Acta Biomaterialia 6 (2010) 4143–4153
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A new sol–gel process for producing Na2O-containing bioactive glass ceramics Qi-Zhi Chen a,b,*, Yuan Li a, Li-Yu Jin a, Julian M.W. Quinn c,d, Paul A. Komesaroff e a
Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia Division of Biological Engineering, Monash University, Clayton, Victoria 3800, Australia c Prince Henry’s Institute, Clayton, Victoria 3168, Australia d Department of Medicine, University of Melbourne, Fitzroy, Victoria 3065, Australia e Department of Medicine, Monash University, Alfred Hospital, Commercial Road, Prahran, Victoria 3181, Australia b
a r t i c l e
i n f o
Article history: Received 9 December 2009 Received in revised form 24 April 2010 Accepted 27 April 2010 Available online 4 May 2010 Keywords: Sol–gel Bioactive glass Glass ceramic Sodium oxide Biological absorbability
a b s t r a c t The sol–gel process of producing SiO2–CaO bioactive glasses is well established, but problems remain with the poor mechanical properties of the amorphous form and the bioinertness of its crystalline counterpart. These properties may be improved by incorporating Na2O into bioactive glasses, which can result in the formation of a hard yet biodegradable crystalline phase from bioactive glasses when sintered. However, production of Na2O-containing bioactive glasses by sol–gel methods has proved to be difﬁcult. This work reports a new sol–gel process for the production of Na2O-containing bioactive glass ceramics, potentially enabling their use as medical implantation materials. Fine powders of 45S5 (a Na2O-containing composition) glass ceramic have for the ﬁrst time been successfully synthesized using the sol–gel technique in aqueous solution under ambient conditions, with the mean particle size being 5 lm. A comparative study of sol–gel derived S70C30 (a Na2O-free composition) and 45S5 glass ceramic materials revealed that the latter possesses a number of features desirable in biomaterials used for bone tissue engineering, including (i) the crystalline phase Na2Ca2Si3O9 that couples good mechanical strength with satisfactory biodegradability, (ii) formation of hydroxyapatite, which may promote good bone bonding and (iii) cytocompatibility. In contrast, the sol–gel derived S70C30 glass ceramic consisted of a virtually inert crystalline phase CaSiO3. Moreover, amorphous S70C30 largely transited to CaCO3 with minor hydroxyapatite when immersed in simulated body ﬂuid under standard tissue culture conditions. In conclusion, sol–gel derived Na2O-containing glass ceramics have signiﬁcant advantages over related Na2Ofree materials, having a greatly improved combination of mechanical capability and biological absorbability. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
1. Introduction Bioactive glasses have been much studied in attempts to develop suitable materials for use as implants in the human body to repair and replace diseased or damaged bone. Such implant materials need mechanical strength, but also the ability to harmlessly degrade over time to allow their gradual replacement with newly formed bone. The best characterized type is 45S5 BioglassÒ (Table 1), which has been used in a number of medical devices approved by the US Food and Drug Administration (FDA) . Commercially produced bioactive glasses have been made by conventional glass powder manufacturing methods, i.e. melting and quenching. Meanwhile, increasing research efforts are being invested in fabrication of bioactive glasses using the sol–gel technique , as it is
* Corresponding author at: Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia. Tel.: +61 3 99053599; fax: +61 3 99054940. E-mail address: [email protected]
an extremely versatile process with many advantages over melting–quenching processes. Using the sol–gel process, ceramic or glass materials can be fabricated in a variety of forms, including ultra-ﬁne spherical powders, thin ﬁlm coatings, ceramic ﬁbres, microporous inorganic membranes, monolithic ceramics and glasses and highly porous aerogel materials . Despite its advantages, the sol–gel technique has not yet been successfully applied to the production of Na2O-containing bioactive glasses or glass ceramics. All members of the 49–86S series of sol–gel derived bioactive glasses, for instance, contain SiO2, CaO and P2O5, but none contain Na2O [3,4]. Inclusion of Na2O in a sol–gel bioactive glass represents a technical challenge due to the high hydrolytic reactivity of sodium alkoxide in water , which is so great that some researchers have instead turned to the use of MgO in the sol–gel fabrication of bioactive glasses, producing SiO2–CaO–P2O5–MgO glasses . However, there are good reasons to believe that inclusion of Na2O in the fabrication of bioceramic materials would offer opportunities to improve mechanical strength without losing a satisfactory biodegradability.
