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j.jallcom.2017.10.132

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Accepted Manuscript
Preparation and structural characterization of bioactive bredigite (Ca7MgSi4O16)
nanopowder
Maryam Rahmati, Mohammadhossein Fathi, Mehdi Ahmadian
PII:
S0925-8388(17)33568-5
DOI:
10.1016/j.jallcom.2017.10.132
Reference:
JALCOM 43529
To appear in:
Journal of Alloys and Compounds
Received Date: 20 February 2017
Revised Date:
17 September 2017
Accepted Date: 17 October 2017
Please cite this article as: M. Rahmati, M. Fathi, M. Ahmadian, Preparation and structural
characterization of bioactive bredigite (Ca7MgSi4O16) nanopowder, Journal of Alloys and Compounds
(2017), doi: 10.1016/j.jallcom.2017.10.132.
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Preparation and structural characterization of bioactive bredigite (Ca7MgSi4O16)
nanopowder
Maryam Rahmati* , Mohammadhossein Fathi, Mehdi Ahmadian
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Biomaterials Research Group, Department of Materials Engineering, Isfahan
University of Technology, Isfahan, 8415683111, Iran.
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Abstract
Nanostructured bioceramics are expected to have better bioactivity than coarser
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crystals. This work aims to prepare, characterize and evaluate the bioactivity of
bredigite nanopowder, synthesized by modified sol–gel method. In vitro bioactivity
assessment was performed by soaking the samples in the simulated body fluid (SBF)
and immersing it for apatite formation on the surface. X-ray diffraction (XRD), and
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field emission scanning electron microscopy (FESEM) techniques were utilized to
characterize the nanopowders. The morphological and compositional changes of the
samples, after soaking in SBF, were studied and analyzed by scanning electron
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microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), energy dispersive
spectroscopy (EDS), and inductively-coupled plasma optical emission spectroscopy
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(ICP-OES). Results showed that the particle size of pure bredigite was around 38– 48
nm. During immersion in SBF, the dissolution rate of the bredigite nanopowder was
higher than that of the bredigite micro size powder, and apatite was formed on it after 3
days of soaking.
Keywords: Sol–gel processes; Grain size; Apatite; Nano powder; Bioactivity; Bredigite.
* Corresponding author. Tel.: +983113912750; fax: +983113912752.
E-mail address: mrahmati2011@yahoo.com (M. Rahmati)
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1. Introduction
Calcium phosphate ceramic such as hydroxyapatite is good candidate for bone tissue
regeneration in biomedical industries due to their biocompatibility, low density,
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chemical stability and their compositional similarity to human bone. Weak mechanical
properties such as low fracture toughness the value of about 1 MPa m1/2 as compared
with 2–12 MPa m1/2 for human bone and higher Young's modulus than cortical bone
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limit the clinical applications of HA Therefore, pure hydroxyapatite cannot be used as
heavy loaded implants[1-3].
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New studies have demonstrated that glass and ceramics containing MgO–CaO–SiO2
components and glass-ceramic have excellent bioactivity for bone tissue engineering
application[4-7]. Mg is one of the important element in human body besides Ca and Si.
It is closely associated with mineralization of calcined tissues[8], and indirectly
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influences mineral metabolism[9]. Due to the enhanced mechanical properties of MgO–
CaO–SiO2 based bioceramics compared to calcium phosphates, there has been an
increasing interest in these ceramics[10]. Bredigite with chemical formula of
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Ca7MgSi4O16 is one of the bioactive ceramics[5,11-12]. Various techniques have been
applied to synthesize this bioceramics including sol–gel method[5,11], combustion
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method[12] and mechanical activation[13-14]. However, the particle sizes of the as
prepared bredigite were mostly in the range of 1-10 µm[5]. Bioceramics at nano-level
have shown interesting functional properties due to larger surface area to volume ratio
and ultrafine structure similar to biological apatite, which would have a great impact on
implant-cell interaction in body environment[8]. Additionally, nanosized bioceramics
have shown better bioactivity compared to coarser crystals[8,15-16].
