Fabrication and characterization of strontium-doped borate-based bioactive glass scaffolds for bone tissue engineering

Accepted Manuscript Fabrication and characterization of strontium-doped borate-based bioactive glass scaffolds for bone tissue engineering Hairong Yin, Chen Yang, Hongwei Guo, Cuicui Wang, Mingyang Li, Yang Gao, Qiang Tong PII:

S0925-8388(18)30100-2

DOI:

10.1016/j.jallcom.2018.01.099

Reference:

JALCOM 44563

To appear in:

Journal of Alloys and Compounds

Received Date: 18 March 2017 Revised Date:

30 November 2017

Accepted Date: 7 January 2018

Please cite this article as: H. Yin, C. Yang, H. Guo, C. Wang, M. Li, Y. Gao, Q. Tong, Fabrication and characterization of strontium-doped borate-based bioactive glass scaffolds for bone tissue engineering, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.099. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Fabrication and characterization of strontium-doped borate-based bioactive glass scaffolds for bone tissue engineering Hairong Yin, Chen Yang*, Hongwei Guo, Cuicui Wang, Mingyang Li,

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Yang Gao, Qiang Tong School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi'an 710021, China.

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Abstract:

Boron is an essential element for bone health, but its cytotoxicity depends on its

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concentration. In the present study, borate-based 13-93B2 glass scaffolds doped with varying amounts of strontium (0-9 mol.% Sr) with a porosity of 80 % and pore size of 200-500 µm were prepared by a polymer foam replication technique. The effect of strontium content on the structure and bioactive properties of scaffolds were investigated

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by FTIR, XRD, ICP and MTT test. The results showed that the appearance of strontium in the glass has no obvious effect on the structure. The as-made scaffolds were amorphous and had a compressive strength of 11 MPa. The scaffolds in vitro bioactivity

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was observed by the conversion of the glass surface to a nanostructured hydroxyapatite layer in SBF. The increase of strontium concentration in borate-based glass accelerated

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the release of Si4+, Ca2+ ions, and significantly reduced the release of B3+ ions in some extent. The results indicated that borate-based bioactive glass doped with particular proportion of Sr suppressed the rapid release of boron and the cytotoxicity of glass scaffolds was minimized at least. This strontium-doped borate-based bioactive glass could be a potential scaffold material for bone tissue engineering. Keywords: Bioactive glass; Strontium; Boron release; Tissue engineering

1. Introduction

ACCEPTED MANUSCRIPT With the development of the society and the change of the diet structure, bone defects become the common diseases, which make patients suffer from physical pain and even lead to disability[1,2]. Helping patients to repair bone defect through developing new bone repair materials is an important problem that biomedical materials science is

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exploring and trying to solve[3, 4]. The bionic preparation of bone originates from bone tissue engineering, which gives an effective approach to generate bone tissue and repair bone defect[5]. Selecting suitable scaffold materials is a crucial problem for bone tissue

biocompatibility, biodegradability and bioactivity[6].

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engineering, and we will give priority to the artificial materials with good

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Bioactive glasses were first produced by Hench in 1969[7] with the general formula (wt%: 45SiO2-24.5CaO-24.5Na2O-6P2O5)[8]. Bioactive glasses such as 13-93 or 45S5 are known to be bioactive, an attractive feature is that it can easily bond to bone tissue in either bulk or particular form via formation of hydroxyapatite (HA) without any

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inflammatory effects[9, 10]. Increasing evidence in the literature indicates that ionic(such as Cu, Zn, Co) dissolution products from inorganic materials is the key of improving the properties of these materials[11-15]. Thus, research efforts are devoted to incorporate

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these ions in bioactive glasses to improved their biological performance[16].

