A reversible covalent bond is a dynamic bond that can be broken under certain conditions and then recombined. When it is introduced into the molecular chain segment of polyurethane, it not only has little effect on the mechanical properties of polyurethane, but also can realize the self-healing functionalization of polyurethane. Common reversible covalent bonds include Diels-Alder reaction, disulfide bond, diselenide bond, borate bond, etc.

Diels-Alder reaction (DA reaction) was discovered by German scientists in 1928. It was first used as an organic synthetic reagent and first applied to polyurethane synthesis in 1992^[18]. This reaction refers to the 1,4-addition of a conjugated diene to an unsaturated compound with a carbon-carbon double bond to form a six-membered cyclic compound. The Diels-Alder reaction has been used many times to prepare self-healing urethanes because of its thermal reversibility, mild reaction conditions, and low degree of side reactions^[19⇓~21]. When the polyurethane molecular chain is broken due to damage, the conjugated dienes and unsaturated carbon-carbon double bonds in the molecular chain at both ends of the broken chain will be recombined through the DA reaction, thus realizing the macroscopic self-healing of polyurethane. Moreover, under the thermal drive, the polyurethane chain containing the DA bond will be broken into short molecular chains due to the reverse reaction of the DA reaction. The resulting short molecular chains move faster than conventional polyurethane molecular chains and easily move from one side of the crack to the other, filling the entire crack (Figure 2). When the temperature drops slightly, the short polyurethane chain can react with DA again, and the long polyurethane chain can be reconstituted to realize the self-healing of polyurethane cracks. Therefore, with the increase of time, the number of molecular chains that can fill the cracks also increases, thus improving the efficiency and strength of polyurethane self-healing. However, if the self-healing time is too long, the resulting short polyurethane chains and small molecules will undergo side reactions such as the decomposition of maleimide groups or the polymerization of their double bonds, which will prevent the DA reaction from continuing, resulting in more loss of mechanical properties of polyurethane after self-healing^[19,22,23]. Therefore, it can be considered to graft the DA bond onto PCL, and use the DA bond as a dynamic crosslinking point to combine high modulus and toughness with low thermally triggered molecular fluidity, so as to endow polyurethane (PU) with excellent mechanical properties (Young's modulus is about 80 ~ 225 MPa,Ultimate tensile strength 16 – 30 MPa, toughness ~26-96 MJ·m^-3), and the ability to spontaneously heal damages such as large scratches, punctures, and complete cuts at low heating temperatures (60 – 70 ° C)^[20]. In addition, the mobility of the molecular chain segment of polyurethane containing DA bonds will also have a huge impact on its self-healing properties. At the same temperature, the faster the mobility of polyurethane molecular segments, the higher the encounter probability of DA bonds, indicating that more DA bonds can be recombined in the same time, thus accelerating the self-healing rate of polyurethane. Ionic liquid (EMITFS) can also be added between the polyurethane molecular chains as a plasticizer to improve the fluidity of the polyurethane chain segments, thereby reducing the self-healing temperature^[21]. The obtained composite has PU-DA as the polymer network, and the EMITFS molecules are dispersed between the polymer networks. The self-healing temperature was reduced from 120 ° C to 60 ° C due to the improved fluidity. In addition, the composites prepared by mixing polyurethane PU-DA containing DA bonds in the main chain with other rigid materials, such as carbon nanotubes, can also achieve self-healing through DA bonds, and the mechanical properties after self-healing can be almost restored to the original level^[24].