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The present study aims to synthesize, characterize bredigite nanopowder by a
modified sol–gel method, and evaluate the in vitro behavior of prepared bredigite
nanopowder in simulated body fluid (SBF). Modified sol–gel method is a combination
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of two methods. This method is essentially a sol-gel method for synthesis of pure
powder, and then a short time ball-milling process was also used for preparing smaller
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nanometer powders
2. Materials and methods
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2.1. Synthesis of bredigite nanopowder
Bredigite nanopowder was prepared using a modified sol–gel method. In this
process, tetraethyl orthosilicate (TEOS) ((C2H5O)4Si, Merck) was mixed with double
distilled water and 2M HNO3 (mole ratio= TEOS:H2O:HNO3=1 : 4 : 0.08) and was kept
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to hydrolyze for 30 min under stirring. Then, magnesium nitrate hexa-hydrate
(Mg(NO3)2.6H2O, Merck) and calcium nitrate tetra-hydrate (Ca(NO3)2·4H2O, Merck)
were added in 150 ml of ethanol as a solvent, while the mole ratio of TEOS:
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Mg(NO3)2.6H2O: Ca(NO3)2. 4H2O was about 4:1:7 . As-prepared solutions were mixed
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and stirred for 5 h at room temperature. The solution was maintained at 60°C for 24 h,
and then dried in an oven at 120 ◦C for another 48 h. The dried gel was calcinated at
1150 ◦C for 3 h at heating rate of 5 °C/min. Finally, the calcinated powder was milled
for 5h using a high-energy planetary ball mill (Fretch Pulverisette 5) with zirconia vial
and balls at ambient temperature. Powder to ball weight ratio of 1:10, and a rotational
speed of 300 rpm were selected.
2.2. Characterization of nanopowders
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Thermal behavior of bredigite powder dried at 120◦C was monitored using a
thermogravimetric and differential thermal analyzer(TG–DTA). The dried powder was
heated from room temperature up to 1100◦C in air with a heating rate of 10◦C/min.
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In order to identify the phase structure and composition of powder, XRD pattern of
the obtained powder was recorded over the 2θ range of 20–60◦ (step size of 0.05 degree
per 1 second) by using a Philips diffractometer(40 kV, 30 mA) with monochromated Cu
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Kα radiation(λ = 0.15406 nm). The pattern was then analyzed by PANalytical X’Pert
High Score software. The obtained experimental patterns were compared with standards
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compiled by the Joint Committee on Powder Diffraction and Standards(JCDPS)[17].
The crystallite size of prepared powder was also determined using Williamson-Hall
equation (Eq. (1))[18].
β cos θ = 0.89 λ D + 2 A
ε
2
sin θ
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Where β is the full width at half maximum(FWHM) of diffraction peaks, λ is the
wavelength of the used X-ray(λ = 0.15406 nm), θ is the diffraction angle, d is the
average crystallite size, ε is internal lattice strain, and A is a coefficient depending on
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the strain distribution.
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. FESEM (Hitachi, S4160) technique was utilized to characterize the morphology of
the synthesized bredigite powder.
2.3. In vitro bioactivity evaluation
The pellet-shape samples with a diameter of 12 mm were prepared by pressing the
nanopowders under a force of 80 kN. Bioactivity evaluation of the obtained
nanopowder was investigated by soaking the samples in the simulated body fluid (SBF)
at pH 7.5 ± .01 for 3, 7, 14 and 28 days. The solid: liquid ratio was 3:2 mg/ml without
refreshing the soaking medium. The SBF was prepared according to the procedure
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described by Bohner and Lemaitre[19]. The soaking experiment was carried out in a
shaking bath maintained at 37°C. At various soaking time, the samples were removed
from SBF and dried at 100°C for 48 h. The apatite formation on the surface of the
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samples was examined by FTIR(Bomem, MB 100, the spectrum was recorded in the
range of 4000–600 cm-1 with 2 cm-1 resolution), SEM(Philips XL30) and EDS. The
concentrations of Ca, P and Mg ions of SBF solutions after soaking the samples were
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determined by ICP-OES(Perkin Elmer, Optima 7300DV). The concentrations of Ca, P
and Mg ions was recorded in the range of 318.049-317.836 nm, 213.555-213.697nm
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and 285.126-285.309nm respectively. Additionally, the changes in pH of soaking
solutions were also measured at pre-determined time intervals for 0–28 days using an
electrolyte-type pH meter.
