Spinning Beta Silks Requires Both pH Activation and Extensional Stress

Andreas Koeppel,Chris Holland,Nicola Stehling,Cornelia Rodenburg

doi:10.1002/adfm.202103295

Abstract

AbstractSynthetic silk production has undergone significant technological and commercial advances over the past 5 years, with fibers from most labs and companies now regularly matching the properties of natural silk by one metric or another. Yet the fundamental links between silk protein processing and performance remain largely unresolved and fiber optimization is commonly achieved through non‐natural methods. In an effort to address this challenge, data that closes this loop of processing and performance is presented by spinning a native silk feedstock ex vivo into a near‐native fiber using just two naturally occurring parameters; pH activation and extensional flow (i.e., spinning rate). This allows us to link previous experimental and modelling hypothesis surrounding silk's pH responsiveness directly to multiscale hierarchical structure development during spinning. Finally, fibers that match, and then exceed, natural silk's mechanical properties are spun and understood by rate of work input. This approach not only provides energetic insights into natural silk spinning and controlled protein denaturation, but is believed will help interpret and improve synthetic silk processing. Ultimately, it is hoped that these results will contribute towards novel bioinspired energy‐efficient processing strategies that are driven by work input optimization and where excellent mechanical properties are self‐emergent.

Highlights

When using the natural system as a biological blueprint, it is both necessary and important to look at the process variables involved in natural fiber formation
Upon exposing silk proteins to acidic vapor during extensional flow, the feedstock viscosity increases (Figure 1a), supporting previous observations in shear experiments.[27]. This increase in viscosity is dependent on the concentration of the acid and was not observed in the absence of acidic vapor, under basic vapor exposure, or if the sample was exposed to the acidic vapor for too long (>15 s, Figure 1a and inset Figure S1, Supporting Information), as this resulted in complete gelation of the proteins
Whilst pH activation appears necessary, to test sufficiency, that is, that structural features in silk can be formed solely by lowering the pH, we exposed a thin hand-drawn protein filament laid upon an Fourier transform infrared (FTIR)-attenuated total reflection (ATR) crystal to acidic vapor without stretching (Figure 1f, inset image)

Summary

Results and Discussion

Upon exposing silk proteins (obtained from fifth instar Bombyx mori silkworms) to acidic vapor during extensional flow, the feedstock viscosity (here indicated by the extensional stress) increases (Figure 1a), supporting previous observations in shear experiments.[27]. A subsequent extensional flow field, in excess of that which can be dissipated through relaxation of the now-gelled protein network, provides the mechanical stress input necessary to drive protein denaturation, leading to the formation of intra- and intermolecular β-sheets and a molecularly oriented, solid fiber.[26,27] Building upon these observations and theory, using just pH activation and extension, we were able to spin silk fibers ex vivo without a specialized spinning device, chemical fixation, or post draw.[4] To replicate the conditions within the silk gland ex vivo we adjusted the diameter of the silk filament to around 50–100 μm prior to vapor exposure. The fibers show a uniform fracture surface with a crack deflection at crystalline regions combined with plastic deformation in the amorphous regions Combined this indicates that the mechanical stress applied during processing may be sufficient to align the proteins parallel to the flow direction, but insufficient to generate complete protein denaturation and ordered β-sheet conversion. This we explain by an increase in the amount of molecular order and multiscale alignment which increases strength, but at the cost of extensibility as the amount of amorphous disordered regions responsible are reduced (Figure 3).[47]

Conclusion

Experimental Section

Data Availability Statement