Synthesis and characterization of pure and clinoptilolite doped hydroxyapatite compounds for bone tissue engineering applications

Calcium phosphate-based hydroxyapatite (HA) compound, the basic inorganic component of bone tissue, is a bioceramics widely used as scaffold and filling material in Bone Tissue Engineering, thanks to its high biocompatibility and biomimetic structure. Although HA has a great bioactive potential, the high crystal stability of synthetic HA limits its bioabsorbability in-vivo, which prolongs the healing process of tissues. Therefore, diverse studies are being carried out to enrich hydroxyapatite with different ionic additives or minerals to adjust its crystal properties and bioactivity. Recently, natural minerals composed of various biomimetic ions have been used as additives to HA. Synthetic and natural zeolites are frequently preferred in Bone Tissue Engineering due to their silicate-rich compositions that trigger angiogenesis and osteogenesis, porous morphology, high surface area and ionic exchange capacity. Clinoptilolite (CLP) is a biocompatible aluminasilicate-based zeolite and is frequently used for biomedical applications. In this study, 5%, 10% and 20% CLP-doped HA was synthesized by the sol-gel method and sintered at different temperatures (800°C, 950°C and 1100°C). The produced pure and CLP doped HA samples have been characterized by various morphological, chemical, mechanical and biological analyses. The sol gel method, among the hydroxyapatite synthesis methods provides the most successful results and is frequently preferred due to its repeatability and ease of monitoring. The biggest advantages of the synthesis are that the reagents are mixed at the molecular level, homogeneous particles are obtained, it can be carried out at low temperatures, does not require pressure, and high purity products are obtained. It is also possible to control properties such as particle size and particle shape in this process. Considering the morphological characterizations made with SEM and S-TEM, nano-sized rod-like biomimetic particles were obtained. Fine HA particles are adhered onto large CLP particles as thin layers. CLP-doped HA samples have a glassy and interconnected porous microstructure at 950°C and 1100°C. In the samples sintered at 800°C, it was observed that 5%, 10% and 20% CLP additives did not affect the crystalline structure much and the pure HA phase was preserved in all samples. FT-IR and XRD analyses results show that thermal stability decreases in doped samples as CLP concentration increases and tri-calcium phosphate compounds are formed in samples sintered at 950°C and 1100°C. Biocompatibility and cytotoxicity tests show that all CLP additives positively affect cell viability and the highest concentration of CLP additive causes the highest rate of cell viability. When the measured BET results of HA and CLP-doped HA were compared, it was seen that the CLP additive reduced the surface area and pore size. In addition, although the compressive strength of all samples was within the value range of trabecular bone and cortical bone compressive strength, it was determined that they had maximum mechanical strength at the highest CLP ratio sintered at the highest temperature.