1742-7061/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2010.04.022
Q.-Z. Chen et al. / Acta Biomaterialia 6 (2010) 4143–4153
Table 1 Composition of the sol–gel derived 45S5, nominal 45S5 composition, crystalline phase Na2Ca2Si3O9 and S70C30 (mol.%).
SiO2 Na2O CaO P2O5
46.14 24.35 26.91 2.60
50 16.667 33.333 0
48.99 20.23 28.03 2.95
70 0 30 0
Firstly, in the glass industry Na2O is added to reduce the melting point of silica-based glasses, whereas other components such as CaO and MgO are added to stabilize theses glasses, which would otherwise be rendered water soluble. Secondly, the presence of Na2O offers advantages in relation to the crystallization treatment that is applied to improve the mechanical properties of bioceramics. Because scaffolds of amorphous bioactive glasses are very fragile, to achieve good mechanical strength bioactive glass foams have to be sintered to form crystalline phases . In bioactive glasses lacking Na2O the crystalline phase is bioinert , which means that mechanical strength is improved, at the cost of sacriﬁcing degradability. In contrast, sintered 45S5 BioglassÒ ceramics possesses both sound mechanical strength and satisfactory biodegradability, which can be attributed to the formation of a crystalline phase, Na2Ca2Si3O9 . Work on such melt-derived Na2O-containing glass ceramics  suggests that Na2O may be a critical component in the production of biodegradable bioceramics with enough mechanical strength to be used as scaffolding materials in bone tissue engineering. As an alternative approach to this problem, sol–gel fabrication of SiO2–CaO–P2O5–Na2O glasses employing an organic solvent has been attempted , by preparing the sol of alkoxide precursors of SiO2–P2O5–CaO–Na2O in ethylene glycol solution under a nitrogen atmosphere. Although promising, the production environment employed in this protocol is inconvenient and difﬁcult to use, and it is probably for these reasons there has been little subsequent development of this technique. Therefore, the primary objective of the present work was to develop a sol–gel based protocol for the production of Na2O-containing bioactive glass ceramics which can be employed under ambient conditions. This would potentially enable these materials to be produced to a high quality but cheaply and in commercially viable quantities. In this work, we chose 45S5, a well-known Na2O-containing composition, as a speciﬁc example to develop such a new sol–gel process, which would in principle be applicable to all Na2O-containing bioactive glass ceramics. The characterization and evaluation of melt-derived 45S5 BioglassÒ ceramics are well documented [6,9–12]. Brieﬂy: (i) the formation of Na2Ca2Si3O9 signiﬁcantly improves the mechanical properties of the material; (ii) crystallization does not inhibit bioactivity, with the bone bonding ability (indicated by the formation of hydroxyapatite) remaining in the fully crystallized ceramics; (iii) when immersed in body ﬂuid the crystalline phase Na2Ca2Si3O9 decomposes and transits to amorphous hydroxyapatite (HA), an easily degradable mineral in vivo. Therefore, the second objective of this work was to establish that Na2O can be successfully incorporated into sol–gel 45S5 glass ceramics produced by this method. To this end, we demonstrate that the sol–gel derived 45S5 material
possesses the above three features that only Na2O-containing glass ceramics have, i.e. formation of Na2Ca2Si3O9 in the sintered material and decomposition of the crystalline phase and formation of amorphous HA throughout the material in simulated body ﬂuid (SBF). Since the new sol–gel protocol involves the use of an acidic catalyst, it is also essential to evaluate the derived 45S5 material in vitro to provide some preliminary assessment of the likely clinical utility of the product. It should be noted, however, that the excellent biocompatibility of melt-derived 45S5 materials is well documented. The last objective of this work was to demonstrate that incorporation of Na2O results in a material with a mechanically sound and yet biodegradable crystalline phase. To this end, the sol–gel derived 45S5 glass ceramic material and a Na2O-free (S70C30 composition, Table 1) bioceramics were compared for the following aspects: crystalline phases, degradation kinetics and formation of HA. 2. Materials and experimental procedures 2.1. Materials The following chemicals were used as precursors for the synthesis of the sol–gel 45S5 and S70C30 materials: tetraethyl orthosilicate (TEOS) (Aldrich, 99%), triethyl phosphate (TEP) (Eastman, 99.8%), sodium nitrate (Sigma–Aldrich, 99%) and calcium nitrate tetrahydrate (Sigma–Aldrich, 99%). 