Hydroxylapatite, also called hydroxyapatite (HA), is a naturally
occurring mineral form of calcium apatite with the formula
Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that
the crystal unit cell comprises two entities.
Hydroxylapatite is the
hydroxyl endmember of the complex apatite group. The OH− ion can be
replaced by fluoride, chloride or carbonate, producing fluorapatite or
chlorapatite. It crystallizes in the hexagonal crystal system. Pure
hydroxylapatite powder is white. Naturally occurring apatites can,
however, also have brown, yellow, or green colorations, comparable to
the discolorations of dental fluorosis.
Up to 50% by volume and 70% by weight of human bone is a modified form
of hydroxylapatite, known as bone mineral. Carbonated
calcium-deficient hydroxylapatite is the main mineral of which dental
enamel and dentin are composed.
Hydroxylapatite crystals are also
found in the small calcifications, within the pineal gland and other
structures, known as corpora arenacea or 'brain sand'.[citation
1 Chemical synthesis of hydroxyapatite
2 Calcium deficient hydroxyapatite
3 Medical uses
5 Use in archaeology
6 Animal structures and potential uses in materials science
7 See also
9 External links
Chemical synthesis of hydroxyapatite
Hydroxyapatite can be synthesized via several methods such as wet
chemical deposition, biomimetic deposition, sol-gel route
(wet-chemical precipitation) or electrodeposition. Yagai and Aoki
proposed the hydroxyapatite nanocrystal suspension can be prepared by
a wet chemical precipitation reaction following the reaction equation
10 Ca(OH)2 + 6 H3PO4 → Ca10(PO4)6(OH)2 + 18 H2O
Several studies have shown that hydroxyapatite synthesis via the
wet-chemical route can be improved by high-power ultrasound. The
ultrasonically assisted synthesis (sono-synthesis) of hydroxyapatite
is a successful technique for the production of nanostructured
hydroxyapatite to high quality standards. The ultrasonic route allows
the production of nano-crystalline hydroxyapatite as well as modified
particles, e.g. core-shell nanospheres and composites.
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. 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 stochiometric hydroxyapatite,
are occupied by phosphate or hydrogen phosphate, HPO42–, anions.
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:
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:
Ca10−x(PO4)6−x(HPO4)x(OH)2−x → (1−x) Ca10(PO4)6(OH)2 + 3x
Flexible hydrogel-HA composite, which has a mineral-to-organic matrix
ratio approximating that of human bone.
A 3D visualization of half of a hydroxyapatite unit cell, from x-ray
Hydroxylapatite can be found in teeth and bones within the human body.
Thus, it is commonly used as a filler to replace amputated bone or as
a coating to promote bone ingrowth into prosthetic implants.
Although many other phases exist with similar or even identical
chemical makeup, the body responds to them very differently. Coral
skeletons can be transformed into hydroxylapatite by high
temperatures; their porous structure allows relatively rapid ingrowth
at the expense of initial mechanical strength. The high temperature
also burns away any organic molecules such as proteins, preventing an
immune response and rejection.
Many modern implants, e.g. hip replacements, dental implants and bone
conduction implants, are coated with hydroxylapatite. It has
been suggested that this may promote osseointegration.[citation
needed] Porous hydroxylapatite implants are used for local drug
delivery in bone. It is also being used to repair early
lesions in tooth enamel.
In spite of attractive biological properties, hydroxylapatite, and
materials based thereon, have some drawbacks, such as low
bioresorption rate in vivo, poor stimulating effect on the growth of
new bone tissues, low crack resistance and small fatigue durability in
the physiological environment. The application of modified
hydroxylapatite opens up the opportunities for the preparation of
artificial bone substances for implants and a large variety of drugs
for curing different lesions of bone, soft and mucous tissues of the
individual. A promising method of modification is the introduction of
fluorine or silicon into the primary structure with the formation of
fluorine- or silicon-substituted hydroxylapatite. The introduction
of fluorine increases the resistance to biodegradation and
improves the adsorption of proteins and adhesion of the coating to the
Hydroxylapatite deposits in tendons around joints resulting in the
medical condition calcific tendinitis.
Microcrystalline hydroxylapatite (MH) is marketed as a "bone-building"
supplement with superior absorption in comparison to calcium. It
is a second-generation calcium supplement derived from bovine
bone. In the 1980s, bone meal calcium supplements were found to be
contaminated with heavy metals, and although the manufacturers
claim their MH is free from contaminants, people are advised to avoid
it because its effect in the body has not been well-tested.
However, the limited tests seem to show positive results. A 1995
randomized placebo-controlled study of 40 people in Europe found that
it was more effective than calcium carbonate in slowing bone loss.
