Biomedical Engineering Reference
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Fig. 3 a Transmission electron micrographic images of mineral nanoplatelets extracted from
bovine bone indicates that mineral crystallites are platelet shaped. Needle-like crystals are not
discernable from TEM images [ 90 ]. b As opposed to TEM, atomic force microscopy can provide
information on the three-dimensional topography of mineral crystallites. Note that the orthogonal
axes do not scale proportionally, such that the height axis is enlarged to bring out crystal
thickness which is otherwise about one tenth of crystal length/width. (Courtesy of Dr. Steven
J. Eppell, Case Western Reserve University. See articles [ 58 , 149 ] for further information on
AFM-based assessment of crystallite morphology.)
Fig. 4 Illustration of
carbonate substitutions
in bone apatite
pure apatite, for example) due to high concentrations of impurities [ 156 ]. The
average dimensions of crystals are 9 9 6 9 2 nm (Fig. 3 )[ 149 ]. The increased
surface area due to nanoplatelet conformation increases non-stoichiometric sub-
stitution. TEM sections of platelets usually reveal 'needle' like crystals and early
literature misinterpreted such sections which concluded that mineral crystallites
are rod-shaped. A similar misinterpretation also emerged from earlier X-ray
diffraction studies which did not essentially apply form factors to X-ray diffraction
patterns to obtain the actual shape. The spatial averaging of platelet morphology
thus resulted in a rod-like perception. Most researchers investigate changes along
the longer or c-axis of the crystal, aligned along collagen fibrils, and sometimes
along the a-axis or the thickness, perpendicular to c-axis.
The most common substitution found in the bone lattice is of carbonate
ions. Bone mineral is initially formed as an amorphous calcium phosphate, which
with maturity transforms into a poorly crystalline form of carbonated apatite
[ 16 , 127 , 128 ]. The carbonate ion is distributed or substituted in three different
locations (Fig. 4 ). When a phosphate ion (PO 4 3- ) present in an apatite crystal is
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