Polymer hydrogels: Classification and recent advances

Hydrogels comprise of a group of crosslinked hydrophilic polymeric materials which are capable of absorbing and holding large quantities of water in their three-dimensional network structure without undergoing dissolution. More importantly, the ability of 'smart' hydrogels to respond to certain environmental changes e.g. pH, heat, light, magnetic field, biomolecules have set them apart as a unique class of materials. A combination of several such useful and unique properties have resulted in a tremendous progress toward development of advanced hydrogel-based materials, as is evident from an explosive amount of research publications available in this area over the last few decades. Owing particularly to their biocompatibility and biodegradability, hydrogels have become a material of prime importance in context to a wide range of applications starting from simple contact lenses to more complex ones e.g. in tissue repair, drug delivery, sensors, 3D bioprinting, soft robotics and agriculture. This review includes i) an overview of hydrogels and its classifications based on source, structure, crosslinking mechanism and stimuli responsiveness, ii) detailed discussion on some of the most advanced research works being carried out in the field over the last few years in context to few smart applications that have become the need of the hour, particularly in the domain of biomedical applications.