Changes in Silk Feedstock Rheology during Cocoon Construction: The Role of Calcium and Potassium Ions

Variation in silk feedstocks is a barrier both to our understanding of natural spinning and biomimetic endeavors. To address this, compositional changes are investigated in feedstock specimens from the domesticated silkworm (Bombyx mori). It is found that the feedstock viscosity decreased systematically by over two orders of magnitude during cocoon construction. Potential factors such as protein concentration, molecular weight, pH, or the presence of trehalose are excluded, whereas a clear correlation appear between viscosity and the relative concentrations of Ca²⁺ and K⁺ ions. It is expected that Ca²⁺ ions would favor "salt bridges" between acidic (Asp and Glu) amino acids, leading to an increased viscosity, whereas K⁺ ions would compete for these sites, thereby reducing viscosity. Thus, these findings suggest a simple, systematic yet sophisticated control of feedstock viscosity in the silkworm, which in turn can be applied to future industrial silk production.