Calcium Phosphate-Based Materials

Calcium phosphate-based systems have wide applications in biomedical areas. Brown has outlined the similarities between the hydration of calcium silicates and calcium phosphates. The hydration products in both systems have high surface areas, variable composition, and poor crystallinity. Pozzolanic reactions and Hadley-like grains form in both systems. The primary cement-water reactions for C3S and tetracalcium phosphate are as follows  

The reaction products in the phosphate system are stoichiometric hydroxyapetite (HAp) and calcium hydroxide (CH). 

The above reactions are specific to one composition of C-S-H or HAp. It is known that C-S-H can exist over a range of compositions (c/s extending from 0.83 to 2.0). The Ca/P ratio varies from 1.40 to 1.84. 

In the pozzolanic reaction analog, tetracalcium phosphate is the source of Ca(OH)2. A variety of acidic calcium phosphates can be considered as the Si02 analog. The analogy is extended by the following reaction  

The hydrolysis reactions of tetracalcium phosphate are coupled and solid CH actually does not form. Calcium deficient HAp then forms through the following reaction  

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Calcium phosphate is a mineral found in several different forms, including hydroxyapatite. (3-whitlockite, and tricalcium phosphate. The crystal structure of hydroxyapatite [Caio(P04)6(OH)2], is similar to that of the calcium and phosphate apatites that are present in the mineral phase of bone. It was found that if a metallic implant is coated with a calcium-phosphate-based material, the formation of bone around the implant is accelerated, and surface contact in the early stages of healing is improved. This is because, in addition to providing a porous surface with which the bone can integrate, the coatings are able to form a direct chemical bond with the bone. 

The teeth and the bones of mammals, the protective shells of mollusks, and the needle-sharp spines of sea urchins (Figure 1.6) and other marine creatures are made from calcium carbonate (caldte or aragonite) and/or calcium phosphate-based materials. 

Because of their outstanding bioactivity and biocompatibility calcium phosphate-based materials have been widely investigated for applications in the biomedical fields (17). Amorphous calcium phosphate nanospheres and hydroxyapatite nanorods have been prepared and hybridized with poly(d,/-lactic acid) in order to fabricate composite nanofibers using an electrospinning technique. 

A calcium phosphate-based material with chemical composition Caio(P04)e (0H)2, rich in bone minerals. 

Vallet-Regi, M. and Arcos D. (2005) Silicon substituted hydroxyapatites. A method to upgrade calcium phosphate based implants. Journal of Materials Chemistry, 15, 1509—1516. 

The major interest in calcium phosphate cements has always been in their potential for biomedical applications. This is because bone contains hydroxyapatite (Ca5(P04)30H), a calcium phosphate mineral. Any material that could be used to bond bone or produce an artificial graft should contain this mineral for compatibility. In fact, much of the research in producing calcium phosphate-based cements or sintered ceramics was motivated by their biomedical applications. We will discuss applications of calcium phosphate cements in detail in Chapter 18. This section describes their materials development. 

The biocompatible CBPC development has occurred only in the last few years, and the recent trend has been to evaluate them as biocompatible ceramics. After all, biological systems form bone and dentine at room temperature, and it is natural to expect that biocompatible ceramics should also be formed at ambient temperature, preferably in a biological environment when placed in a body as a paste. CBPCs allow such placement. We have discussed such calcium phosphate-based cements in Chapter 13. Calcium-based CBPCs, especially those constituting hydroxyapatite (HAP), are a natural choice. HAP is a primary mineral in bone [3], and hence calcium phosphate cements can mimic natural bone. Some of these ceramics with tailored composition and microstructure are already in use, yet there is ample room for improvement. This Chapter focuses on the most recent biocompatible CBPCs and their testing in a biological environment. To understand biocompatible material and its biological environment, it is first necessary to understand the structure of bone and how it is formed. 

Low KL, Tan SH, Zein SH, Roether JA, Mourino V, Boccaccini AR. Calcium phosphate-based composites as injectable bone substitute materials a review. J Biomed Mater Res B Appl Biomater 2010 94B(1), 273-286. 

LC. Chow, Next generation calcium phosphate-based biomaterials, Dent. Mat. J., 28, 1-10 (2009) K. Ishikawa, Bone substitute fabrication based on dissolution-precipitation reactions, Materials, 3, 1138-1155 (2010). 

LeGeros R. Z. Calcium phosphate-based osteoinduaive materials. Chem Rev 2008 108 4742-53. 

Tadic D, Epple M (2004) A thorough physicochemical characterisation of 14 calcium phosphate-based bone substitutimi materials in comparisrat to natural bone. Biomaterials... 

Li, R.H., D Augusta, D., Blake, C., Bouxsein, M., Wozney, J.M., Li, J., Stevens, M., Kim, H., and Seeherman, H. rhBMP-2 delivery and efficacy in an injectable calcium phosphate based matrix. The 28th International Symposium on Controlled Release of Bioactive Materials, Ssn Diego, 2001. 

LeGeros, R. Z. 2008. Calcium phosphate-based osteoinductive materials. Chemical Rewievs 108 4742-53. 

Animal origin hard tissues represent alternative raw materials that can be used for the fabrication of calcium phosphates based products with applications in orthopedics and dentistry. The methods of obtaining calcium phosphates from this kind of biological precursors (constituting sustainable and inexpensive resources) can determine the characteristics of the final products. 

The presence of pores in materials provides anchor points for the bone and improves the mechanical quahty of the bone/implant interface the increase in the specific surface further encourages cell colonization and the revascularization. While calcium phosphate-based bioceramics are excellent materials for bone reconstruction, they have a low mechanical strength (less than that of the bone), not lending themselves to machining. This resistance diminishes while porosity increases, making the utilization of very porous implants very delicate. 

Hydroxyapatite is a class of calcium phosphate-based bioceramic material, frequently used as a bone graft substimte owing to its chemical and stractural... 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

The polysulfide base material contains 50—80% of the polyfunctional mercaptan, which is a clear, amber, sympy Hquid polymer with a viscosity at 25°C of 35, 000 Pa-s(= cP), an average mol wt of 4000, a pH range of 6—8, and a ntild, characteristic mercaptan odor. Fillers are added to extend, reinforce, harden, and color the base. They may iaclude siUca, calcium sulfate, ziac oxide, ziac sulfide [1314-98-3] alumina, titanium dioxide [13463-67-7] and calcium carbonate. The high shear strength of the Hquid polymer makes the compositions difficult to mix. The addition of limited amounts of diluents improves the mix without reduciag the set-mbber characteristics unduly, eg, dibutyl phthalate [84-74-2], tricresyl phosphate [1330-78-5], and tributyl citrate [77-94-1]. 

Bio-nanocomposites based on calcium phosphates can perform other innovative fundions such as acting as a reservoir for the controlled release of bioadive compounds once the material is implanted in the bone defect. For instance, the incorporation of a morphogenetic protein that promotes bone regeneration in an HAP-alginate-collagen system [110] or a vitamin in a Ca-deficient HAP-chitosan nanocomposite [111] are recent examples of this kind of application. 

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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