Facile Preparative Access to Bioactive Silicon Oxycarbides with Tunable Porosity.

Fangtong Xie,Aldo R Boccaccini,Ralf Riedel,Marcela Arango-Ospina,Isabel Gonzalo-Juan,Emanuel Ionescu

doi:10.3390/ma12233862

Fangtong Xie, Aldo R Boccaccini + Show 4 more

Open Access

https://doi.org/10.3390/ma12233862

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Abstract

In the present work, Ca-containing silicon oxycarbides (SiCaOC) with varying Ca content have been synthesized via sol-gel processing and thermal treatment in inert gas atmosphere (pyrolysis). It has been shown that the as-prepared SiCaOC materials with low Ca loadings (Ca/Si molar ratios = 0.05 or 0.12) were X-ray amorphous; their glassy network contains Q3 sites, indicating the presence of Ca2+ at non-bridging-oxygen sites. SiCaOC with high Ca content (i.e., Ca/Si molar ratio = 0.50) exhibits the presence of crystalline calcium silicate (mainly pseudowollastonite). Furthermore, it has been shown that the incorporation of Ca into the SiOC glassy network has a significant effect on its porosity and specific surface area. Thus, the as-prepared Ca-free SiOC material is shown to be non-porous and having a specific surface area (SSA) of 22.5 m2/g; whereas SiCaOC with Ca/Si molar ratio of 0.05 exhibits mesoporosity and a SSA value of 123.4 m2/g. The further increase of Ca content leads to a decrease of the SSA and the generation of macroporosity in SiCaOC; thus, SiCaOC with Ca/Si molar ratio of 0.12 is macroporous and exhibits a SSA value of 39.5 m2/g. Bioactivity assessment in simulated body fluid (SBF) confirms the hydroxyapatite formation on all SiCaOC samples after seven days soaking, unlike the relatively inert ternary silicon oxycarbide reference. In particular, SiCaOC with a Ca/Si molar ratio of 0.05 shows an increased apatite forming ability compared to that of SiCaOC with Ca/Si molar ratio of 0.12; this difference is considered to be a direct consequence of the significantly higher SSA of the sample with the Ca/Si ratio of 0.05. The present work indicates two effects of Ca incorporation into the silicon oxycarbide glassy network on its bioactivity: Firstly, Ca2+ is shown to contribute to the slight depolymerization of the network, which clearly triggers the hydroxyapatite formation (compare the bioactive behavior of SiOC to that of SiCaOC with Ca/Si molar ratio 0.12 upon SBF exposure); secondly, the Ca2+ incorporation seems to strongly affect the porosity and SSA in the prepared SiCaOC materials. There is an optimum of Ca loading into the silicon oxycarbide glassy network (at a Ca/Si molar ration of 0.05), which provides mesoporosity and reaches maximum SSA, both highly beneficial for the bioactive behavior of the materials. An increase of the Ca loading leads, in addition to the crystallization of calcium silicates, to a coarsening of the pores (i.e., macroporosity) and a significant decrease of the SSA, both negatively affecting the bioactivity.

Highlights

Silicon oxycarbide (SiOC) has high biocompatibility [1,2,3] and shows bioactivity, i.e., apatite forming ability upon in vitro simulated body fluid (SBF) exposure [4,5]
Metal-containing silicon oxycarbides have been synthesized via the so-called polymer-derived ceramic method (PDC): The incorporation of metal ions into silicon oxycarbide occurs by modifying its polymer precursors, such as polysiloxanes or polysilsesquioxanes, with suitable metal organic compounds [12,13]
Porous PDC derived SiOC can be achieved by choosing sol-gel synthesized precursors [3,14], using mesoporous templates [15], or applying post-synthesis treatments of HF

Summary

Introduction

Silicon oxycarbide (SiOC) has high biocompatibility [1,2,3] and shows bioactivity, i.e., apatite forming ability upon in vitro simulated body fluid (SBF) exposure [4,5]. The use of metal alkoxides requires the usage of (possibly toxic) organic solvents, which is not recommendable from environmental aspects Metal alkoxides, such as calcium alkoxides, are costly and difficult to handle due to their high tendency to hydrolyze with moisture in air [21]. The investigation of the feasibility of such inorganic metal source (calcium nitrate) for modifying silicon oxycarbides, and its influence on the material porosity, will be the focus of the present study. For this purpose, Ca-containing silicon oxycarbides (SiCaOC) were thermally converted from xerogels derived from sol-gel synthesis of triethoxymethylsilane and calcium nitrate in inert gas atmosphere. In vitro SBF bioactivity assessment has been conducted, in order to investigate the effects of Ca loading and porosity on bioactive behavior, i.e., apatite forming ability

Materials Preparation

Simulated

Materials Characterization

Results and Discussion