Importance of radial line and circulus distributions to the protectoflexibility of scales in fish armors

Fish scales are dermal armors with an attractive combination of resistance to puncture and flexibility. Past work has attributed this unique quality to their composition and the hierarchical stratification of their microstructure. Here, we show that the ridge lines and circuli of scales contribute to the overall damage tolerance and play a key role in the “protectoflexibility” of fish scales. Through application of optical mechanics, we show that the ridge lines channel damage along paths that reduce the effective stress intensity available for bulk fracture of the scales. Furthermore, comparison of the ridge line distribution and circuli in elasmodine scales shows that they are unique across fish species and appear to be tuned for the combined functional requirements, i.e., resistance to threats and locomotion. The importance of these features in the multifunctional requirements of scales is elucidated in detail, including their role in inspiring new approaches for advancing multifunctional structural materials. [Display omitted] •Water improves the damage tolerance of elasmoid scales•Radii and circuli are morphological features that act to delocalize damage•Fibril bridges across radial grooves increase the scale integrity and flexibility•Segmentation of the limiting layer improves the “protectoflexibility” of scales Elasmoid fish scales provide a protective and flexible barrier to threats on the dermis of many modern teleost fish. Jiang et al. show that this “protectoflexibility” is primarily bestowed by the radial grooves and circuli of the mineralized surface layer and the supporting role of water to provide intermolecular lubrication.