Spider silk

Spider silk is a protein fibre or silk spun by spiders. Spiders use silk to make webs or other structures that function as adhesive traps to catch prey, to entangle and restrain prey before biting, to transmit tactile information, or as nests or cocoons to protect their offspring. They can use the silk to suspend themselves from height, to float through the air, or to glide away from predators. Most spiders vary the thickness and adhesiveness of their silk according to its use.
In some cases, spiders may use silk as a food source. While methods have been developed to collect silk from a spider by force, gathering silk from many spiders is more difficult than from silk-spinning organisms such as silkworms.
All spiders produce silk, although some spiders do not make webs. Silk is tied to courtship and mating. Silk produced by females provides a transmission channel for male vibratory courtship signals, while webs and draglines provide a substrate for female sex pheromones. Observations of male spiders producing silk during sexual interactions are common across widespread taxa. The function of male-produced silk in mating has received little study.

Properties

Structural

Silks have a hierarchical structure. The primary structure is the amino acid sequence of its proteins, mainly consisting of highly repetitive glycine and alanine blocks, which is why silks are often referred to as block co-polymers. On a secondary level, the short side-chained alanine is mainly found in the crystalline domains of the nanofibril. Glycine is mostly found in the so-called amorphous matrix consisting of helical and beta turn structures. The interplay between the hard crystalline segments and the strained elastic semi-amorphous regions gives spider silk its extraordinary properties. Various compounds other than protein are used to enhance the fibre's properties. Pyrrolidine has hygroscopic properties that keep the silk moist while warding off ant invasion. It occurs in high concentration in glue threads. Potassium hydrogen phosphate releases hydrogen ions in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungi and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturing in the acidic milieu.
Termonia introduced this basic model of silk in 1994. He suggested crystallites embedded in an amorphous matrix interlinked with hydrogen bonds. Refinements to this model include the discovery of semi-crystalline regions and a fibrillar skin-core model suggested for spider silk, later visualized by atomic force microscopy and transmission electron microscopy. Sizes of the nanofibrillar structure and the crystalline and semi-crystalline regions were revealed by neutron scattering.
The fibres' microstructural information and macroscopic mechanical properties are related. Ordered regions mainly reorient by deformation for low-stretched fibres and the fraction of ordered regions increases progressively for higher fibre stretching.

Mechanical

Each spider and each type of silk has a set of mechanical properties optimised for their biological function.
Most silks, in particular dragline silk, have exceptional mechanical properties. They exhibit a unique combination of high tensile strength and extensibility. This enables a silk fibre to absorb a large amount of energy before breaking.
Strength and toughness are distinct quantities. Weight for weight, silk is stronger than steel, but not as strong as Kevlar. Spider silk is, however, tougher than both.
The variability of spider silk fibre mechanical properties is related to their degree of molecular alignment. Mechanical properties also depend on ambient conditions, i.e. humidity and temperature.

Young's modulus

is the resistance to deformation elastically along the tensile force direction. Unlike steel or Kevlar, which are stiff, spider silk is ductile and elastic, having a lower Young's modulus. According to Spider Silkome Database, Ariadna lateralis silk has the highest Young's modulus with 37 GPa, compared to 208 GPa for steel and 112 GPa for Kevlar.

Tensile strength

A dragline silk's tensile strength is comparable to that of high-grade alloy steel, and about half as strong as aramid filaments, such as Twaron or Kevlar. According to Spider Silkome Database, Clubiona vigil silk has the highest tensile strength.

Density

Consisting of mainly protein, silks are about a sixth of the density of steel. As a result, a strand long enough to circle the Earth would weigh about.

Energy density

The energy density of dragline spider silk is roughly.

Ductility

Silks are ductile, with some able to stretch up to five times their relaxed length without breaking.

Toughness

The combination of strength and ductility gives dragline silks a high toughness, which "equals that of commercial polyaramid filaments, which themselves are benchmarks of modern polymer fibre technology". According to Spider Silkome Database, Araneus ishisawai silk is the toughest.

Elongation at break

Elongation at break compares initial object length to final length at break. According to Spider Silkome Database, Caerostris darwini silk has the highest strain at break for any spider silk, breaking at 65% extension.

Temperature

While unlikely to be relevant in nature, dragline silks can hold their strength below -40 °C and up to 220 °C. As occurs in many materials, spider silk fibres undergo a glass transition. The glass-transition temperature depends on humidity, as water is a plasticiser for spider silk.

Supercontraction

When exposed to water, dragline silks undergo supercontraction, shrinking up to 50% in length and behaving like a weak rubber under tension. Many hypotheses have attempted to explain its use in nature, most popularly to re-tension webs built in the night using the morning dew.

