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Volume 8
Number 4
28 January 2020
Pages 583–828
Journal of
Materials Chemistry B
Materials for biology and medicine
rsc.li/materials-b
ISSN 2050-750X
lin d in
COMMUNICATION
e!
Roberto Orru, Christophe Drouet et al.
ed xe
First successful stabilization of consolidated amorphous
M nde
calcium phosphate (ACP) by cold sintering: toward
highly-resorbable reactive bioceramics
I
Journal of
Materials Chemistry B
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COMMUNICATION View Journal | View Issue
First successful stabilization of consolidated
amorphous calcium phosphate (ACP) by cold
Cite this: J. Mater. Chem. B, 2020,
8, 629 sintering: toward highly-resorbable reactive
Received 27th September 2019, bioceramics†
Accepted 22nd November 2019
Published on 28 November 2019. Downloaded on 1/30/2020 2:11:06 PM.
DOI: 10.1039/c9tb02121c
Marina Luginina,ab Roberto Orru, *a Giacomo Cao,a David Grossin,b
Fabien Brouillet,b Geoffroy Chevallier,bc Carole Thouronb and
rsc.li/materials-b Christophe Drouet *b
In the field of bone regeneration, some clinical conditions require allograft and autograft tissue replacement has been a major
highly-resorbable, reactive bone substitutes to rapidly initiate tissue subject of research.1–3 Hydroxyapatite (HAp), (Ca10(PO4)6(OH)2),
neo-formation. In this view, Amorphous Calcium Phosphates (ACP) due to its good biocompatibility and structural similarity to the
appear as well suited bioceramics taking into account their high mineral part of calcified tissues, is one of the most commonly
metastability. However, the metastability also leads to difficulties of used bioactive ceramics as of today.4 However, it is thermo-
sintering without transformation into crystalline compounds. In this dynamically stable under physiological conditions and thus
work, various calcium phosphate samples (co)doped with carbonate exhibits a low resorption rate in vivo. Additionally, due to the
(CO32 ) and magnesium ions were synthesized by the double decom- low surface area and limited surface reactivity HAp bioactivity is
position method in alkaline media using ammonium and potassium hindered after implantation.5,6 Such differences with respect to
hydroxide solutions. The obtained amorphous powders possess an bone mineral, also called biological apatite, are ascribed to the
exceptionally-high carbonate content up to 18.3 wt%. Spark Plasma fact that the latter consists of nanocrystalline non-stoichiometric
Sintering (SPS) at very low temperature (150 8C) was then utilized to polysubstituted apatite.7 Besides carbonate (CO32 ), which is the
consolidate initial powders with the view to preserve their amorphous main substituting ionic group in biological apatite (5–9 wt%),8–10
character. The influence of the introduction of different apatite growth other ions such as HPO42, Mg2+, Sr2+, Na+ and F are also
inhibitors such as carbonate (CO32 ) and magnesium ions was studied. encountered.11,12 According to Legros et al.,13 the composition
XRD and FTIR analyses revealed that sintered ceramics generally con- of bone mineral is characterized by significant vacancy contents
sisted in highly carbonated low-crystallinity apatites, which are expected in Ca and OH sites, and can be described (omitting trace
to have higher solubility than conventional apatite-based systems. elements) by the general formula Ca8.3(PO4)4.3(CO3,HPO4)1.7-
However, most interestingly, modulation of the doping conditions (CO3,OH)0.3. A recent report analyzing a fresh bone specimen
allowed us to retain, for the first time, the amorphous character of by NMR and vibrational spectroscopies pointed in particular to
ACP powders after SPS. Such consolidated ACP compounds may now a high amount of HPO42 ions besides carbonates.14 In apatites,
be considered as a new family of bioceramics with high metastability whether biological or synthetic, hydroxide (OH ) and phosphate
allowing the fast release of bioactive ions upon resorption. (PO43 or HPO42 ) ionic groups can indeed be replaced by
carbonate ions, leading respectively to A and B-type carbonated
apatites. Additionally, AB mixed-type apatites are often reported.15
One important feature of nanocrystalline apatites, whether in bone
Introduction or dentine or their synthetic biomimetic analogs, is the presence of
In the past few decades, the development of synthetic calcium- a hydrated non-apatitic ionic layer covering the surface of the
phosphate-based bioceramics as an alternative approach for nanocrystals and conferring very reactive properties.16,17 In bone,
this is used for maintaining ion homeostasis in body fluids, and
a
Dipartimento di Ingegneria Meccanica, Chimica, e dei Materiali, Unità di Ricerca in synthetic analogs these characteristics can be tailored and
del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei exploited to modulate the reactivity/bioactivity of the nano-
Materiali (INSTM), Università degli Studi di Cagliari, Via Marengo 2, crystals in view of a variety of biomedical applications, like
09123 Cagliari, Italy. E-mail: roberto.orru@dimcm.unica.it
b
bone regeneration but also nanomedicine.18,19 This may also
CIRIMAT, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France.