Disulfide bonds have been one of the most widely used covalent bonds since they were introduced into polyurethane segments in 1972^[25]. Unlike other polyurethanes containing strong covalent bonds, which need self-healing at high temperature, the disulfide bond has a small bond energy of about 251 kJ/mol, which can be exchanged at room temperature to achieve self-healing^[26]. When the polyurethane containing disulfide bonds is damaged, the polyurethane at the two ends of the damage contacts with each other, and the disulfide bonds in the main chain are exchanged and recombined to reconnect the broken polyurethane molecular chains, which is reflected macroscopically in the disappearance of the gap of the polyurethane, that is, self-repair is realized^[27,28]. This process can be clearly seen in the optical microscope images. At first, the gap is a black line under the microscope. As time goes on, the black line fades, meaning that the gap gradually disappears and becomes almost invisible after a while (Fig. 3). Although disulfide bonds play a key role in the self-healing process of polyurethane, the self-healing efficiency does not necessarily increase with the increase of disulfide bond content. This is because the mobility of the polyurethane segment also affects the efficiency of self-healing, and the two effects on self-healing are mutual^{[26,28⇓ ~30]}. And the position of the disulfide bond also has an effect on the self-healing performance. When the disulfide bond is used as a chain extender in the polyurethane hard segment, if the content of the dynamic disulfide bond is very low, although the mobility of the soft segment is very high, the low-density dynamic bond still cannot repair the material at an obvious speed; If the content of dynamic disulfide bonds is very high, no matter how high the density of dynamic bonds is, the degree of microphase separation in polyurethane is too deep, and the mobility of molecular chains in the hard phase region is severely inhibited due to its T_g higher than room temperature, and the bond exchange of disulfide bonds can not be realized. When disulfide bonds exist in the soft segment of polyurethane, the self-healing efficiency of polyurethane always increases with the increase of disulfide bond content. However, when the content of disulfide bonds is the same and the molecular weight of the soft segment is reduced, that is, the distribution of disulfide bonds is more dispersed, the self-healing efficiency of polyurethane will also be reduced. This is due to the fact that the molecular weight of the soft segment is reduced, the influence of the hard segment on the soft segment is enhanced, and the molecular chain fluidity of the soft segment linking the hard segment is inhibited, thereby reducing the overall self-healing performance of the polyurethane^[31,32]. Therefore, the self-healing ability can be optimized by adjusting the proportion of soft and hard segments of polyurethane or the content of disulfide bonds. A kind of hydrophobic polyurethane (BS-PU) with bis (4-hydroxyphenyl) disulfide (HPS) as chain extender can achieve self-healing at room temperature by controlling the content of disulfide bonds^[29]. Due to the presence of disulfide bonds, BS-PU can achieve self-healing within 360 min at room temperature. In order to further improve the mechanical properties of self-healing polyurethanes containing disulfide bonds for applications in more fields, bis (4-hydroxyphenyl) disulfide (HPS) containing benzene rings can be selected as an aromatic disulfide chain extender to prepare transparent and easy-to-process self-healing polyurethanes (TPU)^[27]. The obtained polyurethane not only has ultrahigh tensile strength and toughness (6.8 MPa and 26.9 MJ·m^-3), but also can recover 99% of the original mechanical properties within 120 min. In addition, dual chain extenders, such as the simultaneous use of bis (2-hydroxyethyl) disulfide (HEDS) and 1,8-alkanediamine (MD) as chain extenders, can also be selected to tailor the mechanical properties of self-healing polyurethanes^[33]. The amino group of MD and isocyanate react to form a group containing hydrogen bonds, providing better mechanical properties, while the rigid ring structure of MD promotes the cleavage and reorganization of disulfide bonds, thereby improving the self-healing efficiency of polyurethane. In addition to being present in the chain extender, the disulfide bond can also be introduced into the polyurethane as part of the soft segment, and the synthesized polyurethane can be completely self-healed after 48 H at room temperature^[32].

In addition to the above covalent bonds, borate ester bonds, as a classical class of dynamic covalent bonds, are also widely used in the synthesis of self-healing polyurethanes^[34]. It can be cleaved into benzene diboronic acid and alcohols at room temperature to realize the bond exchange and recombination of borate ester bonds, thus realizing the self-healing of polyurethane. In addition, another dynamic thiocarbamate bond, which can realize C — S bond exchange and recombination under light and 50 ℃, has also been used as a commonly used dynamic covalent bond in the synthesis of self-healing polyurethane^[35]. Recent studies have shown that the diselenide bond, which exists in biological macromolecules such as proteins, lipids, and nucleic acids, can also be used as a reversible dynamic bond in self-healing polyurethanes (Figure 4)^[36,37]. These studies have shown that reversible covalent bonds are an excellent class of self-healing functional groups that can be used in polyurethane segments.

Unlike reversible covalent bonds, which require shared electron pairs, dynamic non-covalent interactions achieve the self-healing function of polyurethane through the breakage and reconnection of weak interactions between different atoms. Noncovalent interactions, however, are relatively labile, susceptible to disruption under the same conditions, and also susceptible to reassociation^[37].

Therefore, the dynamic non-covalent interaction can not only be used as a dynamic bond to synthesize self-healing polyurethanes, but also sometimes be used as a sacrificial bond to improve the mechanical properties of self-healing polyurethanes. Common dynamic non-covalent interactions are hydrogen bonds, metal coordination bonds, ionic bonds, etc.

