Hydrogels: A Unique Class of Soft Materials

Hydrogels are well-known soft materials that are used to develop soft and wet technology. A solid cross-linked polymer remains in a solvent-entrapped swollen state, and this swollen mass is called a hydrogel. Hydrogel materials having high water absorption and retention capacity are specifically called superabsorbent hydrogels. Such hydrogels are widely used in baby diapers, sanitary towels and agricultural applications. Many hydrogels exhibit the phenomenon of sudden and reversible phase transitions under the influence of external stimuli such as temperature, pressure, electric and magnetic field, light intensity, pH and ionic strength of the medium and chemical triggers: these are called smart hydrogels (also stimuli-responsive gels or intelligent hydrogels). Smart hydrogels have been ‘tailored’ to challenging technological applications such as artificial muscles and organs, drug delivery systems, smart sensors and actuators. The synthesis of functional smart hydrogels having extraordinary activity like sensing, healing, actuation, and other functions, to fulfil the present technological demand of functional soft materials is a challenging task. Syntheses of fast stimuli-responsive and also strong and stretchable hydrogels are two other aspects to consider in the development of smart hydrogels. The present chapter surveys all aspects of hydrogel materials, including their synthesis, characterization, property evaluations, and recent trends in their technological applications.