A 2007 randomized double-blind controlled study of an MH supplement
called the Bone Builder found significant positive effects in bone
mineral density (BMD) compared to control.
been used by
Noel Fitzpatrick to facilitate bionic development in
animals, by coating steel rods in hydroxylapatite to encourage natural
growth of skin around it. As a component of nanocomposites,
hydroxylapatite is finding uses as a potential new bone replacement
The mechanism of hydroxylapatite (HA) chromatography is complicated
and has been described as "mixed-mode" ion exchange. It involves
nonspecific interactions between positively charged calcium ions and
negatively charged phosphate ions on the stationary phase HA resin
with protein negatively charged carboxyl groups and positively charged
amino groups. It may be difficult to predict the effectiveness of HA
chromatography based on physical and chemical properties of the
desired protein to be purified. For elution, a buffer with increasing
phosphate concentration is typically used for application.
Use in archaeology
In archaeology, hydroxylapatite from human and animal remains can be
analysed to reconstruct ancient diets, migrations and palaeoclimate.
The mineral fractions of bone and teeth act as a reservoir of trace
elements, including carbon, oxygen and strontium. Stable isotope
analysis of human and faunal hydroxylapatite can be used to indicate
whether a diet was predominantly terrestrial or marine in nature
(carbon, strontium); the geographical origin and migratory habits
of an animal or human (oxygen, strontium) and to reconstruct past
temperatures and climate shifts (oxygen). Post-depositional
alteration of bone can contribute to the degradation of bone collagen,
the protein required for stable isotope analysis.
Animal structures and potential uses in materials science
Needle-like hydroxyapatite crystals on stainless steel. Scanning
electron microscope picture from University of Tartu.
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 and toughness than any synthetic
composite material; these properties have led to its investigation for
potential synthesis and engineering use. Their dactyl appendages
have excellent impact resistance due to the impact region being
composed of mainly crystalline hydroxyapatite, which offers
significant hardness. Crack propagation of the impact region is
reduced by thin layers of chitosan in between the highly organized
crystal hydroxyapatite structures. Once a hydroxyapatite prism
fractures, the chitosan inter-layers aid in preventing further cracks.
This form of layered crack retardation is also seen in teeth enamel,
where the hydroxyapatite prisms that make up an enamel rod are padded
by thin layers of protein which fulfill the same function. A
periodic layer underneath the impact layer composed of hydroxyapatite
with lower calcium and phosphorus content (thus resulting in a much
lower modulus) inhibits crack growth by forcing new cracks to change
directions. This periodic layer also reduces the energy transferred
across both layers due to the large difference in modulus, even
reflecting some of the incident energy.
Armor models based on
nanoscale composites of hydroxyapatite designed in a similar fashion
would help optimize impact resistance, reducing the weight needed to
The addition of hydroxyapatite particles to a magnesium alloy in the
form of a metal matrix composite stabilized the corrosion rate of the
alloy, resulting in a cyto-compatible, biodegradable material with
adjustable corrosion rates and mechanical properties.
Experimental nanostructure composite air filters containing
hydroxyapatite were found to be efficient in absorbing and decomposing
CO which could eventually lead to utilization in reducing automotive
In 2014 an alginate/nano-hydroxyapatite composite was synthesized and
field-tested as an adsorbent for fluoride. The biocomposite removes
fluoride through an ion-exchange mechanism, and is both biocompatible
Research into hydroxyapatite's proton conductivity resulted in the
diffusion path and conduction mechanism of the mineral, allowing for
potential use as developmental material for energy conversion
Recent developments have allowed the synthesis of ceramic microspheres
made from hydroxyapatite with a diameter of 1.5 micrometers; the
microspheres can be utilized in a variety of fields including filters,
grinding media, and light-weight concrete.
Biomaterials: Mechanical Properties
^ Hydroxylapatite. Mindat
^ Hydroxylapatite. Webmineral
^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols,
Monte C., eds. (2000). "Hydroxylapatite". Handbook of Mineralogy
(PDF). IV (Arsenates, Phosphates, Vanadates). Chantilly, VA, US:
Mineralogical Society of America. ISBN 0962209732.
^ 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 0-07-137829-4. 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
^ Ferraz, M. P.; Monteiro, F. J.; Manuel, C. M. (2004).
"Hydroxyapatite nanoparticles: A review of preparation methodologies".
Journal of applied biomaterials & biomechanics : JABB. 2 (2):
74–80. PMID 20803440.
^ Bouyer, E.; Gitzhofer, F.; Boulos, M. I. (2000). "Morphological
study of hydroxyapatite nanocrystal suspension". Journal of Materials
Science: Materials in Medicine. 11 (8): 523–31.
doi:10.1023/A:1008918110156. PMID 15348004.
