Stabilizing bio-friendly and cyto-mimetic fluid structures has important implications for drug and gene delivery, micro bio-reactors, single cell and microniche studies, and as protocells. The majority of stabilization techniques have been developed for oil-in-water and water-in-oil emulsions, which have limitations in their application to biological systems due to the presence of the oil phase. The structures built in this thesis are therefore made within water-in-water dispersions. These all water dispersions are from aqueous mixtures of two polymers that demix to form two phases, termed aqueous two phase systems (ATPSs). ATPSs are comprised of two water-rich phases and are therefore excellent candidates for hosting or implantation within biological systems. In this thesis, challenges to stabilizing ATPSs are identified and discussed, including the ultra-low interfacial tension characteristic of these systems, and new strategies are developed to overcome such challenges. The first system studied is that of casein and xanthan (a protein and a polysaccharide) which undergoes spinodal decomposition, resulting in a transient bicontinuous structure. Colloids present in this phase-separating mixture attach at the water-water interface owing to capillarity, despite the low interfacial tension. This occurs for both living and passive colloids. The living colloids, Escherichia coli and Pseudomonas aeruginosa bacteria, ultimately break down and restructure the bicontinuous matrix, indicating the bacteria can cooperate with the structure. The second system studied is the aqueous mixture of two polymers, poly ethylene glycol (PEG) and dextran. The PEG-dextran system is used to study emulsion stabilization techniques and understand the nuances of building materials at all water interfaces, specifically using polyelectrolyte complexation. In this study, oppositely charged polyelectrolytes are delivered from droplet and continuous phases to form a complex layer at the interface that stabilizes a water-in-water emulsion. Since the polyelectrolytes are soluble in both phases, successful interfacial complexation requires that polyanion and polycation fluxes be balanced such that they meet at, rather than around, the interface. Otherwise, non-stabilizing complexes are formed either inside or external to the droplet, far from the interface. This strategy is utilized to create microcapsules that can support a microbial community within the lumen. Interestingly, when one of the polyelectrolytes is replaced with a charged nanoparticle, the formation of stabilizing complexes is far less dependent on nanoparticle flux, as nanoparticles remain in the phase in which they are originally dispersed. Intriguingly, nanoparticle-polyelectrolyte complexation favors spontaneous double emulsion formation; we name these structures AWE-somes as they are water emulsion bodies. The mechanism for their formation relies on the osmotic pressure imbalance between the droplet and continuous phases. The encapsulated double emulsion structure is reminiscent of membrane-less organelles within biological cells, which comprise internal membrane-less compartments enclosed by a permeable membrane. This motivates the study of transport within the AWE-some structures. Small and large molecules can diffuse into the lumen and selectively partition to the PEG or dextran phases inside the nanoparticle and polyelectrolyte membrane. One drawback of the nanoparticle-polyelectrolyte shell is less flexible than the membrane formed by the polyelectrolyte pair. To imbue the nanoparticle decorated capsules with both flexible membranes and spontaneous multiple compartments, compound capsules are fabricated in which the continuous phase initially includes both anionic polyelectrolyte and negatively charged nanoparticles. These compound capsules have tunable flexibility and stimulus responsive properties, which are important for fortifying the capsules for various biological environments. Future studies will include the incorporation of biological molecules including DNA and proteins to demonstrate further biological functionality.