In an experiment conducted by Professor Per-Ingvar Branemark, in 1952, the interest for using nano-hydroxyapatite for human purposes was sparked. The experiment involved planting a titanium implant in rabbit bone. When the professor went to remove the implant he reportedly found the implant had integrated with the bone so thoroughly that it could not be removed, this was due to hydroxyapatite.

Hydroxyapatite [HAp, Ca10(PO4)6(OH)2] is a naturally occurring mineral made up of calcium and phosphate and is found in human bones. Scientists have produced a nanoparticulate-based synthetic bone by manipulating the calcium and phosphate molecules. As a result a material has been created that is identical in structure and composition to natural bone hydroxyapatite.

projection of the plane
projection of the plane

What are the properties of nano-hydroxyapatite? Why is it used?

Nano-Hydroxyapatite (HAp) is used in replacing hard tissue in in the human body for the following reasons;

  • In a solid state has the melting temperature of 1250oC- won’t melt in the human body

  • Exhibits hexagonal crystal structure- close packed

  • Bioceramic material- hard and wear resistant, suitable for human body replacements

  • Shows a high resistance to surface reaction with the simulated body fluid- won’t react with other fluids in the body

Nano-hydroxyapatite exists as a nanopowder and forms part of the crystallographic family of apatites, isomorphic compounds with the same hexagonal structure. This is the calcium phosphate compound most commonly used for biomaterial.

Hydroxyapatite can appear to have brown, yellow, or green colorations. In its powder form but is typically white.

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The chemical equation that describes the first precipation reaction is:

10Ca(OH)2 + 6H3PO4=Ca10(PO4)6(OH)2 + 18H2O

Hydroxyapatite consists of Ca2+ ions surrounded by both PO42- and OH- ions, and has the chemical formula Ca5(OH)(PO4)3. It is also known as tricalcium phosphate and calcium hydroxyapatite.

However there is one major disadvantage to hydroxyapatite; poor mechanical properties (in particular fatigue properties) mean that hydroxyapatite cannot be used in bulk form for load bearing applications such as orthopaedic.

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Future and current uses:

Initially the mineral was used for mostly dental implants; however that's not the case anymore. There are many different instances for which nano-hydroxyapatite can be used. Bone voids and/or defects are one. This process involves powders, blocks, or beads of the mineral being placed into or on the affected of areas of bone. Since it is bioactive, it encourages the bone to grow and correct the problem. This process can be an alternative to bone grafts. Also, healing times are shorter than they would be if nano-hydroxyapatite was not used. The mineral is often used for medical implants as well. As it is bioactive, it can integrate into bone structures and support growth without breaking down or dissolving in the human body. By coating a titanium and stainless steel implant with hydroxyapatite the body is tricked into incorporating the implant.

In the future hydroxyapatite could be used to impact the environment; the present experimentation shows that uranium can be effectively removed from groundwater using permeable reactive barriers with hydroxyapatite (HAP) as reactive material. The main factor influencing the removal processes is the composition of the groundwater, namely the concentration of Ca and carbonate.

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However there are health problems surrounding nano-hydroxyapatite; sometimes the molecules can clump together forming crystals. These can form in or around the joints and cause swelling of the joints, tendons, or ligaments.


By Melissa Stanfield