Responsive Acrylamide-Based Hydrogels: Advances in Interpenetrating Polymer Structures

Hydrogels, composed of hydrophilic homopolymer or copolymer networks, have structures similar to natural living tissues, making them ideal for applications in drug delivery, tissue engineering, and biosensors. Since Wichterle and Lim first synthesized hydrogels in 1960, extensive research has led to various types with unique features. Responsive hydrogels, which undergo reversible structural changes when exposed to stimuli like temperature, pH, or specific molecules, are particularly promising. Temperature-sensitive hydrogels, which mimic biological processes, are the most studied, with poly(N-isopropylacrylamide) (PNIPAm) being prominent due to its lower critical solution temperature of around 32 °C. Additionally, pH-responsive hydrogels, composed of polyelectrolytes, change their structure in response to pH variations. Despite their potential, conventional hydrogels often lack mechanical strength. The double-network (DN) hydrogel approach, introduced by Gong in 2003, significantly enhanced mechanical properties, leading to innovations like shape-deformable DN hydrogels, organic/inorganic composites, and flexible display devices. These advancements highlight the potential of hydrogels in diverse fields requiring precise and adaptable material performance. In this review, we focus on advancements in the field of responsive acrylamide-based hydrogels with IPN structures, emphasizing the recent research on DN hydrogels.