By: N N Mahapatra
Spider silk is said to be one of the strongest, toughest materials on Earth. Now, engineers at Washington University in St. Louis have used amyloid silk hybrid proteins to create fibres that are stronger and tougher than some natural spider silks.
To be precise, the artificial silk – dubbed ‘polymeric amyloid’ fibre – was not technically produced by researchers, but by bacteria that were genetically engineered in the lab of Fuzhong Zhang, a professor in the Department of Energy, Environmental & Chemical Engineering.
MANUFACTURING PROCESS
Zhang has worked with spider silk before. In 2018, his lab engineered bacteria that could produce a recombinant spider silk that matched its natural counterparts in all the main mechanical properties.
The research team, which includes first author Jingyao Li, a PhD student in Zhang’s lab, modified the amino acid sequence of spider silk proteins to introduce new properties, while keeping some of the attractive features of spider silk.
A problem associated with the original recombinant spider silk fibre was the need to create β-nanocrystals, a main component of natural spider silk, which contributes to its strength. The natural Spiders have figured out how to spin fibres with a desirable amount of nanocrystals,but when humans use artificial spinning processes, the amount of nanocrystals in a synthetic silk fibre is often lower than its natural counterpart.
To solve this problem, the engineers redesigned the silk sequence by introducing amyloid sequences with a high tendency to form β-nanocrystals. This involved creating different polymeric amyloid proteins using three well-studied amyloid sequences as representatives.
The resulting proteins had less repetitive amino acid sequences than spider silk, making them easier to be produced by engineered bacteria. Ultimately, the bacteria produced a hybrid polymeric amyloid protein with 128 repeating units.
The longer the protein, the stronger and tougher the resulting fibre. The 128-repeat proteins produced a fibre with gigapascal strength (a measure of how much force is needed to break a fibre of fixed diameter), making it stronger than common steel. The fibres’ toughness (a measure of how much energy is needed to break a fibre) is higher than Kevlar and all previous recombinant silk fibres. Its strength and toughness are even higher than some reported natural spider silk fibres.He and his team are ready to produce materials that beat the best material in nature.
Spider silk is one of nature’s most impressive materials, exhibiting impressive strength and toughness. Now, researchers at Washington University in St. Louis claim to have created an artificial version that can outperform some natural spider silks. This isn’t the first rodeo for this research team – back in 2018 they developed a synthetic spider silk that was about on par with the real thing, in terms of tensile strength, extensibility and toughness. To do so, they spliced silk-producing genes into bacteria, and tweaked them so that proteins in the silk would fuse together to make a stronger, tougher material. For the new study, the team built on this prior work to not just match natural spider silk but to surpass it. The key component is beta-nanocrystals, which boost the material’s strength but are hard to reproduce synthetically.
So the researchers once again rolled up their sleeves and tinkered with the protein arrangements in the silk. This time they added new amyloid sequences that tend to produce more beta-nanocrystals, while also using fewer repetitive amino acid sequences, which makes it easier for bacteria to produce them.
In tests, the new synthetic silk showed an average ultimate tensile strength of around 1 Gigapascal (GPa), and an average toughness of around 161 Megajoules per m3. That combination outperforms most synthetic silks, as well as some natural ones, including the dragline silks of elaborate web-spinners like the golden silk spider and the banded garden spider.
And the team says that there’s still plenty of room for improvement. They only worked with three amyloid sequences, meaning better properties could be hiding among the thousands of others waiting to be explored.
Spiders are interesting models because they are able to produce these superb silk fibres at room temperature using water as a solvent . This process of spiders have evolved over hundreds of millions of years, but we have been unable to copy so far.
The lab-made fibres are created from a material called a hydrogel, which is 98 percent water and 2 percent silica and cellulose, the latter two held together by cucurbiturils, molecules that serve as “handcuffs. The silica and cellulose fibres can be pulled from the hydrogel. After 30 seconds or so, the water evaporates, leaving behind only the strong, stretchy thread.
