Customized hydroxyapatites for bone-tissue engineering and drug delivery applications: a review

Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) remains to be the foremost choice in biomedical field right from repair/replacement for the damaged hard tissues to be acting as effective drug delivery agent for tissue healing. Though, HAP is similar in composition with the mineral component of bone, some issues such as lack of mechanical and antimicrobial properties, low degradation, lesser drug loading capability, lower stimuli responsiveness, and targeted deficiency have continuously posed major challenges. However, enactment of various physicochemical, biological, mechanical properties can be improved by articulating particles morphology, size, structure, porosity, synthesis technique, and ionic substitution into HAP structure. Unique structure of HAP permits various anionic and cationic substitutions. Among the available synthesis routes, hydrothermal and microwave-assisted techniques seem to be the most suitable techniques to synthesize HAP with close control over desirable properties. This review primarily focuses on highlighting the customization of desirable properties by controlling particles size, morphology, synthesis parameters, and substitution of mono/multi ions into HAP structure to obtain a product appropriate for bone-tissue engineering and drug delivery applications.