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Mineralized tissues
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<h2 id="evolution">Evolution</h2> <p>The evolution of mineralized tissues has been puzzling for more than a century. It has been hypothesized that the first mechanism of animal tissue mineralization began either in the oral skeleton of <a href="/facts/Conodont/wqA7V7cz">conodont</a> or the dermal skeleton of early <a href="/facts/Agnathans/LohAXfEH">agnathans</a>. The dermal skeleton is just surface <a href="/facts/Dentin/UMpkJ3UV">dentin</a> 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 <a href="/facts/Chondrichthyans/xcW7PJDU">chondrichthyans</a> 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 <a href="/facts/Actinopterygians/3PNaXXmJ">actinopterygians</a> and <a href="/facts/Sarcopterygians/7Glajr07">sarcopterygians</a> during bony fish evolution. It is expected that genetic analysis of <a href="/facts/Agnathans/LohAXfEH">agnathans</a> will provide more insight into the evolution of mineralized tissues and clarify evidence from early fossil records.<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> </p> <h2 id="hierarchical-structure">Hierarchical structure</h2> <p>Hierarchical structures are distinct features seen throughout different length scales.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> To understand how the hierarchical structure of mineralized tissues contributes to their remarkable properties, those for nacre and bone are described below.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> Hierarchical structures are characteristic of biology and are seen in all structural materials in biology such as bone <a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> and nacre from seashells<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p> <h3>Nacre</h3> <p>Nacre has several hierarchical structural levels.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> </p> <h4>The macroscale</h4> <p>Some mollusc shells protect themselves from predators by using a two layered system, one of which is nacre.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a><a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> Nacre constitutes the inner layer while the other, outer, layer is made from <a href="/facts/Calcite/gKMHYQJo">calcite</a>.<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> 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.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> The mineral found in nacre is <a href="/facts/Aragonite/QGcD3SLs">aragonite</a>, CaCO3, and it occupies 95% vol. 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.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a> Furthermore, the nacreous layer also contains some strands of weaker material called growth lines that can deflect cracks.<a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> </p> <h4>The microscale</h4> <p>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.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> 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.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a><a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> 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.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a> </p> <h4>The nanoscale</h4> <p>The 30 nm thick interface between the tablets that connects them together and the <a href="/facts/Aragonite/QGcD3SLs">aragonite</a> 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 <a href="/facts/Chitin/Fo1f8b86">chitin</a>.<a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a> </p><p>To summarize, on the macroscale, the shell, its two layers (<a href="/facts/Nacre/a5ek6xm4">nacre</a> and <a href="/facts/Calcite/gKMHYQJo">calcite</a>), 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.<a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a> </p> <h3>Bone</h3> <p>Like nacre and the other mineralized tissues, <a href="/facts/Bone/kwJaR6IQ">bone</a> has a hierarchical structure that is also formed by the self-assembly of smaller components. The mineral in bone (known as <a href="/facts/Bone_mineral/L6LpbEYE">bone mineral</a>) is <a href="/facts/Hydroxyapatite/SqcXuXVQ">hydroxyapatite</a> with a lot of carbonate ions, while the organic portion is made mostly of <a href="/facts/Collagen/KsyAb72K">collagen</a> and some other proteins. The hierarchical structural of bone spans across to a three tiered hierarchy of the collagen molecule itself.<a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a> Different sources report different numbers of hierarchical level in bone, which is a complex biological material.