Cements, calcium phosphate-based

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. 

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 ... 

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). 

Foam cement is a special class of lightweight cement. The gas content of foamed cement can be up to 75% by volume. The stability of the foam is achieved by the addition of surfactants, as shown in Table 10-9. A typical foamed cement composition is made from a hydraulic cement, an aqueous rubber latex in an amount up to 45% by weight of the hydraulic cement, a latex stabilizer, a defoaming agent, a gas, a foaming agent, and a foam stabilizer [359,362]. Foamed high-temperature applications are based on calcium phosphate cement [257]. 

One great advantage with phosphate bonded ceramics in biomaterial or dental applications is the phosphate ions in their structure. Bones contain calcium phosphate, and hence phosphate bonded ceramics are generally biocompatible with bones. While chemically bonded calcium phosphate ceramics have been difficult to produce, magnesium and zinc based phosphate bonded ceramics have been more easily synthesized and used as structural and dental cements. 

Naturally occurring phosphate cements are also known. Krajewski [3] cites calcium-based phosphate cements in the Albeian condensed Glauconitic Limestone of the Tatra Mountains in Western Carpathians. In recent years methods have been developed to fabricate calcium phosphate ceramics by direct reaction of calcium compounds and either phosphoric acid or an acid phosphate. The mineralogy of the products has also been well studied. Most of these efforts are directed towards development of calcium-based bioceramics containing calcium phosphate compounds, such as hydroxyapatite. These developments are discussed below. 

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. 

C.E. Semler, A quick-setting wollastonite phosphate cement, Am. Ceram. Soc. Bull, 55 (1976) 983-988. T. Sugama and M. Allan, Calcium phosphate cements prepared by acid-base reaction, J. Am. Ceram. Soc., 75 [8] (1992) 2076-2087. 

Tofighi A, Mounic S, Chakravarthy P, Rey C, Lee D. Setting reactions involved in injectable cements based on amorphous calcium phosphate. Key Eng Mat. 2001 192-195 769-72. 

Environmentally responsive Set in situ in response to pH, temperature, or other stimulus Calcium phosphate cements (CPCs) Set in situ by an acid/base reaction... 

Frayssinet P, Gineste L, Conte P, Fages J, Rouquet N. Short-term implantation effects of a DCPD-based calcium phosphate cement. Biomaterials 1998 19(11-12) 971-977. 

M. Otsuka, Development of skeletal drug delivery system based on apatite/collagen composite cement, in B. Ben-Nissan, ed.. Advances in Calcium Phosphate Biomaterials, Springer Series in Biomaterials Science and Engineering, vol. 2, pp. 355-372,2014. 

Ooms, E. M., Wolke, J. G. C., van de Heuvel, R., Jeschke, B., and Jansen, J. A. 2003. Histological evaluation of the hone response to calcium phosphate cement implanted in cortical bone. Biomaterials 24 989-1000. Ooms, E. M., Wolke, J. G. C., van der Waerden, J. P. C. M., and Jansen, J. A. 2002. Trabecular bone response to injectable calcium phosphate (Ca-P) cement. Journal Biomedical Materials Research 61 9-18. Orlovskii, P. V., Komlev, V. S., and Barinov, S. M. 2002. Hydroxyapatite and hydroxyapatite based ceramics. Inorganic Materials 38 1159-72. 

Apatite cement (AC) based on calcium phosphates offer an advantage for being freely moldable aM adaptable to the surface of bone defect. In addition, they have excellent biocompatibility because of their similarity to the inorganic component of the calcified tissue of the body [1,2], The first AC, reported by Prof Moruna and Kanazawa in 1976 was based in a-tricalcirrm phosphate (a-TCP, a-Ca3(P04)2), the cement converted to calcimn deficient HAp (CDHAp, Ca9(P04)s(0H)2) at a temperature below 100°C [3], Setting... 

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