Mineralized tissues
Encyclopedia
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 where mineralized tissues are found.
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.
or the dermal skeleton of early agnathans. The dermal skeleton is just surface dentin
and basal bone, which is sometimes overlaid by enameloid. It is thought that the dermal skeleton eventually became scales, which are homologous to teeth. Teeth were first seen in chondrichthyans and were made from all three components of the dermal skeleton, namely dentin, basal bone and enameloid. The mineralization mechanism of mammalian tissue was later elaborated in actinopterygians and sarcopterygians during bony fish evolution. 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.
. The latter is hard and thus prevents any penetration through the shell, but is subject to brittle failure. On the other hand, nacre is softer and can uphold inelastic deformations, which makes it tougher than the hard outer shell. The mineral found in nacre is aragonite
, CaCO3, and it occupies 95% vol. Interestingly, nacre is 3000 times tougher than aragonite and this has to do with the other component in nacre, the one that takes up 5% vol., which is the softer organic biopolymers. Furthermore, the nacreous layer also contains some strands of weaker material called growth lines that can deflect cracks.
grains detected by scanning electron microscopy from which the tablets themselves are made of together represent another structural level. The organic material “gluing” the tablets together is made of proteins and chitin
.
To summarize, on the macroscale, the shell, its two layers (nacre
and calcite
), and weaker strands inside nacre represent three hierarchical structures. On the microscale, the stacked tablet layers and the wavy interface between them are two other hierarchical structures. 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.
has a hierarchical structure that is also formed by the self assembly of smaller components. The mineral in bone (known as bone mineral
) is hydroxyapatite with a lot of carbonate ions, while the organic portion is made mostly of collagen
and some other proteins. Different sources report different numbers of hierarchical level in bone, which is a complex biological material. The types of mechanisms that operate at different structural length scales are yet to be properly defined. Five hierarchical structures of bone are presented below.
molecules, organic molecules such as lipids and proteins, and finally water. The hierarchical structure common to all mineralized tissues is the key to their mechanical performance.
, silica, calcium oxalate
, whitlockite
, and monosodium urate are examples of minerals found in biological tissues. In mollusc shells, these minerals are carried to the site of mineralization in vesicles within specialized cells. Although they are in an amorphous mineral phase while inside the vesicles
, the mineral destabilizes as it passes out of the cell and crystallizes. 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.
and break easily. Hence, the organic component of mineralized tissues increases their toughness
. Moreover, many proteins are regulators in the mineralization process. They act in the nucleation
or inhibition of hydroxyapatite formation. For example, the organic component in nacre is known to restrict the growth of aragonite. Some of the regulatory proteins in mineralized tissues are osteonectin
, osteopontin
, osteocalcin
, bone sialoprotein and dentin phosphophoryn
. In nacre, the organic component is porous, which allows the formation of mineral bridges responsible for the growth and order of the nacreous tablets.
support.
The silk gel is part of the protein portion and is mainly composed of glycine
and alanine
. It is not an ordered structure. The acidic proteins play a role in the configuration of the sheets. The chitin
is highly ordered and is the framework of the matrix. The main elements of the overall are:
, which then grow to occupy the maximum space available there. The mechanisms of mineral deposition within the organic portion of the bone are still under investigation. 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.
embryo has been used extensively in developmental biology studies. The larvae form a sophisticated endoskeleton
that is made of two spicules. Each of the spicules is a single crystal of mineral calcite
. The latter is a result of the transformation of amorphous CaCO3 to a more stable form. Therefore, there are two mineral phases in larval spicule formation.
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. Some studies perform a finite element model
analysis to investigate the behaviour of the interface. 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. As well, the nanoscale asperities that is on the tablet surfaces provide resistance to interlamellar sliding and so strengthen the material. A surface topology
study has shown that progressive tablet locking and hardening, which are needed for spreading large deformations over large volumes, occurred because of the waviness of the tablets.
