Rheology and Dynamic Light Scattering of Silk Fibroin Solution Extracted from the Middle Division ofBombyx moriSilkworm

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Abstract
Dynamic light scattering (DLS) and rheological measurements were performed on aqueous silk fibroin solutions extracted from the middle division of Bombyx mori silkworm over a wide range of polymer concentration C from 0.08 to 27.5 wt %. DLS results obtained in the dilute region of C less than 1 wt % are consistent with a model that an elementary unit is a large protein complex consisting of silk fibroin and P25 with a 6:1 molar ratio. Rheological measurements in the dilute C region reveal that those units (or clusters) with the hydrodynamic radius of about 100 nm form a network extending over the whole sample volume with small pseudoplateau modulus mainly by ionic bonding between COO- ions of the fibroin molecules and divalent metallic ions such as Ca2+ or Mg2+ ions present in the sample and also that, after a yield stress is reached, steady plastic flow is induced with viscosity much lower than the zero-shear viscosity estimated from creep and creep recovery measurements by 4−6 orders of magnitude. Angular frequency ω dependencies of the storage and the loss shear moduli, G‘(ω) and G‘ ‘(ω), measured in the linear viscoelastic region, indicate that all solutions possess the pseudoplateau modulus in the low ω region and samples become highly viscoleastic for C ≳ 4.2 wt %. Above C = 11.2 wt % another plateau appears at the high ω end accompanied by a distinct maximum of G‘ ‘ in the intermediate ω region. The relaxation motion with τ = 0.5 s corresponding to the maximum of G‘ ‘ is one of characteristic properties of the fibroin solutions in the high C region. Thermorheological behaviors of the solution with C = 27.5 wt % show that the network structure formed in the MM part of the silk gland is susceptible to temperature and a more stable homogeneous network is realized by raising the temperature up to T = 65 °C.