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Boron is a kind of element which can be considered as potential stimulating agent for bone tissue engineering[17]. It was shown that boron affects the RNA synthesis in fibroblast cells[18]. Moreover, Some borate-based glasses have been observed to undergo viscous flow sintering more readily than 45S5 glass, providing a more facile bioactive glass system for producing scaffolds with anatomically relevant shapes[19]. Particles of a boron-modified 45S5 glass, containing 2.5 weight percent B2O3, promoted new bone formation more rapidly than 45S5 glass particles[20]. Scaffolds of a borate glass (molar composition: 20 Na2O, 20 CaO, 60 B2O3) was found to support the growth and

ACCEPTED MANUSCRIPT differentiation of human mesenchymal stem cells[21]. However, the degradation mechanism of borate-based glass and 45S5 bioactive glass is completely different, the poor chemical durability of borate glasses must be taken into consideration to avoid the toxicity of high release of boron[22, 23]. Strontium has

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been suggested as a daily oral supplement for the treatment of osteoporosis, and showed that beneficial effects on bone cells and bone formation in vivo[24, 25]. Thus, bioglass is thought to be an ideal reservoir to deliver strontium, owing to its homogeneous

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composition and controllable degradation[26]. Particularly, strontium has a larger ion radius than calcium ion, the incorporation of strontium by partial substitution of calcium

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is expected to occupy more space and inhibit the movement and release of other ions in the glass network.

The objective of this study is to prepare strontium-doped borate-based bioactive glass scaffolds by a polymer foam replication method, and evaluate the effect of Sr

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content for the release of B ions from glass scaffolds in vitro. The borate glass had a composition designated 13-93 glass but with two-thirds of the molar content of SiO2 replaced by B2O3 and doped with varying concentrations of Sr (0-9 mol.% SrO) in

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accordance with the reviewed literature[27]. The microstructure, compressive strength

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and bioactivity of the scaffolds were evaluated.

2. Experiments

2.1 Preparation of glass scaffolds Scaffolds of borate-based glass (13-93B2 glass composition: 18SiO2, 36B2O3,

22CaO, 6Na2O, 8K2O, 8MgO, 2P2O5; mol.%) doped with 0, 3, 6 and 9 mol.% SrO was prepared by melting a mixture analytical grade SiO2, H3BO3, CaCO3, Na2CO3, K2CO3, MgCO3, and NaH2PO4·2H2O (Sinopharm Chemical Reagent Co., Ltd. Shanghai, China) in a Pt crucible at 1300 °C for 2 h, and quenching in cold deionized water. These particles

ACCEPTED MANUSCRIPT were further ground for 2 h in an attrition mill using high-purity Y2O3-stabilized ZrO2 milling media and ethanol as the solvent, to give particles of size 1.0 ± 0.5 µm, as measured by a laser diffraction particle size analyzer. Scaffolds were prepared using a polymer foam replication technique, as described in detail elsewhere[28]. Briefly,

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polyurethane foam (1 cm × 1 cm × 1 cm) were immersed in a slurry obtained by mixing 25 vol% glass powder and PVA binder for several times. The as-coated foam was dried for at least 8 h at room temperature, and then the samples were heated at 2 °C/ min to

for 1h to density the glass filaments.

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2.2 Characterization of glass scaffolds

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400 °C in air to decompose the foam, and at 5 °C/min to 600 °C kept at this temperature

The structure analysis of glass scaffolds was performed with Fourier Transform Infrared Spectroscopy (FTIR, Bruker VECTOR-22) in the wavenumber range of 4000-400 cm-1 using KBr pellet method. The compressive strength of cylindrical samples

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(6 mm in diameter × 10 mm) was measured using an Instron testing machine (Model 5566, UK) by applying 0-350 N axial load via 500 N load cell at a crosshead speed of 0.5 mm/min, according to GB/T 10700-2006 Standard (China), respectively. The strain at

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which the voids begin to form defined as the strain threshold, glass is typically brittle and

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have no yielding phenomenon so we take the maximum elastic deformation as the strain threshold in this study[29]. The compressive strength of each scaffold was determined from the maximum load recorded during the test divided by the measured initial scaffold cross-sectional area of the specimen[30]. Eight samples were tested, and the mean strength and standard deviation were determined. The porosity of the as-made glass scaffolds was measured using the Archimedes method. The microstructure of the surface and cross-section of the glass scaffolds and individual HA microspheres were observed using scanning electron microscopy (SEM, S4800 Hitachi, Tokyo, Japan) at an

ACCEPTED MANUSCRIPT accelerating voltage of 3.0 kV. 2.3 Assessment of scaffold bioactivity The in vitro bioactivity of the scaffolds was assessed by immersing the scaffolds in simulated body fluid (SBF) with a starting temperature at 37 °C for a week, and

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determining the formation of HA on the surface of the scaffolds using X-ray diffraction (XRD, Model D/MAX2200PC, Japan). The composition of the SBF was identical to that described by Kokubo et al[31]. Acceleration voltage and current used in the analysis were

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0.01° and a scanning rate of 0.5°/min.

