Qinyuan Chai, Yang Jiao and Xinjun Yu

Hydrogels are hydrophilic, three-dimensional networks that are able to absorb large
quantities of water or biological fluids, and thus have the potential to be used as prime candidates
for biosensors, drug delivery vectors, and carriers or matrices for cells in tissue engineering.
In this critical review article, advantages of the hydrogels that overcome the limitations from other
types of biomaterials will be discussed. Hydrogels, depending on their chemical composition,
are responsive to various stimuli including heating, pH, light, and chemicals. Two swelling
mechanisms will be discussed to give a detailed understanding of how the structure parameters
affect swelling properties, followed by the gelation mechanism and mesh size calculation. Hydrogels
prepared from natural materials such as polysaccharides and polypeptides, along with different
types of synthetic hydrogels from the recent reported literature, will be discussed in detail. Finally,
attention will be given to biomedical applications of different kinds of hydrogels including cell
culture, self-healing, and drug delivery.