A hydrogel strip exhibits stress-relaxation when stretched. The decay in tensile stress can be ascribed primarily to strain-induced swelling of the polymer network. This assertion is supported by a bi-phasic continuum model of the gel-solvent system; An equation of motion, a linear constitutive law of the polymer network, Darcy's law and the conservation of mass of the network constituents and interstitial fluid are all solved with boundary and initial conditions appropriate for a stress-relaxation experiment. The model predicts the evolution and distribution of the interstitial fluid pressure, elastic stress, total tensile stress, and isometric force applied to the clamp during the course of a stress-relaxation experiment. It also predicts that the stress-relaxation time constant depends inversely upon the square of the thickness of the gel strip - a non-intuitive result that is confirmed by experiments. The model has recently be extended to explain why crazing or fractures develop in hydrogels during hydration.
