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 45P2O5–50СаО–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 Frenkelʹs 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.