Insights into detailed hierarchical puncture-resistance for Bombyx mori silkworm cocoons

Silkworm cocoons illustrate excellent puncture-resistance performance after an insight into their layers while a clear understanding of the correlation between the excellent puncture property and the silk secondary structure is still lacking. Herein, we peeled silkworm cocoon into eight layers, and a combination of examination techniques including scanning electron microscopy, tensile mechanical test, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction were applied to figure out the morphologies (surface and cross-section view), mechanical properties, secondary structure, the content of β-sheet, and crystallinity of each layer's fibroin after degumming. The results indicated that the fifth layer offers a higher level of puncture-resistance than the other layers except for the eighth layer. Additionally, a high content level of β-sheet structure and high crystallinity gives rise to the high puncture strength as for hierarchical silk fibers. In general, the new finding holds great potential inspiration for the design of puncture-resistant composites.