Electrostatic interactions. Metal coordination represents a powerful energy dissipation mechanism, as it features a large bandwidth of metal-ligand combinations, many of which are also abundant in nature (Figure ). The metal coordination between divalent calcium ions and the biopolymer alginate (Alg) is a prime example for reversible crosslinking; and as illustrated by Sun et al, paved the way towards tough and stretchable hydrogels. In this pioneering work, a Ca 2+ cross-linked Alg network was combined with a second network made from acrylamide (AAm) covalently crosslinked with N,N'-methylenebis(acrylamide) (MBA). Albeit their elastic modulus was limited to E = 29 kPa, the reported double network hydrogels were highly stretchable (ε b = 2300%) and tough (Γ = 9 kJ/m 2 ) at a water content of 90%. Several studies on the mechanical properties of PAAm/Alg-Ca 2+ double networks emerged thereafter. A notable improvement was achieved, when blends of short and long-chain alginates were used as second network. Through incorporation of 33% short-chain alginate, the fracture toughness of the double network rose to Γ = 16 kJ/m 2 with an elastic modulus of E = 98 kPa. Comparing these results to Alg-Ca 2+ single networks reveals the strength of the double network architecture. For example, in a recent study, where Alg-Ca 2+ single networks were generated via slow evaporation of diluted pre-gel solutions, fracture energies were much lower (Γ = 0.5 kJ/m 2 ), despite the gel sample being relatively stiff (E = 1 MPa at water content of 75%). Analogous to other metal-ligand complexes, Alg binds to various metals aside from Ca 2+ , which was firstly explored by Morch et al. in polymer microbeads, and later tested in bulk hydrogels. Both studies indicated that metal-alginate binding affinities increase according to Ca 2+ < Sr 2+ < Ba 2+ < Cd 2+ < Cu 2+ < Pb 2+ < trivalent cations, which was also reflected by the hydrogel's mechanical properties. This knowledge was picked up by Liang et al, who introduced trivalent Fe 3+ cations to their double network. Additionally, the first network, which has been commonly made from a PAAm homopolymer, was adjusted to contain acrylic acid (AAc) groups. As a result, Fe 3+ ions interacted with the acid groups of both the P(AAm-stat-AAc) and the alginate network, interconnecting the two. This synergistic network stabilization significantly enhanced the elastic modulus (E = 24.6 MPa), leaving the fracture energy comparatively behind (Γ = 4.8 kJ/m 2 , water content 51%). Nonetheless, this method of cross-linking also showed a distinct improvement to the fracture stress, which for this system is among the highest values reached (σ b = 24.6 MPa).