Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 9.4
Examples of different Ti coatings obtained with plasma spray technologies: (a) low-roughness APS
coating; (b) high-roughness APS coating; (c) high-porosity SPS coating; (d) sponge-like VPS coating.
that HA can be regarded as nonresorbable in a phys-
iological environment, as long as it remains crystal-
line: HA coatings with high degrees of crystallinity
show low dissolution rates, less resorption, and more
direct bone contact
[40
e
42]
. The amorphous phase,
when located on the outer surface of the coating,
stimulates the growth of osseous tissue, but its fast
dissolution can lead to implant instability when
associated with thick coatings
[43]
. A high degree of
crystallinity in sprayed coatings is thus desirable,
although the presence of small amounts of amor-
phous phase was recognized to promote bone growth.
Generally, plasma-sprayed HA coatings consist of
a mixture of crystalline and amorphous phases,
depending on the initial particle purity, crystallinity
and form, and on the plasma spray process parame-
ters. APS-HA coatings for joint prostheses usually
exhibit a crystallinity level between 65% and 75%
[29]
. Postdeposition heat treatments at 600
e
700
C
have been proposed in order to convert the non-HA
amorphous phase into crystalline HA, but these
processes also induce mechanical degradation of the
coating
[29,44,45]
. Only special hydrothermal
processes have been productively employed for
increasing the crystallinity of HA coating on dental
implants up to 95%
[46,47]
. VPS or LLPS processes
reduce the content of amorphous phase because of
the higher particle velocity and the shorter dwell time
of the particle at high temperature before impacting
the substrate where rapid solidification starts. By
carefully selecting morphology and size of particles,
crystallinity levels around 90% can be reached
[48,49]
. In other biomedical applications, a low
crystallinity may be required. This is the case, for
example, of fixation screws: since these devices are
only temporarily inserted into the bone, the short-
term advantages of a rapid dissolution of the amor-
phous phase are preferred over the long-term stability
of the crystalline phase, which is kept below 20%
[50
e
52]
.
A high roughness is important to achieve a stable
long-term osteointegration of the prostheses. Because
of the brittle nature of HA, the coating thickness must
be kept under certain levels, thus limiting the coating
roughness as well. To reach this goal, double Ti-HA
coatings are used. First, thick Ti coating improves the
overall adhesion to the substrate, forming the rough
outer contour and, if wished, even a three-dimensional
open porosity. Second, a thin HA coating creates the
bioactive layer that accelerates the osteointegration.
Figure 9.5
shows two examples of possible double
layer coatings.
