Gel rupture in a dynamic environment

Polymer hydrogels have been and continue to be used in a wide range of applications in different fields. Due to the structure of these materials, the absorption of water will usually result in uniform swelling, which scales with the degree of cross-linking. These features make hydrogels prime candidates for biosensors, drug delivery vectors, and carriers for cells in tissue engineering. Most past studies into the mechanics of hydrogels typically observed the materials in two separate states: (1) unswollen and without any solvent, or (2) in an equilibrium swelling state where the maximum amount of water has been imbibed. In this experimental study, we observe the dynamic process of swelling and how internal stresses that develop during swelling lead to the subsequent rupture of the poly(ethylene glycol)-based hydrogels. We find that rupture events follow a three-stage process that includes a waiting period, a slow fracture period, and a final stage in which a rapid increase in the velocity of crack propagation is observed. We describe this fracture behavior based on changes in material properties that occur during swelling, and highlight how this rupture behavior can be controlled by straight-forward modifications to the hydrogel network structure.