E-mail: christophe.drouet@cirimat.fr
imply ionic substitutions to modulate the bioactivity of the
c
PNF2, CNRS, Toulouse, France nanocrystals, which may be achieved either for impacting the
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tb02121c stability/degree of crystallinity/thermodynamic properties of
This journal is © The Royal Society of Chemistry 2020 J. Mater. Chem. B, 2020, 8, 629--635 | 629
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Communication Journal of Materials Chemistry B
the apatitic phase,18 or for modulating biological properties, or In this light, the goal of this study was to investigate the
both. For example, doping with Mg2+ ions can activate osteoblast consolidation ability by SPS at low temperature, of amorphous
cells, and Mg-doped apatites also exhibit a less pronounced CaP compounds, with the general aim of producing ACP-based
crystallinity (lower maturation state) than Mg-free ones, thus highly reactive and resorbable biomaterials as a novel family of
leading to a higher reactivity. Like magnesium, carbonate ions bone substitutes. All previous attempts to stabilize ACP after
are also growth inhibitors for the apatite structure, having also an sintering, including by SPS at low temperature,34 have failed since
impact on apatite nanocrystals reactivity/solubility.20 the crystallization into apatite was systematically observed. This
These substitutions and subsequent structural disorder may be related to several factors such as inadequate synthesis
provide thus a way to prepare biomaterials with a higher solubility protocols or too low carbonate contents, thus not playing fully
compared to stoichiometric hydroxyapatite, which may be parti- their crystallization inhibition role.
cularly appealing to rapidly confer constitutive building blocks of In the present work, we have explored alternative synthesis
mineral in view of activated bone regrowth. routes to prepare very highly carbonated ACP powder precursors,
Although the mechanisms of bone mineralization have not eventually also containing magnesium ions, so as to allow
yet been fully clarified, it was suggested that the formation of stabilization and retain the amorphous character even after
Published on 28 November 2019. Downloaded on 1/30/2020 2:11:06 PM.
new bone may involve amorphous calcium phosphate (ACP) SPS consolidation. Indeed, carbonate and magnesium ions are
reactive precursors.21–23 Such ACP are not crystallized but possess two crystallization inhibitors for the apatitic structure, but
short-range order in the form of small organized units with their cumulated effect on low temperature sintering had not
average size of about 0.9 nm, known as Posner’s clusters of been investigated so far. We show here, for the first time, the
chemical formula Ca9(PO4)6nH2O.24 Despite the relevance of ACP possibility to obtain still-amorphous compounds, in this case
to the field of bone regeneration, these amorphous compounds calcium phosphates, after SPS treatment via ‘‘low temperature
have not found as yet the development they deserve – except as an sintering’’. This new family of compounds is intended to allow
ingredient in bone cement formulation.25,26 This can probably be the preparation of highly resorbable CaP ceramics for very fast
linked to the difficulty to process ACP to make granules or 3D release of bioactive building blocks relevant to bone tissue
pieces while avoiding their crystallization into crystallized CaP regeneration. The main physicochemical characteristics of the
such as apatite. Two main techniques have been proposed so far obtained samples are also investigated in this paper by taking
to obtain synthetic ACP, i.e. a wet route in aqueous medium27,28 advantage of complementary techniques.
