Effect of crystallisation of a glass tested in vitro for its bioactivity

The in vitro bioactivity was studied of glassy and crystallised RKKP of molar composition: SiO2=46(.)091; CaO=35(.)967, MgO=4(.)372; CaF2=3(.)997, Na2O=4(.)637, K2O=0(.)127, P2O5=4(.)570, La2O3=0(.)097, Ta2O5=0(.)142. Crystallisation was performed with two different temperature schedules leading to very different microstructures: W non-isothermal (10 degrees C/min to 105000 heat treatment (sample RKKP-NIT) leading to glass-in-glass phase separation and very fine microstructure consisting of fluoroapatite, wollastonite and a residual glassy phase, and (ii) isothermal crystallisation in the temperature range between the solidus and liquidus temperatures, 2 h at T=1250T, (sample RKKP-IT) leading to a rough microstructure consisting of fluoroapatite crystals embedded in a non-demixed glassy phase. Bioactivity was studied in vitro by monitoring the surface structural and compositional changes after soaking in simulated body fluid (SBF). The formation of a phosphate layer crystallised to hydroxyapatite was observed in the case of glassy RKKP and RKKP-IT, with faster kinetics in this last one. In the case of RKKP-NIT, development of a rough surface was observed due to slow dissolution of the separated silicate glassy phase. The different behaviour was linked to compositional changes of the phases formed during the crystallisation process.