**4.1.4** *In vivo* **studies**

Many in-situ forming hydrogels so far prepared for cartilage tissue engineering have been studied in vitro, however, only a few reports have appeared on their performance in vivo. Before clinical application, in-situ forming hydrogels need to be systematically evaluated in animal models. Early *in vivo* studies generally focused on the ability of cell-seeded injectable hydrogels to generate cartilaginous tissue after subcutaneous implantation/injection in mouse models. For example, Elisseeff *et al.* implanted chondrocyte-incorporated PEO-based hydrogels, and showed that the chondrocytes survived during the photopolymerization process and proliferated without any sign of necrosis (Elisseeff et al., 1999). However, partial chondrocyte dedifferentiation and undesired fibro-cartilaginous tissue formation were observed in these gels after 6 weeks' in vivo. To retain the chondrocyte phenotype and improve cartilage regeneration, ECM components and bioactive molecules were incorporated into hydrogels. Na *et al.* showed that cartilage-specific ECM production was significantly higher in poly(Nisopropylacrylamide-co-hydroxyethyl methacrylate) hydrogels containing HA and TGFβ3 compared to those without the growth factor or HA (K.H. Park & Na, 2008). Recent studies have been directed to in-situ forming hydrogels for cartilage regeneration in animal models like rabbit and goat. Hoemann *et al.* tested the residence of in-situ forming hydrogels in rabbit joints and showed that in-situ forming chitosan/glycerol phosphate gels could reside at least 1 day in a full-thickness chondral defect, and at least 1 week in a mobile osteochondral defect (Hoemann et al., 2005). Liu et al. described osteochondral defect repair in a rabbit model using a synthetic ECM composed of hyaluronic acid and gelatin (Liu et al., 2006). At 12 weeks, the defects were completely filled with elastic, firm, translucent cartilage and showed good integration of the repair tissue with the surrounding cartilage. A summary of in vivo studies on in-situ forming hydrogels for cartilage regeneration is presented in Table 3.


Table 3. In-situ forming hydrogels for *in vivo* cartilage regeneration
