Book excerpt: A hydrogel is a three-dimensional network of cross-linked hydrophilic polymer chains swollen in water. The large water content allows hydrogels to be biocompatible. Conventional hydrogels are mechanically soft and weak, which severely limits their applications, especially in structural biomaterials. Studies on double network (DN) hydrogels consisting of two interpenetrating networks of brittle and ductile polymers, revealed a general strategy to toughen network materials. The first network is densely cross-linked, easily breakable, and is brittle in nature, while the second network is loosely cross-linked, highly extensible, and is ductile in nature. During deformation, the first network breaks preferentially to dissipate energy, while the second network keeps the sample intact. As a result, DN hydrogels have high mechanical strength and toughness (~ 1000 J/m2, which is comparable to the cartilage). By adopting a similar strategy of incorporating sacrificial network into polymer network to dissipate mechanical energy, Polyampholyte (PA) gels are developed. In this thesis, a three-dimensional finite strain nonlinear viscoelastic model is developed to study the mechanical behavior of PA gels (both purely physically cross-linked PA gel and chemical PA gel with both physical and chemical cross-links). The time dependent behavior of the PA gel is due to the ionic interactions of oppositely charged monomers randomly distributed along the chain backbone. Our constitutive model connects the strain dependent bond breaking and reforming kinetics in the microscopic regime to the deformation of the gel at the continuum level. We compare the predictions of our model with uniaxial tension, tensile-relaxation, cyclic, and small strain torsional relaxation tests. The material parameters in our model are obtained using least squares error optimization. Our theory agrees well with the experimental behavior of the gel.