The fibres could also be modified in a number of interesting ways. Replacing the cellulose with various polymers could turn the silk into an entirely different material. The basic method could be replicated to produce low-heat, no-chemical-solvents-needed versions of many fabrics.
It’s a generic method to make all fibres, to make any form of artificial fibre green
Unlike silkworms, which can be farmed for their silk, spiders are cannibals who wouldn’t tolerate the close quarters necessary for farming, so turning to the lab is the only way to get significant quantities of the material. Every few years brings headlines about new inroads in the process. A German team has modified E-coli bacteria to produce spider silk molecules. Scientists at Utah State University bred genetically modified “spider goats” to produce silk proteins in their milk. The US army is testing “dragon silk” produced via modified silkworms for use in bulletproof vests. Earlier this year, researchers at the Karolinska Institute in Sweden published a paper on a new method for using bacteria to produce spider silk proteins in a potentially sustainable, scalable way. And this spring, California-based startup Bolt Threads debuted bioengineered spider silk neckties at the SXSW festival. Their product is made through a yeast fermentation process that produces silk proteins, which then go through an extrusion process to become fibers. It’s promising enough to have generated a partnership with outdoor manufacturer Patagonia.
But, as a 2015 Wired story points out, “so far, every group that attempted to produce enough of the stuff to bring it to the mass market, from researchers to giant corporations, has pretty much failed.”
This is the challenge Shah and his team are facing right now.
“Currently we make around a few tens of milligrams of these materials and then pull fibers from them,” he says. “But we want to try and do this at a much larger scale.”
To do so, the team is working on a robotic device to pull and spin fibers more quickly and at a larger scale than previously. They’ve had some success, Shah says, and continue to explore the process.
PROPERTIES OF SYNTHETIC SPIDER SILK
The silk of the humble spider has some pretty impressive properties. It’s one of the sturdiest materials found in nature, stronger than steel and tougher than Kevlar. It can be stretched several times its length before it breaks. For these reasons, replicating spider silk in the lab has been a bit of an obsession among materials scientists for decades.
Spider silks show unique combinations of strength, toughness, extensibility, and energy absorption. To date, it has been difficult to obtain spider silk-like mechanical properties using non-protein approaches. Here, we report on an artificial spider silk produced by the water-evaporation-induced self-assembly of hydrogel fibre made from polyacrylic acid and silica nanoparticles. The artificial spider silk consists of hierarchical core-sheath structured hydrogel fibres, which are reinforced by ion doping and twist insertion. The fibre exhibits a tensile strength of 895 MPa and a stretchability of 44.3%, achieving mechanical properties comparable to spider silk. The material also presents a high toughness of 370 MJ m−3 and a damping capacity of 95%. The hydrogel fibre shows only ~1/9 of the impact force of cotton yarn with negligible rebound when used for impact reduction applications.
The fibers are extremely strong – though not quite as strong as the strongest spider silks – and, significantly, they can be made at room temperature without chemical solvents. This means that if they can be produced at scale, they have an advantage over other synthetic fibers such as nylon, which require extremely high temperatures for spinning, making textile production one of the world’s dirtiest industries. The artificial spider silk is also completely biodegradable. And since it’s made from common, easily accessible materials – mainly water, silica and cellulose – it has the potential to be affordable.
Because the material can absorb so much energy, it could potentially be used as a protective fabric.
APPLICATION OF SYNTHETIC SPIDER SILK
The researchers at the University of Cambridge have created a new material that replicates the spider silk’s strength, stretchiness and energy-absorbing capacity. This material offers the possibility of improving on products from bike helmets to parachutes to bulletproof jackets to airplane wings. The most impressive property is that it is 98 percent water.
“Spiders need that absorption capacity because when a bird or a fly hits their web, it needs to be able to absorb that, otherwise it’s going to break,” Shah says. “So things like shrapnel resistant or other protective military clothing, that would be an exciting application.”
Other potential applications include sail cloth, parachute fabric, hot air balloon material, and bike or skateboard helmets. The material is biocompatible, which means it could be used inside the human body for things like stitches.