Elasticity of Spider Dragline Silks Viewed as Nematics: Yielding Induced by Isotropic-Nematic Phase Transition

Spider dragline silk (SDS), the main structural web silk regarded as the “spider’s lifeline", exhibits a fascinating combination of high tensile strength and high extensibility (1). Its toughness is over 10 times that of Kevlar, the material for making bullet-proof suits (2). In addition, SDS has a unconventional sigmoidal shaped stress-strain curve and shape memory capability after the tension and torsion test (3; 4) (Figure 1). All these intriguing properties have aroused the broad interest of scientists to understand the underlying deformation mechanism of SDS. Leaving the open question of why SDS is that mechanically superb aside, people have already started to produce artificial SDS for numerous applications. Various methods to spin artificial SDS have been explored. These include conventional wet spinning of regenerated SDS obtained through forced silking (5; 6), solvent spinning of recombinant SDS protein analogue produced via bacteria and yeast cell cultures doped with chemically synthesized artificial genes (6; 7), and spinning of silk monofilaments from aqueous solution of recombinant SDS protein obtained by inserting the silk-producing genes into mammalian cells (6; 8). The applications also cover a broad biomedical range. For example, the silk-silica fusion proteins was used for bone regeneration by combining the self-assembling domains of SDS ( Nephila clavipes) as scaffolds and the silaffin-derived R5 peptide of Cylindrotheca fusiformis that is responsible for silica mineralization (6; 9). Improved cell adhesion and proliferation of mouse osteoblast (MC3T3-E1) cells was achieved with films composed of B. mori fibroin and recombinantly produced proteins based upon N. clavipes SDS (incorporating the RGD integrin recognition sequence) in vitro (6; 10). SDS based antibacteria materials was also demonstrated by incorporating silk proteins with inorganic antibacteria nanoparticles such as silver nanoparticles (11) and titanium dioxide nanoparticles (12). In addition, silk films are promising candidates for biocompatible coatings for biomedical implants. For instance, gold nanoparticles, silver nanoparticles, and transition metal oxides/sulfides were dip-coated by SDS proteins and showed novel electrical, magnetic, optical properties, and at the same time, biocompatibility (6; 13–16). Going back to the fundamental question of the structure-property relation of SDS, several experimental studies have been carried out to determine the supra-molecular structure of Elasticity of Spider Dragline Silks Viewed as Nematics: Yielding Induced by Isotropic-Nematic Phase Transition