2.2.2. Covalent Gelation
Covalently cross-linked hydrogels can be prepared from chemically modified alginates [39,40]. To better control the physical properties of alginate gels, covalent cross-linking has been broadly investigated [41]. A covalently cross-linked hydrogel is chemically stable and can provide different modes of stress relaxation. By covalently conjugating methacrylate groups onto the alginate backbone, covalently cross-linked hydrogels can be prepared in the presence of a photoinitiator and UV light. Cells will be evenly distributed throughout the hydrogel if the cells are suspended in the alginate solution prior to photocrosslinking [17,42]. Photocrosslinking has a number of different advantages. By comparison with ionic gelation, photocrosslinking allows formation of more stable alginate hydrogels independent of the level of gel forming ions and non-gelling ions [42]. Mechanical properties and biodegradation rates of a hydrogel can be adjusted by varying the degree of alginate methacrylation [43].