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Effect of Acetic Acid Concentration on Pore Structure for Mesoporous Bioactive Glass during Spray Pyrolysis Bo-Jiang Hong, Chih-Wei Hsiao, Fufa Fetene Bakare, Jung-Ting Sun and Shao-Ju Shih *

ID

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; [email protected] (B.-J.H.); [email protected] (C.-W.H.); [email protected] (F.F.B.); [email protected] (J.-T.S.) * Correspondence: [email protected]; Tel.: +886-2-2730-3716; Fax: +886-2-2737-6544 Received: 25 April 2018; Accepted: 3 June 2018; Published: 6 June 2018

 

Abstract: Mesoporous bioactive glass (MBG) is considered as one of the most important materials in the field of bone implants and drug carriers, owing to its superior bioactivity. In previous studies, tri-block surfactants (e.g., F127 and P123) were commonly used as pore-forming agents. However, the use of surfactants may cause serious problems such as micelle aggregation and carbon contamination and thus decrease bioactivity. Therefore, in this study, we demonstrated the synthesis of MBG using acetic acid (HAc) as a pore-forming agent to overcome the disadvantages caused by surfactants. Both untreated and HAc-treated BG powders were synthesized using spray pyrolysis and various characterizations were carried out. The results show that a mesoporous structure was successfully formed and the highest specific surface area of ~230 m2 /g with improved bioactivity was reported. Keywords: mesoporous bioactive glass; spray pyrolysis; acetic acid

1. Introduction Bioactive glass (BG) is a non-toxic, biocompatible, and bioactive material, developed in the 1960s [1,2]. Among these properties, the bioactivity has attracted much attention and been investigated extensively to enhance BG’s performance [3]. It is well known that BG has the capability to form hydroxyl apatite (HA) layers once immersed in simulated body fluid (SBF), and the formation of HA is influenced by factors such as compositions [4,5] and surface areas [6]. The composition factor has been studied by Li et al. [7], who demonstrated that the composition of 57S presents the best bioactivity. In addition, the factor of surface areas was first reported by Yan et al. in 2004 [8]. By creating a mesoporous structure within BG particles, they increase the surface areas and hence increase bioactivity. To synthesize BG, several studies used the sol-gel method owing to its chemical flexibility [8]. However, the whole process takes a few days and it is a batch production. In contrast, the spray pyrolysis (SP) process offers the benefits of low cost, short process time, and continuous fabrication, which is suitable for mass production [9,10]. In addition, to fabricate mesoporous bioactive glass (MBG), surfactants such as F127 (EO100 PO65 EO100 ) [11] and P123 (EO20 PO70 EO20 ) [12] were commonly used as pore-forming agents, where EO is poly ethylene oxide and PO is poly propylene oxide. However, the use of surfactants may cause problems such as micelle aggregation and carbon contamination. The micelle aggregation will produce particles with a wrinkled surface and hence reduce the porosity and surface area of the resulting specimen [13]. In addition, carbon contamination will inhibit the formation of HA. Both phenomena will result in decrease of bioactivity and hence there is a need to develop a surfactant-free process to synthesize MBG. Materials 2018, 11, 963; doi:10.3390/ma11060963

