Synthesis, biological and environmental applications of hydroxyapatite and its composites with organic and inorganic coatings

The need for biomaterials is in ever-increasing demand for a plethora of applications nowadays. Hydroxyapatite (HAP) is a primary constituent of bones and teeth in vertebrates. HAP and organic/inorganic-coated-HAP composites are fabricated to mimic the natural properties and also endow it with special characteristics like high porosity, surface area, crystallinity, stability, mechanical properties and biocompatibility. These special features make them ideal biomaterial candidates to be employed in diverse biomedical applications such as surgical implants, bone grafts, drug delivery agents and environmental remediation to remove toxic dyes and heavy metals from wastewater. HAP and HAP-containing composites are synthesized by wet, dry and high temperature methods from phosphate and calcium precursors as well as from low-cost natural biowastes. The biowastes include vertebrate bones, egg shells and marine-shells whose usage is not only economical but also aids sustainability and provides value addition. The properties of the fabricated end-product depend on the type of the precursor used and the synthesis protocols employed. The review provides an insight into the various synthesis methods of HAP and HAP-containing composites. It also reviews the most recent developments in the environmental and biomedical applications.