Regarding natural polymers, chitosan (CS) has played an increasing role in the development of ionically driven, tough hydrogels (Figure ). Various reports combined CS with sodium phytate, citrate, polypyrrole, or AAc copolymers. The highest stiffness was observed for the tough hydrogel made from PAAm/CS-polypyrrole (E = 50.1 MPa), where a PAAm network was combined with interpenetrating CS chains, Fe 3+ ions, and in situ forming rigid polypyrrole chains. The authors reported a fracture energy of Γ = 12 kJ/m 2 at a stated water content of 75%; this figure is likely lower as neither the amounts of polypyrrole (20%) nor FeCl 3 are factored in. The highest fracture energy among CS-based systems was measured, when trivalent citrate anions (Cit 3-) were used as ionic cross-linker in a PAAm/CS-Cit 3-double network. Mechanical properties amounted to E = 1.3 MPa and Γ = 14 kJ/m 2 at a moderate water content of 56%. Lastly, the work by Fan et al. should be mentioned, which in addition to ionic interactions introduced hydrophobic domains by use of an aliphatic catechol-functional monomer. While the fracture toughness is showing an average value of Γ = 6.6 kJ/m 2 , the fracture strain exceeds 2500% (the elastic modulus was not reported). Generally, tough polyampholyte hydrogels require higher polymer fractions to maintain strong ionic interactions, which often comes at the expense of the water content in these gels (in the range of 40-60%). Similar to PAAm/Alg or PAAm/Carr double networks, PAAm/CS-Cit 3-double networks achieve good fracture energies along with remarkable stretchabilities.