vaterite composite microparticles towards tuning ion-releasing properties

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Original Research Paper

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Interphase coordination design in carbamate-siloxane/vaterite composite microparticles towards tuning ion-releasing properties

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Jin Nakamura a,⇑, Yoshio Ota b, Yoshio Sakka a, Toshihiro Kasuga c,⇑

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a

Research Center for Functional Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Yabashi Industries Co., Ltd., 226 Akasaka-cho, Ogaki, Gifu 503-2213, Japan c Division of Advanced Ceramics, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b

a r t i c l e

i n f o

Article history: Received 14 May 2016 Received in revised form 9 January 2017 Accepted 5 March 2017 Available online xxxx Keywords: Vaterite Siloxane Carbamate Coordination Ion-releasing Bone regenerative medicine Biomaterials

a b s t r a c t Siloxane-containing vaterite (SiV) microparticles were prepared with controlling the degree of aminopropyl-functionalization in the siloxane; they are aiming for applications as bone regenerative devices. The aim of this work was to evaluate the structure at siloxane/vaterite interphase and to control the solubility of particles by the structural tuning of siloxane. The particles were spherical with average diameters of 1.1–1.4 lm. Differential infrared spectrometry revealed the transformation of aminopropyl terminals in the siloxane into carbamate (NH-COO ) groups. Moreover, the vaterite crystallites in the particles were slightly oriented towards the (0 0 1) plane. These results describe the interphase structure, with the carbamate groups coordinating on the Ca2+ ion face in the (0 0 1) plane of neighbouring vaterite. Upon soaking in buffer solution, the particles exhibited a rapid initial release of Ca2+ ions within 30 min and of soluble silica within 2 h. The vaterite in this particle survived for more than 6 h. The chemical stability of the siloxane was enhanced by incorporating tetraethoxysilane-derived siloxane with fractions of 24 mol% or 50 mol%. This enhancement controlled the initial release of not only soluble silica but also Ca2+ ions. Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

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1. Introduction

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Process of bone formation is regulated by the transaction of growth factors (GFs) among the cells [1,2]. After the report published by Hench et al., an inorganic product made from BioglassÒ 45S5, which dominantly contains soluble silica and Ca2+ ions, has been reported to genetically stimulate osteoblast cells by enhancing GFs expression [3–6]. Osteogenic promoters made of inorganic components have reliability in incorporation into various biomaterials for bone-reconstruction surgery and are expected to possess a long shelf-life because of their chemical stability against GFs. The authors previously developed a method for the preparation of siloxane-containing vaterite (SiV) particles using a carbonation process [7–12]. Vaterite is a polymorph of calcium carbonates with a minimum thermodynamical stability. In SiV, vaterite with 5–20nm-sized lamellae are present, which are enclosed by aminopropyl triethoxysilane(APTES)-derived siloxane (Ap-S) [9]. When the SiV particles are soaked in a physiological pH buffer solution, the

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⇑ Corresponding authors. E-mail addresses: [email protected] (J. Nakamura), kasuga.toshihiro@ nitech.ac.jp (T. Kasuga).

SiAOASi bond in the Ap-S hydrolyses to release soluble silica, following the subsequent release of Ca2+ ions from the dissolution of vaterite. SiV particles were incorporated into poly(lactic acid) (PLA)-based electrospun fibermats [7,8,12]. When implanting at a defect in rabbit calvaria, bone nodules formation was confirmed in the middle part of the fibermat after 4 weeks [8]. The stimulant activities of the soluble silica and Ca2+ ions have been reported to be dose-dependent. Hench et al. reported that enhanced IGF-II expression in human adult osteoblasts was observed at the concentrations of Si from soluble silica and Ca2+ ion ([Si] and [Ca]) of 16.58 and 88.35 lg/ml (0.6 and 2.2 mmol L 1), respectively [3]. Tsigkou et al. reported that cell culture media supplemented with soluble silica by the [Si] of 15 and 20 lg/ml (0.5 and 0.7 mmol L 1) supported the enhanced proliferation and differentiation of foetal osteoblasts [13,14]. For the Ca2+ ions, the local elevation of the ion concentration in extracellular fluid ([Ca2+]o) to levels as high as 40 mmol L 1 is known to enhance the mitosis of osteoblasts [15–17]. The proliferation, differentiation, and mineralisation of murine osteoblasts were individually enhanced at specific levels of [Ca2+]o [18–20]. Therefore, the adjustment of the ion concentrations and their releasing

http://dx.doi.org/10.1016/j.apt.2017.03.003 0921-8831/Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: J. Nakamura et al., Interphase coordination design in carbamate-siloxane/vaterite composite microparticles towards tuning ion-releasing properties, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.03.003

