Abstract
Silk’s outstanding mechanical properties and energy efficient solidification mechanisms provide inspiration for biomaterial self-assembly as well as offering a diverse platform of materials suitable for many biotechnology applications. Experiments now reveal that the mulberry silkworm Bombyx mori secretes its silk in a practically “unspun” state that retains much of the solvent water and exhibits a surprisingly low degree of molecular order (β-sheet crystallinity) compared to the state found in a fully formed and matured fiber. These new observations challenge the general understanding of silk spinning and in particular the role of the spinning duct for structure development. Building on this discovery we report that silk spun in low humidity appears to arrest a molecular annealing process crucial for β-sheet formation. This, in turn, has significant positive implications, enabling the production of a high fidelity reconstituted silk fibroin with properties akin to the gold standard of unspun native silk.
Highlights
The natural silk spinning process entails a silk feedstock experiencing carefully controlled flow stress[1−3] and lowering of pH,[4,5] as well as changing concentrations of metallic ions and salts as it flows down the spinning duct.[5]
Understanding the reverse processing of silk back from solid fiber to liquid feedstock has significant implications for fibroin biomaterial preparation protocols as they typically overlook the process history of their input silk materials.[9−13] Given that reconstituted silk fibroin (RSF) feedstocks rely on the disruption of the solid silk structure, that is, the highly ordered hydrogen-bonded network of fibroin protein molecules, we hypothesized that silk with lower ordered β-sheet crystalinity content would allow for milder solubilization conditions and produce higher fidelity reconstituted silk feedstocks
We found that a silkworm will readily deposit a fiber on the base that contains the Attenuated Total Reflection (ATR)-IR sensor (Figure 1)
Summary
The natural silk spinning process entails a silk feedstock experiencing carefully controlled flow stress[1−3] and lowering of pH,[4,5] as well as changing concentrations of metallic ions and salts as it flows down the spinning duct.[5] far, the general perception is that once silk exits the animal, it is “spun” and its natural processing is more or less completed bar some postprocessing draw-down that further aligns the molecules and supramolecular structures.[3,6] there is evidence that further molecular self-assembly may continue even after the fiber has left the animal.[2,7,8] To examine the roles of in vivo and ex vivo processing, we examined Bombyx mori silk fibers immediately after secretion using infrared spectroscopy This technique allowed us to monitor the β-sheet and water content of silk fibers in controlled environments in order to infer the molecular processes that might underlie the transition of silk from feedstock to filament. RSF prepared from cocoons spun in a dry environment compared to native silk protein feedstocks taken straight from the gland are spectroscopically and rheologically surprisingly similar, as we shall discuss
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