Hydroxyapatite
Hydroxyapatite | ||
---|---|---|
Specific gravity 3.14–3.21 (measured), 3.16 (calculated) | | |
Optical properties | Uniaxial (−) | |
Refractive index | nω = 1.651 nε = 1.644 | |
Birefringence | δ = 0.007 | |
References | [2][3][4] |
Hydroxyapatite (
Up to 50% by volume and 70% by weight of
Chemical synthesis
Hydroxyapatite can be synthesized via several methods, such as wet chemical deposition, biomimetic deposition,
10 Ca(OH)2 + 6 H3PO4 → Ca10(PO4)6(OH)2 + 18 H2O
The ability to synthetically replicate hydroxyapatite has invaluable clinical implications, especially in dentistry. Each technique yields hydroxyapatite crystals of varied characteristics, such as size and shape.[12] These variations have a marked effect on the biological and mechanical properties of the compound, and therefore these hydroxyapatite products have different clinical uses.[13]
Calcium-deficient hydroxyapatite
Calcium-deficient (non-stochiometric) hydroxyapatite, Ca10−x(PO4)6−x(HPO4)x(OH)2−x (where x is between 0 and 1) has a Ca/P ratio between 1.67 and 1.5. The Ca/P ratio is often used in the discussion of calcium phosphate phases.[14] Stoichiometric apatite Ca10(PO4)6(OH)2 has a Ca/P ratio of 10:6 normally expressed as 1.67. The non-stoichiometric phases have the hydroxyapatite structure with cation vacancies (Ca2+) and anion (OH−) vacancies. The sites occupied solely by phosphate anions in stoichiometric hydroxyapatite, are occupied by phosphate or hydrogen phosphate, HPO2−4, anions.[14] Preparation of 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]
- 9.6 Ca(NO3)2 + 6 (NH4)2HPO4 → Ca9.6(PO4)5.6(HPO4)0.4(OH)1.6
Sintering these non-stoichiometric phases forms a solid phase which is an intimate mixture of tricalcium phosphate and hydroxyapatite, termed biphasic calcium phosphate:[16]
- Ca10−x(PO4)6−x(HPO4)x(OH)2−x → (1 − x) Ca10(PO4)6(OH)2 + 3x Ca3(PO4)2
Biological function
Mammals and humans
Hydroxylapatite is present in
Hydroxylapatite deposits in tendons around joints results in the medical condition calcific tendinitis.[18]
Hydroxylapatite is a constituent of calcium phosphate kidney stones.[19]
Remineralisation of tooth enamel
Remineralisation of tooth enamel involves the reintroduction of mineral ions into demineralised enamel.[20] Hydroxyapatite is the main mineral component of enamel in teeth.[21] During demineralisation, calcium and phosphorus ions are drawn out from the hydroxyapatite. The mineral ions introduced during remineralisation restore the structure of the hydroxyapatite crystals.[21] If fluoride ions are present during the remineralisation, through water fluoridation or the use of fluoride-containing toothpaste, the stronger and more acid-resistant fluorapatite crystals are formed instead of the hydroxyapatite crystals.[22]
Mantis shrimp
The clubbing appendages of the
Use in dentistry
As of 2019[update], the use of hydroxyapatite, or its synthetically manufactured form, nano-hydroxyapatite, is not yet common practice. Some studies suggest it is useful in counteracting dentine hypersensitivity, preventing sensitivity after teeth bleaching procedures and caries prevention.[25][26][27] Avian eggshell hydroxyapatite can be a viable filler material in bone regeneration procedures in oral surgery.[28]
Dentine sensitivity
Nano-hydroxyapatite possesses bioactive components which can prompt the mineralisation process of teeth, remedying hypersensitivity. Hypersensitivity of teeth is thought to be regulated by fluid within dentinal tubules.[25] 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.[25] 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.[26] Nano-hydroxyapatite would be preferred as it parallels the natural process of surface remineralisation.[27]
In comparison to alternative treatments for dentine hypersensitivity relief, nano-hydroxyapatite containing treatment has been shown to perform better clinically. Nano-hydroxyapatite was proven to be better than other treatments at reducing sensitivity against evaporative stimuli, such as an air blast, and tactile stimuli, such as tapping the tooth with a dental instrument. However, no difference was seen between nano-hydroxyapatite and other treatments for cold stimuli.[29] 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).[30]
Co-agent for bleaching
Teeth bleaching agents release reactive oxygen species which can degrade enamel.[26] To prevent this, nano-hydroxyapatite can be added to the bleaching solution to reduce the impact of the bleaching agent by blocking pores within the enamel.[26] This reduces sensitivity after the bleaching process.[27]
Caries prevention
Nano-hydroxyapatite possesses a remineralising effect on teeth and can be used to prevent damage from carious attacks.[27] 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.[26] 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.[26]
