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Journal ArticleDOI

Accurate characterization of pure silicon-substituted hydroxyapatite powders synthesized by a new precipitation route

01 Jun 2013-Acta Biomaterialia (Acta Biomater)-Vol. 9, Iss: 6, pp 6992-7004

TL;DR: The results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.

AbstractThis paper presents a new aqueous precipitation method to prepare silicon-substituted hydroxyapatites Ca 10 (PO 4 ) 6-y (SiO 4 ) y (OH) 2-y (V OH ) 2-y (SiHAs) and details the characterization of powders with varying Si content up to y = 1.25 mol mol SiHA −1 . X-ray diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance and Fourier transform infrared spectroscopy were used to accurately characterize samples calcined at 400°C for 2 h and 1000°C for 15 h. This method allows the synthesis of monophasic SiHAs with controlled stoichiometry. The theoretical maximum limit of incorporation of Si into the hexagonal apatitic structure is y < 1.5. This limit depends on the OH content in the channel, which is a function of the Si content, temperature and atmosphere of calcination. These results, particularly those from infrared spectroscopy, raise serious reservations about the phase purity of previously prepared and biologically evaluated SiHA powders, pellets and scaffolds in the literature.

Summary (1 min read)

1. Introduction

  • In order to correctly describe the physical, chemical and biological properties of SiHAs and to compare them to routinely implanted HA and β-TCP, well-characterized pure SiHAs powders first need to be prepared.
  • Therefore, this work was devoted to the development of a new route to synthesize monophasic SiHA powders with controlled stoichiometry.
  • To this purpose, a solution of soluble silicate was first prepared from TEOS via a sol-gel route, and then accurate powder analysis was carried out by means of ICP/AES, Xray powder diffraction, Rietveld refinement, high resolution electron transmission microscopy (HR-TEM) with energy dispersive spectroscopy (EDS) as well as infrared (FT-IR/ATR) and solid-state NMR spectroscopy.
  • Two pH levels of precipitation were studied, as well as six Si/P molar ratios.

2.1 Powder synthesis

  • The as-synthesized powders were heated under air using an alumina crucible.
  • The heating and cooling rate was fixed at 4°C min -1 .

2.2.1 X-ray powder diffraction and Rietveld refinement

  • Crystalline phases were identified by means of a Siemens D5000 θ/2θ X-ray diffractometer.
  • The evolution of the crystallinity of the samples after calcination at 1000°C for 15 h was evaluated by means of the full width at half maximum (FWHM) of the (211) peak at 2θ=31.8°, as it had the highest intensity and minimal overlap with neighboring peaks.

2.2.4 Electron microscopy (HR-TEM, SAED and EDX)

  • Gold was then distributed as crystallized nano-domains which were used as a reference in the selected area electron diffraction (SAED) patterns to calculate as precisely as possible the lattice parameters.
  • The SAED patterns obtained from regions with or without gold on the HA part were the same.

3.2.4 Electron microscopy

  • The results are the average of about ten intervals per pattern.
  • Moreover, other experimental patterns for different zone axes (not shown here) were obtained and compared to theoretical electron diffraction patterns calculated by means of the Java Electron Microscopy Simulation (JEMS) software [87] .
  • The results indicate that the experimental and simulated patterns are perfectly superimposed for 0.734 ≥ c/a ≥ 0.729.

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Submitted on 20 Jun 2013
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Accurate characterization of pure silicon-substituted
hydroxyapatite powders synthesized by a new
precipitation route
David Marchat, Maria Zymelka, Cristina Coelho, Laurent Gremillard, Lucile
Joly-Pottuz, Florence Babonneau, Claude Esnouf, Jérome Chevalier, Didier
Bernache-Assollant
To cite this version:
David Marchat, Maria Zymelka, Cristina Coelho, Laurent Gremillard, Lucile Joly-Pottuz, et al.. Ac-
curate characterization of pure silicon-substituted hydroxyapatite powders synthesized by a new pre-
cipitation route. Acta Materialia, Elsevier, 2013, 9 (6), pp.6992-7004. �10.1016/j.actbio.2013.03.011�.
�hal-00835892�

