Arulrajah, PrasikaPrasikaArulrajahLievonen, Anni ElinaAnni ElinaLievonenSubaşı, DilaraDilaraSubaşıPagal, SubhashreeSubhashreePagalWeuster-Botz, DirkDirkWeuster-BotzHeins, Anna-LenaAnna-LenaHeins2025-06-272025-06-272025-06-05Bioprocess and Biosystems Engineering (in Press): (2025)https://hdl.handle.net/11420/55968In large-scale bioprocesses, gradients in pH, dissolved oxygen level (DO), and substrate concentrations can decrease bioprocess efficiency. Scale-down bioreactors, be it single stirred-tank bioreactors with a special feeding regime, multi-compartment bioreactors, or combinations of bioreactors, offer a promising lab-scale solution for comprehending these gradients, as they allow adjustment of gradients without incurring high costs. However, critical challenges arise when transitioning from large-scale to scale-down bioreactors. Chief among these is realistically approaching the gradient conditions of large-scale bioreactors and choosing appropriate scale-down bioreactor configurations. This review paper begins by addressing the gradients encountered in large-scale bioreactors. Afterward, various types of scale-down bioreactors are characterized and compared, highlighting their advantages and disadvantages. The suitability of scale-down bioreactors is analyzed by examples of bioprocesses with different microorganisms and mammalian cells to underscore the complexities inherent in scale-down bioprocesses and emphasize the influence of cellular responses to these conditions. Finally, the potential of miniaturized and microfluidic bioreactors is briefly discussed for future application in scale-down studies.en1615-76052025Springerhttps://creativecommons.org/licenses/by/4.0/Bioprocess | Bioreactor | C. glutamicum | CHO | E. coli | Fermentation | Gradients | S. cerevisiae | Scale-downTechnology::660: Chemistry; Chemical Engineering::660.6: BiotechnologyNatural Sciences and Mathematics::579: Microorganisms, Fungi and AlgaeTechnology::621: Applied Physics::621.3: Electrical Engineering, Electronic EngineeringReview Articlehttps://doi.org/10.15480/882.1530410.1007/s00449-025-03182-w10.15480/882.15304Review Article