INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 181

include developing water soluble polymers with photoactive groups that can un-
dergo cross-linking when irradiated with light of appropriate wave length and
water soluble polymers with acrylic/methacrylic side groups that can undergo
photo-polymerization via a radical chain polymerization mechanism initiated by
appropriate photo-initiators.

6.3.1.1 Hydrogels from Polymers with Photoactive Side Groups. These
are mainly developed from hydrophilic polymers modifi ed with photo-labile
groups. Due to the mild gelation conditions and absence of non-toxic by-products
or reagents during photo-gelation, this is an attractive strategy for developing in-
jectable biomaterials. Most of the earlier studies using photo-gelling systems were
focused to develop occlusive dressings that conform to the contour of the wound.
Critical design criteria for these materials include fast gelation time, biocompat-
ibility and ability to gel in the presence of low intensity radiation as well as at a
narrow range of physiologically acceptable temperatures [Lu and Anseth, 1999].
Andreopoulos et al., have investigated unique chemical approaches to
modify the hydrophilic, biocompatible, as well as non-degradable polymer
“ poly(ethylene glycol) ” (PEG) to undergo photo - assisted cross - linking to form
hydrogels. These injectable hydrogels were investigated as enzyme immobiliza-
tion matrices, controlled drug delivery vehicles and antithrombogenic surfaces
[Andreopoulos et al., 1996, 1998, 1999]. Photo cross-linked gels were prepared
by the intermolecular photo-dimerization of various photo-labile groups attached
to polymers including cinnamylidene, nitrocinnamate and anthracene. The
PEG-nitro cinnamate system was found to be much more versatile than the
cinnamylidine system due to its better thermal as well as storage stability and
high photo - reactivity ( 350 times) [Zheng et al., 2001]. The effi cacy of PEG-

nitrocinnamate system as a basic fi broblast growth factor delivery system was
recently demonstrated. Long wave ultra violet radiation (365 nm) was used
for photo-gelation and the resulting gels were found to be non-toxic to
human neonatal fi broblast cells. The released growth factor maintained its
activity as well as induced fi broblast proliferation and collagen production
in vitro [Andreopoulos and Persaud, 2006].
In addition to synthetic polymers, natural polymers have also been modifi ed
to form photo-gelling systems. Ono et al., developed photo cross-linkable water
soluble chitosan with a photoactive azide group as a biological adhesive [Ono
et al., 2000]. Chitosan, a polyglycosamine derived from crustacean exoskeleton, is
a highly versatile polymer for chemical modifi cation due to the reactive amine
side groups. Azide groups are unique for photo cross-linking, since upon photo-
irradiation they will be converted into highly reactive nitrene groups which
can under go rapid insertion reactions to form covalent cross-links. The effi cacy
of the chitosan photo-gels was compared to fi brin glue, a clinically used bio-
adhesive. Compared to fi brin glue, the chitosan photo-gels were more effective
in sealing air leakage from pinholes on isolated small intestine and aorta as well
as from incisions on isolated trachea. Moreover, the gels were found to be non-
toxic towards human skin fi broblasts, coronary endothelial cells and smooth

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