Crystallization-controlled pore retention in calcium-phosphate glassceramics from powder sintering of CaO–P2O5–B2O3–Al2O3–TiO2–ZrO2 glass

The formation of a porous structure plays a key role in the synthesis of calcium-phosphate biomaterials for bone implantology and endoprosthesis, since it determines both bioactivity and mechanical strength of the final material. The results of the present investigation demonstrate the feasibility of a crystallization-controlled design of material porosity, without pore-former addition, through a low-cost glass powder sintering process. The method takes advantage of the partial crystallization of glass with molar composition 45P2O5-50CaO-5Al2O3, with added 5B2O3, 5ZrO2, and 5TiO2, for achieving controlled pore retention and mechanical strength. The investigation - comprising differential thermal analysis, X-ray diffraction, scanning electron microscopy, viscosity, density, flexural and compressive strength measurements - gives a quantitative description of how the pore retention is driven by the system viscosity and by the formation of a reinforcing framework of precipitated crystals, the latter ones preventing the collapse into a vitrified non-porous material. The final porosity turns out to be describable by a modified Frenkel's model accounting for the crystallization constraints to the liquid flow. As a result, the present study demonstrates the possibility of obtaining calcium-phosphate glassceramics with 70% of crystal fraction, flexural strength 25 MPa, compressive strength 40 MPa, and a final porosity of 25% with pore sizes selectable from 10 to 180 μm from the starting grain size. Importantly, bioactivity tests show good bio-integration and pore filling with neogenic bone tissue and blood vessels, without toxicity, opening the way to possible applications in small-bone implantology. © 2013 Elsevier B.V.