Synthetic macroscopic materials transforming from bulk solid or semisolid to a closed structure with inner cavities and distinct outer and inner microstructures are rarely reported. Here, we report an in situ method for directing spatial surface-interior separation from bulk dynamic hydrogels to closed three-dimensional (3D) hydrogel containers with inner cavities via constructing a competitively cross-linking gradient within dynamic hydrogels. The initial cross-linking of phenylboronic acid/catechol complexes is disrupted by stronger ferric ions/catechol associations, generating gradually weakened cross-linking from the outside to the inside. Both stronger cross-linking in the outer shells and sequentially weaker cross-linked interior generated during swelling closed the hydrogel container with a tunable dense outer shell, fluffy inner layer, and cavities in the core. Cellulose nanocrystals could be used to significantly improve the spatial distinction of gradient cross-linking within hydrogels, leading to an even denser outer shell with tunable shell thickness. Moreover, cavitary hydrogel containers with diverse shapes can be programmed by designing the initial shapes of dynamic hydrogels and macroscopic assembly of individual dynamic hydrogels based on their self-healing capability after subsequent surface-interior separation. These cavitary hydrogel containers demonstrate thermal-responsive gate systems with unique sustained release at higher temperature and potential reaction containers for oxygen generation on demand. This facile spatial surface-interior separation strategy for fabricating closed cavity systems has great potential for various applications.
Keywords: cross-linking gradient; dynamic hydrogel; hydrogel container; surface-interior separation; sustained release; temperature-responsive.