Calcium-Based CBPC Biomaterials

Mineralization in body fluids. In vitro (outside the body) CBPC formation may not be the same as in vivo (inside the body). The body fluids will affect the mineralization process. Blood may wash away or dissolve the CBPC minerals prior to their setting. The components of blood may also become incorporated within the CBPC, modify its composition, and change the physical properties. 

Lack of carbonated or fluorinated hydroxyapatite. The HAP formation by the reaction in Eq. 13.13 still does not produce a composition exactly the same as that of the bone. Bone contains carbonated and fluorinated apatite, or dahUite, and it is difficult to mimic such a composition by chemical reactions. 

Constantz et al. [9,10], however, have succeeded in producing dahllite-based bioceramic with the stoichiometric formula 

They reacted a mixture of monocalcium phosphate monohydrate (Ca(H2P04)2-H20), a-tricalcium phosphate (Ca3(P04)2), and calcium carbonate (CaC03) with a solution of trisodium phosphate (Na3P04) to produce this rapid-setting cement. They have been able to apply this cement as a paste that sets within minutes under physiological conditions. The mixing time is = 5 min, and the paste sets by crystallizing into dahllite in another 10 min. 

The initial compressive strength is —10 MPa (1428.5 psi) and increases to = 55 MPa (7850 psi) within 24 h. The ultimate tensile strength is =2.1 MPa (300 psi). The compressive strength is approximately the same as that of cancellous bone, while the tensile strength appears to be lower than that of bone. 

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Calcium oxide is the main ingredient in conventional portland cements. Since limestone is the most abundant mineral in nature, it has been easy to produce portland cement at a low cost. The high solubility of calcium oxide makes it difficult to produce phosphate-based cements. However, calcium oxide can be converted to compounds such as silicates, aluminates, or even hydrophosphates, which then can be used in an acid-base reaction with phosphate, forming CBPCs. The cost of phosphates and conversion to the correct mineral forms add to the manufacturing cost, and hence calcium phosphate cements are more expensive than conventional cements. For this reason, their use has been largely limited to dental and other biomedical applications. Calcium phosphate cements have found application as structural materials, but only when wollastonite is used as an admixture in magnesium phosphate cements. Because calcium phosphates are also bone minerals, they are indispensable in biomaterial applications and hence form a class of useful CBPCs that cannot be substituted by any other. 

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