Fine structural analysis of non-thermal degummed fibroin fibers reveals its superior mechanical capabilities

Bombyx mori silk fibroin fibers constitute a class of protein building blocks capable of func-tionalization and reprocessing into various material formats. The properties of these fibers are typically affected by the intense thermal treatments needed to remove the sericin gum coating layer. Additionally, their mechanical characteristics are often misinterpreted by assuming the asymmetrical cross-sectional area is a perfect circle. The thermal treatments impact not only the mechanics of the degummed fibroin fibers but also the structural configuration of the re-solubilized protein, thereby limiting the performance of the resulting silk-based materials. To overcome these limitations, we have developed a non-thermal surface treatment that removes the sericin gum layer while preserving the molecular structure of the fibroin protein, and con-sequently, maintaining the hierarchical integrity of the exposed fibroin microfiber. The precise determination of the initial cross-sectional area of the asymmetrical silk fibers led to a com-prehensive analysis of their mechanical properties. Our findings indicate that the newly de-veloped surface treatment raised the Young’s modulus and tensile strength by increasing the extent of the fibers’ crystallinity by approximately 40% and 50%, respectively, without com-promising their strain. Furthermore, we have shown that this treatment facilitated the produc-tion of high-purity soluble silk protein with rheological and self-assembly characteristics com-parable to those of native silk feedstock, initially stored in the animal’s silk gland. The devel-oped approaches benefits both the development of silk-based materials with tailored proper-ties and the proper mechanical characterization of asymmetrical fibrous biological materials made of natural building blocks.