Probing hydrogel microstructure and porosity with fluorinated liquid NMR nanocapsules

Characterizing the microstructure and porosity of hydrogels in their fully hydrated state remains a major challenge. We present a robust platform based on pulsed field gradient (PFG) NMR combined with 200 nm, liquid-filled nanocapsules to non-invasively measure mesh sizes of hydrogels. By explicitly accounting for particle polydispersity, the method enables accurate interpretation of PFG NMR data. Fluorinated reporter molecules in the liquid core enable the background-free detection <i>via</i> fluorine-19 (¹⁹F) NMR in proton-rich polymer matrices. The microstructure of methacrylated hyaluronic acid (HAMA) hydrogels of varying concentrations is determined <i>via</i> PFG NMR diffusion measurements of the nanocapsules. A clear, HAMA concentration-dependent decrease in mesh size is observed and estimates of the mesh size are obtained using the liquid-core nanocapsules. This approach offers a non-destructive means to probe porous soft materials, offering a valuable tool for the characterization of hydrogels and complex soft tissues.