1
Accurate characterization of pure silicon-substituted hydroxyapatite powders
1
synthesized by a new precipitation route
2
David MARCHAT
1*
, Maria ZYMELKA
1
, Cristina COELHO
2
, Laurent GREMILLARD
3
,
3
Lucile JOLY-POTTUZ
3
, Florence BABONNEAU
4
, Claude ESNOUF
3
, Jérôme
4
CHEVALIER
3
, Didier BERNACHE-ASSOLLANT
1
5
1
Ecole Nationale Supérieure des Mines, CIS-EMSE, CNRS:FRE3312, F-42023 158 cours
6
Fauriel Saint-Etienne cedex 2
7
2
Institut des Matériaux de Paris Centre, FR 2482, Université Pierre et Marie Curie and CNRS
8
Collège de France, 11 place Marcelin Berthelot 75005 Paris, France
9
3
Université de Lyon, INSA-Lyon, MATEIS Laboratory UMR CNRS 5510, Villeurbanne F-
10
69621, France
11
4
Laboratoire de Chimie de la Matière Condensée de Paris, CNRS, Université Pierre et Marie
12
Curie and CNRS, Collège de France, 11 place Marcelin Berthelot 75005 Paris, France.
13
14
E-mail addresses : marchat@emse.fr (David Marchat)*, mzymelka@emse.fr (Maria
15
Zymelka), cristina.coelho@upmc.fr (Cristina Coelho), laurent.gremillard@insa-lyon.fr
16
(Laurent Gremillard), lucile.joly-pottuz@insa-lyon.fr (Lucile Joly-Pottuz),
17
florence.babonneau@upmc.fr (Florence Babonneau), claude.esnouf@insa-lyon.fr (Calude
18
Esnouf), jerome.chevalier@insa-lyon.fr (Jérôme Chevalier), bernache@emse.fr (Didier
19
Bernache-Assollant),
20
21
*Corresponding author: Phone: 0033 (0)4 77 49 97 01, Fax: 0033 (0)4 77 49 96 94
22
Address: Ecole Nationale Supérieure des Mines, Centre Ingénieurie et Santé, 158 cours
23
Fauriel 42023 Saint-Etienne, cedex 2, France.
24
25

2
Abstract
26
This paper presents a new aqueous precipitation method to prepare silicon-substituted
27
hydroxyapatites Ca
10
(PO
4
)
6-y
(SiO
4
)
y
(OH)
2-y
(V
OH
)
y
(SiHAs) and details the characterization of
28
powders with varying Si content up to y=1.25 mol mol
SiHA
-1
. X-ray diffraction (XRD),
29
transmission electron microscopy (TEM), solid-state nuclear magnetic resonance (NMR) and
30
Fourier transform infrared (FTIR) spectroscopy were used to accurately characterize samples
31
calcined at 400°C for 2 h and 1000°C for 15 h. This method allows for synthesizing
32
monophasic SiHAs with controlled stoichiometry. The theoretical maximum limit of
33
incorporation of Si into the hexagonal apatitic structure is y<1.5. This limit depends on the
34
OH content in the channel, which is a function of the Si content, temperature and atmosphere
35
of calcination. These results, particularly those from infrared spectroscopy, express serious
36
reservations about the phase purity of SiHA powders, pellets or scaffolds prepared and
37
biologically evaluated in the literature.
38
39
40
41
42
43
44
45
Keywords: biomaterials; silicon-substituted hydroxyapatite; precipitation method, infrared
46
spectroscopy, NMR spectroscopy.
47
48

3
1. Introduction
49
According to the literature, silicon-substituted hydroxyapatite (SiHA) is a highly promising
50
material in the field of bioactive bone substitutes and bone tissue engineering. It is now well-
51
established that silicon plays an important role in the early stage of cartilage and bone growth
52
[1-4]. Soluble silicon species have been shown to stimulate spontaneous calcium phosphate
53
precipitation (i.e. the mineral bone phase) [5] and to increase bone mineral density [6].
54
Moreover, silicon has been reported to have a positive influence on the synthesis of type I
55
collagen by human osteoblast cells (MG-63 cell line) in vitro [7]. Thereby, it is expected that
56
silicon could enhance the hydroxyapatite (HA) bioactivity [8, 9], and silicon-substituted
57
hydroxyapatites (SiHAs) have become a subject of great interest in bone repair. The SiHA
58
structure corresponds to the substitution of phosphate ions (PO
4
3-
) by silicate ions (SiO
4
4-
)
59
into the HA crystal structure. Different mechanisms for charge compensation have been
60
suggested [8, 10, 11]. The most cited one was proposed by Gibson et al. with the creation of
61
anionic vacancies at OH
-
sites [8, 12]. This mechanism leads to silicon-substituted
62
hydroxyapatites with the general formula Ca
10
(PO
4
)
6-y
(SiO
4
)
y
(OH)
2-y
(V
OH
)
y
, where y
63
represents the molar number of silicate groups introduced into the apatitic structure (0 y
64
2) and V
OH
stands for vacancies maintaining the charge balance. The incorporation of Si into
65
the HA structure in solid solution, i.e. without the formation of other phases, seems to be
66
limited. However, the value and the origin of this limitation are still not known, with for
67
instance the following values: 5 wt% ( 1.7 mol
Si
mol
SiHA
-1
) [13-15], 4 wt% ( 1.4 mol
Si
68
mol
SiHA
-1
) [16, 17], 3.1 wt% ( 1.1 mol
Si
mol
SiHA
-1
) [18], 2 wt% ( 0.7 mol
Si
mol
SiHA
-1
) [11,
69
19], 1.0 wt% (0.36 mol
Si
mol
SiHA
-1
) [20] or 0.28 wt% (0.1 mol
Si
mol
SiHA
-1
) [21]. Additionally,
70
it has been suggested that the concentration of 0.8 wt% of Si ( 0.28 mol
Si
mol
SiHA
-1
) is
71
optimal to induce the development of important bioactivity [22-24]. A value of 2.2 wt% of Si
72
was also reported by Thian et al. [25]. Unfortunately, in spite of extensive studies in recent
73

