Electrostatic interaction is a promising mechanism to expand the range of physiochemical properties of hydrogel materials. However, the versatility of such materials is still limited because of the difficulties associated with harnessing strong electrostatic interactions for controllable hydrogel formation. Here we report a modular approach for programming interactions between positively charged biopolymers and polyoxometalate (POM) anions to create dynamic hydrogels. Fabrication of diverse hydrogels was achieved simply by soaking primary networks with predispersed chitosan in aqueous solutions of POMs with various nuclearity and charges. This resulted in double network (DN) hydrogels with 2–3 orders of magnitude higher toughness compared with the precedent composite hydrogels. In addition, the dynamic electrostatic interactions endowed the DN hydrogels reversible responsiveness and intriguing capabilities to memorize shapes, to actuate, and to change colors upon exposure to specific external cues, which are challenging to achieve in previous hydrogels. Furthermore, the flexibility of our approach is demonstrated by the use of either physically or chemically cross-linked primary networks, which are composed of synthetic polymers, natural biopolymers, and even genetically engineered protein polymers. Consequently, our facile and modular approach establishes new opportunities in design and fabrication of dynamic functional hydrogels for wide applications.
Rubberlike protein hydrogels are unique in their remarkable stretchability and resilience but are usually low in strength due to the largely unstructured nature of the constitutive protein chains, which limits their applications. Thus, reinforcing protein hydrogels while retaining their rubberlike properties is of great interest and has remained difficult to achieve. Here, we propose a fibrillization strategy to reinforce hydrogels from engineered protein copolymers with photo-cross-linkable resilin-like blocks and fibrillizable silklike blocks. First, the designer copolymers with an increased ratio of the silk to resilin blocks were photochemically cross-linked into rubberlike hydrogels with reinforced mechanical properties. The increased silk-to-resilin ratio also enabled self-assembly of the resulting copolymers into fibrils in a time-dependent manner. This allowed controllable fibrillization of the copolymer solutions at the supramolecular level for subsequent photo-cross-linking into reinforced hydrogels. Alternatively, the as-prepared chemically cross-linked hydrogels could be reinforced at the material level by inducing fibrillization of the constitutive protein chains. Finally, we demonstrated the advantage of reinforcing these hydrogels for use as piezoresistive sensors to achieve an expanded pressure detection range. We anticipate that this strategy may provide intriguing opportunities to generate robust rubberlike biomaterials for broad applications.
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