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Gravity-driven microfluidics, which utilizes gravity force to drive liquid flow, offers portability and multi-condition setting flexibility because they do not require pumps or connection tubes to drive the flow. However, because the flow rate decreases with time in gravity-driven microfluidics, it is not suitable for stem cell experiments, which require long-term (at least a day) stability. In this study, gravity-driven microfluidics and a slow-tilting table were developed to culture cells under constant unidirectional perfusion. The microfluidic device was placed on a slow-tilting table, which tilts unidirectionally at a rate of approximately 7° per day to compensate for the reduction in the flow rate. Computational simulations showed that the pulsation of the flow arising from the stepwise movement of the table was less than 0.2%, and the flow was laminar. Hydrophilization of the tanks increased the flow rate, which is consistent with the theoretical values. We showed that vitronectin is better than laminin 511 fragments as a coating material for adhering human induced pluripotent stem cells on a microchamber made of polydimethylsiloxane, and succeeded in culturing the cells for 3 days. It is believed that the system offers easy-to-use cell culture tools, such as conventional multiwell culture vessels, and enables the control of the cell microenvironment.

The “gas–liquid two-phases mixed flow” mechanism that fine bubble generation uses rapidly swirling water, which shear gas and making small bubbles in water. Based on this mechanism, sending liquid such as oil into the swirling water instead of gas and it able to be emulsified. At present, evaluation methods for emulsions are using particle size distribution and dispersion stability. However, the emulsion with bad separation behavior has a problem to catch only some dispersed oil particles and use them as a whole evaluation. In this study we examined emulsion evaluated by chemical oxygen demand (COD) to express numerical evaluation including the oil particles that cannot be dispersed. Using three emulsions of only one oil, an anionic and a nonionic emulsifier were mixed and added to the oil, an anionic and another kind of the nonionic emulsifier were mixed and added to the oil. It was confirmed that COD decreased and type of only one oil was the most evaporate. These results lead to the conclusion that the performance of the emulsion could be expressed numerically, including the oil particles which could not be dispersed. We also directly analyzed evaporate components and compared them with COD results.