Conformational Changes of Water Stable Silk (Bombyx mori) Films by Mechanical Treatment

초록

Elastic and water stable silk-protein films were formed and physically stretched to control structure. These water stable films were formed from regenerated concentrated aqueous silk fibroin solutions (>8%) without crystallization or formation of silk II (beta-sheet) structure. The films were cast and annealed in water vapor at room temperature, resulting in the formation of hydrogels that were highly elastic based on swelling in water. The swelled films could be stretched up to 3 times original length. Conformational changes in the films were assessed by FTIR-ATR and XRD. Cast films were mostly amorphous with some silk I structure. After swelling in water the films were predominately silk I in structure. With methanol treatment and stretching of the films the silk II structure appeared. Stretched films had increased tensile strength that correlated with increased crystallinity due to transitions to silk II (-sheets). The water-stability of the silk films in the absence of silk II was due to the content of silk I structure formed during processing the silk fibroin aqueous solution to high concentration, combined with the slow evaporation of water. Mechanical properties and topographical images according to structural changes of fibroins on the films surface are under study. In conclusion, with the ability to concentrate fibroin in water to high concentrations, mimicking the native process in silkworm and spider glands, water stable and elastic silk fibroin films can be formed even in the absence of beta-sheet formation.