^ Sono-Synthesis of Nano-Hydroxyapatite. hielscher.com
^ a b Rey, C.; Combes, C.; Drouet, C.; Grossin, D. (2011). "1.111 –
Bioactive Ceramics: Physical Chemistry". In Ducheyne, Paul.
Comprehensive Biomaterials. 1. Elsevier. pp. 187–281.
^ Raynaud, S.; Champion, E.; Bernache-Assollant, D.; Thomas, P.
(2002). "Calcium phosphate apatites with variable Ca/P atomic ratio I.
Synthesis, characterisation and thermal stability of powders".
Biomaterials. 23 (4): 1065–72. doi:10.1016/S0142-9612(01)00218-6.
^ Valletregi, M. (1997). "Synthesis and characterisation of calcium
deficient apatite". Solid State Ionics. 101–103: 1279–1285.
^ Sadat-Shojai, Mehdi (2010). Hydroxyapatite: Inorganic Nanoparticles
of Bone (Properties, Applications, and Preparation Methodologies)
(PDF). Iranian Students Book Agency (ISBA).
^ John, Łukasz; Janeta, Mateusz; Szafert, Sławomir. "Designing of
macroporous magnetic bioscaffold based on functionalized methacrylate
network covered by hydroxyapatites and doped with nano-MgFe 2 O 4 for
potential cancer hyperthermia therapy". Materials Science and
Engineering: C. 78: 901–911. doi:10.1016/j.msec.2017.04.133.
^ Starikov, V.V.; et al. (2016). "Properties of magnetron
hydroxyapatite coatings deposited on oxidized substrates". J. Biol.
Phys. Chem. 16 (3): 126–130.
doi:10.4024/14ST16A.jbpc.16.03. CS1 maint: Explicit use of et al.
^ Jeong, KI (2012). "Experimental Study of
Stability of Intentionally Exposed Hydroxyapatite Coating Implants".
Journal of the Korean Maxillofacial Reconstructive Surgery. 34 (1):
^ Kundu, B.; Soundrapandian, C.; Nandi, S. K.; Mukherjee, P.;
Dandapat, N.; Roy, S.; Datta, B. K.; Mandal, T. K.; Basu, D.;
Bhattacharya, R. N. (2010). "Development of new localized drug
delivery system based on ceftriaxone-sulbactam composite drug
impregnated porous hydroxyapatite: a systematic approach for in vitro
and in vivo animal trial". Pharmaceutical Research. 27 (8): 1659–76.
doi:10.1007/s11095-010-0166-y. PMID 20464462.
^ Kundu, B.; Lemos, A.; Soundrapandian, C.; Sen, P. S.; Datta, S.;
Ferreira, J. M. F.; Basu, D. (2010). "Development of porous HAp and
β-TCP scaffolds by starch consolidation with foaming method and
drug-chitosan bilayered scaffold based drug delivery system". Journal
of Materials Science: Materials in Medicine. 21 (11): 2955–69.
doi:10.1007/s10856-010-4127-0. PMID 20644982.
^ Brunton, P. A.; Davies, R. P. W.; Burke, J. L.; Smith, A.; Aggeli,
A.; Brookes, S. J.; Kirkham, J. (2013). "Treatment of early caries
lesions using biomimetic self-assembling peptides – a clinical
safety trial". BDJ. 215 (4): E6. doi:10.1038/sj.bdj.2013.741.
PMC 3813405 . PMID 23969679.
^ Bogdanova, E.A.; Sabirzyanov, N. A. (2014). "Adsorption capacity of
wateroxidized lanthanum-doped aluminum alloy powder" (PDF).
Nanosystems: physics, chemistry, mathematics. 5 (4): 590–596.
^ Cheng, Kui; Weng, Wenjian; Qu, Haibo; Du, Piyi; Shen, Ge; Han,
Gaorong; Yang, Juan; Ferreira, J. M. F. (15 April 2004). "Sol-gel
preparation andin vitro test of fluorapatite/hydroxyapatite films".
Journal of Biomedical Materials Research. 69B (1): 33–37.
^ Zhang, Sam; Xianting, Zeng; Yongsheng, Wang; Kui, Cheng; Wenjian,
Weng (June 2006). "Adhesion strength of sol–gel derived fluoridated
hydroxyapatite coatings". Surface and Coatings Technology. 200
(22–23): 6350–6354. doi:10.1016/j.surfcoat.2005.11.033.
