Hydrogels are substances that absorb and retain water in large quantities. The cross-linked polymers usually swell when placed in a liquid medium. Since its introduction in 1960 by Wichterle and Lím, and have been used in various biological and industrial processes.

Hydrogel products possess some degree of flexibility, like the natural tissues, to hold a significant amount of water. They can be defined as a three-dimensional network made up of hydrophilic polymers that swell and hold large water quantities. Its structure is maintained by the physical or chemical cross-linking of individual polymeric chains.

The hydrogel network’s hydrophilicity is due to the presence of the following hydrophilic groups- CONH, -COOH, -SO3H -OH, -CONH2, and -NH2. Hydrogels exist naturally as gelatin or collagen, or they can even be manufactured synthetically.

Classifications 

Current reports present different views and classification of hydrogels. However, we all understand that they are derived from biopolymers of the polyelectrolytes. According to sources, the types can be defined as either natural polymers or synthetic polymers.

Ionic charges on bond groups can classify hydrogels as either neutral, anionic, or cationic. Additionally, they can be classified according to their structures as semicrystalline, crystalline, amorphous, or the hydrocolloid aggregate.

We can have the interpenetrating network, semi-interpenetrating network, homopolymer, and copolymer based on preparation methods.

The most common types of hydrogels include:

• pH-sensitive 
• Temperature-sensitive 
• Electro-sensitive 
• Light-responsive

What Are The Key Properties Of Hydrogel That Facilitate Its Use?

Hydrogel typically undergoes a significant volume transition or the gel-sol transition in response to chemical or physical stimuli. Physical stimuli might include temperature, magnetic fields, electric currents, light intensity, pressure, or solvent compressions. The biochemical or chemical stimuli might include the ions, chemical compositions, and pH.

Moreover, the confirmatory transitions stated are mostly reversible. Therefore, we can conclude that the hydrogels are well-capable to retain their initial state even after the factor triggering reaction has been removed.

Hydrogels’ response to an external stimulus is determined by its monomer nature, pendant chains, cross-linkage degree, and the charge’s density. The magnitude of hydrogel response to a specific stimulus is directly proportional to the applied stimuli.

How Hydrogels Are Used

Many fields have opted to use hydrogels. Most of the products have good compatibility and excellent specific structure that enable them to survive different conditions. Hydrogels owe their flexibility to water. Water content makes them favorable in different industrial and biological conditions.

The materials used to produce hydrogels also give it its reputable biocompatibility. Its chemical behavior is non-toxic to the biological environment; therefore, its usage has been extended to the medical field.

The major applications include the following:

Drug Delivery Practices

Hydrogels’ use in controlled DDS (drug delivery system) has overcome the limitations of the regular drug formulations. It has been a great choice for drug delivery. It delivers drugs at a given rate for a predefined period.

Hydrogel has the main advantage of releasing pharmaceuticals for a prolonged period. Its sustained release has made it possible to supply active pharmaceutical substances in high concentration to a certain location over time.

Physical and chemical strategies are employed to modify the loaded drug binding with the matrix extending drug release duration. The medical field has appreciated the introduction of hydrogels robustly. This is because of the substance’s ability to store and protect drugs from hostile environments.

It also releases them on the desired kinetics. Drug delivery with hydrogel products can be activated on time of demand by applying local pH, specific enzymes, temperature, or remote stimuli.

Removal of Dye and Heavy Metal Ions

Industrial processes have resulted in vast heavy metal pollutants. It has even caused a threat to the ecological system and general public health.

Removal of the heavy metal ions from the water is a great scientific concern. Polyacrylate hydrogel can be used to aid in the removal of toxic metals in the aqueous medium. A hydrogel is a very effective adsorbent for recovering dyes, heavy metals and removing all toxic effluents’.

Tissue Engineering

Tissue engineering combines improved cells, materials, and engineering to replace and replenish existing biological organs. The procedure needs one to collect the proper cell and culture it in a suitable scaffold. Only one sustainable scaffold is appropriately tuned to appropriate survival conditions.

Hydrogels are the best scaffold materials since they have an extracellular matrix that is similar to various tissues. In tissue engineering, the hydrogel application is used to serve three functions:

• Filling the vacant spaces
• It acts as a bioactive molecules
• Hydrogels form structural support to help the cells build ideal tissues

Manufacturing Of Contact Lenses

The key bio-application of the synthetic hydrogels is ophthalmology, particularly contact lenses. A contact lens is an optical device placed directly above the cornea to alter the eye’s corneal power. The use of a contact lens directly prevents the surface of the cornea from exchanging the atmospheric oxygen.

This disrupts physiological metabolism that occurs naturally at the cornea resulting in hypoxic stress. Therefore, the best contact lens ought to have a maximized oxygen permeability. Mechanical stress, also due to direct placement of the cornel lenses, might lead to hypoxic stress.

Other problems like epithelial cell mitosis and elevated glucosidase and protease activity might come with worsening corneal sensitivity, hydration, and transparency.

To reduce all these stresses, it’s good to consider the choice and shape of contact lenses. hydrogels, therefore, have been used to meet the requirements needed to make an ideal contact lens.

Hydrogel necessitates different physiological processes; therefore, one is always comfortable when using it. The following supports the use of hydrogels in contact lenses:

• Excellent mechanical properties
• Oxygen permeability
• The ability to make the surface wet
• It facilitates good optical properties
• Its non-toxic nature
• Good biological tolerance with the living cells
• Hydrolysis and sterilization stability

Additional uses of hydrogels

• Injectables are used for spinal cord regenerations
• Debridement of fibrotic and the necrotic tissue
• Used as a biosensor

Ultimately, hydrogels are more advantageous than other biomaterials since they have tunable biodegradability, increased biocompatibility, porous structure, and good mechanical strength.