The hydrogen bond is formed by the interaction between a proton donor and a proton acceptor, and is an interaction force that is slightly stronger than the intermolecular force and slightly weaker than other covalent bonds. Because there are a lot of urethane bonds in polyurethane, there are a lot of hydrogen bonds in polyurethane itself. And are most commonly used in the construction of self-healing polyurethanes due to their selectivity, directionality, and reversibility^[38,39]. Unlike the increase in reversible covalent bonds, which depletes the mechanical properties of polyurethanes, the increase in hydrogen bonds, especially the presence of multiple hydrogen bonds, can improve the mechanical properties and self-healing ability of polyurethanes (fig. 5). This is because multiple hydrogen bonds exist between stretchable polymer chains, which aggregate to form geometrically constrained arrays that act as strong but reversible sacrificial and crosslinking bonds, building a physically dynamic crosslinking network. The strength of the hydrogen bond is below 40 kJ/mol, which is much smaller than the chemical bond in the polyurethane chain segment. When polyurethane is stretched, hydrogen bonds are first destroyed, dissipating a part of energy, eliminating stress concentration and promoting molecular chain orientation, thus significantly improving the strength and toughness of polyurethane. Therefore, the broken hydrogen bonds at room temperature can also recombine to form new hydrogen bonds to achieve macroscopic self-healing. It is worth noting that heating can accelerate the self-healing process of hydrogen bonds. This is because some molecular segments are difficult to move at room temperature, and can hardly drive the broken hydrogen bonds on their segments to combine with other broken hydrogen bonds. Therefore, the application of a certain temperature can accelerate the movement of the molecular chain segment and accelerate the combination of the broken hydrogen bonds on the chain segment with other broken hydrogen bonds. The macroscopic manifestation is that the temperature rises and the self-healing speed accelerates. When the heating process is over, the temperature drops, and the hydrogen bonds have reconnected the broken polyurethane molecular chains, which is reflected in the macroscopic self-repair of polyurethane^[38,40,41]. Due to the movement of polyurethane molecular chains, many macromolecules in organisms, such as carnosine, cellulose, spider silk, etc., are also self-healing through hydrogen bonds without loss of mechanical properties^{[42][43,44][45]}. Inspired by this, polyurethane (SPU) with multiple hydrogen bonds, which imitates the structure of spider silk, not only has ultra-high true fracture stress (1. 21 GPa) and ultra-high toughness, but also can self-heal^[46]. The tensile strength and elongation at break of the fractured SPU elastomer reached nearly 100% self-healing efficiency after 36 H at 100 ℃. However, the self-healing speed of this kind of polyurethane needs to be improved, which is difficult to meet the needs of daily applications. Increasing the density of hydrogen bonds can be one of the ways to solve this problem, for example, chain extenders with quadruple hydrogen bonds on the side chain can be used to increase the density of hydrogen bonds in polyurethane chain segments.Then a self-healing polyurethane with higher self-healing efficiency and more robustness was prepared by virtue of the inverse pearl structure and the high-density non-covalent bond interaction at the interface between the dopamine-modified graphene oxide and the polyurethane matrix^{[46⇓~48][40]}. Among them, dopamine can increase the density of hydrogen bonds at the interface and obtain an interwoven network, which endows polyurethane with excellent self-healing ability at ambient temperature. The prepared polyurethane can not only self-heal at room temperature (the healing efficiency reaches 90% after 1 H), but also self-heal at-18 ℃, showing excellent self-healing performance.

As a non-covalent interaction, the bond energy of ionic bond is weaker than that of general covalent interaction, but stronger than that of hydrogen bond and intermolecular interaction, and it is also widely used in self-healing polyurethane due to its excellent reversible recombination reaction (fig. 6)^{[49⇓⇓~52]}. There is an electrostatic interaction between two anions and cations, and when two ions with reversible charges are close to each other, they will attract each other and form an ionic bond^[53,54]. However, the complete recovery of mechanical properties of polyurethane can not be achieved only by ionic bonds. When polyurethane is broken by external force, the more unstable ionic bonds are destroyed first. Only when the unstable ionic bond is completely destroyed, the covalent bond that is difficult to destroy will be destroyed, but the destroyed covalent bond accounts for a small part. When the two ends of the broken polyurethane are in contact, the ionic bonds will attract each other under the electrostatic interaction, which will re-entangle the molecular chains at the two ends of the fracture surface and produce a large number of strong physical entanglement points, accompanied by the process of re-linking the broken polyurethane short chain segments into polyurethane long chain segments. The large number of physical entanglement points can compensate for the loss of mechanical properties of polyurethane caused by the breakage of some covalent bonds. However, a small amount of broken covalent bonds can not self-heal, and the strength of physical entanglement points is slightly lower than that of covalent bonds, so the mechanical properties of polyurethane after self-healing can only be close to 100% of the original, but not consistent^[5,13]. With the ionic bond as the self-healing driving force, the ionic polyurethane (i-PU) with quaternary ammonium salt in the main chain can achieve complete self-healing after standing at room temperature for 400 min, and its mechanical properties can be restored to 97. 5% of the original^[49]. In addition to the ionic bonds on the main chain, the ionic bonds carried on the side chain can also endow polyurethane with excellent self-healing properties. Polyurethanes (ionic PUs) with imidazolium cationic groups on the side chain not only have high tensile strength (about 16. 9 MPa), elongation at break (about 1600%) and toughness (about 198 MJ·m^-3), but also the reversible ionic interaction between the ionic pairs on the side chain of polyurethane helps the polyurethane to achieve crack self-repair at mild temperature (40 ℃)^[55]. In addition, there is not only a single ion in the polyurethane molecular chain, but also a variety of ions to improve the self-healing efficiency and mechanical properties^[50,56]. For zwitterionic polyurethanes (ZSMPUs), due to the special structure of cations in the main chain and anions in the side chain, the polyurethane molecules can form a dynamic crosslinking network through ionic bonds to improve the mechanical properties, and have repeatable self-healing properties, which can recover the original properties after 2 H of self-healing at 50 ℃^[56].

