[14] These calcium-deficient phases can be prepared by precipitation from a mixture of calcium nitrate and diammonium phosphate with the desired Ca/P ratio, for example, to make a sample with a Ca/P ratio of 1.6:[15] Sintering these non-stoichiometric phases forms a solid phase which is an intimate mixture of tricalcium phosphate and hydroxyapatite, termed biphasic calcium phosphate:[16] Hydroxyapatite is present in bones and teeth; bone is made primarily of HA crystals interspersed in a collagen matrix—65 to 70% of the mass of bone is HA.

[23] The clubbing appendages of the Odontodactylus scyllarus (peacock mantis shrimp) are made of an extremely dense form of the mineral which has a higher specific strength; this has led to its investigation for potential synthesis and engineering use.

[26] The movement of this fluid as a result of different stimuli is said to excite receptor cells in the pulp and trigger sensations of pain.

[26] The physical properties of the nano-hydroxyapatite can penetrate and seal the tubules, stopping the circulation of the fluid and therefore the sensations of pain from stimuli.

[30] Hydroxylapatite has shown significant medium and long-term desensitizing effects on dentine hypersensitivity using evaporative stimuli and the visual analogue scale (alongside potassium nitrate, arginine, glutaraldehyde with hydroxyethyl methacrylate, hydroxyapatite, adhesive systems, glass ionomer cements and laser).

[28] In the event of an acid attack by cariogenic bacteria, nano-hydroxyapatite particles can infiltrate pores on the tooth surface to form a protective layer.

[27] Furthermore, nano-hydroxyapatite may have the capacity to reverse damage from carious assaults by either directly replacing deteriorated surface minerals or acting as a binding agent for lost ions.

[32] In addition to remineralisation, in vitro studies have shown that toothpastes containing nano-hydroxyapatite have the potential to reduce biofilm formation on both tooth enamel and resin-based composite surfaces.

[27] It may be used to restore cleft lips and palates and refine existing practices such as preservation of alveolar bone after extraction for better implant placement.

[27] The European Commission's Scientific Committee on Consumer Safety (SCCS) issued an official opinion in 2021, where it considered whether the nanomaterial hydroxyapatite was safe when used in leave-on and rinse-off dermal and oral cosmetic products, taking into account reasonably foreseeable exposure conditions.

It stated:[37] Having considered the data provided, and other relevant information available in scientific literature, the SCCS cannot conclude on the safety of the hydroxyapatite composed of rod–shaped nanoparticles for use in oral-care cosmetic products at the maximum concentrations and specifications given in this Opinion.

This is because the available data/information is not sufficient to exclude concerns over the genotoxic potential of HAP-nano.The European Commission's Scientific Committee on Consumer Safety (SCCS) reissued an updated opinion in 2023, where it cleared rod-shaped nano hydroxyapatite of concerns regarding genotoxicity, allowing consumer products to contain concentrations of nano hydroxyapatite as high as 10% for toothpastes and 0.465% for mouthwashes.

It stated:[38] Based on the data provided, the SCCS considers hydroxyapatite (nano) safe when used at concentrations up to 10% in toothpaste, and up to 0.465% in mouthwash.

The ions present on the surface of hydroxyapatite make it an ideal candidate with unique selectivity, separation and purification of biomolecule mixtures.

The C-sites on the surface of the resin undergo metal affinity interactions with phosphate or carboxyl groups present on the biomolecules.

Generally, hydroxyapatite was used in the polishing step of monoclonal antibodies, isolation of endotoxin free plasmids, purification of enzymes and viral particles.

The mineral fractions of bone and teeth act as a reservoir of trace elements, including carbon, oxygen and strontium.

[43] Due to its high biocompatibility, bioactivity, osteoconductive and/or osteoinductive capacity, nontoxicity, nonimmunogenic properties, and noninflammatory behavior, hydroxyapatite is available and used as a bone filler and as coatings on prostheses.

The 3D composite scaffolds showed good cytocompatibility and osteogenic potential, which is specifically recommended in applications when faster mineralization is needed, such as osteoporosis treatment.