# Bioactive Glasses for Soft and Hard Tissue Healing Applications--A Short Review
## 2 BGs in the Physiological Media of the Human Body  
Leaching, dissolution, and precipitation are the three main processes which take place when BGs are submerged in an aqueous solution. Leaching happens when alkali or alkaline earth elements are released, typically by cation exchange with H\({}^{+}\) or H\({}_{3}\)O\({}^{+}\). Ion exchange happens because these cations do not make up the glass network and merely alter it by creating non-bridging oxygen connections. In the case of silicate BGs, -Si-O-Si-O-Si bonds are broken during network dissolution by the action of hydroxyl (OH-) ions. Then, the silica is locally released into the solution. After this step, a deposit of silica-rich gel is  
Figure 1: (**a**) Publications since the invention of bioactive glass by [PERSON]. (**b**) Article refinement in Scopus showing the distribution of articles referring to given subcategories. formed as an outcome of the hydrated silica (SiOH) which forms on the glass. A calcium-phosphate-rich (CaP) layer is shaped on the surface of the glass during the precipitation step by the release of calcium and phosphate ions from the glass as well as those from the solution. The bioactivity of BG and the nucleation and subsequent production of an apatite layer are attributed to the phosphate species' solubility [16]. This property has prompted a recent major surge in studies dedicated to the use of BGs in soft tissue repair, along with research results that have strongly implied the significance of controllably released ions in soft tissues and their biological functions [17; 18]. Through their dissolution function and release products, BGs can stimulate and improve cellular activity in tissue regenerative processes (released ions, induced biomineral precipitation) [7]. In particular, depending on the kind and concentration of the released ions, several biological processes such as osteogenesis and angiogenesis, as well as antimicrobial and anti-inflammatory effects, can be induced [18]. In addition to the dissolving products, the morphology, surface topography, and chemistry of BGs can regulate some cellular activities, which can further impact tissue regeneration and therapeutic results [19]. Despite the fact that silicate-based BGs such as 45S5 and 13-93 (composition: 53 SiO\({}_{2}\)-20 CaO-6 Na\({}_{2}\)O-12K\({}_{2}\)O-5 MgO-4P\({}_{2}\)O\({}_{5}\) in wt%) have been successful in many therapeutic applications, they also seem to have limits in terms of degradation time. First, CaP conversion is incomplete [20]. Additionally, applications for wound healing need faster solubility. In in vivo experiments, it was observed that 45S5 slowly converts to HA, while 13-93 at an even slower rate. The potential for 45S5 to crystallize after heat treatments is another drawback, making it challenging to produce nanocrystalline 45S5-based 3D scaffolds and fibers [21]. Due to the limitations of silicate-based BGs, other types of BGs have been developed. One of these materials is phosphate-based BGs, which offer a wide solubility range which may be predicted and controlled by varying the glass composition [22]. Compared to silicate-based BGs, the phosphate ones promote faster solubility speeds [23]. Their dissolution rates are ascribed to the hydrolysis of soluble P-O-P bonds and ion leaching of cationic modifiers. These processes can be regulated by modifying the structure and composition (e.g., the quantity of P\({}_{2}\)O\({}_{5}\)) [24]. Phosphate-based BGs can entirely dissolve in aqueous environments, permitting their contents to be liberated as ionic components. The majority of these constituents, PO\({}_{4}\)\({}^{3}\), Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), and Fe\({}^{3+}\), are already present in significant amounts in the human body in the form of ionic species [25]. For cutting-edge biomedical applications, phosphate-based BGs (PBG) may be used alone or in a biodegradable matrix due to their unique material features.