Granulated wollastonite–hardystonite bioceramics

Introduction: Hardystonite (HS, Ca₂ZnSi₂O₇) is a promising compound for bone tissue restoration due to its biocompatibility and good mechanical properties. However, it has too low a rate of dissolution in biological fluids, according to data in the literature. In this work, we propose a method of composite production based on Ca₂ZnSi₂O₇ and CaSiO₃ mixtures to increase the dissolution of HS materials.

Methods: Granules were synthesized by the emulsion method using mechanical mixtures of powdered β-wollastonite and ZnO (0, 0.5, 1.5, 2.5, 5, 10, 12.5, 15, 20, 25 wt.%), with gelatin as a binder. All syntheses were carried out in 10 parallels. Granules were heat-treated in air at 1250°C and analyzed using XRD, FTIR, TGA and SEM. Their fractional composition, true and average density, open porosity and solubility in Tris-HCl buffer were determined.

Results and Discussion: Spherical granules with a diameter of 0.1÷3 mm were synthesized. During heat treatment, the binder burns out (200÷500°C) and hardystonite is formed as a result of solid-state interaction between ZnO and CaSiO₃ (above 750°C). According to XRD and FTIR, variation in ZnO content in the initial powder leads to the formation of α-CaSiO₃ mixed with up to 96 wt.% Ca₂ZnSi₂O₇. The granules had a density >1 g/cm³ and open porosity of 60±5%. Materials can gradually dissolve in Tris-HCl buffer. Concentrations of ions in solutions decrease in the order Ca²⁺ > SiO₃^2– > Zn²⁺ at all time points. The less HS content in granules, the more Ca²⁺ and SiO₃^2– quantities and less Zn²⁺ in the surrounding media. Granules with 96 wt.% HS lost less than 0.5 wt.% in 6 months. A 2-fold decrease in HS content leads to a 25-fold increase in weight loss.

Conclusions: Composites with varying content of Ca₂ZnSi₂O₇ and CaSiO₃ were produced. Mixing with wollastonite allowed for a significant increase in the dissolution rate of HS-containing materials.