  An experimental approach based on design of experiments, process maps and the
analysis of deposition first stages to improve the biocompatibility of
High-Velocity Oxygen Fuel (HVOF) hydroxyapatite (HAp) coatings is here
presented. A two-level design of three factors (23) was performed using the
stand-off distance (SOD), the fuel-oxygen ratio (F/O) and the powder feed rate
(PFR). The effect of these experimental factors on the first stages of the
coating formation was investigated to study the physical state of the particles
before and after impacting the substrate. This study allowed the selection of
the most suitable deposition parameter combinations to obtain HAp coatings with
optimal crystallinity (> 45%), Ca/P ratio (approx. 1.67), phase content (> 95%
of HAp), which guarantee the coatings mechanical stability and
biocompatibility. The behavior of the coating within simulated body fluid (SBF)
and cell culture (hFOB) was studied to analyze the apatite layer formation and
the extracts cytotoxicity on human osteoblasts, respectively. The results show
that the F/O ratio is the most influential factor on temperature and velocity
on the in-flight particles and therefore on the coating properties. The SBF
results confirmed the formation of an apatite layer after 14 days of immersion.
Finally, the mitochondrial activity, measured by the MTS assay, and cell
membrane integrity measures by LDH liberation assays, show that the coating
released material does not induce toxicity on the exposed cells.