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Therefore, we proposed to use acetic acid (HAc) as the new pore-forming agent owing to its unique decomposition behavior, CH3 COOH + 2O2 → 2CO2 + 2H2 O. It generates large amounts of CO2 and H2 O gases, which create the pore structure during calcination [14]. Moreover, the decomposition temperature of HAc (440 ◦ C) [15] is much lower than the calcination temperature of BG (550 ◦ C) [16], which allows the decomposition reaction of HAc to be complete. Hence, HAc can replace the surfactants as the new pore-forming agents for two reasons: (i) It has lower molecular weight (60 g/mol molecular weight [17]) as compared to F127 (12,600 g/mol molecular weight [18]) and P123 (5750 g/mol molecular weight [18]), which minimizes carbon contamination; (ii) since HAc is completely soluble in water, no aggregation effect needs to be considered. In this study, untreated and three HAc-treated BG powders were prepared using SP. Characterizations of phase composition, surface morphology, and inner morphology of all BG powders were obtained using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The detailed pore sizes, pore volumes and surface areas were examined using nitrogen adsorption/desorption isotherms along with Brunauer-Emmett-Teller (BET) [19] and Dollimore–Heal (DH) method [20]. At last, the bioactive tests were carried out and characterized using a Fourier transform infrared reflection (FTIR) spectrophotometer. 2. Materials and Methods 2.1. Synthesis In this study, both untreated (solid) and HAc-treated (mesoporous) BG powders based on 57S composition (57 mol % SiO2 , 33 mol % CaO, and 10 mol % P2 O5 ) were prepared using tetraethyl orthosilicate (TEOS, Si(OC2 H5 )4 , 99.9 wt %, Showa, Tokyo, Japan), calcium nitrate tetrahydrate (CN, Ca(NO3 )2 ·4H2 O, 98.5 wt %, Showa, Tokyo, Japan), and triethyl phosphate (TEP, (C2 H5 )3 PO4 , 99.0 wt %, Alfa Aesar, Heysham, UK) as the sources of Si, Ca and P, respectively. The precursor solution of untreated BG was prepared by dissolving 37.5 g TEOS, 24.78 g CN and 0.73 g TEP in 1.60 g of 0.5 M HCl and 60.00 g of ethanol. As for HAc-treated BG, an additional pore-forming agent of HAc (99.8 wt %, Honeywell, NJ, USA) with various concentrations of 1M, 2M and 3M were added into the precursor solutions. All precursor solutions for untreated and HAc-treated BGs were stirred at room temperature for 4 h for homogeneity before the SP process. For the SP process, all precursor solutions (the mixture of the 200 mL precursor solution and 1000 mL DI-water) were dispersed into fine droplets at the frequency of 1.65 MHz using an ultrasonic nebulizer (KT-100A, King Ultrasonic, New Taipei, Taiwan) with the applied pressure of 1.013 × 105 Pa. With the air carrier gas, the droplets were led into a tube furnace (D-110, Dengyng, New Taipei, Taiwan) with three heating zones of 200 ◦ C, 550 ◦ C, and 300 ◦ C, which underwent preheating, calcining, and cooling to form BG particles [21]. The surfaces of the resulting particles were charged by electrons released from tungsten corona wire at high voltage of 16 kV. Finally, the negatively charged particles were neutralized and condensed inside an earthed stainless steel collector. 2.2. Characterization Characterizations of the phase composition were obtained using a XRD diffractometer (D2 Phaser, Bruker, Karlsruhe, Germany) with collection angles ranging from 20◦ to 80◦ . Surface and inner morphologies were examined using a field-emission scanning electron microscope (SEM, JSM-6500F, JEOL, Tokyo, Japan) and a field-emission transmission electron microscope (TEM, Tecnai G2 F20, FEI, Hillsoboro, OR, USA), respectively. The particle size distributions were measured with more than 300 particles from a couple of SEM images to ensure its reliability. In addition, a constant-volume adsorption apparatus (Novatouch LX2, Quantachrome Instruments, Boynton Beach, FL, USA) was operated at −196 ◦ C to obtain nitrogen adsorption and desorption isotherms. The specific surface areas, pore volumes, and pore sizes of all BG powders were computed using BET method [19] and DH method [20].

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Bioactivity tests of all BG powders were carried out using SBF, which has ionic concentration Bioactivity tests of all [22]. BG powders were carried using by SBF, which hasthe ionic concentration similar to human plasma The specimens were out prepared immersing powders in SBF similar to human plasma [22].ratios: The specimens were by immersing the2powders SBF with different solid-to-liquid 2 mg to 10 mL prepared for untreated BG powder, mg to 42in mL forwith 1M different solid-to-liquid mL2M forHAc-treated untreated BG mg to 42 mL formL 1Mfor HAcHAc-treated BG powder,ratios: 2 mg 2tomg 59 to mL10for BGpowder, powder,2 and 2 mg to 51 3M treated BG powder, 2 mg to 59 mL for 2M was HAc-treated BGatpowder, mg to 51 mL for 3M HAcHAc-treated BG powder. Each specimen immersed 37 ◦ C forand 1 h2and examined using FTIR treated BG powder. Each specimen was immersed at 37 °C for 1 h and examined using FTIR once dry. once dry. 3. Results 3.1. Crystallographic Structure Structure and 3.1. Crystallographic and Morphology Morphology Figure XRD patterns of un-treated and HAc-treated BG powders. A broadAband between Figure 11shows showsthe the XRD patterns of un-treated and HAc-treated BG powders. broad band ◦ to 40 can be observed from all patterns with absence of the crystalline phase, which indicates that between 20° to 40° can be observed from all patterns with absence of the crystalline phase, which all BG powders exhibited amorphous phase after the SP process. indicates that all BG powders exhibited amorphous phase after the SP process. 20◦

Figure 1. XRD patterns of un-treated and HAc-treated BG powders. Figure 1. XRD patterns of un-treated and HAc-treated BG powders.

