Hydroxyapatite

Hydroxyapatite (HAp) is a bioceramic material, also called as hydroxylapatite (HA) which is similar to the inorganic component of human beings. It has attracted many researchers because they are utilized as biomedical materials due to its excellent properties. Implants of hydroxyapatite have been implemented in form of granules, discs, coatings, pastes, cements, dense and block implants.

Properties of Hydroxyapatite

Biocompatibility: Ability to accept the host material and perform some specific functions
Bioactivity: Ability to integrate with bone structures and regenerate the damaged bones without dissolving or breaking down
Osteoconduction: Ability to form cells in bone on a surface
Osteoinduction: Ability to convert preosteoblasts (osteoblast- one of the type of bone cells) from immature cells
Osteointegration: Ability to produce growth of a bone (bone to implant are in contact)
Non toxicity and non inflammatory
Hardness of hydroxyapatite is 5 (Mohs hardness)
Lack of immunogenic response
Slow resorption

Despite its biocompatibility, the mechanical properties of HA include brittleness, low tensile strength and poor impact resistance. Now let us see the mechanical properties for sintered dense hydroxyapatite and bone below:

Material	Tensile strength (MPa)	Compressive strength (MPa)	Yield strength (MPa)	Youngs Modulus (GPa)	Fracture toughness (MPa m^1/2)
Hydroxyapatite	115-120	350-917	-	80-110	1
Cortical bone	78-196	160-250	30-70	4-30	2-12
Cancellous bone	10-20	23	-	0.2-0.5	-

Preparation methods of Hydroxyapatite:

There are several methods to produce hydroxyapatite powders. They are as follows:

· Precipitation method

· Solid state method

· Sol gel method

· Hydrothermal method

Precipitation Method

Precipitation method is the most commonly used method to synthesize Hydroxyapatite. This method involves calcium nitrate [Ca(NO₃)₂] reacts with diammonium orthophosphate [(NH₄)₂HPO₄].These two precursors were mixed in the magnetic stirring for 2 hours with appropriate pH (9-11). After stirring, the mixed precursors were subjected to ageing process for 24 hours until a precipitate is formed. Then the precipitate is taken for filtration in centrifuge filter for 10 minutes at a RPM of 3000. Now, the filtered sample was dried in hot air oven for 24 hours at a temperature of 110^oC to eliminate the impurities still contained. The filtered sample is subjected to sintering at a temperature of 900^oC to get a homogeneous Hydroxyapatite powder sample [Ca₁₀(PO₄)₆(OH)₂]. Finally, the prepared sample was characterized by X-Ray Diffraction (XRD).

Reaction: 10 Ca(NO₃)₂.4H₂O + 6 (NH₄)₂HPO₄ +8 NH₄OH ➝

Ca₁₀(PO₄)₆(OH)₂ + 20 NH₄NO₃+ 6 H₂O

The parameters that influence precipitations are pH, temperature, the calcium to phosphate ratio of reagents, and, resulting in products with minute differences in stoichiometry, crystallinity, and shape, which contribute to varied invitro/invivo behaviour. As a result, it's critical to devise a technique for producing apatites in large quantities while maintaining suitable morphology, impurities, content, and particle, and crystal size.

Advantages

Produce nano HA particles
Industrial production is possible
Simplicity of the experiment
Carried out this method at room temperature
Raw materials were cheaply and easily available
Lower time consumption
Better homogeneity

Disadvantages

Hard to obtain stoichiometric HA
Required high pH to prevent calcium deficient formation
Sensitive to reactive conditions such as pH, stirring rate and drying temperature

Solid State Method

Solid state method is relatively simple method inexpensive when compared to other methods. For solid state method, dicalcium phosphate dihydrate (DCPD, also called as brushite) is reacts with calcium carbonate. However, DCPD contains a small amount of dicalcium phosphate anhydrate (DCPA, also called as monetite). To avoid this, the mixture of DCPD and DCPA can be treated at a temperature of 1000^oC for 10 hours to obtain beta calcium pyrophosphate (Ca₂P₂O₇). It is further treated with calcium carbonate (CaCO₃) for 48 hours in intermediate milling process. After this process, the milled sample was finally sintered to 1050^oC to produce hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂]

Reaction: 3Ca₂P₂O₇ + 4CaCO₃ + H₂O ➝ Ca₁₀(PO₄)₆(OH)₂ + 4CO₂

Advantages

Easy to perform
Inexpensive
Stoichiometric HA formed

Disadvantages

Needs high sintering temperature
Long treatment times

Sol Gel Method

Sol-gel materials can be manufactured by three different methods namely: gelation of colloidal powders, hypercritical drying and then incorporating a drying step at ambient temperature.

Advantages

Homogeneous molecular mixing occurs
Low processing temperature required
Increased control over phase purity

Disadvantages

Expensive
Difficulty to hydrolyse phosphate

Hydrothermal Method

Hydrothermal method involves the reaction between calcium and phosphate solutions at a higher temperature and pressure to produce hydroxyapatite. This reaction is normally conducted at a range of 60-250^oC for 24 hours to yield crystalline HA crystals which are usually agglomerated.

Advantages

Well crystallized and homogeneous powder
Nano HA has been prepared

Disadvantages

Agglomeration of HA powders
Requires high pressure

Sintering behavior of Hydroxyapatite:

If a material is subjected to sintering, the mechanical property increases due to diffusion. The main purpose of sintering is:

To increase densification
To increase the mechanical properties
To increase the crystalline size
To decrease the surface area

Upto 850^oC, the amount of moisture, ammonia, carbonates, nitrates and residues are removed by calcination. The process of sintering involves chemical as well as microstructural transformation by decreasing the surface area and increasing the size of crystal. In the shaping method of hydroxyapatite blocks, sintering plays an essential role in mechanical properties since it causes toughening by increasing the mechanical strength. At a temperature between 900-1000^oC, removal of residuals takes place to generate pores by porogen decomposition and also to persuade the growth of a crystal. If the pressurized powders were subjected to thermal treatments at a temperature between 1000-1200^oC, it will results in grains and blocks with skeletal density similar to the theoretical density. Hydroxyapatite is chemically unstable by dehydroxylation at a temperature above 1250^oC, there is a decrease in mechanical properties due to exaggerated grain growths.

Applications of Hydroxyapatite:

Widely used for orthopaedic, dental and maxillofacial applications.
Used to repair bone injuries related to human hard tissues.
Used to enhance bonding property between metallic implants and bone.
Used as coating to reduces the pain and fast recovery to the patients.
Employed in forms of powders, porous blocks or beads to fill defects or voids (eg: Bone cancer).

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Hydroxyapatite

Table of Contents:

Hydroxyapatite

Properties of Hydroxyapatite