Investigation of the nanofibrils of silk fibers

Abstract

Silk from 3-molted, 4-molted and sex-limited Bombyx mori, Antheraea pernyi and Antheraea yamamai have been investigated using Atomic Force Microscopy and Low Voltage High Resolution Scanning Electron Microscopy. Nanofibrils, bundles of nanofibrils, helical features and a layered structure with a cross angle between nanofibrils in different layers were observed for all silks. Similar log-normal distributions of fibril widths were obtained from all silks with the geometric mean fibril widths covering the range of 90–170nm. There is no correlation of the fibril width with the fiber size from different types of silkworm. Similar normal distributions of cross angles were observed in 30% of the images with the arithmetic means covering the range of 30–50°. There is an apparent correlation of cross angle with fiber size. The larger fibers for which the crystals are primarily alanine exhibit a greater average cross angle, 46°, than do those of the smaller fibers for which the crystals are primarily alanine and glycine, 36°. Initial wide angle X-ray diffraction measurements on a single fiber of A. pernyi are consistent with the fraction of fibrils at different cross angles being 28%. A simplified computer calculation method and molecular modeling were used in a search for packing angles which minimize the interaction energy of packing between beta sheets. Initial results revealed no definitive evidence of epitaxy to account for the cross angles. It is proposed that a multifibrillar fiber offers a number of mechanical advantages over a solid fiber with the same cross sectional area.