The Rheology behind Stress-Induced Solidification in Native Silk Feedstocks.

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.