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span towards specific levels is desired to achieve effective stimulation in the osteoblast cells. We previously demonstrated a simple method for reducing the releasing amount of Ca2+ ions from SiV by enhancing the degree of (0 0 1)-preferred crystal orientation in vaterite crystals [11]. The (0 0 1) plane of vaterite possess a high surface energy as the plane composed of Ca2+ or CO23 ion. Preparation of vaterite with (0 0 1)preferred crystal orientation has been reported in the presence of macromolecules which stabilize the plane [21–25]. The mechanism for the crystal orientation in the presence of Ap-S, however, remained unclear. To the best of our knowledge, the interaction between Ap-S and vaterite has not yet been discussed in any reports. Herein, the structures of SiV particles containing Ap-S with 0– 100 mol% of total siloxane were compared to understand the role of the amino terminals in the Ap-S interphase structure between the vaterite crystals and siloxane. Moreover, the dissolution behaviours of the particles in a physiological pH buffer solution were characterised; this brings a point of view for controlling structures towards tuning the dissolution behaviours of the SiV particles.

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2. Experimental section 2.1. Synthesis and characterisation of SiV particles

3.1. Characteristics of vaterite microparticles containing APTES- and TEOS-derived siloxanes

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The SiV particles were prepared by a carbonation process. Briefly, 25 ml of distilled water and 37.5 g of calcium hydroxide (Yabashi Industries Company, Ltd., Japan) were added to 500 ml of methanol (Wako Pure Chemicals Inc., Japan, Medical grade) while stirring. Then, 60 ml of APTES (Sigma-Aldrich, USA, reagent grade, 4.48  10 3 mol cm 3), tetraethylorthosilicate (TEOS) (Sigma-Aldrich, USA, reagent grade, 4.48  10 3 mol cm 3), or their mixture was added to the slurry. Each reagent was added at 20-min intervals. Carbon dioxide gas (industrial grade, 99.5%) was introduced to the slurry at a rate of 2 L min 1 for 60 min. The resulting slurry was aged for 12 h at room temperature and then heated at 110 °C for 24 h to remove residual solvents. The resulting powder was dissociated with an agate mortar and pestle to form SiV particles. Hereafter, the samples are called SiVxAyT, where x and y represent the volume percent (almost similar to mole percent) of APTES and TEOS in total silane, respectively. The sample codes and recipes of the SiVs are summarised in Table 1. The silicon and calcium contents of the SiV particles were estimated using X-ray fluorescence spectrometry (Hitachi High-Tech Science Corporation, SEA-2210A:XRF, n = 5). Vaterite particles with no silane addition were also prepared using the carbonation process. X-ray diffractometry (PANalytical, X’Pert-MPD: XRD) and attenuated total reflectance Fourier-transform infrared spectrometry (JASCO, FTIR 4000 spectrometer equipped with an ATR PRO 450-S attachment: ATR-FTIR) techniques were used to examine the crystal and chemical structures of the particles. 29Si magicangle spinning nuclear magnetic resonance spectrometry (29Si MAS-NMR; Varian, UNITY Inova 400 plus NMR spectrometer, operated at 79.49 MHz) was used to examine the chemical structure of the siloxane. The morphologies of the particles were examined by

Fig. 1 presents SEM images of the SiV particles. Their mean particle sizes and silicon and calcium contents are summarised in Table 2. The particles exhibited slightly compressed spherical morphologies. SiV100A had the largest mean diameter, i.e., 1.4 lm, among them. The others exhibited the mean diameters of 1.1 lm. All of these particles exhibited narrow size distributions. According to XRF analysis, SiV100A, SiV70A30T, and SiV50A50T contained 2.5 wt% silicon, which was greater than that of SiV100T, namely 1.3 wt%. The Si/Ca mass ratio of SiV100T was estimated to be 0.03, which was only one-third of the raw materials composition (Tables 1 and 2). The ratio in the other SiV particles exhibited minor decreases from the value in their precursor slurries. These results suggested that APTES played a key role in the incorporation of siloxane into vaterite. Fig. 2 presents the 29Si MAS-NMR spectra of the SiV particles. In the SiV100A spectrum, the peaks of the trivalent silicon (Tn) species (T1: 48 ppm, T2: 59 ppm, T3: 68 ppm) attributed to the Ap-S are observed. The peaks of tetravalent silicon (Qn) species (Q2: 94 ppm, Q3: 101 ppm, Q4: 111 ppm) originating from the TEOS-derived siloxane (Te-S) are observed in the SiV100T spectrum. Both Tn and Qn peaks are observed in the spectra of SiV70A30T and SiV50A50T; these SiV particles contained siloxane consisting of both APTES and TEOS. The Tn:Qn molar ratios were estimated to be 76:24 and 50:50 for SiV70A30T and SiV50A50T, respectively, as shown in Table 3. These ratios nearly followed the APTES:TEOS composition ratios in their raw materials. The relations between these structures and the dissolution profiles of SiV particles will be discussed later. Fig. 3 presents the ATR-FTIR spectra of the SiV particles. The spectrum of vaterite is also shown as a reference. Absorption bands