2.2. Sol–gel process The process and a ﬂowchart are provided in Table 2 and Fig. 1, respectively. In brief, the molar ratios of TEOS, TEP, NaNO3 and Ca(NO3)24H2O were designed according to the molar ratio of SiO2, P2O5, Na2O and CaO in 45S5 and S70C30. To achieve a clear sol the molar ratio between water and the four precursor chemicals was set at 10. Each chemical was added reasonably slowly into the HNO3 aqueous solution at room temperature. Each compound in the sequence was added only when the previous solution became clear, and was then stirred for at least 1 h. The resulting gel was dried at 60 and 200 °C for 72 and 40 h, respectively, aged at 600 °C for 5 h and sintered at 1000 °C for 2 h. 2.3. Sintering The primary purpose of sintering the aged gel was to decompose sodium nitrate NaNO3 and calcium nitrate Ca(NO3)2 in order to obtain Na2O and CaO in the material. The full thermal decomposition of NaNO3 and Ca(NO3)2 occurs at about 680 °C and 560 °C, respectively [13,14], and the crystallization temperature of 45S5 BioglassÒ is 600 °C . Hence, crystallization occurs in the glasses during the decomposition treatment of NaNO3 and Ca(NO3)2. The crystallization, however, would not be problematical provided that the crystalline phase is Na2Ca2Si3O9 . To ensure this, the sintering condition was set at 1000 °C in order to achieve both satisfactory mechanical properties and biodegradability of the material .
Table 2 Recipes for the sol–gel 45S5 and S70C30 materials. Sol–gel product
HNO3 solution concentration (M)
Molar ratio water/chemical
Chemical HNO3–H2O (ml)
Q.-Z. Chen et al. / Acta Biomaterialia 6 (2010) 4143–4153
Prepare HNO3 (deionised) water solution
Add TEOS slowly while stirring
When the above solution becomes clear, add TEP, slowl while stirring
When the above solution becomes clear, add NaNO3 slowly while stirring
When the above solution becomes clear, add and Ca(NO3)2⋅4H2O slowly while stirring
Stop stirring, and leave the above sol to gel at the ambient condition
Dry and age the above gel at 60, 200 and 600°C for 72, 40 and 5 h, respectively
Sinter the dried gel at 1000°C Fig. 1. Flowchart of sol–gel fabrication process of 45S5 and S70C30 materials; production of S70C30 omitted the steps in the blue frame. The ramping rate was 0.5 °C min1 below 200 °C, and 1 °C min1 between 200 and 600 °C.
2.4. Characterization using laser diffraction, SEM/EDX, XRD and FTIR Particle size distribution of sol–gel powders was determined by laser diffraction analysis, using a Mastersizer from Malvin Instruments (USA). The sol–gel derived particles were characterized by a thermal ﬁeld emission gun scanning electron microscopy (FEG SEM) (JEOL 7001) before and after immersion in SBF. Samples were gold coated and observed at an accelerating voltage of 15 keV. Energy dispersive X-ray (EDX) spectra (Ka line) were collected at 20 keV in FEG SEM. They were processed using a Bruker program with standard reference spectra. Compositions of the sol–gel derived materials were also determined using elemental analysis conducted by the Campbell Microanalytical Laboratory (New Zealand). Powders were also characterized using X-ray diffraction (XRD) analysis, with the aim of assessing the crystallinity after sintering and possible formation of HA crystals, following different immersion times in SBF. For XRD analysis 0.1 g of the powders was used. A Philips PW 1700 Series automated powder diffractometer was used, employing Cu Ka radiation (at 40 kV and 25 mA) with a secondary crystal monochromator. Data were collected over the range 2h = 5– 80° using a step size of 0.02° and a counting time of 10 s per step. Fourier transform infrared (FTIR) spectroscopy was performed in a Nicolet 6700 spectrometer with an attenuated total reﬂection unit in evanescent mode. The IR spectra were run on KBr pellets, with a weight ratio of sample to KBr of 1:100. The spectrum was recorded at a resolution of 4 cm1. 2.5. Assessment of bone bonding ability in SBF The bone bonding capability of a biomaterial to host bone is associated with the formation of a carbonated HA layer on the surface of the material, either when implanted or placed in contact
Fig. 2. SEM morphology of the sol–gel derived BioglassÒ powders produced using four concentrations of HNO3 aqueous solution: (a) 1.0, (b) 0.5, (c) 0.25 and (d) 0.10 M. All images at the same magniﬁcation.
