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ABSTRACT
Hydrophilic gels are also called as Hydrogels, in which dispersion medium is water. Hydrogels consist of three dimensional networks of hydrophilic homopolymers and co-polymers. Some of the properties of hydrogels are: smartness, softness, swelling, elasticity, absorbent nature, flexibility and the capacity to store water. Hydrogels are natural as well as synthetic. Synthetic hydrogels have replaced natural hydrogels due to their higher water absorption capacity, stability and hydrophilic nature. Hydrogels can be prepared by swelling cross linked polymers in water or in biological fluids containing water. They can be prepared from polymers containing hydrophilic group such as –OH, -COOH, -SO3H, -CONH, -CONH2 either embedded in or grafted to their polymeric back bones. Hydrogels are useful in daily life and their specialized applications are in food, industry, agriculture, cosmetics, medicine and medical treatments etc. Materials that imbibe large volume of water are very useful in daily life such as in glue, napkin, diaper, contact lenses. Aloe Vera cosmetics herbal hydrogels and bioadhesive hydrogels are commonly used in cosmetics. In agriculture hydrogels can be used for amending water storage of sandy loam, loam oil and seedling growth in plants. Hydrogels films are also used for food packaging and wrappings. Other major applications of hydrogels are in medical field such as, in wound dressing, in contact lenses, in breast implants, in drug delivery and diagnosis as in soft tissue substitutes, in tissue engineering scaffolds, in cardiac regeneration, glaucoma treatment. Nerve repair, bone re-engineering, cartilage formation and also as solid supports for holding up collapsed organ. Now a day, hydrogels find wide range of applications because of their lower cost, non- toxic, and eco friendly nature.
INTRODUCTION
Hydrogels are cross linked polymers with the very good ability to swell in a liquid medium. It is three dimensional cross-linked polymer network made up of one or more than one type of monomers by simple reactions. Cross-linking may be formed by Physical or Chemical cross-linking. Physical hydrogels are reversible due to the conformational changes, Chemical hydrogels are permanent and irreversible as a result of configurational changes. [1] Three dimensional architecture could be homo-polymeric, co-polymeric, Semi-interpenetrating and Interpenetrating polymer networks based on preparation methods. Wichterle and Lim (in 1954) synthesized first synthetic hydrogel. [2] Hydrogels can absorb and hold large amount of liquid in their cross linked structure but they are insoluble in water. They can be prepared from polymers containing hydrophilic group such as –OH, -COOH, -SO3H, -CONH, -CONH2 either embedded in or grafted to their polymeric back bones. It is colloidal gel in which water is the dispersion medium. They possess a high degree of flexibility. After swelling they become soft, rubbery and resemble the living tissue and have excellent biocompatibility. Hydrogels are very smart to respond the change in stimuli (Ph, Temperature, Ionic Strength, Electric field, etc.). Hydrogels are hydrophilic gels which are widely applicable in these days in number of industrial and environmental areas. [3]
Hydrogels are of both types Natural and Synthetic. Synthetic type hydrogels replacing fastly the Natural types due to their higher water absorption capacity, low cost, long service life etc.
2. CLASSIFICATION OF HYDROGEL PRODUCTS
The hydrogel products can be classified on different bases as explained below [4,5]:
(a) Classification based on source
Hydrogels classified into two groups based on their natural or synthetic origins
.
(b) Classification according to polymeric composition
The method of preparation makes some important types of hydrogels. These can be categories as following:
(i) Homopolymeric hydrogels are referred to network derived from a single type of monomer, which is a basic structural unit comprising of any polymer network. Homopolymers may have cross-linked structure depending on the nature of the monomer and polymerization technique.
(ii) Copolymeric hydrogels are comprised of two or more different types of monomer with at least one hydrophilic component, arranged in a random, block or alternating configuration to form the chain of the polymer network.
(iii) Multipolymer interpenetrating polymeric hydrogels (IPN), an important class of hydrogels, is made of two independent cross-linked synthetic and/or natural polymer , basically prepared in a network form. In semi-IPN hydrogels, one component is a cross-linked polymer and other component is a non-cross-linked polymer
© Classification based on configuration
On the basis of their physical structure and chemical composition they can be classified as follows:
(i) Amorphous (non-crystalline) type.
(ii) Semi crystalline type.
(iii) Crystalline type.
(d) Classification based on type of cross-linking
Hydrogels can be divided into two separate categories based on the chemical or physical nature of the cross-link junctions. Chemically cross-linked networks have permanent junctions, while physical networks have transient junctions that made from either polymer chain entanglements or physical interactions .
(e) Classification based on physical appearance
Hydrogels appearance as matrix, film, or microsphere depends on the technique of polymerization that involved in the steps of preparation process.
(f) Classification according to network electrical charge
Hydrogels may be categorized into four groups on the basis of presence or absences of electrical charge on the cross linked chains. They are:
(a) Neutral.