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scanning electron microscopy (SEM) using an accelerating voltage of 15 kV (JEOL, JSM-6301F). The mean diameters of the particles were estimated from the SEM images using ImageJ software (http://rsbweb.nih.gov/ij/, n = 50).

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2.2. Dissolution of SiV particles

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Tris buffer solution (TBS) was prepared by dissolving 6.118 g of tris(hydroxymethyl)aminomethane into 900 ml of distilled water at 36.5 °C. The pH was adjusted to 7.40 using 1.0 mol L 1 hydrochloric acid solution. The resulting solution was transferred to a 1000-ml volumetric flask and brought to the volume with distilled water to obtain 0.05 mol L 1 TBS. In polystyrene containers, 0.5 g of SiV particles were immersed in 25 ml of the TBS and incubated at 36.5 °C. After 0.5–24 h of soaking, the samples were filtered and then dried at 50 °C. The dried samples were evaluated by XRD and ATR-FTIR. The [Si] and [Ca] in the soaking solutions were measured using inductively coupled plasma atomic emission spectrometry (Shimadzu, ICPS-7000: ICP-AES) (n = 3).

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3. Results and discussion

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Table 1 Nominal compositions for preparing various SiV particles. Sample

SiV100A SiV70A30T SiV50A50T SiV100T

APTES

TEOS

Vol. [ml]

Mass [g]

Vol. [ml]

Mass [g]

15.0 10.5 7.5 –

1.80 1.26 0.90 –

– 4.5 7.5 15.0

– 0.57 0.94 1.89

Ca(OH)2 [g]

Si/Ca (mass ratio)

37.5

0.09 0.09 0.09 0.09

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Fig. 1. SEM images of SiV particles: (a) SiV100A, (b) SiV70A30T, (c) SiV50A50T, and (d) SiV100T.

Table 2 Particle sizes of SiV particles and their silicon and calcium contents.

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Sample

Mean diameter [µm]

Si [wt%]

Ca [wt%]

Si/Ca (mass ratio)

SiV100A SiV70A30T SiV50A50T SiV100T

1.44 ± 0.03 1.16 ± 0.04 1.07 ± 0.01 1.10 ± 0.01 (n = 50)

2.7 2.3 2.7 1.3 (n = 5)

35.8 37.6 38.0 40.1 (n = 5)

0.08 0.06 0.07 0.03

of Si-O-Si bonding from APTES- and TEOS-derived siloxanes were observed in the range of 1200–950 cm 1. The carbonate ion of vaterite usually exhibits 4 types of bands, namely asymmetric stretching (m3), symmetric stretching (m1), out-of-plane bending (m2), and in-plane bending (m4), in the range of approximately 1500–700 cm 1 [26,27]. These bands were also observed in all of the SiV at approximately 1450, 1085, 870, and 745 cm 1, respectively. The m3 band of SiV100T exhibited a sharp peak top with a narrower width, which was almost identical to the band of pristine vaterite. However, the band was broadened in SiV100A, SiV70A30T, and SiV50A50T. To achieve insight into the mechanism, differential infrared (DIR) spectra were calculated from the SiV and vaterite spectra (Fig. 4). The resulting spectra revealed almost no m1 and m4 bands, indicating the successful subtraction of absorption associated with vaterite. In the DIR spectra of SiV100A, SiV70A30T, and SiV50A50T, strong absorption in the range of 1580–1400 cm 1, originating from the asymmetric stretching and skeletal vibration of carbamate (R-NH-COO ), was observed. The overlap of these absorptions led the broadening of m3 band in fig. 3. These samples also showed the m2 band, originating from the amorphous phase of calcium carbonate, at 868 cm 1 [26].

