Hydroxyapatite – Ca5(OH)(PO4)3– is a complex phosphate of calcium that has chemical and structural resemblance to bone . Thanks to these properties similar to bone and teeth, hydroxyapatite is commonly used for tissue repair. Hydroxyapatite has been widely used for bone defects reparation in dentistry and orthopedics, tooth replacement, alveolar ridges augmentation or and maxillofacial procedures . In order to repair or substitute the bone, the substituting material must be able to create a bond with the host living bone . The position between the implant and the host tissue must be carefully controlled and movements and misalignments are undesirable. Consequently, the patient may be immobilized for as long as the fixation is strong enough to bear load. About 100 days of recovery time have been reported . Hydroxyapatite implants or hydroxyapatite coated implants lead to rapid bone formation between the implant and the surrounding bone tissue. This can shorten the recovery time to 20 days . Other applications of hydroxyapatite include anti-tumor drug delivery and antibodies in the treatment of osteomyelitis 
It is usually obtained by wet precipitation, where calcium and phosphorus ions react in a controlled pH and temperature solution. This forms a precipitation powder, typically calcined at 400-600°C or above . The solution parameters must be carefully controlled in order to obtain a homogeneous structure and avoid deficiencies . Most of these wet methods are not efficient, as the formation of hydroxyapatite and the rinsing of unnecessary anions usually take a lot of time. Sol-gel technique is an alternative to the wet methods, as it is capable to generate glass, glass–ceramic and ceramics powders with very homogeneous molecular mixing at low processing temperatures . The sol-gel process is suitable for hydroxyapatite surface coatings obtention with high homogeneity [10,11]. This process can be used to synthesize hydroxyapatite films and powder under relatively simple environmental conditions . The sol-gel technique allows to achieves a molecular-level mixing of the calcium and phosphorus precursors that improves the homogeneity of the resulting coating in comparison with conventional methods .
Deposits of hydroxyapatite crystal light up bright green where they’ve been tagged with a new peptide created at NIST to bond specifically to the compound. The peptide has been linked to a fluorescent stain for imaging, By National Institute of Standards and Technology (Hydroxyapatite crystal) [Public domain], via Wikimedia Commons
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