The materials for synthesizing organic-organic DN hydrogels are mainly synthetic polymers and natural macromolecules^[17]. Synthetic polymers mainly include polyacrylamide (PAM), poly (2-acrylamide-2-methylpropanesulfonic acid) (PAMPS), polyethyleneimine (PEI), polyacrylic acid (PAA), polyvinyl alcohol (PVA), etc. These synthetic polymers not only have good mechanical properties, but also have the advantages of biocompatibility, low cost and easy availability^[18]. Gong et al. Prepared a DN hydrogel composed of poly (acrylamide-acrylic acid copolymer) (PAM-co-PAA) and polyvinyl alcohol (PVA) by a simple copolymerization and freeze/thaw two-step method^[19]. The copolymer chains associated with hydrogen bonds are entangled to form cross-linking points to build the first network. After being subjected to a freeze/thaw treatment, PVA crystalline domains are formed to serve as knots for the second network. The obtained hydrogel has high strength and toughness (1230 ± 90 kPa and 1250±50 kJ/m³). More importantly, the hydrogel has fast recovery, excellent fatigue resistance, and self-healing properties. Chu et al. Synthesized polyvinyl alcohol/acrylic acid (PVA/PAA) DN hydrogel adsorbent with ammonium persulfate (APS, initiator) and N, N '-methylenebisacrylamide (MBA, crosslinking agent) through a simple two-step method, as shown in Figure 2^[20]. The adsorbent has cheap synthetic raw materials, good stability and good swelling performance, can be repeatedly used for adsorbing heavy metals in wastewater, does not cause secondary pollution, and is an efficient adsorbent for treating wastewater.

DN hydrogels prepared from synthetic polymers have excellent mechanical properties, but synthetic polymers usually form networks through covalent crosslinking, and irreversible covalent crosslinking makes the hydrogels have low self-healing properties^[21]. Hydrogels prepared by physical crosslinking of natural macromolecules have excellent anti-fatigue properties and self-healing properties, so hydrogels prepared by physical crosslinking of natural macromolecules are favored by researchers. Carrageenan (car) is a natural macromolecule commonly used in synthetic hydrogels. It is a water-soluble polysaccharide extracted from marine red algae. It is a hydrophilic colloid with sulfonic acid groups, which can be divided into λ, κ, l, ε and μ types^[22]. The most common car-based DN hydrogel is κ-carrageenan/polyacrylamide (κ-car/PAM) hydrogel. The κ-car-based DN hydrogel was mainly prepared by one-pot method, as shown in Fig. 3^[23]. The κ-car network can be cross-linked by ionic association between K⁺ or other metal ions. The double helix unit of κ-car network is formed at low temperature. Other polymer network synthesis methods mainly include photopolymerization, micellar copolymerization, freeze-thaw cycle and so on. N, N '-methylenebisacrylamide is usually used as a crosslinking agent in the preparation of physicochemical crosslinked DN hydrogels, and the initiator can be selected according to this method.

In addition, there are some other carrageenan based DN gels. Yu et al. Prepared κ-carrageenan/sodium alginate (κ-car/SA) DN hydrogel by calcium hardening method with CaCl₂ as crosslinking agent, as shown in Fig. 4^[24]. Sodium alginate and carrageenan form a large number of hydrogen bonds in a certain proportion, and form a cross-linked network and ionic interaction through hydrogen bonds. The prepared DN hydrogel not only avoided the swelling problem, but also had good adsorption properties for ciprofloxacin CIP (229 mg/G). Guo et al. Prepared a DN hydrogel with ionically crosslinked κ-carrageenan (κ-car) as the first network and covalently crosslinked poly (N-acryloylglycinamide) (PNAGA) as the second network using a one-pot method^[25]. Due to the synergistic interaction between κ-car and PNAGA polymer chains, the hydrogel not only shows excellent mechanical properties, but also has good self-healing properties.

