This dissertation, "All-aqueous Multiphase Microfluidics: From Formation of Liquid Structures to Synthesis of Biomaterials" by Yang, Song, 宋阳, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Droplet microfluidics which enables manipulation on droplets has been widely applied in the production of cosmetics and medicine, fabrication of materials, and bioassays. To form emulsion droplets, organic solvents are frequently used as oil phases. Nevertheless, the use of organic solvents also causes concerns on the biocompatibility and health issues due to their toxicity. In particular, the interface formed by water and oil can cause the denaturation of proteins and even death of cells. Therefore, an alternative set of emulsion droplets without the using of organic solvents is demanded in biomedical applications. In the first chapter of this dissertation, we present an all-aqueous microfluidic approach to generate emulsion droplets from immiscible aqueous phases. Such immiscible aqueous phases take advantages of the phase separation between the incompatible solutes dissolved in water to form all-aqueous emulsions. Compared to the water/oil emulsion systems, the all-aqueous emulsion is advantageous in processing of biomolecules and cells. However, manipulation on the structure of the all-aqueous emulsion is challenged by the ultralow interfacial tension between two immiscible aqueous phases. In the second chapter, we demonstrate the ability to control the structure of the all-aqueous emulsions by combining a perturbation approach and a phase-separation approach. Nevertheless, this approach is only applicable to non-viscous w/w systems. In particular, when biomolecules such as protein and polysaccharide are dissolved to form a viscous mixture, the resultant fluids are often too viscous to be handled by this approach. Alternatively, we investigate using an applied electrical field to manipulate the liquid structures of viscous aqueous phases in chapter 3 and 4. In chapter 3, we present a technique of controlling the folding and unfolding of viscous water-in-water (w/w) jet in the microfluidic channels by electrical charging. In chapter 4, we introduce an all-aqueous electrospray approach to generate w/w droplets from viscous aqueous phases. By using the all-aqueous emulsion droplets as templates, hydrogel particles and core-shell structured capsules can be fabricated with highly tunable structures. The fabricated particles and capsules exhibit good cytocompatibility, which can be used for delivery of bioactive proteins (Chapter 5). However, without the gelation of w/w emulsion droplets, interface of the w/w emulsion are intrinsically unstable. Therefore, we investigate the stabilization of w/w emulsion droplets by using protein nanofibrils with high aspect ratios in Chapter 6. These protein nanofibrils can be efficiently adsorb to the w/w interface and stabilize the w/w droplets. The stabilized droplets can be further used for fabrication of robust protein vesicles. When the concentration of protein fibrils is further increased, a colloidal network forms inside the w/w droplets. Such a composite structure exhibits interesting budding and splitting behaviors upon osmotic shrinking, suggesting the possibility of fabricating functionalized biomaterials by using all-aqueous emulsion droplets as templates. In summary, an all-aqueous multiphase microfluidic technology is developed in this dissertation. This technology enables the manipulation of all-aqueous structures, which provides a versatile and biocompatible platform for fabrication of biomaterials.