Important issues in developing hydroxyapatite (HAp)- and titanium (Ti)-based composite biomaterials for orthopedic or dental devices include the dissociation of HAp during fabrication and its influences in the microstructure and biocompatibility of the final composite. During the densification by sintering of HAp/Ti composites, Ti reacts with -OH freed from HAp to form TiO2 thus dissociated HAp into Ca3(PO4)2, CaO, CaTiO3, TiP, and so forth. To inhibit this reaction, composites were fabricated with Ti and 30, 50, and 70 vol % β-tricalcium phosphate (β-TCP) instead of HAp by spark plasma sintering at 1200°C. It has been observed that after sintering at 1200°C, Ti also reacted with TCP, but unlike HAp/Ti composites, the final TCP/Ti composites contained significant amounts of unreacted TCP and Ti phases. The initial 70 vol % TCP/Ti composite showed compressive strength of 388.5 MPa, Young's modulus of 3.23 GPa, and Vickers hardness of 361.9 HV after sintering. The in vitro cytotoxicity and proliferation of osteoblast cells on the composites surfaces showed that the addition of a higher amount of TCP with Ti was beneficial by increasing cell viability, cell-composite attachment and proliferation. Osteopontin and collagen type II protein expression from osteoblasts cultured onto the 70% TCP-Ti composite was also higher than other composites and pure Ti. In vivo study verified that within 3 months of implantation in an animal body, 70% TCP-Ti had an excellent bone-implant interface compared with a pure Ti metal implant.
Sintering of Ti-biphasic calcium phosphate (BCP) is difficult because of the chemical instability of the phases at high temperature. When the sintering temperature is above 1273 K, Ti reacts with BCP and forms CaO, TiO 2 , CaTiO 3 , TiP etc. Conventional vacuum sintering is common for Ti powder but for Ti-BCP composites, spark plasma sintering in an inert atmosphere is a quick method to overcome the issues associated with a prolonged reaction time. In this study, the effect of two different sintering processes on the sintering reactions and mechanical and biological properties of Ti-30 vol%BCP composites were investigated and compared. Detailed micro-structural and morphological analyses were conducted using scanning electron microscopy (SEM). Mechanical properties were characterized by relative density, Vickers hardness and compressive strength measurement. Phase characteristics were analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Cell viability and biocompatibility were investigated using the MTT assay and by examining cell morphology. In this study, the mechanical properties and biocompatibility for both, spark plasma sintered Ti-Ca-P composites were excellent compare to vacuum sintered composites.
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