what are some of the physical characteristics of this material?
They have a triangular cross section with rounded corners, 5–10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations.
Silk
Silk fibers from the "Bombyx mori" silkworm have a triangular cross section with rounded corners, 5–10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. 
when did silk begin to be produced in china th millennium BC can you name some species from where we obtain this material? Bombyx mori how do these species release that material? By forming a cacoon by this insect is that material what they release- a cellulose or a protein or a fat? Natural protein fiber what type of proteins do they contain? The Protein fiber of silk is composed mainly of fibroin apart from china, which other countries produce that fiber? India is one of them
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Silk [SEP] Physical properties
Silk fibers from the "Bombyx mori" silkworm have a triangular cross section with rounded corners, 5–10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. The flat surfaces of the fibrils reflect light at many angles, giving silk a natural sheen. The cross-section from other silkworms can vary in shape and diameter: crescent-like for "Anaphe" and elongated wedge for "tussah". Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins that act like glue, to form a bave.