Elsevier

Journal of Non-Crystalline Solids

Volume 425, 1 October 2015, Pages 52-60
Journal of Non-Crystalline Solids

Processing and characterization of novel borophosphate glasses and fibers for medical applications

https://doi.org/10.1016/j.jnoncrysol.2015.05.028Get rights and content

Highlights

  • B2O3 replacement for P2O5 increases the stability of the glass toward crystallization.

  • B2O3 enters the metaphosphate structure as BO4 units.

  • Addition of boron delays the formation of the reaction layer.

  • Fibers from boron containing phosphate glass have been drawn into fibers.

  • Fibers can be used to probe the speed of glass dissolution in vitro.

Abstract

In this paper, we investigate the effect of P2O5 substitution by B2O3 in the (50  x)P2O5·20CaO·20SrO·10Na2O·x B2O3 glass system (x from 0 to 5 mol%) on the thermal and structural properties and also on the glass reactivity in simulated body fluid. The goal is to develop new glass candidates for biomedical glass fibers. The addition of B2O3 at the expense of P2O5 increases the refractive index of the glass and also the thermal stability of the glass indicating that these glasses are promising glasses for fiber drawing. Thus, within such glass composition, the core of a core–clad fiber has a larger concentration of B2O3 than the clad of the fiber to enable the light to propagate inside the core. All the investigated glasses form a calcium phosphate layer at their surface when immersed in simulated body fluid. It was found that small addition of B2O3 (1.25 mol%) leads to a decrease in the initial dissolution rate and a delayed layer formation. However, with increasing B2O3 content, the chemical durability decreased slightly but was higher than for the B-free glass. In addition, formation of the calcium phosphate layer was further delayed. This suggests that small contents of B2O3 led to formation of P–O–B bonds and only few BO4 units, increasing the chemical durability. At higher B2O3 contents, the amount of BO4 units increases which makes the glass network slightly more prone to be hydrolyzed. Thus, formation of BO4 units induced by the addition of B2O3 at the expense of P2O5 reduces the reactivity of the glass in SBF. Borophosphate fibers were successfully drawn from preform. As expected from the bioresponse of the bulk glasses in simulated body fluid, the reduction in the intensity of the light transmitted is less and slower in a borophosphate fiber than in a phosphate fiber upon immersion.

Introduction

Biomaterials are an integral and vital part of our modern health-care system. Hand in hand with more sophisticated solutions coming on the market, the demand on biomaterials based on biomedical glass fibers has increased in the past decade. Fibers based on bioactive glasses can be used for reinforcement in composite [1], [2] or biosensing [3]. Due to their biocompatibility, the silica based glasses such as 45S5 [4] and S53P4 [5] are well-known candidates for different biomedical applications including bone-grafting. However, these glasses are prone to crystallize at temperatures typical for fiber drawing [6]. The problems with crystallization may, however, be solved by developing new bioactive glass compositions with thermal properties better suited for fiber drawing processes.

Phosphate glasses are good alternatives to silicate glasses in biomedical applications, such as in bone repair and reconstruction [7]. Recently, new types of bioresorbable borophosphate glasses were found to be good choices for fiber drawing due to their suitable forming properties [8]. Borophosphate glass fibers are good candidates for soft tissue engineering applications involving muscles, ligaments, and tendons. In these, the tissue has, similarly to the glass fibers a high degree of anisotropy.

Borate glasses were found to accelerate the formation of a hydroxyapatite layer and bond to bone [9]. The bioactivity of calcium phosphate glasses was favored by the addition of boron due to the ability of boron to change coordination and to attach hydroxyl groups at the surface of the glasses [10]. The incorporation of boron into the P2O5–CaO–Na2O–MgO glass system showed favorable effects on the cell metabolic activity, proliferation, and morphology [11].

In our previous study, the effect of partial substitution of SrO for CaO on the thermal and bioactivity properties of phosphate glasses in the P2O5–CaO–SrO–Na2O glass system was reported [12]. SrO containing glasses are of interest since traces of Sr are present in the human body [13]. Moreover, it has been shown that Sr can easily replace Ca in the mineral part of the bone to form a stronger bone [14]. In our previous study [12], we showed that SrO-containing phosphate glasses are promising glasses for fiber drawing from preforms. We also found that the addition of SrO at the expense of CaO restrains the leaching of phosphate ions while maintaining similar surface reactivity than the Sr-free phosphate glasses. Additionally, partial to full substitution of CaO for SrO has been found to enhance adhesion and proliferation of cells [15]. However, the composition of the reaction layer forming at the surface of these glasses is closer to dicalcium phosphate dihydrate (DCPD) than hydroxyapatite (HA) [12]. In order to reach Ca/P ratio closer to 1.6 and to increase the bioactivity of the phosphate-based glasses, new glasses were prepared by partially replacing P2O5 by B2O3.

In this work we study the effect of B2O3 addition on the thermal, structural and bioactive properties of phosphate glasses in the P2O5–CaO–SrO–Na2O glass system. We also discuss the impact of the fiber drawing on those properties. Thermal properties of the glasses were measured using a differential thermal analyzer (DTA). Structural characterization of the studied glasses was performed using combination of experimental tools such as NMR, Raman and IR spectroscopies. The in-vitro testing was performed in simulated body fluid.

Section snippets

Experimental section

The glasses with the composition (50  x)P2O5·20CaO·20SrO·10Na2O·x B2O3 with x = 0, 1.25, 2.5, 3.75 and 5 (mol %) (labeled respectively B0, B1.25, B2.5, B3.75 and B5) were prepared using the standard melting method in a platinum crucible. NaPO3, H3BO3, SrCO3, CaCO3 and (NH4)2HPO4 were used as raw materials. Sr(PO3)2 and Ca(PO3)2 precursors were first independently prepared using a slow heating rate up to 900 °C. The precursors were mixed with the other raw materials and melted in the platinum

Results and discussion

New phosphate glasses in the system (50  x)P2O5·20CaO·20SrO·10Na2O·x B2O3 with x = 0, 1.25, 2.5, 3.75 and 5 (mol %) (B0, B1.25, B2.5, B3.75 and B5) were prepared for studying the impact of the replacement of P2O5 by B2O3 on the thermal, structural and bioactive properties.

The thermal properties as well as the density and the refractive indices of the glasses as a function of the boron oxide content are listed in Table 1. An increase in B2O3 induced an increase in the density. Such increase in

Conclusions

The effect of P2O5 replacement by B2O3 on the thermal, structural and bioactive properties of new borophosphate glasses was examined. With increasing substitution of B2O3 for P2O5, the glass transition and crystallization temperature increased indicating an increase in the glass network connectivity. All studied glasses possessed a ΔT greater than 100 °C indicating that they possessed sufficient resistance to crystallization for shaping the glass into fibers from preforms.

The influence of B2O3

Acknowledgments

A part of the research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007–2013 under grant agreement n°264526 through the GlaCERCo Marie-Curie ITN project. The Academy of Finland (grant numbers: #284492; 275427) is gratefully acknowledged for the financial support of J.M.

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