Organic-organic DN hydrogels have been widely studied by researchers because of their high mechanical properties, low cost and certain adsorption capacity for pollutants in water.

Organic-organic DN hydrogels have excellent mechanical properties, but it is difficult to achieve a good balance between mechanical properties and self-healing properties. Therefore, the combination of inorganic and organic materials is considered to be an effective strategy to solve this problem^[26]. The inorganic components in organic-inorganic DN hydrogels are mainly graphene (GO) and its derivatives graphene oxide, reduced graphene oxide, and natural minerals. The organic components are still synthetic polymers and natural macromolecules. The interaction between unfunctionalized graphene and polymer chains is weak, so graphene oxide, a derivative of graphene, is considered to be an ideal substitute^[13]. Graphene oxide is a popular carbon material, which can be self-assembled into reduced graphene oxide (RGO) by hydrothermal treatment, consisting of epoxy, hydroxyl (OH) and carboxyl (COOH), and has been proved to be a good physical strengthening agent^[27][28]. Most researchers have achieved the formation of organic-inorganic DN hydrogels with different structures by two-step or one-pot methods^[14]. The preparation method comprises the following steps of: firstly preparing a specific polymer solution, such as polyacrylic acid, alginate and the like; then dispersing the graphene oxide in deionized water; and performing hydrothermal reduction or reducing agent treatment to obtain the graphene oxide network. Therefore, in the self-assembly process, graphene oxide forms a network, organic materials compose a network, and then DN hydrogel is formed. The prepared DN hydrogel has the advantages of both graphene and hydrogel, and has better adsorption performance and swelling performance^[29].

Huang et al., Zhuang et al., and Yang et al. Synthesized organic-inorganic DN hydrogels using graphene oxide as the inorganic network^[30][28][31]. Huang et al. First attempted to prepare graphene/poly (acrylic acid) (GO/PAA) inorganic-organic DN hydrogel using a two-step synthesis method^[30]. Reduced graphene oxide (RGO) hydrogels were first prepared by reduction-induced in situ self-assembly, and then acrylic monomers were adsorbed into the first network and polymerized therein to form the second PAA network. The prepared RGO/PAA DN hydrogel is different from the traditional polyacrylate hydrogel which is usually brittle and rigid, and it not only shows good elastic properties and mechanical properties, but also has both flexibility and conductivity, which has potential application prospects in the next generation of flexible electronic devices.

Zhuang et al. Also prepared alginate/reduced graphene oxide (SA/RGO) D N hydrogel using a reducing agent^[28]. Compared with single network hydrogel, it has higher Young's modulus and lower swelling ratio. DN hydrogels also showed excellent adsorption capacity (Cu²⁺:169.5 mg·g^-1,Cr₂ {{\mathrm{O}}_7}^{2-} :72.5 mg·g^-1) and regeneration ability, and maintained good reusability after multiple adsorption/desorption cycles, making them promising adsorptive nanomaterials for the removal of pollutants from water.

Yang et al. Immersed graphene/alginate (GO/SA) hydrogel in ascorbic acid solution at 90 ℃ for 8 H to form a DN structure^[31]. The DN hydrogel not only has strong interpenetrability, but also combines the high specific surface area and thermal stability of graphene oxide and the biocompatibility of sodium alginate. Compared with other adsorption materials such as inorganic porous materials, natural and synthetic polymers with large surface area, the adsorption capacity (56.49 mg·g^-1) of graphene oxide/sodium alginate DN hydrogel for Mn (Ⅱ) is superior to them. At the same time, the hydrogel can be quickly separated from the wastewater for recycling.

Compared with many reports on graphene and its derivatives, there are few applications of DN hydrogels composed of other inorganic materials. Sulphoaluminate cement (SAC), silica, hydroxyapatite (HAP), etc., have been used to form the inorganic network in DN hydrogels due to their unique properties and structures.

