Flow analysis of aqueous solution of silk fibroin in the spinneret of Bombyx mori silkworm by combination of viscosity measurement and finite element method calculation

Bombyx mori silk fibers possess outstanding mechanical properties in spite of being spun at room temperature and from the aqueous solution. Therefore, the mechanism of the structural transition has been studied with great attention, but still not be well understood. In this study the flow simulation of the silk fibroin aqueous solution using finite element method was performed on the basis of both the relationship between the viscosity and shear rate of the silk fibroin solution prepared from the silk gland, and the detailed structure of the spinneret including silk press part of the silkworm obtained from the optical micrographs. The viscosity of the silk fibroin solution decreased with power-law till the shear rate, about 1.5 s−1 with increasing shear rate. Then the viscosity increased reversely which is speculated due to the fiber formation as a result of aggregation of the molecules. In the flow simulation analysis, the initiation site of the fiber formation was calculated by regulating the extrusion pressure. The fiber formation occurs in 550 μm from the spigot at 1 MPa and in 600 μm from the spigot at 50 MPa. The extrusion pressure in the range from 1 MPa to 50 MPa induces the fiber formation in the stiff plates (550-600 μm from the spigot), that is, the silk press part in the spinneret.