Figure 2 shows the SEM images of all BG powders and typical spherical particles from the SP Figure the SEM images of all BG typical spherical particles process can 2beshows observed among all images. In powders addition, and it can be seen from Figure 2a from that the the SP process can be observed among all images. In addition, it can be seen from Figure 2a thattwo the untreated BG powder presents only one surface morphology of smooth sphere. In contrast, untreated BG powder presents only one surfacesphere morphology of smooth two surface surface morphologies of smooth and concave (see insets) can besphere. found In in contrast, the HAc-treated BG morphologies of smooth and concave sphere (see insets) can be found in the HAc-treated BG powder powder as shown in Figure 2b–d. as shown in 3Figure Figure shows2b–d. the TEM micrographs of all BG particles and it can be seen from Figure 3a that Figure 3 shows the TEMpresents micrographs of all BG particles andparticle. it can beThis seen indicates from Figure 3a that the untreated BG particle a no-contrast within the there are the no untreated BG particle presents a no-contrast within the particle. This indicates there are no thickness thickness variations and thus represents its inner morphology as solid. In addition, contrast of variations thus represents its inner morphology contrast intensities intensities and can be found in HAc-treated BG particles as as solid. shownIn inaddition, Figure 3b–d. The of dark regions can be found in HAc-treated BG particles as shown in Figure 3b–d. The dark regions represent represent higher absorption of electrons, while bright regions represent with lower absorption of higher absorption of electrons, whilemorphology bright regions represent with lower absorption of electrons, electrons, thus indicating the porous of the HAc-treated BG particles. Combining both thus indicating the porous morphology of the HAc-treated BG particles. Combining both SEM SEM and TEM results, the morphologies of all BG particles can be categorized into three types:and the TEM results, morphologies allmorphology BG particles of cansolid be categorized into three the untreated untreated BGthe particles exhibitsofthe smooth sphere (Typetypes: I), while the HAcBG particles exhibitshave the two morphology of solid smoothsmooth sphere sphere (Type I), while BG treated BG particles morphologies of porous (Type II) the andHAc-treated porous concave particles have III). twoIn morphologies porousofsmooth and porous concave sphereof(Type sphere (Type addition, theofsurface type IIIsphere has a (Type higherII)roughness than the surface type III). In addition, the surface of type III has a higher roughness than the surface of type II. II. The surface roughness may correlates with pore sizes of the internal microstructure. The surface roughness may correlates with pore sizes of the internal microstructure.

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Figure 2. SEM images of (a) untreated BG powders, and (b) 1M, (c) 2M, and (d) 3M HAc-treated BG Figure 2. SEM images of (a) untreated BG powders, and (b) 1M, (c) 2M, and (d) 3M HAc-treated BG powders. The insets areofthe for corresponding morphologies. Figure 2. SEM images (a) enlarged untreatedparticles BG powders, and (b) 1M, (c) 2M, and (d) 3M HAc-treated BG powders. The insets are the enlarged particles for corresponding morphologies. powders. The insets are the enlarged particles for corresponding morphologies.

Figure 3. TEM micrographs of (a) untreated BG powders, and (b) 1M, (c) 2M, (d) 3M HAc-treated BG powders. Figure 3. TEM micrographs of (a) untreated BG powders, and (b) 1M, (c) 2M, (d) 3M HAc-treated BG Figure 3. TEM micrographs of (a) untreated BG powders, and (b) 1M, (c) 2M, (d) 3M HAc-treated powders. BG powders.

