Vapor transport sintering of calcium phosphate ceramics

In modern plastic and reconstructive surgery the application of biocompatible materials with adequate mechanical and chemical properties has become indispensable. For the treatment of skeletal defects, hydroxyapatite (HA) is well-established as a bone graft material. As its chemical composition is very similar to the mineralized part of human bone, this material offers excellent biocompatibility and direct mechanical bonding to the host tissue. The healing process can be further enhanced by mixing HA with the more soluble tricalcium phosphate (TCP) to adjust the implant’s resorption rate to that of new tissue formation. Moreover, to stimulate bone ingrowth, it is crucial to produce ceramic scaffolds with interconnected porosity and pore diameters of 100-500 µm. Various processing methods have been proposed for the fabrication of such scaffolds. However, due to both technical and economic considerations, few of them have been scaled up for industrial production. Our motivation was to explore vapor transport sintering (VTS) as a novel and potentially cost-effective process for the near-net shape manufacturing of custom-made implants. Starting from synthetic powder precursors, the microstructure development of HA and TCP ceramics, as well as biphasic mixtures thereof, was studied for systems with both halide acids and solid chloride agents. Chemical and phase analysis were performed to monitor potential substitutional reactions and phase conversions upon processing. Analyses were carried out with scanning electron microscopy (SEM) with integrated energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR) and X-ray fluorescence (XRF). Furthermore, for gathering a theoretical understanding of the vapor transport process, thermodynamic calculations were carried out for various systems. Among the foremost aims of the investigation was the identification of suitable halide reagents to enhance material transport through the vapor phase, as well as the validation whether large enough pore diameters could be generated by this processing method. As it turned out, pore coarsening stagnates after reaching diameters of about 5 µm. Thus, the combination of VTS with other processing techniques is necessary to create adequate macroporosity. The promising results obtained for sintering in AgCl atmosphere, suggested the combination of macroporous precursors with vapor transport sintering as a potential technique to create a novel composite material. For this purpose, coral skeletons and sea urchin spines were hydrothermally converted into calcium phosphate scaffolds. Subsequent AgCl-sintering not only stimulated enhanced material transport through the vapor phase, but also caused the condensation of fine Ag-bearing particles throughout the ceramic scaffold. The resulting surface modification lends antibacterial properties to the composite material which are expected to decrease post-surgical inflammation risks