processes. Some diseased areas that include the appearance of mineralized tissues include atherosclerotic plaques, tumoral calcinosis
, juvenile dermatomyositis
, kidney
and salivary stones
. All physiologic deposits contain the mineral hydroxyapatite or one analogous to it. Imaging techniques such as infrared spectroscopy
are used to provide information on the type of mineral phase and changes in mineral and matrix composition involved in the disease. Also, clastic cells are cells that cause mineralized tissue resorption
. If there is an unbalance of clastic cell, this will disrupt resorptive activity and cause diseases. One of the studies involving mineralized tissues in dentistry is on the mineral phase of dentin
in order to understand its alteration with aging. These alterations lead to “transparent” dentin, which is also called sclerotic. It was shown that a ‘‘dissolution and reprecipitation’’ mechanism reigns the formation of transparent dentin. The causes and cures of these conditions can possibly be found from further studies on the role of the mineralized tissues involved.
of nacre. This deflection happens because of the weak interfaces between the aragonite
tiles. Systems on the macroscopic
scales are used to imitate these week interfaces with layered composite ceramic tablets that are held together by weak interface “glue”. Hence, these large scale models can overcome the brittleness of ceramics. Since other mechanisms like tablet locking and damage spreading also play a role in the toughness of nacre, other models assemblies inspired by the waviness of microstructure of nacre have also been devised on the large scale.
system. The composites
made by this sequential deposition technique do not have a segmented layered microstructure. Thus, sequential adsorption has been proposed to overcome this limitation and consists of repeatedly adsorbing electrolytes and rinsing the tablets, which results in multilayers.
. Among mollusk shells, the conch
shell has the highest degree of structural organization. The mineral aragonite
and organic matrix are replaced by polysilicon and photoresist
. The MEMS technology repeatedly deposits a thin silicon film. The interfaces are etched
by reactive ion etching and then filled with photoresist
. There are three films deposited consecutively. 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.
occurs on a synthetic surface with some success. The technique occurs at low temperature and in an aqueous environment. Self-assembling films form templates that effect the nucleation of ceramic phases. The downside with this technique is its inability to form a segmented layered microstructure. Segmentation is an important property of nacre used for crack deflection of the ceramic phase without fracturing it. As a consequence, this technique does not mimic microstructural characteristics of nacre beyond the layered organic/inorganic layered structure and requires further investigation.
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
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
Antler
Antlers are the usually large, branching bony appendages on the heads of most deer species.-Etymology:Antler originally meant the lowest tine, the "brow tine"...
bone, tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...
, cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
, tooth enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...
and dentin
Dentin
Dentine is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp...
are some examples where mineralized tissues are found.
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
Biomimetics
Biomimetics is the study of the structure and function of biological systems as models for the design and engineering of materials and machines. It is widely regarded as being synonymous with biomimicry, biomimesis, biognosis and similar to biologically inspired design.-History:The term 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
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
part) in soft protein networks and tissues (the organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
part). There are approximately 60 different minerals generated through biological processes, but the most common ones are calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
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
Nacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
from mollusk shells and bone, which are both high performance natural composites. Many mechanical and imaging techniques such as nanoindentation
Nanoindentation
Nanoindentation is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials...
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.
Evolution
The evolution of mineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of mammalian tissue mineralization began either in the oral skeleton of conodontConodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
or the dermal skeleton of early agnathans. The dermal skeleton is just surface dentin
Dentin
Dentine is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp...
and basal bone, which is sometimes overlaid by enameloid. It is thought that the dermal skeleton eventually became scales, which are homologous to teeth. Teeth were first seen in chondrichthyans and were made from all three components of the dermal skeleton, namely dentin, basal bone and enameloid. The mineralization mechanism of mammalian tissue was later elaborated in actinopterygians and sarcopterygians during bony fish evolution. 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.
Hierarchical structure
Hierarchical structures are distinct features seen throughout different length scales. To understand how the hierarchical structure of mineralized tissues contributes to their remarkable properties, those for nacre and bone are described below.The macroscale
Some mollusk shells protect themselves from predators by using a two layered system, one of which is nacre. Nacre constitutes the inner layer while the other, outer, layer is made from calciteCalcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
. The latter is hard and thus prevents any penetration through the shell, but is subject to brittle failure. On the other hand, nacre is softer and can uphold inelastic deformations, which makes it tougher than the hard outer shell. The mineral found in nacre is aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
, CaCO3, and it occupies 95% vol. Interestingly, nacre is 3000 times tougher than aragonite and this has to do with the other component in nacre, the one that takes up 5% vol., which is the softer organic biopolymers. Furthermore, the nacreous layer also contains some strands of weaker material called growth lines that can deflect cracks.
