**3.3 Biofunctional hydrogels**

Mechanical modulus is in the range of 10 kPa–350 MPa for soft tissues and 10 MPa–30 GPa for hard tissues (S. Yang et al., 2001). Depending on intended application, hydrogel should provide sufficient mechanical strength so as to protect seeded cells and developing neotissue as well as to withstand the physiologic load. However, most of hydrogels reported so far are not qualified especially for bone or cartilage tissue regeneration due to the lack of a high mechanical strength. Thus, robust hydrogels have been developed with on-demand mechanical properties. Mechanical moduli of hydrogels are generally increasing with increasing crosslinking density. Two types of robust hydrogel systems can be classified.

Fig. 6. Hydrogel prepared via the combination of stereocomplexation and photocrosslinking

First, hydrogels can be fabricated by double crosslinking methods. By this approach, the hydrogels have increased crosslinking density, thus improving the mechanical properties without comprising other properties such as permeability and biocompatibility. For example, Feijen and coworkers reported on the in situ hydrogels crosslinked by combining stereocomplexation and photopolymerization (Hiemstra et al., 2007b). Stereocomplexed hydrogels were first formed upon mixing solutions of an 8-arm PEG–PLLA and an 8-arm PEG–PDLLA whichh are partly functionalized with methacrylate groups (40%). These

peptide crosslinker which contained the sequence sensitive to matrix metalloproteinases (Lutolf et al., 2003). The hydrogels were proteolytically degraded via the invasion of primary human fibroblasts. The invasion process depended on MMP substrate activity, adhesion ligand concentration, and network crosslinking density. By mimicking the MMPmediated invasion of the natural provisional matrix, the hydrogels were shown to assist tissue regeneration. These results indicate potential applications of the cell-responsive

Mechanical modulus is in the range of 10 kPa–350 MPa for soft tissues and 10 MPa–30 GPa for hard tissues (S. Yang et al., 2001). Depending on intended application, hydrogel should provide sufficient mechanical strength so as to protect seeded cells and developing neotissue as well as to withstand the physiologic load. However, most of hydrogels reported so far are not qualified especially for bone or cartilage tissue regeneration due to the lack of a high mechanical strength. Thus, robust hydrogels have been developed with on-demand mechanical properties. Mechanical moduli of hydrogels are generally increasing with increasing crosslinking density. Two types of robust hydrogel systems can be classified.

Sterecomplexation

Fig. 6. Hydrogel prepared via the combination of stereocomplexation and photocrosslinking

First, hydrogels can be fabricated by double crosslinking methods. By this approach, the hydrogels have increased crosslinking density, thus improving the mechanical properties without comprising other properties such as permeability and biocompatibility. For example, Feijen and coworkers reported on the in situ hydrogels crosslinked by combining stereocomplexation and photopolymerization (Hiemstra et al., 2007b). Stereocomplexed hydrogels were first formed upon mixing solutions of an 8-arm PEG–PLLA and an 8-arm PEG–PDLLA whichh are partly functionalized with methacrylate groups (40%). These

Post photocrosslinking

hydrogels in tissue engineering and regenerative medicine.

**3.3 Biofunctional hydrogels** 

8-arm Poly(ethylene glycol)

Poly(D-lactide)

Poly(L-lactide)

O

hydrogels can be postcrosslinked by UV-irradiation (Fig. 6). These double-crosslinked hydrogels showed increased mechanical moduli and prolonged degradation times compared to the hydrogels that were formed only by stereocomplexation. The photopolymerization takes place at much lower initiator concentrations (0.003 wt%) than conventional photocrosslinking systems (0.05 wt%), which greatly reduces the possibility of heating effects that can damage cells.

Second, robust hydrogels are produced that consist of two interpenetrated polymeric networks. The hydrogels with double networks contain a subset of interpenetrating networks (IPNs) formed by two hydrophilic networks, one highly crosslinked, the other loosely crosslinked. The double network structure can be obtained by pre- and postcrosslinking through exploiting the disparity of their reaction times. For example, a double netwok composed of two mechanically weak hydrophilic networks based on N, Ndimethylacrylamide and glycidyl methacrylated hyaluronan, provides a hydrogel with outstanding mechanical properties (Weng et al., 2008). Hydrogels containing more that 90% water possessed a compressive modulus and a fracture stress over 0.5 MPa and 5.2 MPa, respectively, demonstrating both hardness and toughness. Besides, it is found that both the concentrations of monomers and crosslinkers are important parameters related to the mechanical strength of double network gels. Therefore, it is easy to control the mechincal properties such as hardness and toughness independently by adjusting the compositions of of the gels for practical applications.
