Hydroxyapatite‐titanium bulk composites for bone tissue engineering applications

Abstract

The research work on bulk hydroxyapatite (HA)-based composites are driven by the need to develop biomaterials with better mechanical properties without compromising its bioactivity and biocompatibility properties. Despite several years of research, the mechanical properties of the HA-based composites still need to be enhanced to match the properties of natural cortical bone. In this regard, the scope of this review on the HA-based bulk biomaterials is limited to the processing and the mechanical as well as biocompatibility properties for bone tissue engineering applications of a model system that is hydroxyapatite-titanium (HA-Ti) bulk composites. It will be discussed in this review how HA-Ti based bulk composites can be processed to have better fracture toughness and strength without compromising biocompatibility. The advantages of the functionally gradient materials to integrate the mechanical and biocompatibility properties is a promising approach in hard tissue engineering and has been emphasized here in reference to the limited literature reports. On the biomaterials fabrication aspect, the recent results are discussed to demonstrate that advanced manufacturing techniques, like spark plasma sintering can be adopted as a processing route to restrict the sintering reactions, while enhancing the mechanical properties. Various toughening mechanisms related to careful tailoring of microstructure are discussed. The in vitro cytocompatibilty, cell fate processes as well as in vivo biocompatibility results are also reviewed and the use of flow cytometry to quantify in vitro cell fate processes is being emphasized.

Research Progress Regarding Interfacial Characteristics and the Strengthening Mechanisms of Titanium Alloy/Hydroxyapatite Composites

“…The remaining tough Titanium phases consumed more energy during fracture process due to plastic stretching effect, and thus mechanical properties were improved. Therefore, adjusting the composition of composites can enhance fracture toughness due to mixed shielding mechanism [74]. Besides, nano-carbon materials can enhance mechanical properties [32,75,76,77].…”

“…Additional energy of crack tip distributed in the fragile second-phase solid was found to dissipate in the shear experiment. Therefore, this reduced the driving force for crack propagation, enhanced the ability of resisting crack propagation, and thus increased fracture toughness of Ti-6Al-4V/HA composites [74]. Besides, crack propagation distributed in a ductile second phase, such as Titanium, was hindered by forcing cracks to pass or change direction, and thus fracture toughness of material was enhanced [20].…”

“…In addition, the in vivo bioactivity and micro-mechanical properties at the interface between biomaterials and bone tissue were also analyzed [74,94,95,96]. Some researchers found that more new bone appears between the surface of Ti-HA composite with higher content of Titanium and the bone tissue during the initial stage after transplantation.…”

“…After being transplanted for 6 months, all of the Ti-HA composites formed a chemical bone-bonding interface with the host bone through apatite layer [94]. However, some researches mentioned that the presence of titanium will reduce the osseointegration of HA-20% Ti composite implanted in the skull of New Zealand white rabbits at the initial stage of transplantation [74]. In fact, it was revealed that the reaction products of HA in HA-Ti composites dissolve faster than HA and can produce more Ca 2+ and PO 4 3− , which will lead to the faster healing at the defectsite after transplantation.…”

“…In fact, it was revealed that the reaction products of HA in HA-Ti composites dissolve faster than HA and can produce more Ca 2+ and PO 4 3− , which will lead to the faster healing at the defectsite after transplantation. Moreover, the presence of HA and its products contributed to the formation of apatite layer between implant and bone tissue, and the presence of TiO 2 on the surface of Titanium also provided more nucleation sites for the formation of apatite, which is beneficial to the increase of bonding strength between implant and bone tissues [74]. In addition, Alzubaydi et al studied the biomechanical properties of HA, Partially Stabilized Zirconia (PSZ), and 50% HA + 50% PSZ coating on Titanium alloy surface [95].…”