Capsules with responsive polymeric shells for applications beyond drug delivery

Polymeric capsules with core-shell structures have been widely explored for molecule separations, energy storage, and catalysis, among other applications. To tailor the application-oriented performance of these structures, researchers have imparted stimuli-responsive properties to the shells. In contrast to capsules with "static" shells, stimuli-responsive capsule shells not only protect the core from the external environment, but also aid in handling and impart properties such as on-demand release of cargo and varied shell permeability, as well as provide a platform for the fabrication of advanced structures. The composition and properties of polymer shells can be tuned through incorporation of dynamic covalent bonds into the polymer backbones, or introduction of segments or pendant side chains which can leverage intermolecular (e.g., hydrogen bonding) or electrostatic interactions to give response. Most reports on responsive polymer capsules focus on their application in cargo delivery, a topic that is heavily reviewed elsewhere. In complement, this minireview addresses responsive polymer capsules and their applications beyond drug delivery, focusing on structure-property relationships. We first highlight the most common fabrication techniques for core-shell structures including hard template, soft template, and microfluidic methods, among which we emphasize our utilization of interfacial polymerization and polymer precipitation within a Pickering emulsion template. We then provide a concise review of commonly employed chemistries for responsive polymer shells based on stimuli, including our contribution on the incorporation of dynamic covalent polymer backbones into thermo-responsive capsule shells. Application-oriented performances of the responsive core-shell structures are then highlighted. Finally, we outline opportunities for advancing the performance-related properties of responsive capsules, wherein we propose new directions where responsive polymer-based capsules can have critical impact in the development of new technologies. This minireview addresses responsive polymer capsules and their applications beyond drug delivery, focusing on structure-property relationships.