37 CHAPTER - II REVIEW OF THE LI TERATURE 2.1 SYNTHESIS OF HYDROXYAPATI TE Bone is a multiphase composite with main constituents of bone collagen matrix and assembled hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2, HAP], in which HAP is the major inorganic constituent [84]. The synthetic HAP is widely used in various biomedical applications in the form of nano powders, films, scaffolds and coatings [85]. It has attracted the attention of researchers from the past 30 years as filler and an implant material because of its excellent biocompatibility and bioactivity with human bone and teeth [86-88]. The chemical species constituting HAP (Ca, P, O and H) are accepted as non-toxic elements. Moreover, their physical properties such as fracture toughness and fracture strength, etc., depend on the structures, compositions and sizes. The HAP of special shape and size could be used as a seed to induce the directional growth of the hydration products, so as to reinforce the strength of the calcium phosphate cement greatly [89]. Nowadays nanomaterials have wide-range of applications in a variety of areas including chemistry, physics, electronics, optics, materials science and biomedical sciences [90]. Therefore, the development of the synthesis protocols for nanomaterials over a range of chemical compositions constitutes a steadily evolving branch of nanotechnology. Hence, researchers have tried to customize its properties such as bioactivity, morphology and particle size [91,92]. The chemical, structural and morphological properties of synthetic hydroxyapatite can be modulated by
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CHAPTER - II
REVIEW OF THE LITERATURE
2.1 SYNTHESIS OF HYDROXYAPATITE
Bone is a multiphase composite with main constituents of bone collagen
matrix and assembled hydroxyapatite [Ca10(PO4)6(OH)2, HAP], in which HAP is the
major inorganic constituent [84]. The synthetic HAP is widely used in various
biomedical applications in the form of nano powders, films, scaffolds and coatings
[85]. It has attracted the attention of researchers from the past 30 years as filler and
an implant material because of its excellent biocompatibility and bioactivity with
human bone and teeth [86-88]. The chemical species constituting HAP
(Ca, P, O and H) are accepted as non-toxic elements. Moreover, their physical
properties such as fracture toughness and fracture strength, etc., depend on the
structures, compositions and sizes. The HAP of special shape and size could be used
as a seed to induce the directional growth of the hydration products, so as to
reinforce the strength of the calcium phosphate cement greatly [89].
Nowadays nanomaterials have wide-range of applications in a variety of
areas including chemistry, physics, electronics, optics, materials science and
biomedical sciences [90]. Therefore, the development of the synthesis protocols for
nanomaterials over a range of chemical compositions constitutes a steadily evolving
branch of nanotechnology. Hence, researchers have tried to customize its properties
such as bioactivity, morphology and particle size [91,92]. The chemical, structural
and morphological properties of synthetic hydroxyapatite can be modulated by
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varying the method and conditions of synthesis. So, the synthesis of HAP for
various applications is being carried out by many researchers.
There are a number of innovative dispensation routes for the synthesis of
hydroxyapatite powders including the precipitation method, ultrasonic irradiation
technique, microwave synthesis, sol-gel method, molten salt method, hydrothermal
technique, freezing method, mechano-chemical synthesis and template method.
2.1.1 Precipitation Method
Tas et al., [93] reported that the synthesis of nano-sized (~ 50 nm),
homogeneous and high-pure HAP ceramic powder from calcium nitrate tetrahydrate
and diammonium hydrogen salts which were dissolved in modified simulated body
fluid (SBF) solutions at 37 °C and pH 7.4 using a novel chemical precipitation
technique. There was no decomposition of HAP into the undesired β-TCP phase
even after heating at 1600 °C in air for 6 h. They observed the superior
high-temperature stability of biomimetic HAP powders.
The research on the synthesis of hydroxyapatite particles using precipitation
method have been reported by Khopade et al., [94]. Although the benefits of low
cost and simplicity of precipitation techniques have provided a direct avenue for
HAP synthesis, the deviation from normal HAP phase can be significant with these
techniques [74,95].
Synthesis of HAP particles using precipitation method have been reported by
Patric et al., [96] has shown that the HAP particle size decreases with increasing
temperature. Liu et al., [97] synthesized HAP nanorods at pH of 4.5 in the presence
of suitable surfactant via wet chemical technique at low temperature. The
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as-synthesized nanorods are found to be pure with the diameter of 50-80 nm and
length of 0.2-1.2 µm. There were no impurities obtained like carbonated HAP
during the process.
