The hydroxyapatite is a calcium phosphate mineral, whose chemical formula is Ca 10 (PO 4 ) 6 (OH) 2 . Together with other minerals and organic matter remnants crushed and compacted, it forms the raw material known as phosphate rock. The term hydroxy refers to the OH anion - .
If instead of that anion it were fluoride, the mineral would be called fluoroapatite (Ca 10 (PO 4 ) 6 (F) 2 ; and so with other anions (Cl - , Br - , CO 3 2- , etc.). Also, hydroxyapatite is the main inorganic component of the bones and dental enamel, predominantly in crystalline form.
So, it is a vital element in the bone tissues of the living beings . Its great stability against other calcium phosphates allows it to withstand physiological conditions, giving bones their characteristic hardness. Hydroxyapatite is not alone: it fulfills its function accompanied by collagen, fibrous protein of connective tissues.
Hydroxyapatite (or hydroxylapatite) contains Ca ions 2+ , but it can also contain other cations in its structure (Mg 2+ , Na + ), impurities that intervene in other biochemical processes of the bones (such as remodeling).
Index
- 1 Structure
- 2 Synthesis
- 3 Hydroxyapatite crystals
- 4 Applications
- 4.1 Medical and dental use
- 4.2 Other uses of hydroxyapatite
- 5 Physical and chemical properties
- 6 References
Structure
The upper image illustrates the structure of calcium hydroxyapatite. All spheres occupy the volume of the half of a hexagonal"box", where the other half is identical to the first.
In this structure the green spheres correspond to the cations Ca 2+ , while the red spheres to the oxygen atoms, the orange spheres to the phosphorus atoms, and the white spheres to the hydrogen atom of the OH - .
The phosphate ions in this image have the defect of not exhibiting a tetrahedral geometry; instead, they look like square-based pyramids.
The OH - gives the impression that it is located far from the Ca 2+ . However, the crystalline unit can repeat itself on the roof of the first one, thus showing the close proximity between both ions. Also, these ions can be replaced by others (Na + and F - , for example).
Synthesis
Hydroxylapatite can be synthesized by the reaction of calcium hydroxide with phosphoric acid:
10 Ca (OH) 2 + 6 H 3 PO 4 => Ca 10 (PO 4 ) 6 (OH) 2 + 18 H 2 OR
Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) is expressed by two units of formula Ca 5 (PO 4 ) 3 OH.
Likewise, hydroxyapatite can be synthesized through the following reaction:
10 Ca (NO 3 ) 2. 4H 2 O + 6 NH 4 H 2 PO 4 => Ca 10 (PO 4 ) 6 (OH) 2 + 20 NH 4 DO NOT 3 + 52 H 2 OR
Controlling the speed of precipitation allows this reaction to generate hydroxyapatite nanoparticles.
Hydroxyapatite crystals
The ions are compacted and grow to form a rigid and resistant biocrystal. This is used as a biomaterial of bone mineralization.
However, it needs collagen, organic support that serves as a mold for its growth. These crystals and their complicated formation processes will depend on the bone (or the tooth).
These crystals grow impregnated with organic matter, and the application of electron microscopy techniques detailed them in the teeth as aggregates with forms of rods called prisms.
Applications
Medical and dental use
Due to its similarity in size, crystallography and composition with hard human tissue, nanohydroxyapatite is attractive for use in prostheses. Also, nanohydroxyapatite is biocompatible, bioactive and natural, in addition to not being toxic or inflammatory.
Accordingly, nanohydroxyapatite ceramics have a variety of applications, which include:
- In the surgery of bone tissue is used in the filling of cavities in orthopedic, traumatological, maxillofacial and dental surgeries.
- It is used as a coating for orthopedic and dental implants. It is a desensitizing agent used after tooth whitening. It is also used as a remineralizing agent in toothpastes and in the early treatment of caries.
- Stainless steel and titanium implants are often coated with hydroxyapatite to reduce their rejection rate.
- It is an alternative to allogenic and xenogenic bone grafts. The healing time is shorter in the presence of hydroxyapatite than in its absence.
- Synthetic nanohydroxyapatite mimics the hydroxyapatite naturally present in dentin and steroid apatite, so its use is advantageous in the repair of enamel and incorporation in toothpastes, as well as mouth rinses
Other uses of hydroxyapatite
- Hydroxyapatite is used in the air filters of motor vehicles to increase the efficiency of these in the absorption and decomposition of carbon monoxide (CO). This reduces environmental pollution.
- An alginate-hydroxyapatite complex has been synthesized that field tests have indicated that it is capable of absorbing fluorine through the mechanism of ion exchange.
- Hydroxyapatite is used as a chromatographic medium for proteins. This presents positive charges (Ca ++ ) and negative (PO 4 -3 ), so it can interact with electrically charged proteins and allow their separation by ion exchange.
- Hydroxyapatite has also been used as a support for the electrophoresis of nucleic acids. Separate DNA from RNA, as well as DNA from a single strand of two-strand DNA.
Physical and chemical properties
Hydroxyapatite is a white solid that can acquire grayish, yellow and green tones. As it is a crystalline solid, it has high melting points, indicative of strong electrostatic interactions; for hydroxyapatite, this is 1100 ºC.
It is denser than water, with a density of 3.05 - 3.15 g / cm 3 . In addition, it is practically insoluble in water (0.3 mg / mL), which is due to phosphate ions.
However, in acidic media (as in HCl) it is soluble. This solubility is due to the formation of CaCl 2 , salt highly soluble in water. Also, phosphates are protonated (HPO) 4 2- and H 2 PO 4 - ) and interact better with water.
The solubility of hydroxyapatite in acids is important in the pathophysiology of caries. The bacteria in the oral cavity secrete lactic acid, product of the fermentation of glucose, which lowers the pH of the tooth surface to less than 5, so the hydroxyapatite begins to dissolve.
Fluoride (F - ) can replace OH ions - in the crystal structure. When this happens, it gives resistance to the hydroxyapatite of tooth enamel against acids.
Possibly, this resistance may be due to the insolubility of CaF 2 formed, refusing to"abandon"the crystal.
References
- Shriver & Atkins. (2008). Inorganic Chemistry (Fourth ed., Pp. 349, 627). Mcgraw Hill.
- Fluidinova (2017). Hydroxylapatite. Retrieved on April 19, 2018, from: fluidinova.com
- Victoria M., García Garduño, Reyes J. (2006). Hydroxyapatite, its importance in mineralized tissues and its biomedical application. TIP Specialized Journal in Chemical-Biological Sciences, 9 (2): 90-95
- Gaiabulbanix. (November 05, 2015). Hydroxyapatite. [Figure]. Retrieved on April 19, 2018, from: commons.wikimedia.org
- Martin.Neitsov. (November 25, 2015). Hüdroksüapatiidi kristallid. [Figure]. Retrieved on April 19, 2018, from: commons.wikimedia.org
- Wikipedia. (2018). Hydroxylapatite. Retrieved on April 19, 2018, from: en.wikipedia.org
- Fiona Petchey Bone . Retrieved on April 19, 2018, from: c14dating.com