Self-assembly. In the final section, tough hydrogels driven by self-assembly are discussed (Figure ). While the formation of helices is often solely associated to strands of DNA, it also occurs in biopolymers such as agar, gelatin or gellan gum. A noteworthy example combined both gelatin and gellan gum in a double network with water contents ranging from 57% to 49%. This reduction was caused by a change from ammonium sulfate to sodium sulfate as immersion salt; the sulfate acted as salting-out agent, while the sodium was able to additionally promote helix-to-helix attractions. Thus, the resulting fracture energy and elastic modulus of the double network was significantly enhanced (Γ = 9.6 to 27.7 kJ/m 2 and E = 2.9 to 42.6 MPa. When it comes to protein folding, only three studies have been published to date, which look into the fracture mechan-ics of the resulting hydrogel materials. A promising source of energy dissipation have been introduced by elastomeric proteins, which function as molecular springs and unfold once a stress is applied. This mechanical unfolding can be observed in smaller elastin-like polypeptides as well as in larger proteins (e.g., G8) and hence, was exploited for the synthesis of tough hydrogels alike (Γ = 0.9-1.33 kJ/m 2 ). Self-assembly between host and guest molecules is a long-known phenomenon and has been exploited in a multitude of materials, yet studies on the fracture mechanics of host-guest mediated hydrogels remain scarce. As a recent innovation, host-guest interactions have been specifically used to furnish networks with slidable cross-links. Slidable cross-links are neither covalent nor non-covalent and can be described as an interlocked topological feature, which has been extensively studied by Ito and co-workers. In their reports, macrocyclic cyclodextrin (CD) host molecules were threaded onto PEG polymer chains (guests) prior to chemically linking threaded CD molecules together. Upon the application of stress to the network, energy can be dissipated by the sliding motion of individual crosslinks along the polymer backbone. Recently, Ito and co-workers constructed networks based on this slide-ring topology and benchmarked them with comparable networks, where the cross-links were fixed. The control sample was extended to ε b = 310% (σ b = 0.45 MPa), while the corresponding slide-ring gel reached an unmatched failure strain of ε b = 13400% representing the highest reported value for a tough hydrogel at the time of writing (σ b = 5.5 MPa, water content: 62%). The same gel showed a fracture energy of Γ = 3.6 kJ/m 2 (vs. Γ = 0.24 kJ/m 2 for the control) and the elastic modulus was not reported. The concept of slide-ring gels was slightly adapted by Chen et al. who produced gels that consisted of a network of CD molecules (connected with epichlorhydrin). This gel was then soaked in a solution containing aliphatic C12 bis(methacrylate), which could enter the CD cavity to form host-guest assemblies. Subsequently, chosen amounts of photoinitiator and 2-hydroxyethyl acrylate (HEA) were added to this host-guest solution to form a tough and stretchable hydrogel upon polymerization. The double network hydrogel demonstrated good strength (σ b = 9.5 MPa) and stretchability (ε b = 1385%), combined with a fracture energy of Γ = 66.3 kJ/m 2 (elastic modulus was not reported). To summarize, it remains difficult to unite excellent fracture energies with high elastic moduli in tough hydrogels. Nonetheless, recent years have produced unique combinations of interactions and materials to yield tough hydrogels with extreme mechanical properties, which are represented throughout all subcategories we have discussed (Figure ). As of today, composite materials have reached some of the highest values (Γ ≳ 100 kJ/m 2 ) by use of clay nanocomposite double networks (P(OEGMA-stat-NiPAAm)/PANI) or glass fiber-reinforced PAAm/Alg double networks (although their water content remains unknown). This has been closely followed by PVA-based hydrogels using microcrystallization: Here, the PVA/PAAc-Fe 3+ double network, and the dual cross-linked PVA-Cit 3-single network reach exceptional fracture energy values, while their elastic moduli remain below E = 10 MPa. As opposed to the composite materials, however, their stretchability is much improved (ε > 1000 vs. < 350%), additionally has the highest water content of this data assembly (90%).