Chu et al. Used inorganic sulphoaluminate cement (SAC) as the first network and organic PAM as the second network at room temperature^[32]. Polyacrylamide/sulphoaluminate cement (PAM/SAC) DN hydrogel was prepared by solution polymerization. The hydrogel not only maintains the swelling property of hydrogel, but also shows excellent tensile properties (stress and strain are 12 MPa and 2 500%, respectively) and compressive strength (stress and strain are 65 MPa and 80%, respectively), which is expected to become a potential grouting plugging functional material.

Kamio et al. Used silica nanoparticles and poly (N, N-dimethylacrylamide) (PDMAAm) as raw materials to prepare a DN ionogel by a one-pot method, in which the brittle silica particle network was used as a "sacrificial bond" to break and dissipate energy under load, resulting in the hydrogel showing extremely high mechanical strength, good thermal stability, and self-healing properties^[33]. Bacterial cellulose-gelatin (BC-GEL)/hydroxyapatite (HAp) D N hydrogel was synthesized by Ran et al., as shown in Figure 5^[34]. The hydrogel combines the advantages of BC/HAp and BC/GEL, so its mechanical strength is greatly enhanced, and it has higher elastic modulus and fracture stress.

Organic-inorganic DN hydrogel not only has excellent mechanical properties of organic-organic DN hydrogel, but also has good self-healing properties. The introduction of inorganic materials such as graphene makes the hydrogel show good adsorption performance, and after adsorption/desorption, it can also be recycled. At present, there are few studies on organic-inorganic DN hydrogels. Considering the application of DN hydrogels in water treatment, it is necessary to find inorganic materials with better adsorption properties.

The preparation methods and properties of various double-network hydrogels are summarized in Table 1.

Excellent mechanical properties are essential for practical applications of hydrogels. This excellent mechanical property can be ductility or compression resistance. Because hydrogels, as an adsorption material, may need to be exposed to groundwater for a long time, they need to ensure the performance of hydrogels under long-term mechanical loading conditions, and they themselves will not be damaged by excessive compression and tension^[36,37]. Therefore, hydrogels with limited mechanical properties cannot be used as long-term adsorbents. Therefore, how to improve the mechanical properties of hydrogels has become a research hotspot in the field of hydrogels^[38]. Methods to improve the mechanical properties of hydrogels are described below.

(1) Explore more network structures. Gong et al. Completed the first chemically crosslinked PAMPS/PAM DN hydrogel^[10]. But this hydrogel, during stretching, the first PAMPS network breaks at minimal strain and thin gel necks begin to appear^[39]. They summarized the design principles for the preparation of high-strength DN hydrogels: ① They should have an asymmetric network structure, in which rigid and brittle polyelectrolytes are the first network, and soft and tough neutral polymers are the second network; ② In the second network, the molar concentration of the neutral polymer monomer should be 20 to 30 times higher than that of the polyelectrolyte monomer in the first network; ③ The first network should have a low concentration of high crosslinks, while the second network should have a lower concentration of weak crosslinks (even non-crosslinks). According to these principles, the preparation method of DN hydrogel was improved. The original two-step polymerization method is generally applicable to the first network of DN hydrogel, which is a strong polyelectrolyte. A first network hydrogel is prepared and then the first network hydrogel is soaked in a solution of a second monomer to polymerize the second network hydrogel within the first hydrogel. When the first network is a neutral polymer or a weak polyelectrolyte, the swelling ability of the first network is poor, which makes the toughness of the DN hydrogel low. Therefore, the "molecular scaffold" method was designed^[40]. The "molecular scaffold" method is similar to the classical two-step polymerization method, in which the linear polyelectrolyte is introduced into the neutral first network by introducing ionic micelles or ionic scaffolds to increase the additional ionic osmotic pressure, thus achieving the purpose of improving the mechanical properties of DN hydrogels.

