INTRODUCTION:

Hench glass with a mass percentage composition of 45SiO₂-24.5CaO-24.5Na₂O-6P₂O₅, is one of the most interesting and widely used cell specific response inducing materials. Since its discovery, various works to optimize its properties have been carried out with the incorporation of trace elements or active ingredients¹. In this purpose, Na₂O has been substituted by K₂O to improve the chemical durability in the Na₂O-CaO-P₂O₅-SiO₂ glass system². For their part, D. Bellucci et al. have shown that the addition of about 3 % by weight of Al₂O₃ to the 45S5 bioglass is sufficient to limit the bone binding capacity of the glass³.

It therefore appears that the addition of oxides such as MgO, K₂O, CuO, ZnO, Al₂O₃ and SrO in the composition of the reference glass could have an effect on its physicochemical and therapeutic properties. In particular, they influence the release kinetics of the oxides in vitro or in vivo, hence the need to control the dissolution rate to allow a progressive colonization of the hydroxyapatite and an efficient release of possible active ingredients.

For this purpose strontium, an alkaline earth metal naturally present in bone tissue, can be incorporated into apatites during the growth phases of its precipitates (formation of calcium deficient apatites). According to the literature, strontium plays a key during cellular reactions and influences the solubility of apatite. Cell culture studies have also indicated that strontium ions facilitate the proliferation of osteoblast cells for bone growth and prevent osteoclast cells from absorbing bone^4-6. In addition to its important biological role, strontium, incorporated into the basic composition of bioactive glasses, has significant effects on the physicochemical reactions of the bioactivity process. In particular, it influences the dissolution of the material, the release of the various elements and the formation of the phosphocalcic layer. Strontium, a network modifier, with calcium like physicochemical properties, the substitution of SrO/CaO becomes a new strategy to design new bioactive materials especially for bone repair and bone regeneration therapy^4,7,8.

As a result, studies have been carried out to explain and understand the properties related to the addition of SrO in bioactive glass structures. For this purpose, Lao et al.⁹ used particle induced X-ray emission (PIXE) to monitor the distribution of strontium after dissolution in simulated fluid environments of several strontium containing silicate glasses synthesized using the sol-gel process. They have shown that the strontium contained in these glasses reduced the dissolution rate compared to those without strontium. Furthermore, Fredholm et al. in a study on the physical properties of the 49.46SiO₂-1.07P₂O₅-(23.08 - x) CaO-26.38NaO-xSrO glass series, has shown the influence of the substitution of strontium for calcium. According to the results of this study, this substitution does not induce any change in the chemical structure of the glass lattice due to the similar chemical properties of SrO and CaO. On the other hand, an expansion of the glass network, associated with the difference in size between the Sr²⁺ and Ca²⁺ cations, has been reported. The consequence is the weakening of the glass network, associated with a decrease in the glass transition temperature¹⁰.

In contrast to the physical property studies conducted by Fredholm et al.⁸, J. Du and Y. Xiang¹⁰ studied the effect of SrO/CaO substitution on the structure, ion scattering and dynamic properties of 45S5 bioactive glasses. Using molecular simulations of constant pressure dynamics, they showed that SrO/CaO substitution leads to a linear increase in glass density and a linear decrease in oxygen density, indicating more or less open compact structures. Similarly, G.S. Lazaro et al.¹¹ used thermal and spectrometric analytical methods to determine how zinc, magnesium and strontium can affect the basic bioactive glass system and influence the reactivity of bioactive glass after immersion in a complex medium such as cell culture medium. They find that SrO/CaO substitution decreases the glass transition temperature while increasing the dissolution rate of the glass. A. D'Onofrio et al.¹² recently studied the effect of SrO/CaO substitution in bioactive glasses used as precursors for injectable bioactive glass cements employing the same methods. They draw the same conclusion as G.S. Lazaro et al.¹¹.

These findings and the prospect of various clinical applications motivated the decision to study strontium doped bioactive glasses. The aim of this contribution is to apply thermal analysis and pycnometry methods to characterize the physicochemical properties of the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO system for comparison with Hench glass.

MATERIAL AND METHODS:

In the present study, nine samples of the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO system were prepared using the standard high temperature fusion method. 0.035% molar SrO was introduced in a range comparable to the average content of this element in human bone tissue in order to better understand the influence of this element on the physicochemical properties of bioactive glasses. For this purpose, commercial products with the characteristics shown in Table 1 were used.

