# Spinning of Endless Bioactive Silicate Glass Fibres for Fibre Reinforcement Applications
## 4 Discussion
Although the glass fibres that were produced have comparable SiO\({}_{2}\) contents to E-glass fibres (53-65 wt% for BG fibres to 50-60 wt% for E-glass fibres), they do not contain Al\({}_{2}\)O\({}_{3}\), but instead have significantly greater alkali metal oxide contents such as Na\({}_{2}\)O and K\({}_{2}\)O, which are present in E-glass only in small amounts up to a maximum of 2 wt%. In addition, the 1-06 and 13-93 glasses contain P\({}_{2}\)O\({}_{5}\), which is responsible for high bioactivity, as hydroxyapatite can form on the surface when the glasses dissolve. Dissolution of the fibres was observed in all glasses. Fibres of glass 18-06 were the most stable and did not dissolve in deionized water or in SBF in the period of 4 weeks. The fibres of glass 1-06 and 13-93, which according to [PERSON] [32] have a medium bioactivity, were dissolved within a few hours. However, all glass fibres showed that storage at 37 \({}^{\circ}\)C in SBF led to an accelerated degradation. In the case of glasses 1-06 and 13-93, the fibres completely dissolved in SBF in less than one day. This result was unexpected, since 13-93 in particular is considered a glass that dissolves more slowly than S53P4 and 45S55 [39]. [PERSON] et al. studied the dissolution of fibres of glass 13-93 SBF even over a period of 40 weeks [40]. The mean strength after fibre drawing was determined to be 1443 MPa by the authors, which was higher than the 877 MPa observed in the present study. However, fibre diameters of 25-160 um were obtained, which explains their higher initial strengths on the one hand and the very large variation ranges on the other. It should be noted, however, that fibres were still present after 40 weeks. The fibres of glass 18-06 with the lowest bioactivity showed the slowest dissolution rate. Since this glass does not contain P\({}_{2}\)O\({}_{5}\), the bioactivity is lower compared to the other glass compositions. Nevertheless, the slow rate of degradation may be an advantage for medical devices. A slow release of ions does not lead to an abrupt change in the local ion composition, and these fibres would be better suited for reinforcing resorbable implants that need to be mechanically stable over a longer period of time. From the results of the study, it is evident that the dissolution capacity of hydrolvically unstable glasses clearly depends on their particular geometry. Therefore, the dissolution behaviour of the produced fibres cannot be compared with bulk materials or powders. The lower bioactivity for 18-06 reported in the literature also suggests that less formation of an apatite layer and thus a decreased bioactivity is to be expected [32]. Preliminary experiments with the murine pre-osteoblast cell line MC3T3-E1 showed that the cells adhered well to all three types of glass fibres after seven days, appearing to serve as a guide structure for the cells. Remarkably, the glass fibres of all compositions were still present after the observation period. Based on these preliminary results, further studies need to follow to evaluate the biocompatibility of the glasses with different cell types in detail. In addition, the influence of the medium and the amount of solvent on the dissolution behaviour of the glass fibres need to be investigated. The issue of sizing is of enormous importance for the fibres and closely related to the dissolution behaviour. If textile-processable resorbable fibres are to be obtained, the size is just as important as the manufacture of the fibres themselves. The key functions of sizings are to protect the glass fibres during production and handling, to establish contact between the individual filaments, as well as to enable bonding between the glass fibre and the polymer matrix. It is also widely proven that the silane component of the sizing is able to repair partially surface flaws of the fibres leading to an increased fibre strength [23; 41]. Furthermore, such surface modifications can be utilised to protect the fibres against environmental degradation [22].