Mineralized tissues
[2] The remarkable structural organization and engineering properties makes these tissues desirable candidates for duplication by artificial means.
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.
[1][2] 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.
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.
[2] 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.
[10][11] 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.
It is expected that genetic analysis of agnathans will provide more insight into the evolution of mineralized tissues and clarify evidence from early fossil records.
[1][2] This waviness plays an important role in the fracture of nacre as it will progressively lock the tablets when they are pulled apart and induce hardening.
Lastly, on the nanoscale, the connecting organic material between the tablets as well as the grains from which they are made of is the final sixth hierarchical structure in nacre.
The first, at a scale of 100 μm to 1 mm, is inside the compact bone where cylindrical units called osteons and small struts can be distinguished.
[17] In bone, studies have shown that calcium phosphate nucleates within the hole area of the collagen fibrils and then grows in these zones until it occupies the maximum space.
Some of the regulatory proteins in mineralized tissues are osteonectin, osteopontin, osteocalcin, bone sialoprotein and dentin phosphophoryn.
[20] In nacre, the organic component is porous, which allows the formation of mineral bridges responsible for the growth and order of the nacreous tablets.
[17] The main structural elements involved in the mollusk shell formation process are: a hydrophobic silk gel, aspartic acid rich protein, and a chitin support.
The mineral nucleates, inside the hole area of the collagen fibrils, as thin layers of calcium phosphate, which then grow to occupy the maximum space available there.
Three possible suggestions are that nucleation is either due to the precipitation of calcium phosphate solution, caused by the removal of biological inhibitors or occurs because of the interaction of calcium-binding proteins.
[22] At the interface, a very large force (>6-5 nN) is needed to pull the protein molecules away from the aragonite mineral in nacre, despite the fact that the molecular interactions are non-bonded.
[7][23] A model has shown that during tension, the back stress that is induced during the plastic stretch of the material plays a big role in the hardening of the mineralized tissue.
Natural structural materials comprising hard and soft phases arranged in elegant hierarchical multiscale architectures, usually exhibit a combination of superior mechanical properties.
For instance, many natural mechanical materials (Bone, Nacre, Teeth, Silk, and Bamboo) are lightweight, strong, flexible, tough, fracture-resistant, and self-repair.
Such a common strategy was perfected by nature itself over millions of years of evolution, giving us the inspiration for building the next generation of structural materials.
[1][27] The large scale model of materials is based on the fact that crack deflection is an important toughening mechanism of nacre.
[1] All hard materials in animals are achieved by the biomineralization process - dedicated cells deposit minerals to a soft polymeric (protein) matrix to strengthen, harden and/or stiffen it.
Thus, biomimetic mineralization is an obvious and effective process for building synthetic materials with superior mechanical properties.
[29] Recently, a lot of bioinspired materials with elegant hierarchical motifs have been built with features ranging in size from tens of micrometers to one submicrometer.
Some examples of efforts in this direction include alternating layers of hard and soft components of TiN/Pt with an ion beam system.
Thus, sequential adsorption has been proposed to overcome this limitation and consists of repeatedly adsorbing electrolytes and rinsing the tablets, which results in multilayers.
[1] Thin film deposition focuses on reproducing the cross-lamellar microstructure of conch instead of mimicking the layered structure of nacre using micro-electro mechanical systems (MEMS).
Although the MEMS technology is expensive and more time-consuming, there is a high degree of control over the morphology and large numbers of specimens can be made.
Successful biomimicking of mollusk shells will depend will on gaining further knowledge of all these factors, especially the selection of influential materials in the performance of mineralized tissues.