In addition to the above common hydrogen and ionic bonds, the metal coordination bond formed by the metal ion and the ligand together is also an excellent dynamic non-covalent interaction for the synthesis of self-healing polyurethanes, and the self-healing of polyurethanes can be achieved through the bond exchange between the metal ion and the ligand (fig. 7)^{[57⇓⇓⇓~61]}. In the polyurethane chain segment, the metal coordination bond formed by the cobalt ion as the metal center and the iminodiol as the ligand can be used as a physical dynamic crosslinking network point, and the self-healing polyurethane HPPU-Co containing the coordination bond not only has excellent tensile properties, but also can be completely self-healed after standing for 12 H at room temperature^[62]. In addition to metal coordination bonds, the Donor-Acceptor structure (DA structure) has attracted much attention because it can endow polymers with significant stretchability, toughness, and self-healing properties (Fig. 7)^{[63⇓⇓~66]}. Moreover, because polyurethane is a special polymer with microphase separation, the synergistic effect of the movement of its molecular chain during microphase separation and D-A self-assembly can further improve the mechanical properties of polyurethane. Inspired by this, we introduced the electron-donating structure of naphthalene ring (D) and the electron-withdrawing structure of imide group (A) into polyurethane, and the synthesized polyurethane DA-PU with alternating donor and acceptor groups along the main chain not only has excellent mechanical properties,At the same time, when the polyurethane is damaged, under the movement of the molecular chain segment, the damaged DA structure and the isolated D and A units will self-assemble again to realize the self-healing of the polyurethane. ^[67]. The polyurethane can completely recover its original properties after self-healing at 60 ℃ for 400 min. In addition, many researchers have introduced ion-dipole moment interaction into polyurethane chain segments, which can also endow polyurethane with excellent self-healing properties^[68,69].

Although the introduction of reversible covalent bonds or dynamic non-covalent interactions in the polyurethane chain segment can endow polyurethane with excellent self-healing properties, the energy of reversible covalent bonds is usually relatively low, resulting in slow self-healing rate, and most of the self-healing bonds can only maintain the original mechanical properties of polyurethane and can not be further improved. In order to obtain polyurethanes with high toughness and fast self-healing ability, reversible covalent bonds and dynamic non-covalent interactions can be introduced into the polyurethane chain segment. The reversible covalent bond is responsible for the self-healing ability, while the dynamic non-covalent interaction acts as a sacrificial bond, which is first destroyed during the stretching process, thus improving the mechanical properties of the self-healing polyurethane as a whole^{[70⇓⇓~73]}. Because the polyurethane segment itself forms one of the dynamic non-covalent hydrogen bonds, the most common combination of multiple driving forces is the combination of hydrogen bonds with other reversible covalent bonds. For example, disulfide bonds can be introduced into polyurethanes containing multiple hydrogen bonds to effectively improve their self-healing efficiency^[33]. Under the synergistic effect of multiple hydrogen bonds and disulfide bonds, the tensile strength of the synthesized polyurethane reaches 24. 8 MPa, the elongation at break reaches 2 143. 7%, and the scratch of the polyurethane disappears completely after 30 min of self-healing at 40 ℃. Another combination effect is the combination of dynamic ditellurium bond and quadruple hydrogen bond, namely 2-ureido-4 [1H] -pyrimidone (UPy), which endows polyurethane with excellent self-healing properties and toughness at the same time^[74]. Quadruple hydrogen bonding (UPy) can make the toughness of polyurethane up to 105.2 MJ·m^-3 by enhancing the physical crosslinking of intramolecular/intermolecular chains; After the introduction of dynamic ditellurium bond, the self-healing efficiency was significantly improved under the synergistic effect of quadruple hydrogen bond, which could be restored to 92. 9%. In addition, the developed high-toughness self-healing polyurethane can also combine the characteristics of high toughness (127.0 MJ·m^-3) and fast self-healing (60 ℃, 2 H) by using the synergistic effect of dynamic imine bond and multi-level hydrogen bond^[75]. In addition to the combination of dynamic covalent bonds, hydrogen bonds and other dynamic non-covalent bonds, such as ionic bonds and metal coordination bonds, can also endow polyurethanes with excellent mechanical properties and self-healing properties^[73][69,72].