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40 kV and 30 mA, respectively. 2θ diffraction angle range was 10°-70° with a step of

Conversion of the glass scaffolds to HA were also followed by measuring the weight loss of the scaffolds and the pH changing of the SBF solution as a function of immersion time. The procedure is described in detail elsewhere[32]. Briefly, scaffold samples were placed in SBF solution with a starting temperature at 37 °C. After given immersion

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times(0.5, 1.0, 1.5, 2.0, 3.0, 4.5, 7, 12, 20 days), scaffolds were removed from the solution, dried at room temperature at least 24 h and weighed. The weight loss was defined as ∆W/W0, where ∆W= W0 – Wt, W0 is the initial mass of the scaffold, and Wt is

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the mass at time t[33]. After removing the scaffolds to be weighed, the solution was

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cooled to room temperature, and its pH was measured. Six samples in each group were tested and repeated three times. For bioactivity studies, each scaffold (size: 1 cm × 1 cm × 1 cm) was immersed in

50 mL of SBF and was stored in an oven at controlled temperature of 37 °C. SBF sample was renewed for various time periods (1, 3, 5, 8, 15, 20 days) up to 20 days. Ion changes of Si, Ca, B and Sr were determined by using an inductively coupled plasma atomic emission spectrometry (ICP-AES, Spectra AA 220FS, THEM, USA) at 27.12 MHz frequency and 167 nm to 800 nm wavelength range. Ar gas was used during

ACCEPTED MANUSCRIPT measurements. The study was repeated three times and six replicates were used within each group. 2.4 Culture of human osteosarcoma cells and MTT detection The use of human osteosarcoma cells line MG-63 in this study was approved by the

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Fourth Military Medical University, Shaanxi, China. The MG-63 cells were cultured in a-MEM medium supplemented with 10% fetal bovine serum plus 100 µg/ml penicillin and 100µg/ml streptomycin sulfate. Prior to seeding with cells, dry-heat sterilized

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scaffolds (6 mm in diameter × 2 mm thick) were soaked for 1 h in a 0.01% solution of polylysine (mol. wt. >150,000) to enhance cell adhesion. The pre-treated scaffolds were

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blotted dry, rinsed twice with phosphate-buffered saline (PBS), placed on a 6 cm diameter Teflon disk and seeded with 50,000 MG-63 cells suspended in 35 µl of complete medium. After incubation for 4 h to permit cell attachment, the cell-seeded scaffolds were transferred to a 24-well plate containing 2 ml of complete medium per

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well. The control group consisted of the same number of cells seeded in wells containing 2 ml of complete medium. All cell cultures were performed at 37 °C in a humidified atmosphere of 5% CO2, with the medium changed every 2 days. Cell proliferation was

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evaluated using the MTT test after 1 d and 3 d of culture. Some of the cell-seeded

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scaffolds were placed in 400 µL serum-free medium containing 100 µg of the tetrazolium salt MTT for the last 4 h of incubation to permit visualization of metabolically active cells on and within the porous glass scaffolds. After the incubation, the scaffolds were briefly rinsed in PBS. 200 µL of dimethyl sulfoxide (DMSO; Sigma, USA) was added into each well and oscillated for 10 min. Finally, 200 µL of DMSO solution from each well was transferred to a 96-well plate, and the absorbance was measured at 570 nm in a multifunctional microplate reader (Wellscan MK3). The study was repeated three times and six replicates were used within each study (n=3).

ACCEPTED MANUSCRIPT 2.5 Statistical analysis All biological experiments (6 samples in each group) were run in triplicate. The data are presented as the mean ± standard deviation. Statistical analysis was performed using Student’s t-test. Values were considered to be significantly different when p < 0.05.

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3. Results and discussion

3.1 Structure, mechanical properties and morphology of glass scaffolds

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Before SBF soaking, the FTIR spectra obtained from all scaffolds were given in Fig. 1. The FTIR graph of the scaffolds showed two characteristic bands of silicates at 1065

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cm-1 and 479 cm-1. The bands in the range 470-512 cm-1 were attributed to bending mode of O-Si-O and also O-P-O. The bands in the range 1050-1100 cm-1 were related to P-O-P and P-O-Si stretching vibrations. The presence of the bands in 570 cm-1 was connected with O-P-O bending vibrations characteristic for [PO4] groups. The peak at 720 cm-1 was

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the main features of B-O stretching of tetrahedral [BO4] units. The band at 1430 cm-1

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corresponding with the C-O vibrations in CO3 groups appeared[34].