3.1. Thermal analysis
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3. Results
In order to determine the mechanism of ceramic formation during heat
treatment
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process, thermo-gravimetric analysis was performed. Figure 1 shows the TG–DTA
curves of the bredigite gel after drying at 120◦C for 48 h. The weight loss occurs within
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three main stages leading to the total weight loss of 16%. The first stage takes place at
below 380°C which might be due to the loss of physically adsorbed water molecule into
the bredigite gel and consequences the weight loss of 5%[7,13-14]. The second stage
occurs below 600°C owing to the loss of organic compounds, i.e. alkoxy groups,
decomposition of precursor and burning of carbon and elimination of residual nitrates
introduced by metal nitrate during the preparation of the sol[6-7,20]. The weight loss in
this region is around 9%. The third stage observed below ~ 850°C, is attributed to the
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crystallization of bredigite powder[6-7,21]. DTA curve of the obtained powder (Figure
1) exhibits one small endothermic peak at 310°C, two sharp endothermic peaks at 570°C
and 600°C, and one exothermic peak at 800°C. The first endothermic peak corresponds
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to the dehydration of the precipitated complex, and the latter two endothermic peaks are
associated with decomposition of compounds; meanwhile, exothermic peak is assumed
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to be the result of crystallization of bredigite powder.
3.2. Phase structure analysis
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Figure 2 shows the XRD patterns of bredigite powder before and after ball-milling
process. The XRD patterns of the sample correspond to bredigite(Ca7MgSi4O16) (PDF
#036-0399) and show a strong peak around 32◦ indicating the formation of a single
phase[17]. according to Williamson-Hall equation, the crystallite size of bredigite
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powder, before ball-milling, is about 280-430 nm which decrease to 36 nm after milling.
The morphological shape and particle size of bredigite powder is shown in figure 3.
Results shows the spherical shapes particles with uniform sizes are synthesized by the
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performed process. The average particle size of the bredigite powders measured from
FESEM was around 38–48 nm. Wu and Chang[5] synthesized a bredigite powder by
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sol-gel method and calcination for 3 h at 1150 °C with particle size in the range of 1-10
µm. In another study, Huang and Chang[12] also synthesized nanocrystalline bredigite
powder by a simple combustion method, and the pure bredigite powder was obtained
with particle size in the range of 234-463 nm after calcination at 650 °C for 4 h. Mirhadi
et al.[13] synthesized bredigite powder with particle size of about 1 µm via 20 h
mechanical activation and annealing at 1200°C for 1 h. In this paper, bredigite
nanopowders were prepared by modified sol-gel method, calcined at 1150 °C for 3 h,
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and ball milled for 5 h with particle size in the range of 38-48 nm. The nanometer-sized
grains and the high-volume fraction of grain boundaries and dislocation in
nanostructured bioceramics may enhance osteoblast adhesion, bone-building cells
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proliferation, osteointegration, and the deposition of calcium phosphate on the surface
of these bioceramics[22-23].
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3.3. In vitro bioactivity evaluation
The FTIR spectra of the bredigite nanopowders before and after soaking in the SBF
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are shown in figure 4. Before immersing the sample in SBF, the bands related to the
characteristic peaks of bredigite could be observed at 1100–1000, 960, and 873 cm−1
(SiO4 stretching) and at 616 cm−1(SiO4 bending) confirming the formation of bredigite
powder. After soaking the samples in the SBF for 28 days, the intensity of silicate
absorption bands decreased and the new peaks are appeared. These peaks consist of v3
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vibration peak of PO4-3 groups located in the range of 1030- 1090 cm-1 and, the bands of
CO32- groups observed in the range of 1415-1490 cm-1 and at 872 cm-1. Additionally,
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the bands assigned to hydroxyl(OH-) groups appear at 3571 cm-1, 632 cm-1 and 1619
cm-1[24]. According to the obtained results, it is obvious that the bone-like apatite layer
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is formed on the surface of bredigite. The presence of carbon in the precipitated apatite
structure plays an important role in forming the chemical bonding to the living bone.
This carbonate-containing hydroxyapatite layer can be reproduced in vitro in the SBF.
3.4. Morphology and distribution of apatite
Figure 5 shows SEM micrographs of bredigite nanopowder, before and after soaking
in SBF for different periods of time. The grains of bredigite exhibited spherical
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morphology before soaking the samples(figure 5a). After soaking for 3 days, numerous
uniform wormlike crystallites with size of smaller than 100 nm which is a typical
morphology of apatites was evident(figure 5b). The number and size of these sediments
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were enhanced by increasing the soaking time upon 7 days(figure 5c). After soaking for
14 days, the lath-like apatite crystallites became more compact and the surface is
continually being covered by apatite(figure 5d). Further soaking upon 28 days results in
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the formation of new and tiny sediments of the apatite(figure 5e). Studies on other
bioceramics such as tri-calcium silicate ceramics showed the same morphology of the
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apatite[25-26]. According to the results, it is obvious that bredigite nanopowder can
develop bone-like apatite on the surface after soaking for 3 days in the SBF, showing
enhanced in vitro apatite forming ability as compared to bredigite powders prepared by
common sol–gel method. The bioceramics of micron size, on the contrary, present a low
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surface area and have a strong crystal-to-crystal bond[27]. Bredigite at nano-level would
have highly functional properties due to its grain size, large surface area to volume ratio
and ultrafine structure similar to biological apatite, which would have a great impact on
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implant-cell interaction in body environment[8]. The EDS spectra of the bredigite
nanopowder, after soaking in the SBF for 28 days, are shown in figure 6. The EDS
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spectrum indicates that the tiny ball-like particles are composed of Ca and P, but Mg
and Si in the EDS spectrum originate from the bredigite.
