Spider Silk Strength, Toughness, and Production Challenges

Spider silk is renowned for its incredible strength and toughness, often compared to steel and Kevlar. This article delves into the science behind spider silk, its unique properties, the challenges of producing it, and the innovative methods scientists are developing to harness its potential.

The Marvel of Spider Silk

Spider webs are not made from a single type of silk. Spiders produce up to seven different kinds of silk, each serving a specific purpose. For instance, one type anchors the web, another forms the stretchy spiral, and a sticky silk catches prey. The strongest of these is dragline silk, also known as major ampullate silk, which provides the structural integrity of the web.

Dragline silk is often compared to steel due to its remarkable strength. This silk is crucial for stopping insects that fly into the web, absorbing the impact and preventing the web from breaking.

Testing the Strength of Spider Silk

To understand its properties, researchers at the Blackledge Spider Lab at the University of Akron, a leading center for spider silk research, conduct tests on dragline silk. Obtaining samples involves carefully extracting silk from spiders, often by gently pulling the dragline silk by hand and attaching it to a custom-made rotating mechanism.

Once a silk strand is obtained, it's subjected to a machine that slowly pulls on it, measuring two key properties:

1. Stress: The force applied over the silk's tiny cross-sectional area.
2. Strain: The amount of stretching, calculated as the change in length divided by the original length.

Initially, stress and strain increase proportionally. If the pulling stops, the silk returns to its original length. However, beyond a certain point, some stretching becomes permanent. Eventually, the silk breaks. The force applied at the breaking point, divided by the cross-sectional area, is called its ultimate tensile strength.

A typical dragline silk can have an ultimate tensile strength of about 600 megapascals. To put this in perspective, a rope of spider silk with a cross-sectional area of one square centimeter (about the size of a little finger) could support approximately 60,000 newtons of force, equivalent to the weight of a fully grown African elephant.

Even more impressive is the silk from the Darwin's bark spider, found in Madagascar. This spider spins webs up to 25 meters long over open water, requiring incredibly strong silk. Its ultimate tensile strength can reach around 1,600 megapascals, more than double that of typical dragline silk.

While experimental ultra-high strength steels can achieve nearly 3,000 megapascals, about three times the strength of typical dragline silk, steel is also six times denser. This means that for the same mass and length, a spider silk rope has six times the cross-sectional area. Consequently, a spider silk rope can withstand roughly twice the force of ultra-high strength steel before breaking, a measure known as specific strength. This makes spider silk a champion for applications where weight is a critical factor.

Toughness: Beyond Strength

Strength alone isn't enough; toughness is also crucial. Toughness refers to a material's ability to absorb energy before breaking. This is analogous to a rock climber's rope: it needs to be strong enough to support weight, but also tough enough to absorb the kinetic energy of a fall without snapping. A stretchy rope absorbs energy gradually, reducing the forces on the climber.

Spider silk is remarkably stretchy. Some dragline silk can stretch up to 70% of its length before breaking. This elasticity allows the web to slowly decelerate an insect, preventing it from bouncing off and ensuring the spider can subdue its meal.

The total energy a material absorbs before breaking is its toughness, represented by the area under the stress/strain curve. Kevlar, used in bulletproof vests, has a toughness of up to 50 megajoules per cubic meter. Experimental ultra-high strength steel reaches around 170 megajoules per cubic meter. However, typical spider silk can have a toughness of about 205 megajoules per cubic meter, and the Darwin's bark spider silk can reach an astonishing 520 megajoules per cubic meter. This is roughly three times tougher than the strongest steel and 10 times tougher than Kevlar. Kevlar is strong but stiff, meaning it breaks before stretching significantly, thus absorbing less energy compared to the stretchy spider silk.

The Secret to Spider Silk's Properties

The extraordinary properties of spider silk stem from its unique molecular structure. Silk fibers are composed of proteins called spidroins. These spidroins have two main types of regions:

• Nanocrystals: Regions where spidroins are uniformly arranged and closely aligned, providing strength. They act like rigid blocks.
• Amorphous regions: Areas where spidroins are less ordered and can move more freely, providing elasticity. They act like elastic cords connecting the rigid blocks.

This network of rigid blocks connected by elastic cords allows the silk to stretch and absorb tremendous amounts of energy before breaking, making it both strong and tough.

Challenges in Producing Spider Silk

Despite its incredible properties, spider silk is not widely used in commercial products. Attempts to produce it on a large scale have faced significant hurdles for centuries.

In 1709, Francois Xavier Bon painstakingly collected spider egg sacs to make silk stockings, but found it impossible to gather enough material for large-scale production. More recently, a project in Madagascar collected over a million golden orb weaver spiders to weave a single golden cape, highlighting the immense effort required.

