Biomimicking the structure of silk fibers via cellulose nanocrystal as beta-sheet crystallite

Inspired by the ordered beta-sheet crystalline structure and molecular orientation of natural silkworm silk, biomimetic silk fibers with refined crystalline structure were creatively produced through incorporating cellulose nanocrystals (CNCs) into silk fibroin (SF) matrix to mimic the beta-sheet crystallites in natural silk via wet-spinning assembly technology. The influence of CNCs and post-draw on the structural characteristics, and thermal and mechanical properties of regenerated cellulose nanocrystal/silk fibroin (CNC/SF) fibers were comparatively studied with those of degummed silk. The resultant CNC/SF fibers exhibited a uniform, circular shape as well as dense morphology. The CNCs were uniformly dispersed into SF matrix and aligned along the fiber axis, which were beneficial to the formation of more ordered structure through intermolecular hydrogen bonding interactions. The crystallinity and overall molecular orientation of CNC/SF fibers were increased with increases of draw ratio and CNC contents, although lower than those of degummed silk. DSC and TGA analysis revealed that thermal decomposition behavior of CNC/SF fibers presented different features depending on the CNC contents, whereas maximum decomposition temperature of CNC/SF fibers improved sharply at first and then decreased slightly with the increase of CNC contents. Moreover, the tan delta peak temperatures of SF were increased with the addition of CNC, indicating restrained SF molecular mobility in the vicinity of the CNC surface. These results indicated that the CNC could substantially enhance the mechanical properties of SF and this enhancement could be attributed to the strong hydrogen bonding interactions as well as CNC-induced crystallization and orientation. Meanwhile, Young's modulus, tensile strength and breaking strain of the CNC/SF fibers with 5 wt% CNC were significantly increased to 28.8 +/- 2.6 GPa, 728.5 +/- 36.4 MPa and 23.1 +/- 1.6% respectively, higher than those of degummed silk. Thus it was demonstrated that biomimicking the natural silk crystalline structure using CNC as beta-sheet crystallite is a promising strategy for production of artificial silk fiber with improved strength without compromising the toughness.