To initiate the gelation of SF solution, ultra-sonication was performed on 3% (w/v) SF aqueous solution of different molecular weights using an ultra-sonic processor (VCX-130, SONICS, USA) at 25% amplitude for 3 min. The treatment was conducted in an ice chamber to prevent the heat elevation during ultra-sonication. Then, ultra-sonicated SF solution was filtered and incubated at 60 °C for 2 days.