and a dry method carried out at high temperatures.29 The
consolidation of ACP represents a difficult goal to achieve due
to the thermal instability of this amorphous precursor. Therefore, Experimental
relatively mild conditions are required to consolidate it while
preserving its metastable character. Overcoming this challenge Synthesis of amorphous (ACP) powders
could then provide a new family of CaP compounds with even The synthesis of five types of ACP-based powders was carried
greater reactivity and bioresorbability (resorbability in vivo being out by double decomposition method,36,37 with and without
directly related to solubility3). carbonate and eventually magnesium ions.
Since years 2000s, the use of Spark Plasma Sintering (SPS) at The starting materials used to this aim are listed in Table 1,
very low temperature (150 1C) was reported to be effective for the along with the selected relative amounts.
consolidation of biomimetic nanocrystalline apatite powders,30–32 Two different solutions, indicated as A and B, respectively,
opening the way to ‘‘cold sintering’’ approaches exploiting the were prepared individually by dissolving the precursors in
water contained in the compounds. These initial experiments deionized water. The pH of both solutions was adjusted to
were performed on non-carbonated apatites. More recently, the B10 (experimental value 9.9). To this purpose, either ammonium
possibility to adjoin natural polymers such as cellulose fibers to hydroxide (NH4OH 20%, Fisher Chemical) (method 1) or potas-
obtain reinforced biomimetic apatite/polymer composites has sium hydroxide solution (KOH, Fisher Chemical) (method 2) were
also been demonstrated.33 In addition, this low temperature SPS utilized as mentioned in the text. As explained in the Result and
exploration has been extended to the obtainment of carbonated discussion section, the Mg30-cACP powders were prepared
apatites.34 according to both methods. To this purpose, di-ammonium
The SPS technique is based on the application of a uniaxial hydrogen phosphate (NH4)2HPO4 was also replaced in the initial
mechanical pressure simultaneously with a pulsed current, which solution B by di-potassium phosphate (K2HPO4), in the same
causes heating by Joule effect through the electrically conductive proportion, leading to the sample denoted Mg30-cACP-K. To
die.35 This method allows consolidating different types of materials reach pH 9.9, KOH solutions with pH = 13 or 15 were added to
including ceramics, metals and polymers under lower temperatures solutions A and B, respectively. Solution B containing HPO42
than those usually employed in conventional hot pressing ions exhibited buffer properties, so that a stronger initial alkaline
techniques. Despite the possibilities of SPS, very few reports have solution was required to increase its pH. The rest of the synthesis
been focused on cold sintering. Sintering at very low temperature procedure was the same for both methods. In each case, solution
(o300 1C) was yet found to be especially relevant for consolidating A was added to solution B at room temperature (B22 1C). The
biomimetic apatites so as to preserve their nanocrystalline, hydrated resulting mixture was stirred for few minutes and then immediately
and thermodynamically metastable features.32 filtered on a Buchner funnel, washed with alkalinized deionized
630 | J. Mater. Chem. B, 2020, 8, 629--635 This journal is © The Royal Society of Chemistry 2020
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Table 1 Starting reactants used to synthesize amorphous calcium phos- were recorded in a range 10–901 with a step of 0.021 and dwell
phates (ACP) time 1 s. In addition, vibrational spectroscopy analysis was
Composition conducted on a Nicolet 5700 Fourier transform infrared (FTIR)
spectrometer. Spectra were recorded in transmission mode in
Solution A Solution B
1
the 400–4000 cm 1 range with 64 scans and 4 cm 1 resolution,
Molar concentration, mol L
Sample name the KBr pellet method was used. Obtained FTIR spectra were
(method number) Ca(NO3)24H2O Mg(NO3)26H2O (NH4)2HPO4 NaHCO3 analyzed using the OMNIC software (Thermo Fisher Scientific).