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a><a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a><a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a> The types of mechanisms that operate at different structural length scales are yet to be properly defined.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> Five hierarchical structures of bone are presented below.<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> </p> <h4>The macroscale</h4> <p>Compact bone and spongy bone are on a scale of several millimetres to 1 or more centimetres.<a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a> </p> <h4>The microscale</h4> <p>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 <a href="/facts/Osteons/fTFx9XDA">osteons</a> and small struts can be distinguished.<a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> The second hierarchical structure, the ultrastructure, at a scale of 5 to 10 μm, is the actual structure of the osteons and small struts.<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a> </p> <h4>The nanoscale</h4> <p>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 <a href="/facts/Hydroxyapatite/SqcXuXVQ">hydroxyapatite</a>, cylindrical <a href="/facts/Collagen/KsyAb72K">collagen</a> molecules, organic molecules such as lipids and proteins, and finally water.<a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a> The hierarchical structure common to all mineralized tissues is the key to their mechanical performance.<a class="footnote-ref" id="fnref:57" href="#fn:57"><sup>57</sup></a><a class="footnote-ref" id="fnref:58" href="#fn:58"><sup>58</sup></a> </p> <h2 id="mineral-component">Mineral component</h2> <p>The mineral is the inorganic component of mineralized tissues. This constituent is what makes the tissues harder and stiffer.<a class="footnote-ref" id="fnref:59" href="#fn:59"><sup>59</sup></a><a class="footnote-ref" id="fnref:60" href="#fn:60"><sup>60</sup></a> <a href="/facts/Hydroxyapatite/SqcXuXVQ">Hydroxyapatite</a>, <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>, <a href="/facts/Silica/A3WILp6J">silica</a>, <a href="/facts/Calcium_oxalate/1NC8NQ01">calcium oxalate</a>, <a href="/facts/Whitlockite/jmE269GJ">whitlockite</a>, and monosodium urate are examples of minerals found in biological tissues.<a class="footnote-ref" id="fnref:61" href="#fn:61"><sup>61</sup></a><a class="footnote-ref" id="fnref:62" href="#fn:62"><sup>62</sup></a> In mollusc shells, these minerals are carried to the site of mineralization in vesicles within specialized cells. Although they are in an <a href="/facts/Amorphous/YTIzmrR0">amorphous</a> mineral phase while inside the <a href="/facts/Vesicle_(biology)/1KjHpjMM">vesicles</a>, the mineral destabilizes as it passes out of the cell and crystallizes.<a class="footnote-ref" id="fnref:63" href="#fn:63"><sup>63</sup></a> In bone, studies have shown that <a href="/facts/Calcium_phosphate/tn03xvdb">calcium phosphate</a> nucleates within the hole area of the collagen fibrils and then grows in these zones until it occupies the maximum space.<a class="footnote-ref" id="fnref:64" href="#fn:64"><sup>64</sup></a> </p> <h2 id="organic-component">Organic component</h2> <p>The organic part of mineralized tissues is made of proteins.<a class="footnote-ref" id="fnref:65" href="#fn:65"><sup>65</sup></a> In bone for example, the organic layer is the protein collagen.<a class="footnote-ref" id="fnref:66" href="#fn:66"><sup>66</sup></a> The degree of mineral in mineralized tissues varies and the organic component occupies a smaller volume as tissue <a href="/facts/Hardness/PtGn6E20">hardness</a> increases.<a class="footnote-ref" id="fnref:67" href="#fn:67"><sup>67</sup></a><a class="footnote-ref" id="fnref:68" href="#fn:68"><sup>68</sup></a> However, without this organic portion, the biological material would be <a href="/facts/Brittle/cn27H2vg">brittle</a> and break easily.<a class="footnote-ref" id="fnref:69" href="#fn:69"><sup>69</sup></a><a class="footnote-ref" id="fnref:70" href="#fn:70"><sup>70</sup></a> Hence, the organic component of mineralized tissues increases their <a href="/facts/Toughness/1gEIUdBu">toughness</a>.<a class="footnote-ref" id="fnref:71" href="#fn:71"><sup>71</sup></a> Moreover, many proteins are regulators in the mineralization process. They act in the <a href="/facts/Nucleation/efUVLsLR">nucleation</a> 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 <a href="/facts/Osteonectin/uwKjq5XD">osteonectin</a>, <a href="/facts/Osteopontin/BUx3CIML">osteopontin</a>, <a href="/facts/Osteocalcin/tP0Ahh7b">osteocalcin</a>, bone sialoprotein and <a href="/facts/Dentin_phosphoprotein/ya4FEyy7">dentin phosphophoryn</a>.<a class="footnote-ref" id="fnref:72" href="#fn:72"><sup>72</sup></a> In nacre, the organic component is porous, which allows the formation of mineral bridges responsible for the growth and order of the nacreous tablets.