Temperature-Responsive, Manipulable Cavitary Hydrogel Containers by Macroscopic Spatial Surface-Interior Separation

Abstract

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.

Self‐Evolution of High Mechanical Strength Dry‐Network Polythiourethane Thermosets into Neat Macroscopic Hollow Structures

“…Based on the above experiments and literature, [10,11] the mechanism of the PTU-0 evolution process could be proposed as follows (see Figure 3 for corresponding schematic representation): the initial film was crosslinked by dense hydrogen bonds between the thiourethane backbones and carboxyl side chains. When the film was immersed into an aqueous NaOH solution, the solution gradually penetrated the polymer networks, destroying the pristine outer dense hydrogen bonds which then resulted in a dissociation of the polymer network.…”

“…Currently, most of the hollow structures were constructed by using template‐assisted methods, [7] microfluidic spinning, [8] and 3D printing technology [9] . Compared to the hollow materials produced by these methods, self‐evolved hollow materials possess the advantage of simple synthetic procedure and thus has attracted increasing attention from material scientists [10, 11] . Till now, the self‐evolved hollow structure materials are mostly hydrogels, the nature of which is soft and wet, resulting in tender mechanical properties [10–13] .…”

“…[9] Compared to the hollow materials produced by these methods, self-evolved hollow materials possess the advantage of simple synthetic procedure and thus has attracted increasing attention from material scientists. [10,11] Till now, the selfevolved hollow structure materials are mostly hydrogels, the nature of which is soft and wet, resulting in tender mechanical properties. [10][11][12][13] Nevertheless, materials with macroscopic hollow structures bearing both easy accessibility and excellent mechanical properties have rarely been reported.…”

“…[10,11] Till now, the selfevolved hollow structure materials are mostly hydrogels, the nature of which is soft and wet, resulting in tender mechanical properties. [10][11][12][13] Nevertheless, materials with macroscopic hollow structures bearing both easy accessibility and excellent mechanical properties have rarely been reported.…”

Self‐healing and shape‐shifting polymers controlled by dynamic bonds

“…With a similar mechanism, an in-situ method was used in a direct spatial surface-interior separation from the bulk dynamic hydrogels to the closed three-dimensional hydrogel containers with inner cavities by constructing a cross-linking gradient within the dynamic hydrogels. [92] The initial cross-linking of phenylboronic acid/catechol complexes was disrupted by stronger ferric ions/catechol associations (Figure 7c), which gradually weakened the cross-linking from outside to inside. Both the stronger cross-linking in the outer shells and sequentially weaker cross-linked interiors were generated during the swelling process of the closed hydrogel container with a tunable dense outer shell, fluffy inner layer and cavities in the core.…”