The research on hydrogels is more than four decades old but there has been a tremendous growth in the recent past because of their unique bulk and surface properties. They form the basis of many novel drug delivery systems.Hydrogels can be made to respond to the environment and the extent of the response can be controlled.The environmental conditions to which a hydrogel can be made responsive pH,temperature,electric field,ionic strength,salt type,solvent,external stress,light or a combination of these.
It is because of these unique properties that these classes of polymer based systems embrace numerous pharmaceutical and bio medical applications. Hydrogels are water swollen three dimensional structures composed of primarily hydrophilic polymers. These are cross linked macro molecular networks that are insoluble but are able to swell rapidly in water or biological fluids.
Classification of hydrogels:
Based on the method of preparation, hydrogels are classified into:
A) Homopolymer hydrogels
B) Co-polymer hydrogels
C) Multi polymer hydrogels
Based on the ionic charges hydrogels can be classified into :
A) Neutral hydrogels
B) Anionic hydrogels
C) Cationic hydrogels
D) Ampholytic hydrogels
Based on the structure hydrogels can be classified into :
A) Amorphous hydrogels
B) Semi-crystalline hydrogels
C) Hydrogen bonded hydrogels
Based on the mechanism controlling the drug release they are classified into:
A) Diffusion controlled release systems
B) Swelling controlled release systems
C) Chemically controlled release systems
D) Environment responsive systems
Structure of hydrogel : The structure of an ideal network of tetra functional covalent cross links.
Materials used in the preparation of hydrogels :
Different polymers can be used for the development of hydrogel systems including cellulose derivatives, natural gums, polycrylates and gelatin.
Preparation : Hydrogels can be prepared by swelling cross linked structures in water or in biological fluids containing water.Commonly hydrogels are prepared by free radical polymerization of hydrophilic vinyl polymers.Cross links can be induced by radiation. Radiation reactions include electron beams, gamma rays, x rays or U.V. light.
Effect of hydrogels water content on chondrocyte signalling.
Opthalmic use : The artificial cornea is made of a dual network hydrogel with a clear center and peripheral pores. Cells infilterate the pores and secrete collagen, which integrates artificial cornea into the surrounding natural tissue.