Dynamic swelling behavior of interpenetrating polymer networks in response to temperature and pH.

Temperature responsive hydrogels based on ionic polymers exhibit swelling transitions in aqueous solutions as a function of shifting pH and ionic strength, in addition to temperature. Applying these hydrogels to useful applications, particularly for biomedical purposes such as drug delivery and regenerative medicine, is critically dependent on understanding the hydrogel solution responses as a function of all three parameters together. In this work, interpenetrating polymer network (IPN) hydrogels of polyacrylamide and poly(acrylic acid) were formulated over a broad range of synthesis variables using a fractional factorial design, and were examined for equilibrium temperature responsive swelling in a variety of solution conditions. Due to the acidic nature of these IPN hydrogels, usable upper critical solution temperature (UCST) responses for this system occur in mildly acidic environments. Responses were characterized in terms of maximum equilibrium swelling and temperature-triggered swelling using turbidity and gravimetric measurements. Additionally, synthesis parameters critical to achieving optimal overall swelling, temperature-triggered swelling, and sigmoidal temperature transitions for this IPN system were analyzed based on the fractional factorial design used to formulate these hydrogels.