# Adipose Stem Cell Response to Borophosphate Bioactive Glass
## 1 Introduction  
Bioactive glasses (BGs) are a class of oxide-based ceramics invented by [PERSON] in the late 1960s [1]. BGs have gained interest due to their bioactivity, biocompatibility, ability to bond and soft tissues, and potential to stimulate tissue regeneration [2; 3; 4]. They are versatile and can be manufactured in different shapes and sizes, such as powders, fibers, and scaffolds [3; 5; 6]. Consequently, several BGs families have emerged with different compositions, including borate BGs (BBGs). When compared to the traditional silicate BG, BBGs generally have faster degradation rates [7; 8; 9], enhanced cell proliferation and differentiation in vitro [10], and tissue infiltration in vivo [11]. BBGs have been shown to help heal chronic wounds and stimulate angiogenesis [12; 13; 14; 15]. However, BBGs quickly release alkaline ions, creating a basic pH environment [5; 16]. This increase in pH is toxic to cells under static conditions in vitro [12; 17]. Blood and most of the interstitial fluids in the human body have an average pH level of 7.35-7.45, which is considered the physiological pH-neutral range [18]. A recent study demonstrated that preconditioning BGs with cell culture medium will prevent the cytotoxic effects associated with the pH shock induced by the dissolution of an alkaline BG [19]. However, it is still unclear how different pretreating periods and glass concentrations affect cell viability and functions. Furthermore, prolonged preconditioning times are not ideal in clinical or laboratory settings. Another way to control the local pH of BG is to add phosphate to the glass composition [5; 6; 20]. Phosphate-based bioactive glasses (PBGs) have generated interest because degradation rates can be controlled, ion release can be customized, and the glass can influence the local pH [21; 22; 23; 24]. A previous study showed that adding phosphate to a calcium-silicate can counteract the release of alkaline ions and reduce the local pH without pretreating the glass before use [25]. A similar effect has been noted for the addition of phosphate to an alkaline BBG [26]. The dissolution of this borophosphate bioactive glass (BPBG) had little effect on the pH of simulated body fluid. Previously, we evaluated the effects of these BPBGs on endothelial cells in vitro and in vivo. We found that pH-neutral glasses supported endothelial cell migration and stimulated greater blood vessel formation than either the basic BBG or acidic PBG [27]. The current study evaluates the effects of three of those glasses on adipose stem cells (ASCs). ASCs are isolated from the stromal vascular fraction of subcutaneous fat. This makes them more accessible than their bone marrow counterpart, bone-marrow-derived mesenchymal stem cells (BMSCs). This less invasive harvest allows cells to be collected from any patient, even patients with advanced age or chronic health disorders, and typically provides a more significant number of cells than the harvest of BMSCs [28; 29; 30]. Additionally, ASCs have a higher proliferative capacity than BMSCs, reducing the time and need for in vitro expansion. Overall, ASCs have a comparable therapeutic effect to the more studied BMSCs and have been shown to have immunomodulatory properties, angiogenic abilities, and differentiation capacity [30; 31; 32]. It is believed that the therapeutic effect of ASCs primarily results from their secretome, which are proteins secreted into the extracellular space [33; 34; 35; 36]. The secretome has an essential role for ASCs as a therapeutic cell, including maintaining homeostasis, tissue and organ development, cell signaling, and organizing extracellular matrix [37]. Compared to BMSCs, the secretome of ASCs is more angiogenic and neuro-regenerative [37]. There is increased interest in isolating the secretome from therapeutic cells because of the advantages of that over cell therapy, including scalability, potential reproducibility, availability, and longer shelf lives. The secretome is often overlooked when evaluating biomaterials.