Table 1: Name, formula, source and purity of products used

Reagents	Formula	Provenance	Purity (%)
Sodium silicate	Na₂SiO₃	Alfa Aesar	99.0
Calcium pyrophosphate	Ca₂P₂O₇ (CPP)	Aldrich	99.9
Tricalcium phosphate	Ca₃(PO₄)₂ (β-TCP)	Prolabo	99.5
Silica	SiO₂ quartz α	Merck	99.9
Strontium oxide	SrO	Aldrich	99.0

The reagents are weighed using a Sartorius balance with 0.01mg accuracy. They are then mixed and finely ground (˂50µm) in a suitable proportion in an agate mortar in the presence of ethanol to achieve homogeneous distribution in the mixture and maximum contact between the grains, thus facilitating vitrification. The resulting mixture is put in a platinum crucible and placed into an oven to undergo a thermal cycle according to the temperature programming defined in the profile described in Figure 1.

Fig. 1: Process of heat treatment cycle of the reagents used

After complete melting, the bottom of the crucible is quenched with water at room temperature. This almost instantaneous cooling results in glass fragments. These glass fragments are kept in a dry atmosphere in a desiccator in the presence of P₂O₅. The nominal compositions expressed in molar and mass percentages are shown in Table 2.

Table 2: Molar percentage compositions of elaborated samples. Equivalent compositions expressed in percent by weight are shown in parentheses.

x	SiO₂	P₂O₅	Na₂O	CaO	SrO
0	41.08 (40.00)	2.60 (5.98)	29.37 (29.50)	26.915 (24.46)	0.035 (0.06)
0.125	43.60 (42.48)	2.60 (5.99)	26.85 (26.99)	26.915 (24.48)	0.035 (0.06)
0.25	46.12 (44.97)	2.60 (5.99)	24.33 (24.48)	26.915 (24.50)	0.035 (0.06)
0.375	48.63 (47.46)	2.60 (6.00)	21.81 (21.96)	26.915 (24.52)	0.035 (0.06)
0.5	51.15 (49.96)	2.60 (6.00)	19.29 (19.44)	26.915 (24.54)	0.035 (0.06)
0.625	53.67 (52.46)	2.60 (6.01)	16.77 (16.91)	26.915(24.56)	0.035 (0.06)
0.75	56.19 (54.97)	2.60 (6.01)	14.25 (14.38)	26.915 (24.58)	0.035 (0.06)
0.875	58.71 (57.48)	2.60 (6.01)	11.73 (11.85)	26.915 (24.60)	0.035 (0.06)
1	61.23 (60.00)	2.60 (6.02)	9.21 (9.31)	26.915 (24.61)	0.035 (0.06)

In order to characterize the crystal structure (crystalline or amorphous) of the glasses, powder X-ray diffraction (XRD) measurements were carried out using a SIEMENS Brücker D5000 diffractometer (Cu linekα, λ = 1.5406 Å).

The measurement of the glass transition temperature (Tg) was carried out using a DSC 111-SETARAM calorimeter. A powder of about 20 mg was placed in a stainless steel crucible and heated under argon at a rate of 5°C.min^-1. The reference is an empty stainless steel crucible, similar to the one containing the sample. The accuracy of the measurement is estimated to be ± 3°C.

The pycnometric method was used to measure the density of the samples at (296.3 ± 0.1) K in a solvent of diethyl orthophthalate (C₁₂H₁₄O₄) with an accuracy of ± 0.03 g.cm^-3.

RESULTS:

Figure 2 shows some X-ray diffractograms of 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO system glasses. No acute diffraction peaks are observed on any of the diffractograms but a diffraction halo around 2q = 33°.

Fig. 2: X-ray diffractograms of some of the strontium containing glass compositions of the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO system; 0 ≤ x ≤ 1

The thermograms are shown in Figure 3. Each of these thermograms has a unique glass transition.

Fig. 3: Thermograms of the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO; 0 ≤ x ≤ 1 system glasses

Measurements of the glass transition temperature (Tg) are shown in Figure 4. For comparison, the Tg values for the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.95CaO¹³ system are also shown in Figure 4. A quasi-linear increase in glass transition temperature with the Na₂O/SiO₂ substitution rate is observed in both cases. The Tg is slightly more accentuated in the case of SrO enriched glass, especially at high silica contents.

Fig. 4: Variations in glass transition temperature (Tg) in 20.15[(2.038 + x)SiO₂-(1.457-x)Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO (Tg(0.035Sr)) and 20.15[(2.038 + x)SiO₂-(1.457 - x)Na₂O]-2.6P₂O₅-26.95CaO (Tg(0.0Sr)) systems; 0 ≤ x ≤ 1

The density of the glasses is shown in Figure 5. The density of these two systems evolves in the same direction with the silica content. Indeed, a decrease in density is observed with the silica content. However, the density is slightly lower in the case of SrO enriched glass, especially at high silica contents.

