*3.4. Chemical Gelation*

Chemical gelation can be mediated by ionotropic or covalent crosslinking. In the first case, the polysaccharides are crosslinked by ions forming a gel network (Figure 6e). In the second case, gels are formed via covalent cross-linking which leads to irreversible chemical networks (Figure 6d). The main problem is that the majority of covalent cross-linking agents are not biocompatible [4,84]. Among them, glutaraldehyde is certainly that with the longest history; it has been widely used to cross-link several biopolymers such as chitosan [85,86], sodium alginate [87–89], cellulose [90,91], guar gum [92,93], collagen [94], collagen-chitosan [95], alginate-guar gum [96], and carrageenan [97,98]. For instance, chitosan microspheres can be produced by mixing chitosan and glutaraldehyde solutions in oil containing surfactants [99,100]. In this case, a Schi ff base reaction between amine and aldehyde occurs and as a result, chitosan chains are covalently cross-linked by the glutaraldehyde molecules. With the same mechanism, other aldehydes such as glyoxal and formaldehyde are able to crosslink chitosan chains [101–103]. Glutaraldehyde has been highly used also for alginate reticulation. Other covalent gelling agents used for this polymer are adipic dihydrazide, lysine, and poly(ethylene glycol)-diamines [104]. Clearly, the type of cross-linking molecule and the cross-linking density determines both the mechanical properties and the degree of swelling in alginate hydrogels. Usually, cellulose is chemically cross-linked in aqueous solution by using epichlorohydrin, dichlorohydroxytriazine, 1,3,5-triacryloylhexahydrotriazine, 2,4-diacrylamido-benzenesulphonic acid, *N*-methylol resins or dialdehydes [4].

As confirmed by the high number of papers present in the literature, among all gelation techniques, ionotropic cross-linking of polysaccharide solutions is the most investigated for the fabrication of biocompatible systems used in biomedical field, due to its a ffordability, versatility and high reproducibility [105,106].
