Chemical and Morphological Changes of Vacuum-Plasma-Sprayed Hydroxyapatite Coatings during Immersion in Simulated Physiological Solutions

The purpose of this investigation was to study the behavior of vacuum-plasma-sprayed hydroxyapatite (VPS-HA) coatings in a defined simulated physiological environment that mimics the conditions that the material experiences after implantation in the human body. Commercially available and clinically used HA coatings on titanium alloy substrates were immersed in an inorganic simulated body fluid (SBF) with ion concentrations that were almost equal to those of human blood plasma and in fetal calf serum (FCS) for time periods of 1, 3, 7, 14, and 28 days. At each time interval, the VPS-HA coatings were analyzed using scanning electron microscopy, X-ray diffractometry, Fourier transform infrared spectroscopy, and inductively coupled plasma-atomic emission spectroscopy. The as-received VPS coatings consisted of HA as the main phase component; in addition, small concentrations of tricalciumphosphate (TCP) were determined to be present. During immersion in SBF, transformation of the peripheral area of the VPS coating to a regular spherical shape was observed. Concentration measurements of the immersion solution indicated that dissolution of the more-soluble TCP started within the first day. At the same time, precipitation of a carbonate-containing, marginally crystalline calcium phosphate with very small coherently scattering domains was observed. In FCS, the dissolution of calcium and phosphorus was observed; however, the precipitation of the new calcium phosphate layer was retarded by the presence of proteins, which indicates their important role in the ion-exchange mechanisms. In both solutions, the coating integrity was not adversely affected, which indicates the high stability of VPS-HA coatings in a simulated physiological environment.