(2) Introducing a new cross-linked network. Chemically crosslinked DN hydrogels show extraordinary mechanical properties, but the irreversible nature of chemical crosslinking as well as the use of toxic chemical crosslinkers result in most hydrogels having poor self-healing properties and biocompatibility. To address these shortcomings, physical-chemical hybrid crosslinking as well as fully physically crosslinked networks have been introduced. Chen et al. Synthesized physicochemical cross-linked Agar (Agar)/polyacrylamide (PAM) DN hydrogel by one-pot method, as shown in fig. 6^[41]. The prepared hydrogel has excellent mechanical properties, can withstand a high level of compression and tension, and also shows good self-healing performance after deformation.

Ye et al. Also prepared a fully physically cross-linked Curdlan/polyacrylamide (PAM) DN hydrogel using a one-pot method^[42]. By using the tough PAM as the second network, not only the energy can be effectively dissipated, but also the mechanical properties can be significantly improved.

And (3) introduce a modified reinforcing fill. In order to overcome the influence of the defects and deficiencies of the prior art on the DN hydrogel, nanomaterials such as graphene, graphene oxide, SiO₂ and the like are introduced to improve the mechanical properties of the hydrogel. After combining the advantages of DN hydrogel and nanocomposite hydrogel, Xu et al. Prepared TM-SiO₂/PAM/PAA nanocomposite DN hydrogel through two-step continuous radical polymerization, as shown in Fig. 7^[43]. TM-SiO₂ nanoparticles have vinyl and hydroxyl groups; therefore, a denser structure and smaller average pore size can be obtained by adding TM-SiO₂ nanoparticles as chemical and physical crosslinkers into PAM/PAA DN hydrogels. The tensile strength and compressive strength of DN hydrogel were increased by introducing appropriate concentration of TM-SiO₂ nanoparticles, and the hydrogel network was more stable.

Liu et al. Successfully prepared graphene oxide-reinforced double-crosslinked alginate/polyvinyl alcohol (SA/PVA) DN hydrogel by a simple freezing/thawing process followed by soaking in Ca²⁺ solution, as shown in Fig. 8^[44]. The addition of GO not only increased the breaking strength of SA/PVA hydrogel from 0.11 MPa of pure SA/PVA to 0.24 MPa, but also improved the adsorption capacity of MB. Therefore, the GO reinforced SA/PVA DN hydrogel can be used as a non-toxic, biodegradable, and low-cost dye adsorbent for dye wastewater treatment.

The improvement of mechanical properties can not only make the DN hydrogel have higher tensile strength and compressive strength, but also improve the swelling property and self-healing property. The improvement of performance leads to the improvement of the recycling rate of hydrogel, which plays a positive role in the practical application of DN hydrogel.

Because of its high hydrophilicity, hydrogel will inevitably swell in water environment, which leads to a sharp decline in its mechanical properties^[45]. The degree of swelling of the hydrogel is controlled by the competition between the osmotic swelling force and the local pressure transmitted by the surrounding grains^[46]. The swelling behavior can cause the rupture of the polymer network formed by irreversible bond crosslinking, which not only destroys the balance between the original osmotic pressure and elastic properties, but also leads to poor fatigue resistance, and then causes the fatigue fracture of the hydrogel under continuous mechanical load, resulting in the destruction of the network structure and shape of DN hydrogel^[47][48]. Therefore, it is necessary to explore the methods to reduce the swelling rate of hydrogel.

At present, chemical crosslinking is considered to be an effective method to reduce the swelling strength of hydrogels. The degree of swelling of the hydrogel is related to the crosslinking density of the network, and a high crosslinking density is beneficial to the reduction of the swelling ratio^[49,50]. With the increase of the amount of chemical crosslinking agent, the interaction between molecular chains becomes closer, and the resistance of water molecules to soak in is also improved, thus reducing the swelling ratio of hydrogels^[51]. The decrease in swelling ratio is also beneficial for hydrogels to have good mechanical properties under physiological conditions. Although chemical crosslinking has good stability, the synthesis process is relatively complex, accompanied by the use/production of toxic and harmful chemical reagents^[52]. Therefore, more and more physically crosslinked DN hydrogels with good swelling resistance have been developed.

