Geoscience Reference
In-Depth Information
and titanium carbide (TiC). Some methods are reported to be available to produce
thin films of HAp. HAp coatings on Ti plates have been reported to yield by elec-
trophoretic deposition.
One of the most important clinical applications of HAp is as a coating on metal
implants, such as hip joint prostheses. This concept combines the mechanical advan-
tages of metal alloys with an excellent biocompatibility and bioactivity of HAp.
Uncoated metal implants do not integrate with bone because bioinert materials are
encapsulated by dense fibrous tissues which prevent proper distribution of stresses
and thereby cause loosening of the implant. In the case of HAp-coated metal, bone
tissue integrates itself completely with the implant, even during early functional load-
ing
[265]
.
HAp coatings provide stable fixation of the implant to bone and minimize
adverse reaction by provision of a biocompatible phase. Moreover, the HAp coat-
ings decrease the release of metal ions from the implant to the body and shield the
metal surface from environmental attack. In the case of porous metal implants, the
HAp coating enhances bone ingrowth into pores. HAp coatings have been applied
to metals like Ti alloys, or Ca
Mo alloy, to carbon implants, to sintered cera-
mics like ZrO
2
and Al
2
O
3
, and even to polymers (Pmma)
[265]
. There are various
methods to fabricate HAp coatings. The common ones are: HIP, spray painting,
oxyfuel combustion spraying magnetron sputtering, flame spraying, ion-beam
deposition, chemical deposition under hydrothermal conditions, electrochemical
deposition, metal-organic CVD, and sol
gel. The coating under hydrothermal con-
ditions has been carried out effectively by many workers and all these references
are listed by Suchanek and Yoshimura
[265]
.
The thickness of the HAp coatings is usually in the range of 40
Cr
m. With
increasing thickness of coating, concentration of metal ions released to the body
decreases; the coatings must be thick enough to resist resorbability of HAp, which
can be as much as 15
200
μ
m per year. Fixation to the bone can be improved if the
HAp coating has an appropriate porosity, which promotes bone intergrowth. It has
been found that the interface between HAp and the metal, ceramics, or polymers
often contain several undesired phases, like phosphides and amorphous phosphates,
which decrease chemical stability and enhance degradation of the coatings
[321]
.
Fujishiro et al.
[321]
have carried out a detailed study of the coating of HAp on
metal plates using thermal dissociation of calcium-EDTA chelate in phosphate
solutions under hydrothermal conditions.
Figure 10.64a and b
illustrates schemati-
cally the mechanism of HAp deposition on iron and titanium plates. The mecha-
nism of deposition, according to the authors, is different in both the cases. Fujishiro
et al.
[293]
have studied the thermodynamics of the homogeneous precipitation of
HAp in Ca(EDTA)
2
2
30
μ
NaH
2
PO
4
solution.
Figure 10.65
shows SEM photographs
of HAp films formed on the surface of iron plates in various concentrations of Ca
(EDTA)
2
2
5 and 150
C for 4 h
[321]
. The direct coat-
ing of HAp and double coating of HAp on titanium plates have been investigated
with the varying pH of the NaH
2
PO
4
solutions. Depending upon the pH and tem-
perature of deposition, different morphologies of the HAp and monetite particles
were obtained.
NaH
2
PO
4
solutions at pH
5
