## 4 Discussion  
First, based on the SEM images of as-prepared BG powders shown in Figure 2, it has been demonstrated that the particle morphology is spherical for all spray dried BG powders. This is due to the typical particle formation mechanism of spray drying method [31], similar to the \"one-particle-per-drop\" mechanism of spray pyrolysis [18]. Moreover, it can be seen from the FIB-prepared BG powders shown in Figure 3 that all BG powders exhibit solid or porous inner structures. During spray drying, hollow particles are often obtained. However, in this study, a low Peclet number in combination with a high solubility of the solutes could result in the formation of solid particles [29]. In addition, the porous structure is possibly formed due to the decomposition of the remaining solvent (e.g. NO\({}_{3}\)\({}^{-}\)) during the calcination process.  
For the particle size distribution, histograms shown in Figure 5 suggest that all BG powders exhibit normal distributions with d\({}_{50}\) around 8 \(\upmu\)m. This is owing to the nature of droplet distribution controlled by the high-speed rotating disc. In addition, the average particle sizes of BG powders are computed as 8.8 \(\pm\) 3.5, 6.4 \(\pm\) 4.2 and 5.9 \(\pm\) 3.5 \(\upmu\)m for 58S, 68S and 76S BG powders, respectively. Meanwhile, it also shows that the particle size of spray dried BG powders is about 10 times bigger than the particles size of spray pyrolyzed BG powders (300-800 nm) [17; 18; 19]. However, owing to the distribution of droplet sizes, the average particle sizes still present standard deviations around 50% similar to the BG particles synthesized using spray pyrolysis [32].  
Figure 6: XRD patterns of 58S, 68S, and 76S BG powders soaked in simulated body fluid (SBF) for 12 h.  
Figure 7: Intensities of hydroxyapatite (HA) (112) peak as a function of SiO\({}_{2}\) concentration.  
The specific surface areas were measured using BET and showed that all BG powders have surface areas ranging from 6 to 40 m\({}^{2}\)/g. Meanwhile, with the density of 2.6 g/cm\({}^{3}\) for 45S5 Bioglass(r) [33], the theoretical surface area of a BG particle with size of 8 \(\upmu\)m can be computed as \(\sim\)0.3 m\({}^{2}\)/g. These results show an increased surface areas of 20 to 100 times as compared to the calculated values, which is due to internal porosity. With more than 20 FIB cross section particles examined, the proportions of porous particles found are 85%, 50% and 20% for 58S, 68S and 76S BG powders, respectively, which shows the same trend as the measured surface areas.  
Finally, the bioactivity tests were carried out by immersing each specimen in SBF. Since the bioactivity of the glasses is directly related to the growth rates of HA, XRD patterns shown in Figure 6 were used to observe the formation of HA. The results show that all spray dried BG powders form HA phases of (211) and (122) after immersion in SBF for 12h. In addition, by performing background subtractions to the XRD patterns, Figure 7 plots the intensities of (112) reflection of HA as a function of SiO\({}_{2}\) concentration. The graph provides a comparison of the bioactivity of each specimen, indicating the order of bioactivity is 58S \(>\) 68S \(>\) 76S.