Sung et al., [98] reported the synthesis of HAP nano powders using a
modified chemical precipitation route. They explained that the dried HAP powder
was almost in amorphous state with very low crystallinity, showing fine particle size
of ~50-100 nm. Due to the enhanced surface area of HAP nano powder showed high
sintered density at 1000 °C. The Ca/P ratio in powder preparation was varied and the
amount of each HAP and β-TCP phase was analyzed. The powder with a Ca/P ratio
of 1.70 showed the formation of the lowest β-TCP content, while that of 1.75
showed the formation of a CaO phase as well as β-TCP. By using a modified
chemical route and varying chemical composition and sintering β-TCP phase
formation behavior of HAP powder could be controlled for artificial hard tissue
applications.
A process for the synthesis of HAP nanocrystals by a wet chemical method
at low temperature in an aqueous medium was reported by Murugan et al., [99].
Further its chemical and crystallographic functionalities were quite similar to
biological apatite. The in-vitro bioresorbability of the nano-HAP is higher than
conventional HAP and close to biological apatite, which can be attributed to its
surface area owing to nanostructure processing. The smoothness of the nano-HAP
would not damage or harm any living biological organism upon implantation.
Yun et al., [100] have synthesized nanocrystalline HAP by chemical
precipitation method with the aid of ultrasonic irradiation using calcium nitrate and
ammonium dihydrogen phosphate as source materials and carbamide (NH2CONH2)
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as precipitator. The crystallinity and morphology of the resulting nanoparticles are
dependent on the Ca/P ratio, concentration of Ca2+
, ultrasonic power and synthetic
temperature. When the concentration of Ca2+
exceeded 0.2mol/L and the ultrasonic
power was lower than 300 W, the monophase of HAP powder could not be obtained.
When the reactive temperature and Ca/P ratio increase from 313 to 353 K and 1.67
to 2.5, respectively the as-prepared crystallites exhibited a preferential growth along
the (002) direction and the nanoparticles showed an acicular morphology. Spherical
nanoparticles could be obtained at high synthetic temperature (353K), ultrasonic
power (300 W) and high Ca/P (2.0-2.5). In addition, the crystallite size of the HAP
nanoparticles decreases with the decrease of [Ca2+
] ions and the increase of synthetic
temperature and ultrasonic power.
Pramanik et al., [101] have studied the synthesis of nanosized HAP by a
precipitation technique at room temperature using an aqueous solution of calcium
nitrate and diammonium hydrogen phosphate as starting materials in presence of
various capping agents like triethanolamine, ethylenediamine tetraacetic acid,
diethanolamine and ethylene glycol separately. The capping agents effectively
restrict the nuclei growth of the HAP. The n-HAP powders synthesized with various
capping agents showed crystallites with small particle size and less agglomeration
compared to the n-HAP synthesized without using any capping agent. This
technique may be used for bulk preparation of nanoHAP powder with high specific
surface area and high aspect ratio.
Wang et al., [102] reported the synthesis of hydroxyapatite nanoparticles by
using chemical precipitation method using various organic modifier such as
polyethylene glycol, tween 20, trisodium citrate and D-sorbitol, they found that
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polyethylene glycol was beneficial for the formation of HAP nanorods with a larger
aspect ratio (average length/average diameter) at high synthesis temperature and
Tween 20 and trisodium citrate favoured the formation of small-sized HAP
nanorods, and D-sorbitol helped the formation of HAP nanorods with long length at
low synthesis temperatures. In their report they also proved that the crystallinity of
the resultant HAP nanorods increased with increase of the synthesis temperature.
The conversion of hydroxyapatite powders into HAP whiskers by using the
refluxing method through the dissolution–reprecipitation process was reported by
Seo et al., [103] in which the amorphous reprecipitates were initially formed in the
Ca(EDTA)2-PO43-
mixed solution and have continuously grown into long and thin
HAP whiskers. It is obvious that the morphology of the final whiskers is altered by
the concentration of H2O2, pH of the starting solution, and refluxing temperatures.
By increasing the H2O2 concentration, pH value and refluxing temperature,
well-shaped whiskers with clean and smooth surfaces were formed. The higher
value of pH was also responsible for the formation of monodispersed HAP whiskers.
Whiskers with high-aspect ratio were obtained from the starting reaction solution of
pH = 9 at the relatively low temperature of 100 °C.
