In this paper, we report a novel technique which enabled us to easily and directly obtain a silk fibroin (SF) solution without the use of organic solvents by applying a simple grinder for silk cocoons. In addition, we successfully prepared cellulose nanofiber/silk fibroin (CNF/SF) nanofiber composite films using a filtration and pressing method from a slurry solution blended with a CNF solution. The peak temperature of the thermal decomposition of the SF nanofibers prepared using the new method developed in this research was improved by more than 13 °C, and that of the composite films compounded with more than CNF 50 wt% showed almost the same temperature as the CNF film. In addition, the mechanical properties of the CNF/SF nanofiber composite films were increased with increasing the amount of CNF. These results strongly suggest that the SF solution prepared using our novel method has advantages of superior thermal and mechanical properties compared with that prepared by a conventional method using a concentrated neutral salt solution. Additionally, this result indicates the film manufacturing method having advantages of water pollution reduction and energy saving.

Introduction

Fibroin is a type of fibrous protein, occupying 70% of the cocoon yarns of insects and spiders, and is a major component in the silk thread of silkworms. In particular, silk fibroin (SF) obtained from domesticated silkworms has been considered in numerous studies, focusing on its detailed molecular structure and higher-order structural changes, its use in artificial skin and culture substrates, its biocompatibility, and its application in supplements, among other areas (Bhardwaj and Kundu, 2011; Chen et al., 2014; Jin et al., 2013; Lee et al., 2018; Mehrabani et al., 2018; Zhou et al., 2017). SF forms a strong hydrogen bond between molecular chains, takes an antiparallel b-sheet type crystal structure, and is insoluble in water (Lu et al., 2010). Therefore, to produce films and nanofibers, it is necessary to prepare an aqueous solution by dissolving the SF in an organic solution and then dialyzing it. In a traditional dissolution method of silk fibroin, the material is dissolved in a concentrated neutral salt solution; the solution is then placed in a cellulose tube and dialyzed with pure water for about 1 week to remove any existing ions (Du et al., 2009; Yamada et al., 2003; Zhu et al., 2012). The higher-order structure of SF in an aqueous solution obtained through this method is of a random coil type (Yamada et al., 2003). However, this sample preparation procedure is very complicated and lengthy, and furthermore it necessitates the use of an organic solvent. From the viewpoint of environmental compatibility and economic efficiency, preparation of bio-based materials in a short time without using organic solvents is strongly desired.