Swelling in CO2-Responsive Hydrogels

Upon exposure to external stimuli such as temperature, pH, or light, stimuli-responsive polymers exhibit changes in physical or chemical properties that can be advantageous for many applications. CO₂ is advantageous for use as a trigger as it is abundant, affordable, and non-toxic. Here, we show that CO₂ can be used to trigger pronounced, reversible swelling of crosslinked hydrogels by modifying their volume phase transition temperature (VPTT), which distinguishes the transition from a collapsed to swollen state. In this study, we synthesized hydrogels that display a CO₂-switchable volume phase transition that enables controlled swelling in response to CO₂ addition. Using UV-visible spectroscopy, we characterized the effect of varying co-monomer content on the resulting VPTT shifts in these samples. Once the VPTTs were determined, isothermal swelling studies were conducted at a temperature between the VPTTs before and after CO₂ exposure. The extent of swelling was dependent on the composition of the gels, leading to gels that swelled to more than double their mass prior to CO₂ exposure. Further swelling tests were conducted that demonstrated the reversible nature of the swelling via the introduction and removal of CO₂. Overall, we show that CO₂ can be used as a trigger for pronounced, reversible swelling in hydrogels displaying a volume phase transition, which may have applications in the field of drug delivery.