## 1 Introduction  
Cell-based therapy is an excellent therapeutic modality that involves the transplantation of cells prepared in vitro into patients, and several basic animal studies have reported remarkable therapeutic effects [1; 2; 3]. In recent years, several types of pluripotent stem cells have been established, including embryonic stem cells and induced pluripotent stem (iPS) cells, and protocols to differentiate these pluripotent stem cells into diverse tissue-specific cells have been developed [4; 5], suggesting the possibility of the mass preparation of therapeutic tissue-specific cells. Cellular drugs, such as TEMCELL HS Injection and Kymriah, have been clinically applied in patients with serious diseases and have shown substantial efficacy. It is expected that the growing number of clinical trials on cellular drugs will lead to an expansion of cell-based therapies in the future [6; 7]. However, the application of cell-based therapy is hindered by a serious challenge, that is, most transplanted cells die immediately post-transplantation, and the low or short survival of transplanted cells reduces the efficacy of cell-based therapy [8; 9]. This cell death is caused by exposure to a severe in vivo environment post-transplantation or the elimination of foreign substances by immune cells [10]. To avoid the elimination of transplanted cells, pre-transplant immunomododulation or immune system deactivation by administering immunosuppressive agents has been investigated and attempted [11; 12]. However, immunosuppression-induced side effects owing to the long-term use of immunosuppressive agents could markedly reduce the quality of life of patients [13]. Moreover, the optimization of the administration route and formulation of transplanted cells is also a significant challenge to achieve the distribution and retention of the cells at a proper site in the body [14]. Therefore, developing methods that prolong cell survival or maintain cell function without inducing potential side effects in patients is highly desirable. Cell encapsulation, in which cells are encapsulated in a biocompatible polymer, physically protects transplanted cells from immune cells in vivo, thereby avoiding rejection post-transplantation [15; 16]. Cell encapsulation renders the use of immunosuppressive agents unnecessary and can solve various challenges associated with cell transplantation. In addition, cell encapsulation with various biofunctional materials can facilitate the optimal distribution and retention of the transplanted cells as well as cell viability. Furthermore, capsule-encapsulated cells can survive and continuously release cytokines, which can be used in disease treatment [17]. Cell encapsulation research was initiated in 1964 by [PERSON], who proposed the use of ultra-thin polymer membrane microcapsules for the immunoprecipitation of transplanted cells [18]. [PERSON] et al. first successfully immobilized pancreatic islets in alginate capsules in 1980 and reported excellent therapeutic effects in a rat model of type 2 diabetes for several weeks post-transplantation [19]. This finding further propagated intensive research on cell encapsulation, and encapsulated cell-based therapies have been explored to address various diseases, including diabetes, hemophilia, cancer, and renal failure [20; 21; 22]. Based on these superior therapeutic results, the application of encapsulated cells to treat human diseases has been promoted [23; 24]. Although cell encapsulation studies have been extensively explored to develop cell-based therapy, advances remain limited in terms of the optimization of preparation protocols [25]. The preparation conditions for cell encapsulation can markedly impact the function of the encapsulated cells [26]; however, no protocol for cell encapsulation has been established, and cell-encapsulating capsules have been prepared under non-standardized conditions. In addition, some previous studies used ambiguous experimental methods, rendering reproducibility impossible.