The mechanical properties of spider silk are determined by the genetic regulation of fibroin proteins and chemical and physical processing during spinning

• Main Author: Guerette, Paul André
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6701

Orb-web building spiders produce a functional variety of fibrous silks that exhibit a range in mechanical properties from Kevlar-like super fibers to rubber-like elastomeric fibers. The mechanisms of silk property control employed by the orb-spider were formerly not known, but were hypothesised to be related to the chemical sequence design of their fibers constituent fibroin proteins and the chemical and physical conditions of fiber spinning. The genome of the orb-spider Araneus diadematus was found to contain members of at least one fibroin gene family which encode proteins containing different proportions of crystal forming poly(alanine) domains and glycine rich amorphous domains. The fibroin genes were found to be expressed differentially in the spiders seven different gland types, indicating that transciptional regulation affords the spider the ability to produce a variety of gland-specific, compositionally distinct silk secretions that are predicted to have different crystallization potentials. Qualitative polarized light microscopy supports this prediction by showing that distinct glandular secretions crystallize differentially under identical shear conditions in the lab. The results therefore indicate that genetic regulation of fibroin genes affords the orb-spider the potential to modulate the mechanical properties of its large repertoire of silks. Chemical micro-environment and physical draw processing conditions were also hypothesised to influence the mechanical properties of orb-spider silk. In the orb-web, the rubber-like native viscid (FL) silk is subject to a distinct chemical micro-environment provided by an inter-penetrating aqueous glue coating that contains water, salts, free amino acids and several water soluble Low Molecular Weight (LMW) compounds. The glue coating was removed by a water wash:dry process in the lab, and the processed FL fibers exhibited an increase in Birefringence and a change in mechanical properties indicative of the development increased supra-molecular order. The data indicate that a LMW plasticizer exists in the aqueous glue that functions either 1) by recruiting water molecules from the atmosphere that in turn plasticize the FL network , 2) by interacting directly with the network and/or 3) by creating free volume to allow for additional mobility of the FL fibroins. The effects of physical draw processing were evaluated by subjecting the native FL fiber to water wash:draw:dry processing. Birefringence increased asymptotically with increased processing draw ratio indicating that an increase in supra-molecular organisation develops with draw. However, the optical data alone fail to reveal weather this new order arises from alignment of amorphous and preexisting crystalline network components or from the creation of new crystal structure. A rudimentary comparison of Retardation values from draw processed FL fibers with published data from hydrated native FL silk at equivalent extensions supports the view that draw processing imparts new crystal structure. The mechanical properties of the native FL silk were also dramatically altered by physical processing, following the general trend of increased stiffness and ultimate strength, and decreased extensibility with increased processing draw ratio. It is suggested that the fibroins which comprise the native FL silk have the potential to strain crystallize and that this phenomenon may explain the exceptional breaking strength of the elastomeric viscid silk. Finally, the optical and mechanical data from processed FL fibers suggest that the orb-spider may employ specific processing regimes in the wild to tune the supra-molecular morphology and thus the mechanics of its variety of silks.