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Production of nanostructured materials, similar to
the complex structure of nano-calcite of hard tissues, eggshell, teeth and
bone, are an attractive field of research. Calcium phosphates with clinical
applications, such as hydroxyapatite (HA) (Ca10 (PO4)6(OH)2)
have been widely used in regeneration of bone and fabrication of medical
implants, mainly due to the chemical composition and structure similarity
between HA and the mineral part of bones and teeth and also as gene and drug
delivery is used. High surface area of HA make it useful for drug release.
Moreover, it has antibacterial property and potential applications in rapid
microbial detection, treatment of heavy metals from aqueous solutions.
Biological synthesis has been attracted more attention for compatibility to
human safety.
Keywords:
Hydroxyapatite, Nano-calcite, Crystalline, Calcium phosphate
INTRODUCTION
Mechanical and
properties of apatite
The properties of hydroxyapatite determine the value and range of
applications of this material, including crystallinity, thermal behavior,
mechanical properties, density, water solubility and biocompatibility
properties [2-5].
X-ray diffraction studies of bone marrow specimens show that
hydroxyapatite enamel crystals are larger than dentin and bone. Obviously, the
density increases with increasing degree of crystallinity, so the density of
hydroxyapatite is more than bone and ivory. Unlike natural hydroxyapatite with
a density of about 3.08 g/cm3, the density of synthetic
hydroxyapatite (synthetic) changes in the range of 3.6-3 g/cm3 [2].
According to the findings of the synthesis of hydroxyapatite studies
morphologically, the size of the crystal and composition is different from that
of hydroxyapatite. Hydroxyapatite is expressed in terms of the degree of
impurity and the rate and amount of temperature received during the synthesis
of different behaviors. These behaviors are divided into 3 categories of fuzzy
transformation, thermal expansion and morphology. For example, if the mineral
hydroxyapatite is heated at a uniform rate, its crystalline structure undergoes
thermal expansion, indicating that with increasing temperature the size of the
crystal lattice constants changes to the extent that crystal lattice [3].
The mechanical properties of hydroxyapatite, among the abrasive
properties that determine the hardness of hydroxyapatite, show that
hydroxyapatite has a low stability in moisture environments and cannot be used
in high stress applications because they are strongly sensitive to cracking.
Therefore, medical applications are limited to covering bio-materials and use in
bone
cements. A mineral-coated
Among the properties of hydroxyapatite as a
biomaterial, the most important feature can be biocompatibility. The biocompatibility
of hydroxyapatite enables the ability to bond with the body's cells precisely.
In addition, hydroxyapatite induces bone growth and ultimately leads to bone
growth. When hydroxyapatite is in the form of pure ceramics or as a coating on
metallic implants in the human body, it can facilitate the recovery and
regeneration of lost or damaged tissue for the body [5].
Chemical synthesis
Chemical production of HA creates processing
defects predominantly introduced through hard-particle agglomeration, which lead
to weakness in the final implant.
Over the past few years, various methods for
the synthesis of hydroxyapatite particles have been reported. These methods are
generally are co-precipitation and sol gel. Some chemical methods can be used
for sedimentary and sol-gel methods. Also, Nano-hydroxyapatite/chitosan
composite is synthesis by solvent casting and evaporation methods for the
function of bone manufacture. The results have shown that the surface roughness
and micropores of the composite membranes increase with HA content, suitable
for adhesion, crawl and growth of cells. The hydroxyapatite holds nano size and
distributes uniformly in the composite membranes [6].
In the sediment method, one or more soluble
salts react with each other to settle to form an insoluble salt in water. The
product solubility of the sediment composition is the most important parameter
for the reaction. In this method, the first product is formed immediately after
completing the reaction. When the concentration of the product passes through
the reaction of the combined product solubility, the particles begin to form.
[6]
One of the most commonly used methods is the
sol-gel method. This method involves the synthesis of an inorganic mineral
network using a mixture of alkaloids and water in the presence of solvents and
catalysts and then processes the hydrolysis, gelling and removal of organic and
aqueous phase residues using low temperatures. Factors such as the effect of
pH, the effect of solvent and water effect can affect the properties of
hydroxyapatite produced by this method [7].
Biological synthesis
Based on the properties mentioned, the
applications of hydroxyapatite can be used in tissue engineering, dentistry,
drug delivery, chromatography column, catalyst, water purification, industrial
effluent and application in sensors. The synthesis of nanoscale hydroxyapatite
by many bacteria included Serratia
marcescens, Enterobacteria, Staphylococcus, Alkanindiges illinoisensis and many other alkaline phosphatase
producer microbe has been reported [8,9]. Enzymatic synthesis by phosphatase
and urease also are reported Calcium phosphate precipitates formed at 37°C and
pH of 7.4 in urea and enzyme urease were found to be amorphous, following
overnight drying at 90°C. Crystallization of these precursors started at above
300°C and after calcination at 1100°C for 6 h, they completely transformed into
single-phase of HA.
