Things fall apart: understanding and controlling self-rupture during dynamic swelling

Hydrogels, e.g., hydrophilic polymer networks that swell but do not dissolve when immersed in water, are ubiquitous materials that are often exploited as platforms for biomaterials and as artificial tissue scaffolds. When a hydrogel begins to imbibe fluid (swell), internal stresses develop in the gel, which can sometimes have dramatic consequences ranging from surface wrinkling and instabilities to complete self-rupture. By adjusting the crosslink density of these gels, one can control the degree of material swelling, as well as the material's ability to withstand imbibement-induced stress. Our work focuses on the dynamics of the swelling process in PEG-based hydrogels, and in particular on self-rupture induced by imbibement. This self-rupture follows a three-stage process: 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 characterize rupture behavior using high-speed imaging and photoelastic measurements, and use this failure behavior as a probe for dynamically changing material properties. We also highlight our recent progress using acoustic signals to better understand this complex process.