INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 177

6.1 OVERVIEW
Injectable in situ cross-linkable gels are clinically highly desirable as they can be
introduced into the body via a minimally invasive manner using endoscopic or
percutaneous procedures. The development of ideal in situ gelling injectable
system for biomedical applications that satisfy various requirements such as
biocompatibility, and fast appropriate physical and mechanical properties as
well as water content, and tunable degradation gelation kinetics is challeng-
ing. Several polymeric systems that respond to stimuli such as light, pH,
ionic concentration, chemical/physical reaction as well as temperature are
currently under development as injectable drug/protein/DNA delivery vehicles
as well as cell carriers for tissue engineering. This chapter focuses on some of
the recently developed injectable hydrogel systems that can undergo solid
to gel transformation in response to light irradiation, chemical or physical
reaction and temperature. Both synthetic and natural polymeric systems are
discussed.
6.2 INTRODUCTION
Hydrogels are three-dimensional cross-linked hydrophilic polymer networks that
have the ability to swell but do not dissolve in water. The unique swelling behav-
ior and three-dimensional structure of hydrogels are derived from specifi c cova-
lent chemical cross-links or a variety of physical cross-links (secondary forces,
chain entanglement, crystalline formation) and therefore can be appropriately
controlled. The interest in hydrogels for biomedical applications started during
the later half of the twentieth centaury when Wichterle and Lim predicted the
potential of hydrogels as unique biomaterials since they are highly compatible
with living tissue [Wichterle and Lim, 1960]. Using 2-hydroxyethyl methacrylate
and glycol dimethacrylate as model polymers, they demonstrated the ability
to rationally design hydrogels for biomedical applications. The study also
demonstrated the excellent tolerance of the body towards hydrogels; the non-
degradable hydrogels got encapsulated in a fi brous capsule without eliciting any
irritation to the surrounding tissue upon subcutaneous implantation. These
hydrogels were subsequently used for developing soft contact lenses and the
biocompatibility was clinically proven. The excellent biocompatibility of hydro-
gels can be attributed to some of its unique properties as summarized by Ulijin
et al., [Ulijin et al., 2007]. These include their ability to:
• Retain a large quantity of water within the matrix, enabling them to behave
quite similar to natural living tissues [Hoffman, 2002].
• provide a semi - wet, three - dimensional environment for molecular - level
biological interactions [Zhang, 2004].
• provide inert surfaces that prevent non - specifi c adsorption of proteins.