Edited by Pichet Adstamongkonkul, AP225, Fall 2011
Since the pioneering work of Wichterle and Lim in the 1960’s on crosslinked HEMA hydrogels, many more studies have been focused on developing hydrogels for a wide range of applications. Hydrogel is a three-dimensional network of hydrophilic polymer chains held together by association bonds such as covalent bonds and weaker cohesive, or intermolecular, forces. They may be chemically stable or may degrade and eventually disintegrate, depending on applications. Therefore, the hydrogels have a degree of flexibility as they have the ability to absorb water and swell, which is similar to natural tissue, due to their significant water content (about 99%).
There are a variety of polymers being used to form hydrogels. These polymers are water soluble, but during the hydrogel synthesis, they are linked by the crosslinker molecules or physical entanglement, and retain the overall structure.
Those hydrogels formed by physical entanglements or crosslinking physically, they are called “physical” or “reversible” hydrogels. There are a number of crosslinking mechanisms involved in forming this type of hydrogels:
- crosslink by ionic interactions
- crosslink by crystallization
- crosslink by hydrophobic interaction
- crosslink by hydrogen bond formation
- crosslink by other specific interactions
Physical hydrogels are not homogeneous, since the molecular entanglements or hydrophobic or ionic domains can form clusters, creating inhomogeneities.
One example of the hydrogels in this class is calcium alginate hydrogel. When a polyelectrolyte is exposed to multivalent ions of the opposite charge, this so-called “ionotropic” hydrogel is formed. The gelation or precipitation depends on the concentration of the ions, the ionic strength, and pH of the solution. The interactions in these hydrogels are reversible, which means they can be disrupted by changes in the physical conditions, including stress, temperature, and pH.
The hydrogels are called “permanent” or “chemical” gels if they form covalently-crosslinked networks. The crosslinkers are small molecules with functional groups, which could be polymerized with the macromers, forming bridges that connect these lengthy polymer chains and eventually result in a hydrogel network. This class of hydrogels can be formed by crosslinking water-soluble polymers or by conversion of hydrophobic polymers to hydrophilic polymers with the addition of crosslinkers. There are many ways one can polymerize the crosslinks:
- Radical polymerization – the crosslinks have double bonds as the functional groups
- Condensation reactions
- Chemical reaction of complementary groups – aldehydes/amine/amide/alcohol
- Enzymatic reactions
One can design these hydrogels to be responsive to the external stimuli, such as pH and temperature, in order to release drugs or other components in a controlled manner. The following is the list of some examples of these hydrogels
- pH-sensitive hydrogels - consisting of poly(acrylic acid) and poly(N,N’- diethylaminoethyl methacrylate)
- Matrix Metalloproteinase (MMP)-responsive hydrogels – the network is cleaved by MMP enzymes, locally respond to the local protease activity at the cell surface
- Light-sensitive hydrogels – involves in the fabrication of artificial lumen inside the hydrogels