Spider silk

Spider silk is a protein fibre spun by spiders. Spiders use their silk to make webs or other structures, which function as sticky nets to catch other animals, or as nests or cocoons to protect their offspring, or to wrap up prey. They can also use their silk to suspend themselves, to float through the air, or to glide away from predators. Most spiders vary the thickness and stickiness of their silk for different uses.

In some cases, spiders may even use silk as a source of food. While methods have been developed to collect silk from a spider by force, it is difficult to gather silk from many spiders in a small space, in contrast to silkworm "farms".

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 one type of silk. Spider silks may be used in many different ecological ways, each with properties to match the silk's function. As spiders have evolved, so has their silks' complexity and diverse uses, for example from primitive tube webs 300–400 million years ago to complex orb webs 110 million years ago.

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

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 (ductility). This enables a silk fibre to absorb a lot of energy before breaking (toughness, the area under a stress-strain curve).

A frequent mistake made in the mainstream media is to confuse strength and toughness, when comparing silk to other materials. Weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than either.

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