Hydroxyapatite (HAp) is a calcium phosphate similar to the human hard tissues in morphology and composition¹. Particularly, it has a hexagonal structure^{2, 3} and a stoichiometric Ca/P ratio of 1.67, which is identical to bone apatite^{2, 4, 5}.

An important characteristic of hydroxyapatite is its stability when compared to other calcium phosphates. Thermodynamically, hydroxyapatite is the most stable calcium phosphate compound under physiological conditions as temperature, pH and composition of the body fluids².

With the development of nanotechnology, a major impact on materials science has been noticed. The production of nanomaterials has gained considerable attention for adsorption, catalysis and optical applications, particularly when biomaterials are involved⁶.

Nano-hydroxyapatite (nano-HAp) is attracting interest as a biomaterial for use in prosthetic applications due to its similarity in size, crystallography and chemical composition with human hard tissue. Bone and teeth enamel are largely composed of a form of this mineral. 

Due to its outstanding properties⁶:

 Biocompatibility
 Bioactivity
 Osteoconductivity
 Non toxicity and non inflammatory nature

The nano-hydroxyapatite bioceramic has got a variety of applications that include⁶:

 Bone tissue engineering
 Bone void fillers for orthopaedic, traumatology, spine, maxillofacial and dental surgery.
 Orthopedic and dental implant coating
 Restoration of periodontal defects
 Edentulous ridge augmentation
 Endodontic treatment like pulp capping
 Repair of mechanical furcation perforations and apical barrier formation
 Fillers for reinforcing restorative glass ionomer cement (GIC) and restorative composite resin
 Desensitizing agent in post teeth bleaching
 Remineralizing agent in toothpastes
 Early carious lesions treatment
 Drug and gene delivery

The literature review shows that the risk associated with an exposition to nanoparticulate calcium phosphate in doses that are usually applied in biomedicine, health care products and cosmetics is very low and most likely not present at all. Moreover, it also states that under all reasonable conditions, calcium phosphate nanoparticles can be considered as safe for humans⁷.

Titanium and stainless steel implants are often covered with hydroxyapatite coatings to trick the body and reduce the implant rejection rate. Hydroxyapatite can also be used in instances where there are bone voids or defects. This process is carried out through powders, blocks or beads of the material being placed in the affected areas of bone.

Due to its bioactivity, it encourages the bone to grow and restores the defect. This process can be an alternative to allogenic and xenogenic bone grafts. It typically results in healing times shorter than those observed if hydroxyapatite was not used. 

Enamel composition is 97 wt.% nano-hydroxyapatite and 3 wt.% organic material and water. In dentin, the nano-hydroxyapatite represents 70 wt.%⁸.

As Nano-hydroxyapatite is the main component of enamel, it gives an appearance of bright white and eliminates the diffuse reflectivity of light by closing the small pores of the enamel surface. 

Synthetic nano-hydroxyapatite mimics the size of natural dentinal hydroxyapatite or enamel apatite.  Experimental results demonstrate the advantages of nano-hydroxyapatite in enamel repair^9-10, which has led to its incorporation in toothpastes and mouth-rinsing solutions to promote the restoration of demineralized enamel or dentin surfaces by depositing hydroxyapatite nanoparticles in the defects¹¹.

Experimental nanostructured composite air filters containing hydroxyapatite were found to be efficient in absorbing and decomposing CO, which could eventually lead to its use in reducing automotive exhaust pollutants¹².

In 2014, an alginate/nano-hydroxyapatite composite was synthesized and field-tested as an adsorbent for fluoride. This biocomposite removed fluoride through an ion-exchange mechanism and is both biocompatible and biodegradable¹³.

Recently, applications in catalysis^14-16 and protein separation¹⁷ were developed and successfully tested using nanostructured calcium phosphates, which suggests that many innovative applications for these materials are yet to come.