Biomedical Engineering Reference
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composed of rod-like apatite crystals measuring 25-100 nm and
an undetermined length of 100 nm to 100 μm or longer along the
c-axis [189-191]. However, the apatite crystals in enamel were
found to exhibit regular sub-domains or subunits with distinct
chemical properties [192]. This subunit structure reflects an
assembly mechanism for such biological crystals [193, 194]. Like
that for bones (Fig. 1.11), seven levels of structural hierarchy have
been also discovered in human enamel; moreover, the analysis of the
enamel and bone hierarchical structures suggests similarities of the
scale distribution at each level [195]. In enamel, nano-sized crystals
of biological apatite at first form mineral nanodimensional fibrils;
the latter always align lengthways, aggregating into fibrils and
afterwards into thicker fibers; further, prism/interprism continua
are formed from the fibers. At the micro-scale, prisms are assembled
into prism bands, which present different arrangements across the
thickness of the enamel layer. These compositional and structural
characteristics endow enamel special properties such as anisotropic
elastic modulus, effective viscoelastic properties, much higher
fracture toughness and stress-strain relationships more similar to
metals than ceramics [196].
Dentin contains ~50% of biological apatite, ~30% of bioorganic
compounds, and ~20% of water. In dentin, the nanodimensional
building blocks (~25 nm width, ~4 nm thickness, and ~35 nm
length) of biological apatite are smaller than those of enamel. Dentin
is analogous to bone in many aspects, for example, it has a similar
composition and a hierarchical structure up to the level of the bone
lamellae [174, 175]. Additional details on the structure, properties,
and composition of teeth are available in section 1.4.2 of this topic,
while even more comprehensive information might be found in
special literature [197].
3.5
The Structure of the Nanodimensional and
Nanocrystalline Apatites
Due to the apatitic structure on natural calcified tissues, apatites
appear to be the best investigated compounds among the available
calcium orthophosphates (Table 1.1). Thus, nanodimensional and
nanocrystalline apatites have been extensively studied by various
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