An opinion on hydroxyapatite based bio-composites as bone-scaffolds

Abstract

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) is regarded as one among most bioactive materials for bone and hard-tissue replacement due to its chemical and structural similarity as that of apatite with Ca/P ratio of 1.67. But, the use of HA is limited due to its poor fracture toughness to the order of 0.5-1.5 MPa.m1/2. Therefore, usually, some additives, such as Al2O3, YSZ, ZnO, Fe3O4, TiO2, Ti, Ag, carbon nanotubes (CNTs), Ti, etc have been incorporated. It is observed that the metallic reinforcement is a better toughening agent than the ceramic reinforcement, but the release of metal ions may also hamper the key metabolic pathways of human cell. Further,β-tricalcium phosphate (β-TCP) and bio glass addition can be used for attaining controlled resorption of material under in vivo conditions so the natural bone can replace the artificial scaffold during healing process. Many additives, such as Ag, ZnO, CuO, TiO2, etc have been incorporated to provide antibacterial efficacy to the scaffolds. The aspect of antioxidant activity obtained from aliovalent ceramics (such as CeO2) may also assist in expedited healing. The design of porosity at multi-length scales can also be envisaged as means of incorporating cell-material interaction at bulk scale (~150-250 µm size, for vascularisation), at micrometer length scale (~10s of µm for cellular alignment) and at molecular length scale (~ few nm for surface protein interaction with implant substrate). Hence, the onus is on interdisciplinary biomedical engineers to aspire and design multifunctional bone-scaffolds with required mechanical integrity, antibacterial efficacy, bioactive response, biosorption for accommodating natural healing, and inducting expedited restoration.