Figure 3|. Mechanical and optical properties of nanofibrils.

a | The scanning electron microscopy (SEM) image shows the cross-sectional morphology of a wood cell wall, which is composed of three layers (S₁, S₂ and S₃). The stress–strain curves illustrate the correlation between the microfibril angle (MFA) and the stress–strain behaviour of different wood tissues. The relationship between the MFA and the tensile stiffness is shown for single wood fibres isolated by different methods. The same trend of decreasing tensile moduli with increasing MFA of the wood fibres is seen under both wet and dry conditions, despite these fibres being isolated by different techniques. b | Structural features of the dactyl club of the peacock mantis shrimp Odontodactylus scyllarus (O. scyllarus). The cross section of the club shows the impact region (blue), the medial periodic region (red), the lateral periodic region (yellow) and the striated region (green). The impact region is formed by mineralized chitin nanofibrils arranged in a herringbone pattern. The periodic region is formed by partially mineralized chitin nanofibrils assembled in a helicoidal structure, as illustrated in the schematics and SEM images. c | Photographs and SEM images of Pollia condensata (P. condensata) and the scarab Chrysina aurigans (C. aurigans) illustrate structural colour enabled through the helicoidal arrangement of biopolymer nanofibrils. A left-handed helicoid reflects left-handed circularly polarized light (blue), in which the wavevector k depends on the pitch p, which is the distance between two planes with the same fibril orientation. The dashed lines indicate the typical arch pattern observed in anatomical cross sections. Right-hand circularly polarized light (red) is transmitted through the left-handed helicoidal structure. E, electric field vector; F, fibre; ML, middle lamella; PCW, primary cell wall; V, vessel. Part a is reproduced with permission from REF 49, Elsevier; adapted with permission from REF. 50, Elsevier; and reproduced from REFS 52,53, Macmillan Publishers Limited. Part b is reproduced with permission from REFS 65,68, AAAS and John Wiley and Sons, respectively. Part c is reproduced with permission from REFS 72,75, IOP Publishing and Proceedings of the National Academy of Sciences, respectively; and adpated from REF. 76, Macmillan Publishers Limited.