Cracking and wrinkling morphomechanics of animal skins

Through the long history of evolution, the skins of animals have developed different geometric patterns that confer multiple functions adapted to various environments. To achieve flexibility, which is critical for their predation and survival, the skins must undergo large deformations, with relatively lower energy dissipation and stress levels. To this end, rich surface patterns can be observed on the skins of different animals, for example, cracked fragments on crocodiles, surface wrinkles on dogs, and intricately patterned scales on fishes. In this paper, we investigate how the skin patterns of animals are determined by morphomechanics and reveal that, apart from wrinkling, cracking is another essential morphomechanical strategy. A core–shell model is established to reveal how the surface patterns of the skins are affected by the biological activities, body sizes, and skin curvatures of the animals. A non-dimensional parameter is defined to differentiate the skin patterns governed by surface wrinkling and fragmentation mechanisms. For thin and soft skins (e.g., humans, frogs, and dogs), surface wrinkling is easier to occur, while for thick and stiff skins (e.g., crocodiles and dinosaurs), they evolve into cracked fragments to avoid high stresses during larger deformation. The theoretical results are in good agreement with a wide range of animals. Furthermore, scaling laws are provided for the geometric features of the morphological patterns of cracking-regulated skins. This work not only helps uncover the secrets underlying the skin morphogenesis of animals, but also hold potential applications in paleontological reconstructions and designs of biomimetic soft robots.