A hydrogel is a network of polymer chains that are hydrophilic, sometimes found as a colloidal gel in which water is the dispersion medium. A three-dimensional solid results from the hydrophilic polymer chains being held together by cross-links. Because of the inherent cross-links, the structural integrity of the hydrogel network does not dissolve from the high concentration of water.[1] Hydrogels are highly absorbent (they can contain over 90% water) natural or synthetic polymeric networks.

The first appearance of the term 'hydrogel' in the literature was in 1894.[2] A hydrogel, sold under the brand Plenity, was approved to help with weight loss in 2019.[3]


Common uses include:


Common ingredients include polyvinyl alcohol, sodium polyacrylate, acrylate polymers and copolymers with an abundance of hydrophilic groups.

The crosslinks which bond the polymers of a hydrogel fall under two general categories: physical and chemical. Physical crosslinks consist of hydrogen bonds, hydrophobic interactions, and chain entanglements (among others). A hydrogel generated through the use of physical crosslinks is sometimes called a ‘reversible’ hydrogel. Chemical crosslinks consist of covalent bonds between polymer strands. Hydrogels generated in this manner are sometimes called ‘permanent’ hydrogels.

One notable method of initiating a polymerization reaction involves the use of light as a stimulus. In this method, photoinitiators, compounds that cleave from the absorption of photons, are added to the precursor solution which will become the hydrogel. When the precursor solution is exposed to a concentrated source of light, the photoinitiators will cleave and form free radicals, which will begin a polymerization reaction that forms crosslinks between polymer strands. This reaction will cease if the light source is removed, allowing the amount of crosslinks formed in the hydrogel to be controlled.[17] The properties of a hydrogel are highly dependent on the type and quantity of its crosslinks, making photopolymerization a popular choice for fine-tuning hydrogels. This technique has seen considerable use in cell and tissue engineering applications due to the ability to inject or mold a precursor solution loaded with cells into a wound site, then solidify it in situ.[14][17]

Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content. As responsive "smart materials," hydrogels can encapsulate chemical systems which upon stimulation by external factors such as a change of pH may cause specific compounds such as glucose to be liberated to the environment, in most cases by a gel-sol transition to the liquid state. Chemomechanical polymers are mostly also hydrogels, which upon stimulation change their volume and can serve as actuators or sensors.


Natural hydrogel materials are being investigated for tissue engineering; these materials include agarose, methylcellulose, hyaluronan, Elastin like polypeptides and other naturally derived polymers. Hydrogels show promise for use in agriculture, as they can release agrochemicals including pesticides and phosphate fertiliser slowly, increasing efficacy and reducing runoff, and at the same time improve the water retention of drier soils such as sandy loams.[19]

In the 2000 there has been an increase in research on the use of hydrogels for drug delivery. Polymeric drug delivery systems have overcome challenge due to their biodegradability, biocompatibility and anti-toxicity.[20] Recent advances have fueled the formulation and synthesis of hydrogels that provide strong backbone for efficient component for drug delivery systems.[21] Materials such as collagen, chitosan, cellulose and poly (lactic-co-glycolic acid) all have been implemented extensively for drug delivery to various important organs in the human body such as: the eye,[22] nose, kidneys,[23] lungs,[24] intestines,[25] skin[26] and the brain. Future work is focused on better anti-toxicity of hydrogels, varying assembly techniques for hydrogels making them more biocompatible[27] and the delivery of complex systems such as using hydrogels to deliver therapeutic cells.[28]


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