Toughening and strengthening mechanisms of biological materials: A review

Biological materials have evolved elegant hierarchical structures composed of various chemical components, imparting them with comprehensive mechanical and physical properties that enable their highly efficient biological functions. Many biological composites (e.g., bones, skins, hoofs, and horns of animals; the exoskeletons of mollusks; the silks of spiders and silkworms; and the beaks of birds) can achieve superior elastic stiffness, strength, and fracture toughness. These properties are crucial for their biomechanical performance in various activities such as locomotion, protection, combat, adhesion, and predation. In this paper, we review the toughening, strengthening, and stiffening mechanisms of biological materials and some related theoretical models. We focus on uncovering how these materials achieve an exceptional combination of high stiffness, toughness, and strength. The relations among the mechanical properties, biological functions, geometric structures, and chemical compositions of biological materials are analyzed through representative examples, including horns, gecko feet, nacres, spider silks, and tendrils. We particularly examine the effects of microstructural sizes, interfaces, structural hierarchy and chirality, and functional gradients. We also provide perspectives on the mechanics of biological materials from the viewpoints of theoretical modeling, experimental characterization, numerical simulations, and biomimetic applications.