Undoubtedly, the first reported PAAm/Alg-Ca 2+ double network inspired many more contributions, which further explored variations of network microstructure, cations, or initiator. Notably, the polysaccharide carrageenan has become a popular replacement for alginate. In the presence of cations, such as Mg 2+ , Ca 2+ , or K + , carrageenan (Carr) polymer strands can aggregate into helices. The aggregation with K + in particular was successfully exploited to generate tough PAAm/κ-Carr double networks. With a water content of 86%, Fei and coworkers reached a fracture energy of Γ = 9.5 kJ/m 2 , albeit the stiffness remained limited (E = 7.2 kPa).[37] A different example by Li and co-workers showed, how a similar system (82% water content) increased the stiffness to E = 130 kPa, yet at the expense of the fracture energy, which fell to Γ = 6.15 kJ/m 2 . Both examples show how an impressive stretchability (beyond 2000% before failure) often comes at the expense of the elastic modulus of the system. Adding gelatin into the formulation increased the stretchability even further, as it provided additional energy dissipation through helix formation at reduced temperatures. In this recent work, such extreme fracture strains (ε b = 5170%) can be a main driving factor to increase the fracture energy (Γ = 16.05 kJ/m 2 ). The highest fracture energy of Γ = 18.5 kJ/m 2 that has been demonstrated for a PAAm/κ-Carr double network has been reported by Yu et al. This improvement was largely attributed to the change from K + to Zr 2+ , which enhanced the binding interactions between the κ-Carr sulfate groups, tightening the helices. As a result, the much stiffer material reached an elastic modulus of E = 1.7 MPa, still resisting a remarkable elongation of ε b = 1870%. Complementary to double networks, metal-binding comonomers have been copolymerized directly into single networks to yield energy-dissipating tough hydrogels. For instance, Xu et al. copolymerized vinyl diaminotriazine with vinyl imidazole (VIm) in the presence of small quantities of PEG diacrylate (PEGDA) as covalent crosslinker. Diaminotriazine dimerizes via hydrogen bonding, while VIm units chelate Zn 2+ cations. Both displayed samples exhibited high stiffness (E = 20.6 and 11.4 MPa at 76% water content), yet the fracture energies remain on the lower scale (Γ = 1.37-1.47 kJ/m 2 ). Wang and co-workers relied on Cu 2+ ions instead. First, they prepared a P(AAmstat-VIm)-based network, which was covalently cross-linked using small amounts of MBA. By soaking this network in aqueous Cu 2+ solutions, the imidazole groups formed copper complexes, leading to a significant increase of both stiffness (E = 15.4 MPa) and fracture energy (Γ = 22.1 kJ/m 2 ). The water content of this hydrogel was reduced to 44%, likely an adverse effect of the soaking process, which may have caused water expulsion from the network.