# Advantages and Limitations of Current Microgravity Platforms for Space Biology Research
## 2 Ground Microgravity Simulators
### Rpm  
Three-dimensional clinostats are constituted by two gimbal-mounted frames that are turned independently by two dedicated motors (Figure 1c). They were first introduced in Japan by [PERSON] for plant research and later improved by Dutch Space (former Fokker Space) in the Netherlands [27]. The term 3D clinostat is appropriate only if the frames operate with constant speed and direction; if speed and direction are different and randomized, it takes the name of a \"random positioning machine\" (RPM) [28]. Dedicated algorithms drive frame rotation, such that the biological samples, fixed on the inner frame, are constantly reoriented and the gravity vector is averaged over time. Thus, even if 1 g is constantly acting on the sample, the averaged gravity vector is close to zero [29]. If the rotation is sufficiently faster than the response time of the biological system, the continuous reorientation will cause the sample to experience a state similar to microgravity [30].  
Rotation induces accelerative forces. Therefore, sample positioning within the RPM is a critical aspect to consider when using this simulator. Where the two rotational axes cross, the forces acting on the sample are negligible; further from the center, the forces acting on the outer edges of the sample become more relevant [27]. Therefore, the choice of an RPM setup imposes limits to sample volume. In addition, compared with static and real microgravity experiments, sample rotation introduces additional fluid motion in the culture flask, producing shear forces and enhancing convection [31]. To reduce shear stress, moderate rotations should be combined with smooth transitions [32]. Enhanced convection leads to improved nutrients and gas provision and waste product removal, which allow to work with higher cell densities. Higher cell density and gravitational unloading increase the chance of cell-cell interaction and formation of multicellular aggregates [33]. Thus, RPM can also be used to induce three-dimensional growth and to obtain spheroid-like structure [30; 34]. Another practical aspect to consider when using the RPM is to avoid the formation of air bubbles in the liquid medium. Air bubbles produce unwanted fluid motion and shear stress [27; 35] and can cause the detachment of adherent cells [30].  
Even if this platform has been used extensively to simulate microgravity for Space biology studies (preparatory tests for flight-experiments, pre- and post-flight experiments, etc.), the RPM has been shown to mimic microgravity effects only partially. Indeed, discrepancies in gene regulation were found between cells exposed to the RPM and those exposed to real microgravity in Space [30]. Therefore, data from RPM experiments must be interpreted carefully and, when possible, compared to experiments in real microgravity.  
In summary, in order to obtain reliable data with the RPM, the experimental conditions must be carefully set. However, depending on the combination of rotation speed and the distance from the center, high quality microgravity conditions can be obtained, in the order of \(10^{-4}\) g (Table 1) [34]. Moreover, thanks to recently introduced algorithms, RPM machines can average Earth gravity partially to simulate Moon- (0.16 g) or Mars-like (0.33 g) gravity conditions (Table 1) [30].