Spider webs are a common sight, yet the material composing them remains a subject of considerable scientific interest. These intricate structures are crafted from a substance, often referred to as spider silk, that possesses remarkable characteristics. The unique properties of spider silk have intrigued researchers for decades, prompting extensive study into its composition and formation.
The Chemical Makeup of Spider Silk
Spider silk is a protein-based material, composed of proteins called spidroins. These spidroins are polypeptides belonging to the scleroprotein group, which also includes collagen and keratin. Their specific composition can vary depending on the spider species and its diet; for example, dragline silk contains spidroin 1 and spidroin 2.
Spidroins are rich in amino acids like glycine (around 42%) and alanine (about 25%). These small amino acids lack bulky side groups, allowing them to pack tightly. This tight packing facilitates the formation of highly ordered crystalline regions within the silk fiber. The arrangement of these proteins, with strong crystalline areas and flexible amorphous regions, gives spider silk its strength and elasticity.
How Spiders Create Silk and Webs
Spiders produce silk within glands in their abdomen. Depending on the species, spiders can possess five to seven pairs of these glands, each producing a distinct type of silk. For example, major ampullate glands produce dragline silk for structural support, while flagelliform glands create sticky capture silk for trapping prey.
The silk begins as a liquid protein solution inside these glands. This liquid silk travels through narrow ducts, where it mixes with hydrogen ions, lowering its pH and initiating the hardening process. The fiber then exits the spider’s body through spinnerets, muscular appendages equipped with tiny valves. These valves control the thickness and speed at which the silk is extruded, allowing the spider to engineer its web.
Unrivaled Strengths of Spider Silk
Spider silk possesses unique physical properties. Its tensile strength, the ability to withstand pulling forces, is high, with dragline silk reaching up to 1.1 gigapascals (GPa). Some spider silk varieties are five times stronger than an equal mass of steel. This strength is complemented by elasticity, allowing silk to stretch significantly before breaking. Dragline silk can extend approximately 27% beyond its original length, while capture silk can stretch two to four times its initial length, absorbing considerable energy.
The combination of high strength and elasticity contributes to spider silk’s toughness, its ability to absorb energy before fracturing. Spider silk is tougher than Kevlar, a synthetic aramid fiber known for its strength. While Kevlar may have higher tensile strength, spider silk’s toughness can be around 180 megajoules per cubic meter (MJ/m³), compared to Kevlar’s 50 MJ/m³. This property makes spider silk resistant to impact and static loads. It is also lightweight, biodegradable, and biocompatible.
Inspired Innovations from Spider Silk
The unique characteristics of spider silk have spurred interest in various scientific and industrial fields. Its strength, elasticity, and biocompatibility make it a material for biomimicry. In medicine, spider silk holds promise for applications such as advanced sutures, drug delivery systems, and tissue engineering scaffolds, due to its ability to naturally degrade within the body. Researchers are exploring its use in wound dressings, with artificial silk woven into bandages showing promise in treating joint injuries and skin lesions in mice.
Beyond medical uses, spider silk’s properties are inspiring innovations in materials science. It could lead to the development of lightweight composites, stronger protective gear, and new biodegradable plastics. However, challenges remain in mass production due to the cannibalistic nature of spiders and the small amounts of silk each spider produces. Current research focuses on overcoming these hurdles through methods like genetically modifying organisms, such as goats, to produce silk proteins in their milk, and developing synthetic silk production techniques using microbes.