INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 182

muscle cells. The accelerating effect of chitosan-azide on wound healing was eval-
uated using a full thickness skin incision on the back of mice and subsequently
covered with the polymer solution followed by photo-irradiation for 90 s . Signifi -
cant wound contraction, accelerated closure, and healing was observed. Histo-
logical examination demonstrated the formation of advanced granulation tissue
and epithelialization on chitosan hydrogel treated wounds [Ishihara et al., 2001].
The ability of the hydrogel to perform as a fi broblast growth factor delivery sys-
tem while functioning as a wound dressing has also been demonstrated using
healing-impaired diabetic mice [Obara et al., 2003].
Another interesting photo-gelling system developed by Haines et al., employs
a uniquely designed peptide that can self-assemble into a hydrogel by forming
intramolecular folded conformational state [Haines et al., 2005]. The peptide (2%
w/v) when dissolved in aqueous medium remains unfolded and is stable to ambient
light. Irradiation of solution with light

360 nm opens the photo - cage to trigger
peptide folding to produce amphiphilic beta-hairpins that assemble to form a visco-
elastic hydrogel. The hydrogel was also found to be cytocompatible as evidenced
from in vitro studies [Haines et al., 2005].
These studies demonstrate the effi cacy of photo-gelling injectable systems as
protein delivery vehicles and wound dressings due to their mild gelation behavior
and controllable swelling properties.

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6.3.1.2 Hydrogels by Photopolymerization. The most common strat-
egy for developing an injectable photo-polymerizable system is by using a
water soluble macromer having pendant acrylate groups along the polymer chain
or as end groups. These photo-sensitive macromers are then polymerized in the
presence of photo initiators (with or without photo sensitizers) upon exposure
to long wavelength ultraviolet or visible radiation. Photo-polymerization further
provides the benefi t of spatial and temporal control of polymerization through
controlling when and where the sample is exposed to the initiating light source.
One of the major concerns in developing photo-polymerizable systems
as injectable biomaterials is the toxicity of the photo initiators. Bryant et al.,
investigated the cytocompatibility of several photoinitiators using cultured
fi broblast cell lines [Bryant et al., 2000]. Photo initiators investigated include
UV initiators such as 2,2-dimethoxy-2-phenylacetophenone (Irgacure 651), 1-
hydroxycyclohexyl phenyl ketone (Irgacure 184), 2 - methyl - 1 - [4 - (methylthio)
phenyl] - 2 - (4 - morpholinyl) - 1 - propanone (Irgacure 907), and 2 - hydroxy - 1 - [4 -
(hydroxyethoxy)phenyl] - 2 - methyl - 1 - propanone (Darocur 2959). The visible light
initiators included camphorquinone (CQ), ethyl 4 - N,N - dimethylaminobenzoate
(4EDMAB) and triethanolamine (TEA) and the photo sensitizer isopropyl
thioxanthone. At low photo initiator concentrations ( 0.01%w/w), all the initia-

tors were found to be cytocompatable except CQ, Irgacure 651 and 4EDMAB.
When the photo initiators were coupled with low intensity initiating light
(approximately 6 mWcm − 2 ) of 365 nm UV light and visible light of 470-490 nm
(approximately 60 mWcm − 2 ), Darocur 2959 at concentrations

<

< 0.05% (w/w) and

CQ at concentrations < 0.01% (w/w) were found to be the most cytocompatible