# Bioreactors: Applications and Innovations for a Sustainable and Healthy Future--A Critical Review
## 6 Recent Advances in Bioreactor Applications
### Cell Culture and Tissue Engineering
Cell therapy has inspired studies on the medical application of bioreactor lymphocytes and macrophages derived from human induced pluripotent stem cells (iPSCs), which have the potential to enable the development of cell-based therapies for numerous disease conditions. However, despite the great potential, the limitation that needs to be overcome is the necessity of ex vivo expansion because of the insufficient number of hMSCs presented within adult organs and the high doses required for transplantation. Thus, it is possible to find different studies in the literature that explore bioreactors for the clinical uses of mesenchymal cells [112]. [PERSON] et al. [113] outline a detailed protocol for mass producing macrophages derived from human induced pluripotent stem cells (iPSC-Mac) in scalable suspension cultures using an orbital shaker or stirred-tank bioreactors (STBRs). The approach is simple and robust, involving the differentiation of primed iPSC aggregates into \"myeloid-cell-forming-complex\" intermediates with the use of a minimal cytokine cocktail. [PERSON] et al. [114], attempted to scale up the production of human macrophages from human induced pluripotent stem cells (hiPSCs) with chimeric antigen receptors (CARs), referred to as CAR-iMacs. This approach is promising due to the challenges posed by the low yields of cancer patients' monocytes, inefficient ex vivo expansion, and the limited efficiency of genetic engineering in primary monocytes. While CAR-macrophage production has been previously demonstrated in 2D systems, the upscaled 3 Dproduction of genetically modified macrophages is a completely new development. The team employed an automated CERO 3D bioreactor(r) to differentiate CAR-iPSCs into CAR-iMacs using a continuous suspension differentiation protocol. The results demonstrated that iMac differentiation protocols can be successfully adapted to upscaling platforms while preserving the functionality and phenotype of the modified cells. However, the scaling up of bioreactors for mammalian cells presents significant challenges that still need to be overcome [115]. The complexity of tissues remains one of the greatest challenges due to each tissue type's unique cellular composition, structure, and function, making artificial replication highly intricate. While the challenges associated with eukaryotic bioreactors are significant, they also present unique opportunities for innovation and advancement in tissue engineering. Recent advances in tissue engineering demonstrating notable progress and innovation include organoids [91], bioprinting [116,117], and the use of the natural extracellular matrix (ECM) structure to bioengineer an organ [118]. [PERSON] et al. [119] were able to scale up cell culture/produce 1 billion cells in 5-day cultures at a 250 mL scale and 4 billion cells in 4-day cultures at a 1 L scale, showing the feasibility of printing tissues of approximately 1 cm \(\times\) 1 cm \(\times\) 0.1 cm; however, to be able to print a solid organ, they point out that, according to other studies, billions or even trillions of cells will be needed. Another alternative being studied is perfusion bioreactors. A special type of bioreactor is used to continuously grow cells in a controlled environment. In cell culture, a perfusion system generally refers to a bioreactor coupled to a CRD, which separates and retains the majority of the cells in the bioreactor while the product-containing harvest is collected [120]. It differs from traditional bioreactors in that it constantly adds fresh nutrients to the culture while removing dead cells and products. Most perfusion bioreactors apply non-uniform flow and shear stress to the biological material due to the geometry of the sample chamber and/or flow channel. Acute expansions or non-rotationally symmetrical geometries tend to generate irregular flow speed areas in the periphery of the sample chamber; disposable rocking bioreactors (wave) are widely used in developing and scaling up in vitro bioprocesses once they overcome these problems.