However, in the self-healing process of polyurethane, the effect of broken covalent bonds on its mechanical properties cannot be ignored. In the polyurethane network, the covalent bond of polyurethane is only a basis for maintaining the mechanical properties of polyurethane, and a single molecular chain of covalent bond alone cannot maintain sufficient mechanical strength.The physical crosslinking points produced by the physical entanglement of polyurethane molecular chains and the dynamic crosslinking points produced by the introduction of self-healing bonds can greatly improve the mechanical properties of polyurethane. In the fracture process of polyurethane subjected to external force, the physical entanglement point absorbs a part of the external force and is destroyed first; Secondly, self-healing bonds such as hydrogen bonds, ionic bonds, and coordination bonds are more unstable than stable covalent bonds such as urethane bonds, and are destroyed as sacrificial bonds after absorbing most of the external force. Only when these sacrificial bonds are completely destroyed, more force is needed to destroy other stable covalent bonds, and the remaining external force can only destroy a very small part of the covalent bonds. When the broken polyurethane is in contact with each other, due to the reversible effect of the self-healing bond, the self-healing bond first contacts with each other to form a new bond, and the broken polyurethane short molecular chain is relinked into a long molecular chain to restore the chain length of the polyurethane to a certain extent. Moreover, at a certain temperature, the polyurethane molecular chains can re-entangle to form a large number of physical entanglement points. The newly formed physical entanglement points can compensate for the loss of mechanical properties of polyurethane caused by the breakage of some covalent bonds. After the above two processes, the polyurethane can almost restore the original mechanical properties. However, the broken covalent bonds such as urethane bonds can not heal themselves, and the strength of physical entanglement points is lower than that of covalent bonds, so the mechanical properties of polyurethane can not be fully restored to 100%, but can only be close to 100%^{[5,13,70⇓⇓ ~73]}.

Stimulus-responsive polymers with similar abilities have attracted much attention, inspired by the shape transformation of organisms in nature in response to environmental changes. Among them, shape memory polymers (SMP) are a representative class of stimuli-responsive polymers, which can be pre-set in shape and restored to their original shape after being stimulated by external stimuli^{[76⇓⇓~79]}. This process has good reversibility. Therefore, after the shape memory function is introduced into the self-healing polyurethane, the obtained shape memory self-healing polyurethane not only has the function of spontaneous healing after being damaged, but also has the shape memory function of restoring the original shape after being stimulated after deformation, and can be designed into a sensor responding to stimulation to be applied in the fields of medical treatment, aerospace and the like (fig. 8)^[80⇓~82].

By introducing Diels-Alder reaction and aniline trimer (AT) into the light-induced programmed shape memory polyurethane, a polyurethane with both target shape memory and precise self-healing properties can be obtained^[78]. Under the action of Diels-Alder reaction, the polyurethane has excellent self-healing performance, and the healing efficiency is still more than 70% after three cycles of self-healing. In addition, the shape memory property of the prepared polyurethane is also excellent, and the polyurethane can restore the original complex 3D structure or perform pre-designed motion under the excitation of infrared rays; At the same time, it also has repeatable shape memory performance, and after three consecutive shape memory cycles, its shaping rate and shape recovery rate are both above 90%. In addition, the introduction of ionic bonds in shape memory polyurethane gels that can be used for 4D printing can also endow them with excellent self-healing properties^[83]. The self-healing property is dependent on the ionic bonds formed between the COO^- groups of the polyurethane nanoparticles and the {{\mathrm{NH}}_3}^{+} groups on the gelatin chains. Moreover, the polyurethane used as the raw material for the shape memory 4D printing structure shows good shape fixation (more than 95%) and shape recovery (more than 98%). This 4D bioprinted, self-healing polyurethane hydrogel with shape memory has very large potential for custom biomanufacturing. In addition to ionic bonds, the introduction of dynamic oxime-carbamate bonds and hydrogen bonds into the molecular chain can also achieve the combination of self-healing properties and shape memory properties of polyurethane^[84]. Polyurethane with 1,4-benzoquinone dioxime (BQDO) and 1,4-butanediol (BDO) as chain extenders has a self-healing effect of 80% within 80 s, and has significant shape memory properties, which can be effectively triggered by near-infrared radiation or direct heating.

