The Remarkable Nature of Spider Silk
Spider silk is a natural fiber with strong mechanical properties. Spiders produce various types of silk, but dragline silk, used for web structures and as a lifeline, is notable for its strength and extensibility. This silk is composed of spidroin proteins, long chains of amino acids like glycine and alanine. These amino acids form segments that contribute to the silk’s strength and flexibility.
Dragline silk is very thin, often thousands of times narrower than a human hair, yet made of thousands of nanostrands. The tight arrangement of these protein molecules gives the silk its characteristics. Spider silk can stretch up to four times its original length without breaking. This elasticity, combined with tension resistance, allows it to absorb substantial energy before breaking.
Understanding Steel’s Strength
Steel is an alloy of iron and carbon, with carbon comprising less than 2% of its composition. The amount of carbon directly influences steel’s hardness and strength, ensuring it remains strong without becoming brittle. Other elements like manganese, chromium, and nickel are added to enhance properties such as corrosion resistance, toughness, and heat strength, customizing steel for diverse applications.
Steel processing, including heat treatments like quenching and tempering, contributes to its mechanical properties. Quenching, a rapid cooling process, increases strength and hardness; tempering restores toughness. Steel’s strength stems from its ability to withstand loads without breaking, its ductility for shaping, and its toughness for absorbing impact. These characteristics make steel a material for structures like bridges and skyscrapers.
Spider Silk Versus Steel: The Direct Comparison
Comparing spider silk and steel requires careful definition, as materials exhibit different types of strength. Spider silk is known for its tensile strength, measuring a material’s resistance to being pulled apart before breaking. Some spider silks can have a tensile strength of about 1 gigapascal (GPa), which typically overlaps with the range of steel, from 0.2 GPa to 2 GPa. Specialized spider silks, like from the Caerostris darwini, can reach up to 1.7 GPa.
The assertion that spider silk is “stronger than steel” refers to its strength-to-weight ratio, also known as strength-to-density ratio. On an equal weight basis, spider silk is stronger than steel. This means a silk strand is stronger for its size than a steel beam, making it a lightweight yet robust material. For instance, dragline silk is five times stronger than a steel strand of the same diameter.
However, steel surpasses spider silk in other properties, such as stiffness and compressive strength. Stiffness refers to a material’s ability to resist elastic deformation. While spider silk is elastic and can stretch up to four times its original length without breaking, steel is stiffer. Steel’s elastic modulus, indicating its resistance to elastic deformation, is higher than spider silk’s. Therefore, while a thin spider silk thread might break easily due to its fine diameter, a scaled-up spider silk cable would be stronger than a similarly sized steel cable, particularly in its ability to withstand tension and absorb energy.