The influence of thermal treatments on the secondary structure of silk fibroin scaffolds and their interaction with fibroblasts

Background Recently, silk fibroin-based biomaterials have received attention for application in tissue engineering and drug delivery systems. The usefulness of heat sterilization methods for silk fibroin-based biomaterials was investigated in this study as all biomaterials are required to undergo a sterilization process when they are used in medical devices. Methods The influence of wet and dry heating on the properties of fibroin molecules in silk fibroin sponges was investigated by measurements of solid-state 13C cross-polarization/magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analyses, strength tests, and cell proliferation/migration assays. Results 13C CP/MAS NMR spectra of wet-heated sponges revealed no changes in the molecular structure below 50 °C. However, above 60 °C, the crystalline structure of the silk proteins transitioned from silk I to silk II; the silk II:silk I ratio increased with temperature. In contrast, dry heating (below 190 °C for up to 180 min) induced no structural changes in the fibroin molecules. These results indicate that, although autoclave sterilization (121 °C for 20 min) induces structural changes in silk fibroin sponges, no such changes are observed with the dry-heat sterilization (180 °C for 30 min). Sterilized sponges with a silk I structure can be obtained using dry-heat method during sterilization. Moreover, the structural differences between the wet- and dry-heated silk fibroin sponges did not influence their interaction with fibroblasts. Discussion This study indicates that both autoclaving and dry heating are acceptable sterilization methods for silk fibroin-based sponges as the scaffold. In particular, dry heating maintains the stability of the secondary structure of the sterilized silk fibroin-based biomaterials.