ACP 27(1) 0.3a 0 0.13b 0 Ca2+ and Mg2+ titrations were carried out by atomic absorption
cACP (1) 0.3a 0 0.65b 0.1b spectroscopy (AAS, in flame mode) using an Analytik Jena
Mg5-cACP (1) 0.29a 0.017a 0.65b 0.1b
Mg15-cACP (1) 0.26a 0.05a 0.65b 0.1b
contrAA 300 high-resolution continuum source spectrometer.
Mg30-cACP (1,2) 0.22a 0.1a 0.65b 0.1b Mechanical evaluations (determination of the tensile strength s)
a b were carried out on the SPS pellets (B8 mm diameter and 2 mm
Refers to a 1 vol% NH3 aqueous solution. Refers to a 2 vol% NH3
aqueous solution. height) via diametral compression tests, aka Brazilian tests, as
previously reported30 on disk-shape samples not suitable for
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regular axial compression tests. These measurements were per-
water (pH 9.9) and freeze-dried. Pure ACP powder was prepared as a formed using a Hounsfield press (model H25K-S) equipped with a
reference using the method described by Heughebaert.27 25 kN force sensor and at a loading speed of 0.5 mm min 1.
Spark plasma sintering (SPS)
SPS experiments for the consolidation of amorphous precursors
Results and discussion
were carried out by means of a 2080 Sumitomo Coal Mining
equipment under argon atmosphere. This equipment provides Powder synthesis and physicochemical characterization
uniaxial pressing and simultaneous heating via DC pulsed The first systems considered in this study were synthesized
current (maximum 8000 A and 10 V). During the experiments, using ammonium hydroxide solution as alkalinization medium.
current was limited to 1000 A, and the sequencing of pulses was XRD patterns of the corresponding powders are shown in Fig. 2.
12 : 2 (12 pulses of 3.3 ms followed by two time intervals without Standard ACP is shown on Fig. 2a. All other compounds were
current). Powder samples (0.35 g) were placed in a graphite die precipitated in the presence of carbonate ions, hence the denomi-
with internal diameter equal to 8 mm covered with a graphite nation cACP, and with increasing magnesium concentrations. For
foil to facilitate sample release after SPS. magnesium contents in the starting powder mixture system equal
Sintering runs were performed with temperature and pressure or less than 15 wt% (Fig. 2b–d), corresponding to experimental
programmed sequences (Fig. 1) similar to those used in previous Mg/(Mg + Ca) molar ratios between 0 and 0.13, our results
studies aimed to the consolidation of biomimetic apatites and indicated that no defined peaks could be detected by XRD,
derived composites.32,33 confirming the preservation of the amorphous character of these
carbonated or non-carbonated ‘‘ACP’’ samples. On the other hand,
Physicochemical characterization the XRD pattern of Mg30-cACP (Fig. 2e) showed, in addition to the
Initial powders and sintered samples obtained after SPS were broad halo with maximum 2y value at around 301, several other
first characterized by X-ray diffraction, using a BRUKER D8 prominent peaks, which are attributed to the presence of a
Advanced diffractometer with CuKa radiation. XRD spectra secondary NH4MgPO46H2O, struvite, crystalline phase (ICDD-
PDF 01-077-2303).
Fig. 2 Diffraction patterns of powders synthesized using ammonia for pH
Fig. 1 SPS conditions (temperature and pressure programs). stabilization: (a) ACP; (b) cACP; (c) Mg5-cACP; (d) Mg15-cACP; (e) Mg30-cACP.
This journal is © The Royal Society of Chemistry 2020 J. Mater. Chem. B, 2020, 8, 629--635 | 631
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The formation of undesired struvite beyond a limit Mg content
may be related to the use of ammonium hydroxide in the synthesis
protocol, thus providing the necessary NH4+ ions. Therefore, this
biphasic Mg30-cACP sample formed in the presence of NH4+ ions
will not be further considered in the study.
