How is spider silk made?
The spiders produce silk from different glands depending upon the species. However, Nephilia clavipes produce dragline silk from both minor and major ampullate glands. The process by which spiders produce silk is called pultrusion wherein the spiders pull the silk thread from the silk-producing gland.
The silk-making glands resemble the spinneret which consists of a number of tubes. These glands contain a watery solution that is nothing but silk protein. This solution is then poured into a long looping duct that tapers as it folds as a result the protein gets aligned in the direction of flow and is transformed into a liquid crystalline state. The duct is further connected to a valve that squeezes the silk for it to form into a thread. The thread then goes into a narrow duck which provides fineness to the thread by stretching it and gives it an acid rinse to remove water. The thread then exits the spigot and the resultant silk solidifies on evaporation of water.
However, the large scale of production of spider silk is difficult and so efforts are made in this field to develop various techniques and bio-material that imitate spider silk which include genetic engineering, stem cell concepts and recycled silk. Some of these are discussed below:
• Chimeric Silkworms:
The resultant fiber produced by this method is a blend of silkworm and spider silk and fibers exhibit higher toughness compared with cultivated silkworm fibers. This technique utilises a piggyBac vector to create transgenic silkworm which produces spider silk protein. But since the silkworm has an inherent property to produce silk, the resultant fiber is a combination of silkworm and spider silk.
• Transgenic Goats:
This method of producing silk was introduced by a Canadian based biotechnology company in the year 2000. The company took genes from the spider dragline genes which was bioengineered and then inserted into the mammary cells of the goat. The milk produced by the goat contained spider silk proteins (1-2 grams of silk proteins per litre of milk). The procured protein was then spun into fiber. The resultant fiber so produced has a tensile strength ranging from 1-2g/den.
• Metabolically engineered Escherichia coli:
This method employs Escherichia coli as an expression system with an aim to produce spider silk protein that resembles spider silk in terms of both molecular weight and mechanical properties.