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Module 1: Release Systems and Hydrogels

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Lesson Summary:
Hydrogels- Three dimensional, hydrophilic polymer networks capable of swelling by absorbing large amounts of fluid.
-can expand to thousand of times their dry weight in fluid
-Insoluble due to physical or chemical crosslinks which provide the network structure and physical integrity
- bioerodible or non-erodible
-Numerous Applications


Advantages of Swelling-controlled Matrix Hydrogels
Low burst effect
Known or predictable swelling rates
Reformulation of vehicle not necessary for different drugs
Disdvantages of Swelling-controlled Matrix Hydrogels
Generally short release periods
Not suitable for all delivery routes or targets

Classification of Hydrogels:

Physical Hydrogels:

- polymer network held together by neutral or ionic bonds
- Not Homogeneous, clusters of molecular entanglements, or hydrophobically/ionically associated
- Single physical cross-link, formed by hydrophobic association/Van der Waals bonding, ionic bonding, or hydrogen bonding between two monomers in water (lower strengths than singular covalent bond)
- Formation of transiently stable hydrogels requires block copolymer structures where cooperative binding can occur
- Formation of one physical bond is immediately followed by bonding of several other adjacent monomer units

Ionic Hydrogels:

- Polymer chains within hydrogels contains pendant groups that are cationic or anionic
- Ionic- Gels are unionized above (cationic) or below (anionic) the pKa of the groups. For pH above (anionic) or below (cationic) pKa, the pendant groups are ionized and repel each other = large swelling