With the increasingly serious environmental problems, countries around the world are formulating different policies to reduce pollution. Among them, the government has put forward measures such as plastic ban and garbage classification to reduce the white pollution caused by material waste. This shows that the use of degradable materials in all aspects is imminent^[85,86]. At present, in addition to natural degradable polymers and biodegradable polymers synthesized by microorganisms, there are also many synthetic degradable polymers, such as polylactic acid (PLA), polycaprolactone (PCL), poly (butylene terephthalate/adipate) (PBAT), etc., which are widely used because of their excellent processability and wide mechanical range. For self-healing polyurethanes, they can prolong their service life by self-healing in the process of use, which is a means to reduce material waste to some extent. However, any material will have the end of its service life, and self-healing polyurethane is no exception. If the self-healing and degradable properties can be integrated into polyurethane, the environmental protection can be achieved with half the effort. In recent years, the introduction of degradable groups into the molecular chain of self-healing polyurethane and the development of degradable self-healing polyurethane decomposed into small molecules or oligomers through unstable bonds under mild conditions have become one of the hot spots to meet the sustainable development strategy (Fig. 9)^{[87⇓⇓~90]}.

Ester bond is one of the most common degradable groups, which can be introduced into the molecular chain of polyurethane to endow self-healing polyurethane with certain degradability^[84⇓~86]. When the ester bond is introduced into the polyurethane molecular chain containing the disulfide bond and the hydrogen bond, the obtained polyurethane can have both self-healing property and degradable property^[91]. Due to the influence of disulfide bonds and hydrogen bonds in the molecular chain segment, the synthesized polyurethane PU-ATs can recover to the original state after self-healing for 3 H at 40 ℃. At the same time, due to the ester bond in the molecular chain segment, PU-ATs could be hydrolyzed in PBS buffer solution (37 ℃, pH = 7.4), and its mass lost 30% in seven weeks. In the case of glutathione (GSH) as a disulfide reducing and degrading agent, polyurethane can be degraded more effectively. GSH is an effective reductant that converts disulfide bonds to thiol groups. In GSH solution, PU-ATs exhibited a faster degradation rate than in PBS medium. In addition, the self-healing polyurethane synthesized from biomaterials can also be degraded under certain conditions. After introducing alginate groups into the polyurethane backbone, alginate can not only act as an ionic bond to endow polyurethane (ASPU) with excellent self-healing properties, but also act as a degradation group to promote the rapid degradation of polyurethane in vivo, and its degradation rate is similar to that of PCL^[92]. Due to the presence of a large number of cations and alginate, the two pieces of ASPU can be completely self-healed after 30 seconds of contact under the action of ionic bonds, and the self-healing efficiency can still reach more than 70% after repeated "breaking-self-healing". When ASPU was put into the solution containing Lipase, the mass loss of ASPU was nearly half after 96 H, which was much faster than the degradation rate of PCL, indicating that ASPU had excellent degradation ability. Similarly, the novel bifunctional polyurethane CS-PU obtained by introducing chitosan into the polyurethane chain containing ester bonds and Schiff bases also has excellent degradability and self-healing properties^[84].

To date, self-healing polyurethanes have been widely used in healthcare, textile industry, and food packaging due to their excellent mechanical properties, water resistance, gas permeability, and biocompatibility^[94⇓~96]. It is worth noting that the antibacterial activity of pure polyurethane is not good, and it is easy to breed bacteria in the long-term use process. Bacterial contamination of various polyurethane surfaces, including implantable devices, surgical equipment and packaging films, may lead to widespread epidemics.It adversely affects our daily lives and, at the same time, poses a devastating threat to human health, causing serious complications for patients and placing a heavy burden on the health care system^[97⇓~99]. In this infection process, bacterial adhesion to the polyurethane surface is a prerequisite. To make matters worse, bacteria that adhere to polyurethane surfaces produce extracellular polymers to form biofilms composed of proteins and other extracellular polymers. It protects bacteria from antimicrobials and is difficult to eliminate. Therefore, in order to improve the safety of self-healing polyurethanes against bacterial infection, it is of great significance to study self-antibacterial polyurethanes.