The statistical measurement of particle morphology types from SEM images is shown in Figure 4. The result shows that the untreated BG powder contains only Type particles, whereas HAcThe statistical measurement of particle morphology types from SEMI images is shown in all Figure 4. The statistical measurement of particle morphology types from SEM images is shown in Figure treated BG shows powders with only Type II and III particles. It can also be whereas seen thatall with an 4. The result thatassociate the untreated BG powder contains only Type I particles, HAcThe result BG shows that the untreated BG1M powder only Type I It particles, HAc-treated increase of HAc concentration from toType 2M,contains the proportion of Type III porous particles treated powders associate with only II and III particles. can alsowhereas be concaved seen all that with an increase of HAc concentration from 1M to 2M, the proportion of Type III porous concaved particles

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BG powders associate with only Type II and III particles. It can also be seen that with an increase of HAc concentration fromto1M to 2M, proportion of Type slightly III porous concaved particles increases from increases from 29.7% 64.1%; thethe proportion decreases to 60.0% for HAc concentration of 29.7% 3M. to 64.1%; the proportion decreases slightly to 60.0% for HAc concentration of 3M.

Figure 4. Statistical measurements of morphology types of untreated and HAc-treated BG powders. Figure 4. Statistical measurements of morphology types of untreated and HAc-treated BG powders.

3.2. Particle Sizes and Specific Surface Areas 3.2. Particle Sizes and Specific Surface Areas The average particle sizes and standard deviations of the untreated and 1M, 2M, 3M HAc-treated The average particle sizes and standard deviations of the untreated and 1M, 2M, 3M HAc-treated BG powders are 842 ± 469, 947 ± 269, 800 ± 183, 1010 ± 280 nm, respectively; all particles are in the BG powders are 842 ± 469, 947 ± 269, 800 ± 183, 1010 ± 280 nm, respectively; all particles are in the same size range. same size range.

Figure 5. Nitrogen adsorption and desorption isotherms of (a) untreated BG powders, and (b) 1M, (c) 2M, and (d) 3M HAc-treated BG powders. The inserts are their corresponding pore size distributions. Figure 5. Nitrogen adsorption and desorption isotherms of (a) untreated BG powders, and (b) 1M, (c) 2M, and (d) 3M HAc-treated BG powders. The inserts are their corresponding pore size distributions.

Figure 5 shows nitrogen adsorption and desorption isotherms with insets of pore size distributions of all BG powders. Based on the original IUPAC classification, all BET isotherms are Figure 5as shows nitrogen adsorption isotherms with insets pore size distributions identified H4 loops, which indicates and that desorption all BG powders contain both openofand partially blocked of mesopores all BG powders. Based on the original IUPAC classification, all BET isotherms are identified as H4 [23]. From the pore size distribution profile, it is evident that the HAc-treated BG powders

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loops, which indicates that all BG powders contain both open and partially blocked mesopores [23]. From the pore size distribution profile, it is evident that the HAc-treated BG powders exhibit Materials 2018, 11, x FOR PEER REVIEW 6 of 9 a mesoporous structure with pore sizes ranging from 2–5 nm. Moreover, specific surface areas and pore Materialsare 2018,computed 11, x FOR PEER REVIEW 6 of 9 6 that volumes using BET methods and shown in Figure 6. It can be seen from Figure exhibit a mesoporous structure with pore sizes ranging from 2–5 nm. Moreover, specific surface areas untreated powder has lowest surface area of 39.7and m2shown /g, while the value increases andBG pore volumes arethe computed using BET methods in Figure 6. It can be seen significantly from exhibit a mesoporous structure with pore sizes ranging from 2–5 nm. Moreover, specific surface areas 2 2 thatm untreated BGthe powder has theof lowest area of 39.7 /g, while theof value increases to moreFigure than 6160 /g with treatment HAcsurface and reaches themmaximum 232.7 m2 /g of 2M and pore volumes are computed using BET methods and shown in Figure 6. It can be seen from significantly to moreIn than 160 m2/gthe with the treatment of HAc and reaches the maximum of surface 232.7 HAc-treated BGuntreated powder. addition, pore area volumes same trendincreases as area Figure 26 that BG powder has theresulting lowest surface of 39.7show m2/g,the while the value m /g of 2M HAc-treated BG powder. In addition, the resulting pore volumes show the same trend as with range from 0.138 cc/g. significantly to0.07 moretothan 160 m2/g with the treatment of HAc and reaches the maximum of 232.7 surface area with range from 0.07 to 0.138 cc/g.

m2/g of 2M HAc-treated BG powder. In addition, the resulting pore volumes show the same trend as surface area with range from 0.07 to 0.138 cc/g.

Figure 6. Specific surface areas and pore volumes of untreated and HAc-treated BG powders.