The microscale
The Microscale can be imagined by a three dimensional brick and mortar wall. The bricks would be 0.5 μm thick layers of microscopic aragonite polygonal tablets approximately 5-8 μm in diameter. What holds the bricks together are the mortars and in the case of nacre, it is the 20-30 nm organic material that plays this role. Even though these tablets are usually illustrated as flat sheets, different microscopy techniques have shown that they are wavy in nature with amplitudes as large as half of the tablet’s thickness. 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.The nanoscale
The 30 nm thick interface between the tablets that connects them together and the aragoniteAragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
grains detected by scanning electron microscopy from which the tablets themselves are made of together represent another structural level. The organic material “gluing” the tablets together is made of proteins and chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
.
To summarize, on the macroscale, the shell, its two layers (nacre
Nacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
and calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
), and weaker strands inside nacre represent three hierarchical structures. On the microscale, the stacked tablet layers and the wavy interface between them are two other hierarchical structures. 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.
Bone
Like nacre and the other mineralized tissues, boneBone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
has a hierarchical structure that is also formed by the self assembly of smaller components. The mineral in bone (known as bone mineral
Bone mineral
Bone mineral is the inorganic component of bone. Bone mineral is formed from carbonated hydroxyapatite with lower crystallinity....
) is hydroxyapatite with a lot of carbonate ions, while the organic portion is made mostly of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and some other proteins. Different sources report different numbers of hierarchical level in bone, which is a complex biological material. The types of mechanisms that operate at different structural length scales are yet to be properly defined. Five hierarchical structures of bone are presented below.
The macroscale
At scale of several millimetres to centimetre is where compact bone and spongy bone can be seen.The microscale
There are two hierarchical structures on the microscale. 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. The second hierarchical structure, the ultrasturcture, at a scale of 5 to 10 μm, is the actual structure of the osteons and small struts.The nanoscale
There are also two hierarchical structures on the nanoscale. The first being the structure inside the ultrastructure that are fibrils and extrafibrillar space, at a scale of several hundred nanometres. The second are the elementary components of mineralized tissues at a scale of tens of nanometres. The components are the mineral crystals of hydroxyapatite, cylindrical collagenCollagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
molecules, organic molecules such as lipids and proteins, and finally water. The hierarchical structure common to all mineralized tissues is the key to their mechanical performance.
Mineral component
The mineral is the inorganic component of mineralized tissues. This constituent is what is what makes the tissues harder and stiffer. Hydroxyapatite, calcium carbonateCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
, silica, calcium oxalate
Calcium oxalate
Calcium oxalate is a chemical compound that forms needle-shaped crystals, known in plants as raphides. A major constituent of human kidney stones, the chemical is also found in beerstone, a scale that forms on containers used in breweries...
, whitlockite
Whitlockite
Whitlockite is a mineral, an unusual form of calcium phosphate. Its formula is Ca96PO3OH. It is a relatively rare mineral but is found in granitic pegmatites, phosphate rock deposits, guano caves and in chondrite meteorites...
, and monosodium urate are examples of minerals found in biological tissues. In mollusc shells, these minerals are carried to the site of mineralization in vesicles within specialized cells. Although they are in an amorphous mineral phase while inside the vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
, the mineral destabilizes as it passes out of the cell and crystallizes. In bone, studies have shown that calcium phosphate
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
nucleates within the hole area of the collagen fibrils and then grows in these zones until it occupies the maximum space.
Organic component
The organic part of mineralized tissues is made of proteins. In bone for example, the organic layer is the protein collagen. The degree of mineral in mineralized tissues varies and the organic component occupies a smaller volume as tissue hardness increases. However, without this organic portion, the biological material would be brittleBrittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
and break easily. Hence, the organic component of mineralized tissues increases their toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
. Moreover, many proteins are regulators in the mineralization process. They act in the nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
or inhibition of hydroxyapatite formation. For example, the organic component in nacre is known to restrict the growth of aragonite. Some of the regulatory proteins in mineralized tissues are osteonectin
Osteonectin
Osteonectin also known as secreted protein acidic and rich in cysteine or basement-membrane protein 40 is a protein that in humans is encoded by the SPARC gene....
