Biomedical Engineering Reference
In-Depth Information
parallel to the long axes of the collagen fibrils [181]. Type I collagen
molecules are self-assembled into fibrils with a periodicity of ~67
nm and ~40 nm gaps between the ends of their molecules, into
which the apatite nano-sized crystals are placed. A biocomposite of
these two constituents forms mineralized fibers. The fibers also may
be cross-linked, which provides a highly dynamic system capable
of modification through the selection of different amino acids to
allow for different mechanical properties for different biomaterial
applications [182]. This is why bone is usually termed a fiber-
reinforced composite of a biological origin, in which nanometer-
sized hard inclusions are embedded into a soft protein matrix [183].
Though dimensions of biological apatite crystals reported in the
literature vary due to different treatment methods and analytical
techniques, it is generally around the nanometric level with values
in the ranges of 30-50 nm (length), 15-30 nm (width), and 2-10
nm (thickness) [184]. Why does the nanometer scale appear to be
so important to bones? It was recently demonstrated that natural
biocomposites exhibit a generic mechanical structure in which the
nanometer sizes of mineral particles are used to ensure the optimum
strength and maximum tolerance of flaws [185, 186]. Furthermore,
nanodimensional apatite has another crucial function for organisms.
It is a huge reservoir of calcium and orthophosphate ions necessary
for a wide variety of metabolic functions, which offer or consume
calcium and orthophosphate ions through a so-called remodeling
process because of a continuous resorption and formation of
nanodimensional apatite by osteoclasts and osteoblasts, respectively,
in a delicate equilibrium [2, 5]. Additional details on the structure,
properties, and composition of bones are available in section 1.4.1
of this topic, while even more comprehensive information might be
found in special literature [5, 177, 187].
3.4.2
Teeth
Teeth are another normal calcium orthophosphate-based calcified
tissue of vertebrates. Unlike bone, teeth consist of at least two
different biominerals: enamel (a crown, the part above the gum
line) and dentin (root, the part below the gum line) [188]. Dental
enamel contains up to 98% of biological apatite, ~1% of bioorganic
compounds, and up to 2% of water. Typical rods in enamel are
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