Hydrogels

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It is also referred to as "Aquagel".

An extremely hydrated polymer gel i.e. colloidal gel giving a jelly like appearance. The polymer chain holds many times its weight in trapped water [1] which can be upto 99%. Hydrogel has water as the dispersion medium or continuous phase.

(N. A. Peppas et al.) Hydrogels resemble, to a large extent, a biological tissue as they are hydrophilic and have the ability to imbibe large amounts of water and other biological fluids. They have a three dimensional macromolecular network. They are insoluble due to chemical and/or physical crosslinking as entaglements and crystallites. The crosslinks are formed by covalent bonds.
Components of hydrogel:
Hydrogel is made by the combination of a hydrophilic component and water i.e.

Hydrogel = Hydrophilic component +  Water

Types:
Hydrogels are
1. natural (Methylcelloluse and agarose)
2. synthetic (Polyacrylamide and polymetha-acrylamide)

(E. Tichy et al.) Two different types of hydrogels:

1. One based on Carbomer.

2. Second based on Polyacrylamide.

Two factors responsible for the extent of swelling and for retaining the water content are as follows:

1. The hydrophilicity of the polymer chain.

2. The crosslinking density. [2]

Properties of Hydrogels:
(Stephen D. Bruck) Properties that can be used to differentiate the hydrogels are

1. Permeability and diffusion co-effecients

2. Some role of quasiorganized water within the hydrogels.

3. Porosity.

4. Presence of functional groups.

5. Types and number of crosslinks.

6. Chemical entities.

Uses:
Hydrogels are very absorbent in nature.
They are also used in sustained release drug delivery systems.

References:
[1]http://www.nature.com/nrmicro/journal/v2/n2/glossary/nrmicro821_glossary.html

[2] 7.2, Engineering Materials For Biomedical Applications (Biomaterials Engineering and Processing Series) by Teoh Swee Hin

E. Tichy, Z. Vitkova, B. Cupkova, Effect of beta-(1,3)-glucan on rheological properties and stability of topical formulations. Die Pharmazie, 2006 Dec;61(12): Pages 1050-1.

N. A. Peppas, P. Bures, W. Leobandung and H. Ichikawa, Hydrogels in pharmaceutical formulations. European Journal of Pharmaceutics and Biopharmaceutics , Volume 50, Issue 1, 3 July 2000, Pages 27-46

Stephen D. Bruck, Aspects of three types of hydrogels for biomedical applications. Journal of Biomedical Materials Research(Volumes 1-23), Volume 7 Issue 5, Pages 387 - 404

Further Reading:
Biodegradable Hydrogels for Drug Delivery by Haesun Park, Kinam Park and Waleed S. W. Shalaby

Hydrogels and Biodegradable Polymers for Bioapplications (Acs Symposium Series, 627) by Raphael M. Ottenbrite, Samuel J. Huang and Kinam Park

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