Measuring hydrodynamic stress in mammalian cell culture across five decades: Cellular responses and the need for mechanistic integration.

Mammalian cell culture constitutes the cornerstone of the modern biopharmaceutical industry, particularly for producing monoclonal antibodies and complex therapeutic recombinant proteins. However, scale-up and intensification of these processes remain critically shaped by hydrodynamic stress (HS), a multifactorial phenomenon arising from agitation and sparging that affects cell physiology, productivity, and critical quality attributes of the products. Despite decades of research, a unified predictive framework for cellular responses to HS remains elusive, partly due to inconsistencies in experimental scale-down approaches. This review summarizes advances reported over the past five decades and highlights a consistent disparity between the stress conditions typically applied in sensitivity studies and the actual operating ranges of industrial bioreactors. While catastrophic cell rupture is largely mitigated in modern systems, the cellular mechanisms governing sublethal and adaptive responses emerge as the more relevant and less understood determinants of process performance. These findings underscore the need for standardized stress metrics and scale-down models that accurately represent the moderate, yet persistent, hydrodynamic environments encountered in contemporary bioprocessing.