<a class="footnote-ref" id="fnref:73" href="#fn:73"><sup>73</sup></a> </p> <h2 id="formation-of-minerals">Formation of minerals</h2> <p>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.<a class="footnote-ref" id="fnref:74" href="#fn:74"><sup>74</sup></a> </p> <h3>Mollusk shell</h3> <p>The main structural elements involved in the mollusk shell formation process are: a <a href="/facts/Hydrophobic/Ifya0CXQ">hydrophobic</a> silk gel, aspartic acid rich protein, and a <a href="/facts/Chitin/Fo1f8b86">chitin</a> support. The silk gel is part of the protein portion and is mainly composed of <a href="/facts/Glycine/3nXI1hqW">glycine</a> and <a href="/facts/Alanine/fphQEdSc">alanine</a>. It is not an ordered structure. The acidic proteins play a role in the configuration of the sheets. The <a href="/facts/Chitin/Fo1f8b86">chitin</a> is highly ordered and is the framework of the matrix. The main elements of the overall are:<a class="footnote-ref" id="fnref:75" href="#fn:75"><sup>75</sup></a> </p> <ol><li>The silk gel fills the matrix to be mineralized before the mineralization takes place.<a class="footnote-ref" id="fnref:76" href="#fn:76"><sup>76</sup></a></li> <li>The highly ordered <a href="/facts/Chitin/Fo1f8b86">chitin</a> determines the orientation of the crystals.<a class="footnote-ref" id="fnref:77" href="#fn:77"><sup>77</sup></a></li> <li>The components of the matrix are spatially distinguishable.<a class="footnote-ref" id="fnref:78" href="#fn:78"><sup>78</sup></a></li> <li>Amorphous <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a> is the first form of the mineral.<a class="footnote-ref" id="fnref:79" href="#fn:79"><sup>79</sup></a></li> <li>Once <a href="/facts/Nucleation/efUVLsLR">nucleation</a> begins on the matrix, the calcium carbonate turns into crystals.<a class="footnote-ref" id="fnref:80" href="#fn:80"><sup>80</sup></a></li> <li>While crystals grow, some of the acidic proteins get trapped within them.<a class="footnote-ref" id="fnref:81" href="#fn:81"><sup>81</sup></a></li></ol> <h3>Bone</h3> <p>In bone, mineralization starts from a <a href="/facts/Heterogeneous/y84KQJxl">heterogeneous</a> solution having calcium and phosphate ions. The mineral nucleates, inside the hole area of the collagen fibrils, as thin layers of <a href="/facts/Calcium_phosphate/tn03xvdb">calcium phosphate</a>, 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.<a class="footnote-ref" id="fnref:82" href="#fn:82"><sup>82</sup></a> </p> <h3>Sea urchin embryo</h3> <p>The <a href="/facts/Sea_urchin/RQrB6XTH">sea urchin</a> embryo has been used extensively in developmental biology studies. The larvae form a sophisticated <a href="/facts/Endoskeleton/UlLveJL0">endoskeleton</a> that is made of two spicules. Each of the spicules is a single crystal of mineral <a href="/facts/Calcite/gKMHYQJo">calcite</a>. 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.<a class="footnote-ref" id="fnref:83" href="#fn:83"><sup>83</sup></a> </p> <h2 id="organic-inorganic-interface">Organic-inorganic interface</h2> <p>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.<a class="footnote-ref" id="fnref:84" href="#fn:84"><sup>84</sup></a> </p><p>At the interface, a very large force (>6-5 nN) is needed to pull the protein molecules away from the <a href="/facts/Aragonite/QGcD3SLs">aragonite</a> mineral in nacre, despite the fact that the molecular interactions are non-bonded.<a class="footnote-ref" id="fnref:85" href="#fn:85"><sup>85</sup></a> Some studies perform a <a href="/facts/Finite_element_method/eUcfP1vn">finite element model</a> analysis to investigate the behaviour of the interface.<a class="footnote-ref" id="fnref:86" href="#fn:86"><sup>86</sup></a><a class="footnote-ref" id="fnref:87" href="#fn:87"><sup>87</sup></a> A model has shown that during tension, the back stress that is induced during the <a href="/facts/Plastic_deformation_in_solids/z1LEmkQX">plastic</a> stretch of the material plays a big role in the hardening of the mineralized tissue. As well, the nanoscale <a href="/facts/Asperity_(material_science)/SBCHaJPL">asperities</a> that is on the tablet surfaces provide resistance to interlamellar sliding and so strengthen the material. A surface <a href="/facts/Topology/2EqW47cU">topology</a> study has shown that progressive tablet locking and hardening, which are needed for spreading large <a href="/facts/Deformation_(engineering)/z1LEmkQX">deformations</a> over large volumes, occurred because of the waviness of the tablets.