Figure 6. Specific surface areas and pore volumes of untreated and HAc-treated BG powders. 3.3. In Vitro Bioactivity 6. Specific surface areas and pore volumes of untreated and HAc-treated BG powders. 3.3. In VitroFigure Bioactivity Figure 7 shows the FTIR spectra for the bioactivity tests. The bioactivities were determined by deviation ofthe theFTIR peak intensities, 2, from the FTIR spectra [16,24], where I1 refers to the intensity 3.3. Inthe Vitro Bioactivity Figure 7 shows spectra forI1/Ithe bioactivity tests. The bioactivities were determined by the of the P-O bending vibration around 566 cm−1 [11], and I2 refers to the intensity of the Si-O-Si bending deviation of the peak intensities, I1 /I2 ,for from the FTIR spectra [16,24], where Iwere to the intensity of 1 refers Figure 7 shows the FTIRInitially, spectra theas-prepared bioactivity tests. The bioactivities determined by −1 [25]. vibration at 482 cm for the BG powders (Figure 7a), the FTIR spectra show −1 [11], and I refers to the intensity of the Si-O-Si bending thethe P-O bending vibration around 566 cm deviation the without peak intensities, I1/I2, from thesuggest FTIR spectra [16,24], I1 refers to only the intensity 2that these a clear I2of , but any I1. These spectra BGwhere surfaces exhibit Si-O-Si −vibration 1 [25]. Initially, −1 [11], vibration at 482 cmno forcm the as-prepared BGtono powders (Figure thesurfaces FTIR spectra of thestructure, P-O bending around 566 and Isuggests 2 refers the intensity of the Si-O-Si bending but P-O structure. Therefore, this result existence of HA on7a), the BG −1 [25]. vibration cmwithout Initially, the as-prepared BG powders (Figure 7a), the FTIR spectra show in SBF. ForIthe immersed BG powders, following Figure 7b, the spectra show the Si-O-Si show a before clearatIimmersing , but any . These spectra suggest that these BG surfaces exhibit only 2482 1for existence I1 and I2any , andI1.the resulting Ithis 1/I2 suggest values untreated and 1M, 2M, of 3MHA HAc-treated BGsurfaces a clear I2, but These spectra that theseno BG surfaces exhibit only structure, but noofwithout P-O structure. Therefore, resultofsuggests existence on theSi-O-Si BG powders were 0.15, 0.35, 0.61, and 0.55, respectively. This indicates that BGofpowder treated with 2M structure, but no P-O structure. Therefore, this result suggests no existence HA on the BG surfaces before immersing in SBF. For the immersed BG powders, following Figure 7b, the spectra show the HAc has the best bioactivity. before immersing in SBF. For the immersed BG powders, following Figure 7b, the spectra show the

existence of I1 and I2 , and the resulting I1 /I2 values of untreated and 1M, 2M, 3M HAc-treated BG existence of I1 and I2, and the resulting I1/I2 values of untreated and 1M, 2M, 3M HAc-treated BG powders were 0.15, 0.35, 0.61, and 0.55, respectively. This indicates that BG powder treated with 2M powders were 0.15, 0.35, 0.61, and 0.55, respectively. This indicates that BG powder treated with 2M HAc hashas thethe best HAc bestbioactivity. bioactivity.

Figure 7. FTIR spectra of untreated and HAc-treated BG powders (a) before and (b) after immersing in SBF for 6 h.

Figure FTIR spectraofofuntreated untreated and and HAc-treated (a)(a) before andand (b) (b) afterafter immersing Figure 7. 7. FTIR spectra HAc-treatedBG BGpowders powders before immersing in in SBF for 6 h. SBF for 6 h.

