Microspheres (MS), such as emulsion droplets, multiple emulsions, microparticles, microcapsules, and liposomes, have been utilized in various industries. However, size control of MS is not so easy. Recently, we proposed a novel method for preparing monodispersed emulsion droplets with a coefficient of variation less than 5% from a microfabricated channel (MC) array. In this study, we analyzed a droplet-formation mechanism from a MC using a microscope high-speed camera system. During droplet formation, the dispersed phase passed through the channel inflated on the terrace in a disklike shape, and the droplets were formed in 0.01 s. A droplet-formation mechanism was suggested in which the distorted dispersed phase on the terrace is cut off spontaneously into spherical droplets by interfacial tension. The mechanism is shown to be an adequate model from the viewpoint of interfacial free energy. This emulsification technique exploits the interfacial tension, which is the dominating force on a micrometer scale. It is a promising technique for producing MS requiring extreme monodispersity because of its simplicity.

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Recently, we proposed a microchannel (MC) emulsification technique, which is a novel method for making a monodispersed emulsion from a microfabricated channel array. The droplet formation mechanism for MC emulsification is a unique one, in which the distorted dispersed phase is spontaneously transformed into spherical droplets by interfacial tension. The objective of this study was to characterize the flow in MC emulsification. We investigated the emulsification behavior at different flow velocities of the dispersed phase using MCs with different sizes and analogous shapes and found a critical flow velocity over which the character of flow changed drastically. The formed droplet diameters were almost constant below the critical velocity, and monodispersed emulsions were formed. The droplet diameters increased drastically above the critical velocity, and polydispersed emulsions were formed. The critical velocities were independent of MC size. Analysis using a dimensionless number revealed that the critical point, at which the character of the flow of MC emulsification changes, is determined by the Capillary number (Ca), which is the ratio of viscous force to interfacial tension force. Below the critical Ca, the interfacial tension, which is the driving force for droplet formation, is dominant, and the flow in MC emulsification was based on spontaneous transformation. Above the critical Ca, viscous force is dominant, and the flow is similar to laminar flow. We experimentally confirmed this idea using the dispersed and continuous phases with different viscosities, MCs with different sizes, and different interfacial tensions. These results suggested a law of similarity in MC emulsification, and the transition of the flow was determined by Ca. The information obtained in this study includes the essential physics underlying spontaneous transformation flow in MC emulsification and is useful for practical applications.

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Recently, we proposed microchannel (MC) emulsification, a novel method for making monodisperse emulsions using a microfabricated channel array. The previous study demonstrated that droplet size is affected by MC geometry. This study proposes a model for the prediction of droplet diameter based on the droplet formation mechanism and on experimental observation. The MC structure used in this study is composed of a narrow channel and a terrace. The terrace is a microfabricated slitlike shape, on which the dispersed phase inflates to a disklike shape. The MC geometry is defined in terms of two variables, terrace length (L) and MC depth (H). First, the relationship between droplet diameter and MC geometry was investigated experimentally. Experimental observation suggests that the dispersed phase, which is within the detachment length (A) from the terrace end, detaches and forms a droplet. The droplet volume was estimated from the volume of the dispersed phase that detaches from the terrace during this process. This volume was calculated using a detachment length parameter, A, assuming the dispersed phase on the terrace to be disk-shaped. Experimental observation and regression analysis indicate that A is independent of L. The prediction curves were fitted by regression analysis as functions of L, using fitting parameters A for each H. The values of A obtained by regression analysis were linearly correlated with H. The prediction curve, which is expressed by two variables L and H, was obtained. The prediction model was correlated with the experimental data. The mean percentage deviation of the calculated values from experimental results was 5.4%. The prediction curve was corrected using the corrected MC depth. The final form of the corrected prediction curve shows a mean percentage deviation from experimental results of 4.6%.

The quick photoresponse of thin hydrogel layers composed of thermoresponsive poly(N-isopropylacrylamide) with an acrylated spirobenzopyran chromophore incorporated in the polymer backbone is reported. The instant formation of microrelief on a thin hydrogel layer is demonstrated by means of micropatterned light irradiation.

This article reports a pressure-driven perfusion culture chip developed for parallel drug cytotoxicity assay. The device is composed of an 8 x 5 array of cell culture microchambers with independent perfusion microchannels. It is equipped with a simple interface for convenient access by a micropipette and connection to an external pressure source, which enables easy operation without special training. The unique microchamber structure was carefully designed with consideration of hydrodynamic parameters and was fabricated out of a polydimethylsiloxane by using multilayer photolithography and replica molding. The microchamber structure enables uniform cell loading and perfusion culture without cross-contamination between neighboring microchambers. A parallel cytotoxicity assay was successfully carried out in the 8 x 5 microchamber array to analyze the cytotoxic effects of seven anticancer drugs. The pressure-driven perfusion culture chip, with its simple interface and well-designed microfluidic network, will likely become an advantageous platform for future high-throughput drug screening by microchip.