FTIR analyses were also carried out for complementary char-
acterization, and the related spectra are presented in Fig. 3. All
the spectra reveal broad bands which are similar to those
reported in previous work for ACP obtained by precipitation in
an alkaline medium.34,38 The broad band between 3700 and
2800 cm 1 and the one at B1640 cm 1 can be attributed to the
presence of water (respectively O–H stretching and H–O–H
bending modes) associated to Posner clusters.39
The amorphous character of the samples leads to poorly
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defined phosphate vibration bands, especially related to the
n3(PO4) and n4(PO4) modes, as opposed to observations with Fig. 4 FTIR spectra of the cACP powder: (a)(I) initial powder, (a)(II) sample
crystalline CaP compounds like apatites,40 and also to a position after SPS; (b) zoom on the n3(CO3) band; (c) zoom on the n2(CO3) band. A,
of the n1(PO4) band close to 959 cm 1 (instead of B961–962 cm 1 B, and L stand respectively for A-type, B-type and Labile carbonates
for apatites).41 For carbonated compounds, additional bands species.
assignable to the CO32 ionic group are also detected (Fig. 3).
In particular, typical regions of the spectra where signals of
amorphous domains and not embedded into the crystallo-
carbonate ions can be observed are 850–900 cm 1 and 1350–
graphic apatitic structure. It is interesting to note that, in case
1550 cm 1, related to the n2(CO3) and n3(CO3) vibration modes,
of this cACP amorphous precursor, carbonate species seem to
respectively. Details of these regions are given in Fig. 4(I) for the
mostly correspond to such labile CO3 (see Fig. 4), pointing to their
cACP synthesized powder. For comparative purposes, the char-
similarity to the labile surface carbonates found in the amorphous
acteristic features of A and B-type carbonate environments in
hydrated layer on nanocrystalline apatites. The same behavior
carbonated apatite, respectively corresponding to carbonate ions
was also observed in the cases of Mg5-cACP and Mg15-cACP
replacing some hydroxyl and phosphate groups in the apatite
initial powders.
structure, have also been pointed out on this figure, and the
The overall content of CO32 ions was estimated for all samples
wavenumbers corresponding to CO32 ionic groups in nano-
by exploiting the FTIR features in a quantitative way, according to
crystalline apatites are summarized in Table 2. In addition, the
Grunenwald et al.’s updated methodology43 (Table 3). In particular,
estimated position of IR bands corresponding to surface carbonates
the ratio rC/P between the integrated intensity of n3(CO3) (1550–
(i.e. CO32 ions located on the hydrated layer on apatite nano-
1350 cm 1 domain) and n3(PO4) band (1230–912 cm 1) was
crystals),42,43 also known as ‘‘labile’’ has also been indicated on
measured and compared to the calibration curve. For the carbo-
Fig. 4 and Table 2.
nated ACP (cACP), the total amount of CO3 was found around
Such labile carbonates are indeed characteristic of meta-
17 wt%, while for Mg15-cACP it reached 18.3 wt%. To the best of
stable chemical environments that may be found in hydrated
our knowledge such highly carbonated amorphous calcium
phosphate powders have been synthesized in the present work
for the first time. By comparison, amorphous powders with carbo-
nate content reaching around 3.1 wt% was reported by Ortali et al.,
although such level was found insufficient to allow preservation of
the amorphous character after SPS consolidation.34 The obtained
augmentation of the carbonation level can be explained on the basis
of several modifications in the synthesis procedure employed here
as compared to literature reports: unheated synthesis (room tem-
perature B22 1C), high alkaline pH raised close to 10, high starting
C/P ratio B1.6 (as opposed to a maximum of 0.5 in ref. 34),
Table 2 Overview of characteristic wavenumbers (in cm 1) of n2(CO3)
and n3(CO3) vibration modes in apatites
Vibrational band A-type B-type Labile CO32
n241,44,45 882 872 866
Fig. 3 FTIR spectra of synthesized powders: (a) ACP; (b) cACP; (c) Mg5- n342,46 1465 1412 1417
cACP; (d) Mg15-cACP. 1542 1462 1480
632 | J. Mater. Chem. B, 2020, 8, 629--635 This journal is © The Royal Society of Chemistry 2020
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Journal of Materials Chemistry B Communication
Table 3 Carbonate contents of as-synthesized powders as well as after
SPS (evaluated by FTIR; uncertainties estimated to 0.5 wt%)
Name of the CO3 wt% samples
sample CO3 wt% initial powders after SPS
ACP 0 0
cACP 17.0 14.0
Mg5-cACP 15.9 11.3
Mg15-cACP 18.3 14.1
low starting Ca/P ratio close to 1.1, and absence of precipitate
Fig. 6 XRD patterns of Mg30-cACP-K synthesized with KOH solution:
maturation. These conditions thus favor carbonation by providing
(a) initial powder; (b) after SPS.
