The host–guest interaction is a type of noncovalent interaction formed by the physical insertion of the guest’s moiety into the host moiety 
. Generally, host molecules include cyclodextrins (CDs), pillar[n]arenes, crown ethers, calix[n]arenes, cucurbiturils and adamantane. The commonly used guest molecules include ferrocene, azobenzene, cholic acid and N-vinylimidazole derivatives.
Among the frequently used host molecules, CD has lipophilic inner cavities and hydrophilic outer surfaces, enabling high-affinity interactions with specific hydrophobic guest moieties. Specifically, as the most important member of the CD family, β-cyclodextrin (β-CD) is most widely produced and possesses a cavity that matches the size of numerous guest molecules and can be easily crystallized, separated and purified. Furthermore, β-CD inclusion complexes can enhance the resistance of the encapsulated guest molecules to various environments, such as acidic and alkaline media, light and heat [110,111,112]
. A self-healing hydrogel was synthesized by the host–guest interaction between the hydrophobic isopropyl group of N-isopropylacrylamide (NIPAM) and β-CD 
. The isopropyl group in NIPAM serves as the guest component and β-CD serves as the host component to form a host–guest complex. Hydrogels have a variety of hydrogen bonds and host–guest interactions. Extensive comparative experiments have shown that the host–guest interaction is the principal factor influencing the self-healing of hydrogels. The hydrogel, cut into two pieces, is capable of rapid self-healing at room temperature. The self-healing ability of the hydrogel was measured by its weight-bearing capacity before and after healing. For example, the original hydrogel (before cutting) could bear 200 g and after healing it could bear 55 g. Therefore, the self-healing efficiency is approximately 28%. Adamantane, as the guest molecule, can form a stable inclusion complex with the β-CD cavity and has a high binding constant with the β-CD cavity compared with other guest molecules. Rodell et al. 
used methacrylate to modify hyaluronic acid and further used it as the main chain of β-cyclodextrin/amantadine (β-CD/Ad) to prepare a double network hydrogel with self-healing properties. The cross-linking point of the first network was formed by the host and guest interaction, and the second network was a methacrylate network. Not only were the mechanical properties greatly improved, but self-healing could also be completed in an instant. The experimental results show that the cut hydrogel fragments heal quickly within about 1 s. However, hydrogels synthesized by chemical processes take a long time and produce toxic byproducts that are unsuitable for biological applications. Therefore, a nonchemical grafting method to prepare hydrogels was proposed 
. In this hydrogel, the amphiphilic substance N,N-dimethyl-1-adamantane (DM-AD) was used as a cross-linking agent, and CMC and poly β-cyclodextrin (β-CDP) were used as the polysaccharide skeleton. One end of DM-AD is the adamantly group, which is wrapped by β-CDP through host–guest interactions. The nitrogen atom at the other end combines with protons to form a quaternary ammonium compound and is electrically attracted to the carboxyl anion. To verify the self-healing ability of the hydrogel, two identical hydrogels were stained with different colors and cut from the middle of hydrogel. Take two different colored hydrogel cut surfaces into contact with each other. After much time (more than 0.5 h), the hydrogel was completely healed, and there was no obvious sign of fracture on the fracture surface of the hydrogel. In addition, stretching the ends of the hydrogel again, the hydrogel did not fracture.
In summary, by changing the host and guest monomers and polymers, different synthetic methods can be utilized to design and prepare host–guest complexes according to the different applications. Self-healing hydrogels containing reversible host–guest interactions exhibit some advantages, such as a repeatable healing process without any external energy, long storage time and high healing rate. Self-healing based on host–guest interactions is still a wide field for research due to the diversity of guest molecules and their reversible nature.