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Hierarchical materials’ structural elements themselves have sub-structure. Such hierarchy is characteristics with multiscality, which means the materials system is formed with multiple phases in different scales. The essence of hierarchy makes it possible for materials scientists and engineers to adjust specific property within different scale-level. , as a result, eliminate the conflict effect between certain properties such as strength verse toughness. Although as a rising materials design strategy in materials design, some concerns remain as structural hierarchy often brings directionality into a system to render it anisotropic

Motivation
The research on hierarchy in materials systems spring up as researchers’ interest in nano-scale and biological materials rising as well as research on fabricating functional multilevel structure materials becoming popular. People found that some difficulty can be solved by imitating the exquisite masterpiece of nature in the aspect of microstructure and composite materials design. The hierarchical structure is one of the nature-inspired ideas that then applied to materials design.

Woods
Wood is composed of amorphous matrix of hemicellulose and so on, embedded with cellulose fibrils. The cellulose ﬁbrils formed by high-modulus and high-strength, crystalline polysaccharide bears most of the loading before deformation occurs. The matrix part suffers almost all the deformation. The chirality of cellulose fibrils and the angle also play important on its’ mechanical properties.

Bones
Bones is hierarchically consisting of nano-plate-shape-particles of hydroxyapatite embedded within a matrix of collagen, non-collagenous proteins, water and so on. Collagens contributes to toughness and mineral offers stiffness, and the hierarchical structure combine these two generally-conflict properties without impacting each other.

Byssal threads
Produced by certain mussels, byssal threads are made of strong fibrous proteins and relatively soft matrix, and possess a good combination of high strength and excellent extensible. Study shows that such good mechanical properties pack is due to the hierarchical structure. Within different levels of hierarchy, two kinds of protein with different density of cross-linking are found, and the two different phases contribute to strength and extensibility respectively.

Hierarchical structure Steel
One of the materials that the hierarchy idea introduced to the design is steel, and plenty of methods are explored and study. Inspired by the observation of byssal threads of specific mussels, a group design multi-hierarchical steel with multi-scale sandwich structure, the nano-scale dual-phase lamellar structure of martensite and austenite. , for which combine the advance of classical gradient steel, dual-phase steel, and TRIP steel. Another steel research design a hierarchical structure of γ matrix, γ’ precipitate, and finer γ precipitates embedded in γ’ precipitate.