Fig. 5: Density variation in 20.15[(2.038 + x)SiO₂-(1.457-x)Na₂O]-2.6P₂O₅-26.915CaO-0.035SrO (Tg(0.035Sr)) and 20.15[(2.038 + x)SiO₂-(1.457 - x)Na₂O]-2.6P₂O₅-26.95CaO systems; 0 ≤ x ≤ 1

DISCUSSION:

Flattened diffraction halos (Figure 2) are characteristic of the scattering phenomenon in amorphous materials, reflecting the absence of order at long distances. This indicates that the samples are devoid of identifiable crystalline species. Tg measurements thus confirm the vitreous character of these samples.

The glass transition is related to the movement of the units in the glass. The glass transition temperature, Tg, ranges from 502°C to 649°C in the bioactive glass system studied at 0.035Sr molar. These Tg are slightly elevated by about 6 % compared to that at 0.0Sr. These variations are due to the association between the increased disturbance of the glass network caused by a slightly larger Sr cation and the lower resistance of the Sr-O bond⁸. For a high Tg, glasses require sufficient energy to rearrange the covalent bonds in the amorphous lattice¹⁴. It is almost accepted that the energy required to break the Sr-O bond in the Si-O-Sr group is higher than that of Ca-O in the Si-O-Ca group due to the higher electronegativity of Sr compared to Ca¹⁵. In this study, the glass transition temperature increases linearly with the SiO₂ level. Due to its degree of connectivity, silica has the effect of increasing the glass transition temperature (Tg). This increase is in agreement with the results of Kouyate et al.¹³ who link the increase in Tg with the increase in the average length of the macromolecular chains. Indeed, the increase in the average length slows down the freedom of movement of the chains in relation to each other and favors the cross linking of the network. Consequently, Tg increases. However, these cross linkages lead to a linear structure, less compact and therefore less dense. Furthermore, the Tg and density measurements evolve in the same direction as those of the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.95CaO system¹³. The substitution of SrO for CaO can influence the structure of the bioactive glasses under study, but both oxides act as modifiers of the glass lattice, composed of SiO₂ andP₂O₅. They transform bridging oxygen into non bridging oxygen by breaking bonds. Our results showed that the low substitution (0.035 % molar) of SrO to CaO does not significantly influence the structure of the glass groups studied, also observed by Lázaro et al. in glasses derived from the SiO₂-CaO-Na₂O-P₂O₅ system¹¹.

These two systems have relatively similar physicochemical characteristics. Density is a property that appears to be closely related to structural changes in glass¹⁶. Indeed, the density of glass is directly related to the density of the elements involved in the system. There is a slight decrease in density and an increase in average molecular weight with the addition of SrO in the bioactive glasses studied compared to the reference bioactive glasses. This decrease in the density of the bioactive glasses with 0.035 molar SrO could be due to the extended structure of the free phosphate glass network resulting from the addition of SrO. The molecular weight and atomic radius of strontium slightly higher than that of calcium could also explain the variations in density¹⁷. The addition of SrO also creates a more compact metaphosphate network due to the higher cationic potential of Sr²⁺ compared to Na⁺ and Ca²⁺¹⁸. In addition, each Sr²⁺ introduced into the glass substitutes a Ca²⁺ resulting in an increase in the reference molar volume and thus a decrease in the density of the glasses of the system under study compared to those of the reference. There may be other factors, namely, a change in the coordination of the phosphate ions, degrees of structural compactness, changes in the geometric configuration of the phosphate glass network and fluctuations in the dimensions of the interstitial holes caused by the dopant ion may influence the density of the glasses¹⁹.

CONCLUSION:

In this work, glass samples were developed by high temperature melting. DRX and DSC measurements confirmed the vitreous nature of these samples. Comparing the present work with the 20.15[(2.038 + x) SiO₂-(1.457 - x) Na₂O]-2.6P₂O₅-26.95CaO system, it appears that the glass transition temperature increases slightly with the SiO₂ content while the density decreases slightly. As a result, strontium inputs do not induce a very significant change in physicochemical quantities such as Tg and glass density. All the results are in good agreement with the rare results in the literature.

ACKNOWLEDGEMENT:

The authors would want to thank Prof. Ange Privat Ahoussou for his precious help.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

REFERENCES:

1. M. Vallet‐Regí, C. Ragel, A. J. Salinas. Glasses with medical applications. European Journal of Inorganic Chemistry. 2003; 6: 1029-1042.