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4. Discussion 4. Discussion Combining SEM and TEM results given in Figures 2 and 3, particle morphology can be Combining SEM and TEM results given in Figures 2 and 3, particle morphology can be categorized categorized into three types (Figure 4 shows their statistical measurements). The formation of Type I into three types (Figure 4 shows their statistical measurements). The formation of Type I particles particles follows typical volume precipitation and has been well documented in our previous work follows typical volume precipitation and has been well documented in our previous work [26]. [26]. Moreover, the porous structure of Type II and Type III particles is formed during the Moreover, the porous structure of Type II and Type III particles is formed during the decomposition decomposition of the HAc. Following CH3COOH + 2O2 → 2CO2 + 2H2O, H2O and CO2 were formed, of the HAc. Following CH3 COOH + 2O2 → 2CO2 + 2H2 O, H2 O and CO2 were formed, while HAc while HAc decomposed in the tube furnace at 440 °C [14,27]. The resulting CO2 gas then releases decomposed in the tube furnace at 440 ◦ C [14,27]. The resulting CO2 gas then releases from the from the droplets, thereby creating a porous structure within the particles. Furthermore, the possible droplets, thereby creating a porous structure within the particles. Furthermore, the possible formation formation mechanism of Type III concave surface is given below. The interaction between HAc and mechanism Type III concave given below. interaction between HAcwill andaggregate TEOS in the TEOS in theof precursor solutionssurface might is form into ethylThe acetate [28]; these products on precursor might form ethyl [28]; theseafter products will aggregate onhigher the surface the surfacesolutions of the particles andinto form the acetate concave surface calcination [29]. The HAc of the particlesinduces and form the concave after calcination [29].results The higher HAc concentration concentration a larger amountsurface of ethyl acetate, and thus in a higher proportion of induces a larger amount of ethyl acetate, and thus results in a higher proportion of Type III particles. Type III particles. Figure 8 shows the correlation of particle morphology, specific surface area, and bioactivity. Figure 8 shows the correlation of particle morphology, specific surface area, and bioactivity. Figure 8a clearly shows that the specific surface area increases when the volume proportion of of Type III Figure 8a clearly shows that the specific surface area increases when the volume proportion Type increases. ThisThis implies that that the concave surface is enhancing the specific surface area of BG particles. III increases. implies the concave surface is enhancing the specific surface area of BG Next, Figure 8b shows the comparison between specific surface areas and bioactivity of all BG powders. particles. Next, Figure 8b shows the comparison between specific surface areas and bioactivity of all The graph shows BGshows powders higher specific surfacespecific area correspond to the higher value of BG powders. Thethat graph thatwith BG powders with higher surface area correspond to the Ihigher greater formation), which agree wellwhich with previous results MBG studies 1 /I2 (i.e., value of I1HA /I2 (i.e., greater HA formation), agree well with on previous results[13,30,31]. on MBG 2 /g exceeds Finally, to the best of our knowledge, the surface area achieved in this study of 232.7 studies [13,30,31]. Finally, to the best of our knowledge, the surface area achieved inmthis study of 2 /g [32] and thus becomes the highest specific surface area the P123-treated MBG particles of 165.0 m 2 2 232.7 m /g exceeds the P123-treated MBG particles of 165.0 m /g [32] and thus becomes the highest reported for SP-derived MBG for powders to date. specific surface area reported SP-derived MBG powders to date.

Figure 8. (a) Correlations between specific surface area and Type III particles and (b) bioactivity and Figure 8. (a) Correlations between specific surface area and Type III particles and (b) bioactivity and specific surface area of all BG powders. specific surface area of all BG powders.

5. Conclusions 5. Conclusions In this study, both untreated and HAc-treated BG powders were successfully synthesized using In this study, both untreated and HAc-treated BG powders were successfully synthesized using SP. The results show that a mesoporous structure is formed with HAc-treated BG powders and SP. The results show that a mesoporous structure is formed with HAc-treated BG powders and surface area is increased up to 230 m22/g. Bioactive tests were carried out, confirming increased surface area is increased up to 230 m /g. Bioactive tests were carried out, confirming increased bioactivity. Therefore, HAc has demonstrated its potential to be used as a new pore forming agent bioactivity. Therefore, HAc has demonstrated its potential to be used as a new pore forming agent for for future studies. future studies. Author Contributions: The study was designed by S.-J.S. The data collection was performed by F.F.B. and J.-T.S. Author Contributions: The study was designed by S.-J.S. The data collection was performed by F.F.B. and J.-T.S. The data data analysis analysis was was performed performed by by B.-J.H. B.-J.H. and and C.-W.H. C.-W.H. The Funding: This research was funded by the Ministry of Science and Technology of Taiwan (Grant numbers of MOST 105-2221-E-011-034, andand fromfrom Tokushima University, Japan, Japan, and National Taiwan 105-2221-E-011-034,and and106-2221-E-011-049) 106-2221-E-011-049) Tokushima University, and National University of Science Technology, R.O.C., (Grant TU-NTUST-106-01). Taiwan University of and Science and Technology, R.O.C.,number (Grant of number of TU-NTUST-106-01). Conflicts of Interest: The authors declare no conflict of interest. Conflicts Interest: The authors declare no conflict of interest.

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