INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 178

• provide a non-adhesive surface towards cells and allow for programmable
cell adhesion upon attaching biological molecules to the hydrogel [Ratner
et al., 2004 ].
• control the mechanical and physical properties by varying the cross-linking
density as well as the type of cross-links.
• maintain a certain extent of structural integrity and elasticity.
• allow for increased diffusion of nutrients into the gel and cellular waste out
of the gel due to the high equilibrium swelling when used as a cell delivery
vehicle.
The commercial success of soft contact lenses raised signifi cant interest in
hydrogels as biomaterials. Subsequently, a wide range of hydrogels were devel-
oped using synthetic and natural hydrophilic polymers for various biomedical
applications such as drug, gene, and protein delivery vehicles, as unique wound
dressing materials, as well as scaffolds for tissue engineering [Peppas and Sahlin,
1996]. Hydrogels are highly suitable as wound dressings due to their ability to
maintain a moist wound environment, to absorb exudate drainage, to allow oxy-
gen transport, high permeability to provide appropriate medication (analgesia,
anti - infl ammatory/antibiotics) and to be easily removed without patient discom-
fort due to their low adherence to tissue. Hydrogels, due to their highly swollen
three-dimensional environment with large pore size, porosity and high water
content, closely resemble the environment of native tissue extracellular matrix
(ECM). Hydrogels are therefore considered as potential materials for developing
scaffolds for tissue engineering. Hydrogels are also suitable as protein delivery
vehicles due to their ECM mimicking hydrated matrix, mild gelation process and
high permeability.
Several synthetic non-degradable hydrogels based on methacrylate and
polyethylene glycols were developed following the studies of Dr. Wichterle.
Poly(ethylene glycol) (PEG) is one of the most extensively investigated synthetic
hydrophilic polymers for biomedical applications due to its hydrophilicity,
good tissue compatibility, non-toxicity and availability of reactive end groups for
chemical functionalization. Among the natural hydrophilic macromolecules
investigated for hydrogel formation, polysaccharides form the most prominent
members [Coviello et al., 2007]. This is due to various advantages of polysaccharides
such as non-toxicity, biocompatibility, availability in large variety of composition
and properties, wide presence in living organisms, as well as due to the fact that
they can be produced using recombinant DNA techniques [Coviello et al., 2007].
In addition to the wide range of hydrogels developed so far, these studies also
led to the development of a major class of hydrogel system with unique stimuli
sensitive properties for therapeutic and diagnostic applications. Stimuli sensitive
polymers exhibit property changes in response to an external stimulus such as
temperature, light, pH, salts, solvents, electric fi eld, chemical as well as internal
stimulus such as biochemical agents [Hoffman, 1991]. Among these, polymers
sensitive to temperature, light, chemical and biological cues are highly preferred