Plate-shaped hydroxyapatite (HAp) particles
with preferred orientation to the c-plane have been synthesized by a homogeneous
precipitation method via an enzyme reaction of urea with urease. The starting
solutions with a Ca/P ratio of 1.67 were prepared by mixing calcium carbonate,
phosphoric acid and urea [10,11].
Nanoparticles of hydroxyapatite were
successfully synthesized by microbial method at proper pressure and
temperature, with calcium chloride and organic acid as reactants.
The effect of alkaline on hydroxyapatite
production from Geloina coaxans shell
has been done by precipitation method. Geloina
coaxans shells and H3PO4 were used to produce HAp.
The result showed that nano-HAp can be obtained however the pH value is
important parameter on synthesis HAp and can influence crystalinity and purity
of HAp
Selected bacteria According to studies, Ghashghaei
and Emtiazi [9] should have enzyme phosphatase, which is more common in
Enterobacteriaceae family. For example, the Gram negative Enterobacter (Serratia marcescens, strain PTCC 1187)
is used for biogeneration of hydroxyapatite. This bacterium is cultured at a
temperature of 30°C and culture medium included phosphate.
DISCUSSION
Compared to other chemical methods, the
sol-gel method is more suitable because in the usual chemical methods, higher
temperatures are applied to crack the prepared crystals. This is despite the
fact that the temperature required for the hydroxylapatite condensation
operation in the sol-gel method is much lower. Another advantage of cell-gel
compared to other chemical methods is the ability of this method to synthesize
one-stage high-level, high-purity hydroxyapatite nanoparticles. In addition,
the above method can be used to apply low thickness hydroxyapatite coating on a
variety of biomaterials [6].
Another major advantage of this method is the
resemblance of hydroxyapatite powder formed with hydroxyapatite in the bone and
tooth, but this advantage cannot compete with the hydroxyl apatite synthesis
bioassay. In the case of bio-hydroxyapatite synthesized by bacteria in the
range of about 25-30 nm, these differences are a motif of morphology that draws
the attention of scientists to the synthesis of bifidyroxyapatite. It has also
been observed that the distribution of particle size and shape in the
biological method is relatively uniform. This can result in smoother sputtering
and the production of a piece with more favorable properties than parts made
from micronutrients when making a piece of hydroxyapatite powder [7,8].
The disadvantages of the sol-gel method
include the possibility of hydrophobic phosphate, the need for accurate pH
adjustment, and the high cost of primary materials plus the accumulation of
powder thus synthesized, often increasing during the synthesis process. Another
disadvantage of this method is that it is usually not possible to achieve a
high degree of crystallinity, so the hydroxylapatite synthesized by the
chemical-tubular-gel method is not uniform in size and shape. This is where the
bacteria synthesized in hydroxyapatite are not observed. In a given grain,
visible crystalline plates are all parallel to each other. The parallelism of
the plates in a powder particle can result in the conclusion that each particle
of powder from a grain Crystalline formed. Meanwhile, the size of the powder
particles observed is approximately the size of the grain obtained from the
Williamson method, which is due to the fact that the particles of powder have
been produced in a single crystalline form, which results in relatively high
uniformity to chemical methods. Moreover, the amount of hydroxyapatite
biosynthesis was lower than the synthesized hydroxyapatites by conventional
chemical methods [12-15]. NHA was prepared using sol-gel technique and added to
the toothpaste with 7% concentration and the toothpaste containing NHA was more
effective than the toothpaste without NHA for the purpose of demineralization
[16].
In
conclusion nature and concentration of the phosphate precursor, solution pH,
treatment duration, ionic and organic additions to the phosphate solution,
mineralogical composition of Sands tones, sulphated stones, gypsum, concrete,
wall paintings, archaeological bones, paper, marble and limestone, manufacture
different HA. This might be useful for cell culture and tooth paste or be toxic
for human safety [17].
In total, the uniform size and shape, and
their low melting, are factors that contribute to the mechanical properties of
the final piece made of hydroxyapatite. Also, the hydroxyapatite powder
produced with organic bases and, along with that particle size, increased the
biocompatibility and bioactivity of hydroxyapatite, biological syntheses with
enzymes or cells are another way for having non-toxic particles.
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JD, Wopenk B, Freeman JJ, Rogers K, Valsami-Jones E, et al. (2004) Lack of OH
in nanocrystalline apatite as a function of degree of atomic order: Implications
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G, Celotti G, Landi E, La Torretta TMG, Sopyan I, et al. (2003) A novel sol-gel
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H, Housaindokht MR, Chahkandi M (2007) Effects of parameters of sol-gel process
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