The environment created on Earth within a clinostat or Rotating Wall Vessel (RWV) bioreactor is often referred to as "simulated microgravity". Both devices utilize constant reorientation to effectively nullify cumulative sedimentation of particles. Neither, however, can fully reproduce the concurrent lack of structural deformation, displacement of intercellular components and/or reduced mass transfer in the extracellular fluid that occur in actual weightlessness. Parameters including density, viscosity, and even container geometry must each be considered to determine the overall gravity-dependent effects produced by either a clinostat or the RWV bioreactor; in addition, the intended application of these two devices differs considerably. A state of particle "motionlessness" relative to the surrounding bulk fluid, which is nearly analogous to the extracellular environment encountered under weightless conditions, can theoretically be achieved through clinorotation. The RWV bioreactor, on the other hand, while similarly maintaining cells in suspension as they continually "fall" through the medium under 1 g conditions, can also purposefully induce a perfusion of nutrients to and waste from the culture. A clinostat, therefore, is typically used in an attempt to reproduce the quiescent, unstirred fluid conditions achievable on orbit; while the RWV bioreactor ideally creates a low shear, but necessarily mixed, fluid environment that is optimized for suspension culture and tissue growth. Other techniques for exploring altered inertial environments, such as freefall, neutral buoyancy and electromagnetic levitation, can also provide unique insight into how gravity affects biological systems. Ultimately, all underlying biophysical principles thought to give rise to gravity-dependent physiological responses must be identified and thoroughly examined in order to accurately interpret data from flight experiments or ground-based microgravity analogs.