3.5. Changes of pH and ions concentration in SBF solution
Figure 7 shows the changes of the concentrations of Ca, P, Si and Mg in the SBF
containing bredigite pellets after different soaking periods of time. The Mg and Ca
concentrations in the SBF were increased, while the P concentration was decreased. The
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decreasing of P ions concentration continued due to the consumption of these ions
during the formation of apatite on the surface of the pellets. The Ca ions concentration
of bioactive ceramics containing CaO in the SBF normally increases rapidly probably
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due to the degradation of bioceramic and Ca2+ dissolution. The released Ca ions may
increase the supersaturating degree of the SBF, and facilitate the apatite nucleation on
the pellet surface. In fact, the Ca ions concentration is controlled by both the release of
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Ca ions from the bioceramics and the formation of the bone-like apatite[28]. While the
sample is bredigite bioceramic, the degradation rate overcomes the deposition rate,
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resulting in a continuous increase in a concentrations of Ca ion. The Mg ions
concentration in SBF from the beginning up to 3 days decreased partially but from 3
days up to 28 days increased again slightly. The increase of Mg ions concentration in
SBF is due to its dissolution and the exchange with the H+ ions. Hafezi-Ardakani et al.
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[7] have stated that the Mg may be able to enter the forming hydroxyapatite nuclei and
thus inhibits their evolution to tiny apatite crystals, because this element cannot be
accommodated in the hydroxyapatite structure. The Mg2+ ions substitution into the
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apatite lattice causes noticeable changes in its physical and chemical properties.
Replacing the magnesium ions with calcium ions leads to formation of the amorphous
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calcium phosphate(Ca, Mg)9(PO4)6. In fact, inorganic ions such as magnesium and
sodium, suppress the crystallization of apatite and at higher concentrations favor the
formation of amorphous calcium phosphate. Consequently, the decrease of Mg2+ ions in
the SBF might be due to replacing with Ca2+ ions in the hydroxyapatite structure[2930]. The researchers reported that the glasses containing magnesium oxide had a lower
rate of apatite growth than the other glasses but they had the potential to serve as the
biodegradable composite implants that can repair load-bearing defects in osseous
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tissue[31]. The small change of Si ions concentration in SBF is due to its dissolution of
bredigite. The dissolution of Si ions plays an important role as nuclies for apatite
formation[32]. Figure 8 shows the changes of pH in SBF after different soaking periods
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of time. The pH of the SBF increased after the first 1 days of soaking; and by increasing
the soaking time to 28 days, it remained almost unchanged. Increasing the pH in the
early stage of soaking is due to the exchange of Ca2+ with the H+ ions and increase in
the OH- ion concentration. Kokubo et al.[33] have shown that apatite formation was
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also much sensitive to the concentration of OH- ions in the surrounding solution, and
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that an increase in the pH of the solution would favor the deposition of apatite on the
surface.
Bredigite powder, synthesized by sol-gel method, does not contain nanoparticles. On
the other hand, although it was reported that the powder prepared via mechanical
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activation is nanostructured, the particle size was measured to be about 1 µm and the
mentioned process requires a long time.
The mechanism of apatite formation on surface bioactive glass, glass-ceramics and
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ceramics containing CaO-SiO2 and MgO-CaO-SiO2 components, after soaking in the
SBF, might be similar to each other[34]. According to figure 9, hydroxyl groups on the
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surface of bioceramics in contact with the SBF are known to act as nuclies for apatite
formation. The ionic exchange between Ca2+ ions of the bredigite and H+ ions within the
SBF increases the ions activity to product the apatite in the SBF solution and led to
formation of silanol(Si-OH-) in the surface layer:
Si-O-Ca-O-Si + 2H+ =2 Si-OH- + Ca2+
(2)
The hydrated silica could provide specific favorable sites for the apatite nucleation
on the surface of bioceramics. Finally, a negative charge is produced on the surface by
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forming the functional group (Si-O-). The following reaction takes place on the surface
of the bioceramics[35-36].
