INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 183

UV and visible initiating systems, respectively. The cytocompatibility of Darocur
2959 was further confi rmed by encapsulating chondrocytes in photo cross-linked
PEG based gel prepared using 0.05% (w/w) Darocur 2959. When photo-
polymerized for ten minutes with approximately 8mWcm − 2 of 365 nm light,
nearly all the chondrocytes survived in Darocur 2959 initiated gels. The good
cytocompatibility of Darocur 2959 (also known as Irgacure 2959) was further
confi rmed using six different cell populations that are commonly used for tissue
engineering applications [Williams et al., 2005]. The study confi rmed that Irgacure
2959 cause minimal toxicity over a broad range of mammalian cell types and
species. However, it has to be noted that different cell types showed variable
responses to identical concentrations of Irgacure 2959 with fast dividing cells
being more sensitive towards the initiator.
Due to the availability of less-toxic photo initiators, extensive research
has undergone to develop photo-polymerizable injectable systems. Most of the
studies were focused on synthetic hydrophilic polymer “ PEG. ” The PEG - based
polymers are highly preferred due to their availability in wide range of molecular
weights as well as functionality, and consistent composition that allows the devel-
opment of predictable gel properties [Cushing and Anseth, 2007]. The feasibility
of transdermal photo-polymerization of polyethylene oxide (PEO) has been
demonstrated [Elisseeff et al., 1999]. The study demonstrated that transdermal
photo-polymerization through human skin is possible and is most effi cient using
visible light in the presence of visible light photo initiators. Mathematical model-
ing predicted that only two minutes of light exposure are required to photo-
polymerize an implant underneath human skin. The effi cacy of the system as a
minimally-invasive injectable construct for plastic surgery, tissue engineering
scaffolds, as well as drug delivery depots, was demonstrated using various animal
models [Elisseeff et al., 1999].
One of the most extensively investigated applications for injectable photo
cross-linkable systems is as cell delivery vehicles due to their ability to control
gel formation, undergo gelation at milder conditions, allow localized delivery,
protect the cells from the immediate external environment, and allow nutrient
diffusion. Since materials that are capable of degrading in vivo are preferred for
drug delivery and tissue engineering applications, most of the studies were fo-
cused on developing degradable, injectable photo cross-linking systems. Degrad-
able, injectable and photo cross-linkable gel systems are usually prepared by in-
corporating ester groups [Davis et al., 2003; Sawhney et al., 1993] or
enzymatically cleavable peptide linkages in the cross-link segments [West and
Hubbell, 1999; Lutolf et al., 2003; Kurisawa et al., 1995].
Anseth, et al., demonstrated the greater effi ciency of a degradable synthetic
photopolymerizable hydrogel compared to a non-degradable hydrogel as a con-
struct for cartilage-tissue engineering. Degradable injectable hydrogels were
developed by photo polymerizing poly(ethylene glycol dimethacrylate) with a
hydrolytically sensitive triblock copolymer, poly(lactic acid)- b - ethylene glycol) -
b -poly(lactic acid) with acrylate end groups. Darocur 2959 was used as the pho-
toinitiator. In vitro cell encapsulation studies using chondrocytes demonstrated