## 1 Introduction  
Adipose tissue has become one of the most promising cell sources for regenerative medicine since it is easily accessible. It is an abundant source of adult stem cells that can differentiate along multiple pathways [1, 2, 3]. This makes definitively human Adipose Stem Cells (hASCs) a major candidate for tissue engineering applications. They are also increasingly used in plastic and reconstructive surgical procedures, where a shift toward tissue-engineering strategies using stem cells [4, 5, 6] is apparent. However, due to the limited number of stem cells that can be extracted from a single donor, one of the major challengesfor the field of bioengineering is the development of production processes that are both compliant with Good Manufacturing Practice (GMP) guidelines and able to generate a high amount of safe stem cells in a reproducible and cost-effective way [7, 8]. This latter aspect, namely the manufacturing costs of cell therapeutics, presents a significant barrier to the widespread implementation of stem cell-based therapies. More specifically, when examining the overall fabrication process, cell expansion represents one of the main cost drivers for cell therapeutics production [8]. One major obstacle is the limited availability of clinical-grade reagents for use in ancillary materials (AMs) [9]. Consequently, it would be essential to develop some of the most crucial AMs needed to manufacture cell therapy products. Therefore, using a defined xeno- and serum-free medium and the development of a biodegradable microcarrier (MC) to produce high numbers of hASCs will represent a step towards the resolution of this issue. Fetal bovine serum (FBS) is still the most used cell culture medium supplement, although it is an undefined cocktail of numerous growth factors and hormones. This ancillary material is unsafe and risky because it may contain harmful contaminants such as prion, viral, or zoonotic agents [10]. Moreover, human primary progenitor cells grown in FBS-containing media become associated with FBS proteins [11], which may cause immune xenogenic rejection upon autologous transplantation in patients, particularly when multiple doses are required. Furthermore, tissue (stem) cells usually are never in contact with serum but in contact with the interstitial fluid, which has a lower concentration of proteins than serum and is specific for each tissue or organ due to the unique composition of secreted polypeptides and metabolic molecules [12, 13]. This is a situation that we can try to reproduce and mimic by using defined serum-free conditions. Finally, there are also serious ethical concerns about FBS' use regarding its harvesting and production process, without forgetting the risk of adulterated composition and unstable prices. Biodegradable and biocompatible microspheres are a continuing focus of research for their numerous applications in biomedicine and biotechnology, including immobilization [14, 15], affinity chromatography [16, 17], drug delivery [18, 19], and MCs for the cultivation of anchorage-dependent cells [20, 21, 22]. MCs have established themselves as a promising technology for meeting current and future cell therapeutic markets' demands combined with stirred single-use bioreactors. MC technology provides a large culture surface in a small volume for the cost-effective production of adherent cells. It enables reproducible Good Manufacturing Practice (GMP) compliant cell expansions in closed, controlled, and scalable processes [7, 23]. A common approach in tissue engineering is to harvest the cells from their growth surface and inject them intravenously. However, for specific applications (such as bone and cartilage repair), there is a need to seed the cells on a scaffold that serves as a substrate and temporary matrix, which can be inserted into the defect to stimulate tissue regeneration. This approach is more time-consuming because it involves three steps: (I) cell expansion in a dedicated system, (II) cell harvesting, and (III) seeding onto a scaffold before implantation.