A double network hydrogel consists of a first network and a second network, where the first network is, or includes, a first polymer that is formed, at least in part, of —CH2—CH(OH)— units, and the second network is, or includes, a second polymer that is formed, at least in part, of carboxyl (COOH)-containing units or salts thereof, sulfonyl (SO3H)-containing units or salts thereof, and at least one of hydroxyl (OH)-containing units or amino (NH2)-containing units, where the hydrogel may be used as a cartilage replacement.