New type of bioactive materials: Hydroxyapatite/α-wollastonite composites

A new type of bioactive materials, hydroxyapatite (HA)/α-wollastonite (α-Wol) composites, were prepared. The sintering behavior, phase evolution, and in vitro bioactivity of hydroxyapatite/α-wollastonite composites were examined. The properties of HA/α-Wol composites were quite different from those of HA ceramics or α-Wol ceramics. HA/α-Wol composites sintered 1300 °C for 2 h exhibited a dense microstructure consisting of grains in the range of 0.3–1.0 μm in diameter. During sintering, a complex phase evolution between HA and α-Wol was observed. At 1300 °C, the formation of the Si substituted HA and the additional α-tricalcium phosphate (TCP) were observed using Fourier transform infrared and x-ray diffraction analysis. Further heat treatment at 1350 °C transformed part of the HA and α-Wol into a new phase with the composition: Ca₁₂P₆Si₂O₃₁. The in vitro bioactivity of the HA/α-Wol composites with a weight ratio of 25:75 and 50:50 sintered at 1300 °C was better than that of α-Wol monophasic ceramics. This result revealed that the silica of α-Wol and the orthosilicate of Si substituted HA provided nucleation sites for the bonelike apatite layer. The phosphate present in the HA or α-TCP phases promoted the nucleation of a bonelike apatite layer on the surface of the composites. The dissolution rate of α-Wol phase in simulated body fluid was faster than α-TCP or HA phase. Therefore, HA/α-Wol composite is the bone replacement material of controllable bioactivity and degradation rate with relative content between HA and α-Wol phase.