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What is hydroxyapatite?

The Hydroxyapatite is a calcium phosphate mineral, whose chemical formula is Ca 10 (PO 4 ) 6 (OH) 2 . Along with other minerals and remains of crushed and compacted organic matter , it forms the raw material known as phosphate rock. The term “hydroxy” refers to the OH  anion .

If instead of that anion were fluoride, the mineral would be called fluoroapatite (Ca 10 (PO 4 ) 6 (F) 2 ; and so on with other anions (Cl  , Br  , CO 2– , etc.). , Hydroxyapatite is the main inorganic component of bones and dental enamel, predominantly present in crystalline form.

So, it is a vital element in the bone tissues of 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, a fibrous protein in connective tissues.

Hydroxyapatite (or hydroxylapatite) contains Ca 2+ ions , but it can also house other cations (Mg 2+ , Na + ) in its structure , impurities that intervene in other biochemical processes of bones (such as their remodeling).

Structure of hydroxyapatite

Hydroxyapatite

The top image illustrates the structure of calcium hydroxyapatite. All the spheres occupy the volume of one half of a hexagonal “drawer”, where the other half is identical to the first.

In this structure, the green spheres correspond to the Ca 2+ cations , while the red spheres to the oxygen atoms, the orange spheres to the phosphorous atoms, and the white spheres to the hydrogen atom of OH  .

The phosphate ions in this image have the defect of not exhibiting a tetrahedral geometry; instead, they look like pyramids with square bases.

OH  gives the impression that it is located far from Ca 2+ . However, the crystalline unit can repeat itself on the roof of the first, thus showing the close proximity between both ions. Likewise, these ions can be replaced by others (Na + and F  , for example).

Synthesis of hydroxylapatite

Hydroxylapatite can be synthesized by reacting calcium hydroxide with phosphoric acid:

10 Ca (OH) 2  + 6 H 3 PO 4 => Ca 10 (PO 4 ) 6 (OH) 2  + 18 H 2 O

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 NO 3   + 52 H 2 O

Controlling the rate of precipitation allows this reaction to generate hydroxyapatite nanoparticles.

Hydroxyapatite crystals

Hydroxyapatite

The ions compact and grow to form a strong and rigid biocrystal. This is used as a biomaterial for the mineralization of bones.

However, it needs collagen, an organic support that acts 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 detail them on the teeth as rod-shaped aggregates called prisms.

Hydroxylapatite uses

Medical and dental use

Due to its similarity in size, crystallography, and composition to hard human tissue, nanohydroxyapatite is attractive for use in prosthetics. Also, nanohydroxyapatite is biocompatible, bioactive and natural, in addition to being non-toxic or inflammatory.

Consequently, nanohydroxyapatite ceramic has a variety of applications, including:

  • In bone tissue surgery it is used to fill cavities in orthopedic, trauma, maxillofacial and dental surgeries.
  • It is used as a covering for orthopedic and dental implants. It is a desensitizing agent used after teeth whitening. It is also used as a remineralizing agent in toothpastes and in the early treatment of cavities.
  • Titanium and stainless steel implants are often coated with hydroxyapatite to reduce their rate of rejection.
  • It is an alternative to allogeneic and xenogeneic bone grafts. Healing time is shorter in the presence of hydroxyapatite than in its absence.
  • Synthetic nanohydroxyapatite mimics hydroxyapatite naturally present in dentin and enamel apatite, making it advantageous for use in enamel repair and incorporation in toothpastes, as well as in mouthwashes.

Other uses of hydroxyapatite

  • Hydroxyapatite is used in motor vehicle air filters to increase their efficiency in absorbing and breaking down 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 ion exchange mechanism.
  • Hydroxyapatite is used as a chromatographic medium for proteins. It has positive charges (Ca ++ ) and negative charges (PO -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 nucleic acid electrophoresis. It is possible to separate DNA from RNA, as well as single-stranded DNA from two-stranded DNA.

Physical and chemical properties

Hydroxyapatite is a white solid that can take on grayish, yellow and greenish hues. As it is a crystalline solid, it has high melting points, indicative of strong electrostatic interactions; for hydroxyapatite, this is 1100ºC.

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 , a highly soluble salt in water. Likewise, phosphates are protonated (HPO 2– and H 2 PO  ) and interact to a better degree with water.

The solubility of hydroxyapatite in acids is important in the pathophysiology of caries. Bacteria in the oral cavity secrete lactic acid, a product of glucose fermentation , which lowers the pH of the tooth surface to less than 5, so the hydroxyapatite begins to dissolve.

Fluorine (F  ) can replace OH  ions in the crystal structure. When this happens, it provides resistance to the hydroxyapatite of the dental enamel against acids.

Possibly, this resistance may be due to the insolubility of the CaF 2 formed, refusing to “leave” the crystal.

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