INJECTABLE HYDROGELS AS BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 185

Figure 6.2. A dense plexus of processes emerged radially from aggregates to penetrate and
grow through the hydrogel environment. Scale bar represents 20 mm.

mesenchymal stem cells in the subcutaneous tissue via transdermal photo polym-
erization [Sharma et al., 2007a]. High molecular-weight hyaluronic acid (HA) was
used as a visco-supplement to increase the effi ciency of injections and to hold the
solution at the injection site during photopolymerization. Injectable PEG hydro-
gel was also used to deliver transforming growth factor-

3) to induce
chondrogenesis of the mesenchymal stem cells. The study demonstrated the fea-
sibility of forming cartilage tissue in situ using stem cells and appropriate inject-
able biomaterial. Figure 6.3 shows photo-polymerized hydrogel in subcutaneous
tissue after three weeks of injection and the construct with a gross appearance
similar to cartilage [Sharma et al., 2007a].
Apart from serving as a minimally-invasive injectable system, the versatility
of a photo polymerization system allows for the development of multilayer
constructs for tissue engineering. A co-culture system using bi-layered photo
polymerized gels to organize zone specifi c chondrocytes in a stratifi ed frame work
has been studied [Sharma et al., 2007b]. The bi-layered constructs were fi rst made
by partially polymerizing the bottom layer followed by quickly adding the second
layer and polymerizing them both. The bi-layered construct showed higher shear
and compressive strength compared to homogeneous constructs and allowed
separation of different cell layers to mimic the natural tissue structure [Sharma
et al., 2007b].
One of the disadvantages of synthetic polymeric materials such as PEG as
scaffolds for tissue engineering is the absence of any bioactive ECM molecules on

β 3 (TGF - β