Synthesis and processing of hydroxyapatite ceramic tapes with controlled porosity

Abstract

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) ceramic sheets with a wide range of porosities (up to 62%) have been prepared. The process is based on the reaction between dicalcium phosphate (CaHPO₄) and calcium carbonate (CaCO₃). When mixed with the appropriate Ca/P ratio, this proves to be a reliable new method for obtaining hydroxyapatite. Moreover, CaCO₃ serves as a gas-forming agent (due to the evolution of carbon dioxide and water during the reaction), which leads to the development of highly porous microstructures. Alternatively, CaHPO₄ and CaCO₃ can be reacted by calcining at 1000°C to produce pure hydroxyapatite powders. When processed in a similar way, a dense ceramic results. By mixing 50 vol% of CaCO₃ and CaHPO₄ with precalcined powders, hydroxyapatite with an intermediate porosity was obtained. Moreover, it should be possible to achieve porosity control by mixing different amounts of uncalcined and precalcined powders. All of these powders are colloidally processed using tape casting to produce thin sheets 150–200 μm thick. This technique can be used to make laminates, with or without porosity gradients, up to several millimetres thick.