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Organ-on-a-chip (OOC) is an innovative microfluidic device mimicking the structure and functionality of real tissue. OOCs typically involve cell culture with microfluidics to emulate the biological forces of different organ tissues and disease states, providing a next-generation experimental platform. When combined with simulated microgravity conditions, such as those produced by random positioning machines, they offer unique insights into disease processes. Microgravity has been shown to affect cellular behaviors, like proliferation and viability, though its influence on cell physiology is not fully explored. The primary objective of this study was to develop an OOC model with continuous flow under simulated microgravity. Cells cultured in static (non-continuous-flow) conditions exhibited clear growth reduction under microgravity conditions, showing more pronounced difference compared to continuous-flow conditions using an OOC setup. Although our results show that A549 cell viability under continuous flow decreased in microgravity compared to normogravity, this study demonstrates the successful development of a system capable of providing continuous flow in organ-on-a-chip (OOC) models within a random positioning machine. Full article

(1) Background: One effect of microgravity on the human body is fluid redistribution due to the removal of the hydrostatic gravitational gradient. These fluid shifts are expected to be the source of severe medical risks and it is critical to advance methods to monitor them in real-time. One technique to monitor fluid shifts captures the electrical impedance of segmental tissues, but limited research is available to evaluate if fluid shifts in response to microgravity are symmetrical due to the bilateral symmetry of the body. This study aims to evaluate this fluid shift symmetry. (2) Methods: Segmental tissue resistance at 10 kHz and 100 kHz was collected at 30 min intervals from the left/right arm, leg, and trunk of 12 healthy adults over 4 h of 6° head-down-tilt body positioning. (3) Results: Statistically significant increases were observed in the segmental leg resistances, first observed at 120 min and 90 min for 10 kHz and 100 kHz measurements, respectively. Median increases were approximately $11\%$ to $12\%$ for the 10 kHz resistance and $9\%$ for the 100 kHz resistance. No statistically significant changes in the segmental arm or trunk resistance. Comparing the left and right segmental leg resistance, there were no statistically significant differences in the resistance changes based on the side of the body. (4) Conclusions: The fluid shifts induced by the 6° body position resulted in similar changes in both left and right body segments (that had statistically significant changes in this work). These findings support that future wearable systems to monitor microgravity-induced fluid shifts may only require monitoring of one side of body segments (reducing the hardware needed for the system). Full article