Random Positioning Machines as a Preflight Research Platform: Physical Foundations, Biological Validity, Experimental Artefacts, and Criteria for Translation to True Microgravity

Random positioning machines (RPMs) are widely employed as terrestrial analogues to investigate biological responses associated with altered gravitational conditions and to support the development of spaceflight experiments. However, their scientific interpretation requires careful consideration of the fundamental differences between simulated and true microgravity. Temporal averaging of the gravitational vector does not eliminate instantaneous terrestrial gravity. Furthermore, sample rotation may introduce inertial accelerations, fluid shear stress, mixing, bubble-mediated effects and other mechanical confounders that influence experimental outcomes. This critical scientific review examines the physical principles, kinematic models, fluid dynamics and biological limitations of RPM-based research. Particular attention is given to mechanotransduction, mammalian cell cultures, three-dimensional cellular structures, extracellular vesicles, regenerative biology, pharmaceutical systems and advanced materials. Published comparisons between ground-based microgravity simulators and orbital experiments are discussed to assess the scientific validity, reproducibility and limitations of extrapolating terrestrial findings to spaceflight conditions. A mechanism-based methodological framework is proposed for experimental design, appropriate controls, sample qualification, physical characterisation, statistical analysis, evidence assessment and preflight go/no-go decisions. The central conclusion is that RPMs can provide valuable preliminary evidence and reduce experimental uncertainty when their operating conditions are compatible with the physical and biological mechanisms under investigation. Nevertheless, simulated microgravity cannot universally reproduce the conditions of orbital free fall, and validation under true microgravity remains essential for research questions requiring direct spaceflight confirmation. Publication status: Scientific preprint, Version 1.0, October 2026. This manuscript is a critical narrative review and methodological proposal. It does not report original experimental data and has not undergone peer review.