, osteopontin
Osteopontin
Osteopontin , also known as bone sialoprotein I , early T-lymphocyte activation , secreted phosphoprotein 1 , 2ar and Rickettsia resistance , is a human gene product, which is also conserved in other species...
, osteocalcin
Osteocalcin
Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein , is a noncollagenous protein found in bone and dentin. In humans, the osteocalcin is encoded by the BGLAP gene.- Function :...
, bone sialoprotein and dentin phosphophoryn
Dentin phosphoprotein
Dentin phophoprotein, or phosphophoryn, is one of three proteins formed from dentin sialophosphoprotein and is important in the regulation of mineralization of dentin....
. In nacre, the organic component is porous, which allows the formation of mineral bridges responsible for the growth and order of the nacreous tablets.
Formation of minerals
Understanding the formation of biological tissues is inevitable in order to properly reconstruct them artificially. Even if questions remain in some aspects and the mechanism of mineralization of many mineralized tissues need yet to be determined, there are some ideas about those of mollusc shell, bone and sea urchin.Mollusk shell
The main structural elements involved in the mollusk shell formation process are: a hydrophobic silk gel, aspartic acid rich protein, and a chitinChitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
support.
The silk gel is part of the protein portion and is mainly composed of glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
and alanine
Alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...
. It is not an ordered structure. The acidic proteins play a role in the configuration of the sheets. The chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
is highly ordered and is the framework of the matrix. The main elements of the overall are:
- The silk gel fills the matrix to be mineralized before the mineralization takes place.
- The highly ordered chitinChitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
determines the orientation of the crystals. - The components of the matrix are spatially distinguishable.
- Amorphous calcium carbonateCalcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
is the first form of the mineral. - Once nucleationNucleationNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
begins on the matrix, the calcium carbonate turns into crystals. - While crystals grow, some of the acidic proteins get trapped within them.
Bone
In bone, mineralization starts from a heterogeneous solution having calcium and phosphate ions. The mineral nucleates, inside the hole area of the collagen fibrils, as thin layers of calcium phosphateCalcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
, which then grow to occupy the maximum space available there. The mechanisms of mineral deposition within the organic portion of the bone are still under investigation. 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.
Sea urchin embryo
The sea urchinSea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
embryo has been used extensively in developmental biology studies. The larvae form a sophisticated endoskeleton
Endoskeleton
An endoskeleton is an internal support structure of an animal, composed of mineralized tissue. Endoskeleton develops within the skin or in the deeper body tissues. The vertebrate is basically an endoskeleton made up of two types of tissues . During early embryonic development the endoskeleton is...
that is made of two spicules. Each of the spicules is a single crystal of mineral calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
. The latter is a result of the transformation of amorphous CaCO3 to a more stable form. Therefore, there are two mineral phases in larval spicule formation.
Organic-inorganic interface
The mineral-protein interface with its underlying adhesion forces is involved in the toughening properties of mineralized tissues. The interaction in the organic-inorganic interface is important to understand these toughening properties.At the interface, a very large force (>6-5 nN) is needed to pull the protein molecules away from the aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
mineral in nacre, despite the fact that the molecular interactions are non-bonded. Some studies perform a finite element model
Finite element method
The finite element method is a numerical technique for finding approximate solutions of partial differential equations as well as integral equations...
analysis to investigate the behaviour of the interface. 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. As well, the nanoscale asperities that is on the tablet surfaces provide resistance to interlamellar sliding and so strengthen the material. A surface topology
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
study has shown that progressive tablet locking and hardening, which are needed for spreading large deformations over large volumes, occurred because of the waviness of the tablets.
Diseased mineralized tissues
In vertebrates, mineralized tissues not only develop through normal physiological processes, but can also be involved in pathologicalPathology
Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....
processes. Some diseased areas that include the appearance of mineralized tissues include atherosclerotic plaques, tumoral calcinosis
Tumoral calcinosis
Tumoral calcinosis is a skin condition characterized by calcification of a pre-existing skin tumor ; it presents as large subcutaneous masses. The original tumor may or may not disappear leaving behind a calcified nodule that may eventually ossify ....
, juvenile dermatomyositis
Dermatomyositis
Dermatomyositis is a connective-tissue disease related to polymyositis and Bramaticosis that is characterized by inflammation of the muscles and the skin.- Causes :...
