Highly stretchable, self-healing elastomer hydrogel with universal adhesion driven by reversible cross-links and protein enhancement.

Engineered hydrogels with excellent mechanical properties and multi-functionality have great potential as soft electronic skins, tissue substitutes and flexible robotic joints. However, it has been a challenge to construct multifunctional hydrogels, especially when integrating high stretchability, toughness and strength, low hysteresis, good self-healing and adhesion abilities into a hydrogel system simultaneously. Here, we successfully developed a structural hydrogel composed of a reversible covalently cross-link-based poly--(2-hydroxyethyl)acrylamide (PHEMAA) network and available plastically deformable casein micelles. Such a design enabled the reversible covalent cross-links and casein micelles to enhance energy dissipation and toughen the PHEMAA/casein hybrid hydrogel synergistically. More importantly, the hydrogel could respond to the imposed strains reversibly by cross-link and micelle deformation induced-network reconstitution, which led to low hysteresis of the hydrogels. The recoverable gel networks still exhibited their effects on energy dissipation at the stress-focused area, endowing the hydrogels with fatigue resistance. As a result, the hydrogels exhibited a compressive strength of 36.5 MPa, high stretchability (1460%), high toughness (∼5.98 MJ m), low hysteresis (<30%) and fatigue resistance with almost completely overlapped hysteresis curves during 10 loading cycles. In addition, the introduction of casein micelles and reversible covalent bonding endowed the elastomer hydrogels with high adhesivity, self-healing abilities and biocompatibility.