Multiple Physical Cross-Linker Strategy To Achieve Mechanically Tough and Reversible Properties of Double-Network Hydrogels in Bulk and on Surfaces

Development of tough and adhesive hydrogels is critical for different applications, including wound dressing, soft robotics, and wearable devices. However, achieving strong and reversible adhesion between hydrogels and surfaces have proved to be a great challenge, because strong adhesion and reversible adhesion are the two highly desirable but opposite properties of hydrogel adhesives. Existing hydrogel adhesives possess either one-time, irreversible, strong adhesion or reversible weak adhesion on diverse surfaces. Herein, we developed a fully physically cross-linked double-network (DN) hydrogel of Agar/pAAEE (N-poly­(acryloylaminoethoxyethanol)) with both high bulk mechanical properties and strong reversible surface adhesion. Synergetic cooperation of high-density hydrogen bonds within both networks enables to simultaneously promote mechanical properties (tensile stress of 1.84 MPa, tensile strain of 2.5 mm/mm, elastic modulus of 1.85 MPa, and tearing energies of 1612 J/m²) and fast mechanical self-recovery (stiffness/toughness recovery of 79%/60% in 5 min and 90%/91% in 30 min at 80 °C). In parallel to high mechanical properties in bulk, Agar/pAAEE DN hydrogels can not only virtually adhere to many untreated solid and soft surfaces (glasses, ceramics, aluminum, titanium, human skin) with high interfacial toughness up to ∼650 J/m² but also retain their strong and durable surface adhesion of 250 J/m² even after 100 times repeating adhesion-on/peeling off tests on glass substrate. Moreover, the hydrogels can also reversibly adhere on a human skin and detach from the skin without causing any damage, pain, and residue. High bulk toughness and strong surface adhesion of Agar/pAAEE gel are attributed to its double-network structure and dynamic/cooperative physical interactions within the networks and between network and surfaces. This free-standing, tough, and adhesive hydrogel could serve as promising materials for many adhesive-related applications.