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Hydrogels are new generation smart materials with multiple uses and applications. In this review we have given a glimpse on the preparation and classification of hydrogels along with the latest developments and advancement in this field which is proving to be very beneficial and a turning point in the field of science and technology.
Hydrogels are made up of polymers which are arranged in a 3D network. They are hydrophilic in nature but do not get dissolved in water. They have physical property of swelling and therefore they can absorb a very large quantity of water. Nowadays synthetic hydrogels are being preferred more over natural hydrogels because of its many characteristics features like it can absorb large amount of water. Gel strength of synthetic hydrogel is higher than the natural hydogels. They can sustain for a long period of time and are stable. More importantly many modifications can be done in synthetic polymers.
a) Based on their origin: They are natural that is they are derived from natural sources and synthetic, which means they are synthesized in the laboratory
b) Depending on the constituent monomer present: On this basis hydrogels can be grouped as homopolymer hydrogel which constitute of single monomer species, copolymer hydrogel which constitute of two or more distinct monomer species and multipolymer interpenetrating polymeric hydrogel in which two independent components of either natural or synthetic polymers are cross-linked to form a network.
c) Configuration based hydrogels: These could be classified as amorphous, semi crystalline, and crystalline hydrogels
d) On the basis of cross linking between polymers: These can be categorized as hydrogels where polymers are chemically cross-linked to each other and the ones in which polymers are physically cross linked to each other.
e) Based on appearance: Based on the different type of techniques employed for the process of polymerization for preparation of hydrogels, they can be categorized as films or matrix.
f) On the basis of charge present on cross-linked polymers they could be non ionic hydrogels, ionic hydrogels, amphoteric hydrogels and zwitter ionic hydrogels.
Hydrogels are commonly prepared by the process of polymerization where a hydrophilic (water soluble) monomer is reacted with a multifunctional cross linker in presence of chemical initiator. The formed hydrogel is then washed to get rid of impurities. Also, direct linkage of water soluble polymer could be achieved in presence of a cross linking agent or in presence of radiation to form a hydrogel.
They are used in hygienic products like sanitary napkins, diapers etc. They are used in drug delivery system. They are used for dewatering of coal. They are used for formation of artificial snow. They have biomedical application like they are used in diagnosis, wound dressing, and for developing regenerative medicines. Nowadays they are highly used for making soft contact lenses. The most recent application of hydrogel is in tissue engineering where they are used as synthetic substitute for an injured or damaged tissue. They are used in sensors as a matrix membrane whose properties like hardness, elasticity, and permeability can be modified.
Hydogels as Effective Drug Delivery System
They are highly porous so drugs can be easily loaded and released from hydrogels. Latest research has been made to improve the delivery system of hydrogels by forming formulations which can exist as polymer solution outside the body to allow easy injection into body and can also exist as gel in the body to increase the rate of release of drug. Drug release can also be controlled by using pH sensitive hydrogels. This type of drug delivery is useful for oral administration of medicine and for masking the bitter taste of medicine. [The pH affects the swelling of Hydrogels, thus, by increasing and decreasing the ph, the amount of swelling can be kept under control which inturn could be used to control and affect the drug release.
Hydrogels In Tissue Engineering
Nowadays, lots of research has been done in direction of using hydrogel as a replacement or a space filling agent for injured tissues and organs. These hydrogels can be naturally derived (eg Agarose, collagen etc) or synthetic (PEO, PVA, PAA). Use of biodegradable Hydrogels is being done for making scaffolds in tissue engineering for growing cells and tissues. In addition to this, many hydrogel scaffolds are used in transplantation of cells and for engineering the growth of many tissues in human body like bone, cartilages, smooth muscles etc. 8. Super porous Hydrogels (SPH)Super porous hydrogels are rapidly swelling materials and highly superabsorbent which are currently being destroyed to be used as gastric retention device. For this purpose, their mechanical strength has been improved by making use of composite materials. The super absorbability of these super porous Hydrogels is due to their porous structure which is open, due to which they swell up easily after coming in contact of water irrespective of their size in dried state. SPHs are grouped into three categories namely; first generation SPHs which have fast and highly swelling rate but have weak mechanical strength. The second generation SPH (SPH composites) has fast & medium swelling but improved mechanical strength. The third generation SPH (SPH hybrids) are the ones which possess elastic properties.
Other Advancements in field of Hydrogels
Hydrogels also have been used for the engineering and development of extracellular matrix which resembles the features and functioning of the native extracellular matrix which functions as a support and helps in regulating the growth of mammalian cells. The latest demand of the industry today is flexible electronic devices. For this, conducting polymer based hydrogels are proving to be very beneficial which show stretchable and transparent properties. Pan et al. have prepared one such conducting structure of hydrogel (poly aniline, PAni ) having excellent electronic conducting but lacks in optical properties. Further Spinks and co-workers have prepared a hydrogel which is both electrically conductive and high in mechanical strength but its shortcoming is being its opaque nature, hence showing no optical properties. Thus, research on inventing a hydrogel which is highly conductive, stretchable and transparent is still under process and next in line of inventions in the field of hydrogels.
Hydrogels are cross linked polymers substances which have the ability to retain large amount of water in them. For this reason they are used in various fields like as absorbents for hygienic products, wound dressing etc. Their structures can be modified easily so they are employed as drug delivery vehicle, in tissue engineering, in regenerative medicines etc.
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