# Advantages and Limitations of Current Microgravity Platforms for Space Biology Research
## 2 Ground Microgravity Simulators  
### Clinostat 1D/2D/Rwv  
The simplest and most affordable tool used to manipulate gravity on Earth is a clinostat. Clinostats were introduced in the late 19 th century by the pioneers of plant gravitropism [19] and, through the years, have evolved to be applied to new technologies and new needs. Clinostats simulate the absence of a directional gravity input by averaging the gravity vector through rotation over time [20] (Table 1). Depending on the rotation speed and the number of rotation axes, basic clinostats fall into different classes and find application in different fields. Clinostats rotate around one axis (1D or 2D clinostat) or two axes (3D clinostats). In 1D clinostats, the vertical axis of the organism is a continuation of the rotation axis of the clinostat (Figure 1a), while in 2D clinostats, the specimen is rotated on a plane perpendicular to the rotation axis (Figure 1b). Slow revolutions (0.3-3 rpm) are common for whole plants to minimise the chances of the centrifugal acceleration mimicking gravity [20]. For the same reason, fast revolving clinostats can only be used for cell suspensions in very narrow vessels (few mm). In these clinostats, the fast rotation (30-150 rpm) eliminates the effect of gravity by stabilizing the fluid around the cell. Rotating wall vessels (RWVs), instead, are larger than the fast-rotating clinostats (5-20 cm diameter) and rotate at 10-20 rpm around the horizontal axis. In RWV, the rotation frequency is matched to the sedimentation velocity, so that the cells remain centred in the vessel and rotate around their central axis [21]. The simplicity of a clinostat setup allows for great adaptability to different experimental applications and clinostats are also used to mimic hypogroavity. This can be achieved simply by non-uniform rotation, by tilting the rotation axis so that a fraction of the gravity vector is not averaged, or by pairing clinostats with centrifuges [22; 23; 24; 25; 26]. This has been used, for example, to demonstrate that the plant gravitropic response is guided by the inclination \begin{table}
\begin{tabular}{l c c c} \hline  & **1D/2D Clinostat/RWV** & **3D Clinostat/RPM** & **Diamagnetic Levitation** \\ \hline
**Microgravity Duration** & Hours to weeks & Hours to weeks & Minutes to hours \\
**Microgravity Quality** & \(\leq\)10\({}^{-3}\) g & 10\({}^{-4}\) g & \(<\)10\({}^{-2}\) g \\
**Hypogroavity** & Y & Y & Y \\
**Biological System** & Cells, microbes, plants & Cells, microbes, plants, animals & Cells, microbes, plants, animals \\
**Cost** & Low & Low & Medium \\
**Accessibility** & Easy & Easy & Easy \\ \hline \end{tabular}
\end{table} 
Table 1: Specifications of ground microgravity simulators. RWV, rotating wall vessel; RPM, random positioning machine. Figure 1: Ground microgravity simulators. Schematic representation of (**a**) 1D clinostat; (**b**) 2D clinostat; (**c**) 3D clinostat/RPM (random positioning machine); and (**d**) diamagnetic levitation system. Clinostats and RPMs simulate the absence of a directional gravity input by averaging the gravity vector through rotation around one or two axes over time. Diamagnetic levitation compensates gravity at molecular level, by balancing the weight of water and biological tissues with the repulsive force exerted by a magnetic field. Within the magnetic field, a specimen can experience from zero (ug) to twice the gravity force (xg) depending on its position within the magnetic field gradient. and not the force of the gravity stimulus [25]. Moreover, cameras and microscopes have been implemented on clinostats to allow for live imaging during clinorotation [25]. Clinostats cancel the directionality of the gravity vector, but are unable to reproduce actual microgravity. Indeed, gravity may be averaged, but is never eliminated and the shift in weight distribution can cause mechanical and bending stress. For these reasons, the results obtained on a clinostat must be viewed in the right prospective.