Since the early days of bioprocess engineering shear associated protein aggregation was believed to be a real thread for proteins causing a decrease in production yield accompanied by higher costs. Although some research indicated that moderate shear rates do not aggregate proteins, the scientific consensus today is still not aligned. Recent results suggested that elongational forces, very similar to shear, can unfold proteins. Hence, there is a strong demand for a technical solution to describe extraordinary high shear rates and investigate their impact on protein aggregation, to answer this unsolved mystery in bioprocess engineering.
The shear rate is the rate at which fluid layers or laminae move past each other. One can think of two layers moving with different velocities.
The differences in velocity (Δv) and the distance between those layers (h) define the amount of shear (γ) occurring in a particular region. Hence a large difference in velocity emerging over a short distance leads to high shear rates.
For cells in a stirred tank reactor and for high pressure homogenization high shear rates are responsible for cell disruption. While for homogenization this effect is desired, the stirrer revolution speed for the cultivation of sensitive cells must possible be reduced. So the question is: are proteins also sensitive to shear?
How to generate high shear rates?
To answer this question, a shear device had to be designed being able to produce isolated and extraordinary high shear rates. Such a prototype was built and tested within a project of the Austrian Centre of Industrial Biotechnology (acib) and the University of Natural Resources and Life Sciences Vienna (BOKU). A micro orifice with a diameter of only 100 µm was placed in the flow path of a piston pump. Proteins were pumped through this device with almost 100 meters per second. One could argue this is fast or?
What is the effect on proteins?
Although we could describe the highest shear rates to be tested on proteins with a validated CFD simulation, not a single of nine structurally different proteins was prone to aggregation. So does this mean all the other researchers were wrong? When testing the impact of something onto protein aggregation, it is very important to exclude all other possible effect. Most probably air/liquid interfaces, cavitation or simply proteases caused false-positive results in early shear studies.
So shear is no thread when processing proteins?
While shear still remains a hazard for cells and larger biological structures we could demonstrate that proteins are simply too small to aggregate in shear gradients applied in common bioprocesses! If you are now curious and maybe want to test other biological structures with this simple methodology: just follow the links below!
This article is based on:
Duerkop, M., Berger, E., Dürauer, A., & Jungbauer, A. (2018). Influence of cavitation and high shear stress on HSA aggregation behavior. Engineering in Life Sciences, 18(3), 169–178. https://doi.org/10.1002/elsc.201700079
Picture credits: Shutterstock; video credits: Mark Dürkop