At present, there are two ways to endow self-healing polyurethane with excellent antibacterial properties: one is to introduce chemical bonds, such as disulfide bonds and ionic bonds, into the molecular chain of polyurethane to achieve antibacterial function (Fig. 10)^[100,101]. The introduction of disulfide bond in the main chain of polyurethane can not only endow polyurethane with excellent self-healing performance (the self-healing efficiency can reach 88. 60% after 30 min of illumination), but also endow polyurethane with excellent antibacterial performance, the antibacterial performance of Escherichia coli can reach 90. 2%, and the antibacterial performance of Staphylococcus aureus can reach 92. 5%^[102]. In addition, the introduction of metal coordination bonds in the polyurethane chain can not only improve the self-healing efficiency of polyurethane, but also improve the antibacterial properties of polyurethane^[103]. These polyurethanes form a covalently crosslinked zinc-dimethylethylenediamine-polyurethane coordination network with a portion of hydrogen bonds, which have triple dynamic bonds. The recombination of hydrogen bonds and metal coordination bonds produces an effective self-healing property; At the same time, the metal coordination bond also endows the polyurethane with excellent antibacterial properties, and the obtained polyurethane film has a bacterial inhibition rate of 99.5% and 98.9% against Staphylococcus aureus and Pseudomonas aeruginosa, respectively. The other is to achieve antibacterial function by mixing antibacterial agents, such as silver nanoparticles, metal cations and so on^[55,104,105]. This method is more common. In order to make the combination of polyurethane and silver nanoparticles more stable, polydopamine nanoparticles with furfural group (FPDA NPs) can be introduced into the polyurethane chain through DA reaction as a crosslinking agent of silver nanoparticles (AgNPs)^[106]. The polyurethane film has excellent self-healing properties (90%) due to the presence of DA bonds. Importantly, due to the presence of FPDA, AgNPs can be completely coated and connected with PU molecular chains. The AgNPs act as antibacterial agents and nanofillers, giving the polyurethane film excellent long-term antibacterial properties against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus.

Self-healing polyurethanes are considered to have great potential in biomedical materials such as wound dressings, surgical sutures and sternal fixation lines because of their controllable mechanical properties^{[93,107⇓⇓ ~110]}. However, once the material is not properly selected, it will cause discomfort to the human body, and even cause a series of complications such as aggravation of injury and tissue infection, causing secondary injury to the human body. Ideal biomedical materials need to meet the following requirements: (1) good histocompatibility and no cytotoxicity; (2) Sufficient physical and mechanical strength to ensure its integrity and avoid the invasion of external bacteria caused by material damage; (3) Appropriate surface microstructure and biochemical properties to promote cell adhesion, proliferation, and differentiation^[109,111]. Therefore, it is necessary for the self-healing polyurethane to be biocompatible and to promote cell growth on the surface without cytotoxicity. At present, the methods to improve the biocompatibility of polyurethane are mainly achieved by introducing biocompatible groups into the polyurethane chain segment (Fig. 11)^[110,112]. The high strength self-healing elastomer (SHE) was synthesized by using biocompatible dimethylglyoxime and glycerol as chain extenders and polytetrahydrofuran (PTMEG) as soft segment, which not only has excellent self-healing properties, but also has excellent biocompatibility^[119]. The self-healing properties of SHE are dependent on the dynamic oxime-carbamate bonds and hydrogen bonds in the polyurethane segment.

In the analysis of acute and chronic subcutaneous inflammation, the inflammatory response of SHE is the smallest, which proves that SHE has excellent biocompatibility and can be used to treat some clinical diseases, such as limiting aneurysms, covering nerves and fixing sternum. The CS-PU containing Schiff base synthesized from chitosan also has excellent self-healing properties and biocompatibility^[93]. CS-PU could completely self-heal after standing at room temperature for 1 H, and the cells proliferated significantly after seven weeks of culture on the polyurethane substrate. Because of these advantages, the polyurethane can be used as a wound dressing to promote the reorganization of skin cells and the generation of new tissue. In addition to the above methods, the polyurethane gel obtained by blending the aloe gel having excellent biocompatibility with the water-soluble self-healing polyurethane also has both biocompatibility and self-healing properties^[113]. The polyurethane hydrogel can be used as a microneedle scaffold after secondary processing, and the microneedle scaffold has excellent self-healing performance due to the existence of dynamic hydrogen bonds and Schiff bases in the polyurethane hydrogel. Subsequently, they were placed in medium containing microneedle scaffolds to culture 3T3 cells. Five days later, it was found that the cells not only grew well in the scaffold part, but also adhered and grew in the microneedle part. This result shows that the prepared polyurethane hydrogel microneedle scaffold has excellent biocompatibility, and the polyurethane hydrogel can be used for wound tissue healing and drug release control.

In addition to introducing a series of functionalizations such as shape memory, degradability, antibacterial and biocompatibility into self-healing polyurethane, other researchers have endowed self-healing polyurethane with a variety of other functions to match more application scenarios^{[114⇓⇓~117]}. The incorporation of a large number of fluorine atoms in the self-healing polyurethane molecular chain can be used to separate immiscible oil/water mixtures and water-in-oil emulsions driven by gravity alone^[118]; The self-healing waterborne polyurethane composite film containing boric acid and borax synthesized by nucleophilic substitution method can be used as a leather coating to improve the abrasion resistance of leather^[119]. Due to the aggregation-induced emission (AIE) characteristics of the specific tertiary amine structure of the side chain, the high-strength self-healing polyurethane with bisamide hydrogen bonds in the side chain not only has good self-healing ability at room temperature, but also can be used as a fluorescent coating for anti-counterfeiting applications^[120].