Q.-Z. Chen et al. / Acta Biomaterialia 6 (2010) 4143–4153
(Ca2+) ions. The ICS-2500 (Dionex) was used for the analysis of phosphate (PO43) and silicon (SiO42) anions.
(a) 0.25 M Percentage / %
5 2.7. Biocompatibility evaluation: elution test method
4 3 2 1 22.4
Particle Size / µm 3.5
Percentage / %
(b) 0.1 M
2.5 2 1.5 1 0.5 4.56
Particle Size / µm Fig. 3. Particle size distribution of the sol–gel derived 45S5 powders at HNO3 aqueous solution concentrations of (a) 0.25 and (b) 0.1 M. The average particle sizes were 4.5 and 5.8 lm, respectively.
with biological ﬂuids [16,17]. Hence, the ability to bond with bone can be preliminarily assessed in vitro in SBF by monitoring the formation of HA on its surface. To do this we used the standard in vitro procedure described by Kokubo et al. . The foams were immersed in 75 ml of acellular SBF in ﬂasks. The ﬂasks were placed inside an incubator at 37 °C. The pH of the solution was maintained constant at 7.25. The size of all samples for these tests was 10 10 10 mm. Two samples were extracted from the SBF solution after given times of 7, 15 and 30 days. The SBF was replaced twice per week because the cation concentration decreased during the course of the experiments as a result of changes in the chemical composition of the samples. Once removed from the incubation medium samples were rinsed gently, ﬁrstly in 100% ethanol, then using pure deionized water, and ﬁnally left to dry at ambient temperature in a desiccator. HA was identiﬁed from XRD diffraction peaks.
2.6. Measurement of pH values and ion concentrations in the tissue culture medium Samples (1 g) of the materials to be tested were soaked in 10 ml of tissue culture medium in 15 ml conical tubes under standard cell culture conditions within a culture incubator (37 °C in humidiﬁed air containing 5% CO2). Medium was collected and transferred to new tubes at different intervals up to 2 days. Acidity was measured by insertion of a pH meter probe into the collected solutions, via a speciﬁcally designed incubator entrance. After pH measurement the same solutions were used to analyse ion concentrations. All solutions were analysed using ion chromatography. An ICS-1000 (Dionex) ion chromatograph with attached autosampler was used for the analysis of sodium (Na+) and calcium
MG63, a human osteoblast-like osteosarcoma cell line (ATCC), was used for biocompatibility assessment because of the well-deﬁned and reproducible proliferative activity of these cells. In addition, the elution test method (ISO 10993) was adopted in the present work. In this method extracts were obtained by placing the test (sol–gel derived BioglassÒ ceramics) and control (HA and NovaboneÒ BioglassÒ) materials in separate aliquots of cell culture medium under standard conditions (0.2 g ml1 of culture medium for 24 h at 37 °C). MG63 cells were cultured in Dulbecco’s modiﬁed Eagle’s medium (DMEM) with 10% heat-inactivated fetal bovine serum, 0.1% penicillin/streptomycin at 37 °C with 5% CO2. Cells were then plated in a 48-well tissue culture plate at a concentration of 2 104 cells per well. After 2 days the cell culture medium was removed and replaced with medium containing the extracts. Cells were placed back in the incubator for a 24 h treatment. Cells were observed under an optical microscope for visible signs of toxicity in response to the test and control materials. Quantiﬁcation of cell viability was achieved by measuring lactate dehydrogenase (LDH) release using a commercial kit (Sigma–Aldrich Tox-7). Culture medium (200 lm per well) was collected after MG63 cells had been exposed to the medium containing extract. The number of dead cells resulting from treatment with extractants was determined from these samples. The number of live cells was measured using the total LDH method of Tox-7, in which live cells were lysed and the medium collected. The LDH levels were determined by measuring the absorbance (A490 A690) using the commercial kit Tox-7 and a spectrophotometer. Our standard curve shows that there is a reasonably good linear relationship between the number of cells and LDH level in the range of 5 103–5 104. Hence, the percentage of dead cells can be simply given by the equation:
%dead cells ¼
LDH in extractant medium total LDH
