Preparing Bombyx mori silk nanofibers using a sustainable and scalable approach

Silk nanofibers have been produced and examined in recent years for a range of advanced biomedical and biotechnological applications. Their fabrication involves the canonical approach of dissolving native silk fibers in a highly concentrated (mostly lithium-ion-based) chaotropic salt, followed by spinning fibers from the solution. Such methods of production involve the use of harmful chemicals and expensive and slow desalting steps. There is an alternative but less explored route to produce silk nanofibers: top-down processing to exfoliate nanofibers directly from native silk fibers. This approach takes advantage of the assembled bundles of silk nanofibers present in native silk fibers. Here, we report the use of this approach to produce silk nanofibers for the first time at a significant scale. We have demonstrated that tens of grams per batch of nano-microfibrillated silk (NMFS) can be produced without the use of chemicals; this is in sharp contrast to other reported methods that typically involve a microgram scale and/or chemically intensive top-down processing. We have adapted a combination of milling and the homogenization approach to prepare NMFS containing approximately 80% fibers with a diameter of less than 300 nm. Such fibers (1% w/v) produced a stable gel-like continuous network in water. The average aspect ratio was more than 50.