Chiang Mai Journal of Science

 In this study, gel retarding agents namely ammonium hydroxide and ammonium hydrogen carbonate were incorporated in the sol-gel system based on calcium nitrate tetrahydrate and phosphorus pentoxide aiming to extend the gelling time of the sol and improve the coating performance.  It was found that mixing ammonium hydroxide and ammonium hydrogen carbonate could increase the gelling time to 1 week and 1 month respectively compared to 24 hours for non-mixed sol and thereby increasing the storage stability of the sols.  The coating produced by dipping a titanium disc in the non-mixed sol was highly hygroscopic, non-uniform and showed high shrinkage whereas both ammonium hydroxide and ammonium hydrogen carbonate mixed sols produced low hygroscopic, uniform and non-shrank coatings.  All the sols were similarly converted to carbonated calcium deficient hydroxyapatite after sintering at 550 ^oC for 1 hour indicating that the use of gel retarding agents in the sol only enhanced the coating property and sol stability, but did not interfere with the formation of hydroxyapatite phase.  Tensile bonding test of coating using ammonium hydroxide and ammonium hydrogen carbonate mixed sols were 54 and 65 MPa respectively and decreased slightly to 40 and 41 MPa after soaking in simulated body fluid for 56 days.  These values were well above the recommendation of ISO standard for biomedical hydroxyapatite coating. Osteoblasts proliferation on the coated samples were observed to be comparable to the uncoated one at short culture times, but showed higher proliferation rate at longer times.