Biomedical Engineering Reference
In-Depth Information
HAp
Ti
FIGURE 3.13
SEM image of the cross-section of a dense nano-HAp coating on a Ti substrate. (Reproduced by courtesy of
Journal of Biomedical Materials Research B: Applied Biomaterials .)
of 18 MPa was obtained, with a maximum of 47 MPa [103]. Figure 3.13 shows the dense
nano-HAp coating that was formed on the surface of a Ti substrate using electrophoretic
deposition.
In order to reduce the thermal expansion mismatch between the HAp coating and the
underlying metallic substrate, glass has been codeposited with HAp for improved bonding
strength [133,142]. Homogenous and crack-free HAp-glass composite coatings on acetabu-
lar cup implants were fabricated by electrophoretic deposition [142]. A composite coating
of calcium phosphate glass and HAp powder was used as an adhesive layer adjacent to
the substrate for improvement in the bonding strength of a multilayered apatite composite
coatings [133].
Recently, multiwalled carbon nanotubes (MWNTs) have been added to the suspension
and codeposited with HAp onto the substrate to form composite coatings [135]. The addi-
tion of carbon nanotubes not only substantially improved the microhardness and elastic
modulus of the coating but it also significantly increased the shear strength of depos-
ited layers. The MWNTs provided reinforcement to the deposit and helped prevent its
peeling.
Crack-free coatings as thick as 25 µm were produced using codeposited nano-HAp
(20 nm) and MWNTs on Ti6Al4V substrates [170]. The samples were heated for 2 h at the
unusually low heat treatment temperature of 600°C, which probably explains the relatively
low bond strength of 2.9 MPa. In a similar study, MWNT-reinforced nano-HAp (30-50 nm)
was electrophoretically deposited on Ti sheet at thicknesses of 10 to 20 μm and heated at
700°C for 2 h in a flowing argon [136]. A shear strength as high as 35 MPa was obtained.
The corrosion resistance of the coated metal also was said to be superior to that of the bare
metal. Coatings of CNT-reinforced HAp showed increased microhardness while retaining
adequate corrosion resistance and adhesive strength [134].
Hydroxyapatite-Polymer Composite Coatings
In 2005, the organic phase chitosan was introduced to HAp for the first time to produce
HAp/polymer composites at room temperature [172]. In this method, the chitosan was
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