Although hollow microscopic capsules have a variety of potential biomedical applications, reports of organic-solvent-free methods for their preparation are rather limited. Herein, a novel approach is demonstrated for organic-solvent-free preparation of giant unilamellar vesicles utilizing the unique response of polyion complexes (PICs) to changes in additive salt concentration. A microfluidic device consisting of a main channel bearing side pockets that work as microscale reaction chambers is designed for facilitating the preparation process under an optical microscope. With this device, real-time observation of morphological transformation of individual PIC microparticles is carried out during rapid reduction of the additive salt concentration and direct formation of giant vesicles from PIC microparticles is shown. There is a quasilinear relationship between the surface areas of the formed vesicles and the volumes of the PIC microparticles, and the thickness of the vesicle membrane estimated by the relationship is indicative of the formation of a uniform unilamellar structure of the PIC membrane. Furthermore, detailed properties of the formed PIC vesicles with regard to salt response, loading of guest molecules, and permeability of the PIC membrane with/without modification of the PIC membrane by cross-linking are investigated using the microfluidic chamber. Thus, the usefulness of the microfluidic chamber for visualization and investigation of dynamic responses of microscale soft materials during changes in surrounding conditions is also demonstrated.
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