not only fewer cations and more carbonate ions per unit volume,
but also by providing a more stable chemical environment for
limiting CO2 release (high pH, absence of maturation). that the applications aimed with these compounds are not to
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In order to avoid the precipitation of Struvite during the treat large bone defects in load-bearing sites. Typical values of
synthesis of Mg30-cACP, the preparation of the latter was tensile strength s evaluated via diametral compression tests
attempted substituting NH4OH by KOH as described in the reached 13 3 MPa, which are comparable to previous data on
Materials and methods section. FTIR spectra of the synthesized CaP compounds (ACP and nanocrystalline apatite) measured in
powder and the corresponding sample consolidated by SPS are similar conditions.30 Exploration of the SPS data (ESI,† Fig. SI1)
given in Fig. 5a and b, respectively. The carbonation level of raw confirmed the occurrence of an actual sintering stage when
Mg30-cACP-K powder was found to be around 13.8 wt%, while approaching the 150 1C plateau, totally distinct from the powder
its amorphous character was confirmed by XRD analysis compression stage noticeable upon applying mechanical pressure.
(Fig. 6a); and its Mg/(Mg + Ca) molar ratio was close to 0.14. The XRD pattern of the ceramic obtained from pure ACP
The above data thus allowed us to prepare single-phased, highly (Fig. 7a) revealed, as anticipated, that crystallization into an
carbonated ACP powders, with or without co-incorporation of Mg2+ apatite phase (ICDD-PDF 00-009-0432) occurred on this sample
ions. The next step therefore consisted of checking their con- during SPS. Attempts to avoid this crystallization were then
solidation by low temperature SPS treatment, and to char- made by investigating the effect of the carbonation and magnesium
acterize the obtained bioceramic pellets. This is the object of contents of the amorphous precursor powder. Apatite formation
the following section. was also noticed for the cases of cACP exempt of magnesium
(Fig. 7b) as well as for low amounts of Mg, namely for samples
Characterization of spark plasma sintered bioceramics
Mg5-cACP (Fig. 7c) and Mg15-cACP (Fig. 7d). However, it may be
SPS was tested on each of the carbonated/magnesium ACP noted that the related low resolution of the peaks evidences their
compositions prepared (see Fig. 1). In all cases, successful low crystallinity character (similarly to bone mineral47) as
consolidation was achieved, with the obtainment of hardened opposed to pure ACP. This low crystallinity can be related to
pellets. Densification rates (ratio of apparent density to theore- several factors as nonstoichiometry, ion substitutions and nano-
tical) of ca. 60% were attained for cACP, with a decreasing trend sized crystallites. This tendency toward lower apatite crystallinity
down to ca. 45% when increasing the Mg initial content up to is all the more obvious that the Mg content increases (Fig. 7). It can
30%. These values point to the residual porosity within the
pellets, which can be advantageously exploited for the loading
and release of drug/active molecules, also taking into account
Fig. 7 Diffraction patterns of the SPS samples obtained from the amorphous
Fig. 5 FTIR spectra of Mg30-cACP-K synthesized with KOH solution: powders in presence of ammonium hydroxide solution: (a) ACP; (b) cACP;
(a) initial powder; (b) after SPS. (c) Mg5-cACP; (d) Mg15-cACP.
