Date Published: January 30, 2017
Publisher: Public Library of Science
Author(s): Simon L. Wuest, Philip Stern, Ernesto Casartelli, Marcel Egli, Roi Gurka.
Random Positioning Machines (RPMs) are widely used as tools to simulate microgravity on ground. They consist of two gimbal mounted frames, which constantly rotate biological samples around two perpendicular axes and thus distribute the Earth’s gravity vector in all directions over time. In recent years, the RPM is increasingly becoming appreciated as a laboratory instrument also in non-space-related research. For instance, it can be applied for the formation of scaffold-free spheroid cell clusters. The kinematic rotation of the RPM, however, does not only distribute the gravity vector in such a way that it averages to zero, but it also introduces local forces to the cell culture. These forces can be described by rigid body analysis. Although RPMs are commonly used in laboratories, the fluid motion in the cell culture flasks on the RPM and the possible effects of such on cells have not been examined until today; thus, such aspects have been widely neglected. In this study, we used a numerical approach to describe the fluid dynamic characteristic occurring inside a cell culture flask turning on an operating RPM. The simulations showed that the fluid motion within the cell culture flask never reached a steady state or neared a steady state condition. The fluid velocity depends on the rotational velocity of the RPM and is in the order of a few centimeters per second. The highest shear stresses are found along the flask walls; depending of the rotational velocity, they can reach up to a few 100 mPa. The shear stresses in the “bulk volume,” however, are always smaller, and their magnitude is in the order of 10 mPa. In conclusion, RPMs are highly appreciated as reliable tools in microgravity research. They have even started to become useful instruments in new research fields of mechanobiology. Depending on the experiment, the fluid dynamic on the RPM cannot be neglected and needs to be taken into consideration. The results presented in this study elucidate the fluid motion and provide insight into the convection and shear stresses that occur inside a cell culture flask during RPM experiments.
The behavior and development of biological cells are largely governed by their complex interaction with external stimuli. Besides a variety of signals, including electrical and chemical signals, cells also respond to mechanical stimulations. Cells react to a variety of mechanical stresses, such as tensile, compressive and shear stresses [1–5]. The influence of mechanical stimulation on cells is of such great importance and complexity that a new research field called mechanomics evolved [6, 7]. In order for cells to respond to mechanical stimuli, they need to adhere through focal adhesion (FA) junctions [8–10], for example. Most cells communicate with the extracellular matrix by bridging FA with intracellular connectors such as fibronectin and other related bridge molecules. Research on cell adherence has revealed many findings on how cells can sense their environment and transduce mechanical forces into intracellular signals [2, 11].
Even though RPMs have been in use for several years, the fluid dynamic appearing in the culture flask has never been analyzed deeply; thus, it has been widely neglected. Besides distributing the Earth’s gravity vector, the motion of the RPM induces enhanced convection and increased shear stresses. In this study we elucidated the fluid velocity, shear stresses and convection for a specific and simplified case: (1) The flask has the approximate geometry of a standard T25 cell culture flask. (2) It is placed at the center of rotation. (3) The two frames of the RPM rotate with equal and constant velocity. Multiple RPMs have been developed with different controlling algorithms implemented. Some RPMs rotate the frames with random velocities, while other RPMs rotate with constant velocities, inverting periodically the rotational direction (reviewed in ). Therefore, some deviation from the results presented here should be expected on the various RPM systems. In general, larger shear stresses can be expected with increasing rotational velocities. The largest shear stresses in the “bulk volume” are in the order of a magnitude of 10 mPa. The highest shear stresses always appear along the flask’s wall. In the simulations, they were around 50 mPa, 100 mPa and 200 to 300 mPa for rotational velocities of 40 deg/s, 60 deg/s and 90 deg/s, respectively. Thus, the shear stress appearing on the RPM was in the range that provoked a cellular effect in previously published experiments (compare to Table 2). However, only a small portion of the cell population is exposed to the highest shear stresses along the flask walls. This means that for adherent cells less than 5% of the cell population is exposed to shear stresses greater than 50 mPa for rotational velocities up to 60 deg/s. Even rotational velocities up to 90 deg/s do not expose more than 5% of the cell population to shear stresses greater than 100 mPa.