4
years, these results remain heterogeneous, confusing and sometimes misleading. For instance,
74
Hing et al. revealed faster bone apposition and improved adhesion and proliferation of
75
osteoblast-like cells for SiHA compared to stoichiometric HA [23, 26], whereas Palard et al.
76
found no significant difference in the behavior of MG-63 osteoblast-like cells between pure
77
HA and SiHA pellets (three compositions: y=0.2, 0.4 and 0.6 mol
Si
mol
SiHA
-1
) [27]. Recent
78
critical analyses of the published results regarding SiHAs have highlighted the lack of
79
experimental evidence which could explain the real effects of Si substitution on biological
80
activity in a biological environment [28, 29]. In particular, it has been criticized that the
81
physico-chemical characterizations of SiHA bioceramics are not detailed (purity, solubility,
82
rate of incorporation of Si inside the crystal lattice, etc.). Therefore, the available data do not
83
provide sufficient information to establish the origin of the improved biological performance
84
of SiHA: (i) a direct effect of SiHA by Si release, (ii) an indirect effect of SiHA by changes in
85
the physico-chemical properties of HA due to Si substitution (microstructure, superficial
86
chemistry, topography, etc.) or (iii) an effect of second phases (crystalline and/or amorphous).
87
According to Boanini et al., the term “ion-substituted” is quite often used without any
88
experimental proof regarding the incorporation of ions inside the crystal lattice of calcium
89
orthophosphates [28]. The unclear bioactivity of SiHA ceramics could be explained by
90
variations in the phase composition. The first evidence for this was provided by the few
91
accurate analyses available in the literature which show that SiHA powders can contain
92
crystallized [16, 30-35] and amorphous [16, 34, 36-38] impurities. The study by Kanaya et al.
93
is representative of the characterization problems of SiHA samples [38]. Indeed, while the X-
94
ray diffraction patterns show only the characteristic lines of the HA phase (PDF: 09-432), the
95
29
Si MAS NMR spectrum revealed that only 10% of Si was incorporated into the HA lattice;
96
the rest was on the particle surface in the form of polymeric silicate species [38]. An
97
equivalent observation was made by Gasquères et al. [16]. Most studies do not evidently show
98

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  • ...0 powders mainly present bands characteristic of 432 hydroxyapatite with the ν1 (962 cm ), ν2 (473 cm ), ν3 (1021 and 1085 cm ) and ν4 (562 and 433 600 cm) modes of PO4 , as well as the stretching ( νS: 3572 cm ) and librational ( νL: 629 cm 434 (1)) modes of hydroxide groups [81, 92, 94]....

    [...]

  • ...and ν4), and the stretching ( νS) and librational ( νL) modes of the hydroxide groups [81, 92-94]....

    [...]

  • ...The former was chosen 182 according to phosphate and silicate speciation curv es, in order to have HPO 4 2- [81] and 183 H3SiO4 - ions [82] in solution as the main phosphate and si licate species, respectively....

    [...]


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"Accurate characterization of pure s..." refers background in this paper

  • ...Two other SAED patterns were 324 obtained for the [1-10] zone axis....

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  • ...established that silicon plays an important role in the early stage of cartilage and bone growth 52 [1-4]....

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TL;DR: In conclusion, orthosilicic acid at physiological concentrations stimulates collagen type 1 synthesis in human osteoblast-like cells and enhances osteoblastic differentiation.
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  • ...collagen by human osteoblast cells (MG-63 cell line ) vitro [7]....

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