Co-author: Verena Beck
While for scaling up a production process the main goal is to keep the quality and quantity of a product stable, scaling-down is often used for troubleshooting and testing unit operations. At the microscale various process parameters such as temperatures, buffer additives or mixing conditions can be tested much faster and with lower material consumption compared to large scale. Researchers of acib investigated the most crucial parameters affecting the mixing behaviour at the microscale and how mixing of fluids in small scale can be compared to large vessels.
Why scale-down to the microscale?
The need for rapid and reliable process development platforms is increasing. High throughput screening methods, using microtiter plates, meet the demands: various conditions can be tested in only one microtiter plate whereas more than one stirred tank reactor would be needed to perform a comparable experimental set-up. Microtiter plates are available in a broad range of different formats (4 to 9600 wells) and geometries (rectangular or circular, deep or shallow plates). However, mixing properties are completely different in microtiter plates, where mixing is achieved by shaking, compared to stirred tanks, even small ones. This difference has to be considered for scale up of a mixing process. Currently, the transfer of processes from shaken microtiter plates to stirred industrial dimensions is mostly performed empirically without verification of the engineering parameters.
A crucial parameter that has to be considered for scale up of mixing processes is the effective power input, which is directly proportional to the temperature increase in the system.
How could scientists compare mixing by shaking and stirring?
Two different methods were applied to investigate fluid dynamics when using shaken microtiter plates. On the one hand acib researchers designed a clay/polymer method to measure hydrodynamic stress in solutions, which is directly correlated to the power input. Flocks with a specific diameter were generated and transferred to microtiter plates with different formats and geometries. By shaking they disassemble and the average end flock diameter was measured with a special imaging software after shaking at a certain amplitude and time. Using an equation which correlates the total volume of a microtiter plate well to the average end flock diameter, the maximum power input could be calculated.
On the other hand, by micro calorimetry the specific power consumption and therefore the input of energy can be determined directly. Under adiabatic conditions (temperature-isolated) the samples were thermodynamically isolated from the environment. For that purpose, researchers came up with the idea to place the plates into a vacuum manifold device which was covered with Styrofoam to avoid influences of the environmental temperature. The final design was fixed on an orbital shaker and all temperature sensors were hermetically sealed as well. Now they were able to measure the temperature increase between the starting and equilibrium temperature which leads to the specific power input.
All clear now? If you are eager to know more about micro calorimetry watch the video from CrashCourse Chemistry.
To sum it up hydrodynamic stress differs significantly between the different formats of MTP. Thus, the transfer of mixing conditions from the microtiter plate to small-scale and pilot-scale reactors must be undertaken with care. Comparing mixing at the microscale to laboratory- and pilot-scale reactors shows that the hydrodynamic conditions of stirred tank reactors cannot be reached in shaken 96-well microtiter plates.
This work is based on the following paper:
Astrid Dürauer, Stefanie Hobiger, Cornelia Walther and Alois Jungbauer: Mixing at the microscale: Power input in shaken microtiter plates; Biotechnology Journal, Volume 11, Issue 12, December 2016, Pages 1539–1549, DOI: 10.1002/biot.201600027
Picture credits: Pixabay