Many biological tissues, such as wood and bone, are fiber composites with a hierarchical structure. Their exceptional mechanical properties are believed to be due to a functional adaptation of the structure at all levels of hierarchy. This article reviews the basic principles involved in designing hierarchical biological materials, such as cellular and composite architectures, adapative growth and as well as remodeling. Some examples that are found to utilize these strategies include wood, bone, tendon, and glass sponges – all of which are discussed.

Natural materials such as bone, tooth, and nacre are nanocomposites of proteins and minerals with superior strength. Why is the nanometer scale so important to such materials? Can we learn from this to produce superior nanomaterials in the laboratory? These questions motivate the present study where we show that the nanocomposites in nature exhibit a generic mechanical structure in which the nanometer size of mineral particles is selected to ensure optimum strength and maximum tolerance of flaws (robustness). We further show that the widely used engineering concept of stress concentration at flaws is no longer valid for nanomaterial design.

Structural materials in nature exhibit remarkable designs with building blocks, often hierarchically arranged from the nanometer to the macroscopic length scales. We report on the structural properties of biosilica observed in the hexactinellid sponge Euplectella sp. Consolidated, nanometer-scaled silica spheres are arranged in well-defined microscopic concentric rings glued together by organic matrix to form laminated spicules. The assembly of these spicules into bundles, effected by the laminated silica-based cement, results in the formation of a macroscopic cylindrical square-lattice cagelike structure reinforced by diagonal ridges. The ensuing design overcomes the brittleness of its constituent material, glass, and shows outstanding mechanical rigidity and stability. The mechanical benefits of each of seven identified hierarchical levels and their comparison with common mechanical engineering strategies are discussed.

Bone is a hierarchically structured material with remarkable mechanical performance which may serve as a model for the development of biomimetic materials. Understanding its properties is essential for the assessment of diseases such as osteoporosis. This will lead to a critical evaluation of current therapies and aid in their more targeted development. While the full hierarchical structure of bone is extremely complex and variable, its basic building block, the mineralized collagen fibril, is rather universal. Due to the progress in experimental methods to characterize materials at the nanoscale, new insights have been gained into the structure/mechanical function relation in this nanocomposite. The amount of mineral is usually thought to determine the stiffness of the material, but recent results suggest that the properties of the organic matrix as well as the geometrical arrangement of the two components might have a much larger influence on the properties than traditionally assumed. Some recent results from experiment and numerical modeling leading to these ideas are reviewed.

Mussel Fibers: While it is possible to make strong fibers or threads from organic materials, most suffer from high wear abrasion. Marine mussels attach themselves to rocky seashores using a series of byssal threads. Despite the constant rubbing caused by the motion of the tides, the threads show high wear resistance. Harrington et al. (p. 216 , published online 4 March; see the Perspective by Messersmith ) now find that the threads are protected by a proteinaceous outer cuticle that is rich in the amino acid 3,4-dihydroxyphenylalanine (dopa), which is known to be a strong adhesive. The cuticle is also rich in metal ions, primarily Fe ³⁺ . The dopa-metal crosslinks helped to form the tough outer coating.