Stiff, strong, and tough hydrogels with good chemical stability

Recent developments in the area of hydrogels promise to greatly expand their scope of applications. Many applications require hydrogels to endure signicant mechanical loads in aggressive environments. Examples range from biomedical applications such as articial cartilage in tissue engineering, to engineering applications such as swellable packers in the oil industry, or articial nerves and muscles in the nascent eld of so machines. Substitutes for cartilage require high stiffness (1 MPa), high toughness (1000 J m ), and high water content (60– 80%). Materials for oil packers require high stiffness, high strength, and chemical stability; they need to resist deformation, sustain sealing pressure (up to 34 MPa) and be stable in saline water. Materials for articial nerves and muscles require high resistance to mechanical damage, and tolerance of concentrated electrolyte for ionic conductance. Most hydrogels have low stiffness (10 kPa), strength (100 kPa) and toughness (10 J m );7 and some of them degrade in electrolyte solutions. There is a strong need for mechanically robust hydrogels with good chemical stability. Despite recent progress, developing hydrogels that are both mechanically robust and chemically stable is still a challenge. Breaking covalent bonds in double network gels results in permanent and irreversible damage to the network. Formation of hydrophobic associations is limited by low solubility of the