Fig. 1 FTIR spectra of the as-prepared glass scaffolds.

Furthermore, with the presence of strontium in the borate-based glass structure, the

ACCEPTED MANUSCRIPT intensity of bands in the range 1050-1100 cm-1 assigned to Si-O vibrations was decreased. Moreover, when 13-93B2 glass scaffolds were doped with 9 mol.% strontium, the band at 1065 cm-1 in spectra was split into two bands: 1088 cm-1 and 986 cm-1. That effect was probably related to the substitution of strontium for calcium resulting in slight expansion

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of the glass network caused by the larger ionic radius of Sr ions (1.16 Å) compared with Ca ones(1.00 Å)[35]. The intensity of peak in 1430 cm-1 attributed to C-O vibrations was decreased. However, the intensity of B-O stretching in 720cm-1 was not variable.

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XRD patterns of the as-prepared glass scaffold and the glass scaffold soaking for 7 days in SBF were shown in Fig. 2. The pattern of the as-formed scaffold showed broad

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bands of a typical amorphous material. After immersion for a week in SBF, the pattern of the scaffold contained peaks corresponding to those of a standard HA (JCDPS 72-1243), indicating the 13-93B2 glass scaffolds doped with 6 mol.% strontium have a good bioactivity. The width of the peaks in the XRD pattern may indicate the formation of

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nanometer-sized HA crystals.

Fig. 2 X-ray diffraction of 13-93B2 glass scaffolds doped with 6 mol.% strontium before and after being soaked for a week in SBF.

Conversion of the glass to HA in the SBF leaded to a weight loss, so the data can be

ACCEPTED MANUSCRIPT used to follow the kinetics of the conversion reaction[36]. Data for the weight loss of the borate-based bioactive glass scaffolds as a function of immersion time in the SBF was shown in Fig. 3. It can be observed that weight loss values increased rapidly during the

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first 5 days of soaking. Between 5-20 days, the weight loss was nearly constant, and it keeps at a value of 17.0 % after 20 days when the experiment was stopped. The change in the pH value of the SBF solution, resulting from the reactions accompanying the

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conversion of the glass to HA (Fig. 2), also increased rapidly during the first 5 days, from 7.1 to 8.0. After five days, the pH increased slowly and reached a final limiting value of

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8.1. That indicated the prepared borate-based bioactive glass scaffolds were alkaline.

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Fig. 3 Weight loss of scaffolds and pH change of the solution, resulting from soaking of 13-93B2 glass scaffolds doped with 6 mol.% strontium in SBF with a starting pH=7.1.

The typical stress–strain behavior of a fabricated glass scaffold in compression was

shown in Fig. 4. The peaks and valleys in the curve may be related to progressive breaking of the solid particulate network. The response showed linear elastic behavior during the initial compression, followed by a decrease in stress, possibly because of the fracture of some struts in the solid network. As the strain increased, the additional struts were fractured. Finally, the macro-cracks propagated in the scaffold, leading to a decrease

ACCEPTED MANUSCRIPT in stress to almost zero. Taking the compressive strength as the highest stress on the

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stress-strain curve, the compressive strength was 11.2 ± 0.5 MPa.

Fig.4 Stress-strain response of 13-93B2 glass scaffolds doped with 6 mol.% strontium in compression. Table 1 Compressive strength and porosity of samples Compressive strength (MPa)

Porosity (%)

13-93B2

10.2 ± 0.4

78 ± 2.1

13-93B2+3%Sr

10.8 ± 0.1

83 ± 1.5

13-93B2+6%Sr

11.2 ± 0.5

82 ± 3

13-93B2+9%Sr

12.3 ± 0.2

80 ± 2.7

human trabecular bone

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50-90

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Samples

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The earlier studies indicated that scaffold should have a porosity and compressive strength in the range of 50-90 % and 2-12 MPa in order to be used in tissue engineering applications as a trabecular bone[6]. In this study, the compressive strength and porosity of samples were shown in Table 1. The effect of the concentration of strontium on the compressive strength and porosity were investigated. High porosity is important for cell proliferation. However, some scaffolds with better degradation rate should have less porosity than 90% as they should preserve their structure until the new bone replacement[6]. As shown in Table 1, all glass scaffold samples had a compressive

ACCEPTED MANUSCRIPT strength around 11MPa and the porosity is around 80%. Furthermore, the compressive strength of scaffold slightly increased with increasing strontium concentration, but the

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porosity of scaffold have no obvious changing trend.