<a class="footnote-ref" id="fnref:88" href="#fn:88"><sup>88</sup></a> </p> <h2 id="diseased-mineralized-tissues">Diseased mineralized tissues</h2><p> In <a href="/facts/Vertebrates/tT54FSTT">vertebrates</a>, mineralized tissues not only develop through normal physiological processes, but can also be involved in <a href="/facts/Pathology/U9b18LJG">pathological</a> processes. Some diseased areas that include the appearance of mineralized tissues include <a href="/facts/Atherosclerotic/WIS8QQ6v">atherosclerotic</a> plaques,<a class="footnote-ref" id="fnref:89" href="#fn:89"><sup>89</sup></a><a class="footnote-ref" id="fnref:90" href="#fn:90"><sup>90</sup></a> <a href="/facts/Tumoral_calcinosis/2lJvrbvL">tumoral calcinosis</a>, juvenile <a href="/facts/Dermatomyositis/VDJk2fE5">dermatomyositis</a>, <a href="/facts/Kidney_stone/8EowkGyr">kidney</a> and <a href="/facts/Sialolithiasis/0q7SMv1S">salivary stones</a>. All physiologic deposits contain the mineral <a href="/facts/Hydroxyapatite/SqcXuXVQ">hydroxyapatite</a> or one analogous to it. Imaging techniques such as <a href="/facts/Infrared_spectroscopy/u1Hzx617">infrared spectroscopy</a> are used to provide information on the type of mineral phase and changes in mineral and matrix composition involved in the disease.<a class="footnote-ref" id="fnref:91" href="#fn:91"><sup>91</sup></a> Also, clastic cells are cells that cause mineralized tissue <a href="/facts/Bone_resorption/cMx23mGW">resorption</a>. 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 <a href="/facts/Dentin/UMpkJ3UV">dentin</a> 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.<a class="footnote-ref" id="fnref:92" href="#fn:92"><sup>92</sup></a> The causes and cures of these conditions can possibly be found from further studies on the role of the mineralized tissues involved. </p> <h2 id="bioinspired-materials">Bioinspired materials</h2> <p>Natural structural materials comprising hard and soft phases arranged in elegant hierarchical multiscale architectures, usually exhibit a combination of superior <a href="/facts/Mechanical_properties/Cg2LVoQe">mechanical properties</a>. For instance, many natural mechanical materials (<a href="/facts/Bone/kwJaR6IQ">Bone</a>, <a href="/facts/Nacre/a5ek6xm4">Nacre</a>, <a href="/facts/Teeth/Wc6IqwVf">Teeth</a>, <a href="/facts/Silk/XhxPpvvS">Silk</a>, and <a href="/facts/Bamboo/wGPJ2cEA">Bamboo</a>) are lightweight, strong, flexible, tough, fracture-resistant, and self-repair. The general underlying mechanism behind such advanced materials is that the highly oriented stiff components give the materials great <a href="/facts/Mechanical_strength/CE77cUcB">mechanical strength</a> and <a href="/facts/Stiffness/xlabLkfT">stiffness</a>, while the soft matrix “glues” the stiff components and transfer the stress to them. Moreover, the controlled <a href="/facts/Plastic_deformation/z1LEmkQX">plastic deformation</a> of the soft matrix during <a href="/facts/Fracture/nxDNAj0d">fracture</a> provides an additional toughening mechanism. 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. There are several techniques used to mimic these tissues. Some of the current techniques are described here.<a class="footnote-ref" id="fnref:93" href="#fn:93"><sup>93</sup></a><a class="footnote-ref" id="fnref:94" href="#fn:94"><sup>94</sup></a> </p> <h3>Large scale model materials</h3> <p>The large scale model of materials is based on the fact that crack deflection is an important <a href="/facts/Fracture_toughening_mechanisms/stup0Kd3">toughening mechanism</a> of nacre. This deflection happens because of the weak interfaces between the <a href="/facts/Aragonite/QGcD3SLs">aragonite</a> tiles. Systems on the <a href="/facts/Macroscopic/NLwPDtLe">macroscopic</a> 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.<a class="footnote-ref" id="fnref:95" href="#fn:95"><sup>95</sup></a> </p> <h3>Biomimetic mineralization</h3> <p>All hard materials in animals are achieved by the <a href="/facts/Biomineralization/m5mGPaTM">biomineralization</a> 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. The general strategy is started with organic scaffolds with ion-binding sites that promote heterogeneous nucleation. Then localized mineralization can be achieved by controlled ion supersaturation on these ion-binding sites. In such a <a href="/facts/Composite_material/UBysQldh">composite material</a>, mineral function as a highly strong and highly wear- and erosion-resistant surface layer. While the soft organic scaffolds provide a tough load-bearing base to accommodate excessive strains. </p> <h3>Ice templation/Freeze casting</h3> <p>Ice temptation/<a href="/facts/Freeze_casting/6m3UDlWU">Freeze casting</a> is a new method that uses the physics of ice formation to develop a layered-hybrid material. Specifically, <a href="/facts/Ceramic/9JKudpqX">ceramic</a> suspensions are directionally frozen under conditions designed to promote the formation