Si-OH- +OH- = Si-O- + H2O
(3)
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There is also the release of Mg2+ ions. Previous studies of researchers showed that Mg
in bioglass could cause a decrease of the solubility of the glass. The higher Mg-O bond
energy makes it difficult to release from crystal lattice when compared with the Ca-O
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bond. In addition, Mg atoms in crystal lattice inhibit the Ca atoms releasing, which also
decrease the solubility of the glass. The Mg content indirectly affects the apatite
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formation ability and the dissolution of the bioactive ceramics in CaO-SiO2-MgO
system. The Ca ions in the SBF are initially attracted to the interface between the
powder and solution, and are combined with the negative phosphate ions in the fluid.
This results in the formation of an amorphous calcium phosphate, which later
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crystallizes into bone-like apatite[36-37, 11].
The approach utilized in this study, i.e. combination of sol-gel and mechanical
activation methods, led to the production of nanoparticles with size of about 38-48 nm.
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Besides, this approach provides several advantages such as economic benefits, saving
time and the desired purity of the final product. The nanoparticles of bredigite
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bioceramic are expected to have higher bioactivity and greater capability to form bonelike apatite layer as compared to micron-sized bredigite. Furthermore, the profile of Ca,
Mg and P concentration and pH in SBF is similar to that of CaO-SiO2-MgO based
bioactive ceramics such as diopside. The diopside and bredigite are similar in terms of
their composition.
Obtained bredigite nanopowder can be a good candidate for biomedical applications
such as preparing polymer-bioceramic composite foam and scaffolds, nanoparticles and
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nanofibers for drug delivery, surface treatment of biodegradable Mg alloy, and
electrophoretic deposition nanostructured coating for human body implants[34,38-40].
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4. Conclusions
Single-phase bredigite nanopowder was synthesized by a modified sol–gel method.
Particle size of the prepared bredigite nanopowder was in the range of 38– 48 nm. The
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in vitro bioactivity test showed that bredigite nanopowder possessed bone-like apatite
layer formation ability earlier compared to the micro size bredigite. After 3 days
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soaking, the numerous uniform wormlike of apatite crystallites were formed on the
surface with the size of smaller than 100 nm. Further soaking upon 28 days resulted in
the formation of new and tiny sediments of the apatite. The mechanism of apatite
formation on the surface of bredigite in the SBF seems to be related to the dissolution of
Ca(II) ions from the bioceramics in the early stage of immersion and exchange with H+
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ions leading to the formation of silanol (Si-OH-) in the surface layer. The Ca ions are
attracted to the interface, and are combined with the negative phosphate ions in the
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fluid. It is suggested that bredigite nanopowder was able to present the improved
behavior of bioactivity, and could be a potential candidate for biomedical applications
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such as bone repair tissue engineering.
Acknowledgement
This work is supported by Isfahan University of Technology. The authors are
grateful to the technical and administrative staff for their support and cooperation.
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Figure captions
Figure 1. TG–DTA curves of gel bredigite nanopowders dried at 120 ◦C for 48 h.
Figure 2. XRD patterns of prepared bredigite nanopowders via modified sol-gel
bredigite by sol-gel method.
Figure 3. FESEM micrographs of bredigite nanopowders.
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method, calcined at 1150°C for 3 h, and finally ball milled for 5 h, and prepared
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Figure 4. FTIR spectra of bredigite nanopowder soaked in SBF solutions for various
period of times.
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Figure 5. SEM micrograph of the bredigite powders after soaking in the SBF for
various period of times: (a) 0 day, (b) 3 days, (c) 7 days, (d) 14 days and (e) 28 days.
Figure 6. EDS spectrum of the bredigite powder after soaking in the SBF for 28 days.
Figure 7. Changes of ions concentrations in the SBF solution after soaking the bredigite
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for various periods: (a) Ca, (b) P, (c) Mg, and (d) Si.
Figure 8. Changes of pH in the SBF solution after soaking the bredigite for various
periods.
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Figure 9. Schematic representation of the mechanism of apatite formation on the
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surface of bredigite in the SBF.
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The dear reviewer,
Journal of Alloys and Compounds Elsevier
nanopowder
Highlights:
1. The particle size of pure bredigite was around 38– 48 nm.
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Preparation and structural characterization of bioactive bredigite (Ca7MgSi4O16)
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2. The dissolution rate of the bredigite nanopowder was higher than that of the bredigite micro
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size powder
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3. The bone-like apatite was formed on the surface bredigite after 3 days of soaking.
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