Cengiz et al., [104] have synthesized hydroxyapatite nanoparticles using CaP
tris solution as a calcium phosphate medium by precipitation method. Their results
fit very well with the reference and the crystallite they synthesized in the presence of
CaP tris solution varied in a range of very fine size from 15.88 to 16.12 nm. In terms
of (d002/d300) ratio, which is a measure of uniformity of crystallites, the ratio was
0.99 for the particle synthesized using CaP tris as a medium. As the powder
(nanoparticles) produced from SBF contains large fraction of needle-shaped
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particles, nanoparticles synthesized in the presence of CaP tris medium almost
consist of desultory structure. There is an acceptable agreement between the results
for CaP tris. They concluded from the results of HAP particles (CaP tris) the length
and width of the particles were less than 500 and 100 nm, respectively and the
composition of the final product produced from CaP tris (1.58) is closer to the ideal
HAP (1.67). Therefore it showed that CaP tris solution can act as a medium to
synthesize HAP nanoparticles.
Shanthi et al., [105] have obtained nano-crystalline HAP rods by
co-precipitation method using cationic surfactant CTAB as template, at ambient
temperature and pressure. The temperature controls like hydrothermal treatment or
refluxing process, which hurdle the bulk production was eliminated. HAP rods with
diameter by 20 nm and length in the range of 100-200 nm were obtained.
Calcium nitrate and tri ammonium phosphate as starting materials for the
preparation of water-dispersible HAP nanorods was reported by Tan et al., [106].
They prepared HAP nanorods with length of 300-400 nm and width 40-60 nm by
chemical precipitation at 90 °C. The sodium citrate used in the reaction medium
improved the colloidal stability of HAP dispersions by adsorbing citrate ions on
HAP surface.
Catros et al., [107] have synthesized nano-HAP using wet chemical
precipitation method at room temperature. The morphological, physico-chemical
and crystallographic analysis revealed the specific features of HAP. Biological
in-vitro experiments revealed that the high affinity and proliferative ability of MG63
cells cultured on to the material.
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Wu et al., [108] in their study reported the synthesis of mesoporous
nano-hydroxyapatite (n-HAP) particles by low-temperature co-precipitation method
in the presence of CTAB. The cationic surfactant of CTAB was used as a template
to regulate n-HAP crystal nucleation and growth. The results show that the
synthesized particles have the features of high pure phase, low crystallinity and
mesoporous structure. The ratio of surfactant effectively influences the mesoporous
structure of n-HAP particles, including the surface area, the pore volume and the
pore size. The adsorbed amount of Bovine Serum Albumin (BSA) on n-HAP
increases with the specific surface area and the pore volume, and the release rates of
BSA are different due to the different pore sizes and pore structures. The n-HAP
particles synthesized with 0.5 % CTAB exhibited the highest BSA loading and the
slowest release rate due to its highest specific surface area and the smaller pore size,
indicating that it has optimal mesoporous structure for good loading and well
controlled release of BSA. These mesoporous n-HAP materials demonstrate a
potential application in the field of protein delivery due to their bioactive,
biocompatible and mesoporous properties.
2.1.2 Ultrasonic Irradiation Technique
Ultrasound irradiation can be utilized to synthesize bioceramic materials
such as hydroxyapatite too. It was well demonstrated by synthesizing hydroxyapatite
from the hydrolysis of calcium oxy phosphates by Fang et al., [109]. The reaction
time required to prepare hydroxyapatite from a mixture of Ca4(PO4)2O and
CaHPO4(H2O) (brushite) and 38 ± 0.5 °C was reduced from 3 h without sonication
to 15 min with sonication. The morphologies of these two products were however
quite different. Meissner et al., [110] in their studies have observed that the size and
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morphology of HAP can be controlled and are dependent on the precipitation and
ultrasonic power. Kim and coworkers have also reported the synthesis of
hydroxyapatite particles from H3PO4 and Ca(OH)2 using sonication has better
thermal behavior than the conventional counterparts [111]. Further, the morphology
of the resulting powders could also be conveniently controlled by this method.
Gopi et al., [112] have reported the synthesis of hydroxyapatite nanoparticles
by ultrasonic coupled sol-gel method. They found that the HAP synthesized by this
method possesses an excellent purity and the particle sizes are nano sized.
Gopi et al., [113] have also reported the synthesis of hydroxyapatite
nanoparticles by a novel ultrasonic assisted mixed template directed method. In this
method glycine–acrylic acid (GLY–AA) hollow spheres were used as an organic
template which could be prepared by mixing of glycine with acrylic acid. The effect
of ultrasonic irradiation time on the crystallinity and size of the HAP nanoparticles
in presence of glycine–acrylic acid hollow spheres template were investigated. From
the inspection of the above results it is confirmed that the crystallinity and size of the
HAP nanoparticles decrease with increasing ultrasonic irradiation time.
2.1.3 Microwave Synthesis
Hydroxyapatite powders prepared by reacting calcium chloride and
diammonium hydrogen phosphate in three types of cyclohexane, surfactant