The volume phase transition temperature (VPTT) marks the transition from a shrunken, hydrophobic state to a swollen, hydrophilic state. This transition is traditionally modulated by a temperature change; however, in certain systems, it may be triggered by changes in pH, thereby regulating the uptake and release of water without exposing the system to temperature changes. Carbon dioxide (CO
2) gas forms carbonic acid in water, reducing its pH. Purging with an inert gas reverses this pH change thereby avoiding the use of traditional acids and bases, which results in salt byproducts that have been shown to hinder reversibility in certain CO
2-switchable systems. In this study, we conferred CO
2 responsivity to poly(
N-isopropylacrylamide) (pNIPAM) hydrogels and demonstrated that these gels display a CO
2-switchable volume phase transition to enable isothermal and reversible gas-triggered swelling. We systematically measured the effect of hydrogel composition on its VPTT before and after exposure to CO
2 using UV-Visible spectroscopy. Over the compositional range studied, we observed shifts of up to 8.6ºC upon CO
2 exposure. These VPTT shifts enabled CO
2-induced swelling of hydrogels of up to 5 times their initial mass. The morphology of the hydrogels throughout a swell-shrink cycle was analyzed using scanning electron microscopy (SEM), showing that the average pore size of the hydrogels more than doubled after exposure to CO
2 and that upon removal of CO
2, the pores returned to their initial size. The CO
2-responsive system described in this study can be tuned and applied to induce pronounced, isothermal swelling in hydrogels for use in a variety of biomedical applications.
