Na₂CaSi₂O₆–P₂O₅ based bioactive glasses. Part 1: Elasticity and structure

Abstract

The glass structure and elastic properties of two bioglasses having bulk compositions near Na₂CaSi₂O₆ (45S5.2) and Na₂CaSi₃O₈ (55S4.1) were studied using both Raman and Brillouin scattering techniques. The annealed 45S5.2 glass has more Q² and Q⁰ but less Q³ species than 55S4.1 glass due to lower (Si⁴⁺ + P⁵⁺)/(Na⁺ + Ca²⁺) ratio. Brillouin scattering measurements of the as-annealed glasses indicated that 45S5.2 glass is ca. 2% and 9% higher in Young’s and bulk moduli than 55S4.1 glass due to more modifiers in the 45S5.2 glass. Nearly full crystallization of 45S5.2 glass was observed after treating it at 715 °C for ca. 30 min. Devitrification of the 45S5.2 glass caused an increase in the elastic moduli up to ca. 30% (fully crystallized) but a negligible change in density. This 45S5.2-derived crystalline phase displayed at least 17 Raman bands, and has the average elastic moduli of 72.4 (bulk), 41.6 (shear), and 104.7 (Young’s) GPa. The comparable elastic moduli with hydroxyapatite and the ability for developing a HCA layer in simulated body fluid indicate that the 45S5.2-derived phase may be better for using as a substitute of bone than its parent glass.

Introduction

Most of the bioactive materials developed for applications in prosthesis or bone implantation are the (Ca, P)-bearing silicate glasses and ceramics. Hench and co-workers had developed a series of Na₂O–CaO–P₂O₅–SiO₂ bioactive glasses (the so-called Bioglass^®) with acceptable in vivo bioactivity index [1], [2]. This type of bioactive materials is attractive because of no elicitation of foreign body response and strong bonding to bone tissue. The bioactivity and biocompatibility of a bioactive glass are due to the presence of phosphorus and calcium – the typical surface active sites in a bioglass [3]. Sodium ions in the bioglass composition can also contribute to two effects. First, Na₂O is an effective flux in the glass melting process, making it easier to homogenize, cast and flame spray the glasses. Second, dissolution of Na⁺ and Ca²⁺ from the glass will result in the formation of a silica-rich and CaO–P₂O₅-rich bilayer and finally the formation of hydroxycarbonate apatite (HCA), which is necessary for the tissues to bond to the implant as of concern to the bone-bonding ratio [2], [4]. The leached Na⁺ and Ca²⁺ also affect the physiological balance of solution at the glass–ceramic interface and modify the local pH [1]. In fact, the alkalinization may promote synthesis and cross-linking of collagen and the formation of hydroxyapatite – a beneficial effect for in vivo bone growth and repair [5]. The network former (i.e., SiO₂) in the bioactive glass holds the three-dimensional non-periodic glass structure during selective dissolution of cations (e.g., Na⁺) by suppressing the detachment of some other ions [1]. The presence of SiO₂ also helps the precipitation or surface reconstruction of the loose silica-rich layer, and hence enhances the formation of hydroxyapatite layer [2], [6], [7]. The interactions of bone tissues and the Na₂O–CaO–SiO₂–P₂O₅ bioactive implants, in particular the interfacial reaction kinetics and the sequence of reactions, have been critically reviewed by Hench [2], [8].

An ideal biomaterial requires both good biochemical and biomechanical compatibility. A particular advantage of bioglass is the ability to bond to both hard and soft tissues. However, a disadvantage of bioglass is its brittleness. As a consequence, its handling and mechanical properties are not adequate for significant load bearing applications. The monophase bioactive glass, with low strength, was generally restricted to clinical non-load bearing situation. On the other hand, the glass–ceramics, derived from bioglasses by devitrification or combination with other materials (e.g., polymers, inert alloys and ceramics), have offered a better mechanical/biological performance [9], [10]. It is also noteworthy that bone must be loaded in order to remain healthy. The implant with too much higher Young’s modulus than bone tends to carry the load. The resultant stress shielding of bone would induce biological change and even bone resorption. The interface between a stress shielded bone and the implant would then deteriorate as the bone structure is weakened [9]. Thus, elastic properties should not be neglected while searching biological implant materials based on chemical criterion.

The intrinsic strength of a bioglass can be used as a reference for development of new bioactive materials, e.g., a bioactive glass–ceramic with a specified crystallinity or a composite with specified mechanical strength based on that glass. The mechanical strength of a glass product depends on its elastic constant, shape/architecture, surface/internal defects (e.g., scratches, flaws, and bubbles) and the character of the force to which it is subjected. These factors inevitably degrade the mechanical strength of a material to 1/10 or lower of its intrinsic (theoretical) strength. Thus, one cannot obtain the intrinsic strength by using conventional and destructive mechanical test on a big specimen. Brillouin spectroscopy is a powerful alternative for elasticity measurement of transparent materials without a second phase [11], [12], [13]. The surface defects of a small and bubble-free glass specimen causes only a slightly higher background but no frequency shift for the Brillouin signals. Therefore, the elastic constants/moduli determined by Brillouin spectroscopy represent the intrinsic and highest strength of the glass.

Raman spectroscopy has been widely used to study the structure of glasses and supercooled melts. Recently, Raman spectroscopy was used to study the structure in several bioglasses [14] and to detect the formation of HCA on bioactive materials after exposure to simulated body fluid (SBF) [15]. A combination of Raman and Brillouin scattering methods allows researchers to investigate the detailed relations among composition, elastic properties, and glass structure of a bioactive glass. This information may be applied to screen the optimal biomaterial from a series of candidates.

Among the Na₂O–CaO–P₂O₅–SiO₂ glasses, 45S5 material (a composition 24.5Na₂O · 24.5CaO · 6P₂O₅ · 45SiO₂ (in wt%), where S denotes the network former SiO₂ in 45% followed by a specific Ca/P molar ratio 5.2) has been the most popular one and its properties and applications widely studied. The 45S5 glass is non-toxic and biocompatible [16], and in fact clinically used for middle ear prostheses and as endosseous ridge implants [9]. Nevertheless, the intrinsic strength of this glass or its glass–ceramics counterpart was not explored till now.

Here, we report the anionic structure and elastic properties of the as-annealed 45S5.2 glass and its thermal evolution using both Raman and Brillouin spectroscopic methods. Another bioglass, 55S4.1, which has an in vivo bioactivity index lower than that of 45S5.2 glass but close to that of Ceravital^® glasses [2] was also characterized in order to study the effect of composition on the structure and elastic properties of this popular bioglass system. The results of specific devitrification treatments to form glass–ceramics with crystals dispersed in the glass matrix are reported in the interrelated paper [17].