Biomedical Engineering Reference
In-Depth Information
Fig. 3
Schematic of hydrogel formation. Crosslinked network is created through physical
associations, ionic bonds, or covalent bonds
the osmotic forces and the elastic retractivity, defines the properties of the
gels. Depending upon their chemical composition, crosslink density, and hy-
drophobicity, hydrogels can vary in consistency from viscous fluids to fairly
rigid solids. Basically, hydrogels are wet, soft, and capable of undergoing
large deformation. Thus, hydrogels are very similar in nature to mammalian
tissues, which are essentially aqueous gels made up of proteins and polysac-
charide networks. The hydrogel-like nature of tissues helps the organisms to
transport oxygen, nutrients, and other bioactive moieties easily and effec-
tively while retaining their solid nature.
Chemically crosslinked hydrogels are usually created by radical polymer-
ization which enjoys the presence of a vinyl moiety in the starting monomer
and oligomer mixtures [94, 95]. Hydrogels have also been created by tak-
ing advantage of the reaction between different functional groups present
in the reacting oligomers [96-98]. Recently, Sperinde et al. have reported
the formulation of a novel PEG-based hydrogel in which the network is
formed by enzymatic crosslinking which uses transglutaminase, a natural
enzyme, as a catalyst [99, 100]. Tranglutaminase is a calcium-dependent en-
zymeanditisubiquitousinbodyfluid[99].Anotherapproachthathas
been widely used to create chemically crosslinked hydrogels involves pho-
topolymerization through exposure to UV light [101], visible light (Varghese
and Hill, 2005, personal communication), and
-radiation [102]. Other ap-
proaches involving ionic interactions [103, 104], hydrophilic-hydrophobic in-
teractions [105-108], crystallization [109-111], hydrogen bonding [112, 113],
molecular recognition [114, 115], and self-assembly [116-120] have been
widely explored to create physically crosslinked hydrogels.
Conventional hydrogels are known for their super-absorbing properties
and they have found applications in personal care products and agricultural
aids [121, 122]. There is another class of hydrogels known as “smart” hy-
drogels. The smartness of these hydrogels is characterized by their ability
to respond to external stimuli such as temperature, pH, chemical residues
γ