In some toothpaste hydroxyapatite can be found in the form of nanocrystals (as these are easily dissolved). In recent years, hydroxyapatite nanocrystals (nHA) have been used in toothpaste to combat dental hypersensitivity. They aid in the repair and remineralisation of the
As a dental material
Hydroxyapatite is widely used within dentistry and oral and maxillofacial surgery, due to its chemical similarity to hard tissue.[33]
In the future, there are possibilities for using nano-hydroxyapatite for tissue engineering and repair. The main and most advantageous feature of nano-hydroxyapatite is its biocompatibility.[34] It is chemically similar to naturally occurring hydroxyapatite and can mimic the structure and biological function of the structures found in the resident extracellular matrix.[35] Therefore, it can be used as a scaffold for engineering tissues such as bone and cementum.[26] 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.[26]
Safety concerns
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:[36]
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. However it warns of needle-shaped nano hydroxyapatite and of inhalation in spray products. It stated:[37]
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. This safety evaluation only applies to the hydroxyapatite (nano) with the following characteristics:
– composed of rod-shaped particles of which at least 95.8% (in particle number) have an aspect ratio of less than 3, and the remaining 4.2% have an aspect ratio not exceeding 4.9;
– the particles are not coated or surface modified.
Chromatography
Along with its medical applications, hydroxyapatite is also used in downstream applications under mixed-mode chromatography in polishing step. The ions present on the surface of hydroxyapatite make it an ideal candidate with unique selectivity, separation and purification of biomolecule mixtures. In mixed-mode chromatography, hydroxyapatite is used as the stationary phase in chromatography columns.
The combined presence of calcium ions (C- sites) and phosphate sites (P-sites) provide metal affinity and ion exchange properties respectively. The C-sites on the surface of the resin undergo metal affinity interactions with phosphate or carboxyl groups present on the biomolecules. Concurrently, these positively charged C-sites tend to repel positively charged functional groups (e.g., amino groups) on biomolecules. P-sites undergo cationic exchange with positively charged functional groups on biomolecules. They exhibit electrostatic repulsion with negatively charged functional groups on biomolecules. For the elution of molecules buffer with high concentration of phosphate and sodium chloride is used. The nature of different charged ions on the surface of hydroxyapatite provides the framework for unique selectivity and binding of biomolecules, facilitating robust separation of biomolecules.
Hydroxyapatite is available in different forms and in different sizes for the purpose of protein purification. The advantages of hydroxyapatite media are its high product stability and uniformity in various lots during its production. Generally, hydroxyapatite was used in the polishing step of monoclonal antibodies, isolation of endotoxin free plasmids, purification of enzymes and viral particles.[38]
Use in archaeology
In
Defluoridation
Hydroxylapatite is a potential
See also
References
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- ^ Hydroxylapatite on Webmineral
- ISBN 978-0962209734. Archived(PDF) from the original on 2018-09-29. Retrieved 2010-08-29.
- ^ "The official IMA-CNMNC List of Mineral Names". International Mineralogical Association: COMMISSION ON NEW MINERALS, NOMENCLATURE AND CLASSIFICATION. Retrieved 24 August 2023.
- ISBN 978-0-12-811982-2, retrieved 2020-11-18
- ^
Junqueira, Luiz Carlos; José Carneiro (2003). Foltin, Janet; Lebowitz, Harriet; Boyle, Peter J. (eds.). Basic Histology, Text & Atlas (10th ed.). McGraw-Hill Companies. p. 144. ISBN 978-0-07-137829-1.
Inorganic matter represents about 50% of the dry weight of bone ... crystals show imperfections and are not identical to the hydroxylapatite found in the rock minerals
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- ^ "CALCIUM PHOSPHATE STONES: Causes and Prevention | Kidney Stone Evaluation And Treatment Program". kidneystones.uchicago.edu. Retrieved 2023-01-14.
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- ^ European Commission Scientific Committee on Consumer Safety, Opinion on Hydroxyapatite (nano), SCCS/1624/20 – 30–31 March 2021
- ^ European Commission Scientific Committee on Consumer Safety, Opinion on Hydroxyapatite (nano), SCCS/1648/22 – 21–22 March 2023
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- S2CID 4366155. Archived from the original(PDF) on 2011-03-07. Retrieved 2015-08-28.
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External links
Media related to Hydroxylapatite at Wikimedia Commons