Hydrogels

Hydrogels are three-dimensional, crosslinked networks of hydrophilic polymers that can absorb and retain large amounts of water while remaining insoluble. Their high water content gives them soft, gel-like properties that resemble natural tissue.

They can be formed from natural polymers such as alginate, collagen, chitosan, and gelatin, or from synthetic polymers such as polyacrylamide, poly(ethylene glycol), and poly(vinyl alcohol). Crosslinking, which stabilizes the network, can be chemical (covalent bonds) or physical (ionic interactions, hydrogen bonding, or crystallization).

Hydrogels may be non-responsive or stimuli-responsive (smart hydrogels) and can change their volume or permeability in response to temperature, pH, ionic strength, enzymes, or light. This tunability, along with biocompatibility, makes them useful in biomedical and environmental applications.

Common properties include high porosity, tunable mechanical strength, and controllable diffusion of solutes. Limitations include relatively weak mechanical strength for load-bearing applications, drying and shrinking upon dehydration, and potential degradation or leaching of residual monomers in some systems.

Applications span drug delivery, tissue engineering scaffolds, wound dressings, contact lenses, sensors, and agriculture. They are manufactured by chemical crosslinking or physical gelation, and advanced designs pursue degradability and multi-functional responsiveness.

The first hydrogel was developed in the 1960s by Wichterle and Lim for soft contact lenses, and the field has since expanded to diverse biocompatible and smart materials.