Use of CO2 to Trigger Reversible Volume Phase Transitions in Hydrogels

Smart materials are often inspired by natural processes that are responsive to environmental stimuli. CO₂ has garnered interest as a stimulus as it is abundant, inexpensive, and non-toxic. Swelling, or the process of absorbing water, is a fundamental property of hydrogels yet is often not controlled using an environmental stimulus. In this work, CO₂ is used to trigger pronounced, reversible swelling of a crosslinked hydrogel. Specifically, CO₂ responsivity was conferred to hydrogels with a volume phase transition temperature (VPTT), which demarcates the transition from a collapsed to swollen state. We show that these gels display a CO₂-switchable volume phase transition that enables gas-triggered swelling at a constant temperature. To tune the VPTT, compositional studies were conducted to evaluate the effect of varying co-monomer content on the resulting VPTT shifts in these samples. Increasing the concentration of CO₂-responsive moiety resulted in an increase in the initial VPTT as well as a greater shift in the VPTT after CO₂ exposure. Isothermal swelling studies conducted at a temperature between the VPTTs before and after CO₂ exposure show that CO₂ triggers a reversible volume transition and controls swelling. The results presented here demonstrate the promise of CO₂ as a stimulus for the design and study of smart materials, which may find applications in drug delivery, microfluidics, and soft actuators.