The formation of the carbamate group has been reported as a phenomenon occurring during the chemisorption of CO2 gas by aminopropyl-functionalised mesoporous silica [28,29]. For SiV, carbamate is expected to form during the carbonation process of the precursor slurry; the amino terminals of Ap-S forms an amide bond with a CO2 gas molecule to form the group. During the chemisorption process, carbamic acid (R-NH-COOH) and an aminopropyl cation (Si-(CH2)3-NH+3) also form to maintain electrical neutrality with the carbamates [28,29]. However, neither a C@O band nor an NH3+ band from these by-products were observed in the DIR spectra. As described elsewhere [30], the (00l) plane of vaterite is the uni-ionic face of Ca2+ or CO23 ions. At the siloxane-vaterite interphase in the SiV100A, SiV70A30T, and SiV50A50T, the carbamate groups in the Ap-S are suggested to coordinate with the Ca2+ uniionic face of vaterite for the charge compensation, as depicted in Fig. 5. Fig. 6 presents the XRD patterns of SiV particles. All of the particles exhibited peaks corresponding to vaterite with a hexagonal P63/mmc structure (ICDD No. 33-0268). SiV100A and SiV100T particles showed a peak originated from the (1 0 4) plane of calcite at 29°. The I(0 0 4)/I(1 1 0) ratios were estimated to be 0.18–0.24 for the SiV particles containing Ap-S, which were greater than the ratio

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Fig. 2.

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Fig. 3. ATR-FTIR spectra of SiV and vaterite particles. The absorbances of all the spectra were normalised at 1410 cm 1 (r).

Si MAS-NMR spectra of SiV particles.

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of SiV100T, I(0 0 4)/I(1 1 0) = 0.14. This result indicates that the former 3 types of SiV particles exhibited preferred crystalline orientations towards their (0 0 l) planes. This behaviour is consistent with previous reports depicting the formation of (0 0 1)-plane-oriented vaterite by the coordination of ionic species, including ammonium ion [21,22], phosphonates [23] and sulfonates [24,25]. It is noteworthy that the ratio of SiV100T was almost equivalent to that of the pristine vaterite [9]. The relative integrated peak area ratios of the main peak (1 1 2) over the silicon standard (I(1 1 2)/I(Si)) were estimated to be 3.8–4.5 for SiV100A, SiV70A30T, and SiV50A50T (Table 4). The full width at half maximum (FWHM) of their (1 1 2) peaks were calculated to be 0.53–0.58. The I(1 1 2)/I(Si) and FWHM(1 1 2) of SiV100T were estimated to be 6.2 and 0.47, respectively. These results suggest that SiV100A, SiV70A30T, and SiV50A50T contained relatively premature vaterite with lower crystallinities and smaller crystallite sizes compared with the SiV100T. As previously characterised, in the carbonation process of SiV particles, the vaterite gradually crystallises from an amorphous precursor [9]. The coordination of the carbamate groups into vaterite might occur at the early stage of this reaction to interrupt the crystal growth of vaterite.

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3.2. Soaking test of SiV particles in physiological buffer solution

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Fig. 7(a) and (b) show the [Si] and [Ca] in the solution after soaking the SiV particles. Table 5 lists the mass percentages of siloxane released during the soaking periods of 0–2 and 2–24 h. All of the SiV particles exhibited two releasing regimes of Ca2+ ions, namely, the relatively fast increase of the concentration with a peak during the initial 4 h, followed by a stable stage with no per-

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Fig. 4. Vaterite-subtracted differential infrared spectra of SiV particles.

turbations in the concentration after 6 h of soaking. Fig. 8 presents the XRD patterns of SiV particles retrieved from TBS. The patterns showed a diffraction peak of calcite at 2h = 29° within 4 h. The former term (i.e., initial 2 h), therefore, was associated with the dissolution of vaterite and the induction of calcite re-crystallisation. In the latter term, diffraction peaks from calcite became dominant in the patterns. The stable stages in the concentration were considered to be the solution equilibria of calcite.

Table 3 Molar fractions of Tn and Qn species calculated from Fig. 2. Sample

T1

T2

T3

Q2

Q3

Q4

Tn total

Qn total

SiV100A SiV70A30T SiV50A50T SiV100T

7 8 11 –

32 25 20 –

61 43 19 –

– 1 5 14

– 2 13 30

– 22 32 56

100 76 50 0

0 24 50 100

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Fig. 5. Schematic of interphase structure between siloxane and vaterite.

Fig. 7. (a) Si from soluble silica ([Si]) and (b) Ca2+ ion ([Ca]) concentrations in TBS after soaking SiV particles.

Table 5 Mass percentages of siloxane released from SiV particles after soaking in TBS.

Fig. 6. XRD patterns of SiV particles. The broken lines represent the Voigt-functionfitted peaks.

Table 4 Peak integrated area ratios of I(0 0 4)/I(1 1 0) and I(1 1 2)/I(Si) and FWHM of (1 1 2) peak estimated from Fig. 1.

