Urea treatment. Freshly cut cocoon shells were soaked into 8 M urea aqueous solution for
30 min and then refluxed at 85C for 30 min. Centrifugation and filtration were used to remove all insoluble residues. The solution was thoroughly dialyzed in distilled water using cellulose membrane (Cellusep T2, MWCO 6,000-8,000, Sequin, Texas, USA) for 3 days.