2.8. Cell proliferation: Alamar blue Cell proliferation was assessed using a commercial Alamar blue™ assay kit (Life Technologies). Alamar blue™ is non-toxic to cells. The assay does not interrupt cell culture, allowing continuous measurement of cell proliferation kinetics. Hence, the Alamar blue™ assay is appropriate to evaluate the long-term cytotoxicity of biomaterials due to biodegradation under physiological conditions. MG63 cells were seeded at 5000 cells ml1 each well in a 48-well plate and cultured in the medium with extract. Incubated medium in wells with neither cells nor testing materials extracts were used as negative controls. After culture for 48 h, 100 ll of Alamar blue™ indicator was added to each well (except background controls) and incubated under culture conditions for 6 h. The medium was then transferred to a new plate, followed by absorbance determination at wavelengths of 570 and 600 nm in a spectrophotometer (Thermo Scientiﬁc, Pathtech Australia). The above procedures were repeated every 48 h until conﬂuence was reached (between days 6 and 8). Cell proliferation was quantiﬁed by the percentage reduction of Alamar blue, i.e.
%reduced Alamar blue ¼
eox ðk2 ÞAðk1 Þ eox ðk1 ÞAðk2 Þ 100; eRED ðk1 ÞA ðk2 Þ eRED ðk2 ÞA ðk1 Þ ð2Þ
where A(k1) and A(k2) are the absorbance values of test wells measured at wavelengths k1 and k2 and A ðk1 Þ and A ðk2 Þ are the values
Q.-Z. Chen et al. / Acta Biomaterialia 6 (2010) 4143–4153
0.3 0.2 0.1
0.4 0.3 0.2 0.1
0.1 PO4 Crys
0 0.4 0.3 0.2
Sintered NovaBone Bioglass®
0 PO4 Crys
0.4 0.3 0.2
Sol-gel derived and sintered Bioglass , 0.25M
Wavenumber/ cm-1 Fig. 4. FTIR spectra of two nitrate precursors and commercial BioglassÒ (with and without sintering as indicated) and the sol–gel derived BioglassÒ sintered at 1000 °C for 2 h. The FTIR spectra proﬁles of the four powders are the same. The spectra of the sol–gel derived BioglassÒ ceramics have the same proﬁle as that of the commercial NovaBone 45S5 BioglassÒ sintered at 1000 °C for 2 h.
of absorbance of negative control wells containing only medium and Alamar blue™ without cells. All values were blanked with the readings of background controls. The other parameters in Eq. (2) are as follows: eox ðk1 Þ ¼ 80:586, eox ðk2 Þ ¼ 117:216, eRED ðk1 Þ ¼ 155:677 and eRED ðk2 Þ ¼ 14:652. 2.9. Statistics All experiments were run with ﬁve samples and the data were represented as means ± SE. Statistical difference was analysed using one-way analysis of variance (ANOVA) with Tukey’s post hoc test, and a P value of 0.05), and the sintered S70C30 glass ceramic caused the least increase in pH value of the culture medium. The increment in pH values due to 45S5 materials can be attributed to the fast release of Na+ ions, as shown in Fig. 12. A comparison of Fig. 12a and b reveals that ion release was slower in the crystallized 45S5 glass ceramic. This was also observed with the S70C30 material (Fig. 12c and d). The ion release from 45S5 BioglassÒ observed in the present work was generally slower compared with those previously reported in studies using deionized water . This can be explained by certain differences in the experimental procedures, namely that in the previous study water was replaced with fresh deionized water after each measurement, whereas in the present work medium was not changed after each measurement during the 2 day experiment. The procedure used in this work is in accordance with standard tissue culture practice, i.e. culture medium is changed every 2 days. Hence, the slower ion re-
100 90 80 70 60 50 40 30 20 10 0
HA NovaBone Sol-gel 45S5 (0.1M) Sol-gel 45S5 (0.25M)
Tissue Culture Duration Fig. 13. MG63 cell proliferation kinetics measured by the Alamar blue™ technique. The initial plating density was 5000 cells ml1 in each well of a 48-well plate (n = 3). Medium with hydroxyapatite and commercial 45S5 BioglassÒ (NovaBone) extracts were positive controls. Two types of sol–gel derived and sintered 45S5 glass ceramics were made with 0.1 and 0.25 M HNO3 solutions as indicated. Sol–gel derived 45S5 and Novabone™ 45S5 materials were sintered under the same conditions (1000 °C for 2 h). The differences between NovaBone™ and the sol–gel derived 45S5 materials were not signiﬁcant (P > 0.05) on days 2–6. The differences between HA, NovaBone™ and the sol–gel derived 45S5 materials were not signiﬁcant (P > 0.05) on days 2–4. On day 6 the signiﬁcance values were HA vs. BG_0.1 M (P < 0.01), HA vs. Novabone (P < 0.01), HA vs. BG_0.25 M (P > 0.05).
