  Hydrogels have had a profound impact in the fields of tissue engineering,
drug delivery, and materials science as a whole. Due to the network
architecture of these materials, imbibement with water often results in uniform
swelling and isotropic expansion which scales with the degree of cross-linking.
However, the development of internal stresses during swelling can have dramatic
consequences, leading to surface instabilities as well as rupture or bursting
events. To better understand hydrogel behavior, macroscopic mechanical
characterization techniques (e.g.\ tensile testing, rheometry) are often used,
however most commonly these techniques are employed on samples that are in two
distinct states: (1) unswollen and without any solvent, or (2) in an
equilibrium swelling state where the maximum amount of water has been imbibed.
Rarely is the dynamic process of swelling studied, especially in samples where
rupture or failure events are observed. To address this gap, here we focus on
rupture events in poly(ethylene glycol)-based networks that occur in response
to swelling with water. Rupture events were visualized using high-speed
imaging, and the influence of swelling on material properties was characterized
using dynamic mechanical analysis. 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.
