Microcapsules are appealing containers for delivering active ingredients, such as drugs and cosmetics, or conducting chemical and biological reactions on a small scale. To control the timing and location of release of these encapsulants, the shells of microcapsules must exhibit a controlled permeability, thus ensuring a spatial separation between the substances within the capsule cores from reagents in the surrounding environment. However, most microcapsules function as single-use carriers since cargo is only released in significant amounts if their shells are damaged or the release relies on passive diffusion. It remains a challenge to develop mechanically stable and dynamic microcompartments that they can repetitively and selectively uptake and release the desired reagents on demand. In this thesis, we develop ionically crosslinked, viscoelastic, self-healing capsule shells inspired by the adhesive chemistry of the mussel byssus. In the first part, we introduce viscoelastic, self-healing capsules displaying a charge-selective permeability that can be tuned with the choice of the crosslinking ion cluster. Capsules are made from water-oil-water double emulsion templates and converted into capsules through the ionic crosslinking of the chelator functionalized surfactants localized in proximity of the liquid-liquid interfaces, resulting in the formation of viscoelastic shells. We demonstrate that these capsules can selectively uptake and release well-defined substances multiple times. This feature enables them to be applied as reusable selectively permeable filters ideal for wastewater treatment by extracting particular pollutants from aqueous solutions, or as microreactors to conduct chemical reactions within capsule cores while continuously supplying new reagents. In the second part, we leverage the high pH-stability of charge-selectively permeable double emulsions stabilized by chelator-functionalized surfactants to investigate the guanine crystallization. Our results indicate that we can influence the structure of the resulting guanine crystals formed under confinement by adjusting the pH. Similarly, the crystal structure depends on the guanine concentration within the cores of double emulsions that can be tuned by subjecting them to different osmotic pressures. Thereby, we for the first time demonstrate that the crystal morphology and polymorphs of guanine formed under confinement can be controlled by pH and osmotic pressures.