Introduction to Hydrogels

Hydrogels are versatile polymer networks whose mesh size and responsiveness can be engineered for biomedicine, environmental systems, and soft electronics. This chapter emphasizes crosslinking density as a unifying parameter governing swelling, transport, and mechanical performance. We synthesize the field from early developments to current design strategies, organizing hydrogels by source, network formation, and environmental responsiveness. Particular attention is given to stimuli-responsive systems, including temperature, pH, light, enzymatic cues, and emerging magnetic, electrical, and redox triggers, which enable controlled release, actuation, and self-healing. Advances in nanocomposites, dynamic covalent chemistry, and bioinks are highlighted as drivers of multifunctionality. Remaining challenges include balancing toughness with biocompatibility, achieving predictable degradation, scaling fabrication, and ensuring reliable integration with sensors and biological tissues. Looking forward, opportunities lie in sustainable green synthesis, 3D biofabrication, and next-generation smart hydrogels for theranostics, regenerative medicine, and wearable bioelectronics.