(b) Ionic ( anionic or cationic).
© Amphoteric electrolyte (ampholytic) containing both
Acidic and Basic groups.
(d) Zwitterionic (polybetaines) containing both anionic and
cationic groups in each repeating unit.
3. SOME EXAMPLES OF DIFFERENT TYPES OF HYDROGELS
These are given some hydrogels, which are widely used in different areas of our daily need. They are: Hydrogels Based on Acrylamide Derivatives, PEG-Based Hydrogels, Acrylic Based Hydrogels, Chitosan-Based Hydrogels, Hydrogels Based on Alginic Acid, Scleroglucan-Based Hydrogels, Hydrogels Based on Hyaluronic Acid, Pectin Based Hydrogels, Poly (Ethylene Glycol) Hydrogels, Starch-Based Hydrogels, Hydrogels Based on Polyvinylpyrrolidone.
4. IMPORTANT FEATURES
Hydrogels are characterized by their ultimate capacity to absorb and hold liquids (swelling thermodynamics), the rate at which the liquid is absorbed into their cross-linked structure (swelling kinetics), as well as their mechanical property in wet or hydrated state (wet strength). Hydrogels contains many important types of features which makes it a novel material in different areas of application. Some important features are as follows: [6]
1. The highest absorption capacity
2. The lowest soluble content and residual monomer
3. The lowest cost
4. Durable and stable in swelling medium
5. Biodegradable nature
6. Colorless, Odorless and absolute nontoxic in nature
7. Re-wetting capability present
8. Stability in different environment where placed
5. APPLICATIONS
Hydrogels are in application from last 50 years due to their excellent properties which are helpful in using wide range of areas. [7] Various hydrogels have been prepared for various applications in pharmaceutical and biomedical fields their resemblance to living tissue makes many opportunities for applications in general and biomedical areas. Currently, hydrogels are used for manufacturing contact lenses, hygiene products, tissue engineering, scaffolds, drug delivery systems, agriculture and wound dressings.[8-15]
(5.1) Hydrogels in Drug Delivery
Hydrogels have various properties for their use in Drug Delivery. It can be easily be adjusted by changing the density of cross-links in their matrix and the affinity aqueous solution. They give sustained release to drug. loading of drug to hydrogels is very easy and can be done by several mechanisms. It can also act as reservoir of drug. It is very suitable for oral drug delivery also.
(5.2) Tissue Engineering
Tissue Engineering is the improvement of specific tissues or organs using engineered materials and synthetic strategies. In this, Hydrogels act as space filling agents. Scaffolds of hydrogels have also been applied to transplant cells and to engineer large number of tissues in the body including cartilage, bone and smooth muscle. Some examples of hydrogels for this purpose are Poly (ethylene oxide), Poly (acrylic acid), Polypeptides, Agarose, Chitosan, etc.
(5.3) Hygiene Products
Hydrogels as superabsorbent polymers (SAPs) have been commercially produced in Japan in 1978 for use in Napkins. SAPs are widely used in making Diapers and many domestic products. They are also able to stop growth of microorganisms by locking the urine in their cross-linked structure.
(5.4) Contact Lenses
Wichterle and Lim were the first to describe a hydrogel based on poly-2-hydroxyethylmethacrylate (PHEMA) as a synthetic biocompatible material useful for contact lens applications. According to their elasticity Contact lenses are mainly classified as ‘hard’ or ‘soft’ .Hard lenses are made up of Hydrophobic material but Soft lenses are made by Hydrophilic material mainly Hydrogels. They can easily adjust and function in eyes due to their flexible and soft nature.
(5.5) Wound Dressing
Hydrogels are widely used as debriding agents, moist dressings, and components of pastes for wound care. One example of commercially using hydrogel is Granugel (Conva Tec).It is a clear, viscous hydrogel for the management of partial and full-thickness wounds, may be used as filler for dry cavity wounds to provide a moist healing environment.
(5.6) Some other applications
Hydrogels are the group of many different types of chemically composed hydrophilic gels that are using also as flocculator and thickener .A very good example is Polyacrylamide (PAAm) based hydrogels. They are widely using as adhesive in paper industry ,thickener in oil recovery, in food preparation, as flocculator in sewage water treatment, in hydrometallurgical process, controlling soil erosion, fire retardant, in gel electrophoresis and in preparation of Cosmetics. It is also used to reduce drag and friction in multiple pipelines containing both oil and water.
CONCLUSION
Recently, hydrogels are studied for different type of structure and applications. Now, we can say that Hydrogels are one of the most important material for development of human lifestyle. It is applicable in all the areas of need without having any harmful aspects. Hydrogels can prepare very easily and with low cost so easily available at the time of demand. Swelling and mechanical features of hydrogel polymers have enabled them to find extensive applications in all the areas. Desirable hydrogel properties for a particular application can be achieved by selecting a proper hydrogel material, crosslinking method, as well as processing techniques.