Bioinspired Robust All-Aqueous Droplet via Diffusion-Controlled Interfacial Coacervation

All-aqueous droplets demand structural and functional robustness to provide tailored microenvironments for various biomedical applications. Inspired by the structural supporting properties of eggshell membranes, all-aqueous droplets with robust shells featuring optical transparence, compound permeability, and swelling resistance are developed. The dense coacervate shell encapsulates sodium alginate (Alg) droplets in a continuous ε-poly-l-lysine (ε-PL) solution, via diffusion-controlled interfacial coacervation between Alg and ε-PL. Benefiting from the permeability of the coacervate shell, the core of the droplet presents embryo-like stiffness-responsiveness between the fluid and gel states via the perception of external microenvironment. Particularly, in a benchmarking comparison with Alg-Ca²⁺ hydrogel, the coacervate encapsulation presents boosted durability, mechanical robustness, and selective permeability in diverse conditions. This design strategy not only deepens the understanding on the design of all-aqueous droplets with adaptive functions via one-step interfacial coacervation between two miscible aqueous phases, but also broadens their feasibility as integrated platforms for biomedical applications.