Bio-inspired tailored hydroxyapatite-based powder composites for dental applications

This study includes two interconnected parts. The first part describes the characterization of the natural materials including the teeth of predator and preys' aspects and Arapaima gigas scale. Based on the first part, several special features presented in the natural materials were investigated. In the second part, the special procedures of the fabrication of tailored hydroxyapatite (HAP) based composites by the novel consolidation technique of spark plasma sintering are developed. These procedures are bio-inspired by the findings of the first part of the study. A natural tooth is composed of the external layer, hard enamel which has high degree of mineralization and the internal region - tough dentin which has collagen fibrils and carbonated apatite mineral. The main feature in the dentin is the tubule structure. Arapaima gigas scales serve as armor- like materials to protect Arapaima gigas from being bitten by piranha. The scales have a laminate structure with the hard external layer and soft internal layers. Besides the hydroxyapatite mineral, the main building block of the scale is collagen fibers. The structure of teeth and scale were characterized using scanning electron microscopy and optical microscopy. The chemical compound was analyzed by X-ray diffraction, Fourier transform infrared spectroscopy and Energy-dispersive X-ray spectroscopy. Mechanical tests (microindentation, nanoindentation, tensile and compression tests) were performed to investigate the mechanical properties of those biological materials. The penetration test of piranha teeth on the arapaima scale was conducted to prove that the scale is stronger than the piranha tooth. In order to fabricate artificial dental materials inspired by the investigated structure of the natural teeth, spark plasma sintering involving high heating rates, shorter sintering times and low sintering temperatures was utilized to fabricate hydroxyapatite composites. Hydroxyapatite is widely used in biomedical applications, but its low mechanical strength limits its applications in heavy loaded implants. Carbon nanotube (CNT) - HAP composites were fabricated by spark plasma sintering and indicated that the addition of CNT can improve the mechanical strength of pure HAP. Unaxial freeze drying process was utilized to create a micro channel structure in the hydroxyapatite components. To maintain the porous structure of the tailored hydroxyapatite green specimens, a newly developed sintering process, Free Pressureless Spark Plasma Sintering(FPSPS), was used to consolidate the tailored hydroxyapatite without applying pressure. The comparison of the outcome of the treatment of hydroxyapatite components by conventional sintering and free pressureless spark plasma sintering was conducted showing that FPSPS can provide a better approach to consolidate the material with higher micro hardnes