen where inside the glands the silk starts out as liquid as it travels down the cavity to the spigots where a combination of water extraction acid and tension turns it into its final solid silk form where this silk then travels through the spinnerets where it’s spun into a fiber demonstrating how spiders achieve precise control over protein polymerization through physicochemical gradients.

Orb weavers produce at least seven different kinds of silk where silk from the major ampullate is the structural silk like the bridge line and the radial spokes where then there’s silk from the flagelliform gland which makes the spiral where then there’s the aggregate silk which creates the sticky balls which capture prey where but of the many types the most extensively studied is dragline silk aka the spider’s lifeline demonstrating how spiders have evolved specialized gland systems for producing mechanically distinct silk types optimized for specific web architecture functions.

Dragline silk enables the spider to hang from ceilings and serves as a constant connection to the web allowing it to drop away if it’s in danger where and this particular silk is perhaps the most remarkable material from the natural world when it comes to its strength where dragline silk combines toughness strength and elasticity to an extraordinary degree demonstrating how a single biological material can simultaneously optimize multiple mechanical properties for survival.

Dragline silk is a composite material composed of two different proteins spidroin one and spidroin two where each protein contains an amorphous non-crystalline matrix and this is the stretchable part of the silk that gives it its elasticity where then embedded in the amorphous portions are crystalline regions that link together and toughen the silk demonstrating how spider silk achieves exceptional mechanical properties through a semicrystalline protein architecture combining flexible and rigid domains.

Tensile strength is the resistance to breaking under tension where tensile strength is measured in pascals the pressure of one newton per square meter and for a material that seems rather delicate it surprisingly has a similar or sometimes even better tensile strength than steel where when measuring tensile strength the diameter of the material is accounted for demonstrating how spider silk’s exceptional performance emerges from molecular architecture rather than bulk dimensions.

Steel exhibits substantial tensile strength variation across different alloy compositions where specialized steel composites achieve up to 2.5 gigapascals while the most common construction material mild steel or plain carbon steel averages 0.4 gigapascals where this baseline structural material provides a meaningful benchmark for evaluating spider silk’s exceptional mechanical performance demonstrating how standardized construction materials establish reference values for biological material comparisons.

While spider silk’s tensile strength can also vary one of the strongest spider silks comes from the golden orb spider which has a tensile strength of up to 1.6 gigapascals where when measuring tensile strength the diameter of the material is accounted for which means a hair-like strand a pencil width strand or a thick beam of the same material will have the exact same tensile strength demonstrating how tensile strength is an intensive material property independent of specimen dimensions.

Spider silk is about one sixth as dense as steel where this means a strand of silk is much stronger for its weight than steel is where another area where spider silk excels is its ductility its ability to be stretched and deformed without breaking where therefore its toughness is remarkably high where single strand of spider silk can outperform many types of steel of the same diameter which is impressive enough but when you consider the densities of each material it’s even more so demonstrating how specific strength rather than absolute strength determines performance in weight-critical applications.

Spider silk’s ductility is its ability to be stretched and deformed without breaking where therefore its toughness the area under the stress strain curve is remarkably high higher than even Kevlar where for spiders this level of strength and flexibility is essential to their survival where when an insect strikes their web the stretching of the matrix enables the web to withstand the impact of the flying prey demonstrating how evolution has optimized spider silk for energy absorption during high-velocity prey capture events.

Over hundreds of millions of years these properties were favored during the process of natural selection giving us the extraordinary material we know today where its potential uses in textiles medicine and even aviation are boundless and so it’s no surprise that now scientists are working tirelessly to recreate it demonstrating how evolutionary optimization over geological timescales produces materials that inspire modern engineering applications across diverse industries.

The mass production of spider silk is incredibly challenging where trying to farm them at scale has been attempted and proven to be basically impossible where spiders are carnivorous animals that don’t like to live in groups as it results in large-scale cannibalism but collecting milking and then releasing wild spiders is also a huge challenge where collecting the amount of silk needed to make anything of significance takes a very long time such as the three years it took to make one cape and shawl demonstrating how spider behavioral ecology fundamentally constrains industrial silk production.

Historically there has been one major problem with recreating spider silk where it has been very difficult to find a way to produce large enough proteins and large proteins are important where the tensile strength and toughness of spider silk are positively correlated with its molecular weight where the bigger the molecule the stronger the silk demonstrating how protein polymer length directly determines mechanical performance through increased chain entanglement and crystalline domain formation.

One common method is to genetically modify a different host organism to produce spider silk proteins where goats are domesticated mammals that already naturally produce large volumes of protein-rich milk where by targeting the gene for milk protein genetically modified spider goats can produce spider silk proteins in their milk where first attempts at this method were promising where the milk contained significant quantities of spider silk proteins however the size of the proteins was still too small just 65 kilodaltons compared to spider silk’s 350 demonstrating how mammalian expression systems face molecular weight limitations for large repetitive proteins.

So scientists turned to silkworms where they offer a great advantage since they already have the necessary spinning apparatus needed to turn silk proteins into silk where inefficient artificial spinning methods aren’t needed where studies have found that modified silkworms can produce large amounts of spider silk with their resulting silk content consisting of 35% spider silk protein but spider silk made from modified silkworms isn’t perfect where the protein is still smaller than natural spider silk proteins demonstrating how transgenic insect hosts provide biological spinning capability but remain limited by protein size constraints.

But perhaps the most promising method yet a method capable of making the biggest proteins comes from something much smaller where genetically modified bacteria can be used to create large amounts of a desired protein because the bacteria E coli replicates itself pretty quickly so a large modified colony could be made in no time however E coli too has some limitations where after the DNA sequence reaches a certain size the bacteria can’t handle it and they chop the sequence into smaller pieces so that the resulting protein doesn’t get too large but in 2018 scientists found a workaround where they added a short genetic sequence to the silk DNA that promotes a chemical reaction between the resulting proteins fusing them together to form a bigger protein where this method was so effective that the resulting proteins were larger than anyone’s ever been able to make at 556 kilodaltons in fact they were larger than most natural spider proteins demonstrating how protein fusion engineering overcame bacterial expression size limitations to achieve unprecedented spidroin molecular weights.

Once the proteins are collected they can be spun into fibers with properties similar to natural spider silk but the process still has some drawbacks where mostly that the spinning process is hard to replicate where the transformation from a liquid state inside the silk glands to a solid state outside of the spigots is pretty complicated but scientists have figured out that a sudden drop in pH a particular salt concentration and shear forces allow the spider silk to materialize where by mimicking these conditions scientists are getting close to an artificial spinning process that’s as good as the real thing demonstrating how biomimetic engineering approaches natural physicochemical gradients to achieve controlled protein polymerization.

So far every group that’s attempted to produce enough spider silk to bring it to the mass market has failed but with these recent advances spider silk may very well soon replace petroleum-based synthetic fibers used in many industries where in 2016 AM Silk collaborated with Adidas to develop a concept sneaker and Omega recently created a special edition spider silk watch strap where Airbus is also exploring how it can be used in aviation where it soon could take on a role much like carbon fiber in today’s world demonstrating how biotechnology advances are enabling commercial applications of bio-inspired materials to displace petroleum-derived synthetics.