## 1 Introduction  
Bioactive glasses (BGs) have attracted considerable attention since they was first developed by [PERSON] et al. [(1969)] in 1969. Various studies have demonstrated that when BGs are implanted into human body, layers of hydroxyapatite (HA), constituting the main mineral component of human bones, are formed on the surface of BGs [(1969); (1979)]. These HA layers can chemically bond to human bones, which reduces rejection and inflammation [(1979); (1979)]. Owing to properties such as bioactivity, non-toxicity and degradability, BGs have been applied in various biological applications such as toothpaste fillers [(1979)], bone implants [(1979)], drug carriers [(1989)], and radioisotope vectors [(1999); (2000)]. The first BG reported by [PERSON] et al. [(1999)] was fabricated by a conventional glass melting process; the process involved melting the raw precursors at high temperatures of 1250-1400 \({}^{\circ}\)C and casting the melts into bulk implants. However, the required high temperatures lead to disadvantages, such as high cost of energy consumption and platinum crucibles [(1999)], partial crystallization due to different melting points and also potential contamination during grinding and sieving [(1999)]. As BGs are fabricated for medical use, contamination must be avoided and purity must be assured. To avoid the disadvantages of the conventional glass melting process, [PERSON] et al. [(1999)] synthesized BGs using a sol-gel method in 1991, which subsequently became one of the most popular BG fabrication methods, due to its lower heat treatment temperature (around 600 \({}^{\circ}\)C) and chemical flexibility (better control of the product's properties). In addition, by adding templates or surfactants, different morphologies can be prepared,including solid, hollow or mesoporous structures. For example, [PERSON] et al. [15] used a tri-block surfactant to successfully synthesize a well-ordered mesoporous bioactive glass in 2004. Although the sol-gel method has been widely used for BG fabrication, the overall process follows a batch process that is, it is difficult to adapt to mass production, which usually requires several steps, some of which need long periods of time, up to 4-7 days [16]. Spray pyrolysis offers the advantages of both processes described above; it is suitable for mass production, operates lower temperature of around 500-600 \({}^{\circ}\)C and produces high purity BGs within a period of 1-2 days [17]. The process involves dissolving the precursor ingredients in a solvent to form a precursor solution, which is then atomized into fine droplets using ultrasound and introduced into a tube furnace to undergo stages of solvent evaporation, solute precipitation and precursor decomposition [18]. Synthesized BG particles are then collected in a high voltage electrostatic deposition system located after the furnace. However, owing to the nature of the ultrasonic atomizer, particle sizes of spray pyrolyzed BG lay around the sub-micron (300-800 nm) range [17; 18; 19]. This limits its applications, such as dermal fillings [20] and radioisotope vectors [10; 11], which require a particle size up to few microns. Studies have shown that microspheres with larger particle size could survive the ingestion process of phagocytosis [20], while the mechanical properties can also be controlled by the size effects [21; 22]. In contrast, the spray drying method is a widely applied manufacturing process which uses the aerosol phase to dry particles [23]. It is a well-establish technique that has been used for over a century in the fields of pharmaceuticals, polymers and ceramics [24]. Yet, although some authors have already produced BG with spray drying method [25; 26], the particle sizes reported are only within 1 \(\upmu\)m, which is still too small for the above applications. Therefore, in this study, we aimed for the preparation of micron-sized BG powders of up to 20 \(\upmu\)m using the spray drying method instead of spray pyrolysis.