“…Synthetic methods include co-precipitation [42], sol-gel, sonochemical, ultrasound microwave, calcination, alkaline hydrolysis, hydrothermal, or combined approaches. A vast number of natural sources have been explored as starting materials to extract HAp [43][44][45]. As opposed to the numerous synthetic routes, alkaline hydrolysis, hydrothermal treatment, and calcination are regarded as the only relevant techniques to extract HAp from a series of biological sources or wastes, such as mammalian bones (e.g., bovine, camel, and horse), marine or aquatic sources (e.g., fish bone and fish scale), shell materials (e.g., cockle, clam, eggshell, and seashell), plants, algae, and also from mineral sources (e.g., limestone) [46][47][48].…”