Figure 1|. Hierarchical structure of silk, cellulose and chitin.

Silk, wood and exoskeletons of arthropods are formed following the same multiscale construction principle. The smallest units of silk fibrils are highly repetitive core sequences of amino acids with hydrophilic and hydrophobic segments (repetitive A–B domain), which are flanked by terminal domains (N-terminal and C-terminal)¹⁴. The hydrophilic domains assemble into random coils and/or helical structures, and the hydrophobic domains transition into β-sheet structures. Cellulose consists of a linear chain of β-(1,4)-linked d-glucose units^4,5, and chitin is made of a long-chain polymer of (1,4)-β-N-acetylglucosamine¹⁵⁵. The nanocrystal size of the respective crystalline units is confined to several nanometres. The nanocrystals assemble into fibrils with an orientation along the longitudinal axis, which can be further organized into 2D laminar and 3D helicoidal architectures at the micrometre scale in silk fibres, cell wall layers and exoskeletons. S, secondary wall layer. The wall layers are reproduced from REF. 156, Macmillan Publishers Limited. The helicoid structure is adapted with permission from REF. 63, Elsevier.