Highest-performance

The toughest known spider silk is produced by the species Darwin's bark spider : "The toughness of forcibly silked fibers averages 350 MJ/m³, with some samples reaching 520 MJ/m³. Thus, C. darwini silk is more than twice as tough as any previously described silk and over 10 times tougher than Kevlar".

Adhesive

Silk fibre is a two-compound pyriform secretion, spun into patterns using a minimum of silk substrate. The pyriform threads polymerise under ambient conditions, become functional immediately, and are usable indefinitely, remaining biodegradable, versatile and compatible with other materials in the environment. The adhesive and durability properties of the attachment disc are controlled by functions within the spinnerets. Some adhesive properties of the silk resemble glue, consisting of microfibrils and lipid enclosures.

Uses

All spiders produce silks, and a single spider can produce up to seven different types of silk for different uses. This is in contrast to insect silks, where an individual usually only produces a single type. Spiders use silks in many ways, in accord with the silk's properties. As spiders have evolved, so have their silks' complexity and uses, for example from primitive tube webs 300–400 million years ago to complex orb webs 110 million years ago.

Use	Example	Reference
Prey capture	Orb webs produced by the Araneidae ; tube webs; tangle webs; sheet webs; lace webs, dome webs; single thread used by the Bolas spiders for "fishing".
Prey immobilisation	"Swathing bands" to envelop prey. Often combined with immobilising prey using a venom. In species of Scytodes the silk is combined with venom and squirted from the chelicerae.
Reproduction	Male spiders may produce sperm webs; spider eggs are covered in silk cocoons.
Dispersal	"Ballooning" or "kiting" used by smaller spiders to float through the air, for instance for dispersal.
Food	The kleptoparasitic Argyrodes eats the silk of host spider webs. Some daily weavers of temporary webs eat their own unused silk, thus mitigating an otherwise heavy metabolic expense.
Nest lining and nest construction	Tube webs used by "primitive" spiders such as the European tube web spider. Threads radiate out of the nest to provide a sensory link to the outside. Silk is a component of the lids of spiders that use "trapdoors", such as members of the family Ctenizidae, and the "water" or "diving bell" spider Argyroneta aquatica forms a silk diving bell.
Guide lines	Some spiders that venture from shelter leave a silk trail by which to find their way home again.
Drop lines and anchor lines	Spiders such as the Salticidae venture from shelter and leave a trail of silk, use that as an emergency line in case of falling from inverted or vertical surfaces. Others, even web dwellers, deliberately drop from a web when alarmed, using a silken thread as a drop line by which they can return in due course. Some, such as species of Paramystaria, hang from a drop line while feeding.
Alarm lines	Some spiders that do not spin actual traps build alarm webs that the feet of their prey can disturb, cueing the spider to pounce on prey or flee a formidable intruder.
Pheromonal trails	Some wandering spiders leave a largely continuous trail of silk impregnated with pheromones that the opposite sex can follow to find a mate.

Silk types

Meeting the specification for all these ecological uses requires different types of silk presenting different properties, as either a fibre, a structure of fibres, or a globule. These types include glues and fibres. Some types of fibres are used for structural support, others for protective structures. Some can absorb energy effectively, whereas others transmit vibration efficiently. These silk types are produced in different glands; so the silk from a particular gland can be linked to its use.

Gland	Silk Use
Ampullate	Dragline silkused for the web's outer rim and spokes, also for lifeline and for ballooning
Ampullate	Used for temporary scaffolding during web construction
Flagelliform	Capture-spiral silkused for the capturing lines of the web
Tubuliform	Egg cocoon silkused for egg sacs
Aciniform	Used to wrap and secure prey; used in male sperm webs; used in stabilimenta
Aggregate	Sticky globules
Piriform	Bonds between separate threads for attachment point.

Many species have different glands to produce silk with different properties for different purposes, including housing, web construction, defence, capturing and detaining prey, egg protection, and mobility.

Silk	Use
Major-ampullate silk	The web's outer rim and spokes and the lifeline. Can be as strong per unit weight as steel, but much tougher.
Capture-spiral silk	Capturing lines. Sticky, stretchy, and tough. The capture spiral is sticky due to droplets of aggregate that are placed on the spiral. The elasticity of flagelliform allows enough time for the aggregate to adhere to the aerial prey flying into the web.
Tubiliform silk	Protective egg sacs. Stiffest silk.
Aciniform silk	Wrap and secure prey. Two to three times as tough as the other silks, including dragline.
Minor-ampullate silk	Temporary scaffolding during web construction.