Synthesis, properties, and applications of hydroxyapatite

Abstract

Hydroxyapatite (HA) has been extensively investigated and used in bone clinical application for more than four decades. The increasing interest in HA is due to its similar chemical composition to that of the inorganic component of natural bone. HA displays favourable properties such as bioactivity, biocompatibility, slow-degradation, osteoconduction, osteointegration, and osteoinduction. HA is commercially available either from a natural source or as synthetic HA. Various methods have been reported to prepare synthetic HA powders which include solid state chemistry and wet chemical methods. For bone applications, pure HA, biphasics with ß-tricalciumphosphate (ß-TCP) and HA composites have been widely investigated. HA is processed into dense bodies by sintering and sintering temperature, stoichiometry, phase purity, particle grain size, And porosity are important processing parameters. Furthermore porosity in particular pore size; macro and microporosity; pore interconnectivity; morphology; pore size distribution, and surface properties influence bone remodelling. At high sintering temperatures, HA is transformed primarily into ß-TCP which is amorphous and resorbable. Despite the success of HA derived implants one of the major drawbacks of this material is its poor tensile strength and fracture toughness compared to natural bone. This makes HA unsuitable for several load-bearing applications. HA has been reinforced with a number of fillers including polymers such as collagen, metals and inorganic materials such as carbon nanotubes, and HA has also been applied as coatings on metallic implants. To improve the biomimetic response of HA implants, nano-HA powder has been synthesised, and HA nanocomposites containing electrospun nanofibers, and nanoparticles have been produced. Nano-HA displays a large surface area to volume ratio and a structure similar to natural HA, which shows improved fracture toughness, improved sinterability, and enhanced densification. Biological entities such as bone morphogenic proteins (BMP s), stem cells, and other growth factors have also been incorporated into HA nanocomposites. HA implants have been applied in the form of dense and porous block implants, disks, granules, coating, pastes, and cements. Some of the frequent uses of HA include the repair of bone, bone augmentation, acting as space fillers in bone and teeth, and coating of implants. In this book chapter, we will focus on the synthesis and properties of HA powders and HA implants with specific application in bone engineering. We will also share our experience over the past 20 years in dental and craniofacial reconstruction.