of lamellar <a href="/facts/Ice_crystals/By1nPMgF">ice crystals</a>, which expel the ceramic particles as they grow. After sublimation of the water, this results in a layered homogeneous ceramic scaffold that, architecturally, is a negative replica of the ice. The scaffold can then be filled with a second soft phase so as to create a hard–soft layered composite. This strategy is also widely applied to build other kinds of bioinspired materials, like extremely strong and tough <a href="/facts/Hydrogels/t7eStnK0">hydrogels</a>,<a class="footnote-ref" id="fnref:96" href="#fn:96"><sup>96</sup></a> metal/ceramic, and polymer/ceramic hybrid biomimetic materials with fine lamellar or brick-and-mortar architectures. The "brick" layer is extremely strong but brittle and the soft "mortar" layer between the bricks generates limited deformation, thereby allowing for the relief of locally high stresses while also providing <a href="/facts/Ductility/SBJsahEk">ductility</a> without too much loss in strength. </p> <h3>Additive manufacturing</h3> <p><a href="/facts/Additive_manufacturing/lbvWU3t0">Additive manufacturing</a> encompasses a family of technologies that draw on computer designs to build structures layer by layer.<a class="footnote-ref" id="fnref:97" href="#fn:97"><sup>97</sup></a> 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. Therefore, the <a href="/facts/Fracture/nxDNAj0d">crack</a> of materials only can happen and propagate on the microscopic scale, which wouldn't lead to the <a href="/facts/Fracture/nxDNAj0d">fracture</a> of the whole structure. However, the time-consuming of manufacturing the hierarchical mechanical materials, especially on the nano- and micro-scale limited the further application of this technique in large-scale manufacturing. </p> <h3>Layer-by-layer deposition</h3> <p>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 <a href="/facts/Ion_beam/e23WW7uC">ion beam</a> system. The <a href="/facts/Composite_material/UBysQldh">composites</a> 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 <a href="/facts/Electrolytes/4YE8bIVa">electrolytes</a> and rinsing the tablets, which results in multilayers.<a class="footnote-ref" id="fnref:98" href="#fn:98"><sup>98</sup></a> </p> <h3>Thin film deposition: microfabricated structures</h3> <p>Thin film deposition focuses on reproducing the cross-lamellar microstructure of conch instead of mimicking the layered structure of nacre using <a href="/facts/Microelectromechanical_systems/TkhEzDpm">micro-electro mechanical systems (MEMS)</a>. Among mollusk shells, the <a href="/facts/Conch/Ie4rhJQx">conch</a> shell has the highest degree of structural organization. The mineral <a href="/facts/Aragonite/QGcD3SLs">aragonite</a> and organic matrix are replaced by <a href="/facts/Crystalline_silicon/nvx05hmr">polysilicon</a> and <a href="/facts/Photoresist/XNnIkCjU">photoresist</a>. The MEMS technology repeatedly deposits a thin silicon film. The interfaces are <a href="/facts/Etching_(microfabrication)/kPa01dgL">etched</a> by reactive ion etching and then filled with <a href="/facts/Photoresist/XNnIkCjU">photoresist</a>. 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.<a class="footnote-ref" id="fnref:99" href="#fn:99"><sup>99</sup></a> </p> <h3>Self-assembly</h3> <p>The method of self-assembly tries to reproduce not only the properties, but also the processing of <a href="/facts/Bioceramics/IWTfHSUf">bioceramics</a>. In this process, raw materials readily available in nature are used to achieve stringent control of nucleation and growth. This <a href="/facts/Nucleation/efUVLsLR">nucleation</a> 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.<a class="footnote-ref" id="fnref:100" href="#fn:100"><sup>100</sup></a> </p> <h2 id="the-future">The future</h2> <p>The various studies have increased progress towards understanding mineralized tissues. However, it is still unclear which micro/nanostructural features are essential to the material performance of these tissues. Also constitutive laws along various loading paths of the materials are currently unavailable. For nacre, the role of some nanograins and mineral bridges requires further studies to be fully defined. 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. Also the final technique used for artificial reproduction must be both cost effective and scalable industrially.<a class="footnote-ref" id="fnref:101" href="#fn:101"><sup>101</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Shell_growth_in_estuaries/WtAUW1S2">Shell growth in estuaries</a></li> <li><a href="/facts/Biomineralization/m5mGPaTM">Biomineralization</a></li></ul> <h2 id="bibliography">Bibliography</h2> <ul><li>T.G. Bromage (1991). 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