Hydroxyapatite bond coats

Kurzweg, H., Heimann, R.B., Troczynski, T., (1998), Adhesion of thermally sprayed hydroxyapatite-bond-coat systems measured by a novel peel test , J. Mat. Sci. Med., 9(1), 9-16. 

Heimann, R.B. (1999) Design of novel plasma-sprayed hydroxyapatite-bond coat bioceramic systems. J. Therm. Spray Technol., 8 (4), 597-604. 

According to Rolla, ionic bonds are important in the associations between bacterial polysaccharides and protein-coated tooth surfaces (21). This was based on in vitro experiments on the afiinity of dextran for hydroxyapatite powder coated with salivary glycoprotein specifically, adsorption of dextran was inhibited by 0.5M. Prior treatment of the coated hydroxyapatite with neuraminidase also reduced adsorption of dextran. Neuraminidase would be expected to reduce the negative charge of the protein coat by removing ionized sialic acid moieties. Of course, reduced adsorption of dextran could result from conformational changes induced in the pellicle protein by the neuraminidase treatment, as was apparently effected by 4M or 8M urea, in other experiments. 

J., and Yakovin, S. (2014) Formation of solution-derived hydroxyapatite coatings on titanium alloy in the presence of magnetron-sputtered alumina bond coats. Open Bioeng. J., in press. 

Extensive work on titania bond coat/hydroxyapatite top coat duplex systems has been performed by one of the authors and his co-workers during the last two decades (Heimann and Vu, 1996, 1997 Vu and Heimann, 1997 Heimann,... 

Figure 6.19 (a) STEM image of a cross section of a plasma-sprayed titania (brookite) bond coat-hydroxyapatite topcoat assembly. On the substrate side, embedded alumina (corundum) particles are visible. The sample was produced by focused ion beam (FIB) cutting to conserve the true geometry of the coating interfaces. The black strip at the top surface consists of platinum deposited by decomposing an organic compound to... 

These challenges notwithstanding, several attempts have been made in the past to use zirconia as a bond coat material to improve the performance of biocon-ductive hydroxyapatite coatings for hip endoprostheses. The microstructures of zirconia bond coat/HAp coating cross-sections were studied by 2D-SIMS imaging to obtain information on plasma spray-induced diffusional processes at the coating interfaces as well as the spatial distribution of minor and trace elements across the coating cross-sections (Heimann et al., 1998). 

Figure 6.20 SEM image of an as-sprayed APS Ti6AI4V substrate (right) - zirconia bond coat (centre) - hydroxyapatite top coat (left) system (a) and 2D-SIMS images using an 0" ion beam at 12.5 kV, 20 nA, 250 pm2 raster area (b). In the elliptic SIMS images the substrate is at the bottom half and calcium...

Chou, B.Y. and Chang, E. (2002) Plasma-sprayed zirconia bond coat as an intermediate layer for hydroxyapatite coating on titanium substrate. /. Mater. Sci. Mater. Med., 13, 589-595. 

Lamy, D., Pierre, A.C., and Heimann, R.B. (1996) Hydroxyapatite coatings with a bond coat of biomedical implants by plasma projection. /. Mater. Res., 11 (3), 680-686. 

Figure 7.10 Raman spectra of as-sprayed hydroxyapatite coatings without a bond coat (a) and with a Ti02 bond coat (b). (A,) TTCP + p-TCP + ACP + OHAp, (A2) well-ordered HAp and (A3) p-TCP + OHAp.

Table 7.2 Quantitative composition in mass% of hydroxyapatite coatings without and with a Ti02 bond coat as-sprayed and after incubation in r-SBF for 1 and 12 weeks (based on Raman data) (Heimann, Tran and Hartmann, 2003).

A metal foil such as Ti6Al4V attached to a copper block by a 1 2 mixture of a silicone sealant (Dow Corning 732) and copper powder (ALCAN 154, grain size range < 44 pm) will act as a heat sink. The adhesive is cured at ambient temperature for 12 h. Then the foil is coated by plasma spraying, for example with either hydroxyapatite or a hydroxyapatite top coat/titania bond coat system (Heimann, 1999). The procedure is shown in Figure 7.27. 

The procedure to evaluate adhesion strength of plasma-sprayed hydroxyapatite coatings has been described earlier. The presence of a thin plasma-sprayed titania bond coat (see Chapter 6.3.2.2) considerably enhances the adhesion strength to a Ti6Al4V substrate of titania/HAp tandem coatings compared to HAp coatings without a bond coat. 

Figure 7.32 Peel adhesion strength of plasma-sprayed hydroxyapatite coating on a Ti6AI4V foil without (circles) and with a titania bond coat (crosses). The size of the symbols reflect the statistical error of the measurements. Extension of the regression...

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