Mineralized tissues

Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusc shells, deep sea sponge Euplectella species, radiolarians, diatoms, antler bone, tendon, cartilage, tooth enamel and dentin are some examples of mineralized tissues.

These tissues have been finely tuned to enhance their mechanical capabilities over millions of years of evolution. Thus, mineralized tissues have been the subject of many studies since there is a lot to learn from nature as seen from the growing field of biomimetics. The remarkable structural organization and engineering properties makes these tissues desirable candidates for duplication by artificial means. Mineralized tissues inspire miniaturization, adaptability and multifunctionality. While natural materials are made up of a limited number of components, a larger variety of material chemistries can be used to simulate the same properties in engineering applications. However, the success of biomimetics lies in fully grasping the performance and mechanics of these biological hard tissues before swapping the natural components with artificial materials for engineering design.

Mineralized tissues combine stiffness, low weight, strength and toughness due to the presence of minerals (the inorganic part) in soft protein networks and tissues (the organic part). There are approximately 60 different minerals generated through biological processes, but the most common ones are calcium carbonate found in mollusk shells and hydroxyapatite present in teeth and bones. Although one might think that the mineral content of these tissues can make them fragile, studies have shown that mineralized tissues are 1,000 to 10,000 times tougher than the minerals they contain. The secret to this underlying strength is in the organized layering of the tissue. Due to this layering, loads and stresses are transferred throughout several length-scales, from macro to micro to nano, which results in the dissipation of energy within the arrangement. These scales or hierarchical structures are therefore able to distribute damage and resist cracking. Two types of biological tissues have been the target of extensive investigation, namely nacre from mollusk shells and bone, which are both high performance natural composites. Many mechanical and imaging techniques such as nanoindentation and atomic force microscopy are used to characterize these tissues. Although the degree of efficiency of biological hard tissues are yet unmatched by any man-made ceramic composites, some promising new techniques to synthesize them are currently under development. Not all mineralized tissues develop through normal physiologic processes and are beneficial to the organism. For example, kidney stones contain mineralized tissues that are developed through pathologic processes. Hence, biomineralization is an important process to understand how these diseases occur.

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