Liu et al. Synthesized PVA/P (AM-co-AA)/CS DN hydrogel by reversible physical interaction of PVA and polyacrylamide-co-acrylic acid/chitosan (P (AM-co-AA)/CS)^[53]. The synergistic effect of the two different physically crosslinked networks results in high extensibility and strong self-healing properties of the hydrogel. In addition, the strong electrostatic interaction can also prevent water molecules from attacking the polymer chains of the gel, thus greatly reducing the swelling ratio of the hydrogel, so that the swelling ratio of the hydrogel is stabilized at about 1.4 G/G.

Shi et al. Prepared a completely physically crosslinked DN hydrogel of sodium alginate/chitosan/zinc ion (SA/CS/Zn²⁺) without toxic chemical reagents^[54]. Compared with the sodium alginate/chitosan single network hydrogel crosslinked by electrostatic interaction, the mechanical properties of the DN hydrogel are obviously improved. The swelling ratio of the hydrogel can be effectively adjusted by adjusting the content of zinc ions, and the swelling ratio of the hydrogel decreases with the increase of zinc content. Because a higher zinc content results in a higher crosslinking density, the exposure of the polymer chain to water molecules is reduced, thereby reducing the swelling rate. SA-CS-10 (10 represents the content of Zn) exhibited rapid water absorption (over 500% after 1 H) and reached about 1500% after 10 H. In contrast, SA-CS-30 achieved only 500% swelling.

In addition to the introduction of physical crosslinking network, Wang et al. Found that the introduction of graphene oxide into DN gel can also reduce the swelling rate of hydrogel^[55]. By incorporating the modified reinforcing filler graphene oxide into calcium alginate/polyacrylamide (CA/PAM) DN hydrogels, providing substantial physical interpenetration and chemical bonding between long polymer chains and graphene oxide nanosheets,The graphene oxide network induced by Ca²⁺ coordination was constructed, which improved the mechanical properties of the original DN gel, and its swelling properties were significantly inhibited, with smaller swelling changes in the visual model diagram. At the same time, the hydrogel also showed excellent fatigue resistance and high biocompatibility.

Hydrogels are highly deformable due to their swelling properties, but they are generally brittle and conventional strengthening or toughening methods tend to reduce their stretchability. There are now several literature reports on strategies to fabricate tough but deformable hydrogels, enhancing their mechanical strength while improving their swelling properties. Kim et al. Developed a new three-step strategy to prepare PVA/poly (AM-co-AA sodium salts) DN hydrogel^[56]. Dense entanglement was achieved by using unusually low amounts of water, crosslinker, and initiator during synthesis. A super-tough DN hydrogel with remarkable swelling characteristics (1200 ± 20% swelling ratio) and an exceptionally high compressive modulus (10.12 ± 0.31 MPa) was obtained using this strategy. Wu et al. Achieved swelling-enhanced hydrogel (SSH) via a biomembrane barrier-inspired strategy, and liposomal membrane nanocarriers covalently embedded in a cross-linked network were used to regulate transmembrane transport^[57]. The designed SSHs automatically switch from a single network to a double network structure via a catalyst-free click reaction without the help of an external trigger, and present an increased mechanical strength after swelling (the compressive modulus of SSH increases by 15.6% ± 4.5% at a swelling ratio of 25%, and SSH can maintain its initial mechanical strength even when the swelling ratio is increased to 75%). Wang et al. Carried out a force-coupling reaction by forming chain elongation in a polymer network, triggering the chain to reach its nominal scission point^[58]. Reactive chain extension of up to 40% results in a further stretching of the hydrogel by 40% ∼ 50% and exhibits twice as large tearing energy compared to a network made of similar control chains. These enhancements are synergistic with those provided by the dual-network architecture and complement other existing fortification strategies.

