Biomedical Engineering Reference
In-Depth Information
surrounding tissues. On the other hand, a rough implant surface appears to be favorable
for cell attachment and particularly suitable for primary implant stability as compared to a
smooth implant surface. HA coatings with controllable degradation rate are much desired
and can be realized by altering the phase composition, doping of other ions (F
−
, Mg
2+
, Zn
2+
,
etc.) (Cai et al. 2009; Wang, Sam et al. 2007; Miao et al. 2005), and controlling the residual
stress.
In Vitro Test in Acellular Simulated Body Fluid
Currently, Simulated Body Fluid (SBF), which was first developed by Kokubo et al. in 1991,
has been routinely used as an effective
in vitro
testing method to predict the
in vivo
bone
bioactivity of various biomaterials (Kokubo and Takadama 2006). The development of SBF
is based on the concept that the essential requirement for an artificial material to bond
to living bone is the formation of bonelike apatite on its surface when implanted into the
living body. The acellular SBF has similar inorganic ion concentrations to those of human
blood plasma (as shown in Table 1.6) and can reproduce the formation process of bone-
like apatite on biomaterials
in vitro
. Furthermore, this test can be used for the determi-
nation of the number of animals used during
in vivo
testing, as well as the duration of
animal experiments. For a material to be bioactive
in vivo
, it must possess the capability
to induce bonelike apatite formation on its surface in SBF. Many studies have been done
on HA-coated metallic implants in SBF, which verified the intrinsic bioactivity of HA to
be used as coatings (Gu, Khor, and Cheang 2003; Nagarajan, Raman, and Rajendran 2010;
Stoch et al. 2005). On the other hand, the response of HA coatings in SBF was observed to be
highly affected by coating phase composition, crystallinity, morphology, etc. (Ducheyne,
Radin, and King 1993; Khor et al. 2003), and the precipitation rate of bonelike apatite was
directly dependent on the Ca
2+
ion concentration in the SBF at the vicinity of the coat-
ing surface (Lee et al. 2005; Montenero et al. 2000). As mentioned above, besides the HA
phase, there also exists some other phases, including TCP, TTCP, and CaO, which possess
higher solubilities than that of the HA phase. Therefore, in SBF testing, the dissolution
of such impure phases will accelerate the precipitation rate of bonelike apatite through
significantly enhancing the Ca
2+
ion concentration at a localized area near to the implant
surface (Gu, Khor, and Cheang 2003; Khor et al. 2003). To some extent, this indicates that
the existence of impure phases in HA coating could improve the bioactivity of HA-coated
metallic implants.
However, actual body fluid contains not only the inorganic components, but also various
kinds of organic components (Table 1.6), and the organic components would exert notice-
able influence on the implants (Dorozhkin, Dorozhkina, and Epple 2003; Wang, Sam et al.
2007). Therefore, it is unwise to neglect the influences of organic components in
in vitro
tests. Jaou et al. (2000) and Balint et al. (2001) reported that although sugar and/or glucose
have a minor influence on crystallization of HA, they significantly inhibit the crystalli-
zation process of fluoridated apatite (FA). This effect was attributed to the formation of
nonstoichiometric apatite in the presence of sugar. The inhibition effects of carbohydrates
on the bone mineralization were also reported by other researchers (Pearce, Hancock, and
Gallagher 1984; Balint et al. 2001). Dorozhkin, Dorozhkina, and Epple (2003) concluded
that glucose exhibited negligible influence on crystallization of calcium phosphate based
on
in vitro
tests with the glucose-modified SBF solution. On effects of proteins, extensive
investigations have been done on CaP biomaterials, especially on HA (Luo and Andrade
1998; Xie, Riley, and Chittur 2001). It has been reported that plasma proteins would adsorb
immediately on the surface of HA after it was implanted
in vivo
, and the initial cellular