The primary challenge with farming spiders is their cannibalistic nature. They cannot be kept in close proximity without eating each other, requiring vast amounts of space for a spider farm. Additionally, milking spiders for their silk is a difficult and inconsistent process, as spiders vary in cooperation and yield. Natural spider silk is therefore extremely scarce and expensive, costing around $7 million per kilogram, about 50 times the price of gold.

Engineering Spider Silk

Given the difficulties of natural production, scientists have turned to genetic engineering to create spider silk proteins.

• Early Attempts: In the late 1990s, DuPont researchers inserted spider silk genes into E. coli and yeast, turning these microorganisms into protein factories. In 2001, a German team tried implanting these genes into plants like tobacco and potatoes. A Canadian company, Nexia, even created "spider goats" by inserting spider silk genes into goats, causing them to produce spider silk proteins in their milk.
• The Processing Problem: While these methods successfully produced spider silk proteins, they all faced the same issue: the proteins did not resemble actual spider silk. Whether purified as a powder, a viscous gel, or dissolved in milk, they lacked the intricate structure of natural silk. The key challenge lies in replicating the spider's natural spinning process.

The Spider's Spinning Process

A spider's silk gland is a marvel of natural engineering. The process begins in the tail region, where cells produce spidroins and secrete them into a fluid. Each spidroin has an N-terminal, a C-terminal, and a long repetitive region of amino acids in between.

1. Initial Assembly: In the tail, C-terminal ends pair up, linking spidroins, while N-terminal ends remain separate, allowing flexibility.
2. Storage and pH Control: The spidroins move to a storage sac where they are highly concentrated. To prevent clumping, the pH is carefully tuned, giving N-terminal ends a net negative charge. Like charges repel, keeping the proteins from sticking together.
3. Micelle Formation: In the aqueous solution, spidroins rearrange. Hydrophilic N-terminals face outward towards water, while hydrophobic repetitive regions tuck inward, forming tiny structures called micelles, which then cluster into larger globules.
4. Shear and Chemical Changes: These globules enter the spinning duct, which is S-shaped and narrows. As the liquid flows, shear forces stretch and deform the globules. Water is removed, and the pH drops, making the solution more acidic. This drop in pH neutralizes the negative charges on the N-terminals, allowing them to pair up and link spidroins into long chains.
5. Final Stretching and Structure: The chains are pulled through a tiny narrowing called the drawdown taper, leading to the spigot. This final stretch aligns neighboring spidroins. Depending on the amino acid sequence, some sections fold into zigzag strands that interlock with hydrogen bonds, forming rigid nanocrystals (for strength). Other sections remain less ordered, forming amorphous regions (for stretch).

This complex process, which no one fully understands yet, is what makes recreating spider silk in a lab so difficult.

Transgenic Silkworms: A Promising Solution

Researchers at Kraig BioCraft Laboratories in Michigan are exploring a different approach: using an animal that already has a spinner – the silkworm. Humans have farmed silkworms for nearly 5,000 years, and they are highly efficient at producing silk.

The idea is to genetically modify silkworms to spin spider silk. This involves:

1. Microinjection: Spider silk genes are injected into silkworm eggs using tiny needles.
2. Jumping Genes (Transposons): Scientists use "jumping genes" or transposons, like piggyBac, which naturally move DNA sequences within a genome. By replacing the middle section of piggyBac with the spider silk gene and injecting it along with an enzyme, the spider silk gene can be inserted into the silkworm's DNA.
3. Targeted Insertion: Early transgenic silkworms produced silk that was only 6-10% spider DNA. While this still achieved significant mechanical performance (around 60% of pure spider silk's strength), the goal is to create "knock-in/knock-out" transgenics where the silkworm's native silk genes are completely replaced with spider silk genes.
4. CRISPR-Cas9: To achieve more targeted gene insertion, leading labs are exploring CRISPR-Cas9. This tool uses a specific guide RNA to match and cut a precise DNA sequence (e.g., within the silkworm's own silk gene). Scientists then inject donor DNA containing the spider silk gene, which the cell incorporates during repair.

Kraig BioCraft Laboratories has successfully produced half a ton of transgenic spider silk cocoons in a year. Other companies like AMSilk (Germany) and Spiber (Japan) are also developing protein fibers through fermentation. These engineered silks are already being used in clothing by brands like Goldwin and The North Face. The US Army has funded Kraig to develop ballistic shoe packs for potential body armor, and spider silk is even being explored in medicine for nerve repair by companies like Newrotex.

The Spider-Man Test

To demonstrate the strength of transgenic spider silk, a continuous filament made of 10 individual silk strands was put to the ultimate test: a human swing. While the silk successfully held the weight of a person and allowed for a swing, its extreme thinness and strength caused it to cut into the skin. This highlights a practical challenge: while incredibly strong, the thinness of the silk can make it impractical for direct human interaction without proper handling or weaving into thicker materials.

The potential applications of spider silk are vast, from clothing and armor to medical implants and nerve repair, provided it can be produced affordably and at scale.