Fig. 5 SEM images of: (a) polyurethane foam used in experiments; (b), (c) 13-93B2 glass scaffolds

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doped with 6mol.% strontium fabricated by a polymer foam replication method; (d) surface of the scaffolds soaking in SBF for 5 days.

Fig. 5a showed the microstructure of the polyurethane foam we used in experiments,

which had a structure close to the trabecular bone. The microstructure of a fractured cross-section of the glass scaffold (Fig. 5b and c) consisted of a dense network of glass and interconnected cellular pores. Generally speaking, the microstructure of the glass scaffold was almost same as those of polyurethane foam, and human trabecular bone as well as. The scaffold had a porosity of 82±3%, which consisted of an interconnected network of pores of size 200-500 µm. The SEM images of 13-93B2 glass scaffolds doped

ACCEPTED MANUSCRIPT with 6 mol.% strontium after immersion in SBF for 5 days were shown in Fig. 5d. Compared to the smooth surface of the as-made glass scaffold, a fine particulate layer produced on the surface of the scaffold after soaking in SBF for 5 days, it consisted of a nanometer-sized crystal, which is presumably HA based on the XRD pattern described in

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Fig. 2. 3.2 In vitro bioactivity

Fig. 6 showed the changes of the concentration of the ions (Si4+, Ca2+, B3+and Sr2+)

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before and after the scaffolds were soaking in SBF for 1, 3, 5, 8, 15 and 20 days determined by ICP analysis. SBF control did not contain Si4+ ions, after soaking of

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scaffolds into SBF throughout 20 days, the concentration of Si4+ ions in SBF sharply increased during the first 8 days, and tended stability from 15 days to 20 days (Fig. 6a). What’s more, with the appearance of strontium in glass scaffolds, the concentration of Si4+ was increased significantly (when 9mol.% strontium was doped, Si concentration

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values, respectively, increased from 14 ppm to 24 ppm on the 20th day). The changes the concentration of Ca2+ and B3+ions of strontium-doped borate-based glass scaffolds in SBF (Fig. 6b and Fig. 6c) shown a similar trend with Si4+. On the contrary, B3+ ions

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concentration was decreased gradually with strontium doped in 13-93B2 glass. The

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concentration of Sr2+ ions of all scaffold samples gradually increased until the 15th day (Fig. 6d), and had no significant change from 15 to 20 days. Sharp rise of Si4+ and Ca2+ ions concentration at the beginning of the experiment

showed that silica network of glass scaffolds dissolved faster. This situation pointed out the earlier formation of HA on the scaffolds surface, and this may be the proof of higher bioactivity of strontium-doped borate-based glass than 13-93B2 glass scaffolds. The concentration of Si4+ ions of the glass scaffolds at the end of the 15th day approximately trended to be stable in the SBF due to the Si(OH)4 formation on the surface of the

ACCEPTED MANUSCRIPT scaffolds showing the second stage of HA formation[37]. Boron is an essential element for bone health, but its cytotoxicity depends on its concentration. Fu et al[27] found that boron concentration above 0.65 mM inhibit the growth and proliferation of cells (B3+ ions concentration of 13-93B2 glass scaffolds at the

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end of 20th day is 680 ppm). As discussed in Fig. 1, the presence of strontium in the borate-based glass decreased the intensity of Si-O vibrations; meanwhile, the intensity of B-O stretching is not variable. It’s reported that the appearance of strontium in

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borate-based glass will substitution calcium, thus it occupies more space in the glass network, which effectively inhibits the movement and release of boron from glass

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scaffolds[38]. This may be another proof of higher bioactivity of strontium-doped

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borate-based glass than 13-93B2 glass scaffolds.

