Carbonated hydroxyapatite coatings were formed on seeded titanium substrates by hydrothermal crystallization in the presence of urea. Decomposition of urea to ammonia and carbon dioxide under hydrothermal conditions was found to have a dramatic effect on crystal morphology and composition of the coatings. Some of the carbon dioxide released by urea decomposition was converted to carbonate ions that substitute for a small fraction of the phosphate groups in the hydroxyapatite crystals. The added urea also suppressed growth of the crystallographic c-axis relative to a-axis, resulting in a decrease in thickness and an increase in density of the coatings. Repeated hydrothermal growth steps in the presence of urea resulted in dense, highly crystalline coatings with near perfect alignment of the crystallographic c-axis normal to the coating surface. The dense coatings can be doped with a variety of other ions, including magnesium, potassium, fluoride, and lanthanum, by addition of appropriate salts in the hydrothermal synthesis. Recent reports have shown that ions move preferentially along the c-axis of carbonated apatites at elevated temperature. The morphology of the coatings is therefore ideal for promoting ion transport through the coating to produce electrically polarized bioelectret coatings or ion conducting membranes for electrochemical devices.
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