The Rheology behind Stress-Induced Solidification in Native Silk Feedstocks

Abstract

The mechanism by which native silk feedstocks are converted to solid fibres in nature has attracted much interest. To address this question, the present work used rheology to investigate the gelation of Bombyx mori native silk feedstock. Exceeding a critical shear stress appeared to be more important than shear rate, during flow-induced initiation. Compositional changes (salts, pH etc.,) were not required, although their possible role in vivo is not excluded. Moreover, after successful initiation, gel strength continued to increase over a considerable time under effectively quiescent conditions, without requiring further application of the initial stimulus. Gelation by elevated temperature or freezing was also observed. Prior to gelation, literature suggests that silk protein adopts a random coil configuration, which argued against the conventional explanation of gelation, based on hydrophilic and hydrophobic interactions. Instead, a new hypothesis is presented, based on entropically-driven loss of hydration, which appears to explain the apparently diverse methods by which silk feedstocks can be gelled.

The rheological properties of native sericin

“…Our findings are consistent with previous studies of this strain, where fibroin deficient individuals produce a native feedstock that is composed of ~98.5% sericin [102] of molecular weights 24-400 kDa [37,[99][100][101]. Given we are primarily interested in the rheological properties of sericin, beyond confirmation of sericin presence, Light specimens also serve as a useful internal control, and their feedstocks are consistent with regular B. mori fibroin [17,42,89,[93][94][95]98]. Hereafter, when we discuss native sericin's properties, these relate to samples from the Dark specimens, whereas native fibroin's properties are gleaned from Light specimens and the wider literature.…”

“…Specimens were subjected to a period (100 s) of pre-shear (1 s -1 ) to improve sample homogeneity between the plates [42,[93][94][95]. Apparent viscosity measurements were made by averaging the final 30 s of this test.…”

Biophotonics of Native Silk Fibrils

“…We examined the aggregation of soluble NSF under various destabilizing conditions and found that the transformation of native silk from a disordered random coil to highly ordered‐rich β‐sheet‐rich structure is accompanied by the conformation appearance of a fluorescence signal a in the blue–green region of the visible spectrum, which features a long fluorescence lifetime ( Figure 1 ). Interestingly, the observed spectral and lifetime characteristics are very similar to those reported for amyloid fibrils, such as amyloid‐β, lysozyme, and α‐synuclein, suggesting that there is a common structural origin for the intrinsic fluorescence in NSF and in amyloid fibrils, independent on the presence of a chemical chromophore. Moreover, we show that aggregation of native silk produces a material with remarkably strong and stable fluorescence characteristics.…”

“…Native silk samples were extracted from fifth instar B. mori larvae during the early stages of cocoon construction as described previously . In brief, silk glands were removed and the epithelial cells covering the gland were peeled off under cold (≈5 °C) distilled water, using fine tweezers and a dissection microscope (Olympus SZ 40, Japan).…”

“…Native silk samples were extracted from fifth instar B. mori larvae during the early stages of cocoon construction as described previously. [30][31][32] In brief, silk glands were removed and the epithelial cells covering the gland were peeled off under cold (≈5 °C) distilled water, using fine tweezers and a dissection microscope (Olympus SZ 40, Japan). Both posterior sections of the middle division of the silk glands were then placed into a single 2 mL microcentrifuge tube and diluted in distilled water to create 1% W/W native liquid silk solution to be used for processing.…”