biostable, and bio-adoptable natural ceramic without viral effects [9, 10]. Moreover,
it has a unique characteristic, i.e., osteoconduction [11–14]. HAp also facilitates new
bone formation without resorption and interaction with the living system. Moreover,
nanostructured HAp has high surface area and enhanced bioactivity. For this reason,
it interacts with the lowest hierarchical levels of bone structure more intensively. As
a result, several polymer/HAp nanocomposites have been employed to resemble the
structure and properties of human bone tissue. In this chapter, the structure of natural
bone, polymer blends, and recent advances in polymer/HAp nanocomposites for
application as synthetic bone are described.
2 Structure of Natural Bone
Bone is a vascularized, dense connective tissue that forms the endoskeleton of
vertebrates. There are 206 bones in adult humans [15]. The extracellular matrix of
natural bone is composed of two major phases: organic, consisting mainly
of collagen; and inorganic, consisting of minerals (mainly HAp). The inorganic
phase accounts for about 70% of the dry weight of bone and the organic matrix
makes up the rest [16]. Collagen fibers are seen at the nanoscale level and are
surrounded and infiltrated by minerals. The hierarchical structure of bone has been
constructed from these nanostructured building blocks. However, compositional
differences in bone exist between different species and age groups. The composition of the two phases of bone are discussed in Sects. 2.1 and 2.2.
2.1 Organic Phase of Bone
The organic matrix mostly consists of type I collagen (90%). Bone collagen is a
fibrous molecule of 100–2,000 nm length. The collagen fibers provide the framework and architecture of bone, while the HAp crystals are located in the fibers and
between the fibers. The noncollagenous proteins of the organic matrix consist
of proteoglycans, glycoproteins, and γ-carboxyglutamic-acid-containing proteins
[17, 18]. Type I collagen is synthesized by osteoblasts (bone-forming cells). These
molecules have a triple helical structure. After secretion, the globular ends are
cleaved off by enzymes and the triple-helical molecules undergo a self-assembly
process. In general, there are 12 types of collagen found in the body and each
collagen comprises three polypeptide chains containing approximately 1,000 amino
acid units in each chain. Specifically, type I collagen (MW 139,000 Da) possesses
two identical α1(I) chains and one unique α2 chain; this configuration produces a
fairly rigid linear molecule that is 300 nm long [19]. These self-assembled triple
helix bundles have a periodicity of 67 nm, with 40 nm gaps between the ends of the
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A. Bhowmick et al.
it has a unique characteristic, i.e., osteoconduction [11–14]. HAp also facilitates new
bone formation without resorption and interaction with the living system. Moreover,
nanostructured HAp has high surface area and enhanced bioactivity. For this reason,
it interacts with the lowest hierarchical levels of bone structure more intensively. As
a result, several polymer/HAp nanocomposites have been employed to resemble the
structure and properties of human bone tissue. In this chapter, the structure of natural
bone, polymer blends, and recent advances in polymer/HAp nanocomposites for
application as synthetic bone are described.
2 Structure of Natural Bone
Bone is a vascularized, dense connective tissue that forms the endoskeleton of
vertebrates. There are 206 bones in adult humans [15]. The extracellular matrix of
natural bone is composed of two major phases: organic, consisting mainly
of collagen; and inorganic, consisting of minerals (mainly HAp). The inorganic
phase accounts for about 70% of the dry weight of bone and the organic matrix
makes up the rest [16]. Collagen fibers are seen at the nanoscale level and are
surrounded and infiltrated by minerals. The hierarchical structure of bone has been
constructed from these nanostructured building blocks. However, compositional
differences in bone exist between different species and age groups. The composition of the two phases of bone are discussed in Sects. 2.1 and 2.2.
2.1 Organic Phase of Bone
The organic matrix mostly consists of type I collagen (90%). Bone collagen is a
fibrous molecule of 100–2,000 nm length. The collagen fibers provide the framework and architecture of bone, while the HAp crystals are located in the fibers and
between the fibers. The noncollagenous proteins of the organic matrix consist
of proteoglycans, glycoproteins, and γ-carboxyglutamic-acid-containing proteins
[17, 18]. Type I collagen is synthesized by osteoblasts (bone-forming cells). These
molecules have a triple helical structure. After secretion, the globular ends are
cleaved off by enzymes and the triple-helical molecules undergo a self-assembly
process. In general, there are 12 types of collagen found in the body and each
collagen comprises three polypeptide chains containing approximately 1,000 amino
acid units in each chain. Specifically, type I collagen (MW 139,000 Da) possesses
two identical α1(I) chains and one unique α2 chain; this configuration produces a
fairly rigid linear molecule that is 300 nm long [19]. These self-assembled triple
helix bundles have a periodicity of 67 nm, with 40 nm gaps between the ends of the
138
A. Bhowmick et al.