Tuning the microstructure and mechanical properties of lyophilized silk scaffolds by pre-freezing treatment of silk hydrogel and silk solution

This work aims to understand how pre-freezing treatments (-20 °C, -80 °C or -196 °C (liquid nitrogen)) affect the microstructure, mechanical properties and secondary structure of silk scaffolds prepared from lyophilization of silk hydrogels and silk solutions. It is found that in comparison with silk solutions, silk hydrogels at the same silk fibroin concentrations produce scaffolds with more nanofibrous structures when they are pre-frozen at the different temperatures. Although pre-freezing with liquid nitrogen can produce nanofibrous scaffolds from either a silk solution (low concentration of 2%) or silk hydrogel (produced from 2 to 6% silk fibroin solutions), aligned macro-channels can be produced only from silk hydrogels. In addition, scaffolds obtained from silk hydrogels are dominated by β-sheets due to the crystallization process for gel network formation, while scaffolds prepared from silk solutions are largely amorphous. The findings of this work are important to tune the microstructure and mechanical properties of silk scaffolds.