Modification of Hydrophobic Hydrogels into a Strongly Adhesive and Tough Hydrogel by Electrostatic Interaction

Synthetic hydrogels with hydrophobic interactions, which show excellent mechanical performance and good anti-swelling ability in saltwater, have great potential in various industries, such as soft robots, 3D printing, and wearable sensors. Normally, hydrophobic molecules inside a hydrophobic hydrogel tend to aggregate to form a large hydrophobic domain, leading to a phase separation phenomenon because water is a poor solvent of the hydrophobic domain. This aggregation, however, inhibits the adhesion of the hydrophobic hydrogel to various dry materials and thus limits its application in device and sensor industries. In this study, we report the synthesis of hybrid hydrogels with ionically and hydrophobically cross-linked networks. This novel hybrid hydrogel can strongly adhere to various substrates, such as glass, polypropylene, silicone, wood, and polytetrafluoroethylene, with a maximum adhesion strength measured to be 100 kPa. Meanwhile, this hybrid hydrogel can be stretched beyond 8-10 times of its initial length. We attribute this observed strong adhesion and high toughness properties to the synergy of electrostatic interactions and hydrophobic associations. With the strong adhesion and excellent tensile performance, these hydrogels may serve as a model system to explore the strong adhesion mechanism of hydrophobic hydrogels and expand the scope of hydrogel applications.