Bone as a Composite Material

Bone is composed of living cells in a ceramic matrix. The matrix itself consists of inorganic (ceramic) and organic matter, along with water. The composition and structure of bones are very complex, and simulating a material similar to bone by artificial means is very difficult. CBPCs, however, hold promise toward producing materials similar to bone in composition, if not in exact stmcture. 

Typically, bone has a solid outer portion called cortical bone and a porous inner part called cancellous bone. The amounts of each vary with location in the body. The cortical bone is a ceramic containing calcium compounds and viscous liquids, a protein called collagen , and an organic polymer. In addition to HAP, bone consists of calcium carbonate and calcium phosphate. HAP is 69 wt.% of total calcium phosphate compounds [4]. Part of the Ca in these compounds is substituted by Na, K, Mg, and Sr. Hydroxyl ions in the HAP are also substituted by F, CO3, or Cl, which makes the apatite a fluoroapatite, dahllite or chloroapatite, respectively. These substitutions are considered to play significant roles in the structure and mechanical properties of bones. 

New bone material is formed chemically by cells called osteoblasts . These cells contain water in place of minerals. Gradually, calcification of the organic matrix (osteoid) occurs due to catalysis of the elements of the collagen structure, and this matrix grows to form the ceramic portion of the bone. The initial mineralization in humans is fast, 70% within the first four days. It takes months to reach normal mineral capacity thereafter. 

Bone is an anisotropic and viscoelastic ceramic matrix composite and is distinct from conventional ceramics. Its mechanical properties depend on its porosity, degree of mineralization, collagen fiber orientation, and other structural details. The data in Table 18.1 may be used to compare the physical and mechanical properties of bone, hydroxyapatite (the major mineral in bone, and hence, the most relevant material as a bioceramic), and CBPCs. 

As shown in Table 18.1, bone is a lightweight (low density) material. Most of the orthopedic substitutions currently used, such as steel, titanium, and alumina, are much denser than bone. Comparatively, CBPC materials come close to the density of compact bone. 

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Since the X-ray-opaque filler (e.g., Zr02) is one of the basic components of bone cement, the physical properties of bone cement should be treated as a composite material of polymethyl methacrylate (PMMA) and fillers. The physical properties of composite materials depend on the matrices, fillers, and interfaces between them. The most desirable situation may be the combination of good properties of each component material. For this purpose, the role of interface is very important for an efficient stress transfer from the matrix to the fillers [41,42]. 

A composite, as we mentioned in the chapter opening, is a material constructed of two or more different kinds of materials. We mentioned bone as a natural composite of calcium phosphate crystals and a protein collagen. Bone, as a composite, is much stronger than either of its components and less subject to fracture than the calcium phosphate structure would be by itself. An example of a commercial composite is epoxy plastic reinforced with carbon fibers it is used in aircraft parts, golf clubs, fish-... 

Bone tissue can be defined as a composite material. Collagen, one of the most common proteins of the body, represents almost 90% of the bone organic component and endows elasticity and impact resistance to the tissue. The second component, the inorganic one, stiffens and hardens the organic matrix and is formd in form of crystals from the hydroxyapatite mineral family [1-4]. 

The bones and teeth of humans and other vertebrate animals, for example, consist mainly of a composite material made up of an organic substance, collagen, and a biomineral, calcium carbonate phosphate (see Textboxes 32 and 61). The latter, which makes up about two-thirds of the total dry weight of bone, is composed of calcium phosphate containing between 4-6% calcite (composed of calcium carbonate) as well as small amounts of sodium, magnesium, fluorine, and other trace elements. The formula Ca HPChXPChMCChXOH) approximately represents its composition its crystal structure is akin to that... 

Bone is also a composite material composed of about 70% HA, as well as collagen (see Section 1.5.3), but in contrast to enamel, bones also contain blood vessels that help... 

Hydroxyapatite (HAP), the molecular formula of which is Ca5(P04)3(0H) or Caio(P04)6(OH)2, is the major inorganic constituent in bone, teeth, etc. in the human body. HAP has essentially the same chemical composition and crystalline structure as those of human bone and so has good bio-compatibility. For a long time, it has been widely used as a sclerotin material in setting broken bone, filling teeth, etc. [215]. In addition, HAP can also be used as a food additive and moisture-sensitive element, etc. 

R.B. Martin, Bone as a ceramic composite material, in Bioceramics, ed. J.F. Shackleford (Trans Tech Pub, Brandrain, 1999), p. 9. 

Another interesting application is a composite material made of hydroxyapatite-chitin-chitosan as bone filling material for guided tissue regeneration, for treatment of periodontal bone defects it forms a self-hardening paste [87,88]. 

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