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Mechanism of silk processing in insects and spiders

Abstract

Silk spinning by insects and spiders leads to the formation of fibres that exhibit high strength and toughness1. The lack of understanding of the protein processing in silk glands has prevented the recapitulation of these properties in vitro from reconstituted or genetically engineered silks. Here we report the identification of emulsion formation and micellar structures from aqueous solutions of reconstituted silkworm silk fibroin as a first step in the process to control water and protein–protein interactions. The sizes (100–200 nm diameter) of these structures could be predicted from hydrophobicity plots of silk protein primary sequence2. These micelles subsequently aggregated into larger ‘globules’ and gel‐like states as the concentration of silk fibroin increased, while maintaining solubility owing to the hydrophilic regions of the protein interspersed among the larger hydrophobic regions. Upon physical shearing or stretching structural transitions, increased birefringence and morphological alignment were demonstrated, indicating that this process mimics the behaviour of similar native silk proteins in vivo. Final morphological features of these silk materials are similar to those observed in native silkworm fibres.

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Figure 1: Optical polarizing microscope image of silk/PEO blend film cast from aqueous solution at ambient conditions.
Figure 2: Model of chain folding, micelle formation, globule formation and curing, and shear processing of silk proteins.
Figure 3: Scanning electron micrographs of silk/PEO (80/20 wt%) blend films after PEO extraction in water at room temperature for 24 h.
Figure 4: Fractured fibres.

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Acknowledgements

We thank R. Valluzzi and J. Park for technical input. This work was supported by the NIH, the NSF and the DoD (Air Force).

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Correspondence to David L. Kaplan.

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Jin, HJ., Kaplan, D. Mechanism of silk processing in insects and spiders. Nature 424, 1057–1061 (2003). https://doi.org/10.1038/nature01809

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