ABSTRACT
During evolution, silk has evolved multiple times (Sutherland et al. 2010). Since all known silks are heterogeneous in both molecular structure and biological function, it is diffi cult to postulate a distinct defi nition of silk: Silk belongs to the family of structural extracorporeal proteins with highly repetitive sequences (Craig 1997), which are processed starting with a concentrated silk solution. This defi nition excludes, e.g., hair which is fabricated out of low concentrations of keratin. Further, silk glands are required to store the concentrated silk dope. In particular, silks are fi bers, which distinguish them from other secreted structural proteins such as glue produced by marine organisms (Waite 2002). On a molecular level, a specifi c characteristic of silk is the semicrystalline structure, based on a distinct amino acid sequence (Craig 1997). The crystallites contribute to the unique mechanical properties of silk (Porter and Vollrath 2009). Throughout all known silk types, the fraction of crystalline structures varies from low to high amounts (Walker et al. 2013). Finally, a spinning process is associated with silk, which describes a highly controlled phase transition of the liquid silk dope towards a solid fi ber induced by shear forces (Porter and Vollrath 2009, Greving et al. 2012). In most cases, these
Lehrstuhl Biomaterialien, University of Bayreuth, 95440 Bayreuth, Germany. a Email: martin.neuenfeldt@bm.uni-bayreuth.de b Email: thomas.scheibel@bm.uni-bayreuth.de
shear forces are applied within the organism’s body, and the proteins are assembled in form of a fi ber. However, the spinning process can also take place after the excretion of the silk dope (Weisman et al. 2009, Ashton et al. 2012); therefore, the assembly trigger can be generalized as rheological stress.
