Fig. 1

Schematic depiction of the putative spider silk formation process in vivo and two theories explaining the experimental observations, namely, the liquid crystal theory [14] and the micelle theory [16]. Spidroins consisting of a long repetitive core region flanked by nonrepetitive N- and C-terminal domains are excreted into the spider’s gland tail, wherein the N-terminal domains are in a monomeric state, while the C-terminal domains adopt a dimeric structure. In the ampulla, according to the liquid crystal theory, spidroin molecules form a nematic liquid crystalline phase under storage conditions. During fiber formation along the spinning duct, the spidroin molecules experience ion exchange, a gradual pH drop, water removal, and elongational flow, resulting in a specific assembly process of the spidroins from a cellular optical texture to β-sheet-rich nanofibrils. In contrast, the micelle theory claims that the dimeric spidroin molecules assemble into micellar constructs due to hydrophilic/hydrophobic interactions under storage conditions in the ampulla. These micellar structures coalesce into larger condensates that are elongated and compressed as they pass through the spinning duct. In combination with the pH decrease, ion exchange, and water removal, molecular interactions lead to the formation of β-sheet-rich nanofibrils. In both theories, the last step involves the transfer of the liquid-like nanofibrils to a solid-state by drawing the silk thread from the exit spigot (i.e., applying shear forces)