Fig. 1: Hierarchical structure of different natural protein-based fibers.

a B. mori fibers contain a core of nanofibrils with β-sheet structure forming crystallites responsible for the mechanical strength of the fiber. In between are amorphous parts responsible for fiber elasticity. The nanofibrils are covered by layers consisting of silk fibroins, and sericin. b On the macroscopic level, spider silk fibers also possess a skin-core structure, with a core formed by nanofibrils. The fibrils contain often β-sheet crystals aligned with the fiber axis responsible for the strength of the fiber. The amorphous phase is formed by GPGXX and GGX motifs being necessary for the elasticity of the fibers. c Keratin fibers show a hierarchical structure consisting of a cortex and a cuticle. Within the cuticle there are spindle-shaped macrofibrils based on two main structures, the protofibrils and the matrix. The protofibrils are formed by a mostly α-helical protein with low cysteine content in case of α-keratin or a β-sheet-rich structure with high cysteine content in case of β-keratin. d Each collagen molecule is made of three α-chains forming the ~300 nm long triple helical collagen molecule. Upon lateral and longitudinal assembly fibrils are achieved, which assemble into the final collagen fiber. e Elastin fibers are part of the extracellular matrix (ECM) and responsible for the elasticity of tissues. Elastin fibers are composed of microfibrils consisting of tropoelastin. The fibers are formed by enzymatically driven cross-linking of the microfibrils.