Diffusive and Electro-osmotic Swelling of Neutral and Ion-Containing Poly(ethylene glycol) Hydrogels

This study presents a comprehensive analysis of the swelling behavior of poly-(ethylene glycol) (PEG)-based hydrogels of different molecular weights under various conditions. The rheological response and swelling kinetics of PEG hydrogels with molecular weight between cross-links ranging from 700 to 10 000 g/mol reveal the connection between architecture and material properties that are important for soft actuators. In addition to providing insight into the network structure and cross-linking density, rheological measurements find that the shear moduli of the networks increase with the degree of water swelling. Furthermore, the moving boundary of the gels during swelling, which is captured using a numerical model, is found to result in apparent super Case II water transport behavior. Finally, the application of an electric field in the presence of a salt solution is found to increase the rate of swelling by an order of magnitude when electro-osmosis is in the same direction as diffusion or decrease the swelling rate by an even greater amount when directions are opposed. The integration of experimental data with numerical models contributes to a deeper understanding of hydrogel behavior, guiding the design of PEG-based systems for biomedical or soft robotics applications. PEG hydrogels with dense network structures can do useful work and are promising for applications such as twisted and coiled actuators due to their ability to maintain a high modulus while radially swelling.