Dual-Responsive poly(N-isopropylacrylamide-co-methacrylic acid) Hydrogels: Role and Synergies of pH and Temperature in Tailoring Hydrogel Responsiveness

Stimuli-responsive hydrogels have attracted significant attention as versatile platforms for biomedical applications, among which the most important include drug delivery, tissue engineering, and wound healing. In this article, we explore the swelling properties of dual pH and thermoresponsive hydrogels. Crosslinked hydrogels of poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAMm-co-MAAn)) were synthesized by photopolymerization from varying proportions of the thermoresponsive monomer NIPAM and the pH-responsive monomer MAA (10, 20, or 30 mol% MAA), using 1 mol% N, N′-methylenebisacrylamide (BIS) as crosslinker. Swelling, compressive elastic modulus, water contact angle, pore size, transition temperature, and enthalpy of phase transition were studied as a function of temperature and pH for the P(NIPAMm-co-MAAn) hydrogels. The pH-dependent behaviour of the thermoresponsive copolymer hydrogels is attributed to hydrogen bonding interactions between NIPAM and MAA units, predominant at low pH, and to the enhanced hydration of ionized MAA carboxylate groups at high pH. Although the MAA content did not exceed 30 mol%, it played a key role in determining hydrogel properties.