Hollow microgels: Interplay of structure, mechanics, and transport.

Responsive hollow nano- and microgels, cross-linked polymer networks containing a solvent-filled cavity, have attracted significant interest in both fundamental science and emerging technologies. The presence of these internal cavities imparts enhanced softness and deformability, enabling pronounced and tunable size and shape transformations in response to external stimuli. As a result, these systems are particularly well suited for biomedical applications such as drug delivery, controlled release, and encapsulation. By tuning synthesis conditions and incorporating appropriate functional moieties, hollow microgels can be engineered to respond to environmental triggers, including pH, temperature, and ionic strength. This review summarizes recent advances in the synthesis, properties, and applications of hollow nano- and microgels, with a particular focus on their mechanical and interfacial behavior. When subjected to external stresses, such as osmotic pressure, these systems can undergo significant shape transformations, including deswelling via inward network reconfiguration or buckling instabilities, depending on their size and core-to-shell ratio. At interfaces and under confinement, their high deformability enables complex morphological adaptations that govern their interactions and assembly. We further discuss these behaviors in the context of theoretical and computational studies. Finally, emerging features such as shape anisotropy and their implications for future applications are highlighted.