NPs. In an example by Qiu, Wang and co-workers, vinyl-functional SiO 2 NPs are first decorated with vinylfunctional chondroitin sulfate polymers. Next, the decorated SiO 2 NPs were copolymerized with AAm and small quantities of MBA (first network) in the presence of agar, which upon cooling formed the second network. At a water content of 77%, a strong double network was obtained (σ b = 20.4 MPa), whose fracture energy and stiffness unexpectedly lacked behind (Γ = 3.544 kJ/m 2 , E = 0.321 MPa). Shi et al. copolymerized a solution containing vinyl-functional SiO 2 NPs, AAm, and SMA in the presence of SDS as surfactant. The resulting polymer network is covalently cross-linked with the SiO 2 NPs and dissipates energy via the micellar aggregates of the SMA. The fracture energy of the bespoke hydrogel reached a value of Γ = 12.1 kJ/m 2 despite the rather low elastic modulus of E = 9.2 kPa. The latter was key to enhance the gel's stretchability, which amounted to ε b = 2820% at a water content of 90%. In a study by Guo et al, a PVA network is first established via freeze-thaw cycles, followed by soaking in sodium silicate solutions to fabricate robust dual cross-linked networks. Their toughest hydrogel reached a fracture energy of Γ = 10.7 kJ/m 2 and an elastic modulus of E = 2.1 MPa at a water content of 72%. An increasingly popular strategy to reinforce hydrogels has been the in situ mineralization of calcium phosphate (CaP), as well as CaCO 3 . In 2020, Yu et al. presented a film-casting technique to generate robust tough hydrogels consisting of PVA, NaAlg, and CaP. During the drying of the precursor film, CaP oligomers formed and transformed into hydroxy apatite nanocrystals through the interactions with surrounding PVA and Alg chains. The mineralization process produced a strong gel (σ b = 17.84 MPa) and improved both the fracture and mechanical properties (Γ = 8.97 kJ/m 2 , E = 26.93 MPa), while the water content was maintained at 75%. Notably, while this gel had nearly identical failure stress and strain (σ b = 17.8 MPa, ε b = 96%) as the double network made from chondroitin sulfate/SiO 2 /agar, its elastic modulus is 90-fold higher. On other occasions, tough hydrogels were supplemented with metal organic frameworks (MOF), or biochars NPs. As opposed to hard sphericles particles, soft polymer microspheres have been used to create strong and tough composite hydrogels. Tang et al. used sodium 2-acrylamido-2-methyl-1-propanesulfonate (AMPS) in the presence of AAm and MBA to create a hydrogel sheet, which was subsequently freeze-dried and extensively ground to yield a microgel powder. The final hydrogel was fabricated using a formulation consisting of AAm/MBA, clay and magnetic NdFeB nanoparticles, as well as various amounts of the ground microgel particles. While the NdFeB content slightly improved the overall strength and fracture energy of the gel, major stress-strain enhancements stemmed from the incorpo- ). PAMPS microgels were also used in a report by Yan and co-workers, except emulsion polymerization was used for their preparation. Using a water content of 67% and microgels (cured with 3.6 mol% crosslinker) generated an extremely stretchable (ε b = 5000%) hydrogel, which was capable of dissipating extensive amounts of energy (Γ = 157 kJ/m 2 ). Comparatively, its stiffness (E = 0.18 MPa) and failure stress (σ b = 0.237 MPa) lacked considerably behind. Tough hydrogels reinforced by nanosheets have primarily used clay (Laponite) (Figure ). In one notable report, a physical gel containing clay and a copolymer based on oligoethyleneglycol methacrylate (OEGMA) and NIPAAm was rapidly extruded into a fiber. Next, the authors immersed this gel fiber in aniline (ANI) solutions in the presence of phytic acid and a thermoinitiator to form the sec-ond network. The final double network fibers were swollen in water, however, the final water content was not stated. The combination of the soft non-covalent nanocomposite network and the rigid PANI (cross-linked with negatively charged phytic acid) brought forward a unique set of mechanical properties. Increasing the ANI immersion time from 6 to 8 minutes prior to the polymerization of the second network caused a significant change in fracture energy and stiffness (Γ = 172 to 85.7 kJ/m 2 , E = 11.4 to 24.4 MPa). In addition, the failure strain was approximately halved (ε b = 356 to 153%), while the gel's strength was not affected (σ b = 6.9 to 7.2 MPa).