Biomedical Engineering Reference
In-Depth Information
The smaller amount of other phases, the higher degree of crystallinity, and the lower the
dissolution rate, the better the performance of the implants, and vice versa. Thereby, from
the viewpoint of phase purity, the sol-gel method is preferred in depositing the expected
HA coatings on metallic substrates.
X-ray diffraction (XRD) is a powerful, nondestructive analysis tool and has been widely
used as a means of determining the phase composition and crystallinity of the HA coatings.
That is, it estimates the percentage of crystallinity and identifies the secondary phases gen-
erated during the preparation of HA coatings from the relative peak intensity of different
phases (Löbmann 2007). The new phases can be identified from the plot of the intensity vs.
2
θ
, or from the lattice parameter change determined by XRD results. Also, other analytical
techniques, such as differential thermal analysis (DTA)/differential scanning calorimetry
(DSC), Fourier transform infrared spectroscopy (FTIR), can help in this analysis.
Surface Chemistry and Composition
The surface chemistry of sol-gel derived HA coatings, as one of the most significant fac-
tors for the coating in a clinical application, has received considerable attention because of
its dominant role in osseointegration (Hench and Wilson 1993; Ka
č
iulis et al. 1998). X-ray
photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA)
is the most famous technique for the analysis of surface chemistry, since it can detect all
elements except hydrogen and helium with high sensitivity even for the trace amount of
contaminants in the materials (Briggs and 2003; Brundle, Evans, and Wilson 1992). XPS is
an important analytical tool in the area of coatings/thin films, and it can provide useful
information, including compositions (elements), chemical states, and coating thickness.
Based on the survey and narrow scan, the possible elements can be accurately ascertained
since each element has its own characteristic binding energy. The chemical state (valence)
or bonding information of each element can be determined through the shift of corre-
sponding peak position (binding energy).
There are two main properties of concern regarding surface chemistry: the composi-
tion of the HA coating and the chemical status of the elements existing in the HA coating
(Ka
č
iulis et al. 1999). As for the composition of HA coatings, it is mainly the three elements
of Ca, P, and O and their concentration (commonly described as Ca/P and O/Ca molar or
weight ratio). The Ca/P and O/Ca peak ratios can help distinguish and quantify different
Ca/P phases in the mixtures. With respect to the stoichiometry of the HA coatings, the
Ca/P ratios for the sol-gel samples were in good agreement with the stoichiometric val-
ues compared with other coating techniques (Massaro et al. 2001; Metikos-Hukovi
c
et al.
2003). However, as Haddow, James, and Van Noort (1996) have pointed out, the Ca/P ratio
could have a great difference in response to different precursors selected. For example,
the Ca/P ratio was 1.46 for the inorganic-route precursors and 2.10 for the organic-route
precursors in the work of Gan and Pilliar (2004). All these changes on the Ca/P ratio can
be demonstrated by other analytical methods, such as XRD and FTIR. As for the chemical
states analysis, it is becoming more and more important in the biological development.
Each element has the “fingerprint” characteristics that can be determined by some ana-
lytical techniques (Ka
č
iulis et al. 1998). According to the analysis, some new phases can be
determined. For HA coatings, many studies have been done on this point. Generally, Ca2p
(or Ca2p
1/2
and Ca2p
3/2
), P2p, and O1s are the most studied for determining the chemical
properties of the coating. For example, in Figure 1.3, the O1s peaks of the HA coatings
deposited by different methods are presented (Massaro et al. 2001). As indicated, peak 1
is the characteristic O1s peak in HA structure with a binding energy of 531.4eV; peak 2 is