2. S. M. Salman, S. N. Salama, H. A. Abo-Mosallam. The role of strontium and potassium on crystallization and bioactivity of Na₂O–CaO–P₂O₅–SiO₂ glasses. Ceramics International. 2012; 38:(1) 55–63.

3. D. Bellucci, V. Cannillo, A. Sola. An overview of the effects of thermal processing on bioactive glasses. Sci. Sinter. 2010; 42: 307–320.

4. J. Lao, E. Jallot, J. Nedelec. Strontium-Delivering Glasses with Enhanced Bioactivity A New Biomaterial for Antiosteoporotic Applications? Chem. Mater. 2008; 20: 4969-4973.

5. Devis Bellucci, Valeria Cannillo. A novel bioactive glass containing strontium and magnesium with ultrahigh crystallization temperature. 2018; 213: 67-70.

6. P. J. Marie. Laser Technology in Biomimetics: Basics and Applications. Bone 2007; 40: 55–58.

7. M. D. O'Donnell, R. G. Hill. Influence of strontium and the importance of glass chemistry and structure when designing bioactive glasses for bone regeneration. Acta Biomater. 2010; 6(7): 2382–2385.

8. Y. C. Fredholm, N. Karpukhina, R. V. Law, R. G. Hill. Strontium containing bioactive glasses: Glass structure and physical properties. Journal of Non-Crystalline Solids. 2010; 356(44-49): 2546–2551.

9. J. Lao, J. M. Nedele, E. Jallot. New strontium-based bioactive glasses: physicochemical reactivity and delivering capability of biologically active dissolution products. J. Mater. Chem. 2009; 19: 2940–2949.

10. Jincheng Du, Ye Xiang. Effect of strontium substitution on the structure, ionic diffusion and dynamic properties of 45S5 Bioactive glasses. Journal of Non-Crystalline Solids. 2012; 358(8): 1059 –1071.

11. G. S. Lázaro, S. C. Santos, C. Xavier Resende, E. A. dos Santos. Individual and combined effects of the elements Zn, Mg and Sr on the surface reactivity of a SiO₂·CaO·Na₂O·P₂O₅ bioglass system. Journal of Non-Crystalline Solids. 2014; 386: 19–28.

12. E. Dietrich, Oudadesse H., Lucas-Girot A., Le Gal Y., Jeanne S., Cathelineau G. Effects of Mg and Zn on the surface of doped melt-derived glass for biomaterials applications. Applied Surface Science. 2008; 255(2): 391–395.

13. A. Kouyate, A. P. Ahoussou, J. Rogez, P. Benigni. Application of Solution Calorimetry to the Prediction of 20.15[(2.038+x) SiO₂-(1.457−x) Na₂O]-2.6-P₂O₅-26.95CaO Glass Bioactivity. Advances in Chemical Engineering and Science. 2013; 3: 123-129.

14. M. Ojovan. Configurons: Thermodynamic Parameters and Symmetry Changes at Glass Transition. Entropy; 2008; 10(3): 334-364.

15. D. Boyd, M. R. Towler, S. Watts, R. G. Hill, A. W. Wren, O. M. Clarkin. The role of Sr²⁺ on the structure and reactivity of SrO-CaO-ZnO-SiO₂ ionomer glasses. J. Mater. Sci. Mater. Med. 2008; 19(2): 953-7.

16. Mohamed Atef Cherbib, Ismail Khattecha, Lionel Montagne, Bertrand Revel, Mohamed Jemala. Effect of SrO content on the structure and properties of sodium-strontium metaphosphate glasses. Journal of Physics and Chemistry. 2017; 102: 62–68.

17. Y. C. Fredholm, N. Karpukhina, D. S. Brauer, J. R. Jones, R. V. Law, R. G. Hill. Influence of strontium for calcium substitution in bioactive glasses on degradation, ion release and apatite formation. J. R. Soc. Interface. 2012; 9: 880–889.

18. T. Satyanarayana, I. V. Kityk, M. Piasecki, M.G. Brik, Y. Gandhi, N. Veeraiah, J. Structural investigations on PbO-Sb (2)O(3)-B (2)O(3): CoO glass ceramics by means of spectroscopic and dielectric studies. J. Phys. Condens. Matter. 2009; 21: 245104.

19. F. H. ElBatal, Y. M. Hamdy, S. Y. Marzouk. Gamma ray interactions with V₂O₅-doped sodium phosphate glasses. Mater. Chem. Phys. 2008; 110(3): 991-1000.

Received on 27.05.2020 Modified on 22.06.2020

Asian J. Research Chem. 2020; 13(5):352-356.

DOI: 10.5958/0974-4150.2020.00066.8