Fig. 6 (a) Si ion, (b) Ca ion, (c) B ion and (d) P ion changes of concentration after immersion in SBF

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3.3 Cell proliferation of glass scaffolds The result of MTT assay on MG-63 cells with glass scaffolds was shown in Fig. 7.

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On the first day, compared with control group, the MG-63 cells proliferation of 13-93B2 glass scaffolds had no obvious difference. However, the samples doped with strontium elevated the proliferation of MG-63 cells significantly (p＜0.05). On day 3, 13-93B2

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glass scaffolds inhibited cell proliferation obviously, and the proliferation of other samples was further improved with the appearance of strontium. The MTT assay

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provided further evidence that strontium doped in 13-93B2 glass suppressed the rapid

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release of boron and the cytotoxicity of glass scaffolds was minimized at least.

Fig.7. Proliferation of MG-63 cells cultured with the scaffolds for 1 and 3 days, data were represented as mean ± SD, n=3; differences were considered significant when *P<0.05.

4. Conclusions Strontium-doped borate-based bioactive glass scaffolds were created by a polymer foam replication method in the present investigation. The FTIR results showed that the addition of Sr in the borate-based glass decreased the intensity of Si-O vibrations and

ACCEPTED MANUSCRIPT C-O vibrations; there was no significant change in the glass structure. The as fabricated scaffold was amorphous and had a compressive strength of 11 MPa, and after immersing the scaffold in a simulated body fluid for 7 days, a nanostructured hydroxyapatite layer formed on the surface. The in vitro studies showed that the scaffolds converted into HA

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by losing 17% of weight and the pH of the solution increased from 7.0 to 8.1 after 20 days. A dopant of Sr in scaffolds accelerated the release of Si4+ and Ca2+ ions, and significantly reduced the release of B3+ ions in some extent, and this is an indicator of

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higher bioactivity for glass scaffolds. In vitro cell culture showed that the appearance of Sr in scaffolds had an excellent ability to support the proliferation of MG-63 cells. Hence,

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particular proportion of Sr could eliminate the cytotoxicity of the glass scaffolds by suppressing the rapid release of boron. Thus, strontium-doped borate-based bioactive glass scaffolds can be stated to have higher bioactivity for bone tissue engineering.

Acknowledgements

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This work was supported by the International S&T Cooperation Program of China (No. 2009DFR50520), Natural Science Foundation of China (NO. 51472151), Science

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and Technology Research Project of Xianyang (NO. 2016K02-28), Science and Technology R & D Program Focused on Special Project (NO. 2017YFB0310201),

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Shaanxi Province focus on R & D projects (NO. 2017ZDXM-NY-048).

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[27] H. Fu, Q. Fu, N. Zhou, W. Huang, M.N. Rahaman, D. Wang, X. Liu, In vitro evaluation of borate-based bioactive glass scaffolds prepared by a polymer foam replication method, Materials Science & Engineering C, 29 (2009) 2275-2281. [28] Q. Fu, M.N. Rahaman, B.S. Bal, R.F. Brown, D.E. Day, Mechanical and in vitro performance of 13-93 bioactive glass scaffolds prepared by a polymer foam replication technique, Acta Biomaterialia, 4 (2008) 1854-1864. [29] W. Wang, B. Guo, Y. Ji, C. He, X. Wei, The study on the threshold strain of microvoid formation in TRIP steels during tensile deformation, Materials Science & Engineering A, 546 (2012) 272-278. [30] R. Saidi, M. Fathi, H. Salimijazi, Synthesis and characterization of bioactive glass coated forsterite scaffold for tissue engineering applications, Journal of Alloys & Compounds, 727 (2017) 956-962. [31] T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro, Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W, Journal of Biomedical Materials Research, 24 (1990) 721-734. [32] W. Huang, D.E. Day, K. Kittiratanapiboon, M.N. Rahaman, Kinetics and mechanisms of the conversion of silicate (45S5), borate, and borosilicate glasses to hydroxyapatite in dilute phosphate solutions, Journal of Materials Science: Materials in Medicine, 17 (2006) 583-596. [33] S.K. Misra, D. Mohn, T.J. Brunner, W.J. Stark, S.E. Philip, I. Roy, V. Salih, J.C. Knowles, A.R. Boccaccini, Comparison of nanoscale and microscale bioactive glass on the properties of P(3HB)/Bioglass ®; composites, Biomaterials, 29 (2008) 1750-1761. [34] Li A, Ren H, Cui Y, et al. Detailed structure of a new bioactive glass composition for the design of bone repair materials[J]. Journal of Non-Crystalline Solids, 2017, 475: 10-14. [35] A.K. Varshneya, Fundamentals of inorganic glasses, Fundamentals of Inorganic Glasses, (1994). [36] L.M. Grover, A.J. Wright, U. Gbureck, A. Bolarinwa, J. Song, Y. Liu, D.F. Farrar, G. Howling, J. Rose, J.E. Barralet, The effect of amorphous pyrophosphate on calcium phosphate cement resorption and bone generation, Biomaterials, 34 (2013) 6631-6637. [37] L.L. Hench, Bioceramics: From Concept to Clinic, Journal of the American Ceramic Society, 74 (2010) 1487-1510. [38] Z.Y. Li, W.M. Lam, C. Yang, B. Xu, G.X. Ni, S.A. Abbah, K.M. Cheung, K.D. Luk, W.W. Lu, Chemical composition, crystal size and lattice structural changes after incorporation of strontium into biomimetic apatite, Biomaterials, 28 (2007) 1452-1460.

