Evidence of a Cholesteric Liquid Crystalline Phase in Natural Silk Spinning Processes

A look inside the silk spinning process along the length of a silk gland has been achieved by the cryogenic quenching and subsequent microtoming of live silk-spinning animals, Nephila clavipes (spider) and Bombyx mori (silkworm). Observations made using transmission electron microscopy, electron diffraction and atomic force microscopy indicate a cholesteric liquid crystalline phase of aqueous silk fibroin in the early duct portion of the major silk-producing gland in both species. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide evidence for the cholesteric intermediate phase. The fracture surface produced by the diamond microtoming knife follows the twist of the director field, yielding thin sections with an undulating surface topography which produces a characteristic banding, on the order of 200−600 nm, in TEM and AFM images. Electron diffraction results also support the picture of the aqueous silk existing as a cholesteric at an intermediate stage in the spinning process.