Q.-Z. Chen et al. / Acta Biomaterialia 6 (2010) 4143–4153
Acknowledgement Q.-Z.C. would like to acknowledge support from the New Staff Research Grants and Small Research Grants of the Faculty of Engineering at Monash University.
Appendix A. Figures with essential colour discrimination
Fig. 14. Cytotoxicity of the test materials using MG63 cells, detected by measuring the release of lactate dehydrogenase (LDH) into medium containing the extracted substances. Medium with HA and commercial BioglassÒ (NovaBoneÒ) extracts served as positive controls. Two types of sol–gel derived and sintered 45S5 glass ceramics were made with 0.1 and 0.25 M HNO3 solution as indicated. Sol–gel derived 45S5 and Novabone™ 45S5 materials were sintered under the same conditions (1000 °C for 2 h). No signiﬁcant differences were found in the percentage of dead cells between any two of the four groups (P > 0.05).
3.6. Evaluation of cytocompatibility The biocompatibility of melt-derived 45S5 and sol–gel derived S70C30 has been extensively evaluated previously, both in vitro and in vivo [28–30]. Given that the current sol–gel derived 45S5 was produced by a completely new process which involved an acidic catalyst HNO3, it was essential to evaluate the biocompatibility of the materials, using the cytocompatibility standard described by ISO 10993 and required by the FDA. Melt-derived 45S5 BioglassÒ (NovaBone) and HA were used as controls in this work. Medium containing the extracts of sintered powders was found to support proliferation of MG63 cells and visual microscopic observation could identify no gross differences in cell proliferation in the following four groups of samples: HA, commercial (NovaBoneÒ) BioglassÒ and sol–gel derived BioglassÒ ceramics (0.1 and 0.25 M). This was quantitatively conﬁrmed using the Alamar blue™ technique (Fig. 13). This revealed that none of the growth parameters of MG63 cells were statistically different between the four groups of medium tested, with no signiﬁcant difference seen between any two of the cellular growth kinetic curves (P > 0.05). The sol–gel derived and sintered glass ceramics were found to have very similar cytocompatibilities to both HA and commercial NovaBoneÒ BioglassÒ, as indicated in Fig. 14. The proportions of dead cells observed in MG63 cultures exposed to the different types of BioglassÒ or HA were not signiﬁcantly different (P > 0.05).
4. Conclusions Fine powders of Na2O-containing glass ceramics have been successfully synthesized using the sol–gel technique in aqueous solution under ambient conditions. Microspheres of size 5 lm can be easily achieved at low cost, eliminating the time-consuming processes, i.e. grinding and sieving. The sol–gel derived and sintered 45S5 glass ceramic materials possess the essential features of Na2O-containing bioactive materials, namely the formation of crystalline phase Na2Ca2Si3O9 during sintering, a decrease in the crystalline phase and extensive formation of amorphous bone-like apatite. A comparative study on Na2O-containing and Na2O-free bioactive glass ceramics indicated that Na2O (or oxides of other active elements, such as potassium) could be an important constituent enabling achievement of an optimal combination of sound mechanical properties and good biodegradability in one material.
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