ACCEPTED MANUSCRIPT Tables Table 1 Compressive strength and porosity of samples Compressive strength (MPa)

Porosity(%)

13-93B2

10.2 ± 0.4

78 ± 2.1

13-93B2+3%Sr

10.8 ± 0.1

83 ± 1.5

13-93B2+6%Sr

11.2 ± 0.5

82 ± 3

13-93B2+9%Sr

12.3 ± 0.2

80 ± 2.7

human trabecular bone

2-12

50-90

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Samples

ACCEPTED MANUSCRIPT Figure captions Fig. 1 FTIR spectra of the as-prepared glass scaffolds. Fig. 2 X-ray diffraction of 13-93B2 glass scaffolds doped with 6 mol.% strontium before

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and after being soaked for a week in SBF. Fig. 3 Weight loss of scaffolds and pH change of the solution, resulting from soaking of 13-93B2 glass scaffolds doped with 6 mol.% strontium in SBF with a starting pH=7.1.

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Fig.4 Stress-strain response of 13-93B2 glass scaffolds doped with 6 mol.% strontium in compression.

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Fig. 5 SEM images of: (a) polyurethane foam used in experiments; (b), (c) 13-93B2 glass scaffolds doped with 6 mol.% strontium fabricated by a polymer foam replication method; (d) surface of the scaffolds soaking in SBF for 5 days.

Fig. 6 Fig. 6 (a) Si ion, (b) Ca ion, (c) B ion and (d) P ion changes of concentration after

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immersion in SBF for various time periods (1, 3, 5, 8, 15 and 20 days) Fig.7. Proliferation of MG-63 cells cultured with the scaffolds for 1 and 3 days, data were

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Fig. 1 FTIR spectra of the as-prepared glass scaffolds.

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Fig. 2 X-ray diffraction of 13-93B2 glass scaffolds doped with 6 mol.% strontium before

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Fig. 3 Weight loss of scaffolds and pH change of the solution, resulting from soaking of

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13-93B2 glass scaffolds doped with 6 mol.% strontium in SBF with a starting pH=7.1.

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Fig.4 Stress-strain response of 13-93B2 glass scaffolds doped with 6 mol.% strontium in

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compression.

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Fig. 5 SEM images of: (a) polyurethane foam used in experiments; (b), (c) 13-93B2 glass scaffolds doped with 6mol.% strontium fabricated by a polymer foam replication method; (d) surface of the scaffolds soaking in SBF for 5 days.

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Fig. 6 (a) Si ion, (b) Ca ion, (c) B ion and (d) P ion changes of concentration after immersion in SBF for various time periods (1, 3, 5, 8, 15 and 20 days)

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Fig.7. Proliferation of MG-63 cells cultured with the scaffolds for 1 and 3 days, data were

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ACCEPTED MANUSCRIPT 1. Strontium-doped borate-based bioactive glass scaffolds were prepared by a polymer foam replication method; 2. Appearance of strontium has no obvious effect on borate-based glass structure; 3. Increase the concentration of Sr accelerates the release of Si4+, Ca2+ ions;

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cytotoxicity of scaffolds.