Acid treatment. Citric acid solution was used to prepare acid-degraded silk sericin solution.Cocoons of Bombyx mori silkworms were cut into small pieces, added with 1.25% citric acid solution and boiled for 30 min. After filtration to remove insoluble fibers, the clear filtrate was immediately dialyzed in distilled water for 3 days using cellulose membrane (Cellusep T2, MWCO6,000-8,000, Sequin, Texas, USA