Programmable Electrostatic Interactions Expand the Landscape of Dynamic Functional Hydrogels

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.