Spider Silk

Spider silk is a protein-based fibre produced by all known species of spiders and used in an extraordinary range of biological functions. It is most recognisable as the material from which spiders construct webs, but its roles extend far beyond prey capture. Silk may be employed to entangle victims, transmit tactile signals, form protective egg cocoons, create shelters and serve as safety lines or ballooning threads that allow spiders to disperse through the air. Many spiders adjust the thickness, elasticity and adhesive properties of their silk depending on its specific use, demonstrating an evolved versatility unmatched by most natural fibres.
Silk also plays a significant part in spider courtship and reproduction. The draglines and webs produced by females serve as conduits for male vibratory signals and as platforms carrying pheromones that guide mating attempts. Observations across diverse spider taxa show males producing silk during sexual interactions, although its precise role in mating dynamics requires further study.

Structural Properties

Spider silks are hierarchical materials. Their primary structure consists of spidroins—proteins dominated by repeated glycine- and alanine-rich blocks. These repetitive sequences give the fibres the characteristics of a copolymer. Secondary structure organisation divides the fibre into regions of differing order: alanine-rich segments form tightly packed β-sheet crystallites, while glycine-rich regions constitute amorphous matrices composed of helical and β-turn motifs. The interplay of these domains yields a composite material combining rigidity and elasticity.
Non-protein components further enhance silk function.

• Pyrrolidine maintains moisture and helps deter ants.
• Potassium hydrogen phosphate renders the silk acidic, inhibiting microbial digestion.
• Potassium nitrate stabilises proteins in this acidic environment.

Early models characterised silk as crystallites embedded in an amorphous matrix linked by hydrogen bonds. Later investigations revealed semicrystalline regions and a fibrillar skin–core structure, visualised through technologies such as atomic force microscopy and transmission electron microscopy. Neutron scattering studies have clarified the nanoscale organisation of fibrils and crystalline domains, connecting microscopic orientation with macroscopic mechanical performance.

Mechanical Properties

Each silk type is specialised for its biological function, but dragline silk is particularly notable for its mechanical excellence. Spider silk combines high tensile strength with considerable extensibility, giving it exceptional toughness—the capacity to absorb energy before breaking.
Key mechanical characteristics include:

• Young’s modulus: Lower than steel or Kevlar, indicating greater elasticity and ductility. Some species, such as Ariadna lateralis, possess silks with comparatively high stiffness for spider fibres.
• Tensile strength: Dragline silk rivals that of high-grade steel and approaches half the strength of high-performance aramids like Kevlar, though silk is significantly less dense.
• Density: Approximately one-sixth that of steel, giving high strength-to-weight ratios.
• Ductility: Many silks stretch several times their resting length without breaking.
• Toughness: The toughest known natural silk is produced by Caerostris darwini, with toughness values far surpassing Kevlar.
• Elongation at break: Certain species, such as Caerostris darwini, show extreme extensibility, reaching up to 65 per cent strain.
• Thermal performance: Dragline silk retains strength over a wide temperature range (−40 °C to 220 °C).
• Supercontraction: Exposure to water can shrink dragline silk by up to half its length, aiding in web maintenance and tensioning.

The mechanical variability seen across silk types arises primarily from differences in molecular alignment and environmental conditions such as humidity and temperature.

Adhesive Silk

Some silks function as adhesives rather than structural fibres. Pyriform silk, for example, forms attachment discs that anchor webs and other silk structures to substrates. These secretion-based adhesives polymerise rapidly, remain functional under ambient conditions and are biodegradable. Their design includes microfibrils and lipid layers that contribute to adhesion and durability, mediated by specialised spinneret mechanisms.

Biological Uses

Spiders can produce up to seven distinct silk types, each matched to a specific ecological or behavioural need. These include:

• Structural fibres for web frames and draglines
• Sticky capture spirals for prey entanglement
• Cocoon silks for protecting eggs
• Safety lines for rapid descent or movement
• Ballooning threads for aerial dispersal
• Signal threads optimised for vibration transmission
• Protective coverings for shelters and retreats

Synthesis and Fibre Spinning

Spider silk production differs markedly from methods used in the production of insect silks. Although forced extraction of spider silk has been achieved experimentally, the biology of spiders makes large-scale cultivation difficult. Within the spider, silk is stored as a concentrated protein solution and undergoes controlled biochemical and mechanical transformations as it moves through the spinning ducts. These processes align, fold and solidify spidroin molecules into fibres with precisely tuned properties.
The spinning apparatus enables the spider to adjust the composition and structure of the silk in response to environmental cues or behavioural requirements. This natural fibre-spinning process is more refined than most synthetic fibre technologies and remains a major inspiration for biomimetic research.