This journal is © The Royal Society of Chemistry 2020 J. Mater. Chem. B, 2020, 8, 629--635 | 633View Article Online
Communication Journal of Materials Chemistry B
thus be stated that in these conditions, carbonate and magnesium SPS is thought to be related to the strong inhibiting effect of the
ions limit, but do not block the progression of apatite crystal- Mg2+ ions on apatite crystallization combined to that of carbonates.
lization during SPS. This conclusion agrees with previous results The carbonation level on this sample after SPS was found to be kept
reported in the literature;20,48 however these findings indicate around 9 wt%. The typical morphology of such sintered ACP is
that these starting CO3/Mg compositions were not sufficient to shown on Fig. 8, evidencing in particular a submicron porosity.
stabilize amorphous CaP after SPS. The mechanism of sintering at such low temperatures is not
FTIR spectra also revealed the occurrence of the transformation fully elucidated. But, similarly to the case of biomimetic apatite
of amorphous powders into low-crystallinity apatite after SPS treat- nanocrystals which contain water molecules in a surface hydrated
ment for these samples (Fig. 4a-II). Carbonate ions can then enter layer as well as labile surface ions,30–32 the presence of water
the apatite lattice leading to the formation of either A or B-types of molecules associated to amorphous clusters of ACPs most likely
substitution. In particular, based on the FTIR spectral analysis, plays a key role in the formation of interparticle bridges resulting
partial transformation into mostly B-type apatite can be noted in ACP cold sintering. The role of (added) water was also shown
after SPS of these samples in domains 1462 cm 1 (Fig. 4b-II) and to favor sintering (although not via SPS) in the case of crystallized
872 cm 1 (Fig. 4c-II). FTIR spectra relative to the specimens calcium carbonate, when using high pressure compaction.49 In
Published on 28 November 2019. Downloaded on 1/30/2020 2:11:06 PM.
consolidated by SPS evidenced also a decreasing in the carbonation ACPs, the absence of long-range order could allow a high mobility of
level (Table 3), albeit samples still remained highly carbonated. the constitutive ions and H2O molecules, which could facilitate ion
This diminution can be linked to the partial decomposition of diffusion despite the low temperature, and thus the consolidation
carbonate ions followed by expulsion of CO2 gas. process. The occurrence of dissolution-reprecipitation on the other
In order to continue our attempts to stabilize ACP even after hand cannot be, at this stage, neither eliminated nor confirmed in
SPS, greater levels of Mg were tested. SPS treatment at 150 1C the absence of direct evidence.
was again found to be effective in the consolidation of the
sample Mg30-cACP-K synthesized with the KOH solution. However,
in this case, the initial amorphous character of calcium phosphate
Conclusions
powder could be successfully maintained even after SPS, as con- Carbonated and magnesium co-doped amorphous calcium
firmed by XRD analysis (Fig. 6b). To the best of our knowledge, this phosphate powders with different compositions were synthesized
outcome is obtained for the first time in the literature, thus opening by an adapted precipitation method and then consolidated by Spark
the way for identifying new families of bioceramic/composite Plasma Sintering at low temperature in view of cold sintering. The
materials with high metastability/surface reactivity/resorption high content of carbonate ions was demonstrated (maximum
abilities. The preservation of this amorphous state even after carbonate content B18.3 wt%). Cold-sintering at 150 1C by SPS
was effective to consolidate all powders prepared. The influence of
the introduction of carbonate and magnesium ions in ACP on the
consolidation process was also investigated. The high carbonation
was preserved after SPS allowing one to expect high resorption rates
in vivo.50 Our main finding is the determination of experimental
conditions allowing, for the first time, to retain the amorphous
nature of ACP compounds after SPS. Suitable SPS conditions
associated to adequate Mg2+ and CO32 substitutions are needed
to obtain this original outcome. These findings open the way to
the development of consolidated ACP-based bioceramics with
high (bio)reactivity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
One of the authors (M. L.) performed her activity in the framework
of the International PhD in Innovation Sciences and Technologies at
the University of Cagliari, and wishes also to thank the ERASMUS
Placedoc program for allowing mobility to France.
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