The introduction of physical crosslinking and the strategy of making tough but deformable hydrogels into DN hydrogels has improved the swelling resistance of DN hydrogels, and also made them have good mechanical properties, excellent fatigue resistance and high biocompatibility under physiological conditions. This greatly extends the service life of the hydrogel under long-term mechanical loading conditions.

Self-repairing performance refers to the ability of materials to repair themselves after being damaged, which is the essential attribute of some materials. As a new type of smart material, self-repairing hydrogel can effectively prolong the service life of hydrogel, and has great application potential in biomedicine, tissue engineering, environmental management and other fields^[59]. The ability of hydrogels to repair themselves is closely related to the way they are cross-linked. Traditional DN hydrogels usually contain one or two irreversible covalent crosslinking networks, but the covalent crosslinking is irreversible, which will lead to permanent breakage of covalent bonds under high strain, which not only destroys their mechanical properties, but also makes them have low self-healing properties and compatibility^[60][11]. On the contrary, the physical crosslinking is reversible. After the hydrogel is subjected to large deformation or damage, the rupture of the physical crosslinking bond can effectively dissipate energy, providing high mechanical strength and toughness for the DN hydrogel, which can be reformed in the subsequent process. Therefore, the physically crosslinked hydrogel has high fatigue resistance and self-healing properties^[53]. In general, the self-healing properties of fully physically crosslinked DN hydrogels are better than those of fully chemically crosslinked and physical-chemical hybrid crosslinked DN hydrogels^[61].

Sun et al. Introduced physical crosslinking into DN hydrogel, and prepared alginate/polyacrylamide (SA/PAM) DN hydrogel with Ca²⁺ as ionic crosslinking agent^[62]. After the first loading, the hydrogel was kept at 80 ℃ for 1 d, and 74% of the initial value was recovered, indicating that the hydrogel had a certain degree of self-recovery, and the hydrogel also showed high extensibility and toughness. Zheng et al. Prepared PAM/SA-Fe physically crosslinked DN hydrogel by one-pot method, as shown in Fig. 9^[63]. After one loading and unloading experiment, the fracture strength and toughness of the hydrogel recovered by 103. 85% and 75. 54%, respectively, within 1 min. This is better than the self-healing properties of most hybrid crosslinked DN hydrogels. Fully physically crosslinked hydrogels also exhibit excellent mechanical properties and fatigue resistance. Wei et al. Prepared a fully physically crosslinked Agar (Agar)/polyacrylamide (PAM) DN gel by using hydrogen bond crosslinking, as shown in fig. 10^[64]. Physically crosslinked DN gel also showed rapid self-healing properties at room temperature without external stimuli (toughness recovery of 83% after standing for 2 min). The tensile strength of the repaired gel is 0. 38 MPa (repair efficiency is 75%), and the tensile strain is 420%. DN gel has high strength, rapid self-healing and self-healing properties, and has broad biological application prospects under physiological conditions.

Inspired by Chinese people's use of gelatinized starch to paste spring couplets during the Lunar New Year, Shang et al. Introduced cassava starch (ST) into a chemically cross-linked polymer hydrogel based on polyacrylic acid (PAA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) to form a hybrid hydrogel composite containing a covalent network^[65]. When the sample is cut into two pieces and then contacted at the fracture surface, the damage at the cut interface will slowly but steadily self-heal to the initial state, with stress and tensile strength of about 99% and 96%, respectively, after 24 H. Such a high self-healing ability is very rare for previous PAA matrix based hydrogels.

Self-repairing DN hydrogel has many advantages. Its high self-repairing performance can not only prolong the service life of the hydrogel, but also show excellent mechanical properties and anti-fatigue characteristics, which makes the hydrogel have broad application prospects under physiological conditions.

The mechanical properties of various double-network hydrogels are summarized in Table 2.