Novel Bioactive Borate Glasses for Biomedical Applications

Muhammad Sami Hasan (Department of Applied Oral Sciences, Dalhousie University, Halifax, NS, Canada)
Daniel Boyd (Department of Applied Oral Sciences, Dalhousie University, P.O. Box 15000, Halifax, NS, Canada B3H 4)

Introduction

Borate glasses have been known for their bioactivity and degradability1. Doping with various network formers (e.g. SiO2, P2O5) could help to tailor the durability and bioactivity -in terms of therapeutic effects or conversion into hydroxyapatite1 2. Vitreous borate glass is made of boroxol (B3O6) rings that break into BO3 and BO4 unit with the addition of network modifiers. Alkali and alkaline earth metal oxides have been incorporated in biodegradable glasses to control their degradation rate and to induce bioactivity2. For current project, strontium, due to its therapeutic effect on bone cells, and titanium, for its antibacterial properties, alongside their effects on durability have been selected as dopants. Investigations into chemical-structural changes, thermal properties variation, effect on durability, quantities of ions released and cytocompatibility per systematic compositional changes were made.

Materials and Methods

A melt-quenching technique was used to produce 11 glass compositions using: B2O3, SrCO3, TiO2 and Na2CO3. The glasses were ground using an automated grinder and sieved <100 um. Glass rods were melt-casted in a graphite mold preheated at 5C above Tg. and cut into discs using a diamond saw. Digesting the glass in strong acid and detecting constituent oxides in an ICP-AES provided the actual compositions. The amorphicity of all the glass compositions was confirmed using X-ray diffraction spectra obtained for angular range 2𝜃 for each scan15∘ to 100∘ with a step size of 0.02∘. 11B magic-angle spinning nuclear magnetic resonance (MAS-NMR) studies were carried out on a Bruker Avance NMR spectrometer. The thermal characteristic temperatures - glass transition (Tg), crystallization (Tc) and melting (Tm) were characterized by a TA Instruments-DQ200 differential scanning calorimeter operated at a heating rate of 10 °C min-1 from 25 to 1100 °C. Density of glasses was measured using He-Pycnometer. Glass discs (9 mm diameter, 4 mm thick) were immersed into vials containing 30 ml of phosphate buffered saline and incubated in a shaking incubator at 37°C for 30 days. At specific time points, pH was measured, the discs were taken out of their respective containers and blot-dried and weighed. The solution was not changed at during the study. Glass extracts were prepared in tissue culture water with a ratio of 0.2g/ml. The glass powders for each of the composition (n=3) were immersed in extraction medium for up to 28 days. The specimen incubated at 37 °C in a shaker incubator, agitated at 2 Hz. After each incubation period, the extracts were separated from spheres and diluted (1:100) in 2% nitric acid solution in deionized water. The following elements were detected using ICP-AES: Na as Na2O, Sr as SrO, Ti as TiO2 and B as B2O3. The sample solutions were analyzed against the calibration curve. In vitro cytocompatibility of L929 mouse fibroblast was evaluated using the MTT assay with evaluations being on the basis of indirect exposure via the same extracts prepared for ICP-AES.

Results

Borate glasses in system (S1: 70B2O3-30-XSrO2-XTiO2 and S2: 70B2O3-20SrO2-10-XNa2O-XTiO2, where X=2,4,6,8,10) were prepared. The XRD traces confirmed the amorphicity of the glasses. Compositions of the glass as measured by ICP-AES were within 5-8% of theoretical compositions (Table 1). For glasses S1, a downward trend and for S2 no significant change in Tg, Tc and Tm was observed with increasing Ti content. Density appeared to have reduced in S1 and unchanged in S2. In S1, an increased solubility was observed with increased Ti content. However, no change in durability was observed for S2 (Figure 1). The pH was observed to change from 7.5 to 8.1-8.5 during the study. NMR investigations revealed structural changes in the glass series with increasing Ti content. The peak position, (0.54-0.6ppm) represents the BO3 and peak position (13.5-14ppm) represent BO4 units, did not change significantly. However, the integral of peak 1 significantly reduced (from 43 to 32 in S1 and 43-32 in S2) in favor of peak 2 (58 to 68 in S1 and 57-68 in S2). This indicates the shift in the borate structure from a structure with greater BO3 units to a modified structure that is predominantly comprised of BO4 units. BO3 is less stable than BO4, which explains the observed changes in the properties (Thermal, density) in S1. The effect of Sr on durability was clearer than Ti. On adding a network modifier such as alkali oxide, the glass properties do not change as a monotonic function of composition – the so-called boron oxide anomaly. The addition of a network modifier to B2O3 initially leads to an increase in the coordination number of B atom from 3 to 4, rather than formation of non-bridging oxygen. (i.e. conversion of BO3 triangles to BO4 tetrahedra). For both S1 and S2 a burst release up to 3rd day and then a plateau due to saturation of all constituent ions was observed which was directly related to the durability of the glass composition. Cytocompatibility assessments via MTT assay are under investigation.

Discussion and Conclusion

Borate glass with controlled durability and continuous release of therapeutic levels of ions can be prepared. Alkaline earth metal (Sr) is more effective in controlling the durability. Variation of modifiers/intermediates induced the structural changes in the glasses that lead to the change in characteristic properties.

Theoretical and actual compositions of borate glass series alongside their thermalproperties (Tg, Tc, Tm), density and integral of BO3/BO4 NMR peaks.

Figure1: Commulative degradation of two borate glass series over 30 days in PBS at 37 degree Celsius.

References

1. Ouis MA, Abdelghany AM and ElBatal HA. Corrosion mechanism and bioactivity of borate glasses analogue to Hench’s bioglass. Processing and Application of Ceramics 2012; 6: 141–9. 2. Sinouh H, Bih L, Azrour M, et al. Effect of TiO2 and SrO additions on some physical properties of 33Na2O–xSrO–xTiO2–(50 − 2x)B2O3–17P2O5 glasses. J Therm Anal Calorim. 2013; 111: 401-8.

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