Biomedical Engineering Reference
In-Depth Information
20
30
40
2
θ
(deg)
50
60
FIGURE 1.2
Phase composition of sol-gel derived HA coatings on Ti6Al4V substrate after calcination at 900°C. (From
Montenero et al.,
Journal of Materials Science
, 35, 11, 2791-2797, 2000. With permission.)
derived HA coatings on Ti6Al4V after being calcined at 900
o
C by using Ca(NO
3
)
2
•4H
2
O
and (NH
4
)
2
HPO
4
as corresponding precursors (Montenero et al. 2000).
On the other hand, the phase purity and the degree of crystallinity of sol-gel derived
HA coatings is reported to depend on the kind of precursors used for the preparation of
HA coatings (Haddow, James, and Van Noort 1996). For instance, Gan and Pilliar (2004)
pointed out that the HA coating prepared with an organic route (precursors: calcium nitrate
tetrahydrate and triethyl phosphite) possesses higher crystallinity than that obtained with
an inorganic route (precursors: Ca(NO
3
)
2
•4H
2
O and ammonium dihydrogen phosphate).
Moreover, besides the selection of temperature, some other factors, including time of heat
treatment, heating rate, and surrounding atmosphere, are also quite important to control
the final phase composition and crystallinity (Chen et al. 1997; Wang, Chen, and Wang
2009; Hsieh, Perng, and Chin 2002). For instance, water molecules in the firing atmosphere
could promote HA crystallization, whereas in a dry atmosphere TCP and TTCP are more
stable than HA in a higher temperature (Chen et al. 1997). Comparatively, the phase com-
position of HA coatings produced by the sol-gel method is simpler than those coatings
obtained by using high temperature deposition methods, which usually contain a certain
amount of other impure phases, such as amorphous hydroxyapatite (ACP), oxyapatite, tri-
calcium phosphate (TCP), tetracalcium phosphate (TTCP), and calcium oxide (CaO) (Ong
et al. 2006; Yan, Leng, and Weng 2003; Lima et al. 2005).
No matter the impure phases, all have crucial effects on the performance of coated
implants, especially on the dissolution behavior of the coatings (Sun et al. 2001; You, Oh,
and Kim 2001; Yang, Kim, and Ong 2005). All of the others' phases in the coating have
larger solubility than that of HA (Ducheyne, Radin, and King 1993; Wang, Lu et al. 2007;
Khor et al. 2003). Although the faster dissolution produces a supersaturated environment,
which allows physiologically produced HA to precipitate on the coating and enhance the
bone ingrowth, it also leads to the serious resorption or degradation of the coatings, and
even to the failure of the implants (Cheng, Zhang, and Weng 2007; Kim, Kim, and Knowles
2005). On the other hand, the impure phase, CaO, has no biocompatibility and dissolves
significantly faster than TCP. Thus, it is a detrimental phase that should be avoided (Wang,
Chen et al. 2008; Sun et al. 2001). As such, both the purity and the crystallinity of the coat-
ing should be strictly controlled in order to obtain the expected effective HA coating layers.
