Shear Sensitivity in Animal Cell Culture — Why are many animal cell cultures not severely impacted by bulk turbulent shear in well-designed bioreactors?

Introduction:
Animal cells (mammalian, insect) are shear-sensitive due to the absence of rigid walls, yet many cultivations succeed in stirred tanks. The resolution lies in turbulence micro-scales: damaging shear is transmitted when eddy sizes approach or are smaller than the cell, producing high local velocity gradients. If the smallest eddies are still larger than cells, the cells translate with the fluid with less deformation.

Given Data / Assumptions:

• Well-designed bioreactor with appropriate impeller speed and aeration.
• Kolmogorov eddy size larger than typical cell diameter for chosen operating conditions.
• No excessive bubble-cell collisions or air-liquid interface exposure (which can also damage cells).

Concept / Approach:

The Kolmogorov micro-scale sets the lower limit of eddy size. When eta > cell diameter, local shear gradients across a cell are modest, lowering deformation and lysis risk. Microcarriers, shear protectants, and controlled aeration further mitigate stresses arising from bubble breakup and impeller tip vortices.

Step-by-Step Solution:

Verification / Alternative check:

Empirical observations show improved viability when operating at moderate power input per volume and using spargers/impellers that limit small-scale turbulence and bubble shear; viability drops when eta approaches or falls below cell size.

Why Other Options Are Wrong:

B: If cells are larger than eta, shear impact generally increases, not decreases. C: Animal cells lack rigid walls. D: Membranes confer fragility, not protection. E: Bubbles can increase shear and interfacial damage; they do not eliminate shear.

Common Pitfalls:

Ignoring bubble-related stresses and assuming turbulence alone dictates damage; both eddy scale and gas-liquid interactions matter. Over-speeding to boost kLa can inadvertently reduce eta below safe limits.

Final Answer:

Because typical cell diameters are smaller than the critical turbulent eddy size that transfers damaging shear at the cell scale.