Influence of preparation conditions on the microstructure and bioactivity of alpha-CaSiO(3) ceramics: formation of hydroxyapatite in simulated body fluid.

Two different reagents, NaOH and NH(4)OH, were used to precipitate CaSiO(3) precursor powders from ethanol solutions of Ca(NO(3))(2). 4H(2)O and Si(OC(2)H(5))(4). The resultant powders of different Ca/Si ratio and residual Na(2)O content exhibited significant differences in the microtexture of the resulting sintered alpha-CaSiO(3) ceramics. The microtexture of the ceramics from the NaOH system (CS-Na) contained smaller grain sizes and a thicker glassy phase at the grain boundaries than those produced using NH(4)OH (CS-NH). The CS-Na ceramics were soaked in a simulated body fluid (SBF) at 36.5 degrees C for 2 h and 1, 5, 6, 10, 21, and 30 days while the CS-NH ceramics were soaked for 1, 5, 7, 15, 20, and 25 days using the same conditions. Hydroxyapatite (HAp) formed on the surfaces of both samples but at different formation rates due to differences in the microstructure. The CS-Na ceramics showed faster HAp formation because their smaller alpha-CaSiO(3) grains dissolved more readily, allowing the calcium concentration in the SBF quickly to approach the appropriate condition for nucleation of HAp. In addition, the thicker glassy phase at the grain boundaries facilitated a faster formation of silanol on the surface of the amorphous SiO(2) interlayer, a reaction that is considered to be a prerequisite for HAp formation. The formation of the HAp layer on the CS-Na ceramics therefore was very fast (12 microm/day), and their surfaces were covered completely within 5 days. A layer thickness of about 110 microm was achieved in 30 days, in contrast with the CS-NH ceramics, which took about 25 days to be fully covered with a 60-microm layer of HAp. Copyright 2000 John Wiley & Sons, Inc.