China
Africa
Explore chapters and articles related to this topic
Oxygen Transfer in Cell Culture Bioreactors
Published in Wei-Shou Hu, Cell Culture Bioprocess Engineering, 2020
In a hollow fiber bioreactor, the oxygen concentration gradient exists in both axial and radial directions. It was realized very early on, when various reactors were being explored for cell culture bioprocessing, that oxygen transfer limitations would restrict the widespread application of hollow fiber bioreactors. Hence, oxygen transfer in hollow fiber bioreactors was extensively studied.7 As the medium flows downstream in the lumen, oxygen is diffused through the membrane into the cell compartment. Oxygen concentration therefore decreases along the axial direction of the medium flow (Figure 11.13). In the radial direction, oxygen first diffuses from the bulk liquid in the fiber lumen to the wall of the fiber, and then through the fiber wall into the cell chamber. In the cell chamber, oxygen travels through cell clusters where it is consumed by cells before reaching the cells distal to the fiber. At a high cell concentration, the drop in oxygen concentration is steep in both the axial and radial directions (Panel 11.19). Figure 11.14 is a qualitative description of oxygen concentration in a single fiber, in the direction of medium flow. Under some conditions, an oxygen concentration difference can be seen between the center point of the fiber lumen and the inner wall. The fiber wall often presents a large resistance, causing oxygen concentration to decrease across significantly. In the cell chamber, oxygen decreases rapidly due to a high consumption rate. While the dissolved oxygen concentration in a stirred tank bioreactor is often maintained at 30% of the saturation, that in the exit stream of a hollow fiber bioreactor is typically kept at a much higher level to avoid cells in the downstream suffering from oxygen starvation. To improve oxygen transfer, one may increase the flow rate of medium recirculation or reverse the flow direction periodically to alternate the upstream and downstream ends. Even with those practices, the longitudinal length of a hollow fiber bioreactor is limited due to oxygen transfer limitations. A hollow fiber bioreactor with two sets of hollow fibers, each supplying medium and oxygen respectively, can alleviate the problem of oxygen transfer limitation. The demand for such bioreactors comes during scale-up.
Bioreactor Instrumentation and Control for 3D Cellular and Tissue Systems
Published in Karen J.L. Burg, Didier Dréau, Timothy Burg, Engineering 3D Tissue Test Systems, 2017
A number of creative bioreactor embodiments exist that are either variations on the themes noted above or are novel designs matched to a specific tissue need. A few selected examples follow: In a rocker platform, or wave bioreactor vessel, a flexible bag holding the culture medium is rocked back and forth. The tissue medium is aerated due to agitation from surface wave action, and nutrients are provided to the tissue construct through convective flow (Amrollahi and Tayebi 2016; Blose et al. 2014; Lyons and Pandit 2005; Yuk et al. 2011).A hollow fiber bioreactor vessel consists of a closed, medium-filled vessel filled with semipermeable hollow fibers. These fibers emulate blood vessels by providing nutrients and removing waste. Primarily intended for mammalian cell growth, this design offers the advantage of better nutrient delivery to the center of the tissue, but oxygen delivery can be a challenge (Martin and Vermette 2005; Tharakan and Chau 1986). Monitoring cell growth and determining cell removal efficiency at harvest can also be a difficult.In an airlift reactor vessel, oxygen is supplied to the tissue through the delivery of air at the bottom of the vessel. A draft tube is needed to prevent sparging due to direct air contact with the developing tissue. Nutrient availability can be a challenge due to the lack of mixing of the glass spheres or polystyrene beads upon which cells are seeded, since they tend to settle on the bottom of the vessel (Al-Mashhadani et al. 2015; Martin and Vermette 2005).A double-chamber bioreactor vessel is employed for the growth of larger, more complex tissue constructs such as osteochondral grafts and tracheal allografts. Two chambers, for example, a chondral compartment and a bone compartment, each with their own mixing apparatus, are separated by a membrane, and the tissue scaffold spans the membrane so that it resides in both chambers (Chang et al. 2004; Haykal et al. 2014; Wendt et al. 2005).Microfluidic bioreactors based on the integration of electronics at, for example, the CMOS level are becoming available for portable and implantable applications (Christen and Andreou 2007; DeBusschere and Kovacs 2001; Kim et al. 2011).
Advances of engineered extracellular vesicles-based therapeutics strategy
Published in Science and Technology of Advanced Materials, 2022
Hiroaki Komuro, Shakhlo Aminova, Katherine Lauro, Masako Harada
With their promising clinical applications, there is a demand for creating a standardized method for large-scale EV production. In the laboratory, EV isolation is typically done by UC or filtration techniques. However, even though it varies slightly based on the donor cell type used, the yield of EVs produced from these methods is often too low [108]. Therefore, next-generation approaches are necessary for developing EV mass production techniques to manufacture EVs at an appropriate scale for applicational use. Various cell types have been investigated as possible donor cells. The majority of these EVs are produced from different types of stem cells, epithelial cells, and cancer cells. However, the use of these cell types for large scale EV production comes with a lot of challenges. One of these challenges is that in culture, these cells often grow as adherent monolayers that stop dividing once, they become confluent. This severely limits the number of cells that can be maintained since they require so much space and upkeep. Another challenge is that after a certain number of passages, cell lines build up genotypic and phenotypic changes that lead to senescence, where they eventually stop dividing. This means that new cells need to be constantly obtained from the donor or a multitude of donors, both of which introduce variability [92,106,108]. For example, bone marrow stem cells from older donors proliferate and differentiate slower than those from younger donors [109]. The third challenge is that stem cell lines naturally change and differentiate, which does not warrant a homogenous population of producer cells, thus not producing uniform EVs. This is one of the biggest challenges with MSCs, the biggest source of EVs used for mass production today [92]. Different processing conditions have been investigated to increase the efficiency of EV production. Most of these conditions involve the substitution of culturing cells in T-flasks for bioreactors, which are multilayered systems that have mechanisms for performing functions to keep the cells alive [92]. A hollow-fiber bioreactor system resulted in 40-fold the amount of EVs produced per volume of conditioned media in comparison to traditional cell culture techniques [110]. Bioreactors allow for more cell layers to be grown under identical conditions while taking up a smaller amount of space, minimizing manipulation and culture time, and reducing the number of consumables used in traditional culturing methods [111]. However, the bioreactor can also introduce batch variability due to difficulty in monitoring cell conditions. It has also been established that different culturing conditions and stress put on producer cells can influence EV production [92]. Stressors like hypoxia, irradiation, serum starvation, and physical and chemical stress have been known to increase EV production, however, these EVs have the potential to be physiologically different than naturally produced EVs, which is not ideal for the uniformity needed for successful mass production [111]. Due to all these challenges, more work is still needed to standardize EV mass production.