Fibroin is a type of protein created by Bombyx mori (silkworms) in the production of silk. Silk emitted by the silkworm consists of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it.

The fibroin protein consists of layers of antiparallel beta sheets. Its primary structure mainly consists of the recurrent amino acid sequence (Gly-Ser-Gly-Ala-Gly-Ala)_n. These sheets are largely responsible for the tensile strength of the material. In addition to being stronger than Kevlar, fibroin is known to be highly elastic. These attributes make it a material with applications in several areas, including biomedicine and textile manufacture.

Fibroin is known to arrange itself in three structures, called silk I, II, and III. Silk I is the natural form of fibroin, as emitted from the Bombyx mori silk glands. Silk II refers to the arrangement of fibroin molecules in spun silk, which has greater strength and is often used commercially in various applications. Silk III is a newly discovered structure of fibroin, first observed by Professor Regina Valluzzi et al. at Tufts University. Silk III is formed principally in solutions of fibroin at an interface (i.e. air-water interface, water-oil interface, etc.). Research on silk III in particular continues to better understand its physical structure and properties.

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• MeSH Fibroin

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