^ a b c d e Straub, D.A. (2007). "Calcium Supplementation in Clinical
Practice: A Review of Forms, Doses, and Indications". NCP- Nutrition
in Clinical Practice. 22 (3): 286–96.
doi:10.1177/0115426507022003286. PMID 17507729.
^ Tucker, L. A.; Nokes, N.; Adams, T. (2007). "Effect of a Dietary
Supplement on Hip and Spine BMD". Medicine & Science in Sports
& Exercise. 39: S230.
^ Richards, M. P.; Schulting, R. J.; Hedges, R. E. M. (2003).
"Archaeology: Sharp shift in diet at onset of Neolithic" (PDF).
Nature. 425 (6956): 366. Bibcode:2003Natur.425..366R.
doi:10.1038/425366a. PMID 14508478.
^ Britton, K.; Grimes, V.; Dau, J.; Richards, M. P. (2009).
"Reconstructing faunal migrations using intra-tooth sampling and
strontium and oxygen isotope analyses: A case study of modern caribou
(Rangifer tarandus granti)". Journal of Archaeological Science. 36
(5): 1163–1172. doi:10.1016/j.jas.2009.01.003.
^ Daniel Bryant, J.; Luz, B.; Froelich, P. N. (1994). "Oxygen isotopic
composition of fossil horse tooth phosphate as a record of continental
paleoclimate". Palaeogeography, Palaeoclimatology, Palaeoecology. 107
(3–4): 303–316. doi:10.1016/0031-0182(94)90102-3.
^ Van Klinken, G. J. (1999). "Bone Collagen Quality Indicators for
Palaeodietary and Radiocarbon Measurements". Journal of Archaeological
Science. 26 (6): 687–695. doi:10.1006/jasc.1998.0385.
^ Weaver, J. C.; Milliron, G. W.; Miserez, A.; Evans-Lutterodt, K.;
Herrera, S.; Gallana, I.; Mershon, W. J.; Swanson, B.; Zavattieri, P.;
Dimasi, E.; Kisailus, D. (2012). "The Stomatopod Dactyl Club: A
Formidable Damage-Tolerant Biological Hammer". Science. 336 (6086):
1275–80. Bibcode:2012Sci...336.1275W. doi:10.1126/science.1218764.
^ Simmons, L. M.; Al-Jawad, M.; Kilcoyne, S. H.; Wood, D. J. (2011).
"Distribution of enamel crystallite orientation through an entire
tooth crown studied using synchrotron X-ray diffraction". European
Journal of Oral Sciences. 119: 19–24.
^ Tanner, K. E. (2012). "Small but Extremely Tough". Science. 336
(6086): 1237–8. Bibcode:2012Sci...336.1237T.
doi:10.1126/science.1222642. PMID 22679085.
^ Barthelat, F.; Rabiei, R. (2011). "Toughness amplification in
natural composites". Journal of the Mechanics and Physics of Solids.
59 (4): 829–840. doi:10.1016/j.jmps.2011.01.001.
^ Witte, F.; Feyerabend, F.; Maier, P.; Fischer, J.; Störmer, M.;
Blawert, C.; Dietzel, W.; Hort, N. (2007). "Biodegradable
magnesium–hydroxyapatite metal matrix composites". Biomaterials. 28
(13): 2163–2174. doi:10.1016/j.biomaterials.2006.12.027.
^ Nasr-Esfahani, M.; Fekri, S. (2012). "Alumina/TiO2/hydroxyapatite
interface nanostructure composite filters as efficient photocatalysts
for the purification of air". Reaction Kinetics, Mechanisms and
Catalysis. 107: 89–103. doi:10.1007/s11144-012-0457-x.
^ Pandi, K.; Viswanathan, N. (2014). "Synthesis of alginate
bioencapsulated nano-hydroxyapatite composite for selective fluoride
sorption". Carbohydrate Polymers. 112: 662–667.
^ Yashima, M.; Kubo, N.; Omoto, K.; Fujimori, H.; Fujii, K.; Ohoyama,
K. (2014). "Diffusion Path and Conduction Mechanism of Protons in
Hydroxyapatite". The Journal of Physical Chemistry C. 118 (10):
^ Li, S.; Wu, H. H.; Xu, G. J.; Xiao, X. F. (2014). "Facile Biomimetic
Fabrication of Hollow Hydroxyapatite with Hierarchically Porous
Microstructure Using Hyperbranched Gemini Surfactant as Template".
Advanced Materials Research. 1015: 355–358.
^ Korolev, E. V.; Inozemtcev, A. S. (2013). "Preparation and Research
of the High-Strength Lightweight Concrete Based on Hollow
Microspheres". Advanced Materials Research. 746: 285–288.
Media related to Apatit-(CaOH) (Hydroxylapatite) at Wikimedia Commons