, kidney
Kidney stone
A kidney stone, also known as a renal calculus is a solid concretion or crystal aggregation formed in the kidneys from dietary minerals in the urine...
and salivary stones
Sialolithiasis
Sialolithiasis refers to the formation of stones in the salivary glands. Stones are most commonly found in the submandibular gland, where stones can obstruct Wharton's duct...
. All physiologic deposits contain the mineral hydroxyapatite or one analogous to it. Imaging techniques such as infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
are used to provide information on the type of mineral phase and changes in mineral and matrix composition involved in the disease. Also, clastic cells are cells that cause mineralized tissue resorption
Bone resorption
Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood....
. If there is an unbalance of clastic cell, this will disrupt resorptive activity and cause diseases. One of the studies involving mineralized tissues in dentistry is on the mineral phase of dentin
Dentin
Dentine is a calcified tissue of the body, and along with enamel, cementum, and pulp is one of the four major components of teeth. Usually, it is covered by enamel on the crown and cementum on the root and surrounds the entire pulp...
in order to understand its alteration with aging. These alterations lead to “transparent” dentin, which is also called sclerotic. It was shown that a ‘‘dissolution and reprecipitation’’ mechanism reigns the formation of transparent dentin. The causes and cures of these conditions can possibly be found from further studies on the role of the mineralized tissues involved.
Bioinspired materials
The attractive properties of mineralized tissues like nacre and bone have led to the creation of a large number of biomimetic materials. Although improvements can be made, there are several techniques used to mimic these tissues. Some of the current techniques are described here for nacre imitation.Large scale “model materials”
The large scale model of materials is based on the fact that crack deflection is an important toughening mechanismFracture toughening mechanisms
In materials science, fracture toughening mechanisms are processes that increase energy absorption during fracture, resulting in higher fracture toughness.-Intrinsic toughening mechanisms:...
of nacre. This deflection happens because of the weak interfaces between the aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
tiles. Systems on the macroscopic
Macroscopic
The macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
scales are used to imitate these week interfaces with layered composite ceramic tablets that are held together by weak interface “glue”. Hence, these large scale models can overcome the brittleness of ceramics. Since other mechanisms like tablet locking and damage spreading also play a role in the toughness of nacre, other models assemblies inspired by the waviness of microstructure of nacre have also been devised on the large scale.
Ice templation
Ice Templation is a new method that uses the physics of ice formation to develop a layered-hybrid material. In this system, ceramic particles in a concentrated suspension are frozen using carefully controlled freezing kinetics. As a result, a homogeneous, porous scaffold can be made, which is then filled with a second organic or inorganic phase to build dense layered composites.Layer-by-layer deposition
Layer-by-layer deposition is a technique that as suggested by its name consists of a layer-by-layer assembly to make multilayered composites like nacre. Some examples of efforts in this direction include alternating layers of hard and soft components of TiN/Pt with an ion beamIon beam
An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
system. The composites
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
made by this sequential deposition technique do not have a segmented layered microstructure. Thus, sequential adsorption has been proposed to overcome this limitation and consists of repeatedly adsorbing electrolytes and rinsing the tablets, which results in multilayers.
Thin film deposition: microfabricated structures
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)Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
. Among mollusk shells, the conch
Conch
A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....
shell has the highest degree of structural organization. The mineral aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
and organic matrix are replaced by polysilicon and photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
. The MEMS technology repeatedly deposits a thin silicon film. The interfaces are etched
Etching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
by reactive ion etching and then filled with photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
. There are three films deposited consecutively. 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.
Self-assembly
The method of self assembly tries to reproduce not only the properties, but also the processing of bioceramics. In this process, raw materials readily available in nature are used to achieve stringent control of nucleation and growth. This nucleationNucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
occurs on a synthetic surface with some success. The technique occurs at low temperature and in an aqueous environment. Self-assembling films form templates that effect the nucleation of ceramic phases. The downside with this technique is its inability to form a segmented layered microstructure. Segmentation is an important property of nacre used for crack deflection of the ceramic phase without fracturing it. As a consequence, this technique does not mimic microstructural characteristics of nacre beyond the layered organic/inorganic layered structure and requires further investigation.