270 271 272 273 274 275

Sample

I(0 0 4)/I(1 1 0)

I(1 1 2)/I(Si)

FWHM(1 1 2)

SiV100A SiV70A30T SiV50A50T SiV100T

0.21 0.24 0.18 0.14

4.52 4.22 3.76 6.21

0.58 0.53 0.53 0.47

The Ca2+ ion-releasing behaviours in the former term were sensitive to the coordination structures of siloxane and vaterite in the SiV particles. As observed in Fig. 7(b), either SiV100A or SiV100T reached the highest Ca2+ ion concentrations at 30 min after soaking. No diffraction peaks of vaterite were observed in the diffraction pattern of SiV100T after 2 h of soaking, whereas the

Sample

0–2 h [wt%]

2–24 h [wt%]

Total [wt%]

SiV100A SiV70A30T SiV50A50T SiV100T

74 39 28 10

26 30 20 13

100 69 48 23

peaks were still visible for SiV100A (Fig. 8(a) and (d)). At this soaking time, more than 70 wt% of Ap-S was estimated to leach out from the SiV100A. In our previous report, vaterite primary particles in a SiV particle were observed to be enclosed in siloxanes [9]. Even a trace amount of Ap-S, which coordinated to the vaterite, might suppress the contact of TBS and delay the dissolution. However, Te-S has no coordination with vaterite, resulting in a complete dissolution of vaterite and re-precipitation into calcite. The 29Si MAS-NMR spectrometry revealed that the SiV70A30T contained both Ap-S and Te-S with a molar ratio of 76:24 (Table 3). As observed in Table 5, this particle was estimated to release 39 wt % of its siloxane within the initial 2 h of soaking; the incorporation of Te-S into the Ap-S enhanced its chemical stability. This particle gradually released Ca2+ ions, and the concentration reached a maximum after 2 h (Fig. 7(b)). The initial rate of this ion-release was the slowest among the 4 types of SiV particles. Diffraction peaks associated with calcite were almost negligible for this soaking period, as observed in Fig 8(b). These results indicate that (i) the carbamate group in siloxane promotes the coordination/integration with the vaterite primary particle at their interphase and (ii) the chemical stability of siloxane-shell strongly influences the dissolution rate of vaterite. When equimolar Ap-S and Te-S were included in SiV (SiV50A50T), its I(0 0 4)/I(1 1 0) ratio decreased slightly compared with that of SiV70A30T. When this SiV was soaked in TBS, the Ca2+ ion concentration reached to the peak within 30–60 min, as observed in Fig. 7(b). Noteworthy, small diffraction peaks of vaterite were observed only

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Fig. 8. XRD patterns of SiV particles after being soaked in TBS for 2–24 h: (a) SiV100A, (b) SiV70A30T, (c) SiV50A50T, and (d) SiV100T.

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in the XRD pattern of SiV50A50T particles after 24 h of soaking. This particle released minimum amount of siloxane among the 3 kinds of SiV particles containing Ap-S after 24 h of soaking, according to Fig. 7(a). These results suggested that, within initial 2 h the dissolution rate of vaterite in the SiVxAyT is affected dominantly by the I (0 0 4)/I(1 1 0) ratio, while the rate is influenced by the chemical stability of siloxane at later period.

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4. Conclusions

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SiV particles, which containing siloxane functionalised with carbamate groups, were prepared by the carbonation process in the presence of APTES. At the interphase between the siloxane and vaterite, the carbamate groups coordinated to the (0 0 1) plane-oriented vaterite crystallites. Neither the carbamate group nor the crystal orientation was observed in the SiV particles, which contained TEOS-derived siloxane. To incorporate both APTES and

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TEOS during the process, a carbamate-functionalised siloxane consisting of Ap-S and Te-S formed within the SiV particles. Upon soaking in a physiological pH buffer solution, the SiV particles gradually hydrolysed to release soluble silica and Ca2+ ions. In particular, vaterite crystallites in the SiV particles, which were coordinated by the carbamate-functionalised siloxane, survived for at least 6 h. Enhanced chemical stabilities were observed in the SiV particles containing both Ap-S and Te-S. In particular, the SiV particles with an Ap-S:Te-S molar ratio of 74:26 exhibited the slowest initial increase in their Ca2+ ion concentration.

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Acknowledgements

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This work was supported in part by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (#23
8842, #26
9064).

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Please cite this article in press as: J. Nakamura et al., Interphase coordination design in carbamate-siloxane/vaterite composite microparticles towards tuning ion-releasing properties, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.03.003

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