INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 180

effi ciency, ability to deliver protein in a non-invasive manner, suitable for sensi-
tive proteins due to the mild gelling process, possibility of sustained or controlled
protein delivery and the absence of any organic solvents.
Even though most of the earlier works using hydrogels were focused at using
them as drug/protein delivery vehicles, the application of hydrogels as scaffolding
materials for tissue engineering has attracted much attention recently. The con-
cept of in situ generated implant strategy is highly unique and attractive since the
implants (either cell or factor loaded) can be processed in the operating room,
administered using a minimally invasive surgical procedure, and can be used to fi ll
irregularly shaped defects as the implant is formed at the defect site [Cheung
et al., 2007].
Hydrophilic polymers that can undergo gelation in response to different
types of stimuli have been developed either by modifying existing polymers or by
specifi cally designing polymers with stimuli sensitive units along the polymer
chain. The various stimuli of interest include light (photo-polymerizing/photo-
gelling systems), chemical agents (chemical and ionic cross-linking systems) as
well as environmental stimuli present under the physiological condition (tem-
perature, pH and ionic strength).
Visible or near ultra violet light induced photo-polymerization is one of the
most extensively investigated in situ gelation process for developing injectable
hydrogels. The photo-polymerizable systems are currently investigated for devel-
oping depot formulations [An and Hubbell, 2000], as biological adhesives [Ono
et al., 2000] as well as for orthopaedic tissue engineering [Elisseeff et al., 2001].
Various chemical as well as physical processes are being investigated that
enable the formation of hydrogels from hydrophilic polymers. These include
using chemical polymerization reagents [Holland et al., 2007] or cross-linking
agents as well as physical interactions between molecules.
Among the stimuli sensitive gelling systems, those responding to environ-
mental stimuli are considered more biocompatible as they are able to change
from liquid aqueous solutions to gel under physiological conditions without the
addition of any chemicals or external stimulus. Temperature sensitive gelling sys-
tems, particularly those undergoing thermal transition at or near physiological
temperature (37 °C), are the most extensively investigated and are the most pre-
ferred systems in this class [Ruel-Gariepy and Leroux, 2004].
6.3.1 Photo-Gelling Polymers
Photo-gelling biocompatible polymers form a versatile class of injectable biomate-
rials as the aqueous polymer solution can be introduced to the desired site via
injection followed by photo-curing in situ using fi ber optic cables and enabling a
controllable as well as rapid gelation at physiological temperature and pH
[Baroli, 2006]. Injectable photo-gelling systems have been extensively investigated
as protein/cell delivery vehicles and as scaffolds for tissue engineering.
Different processes have been investigated to develop hydrogels from photo-
labile polymers [Fisher et al., 2001]. The most extensively investigated processes