Self-healing polyurethane is very suitable as the substrate material of flexible sensors because of its strong plasticity, good wear resistance, strong corrosion resistance and self-healing. In detail, polyurethane has excellent plasticity and can be made into different shapes for complex sensor designs. The good wear resistance can make the polyurethane-based sensor maintain its good performance in the long-term use process, which can be used in long-term monitoring and other fields. In addition, the excellent corrosion resistance ensures that the polyurethane-based sensor is free from chemical interference during use. Most importantly, the excellent self-healing performance makes the polyurethane-based sensor heal spontaneously after being damaged by external force, which further prolongs the service life of the flexible sensor. Due to these advantages, polyurethane has been widely used as a flexible sensing material.

Because the self-healing polyurethane itself has a certain dielectric constant, it can be directly used as a flexible sensing material to sense small physical changes (Figure 12)^{[29,30,35,67]}. A kind of pressure sensor with self-healing polyurethane (BS-PU) containing disulfide bond as dielectric layer and liquid metal as conductor can be used to sense the change of different pressures^[29]. The sensor can identify a specific pressure by a change in capacitance. Moreover, due to the disulfide bond in BS-PU, the sensor showed obvious toughness and self-healing ability at room temperature. After self-healing at room temperature for 6 H, the sensor recovered its function as before. In addition, the sensor with polyurethane as the sensing material can sense not only the change of pressure, but also the change of strain. The sensor with self-healing polyurethane containing DA structure as the sensing material can produce the change of capacitance signal under different strains, which can be used to monitor the standard degree of athletes'movement^[67]. Due to the DA structure in the polyurethane backbone, the sensor can self-heal at room temperature, and the sensing performance is hardly affected after self-healing.

Although the sensor directly using self-healing polyurethane as sensing material can sense different physical changes, due to the limitation of the dielectric constant of self-healing polyurethane itself, the sensitivity of the sensor is low, and it can not be applied to more sophisticated environments. In order to overcome the defect of sensitivity while retaining the self-healing performance of polyurethane, we can achieve it by filling the conductive material into the self-healing polyurethane. Common conductive fillers can be divided into two categories, one is to transmit signals through electron migration, such as graphene, carbon nanotubes, etc^{[121⇓⇓~124]}. The other is to transmit signals through ion migration, mainly ionic liquids (IL)^[34,125]. The first type of research is more extensive, for example, the self-healing polyurethane containing dynamic oxime-carbamate bonds blended with graphene as a dielectric layer is integrated into the corresponding sensor, which can be used to monitor human movement and various mechanical movements sensitively and stably^[121]. The sensor relies on electron migration to transmit signals, and different strains will change the distribution of the conductive network in the sensor, resulting in different electrical signals to distinguish different actions. In addition, the polyurethane molecular chain contains abundant dynamic bonds, so that the flexible sensor has excellent self-healing ability, and still has excellent strain response ability after self-healing. The second type of research is later than the first type, but it has a more significant effect on improving sensitivity^{[34,49,52,126]}. For example, the ionic sensing material developed by us is obtained by blending ionic self-healing polyurethane (i-PU) and ionic liquid, and a small pressure will produce a significant change in capacitance^[49]. Therefore, the sensor I-Skin-i prepared with this dielectric layer has excellent sensitivity. Moreover, because the polyurethane as the substrate material contains a large number of ionic bonds, I-Skin-i can self-heal at room temperature, and the sensitivity after self-healing can still be restored to its original state without affecting the subsequent use of the sensor.

For flexible sensors, electrode materials for electrical signal transmission by electron migration are an integral part. At present, most of the electrode materials used are rigid metal materials, such as gold, silver, copper, and indium tin oxide (ITO). Although these electrode materials have very low resistance and strong conductivity, they have little stretchability and poor fatigue resistance, and are easy to break and fail after multiple deformations, which seriously hinders their use in the field of flexible sensors. At the same time, this kind of electrode lacks self-healing performance, once damaged, it can not self-heal, affecting the transmission of electrical signals, and ultimately affecting the service life of flexible sensors. The integration of flexible substrate materials and conductive materials has become one of the breakthroughs in the development of flexible conductive electrodes. Self-healing polyurethane has great potential as a flexible electrode substrate material because of its excellent stretchability, fatigue resistance, and self-healing ability (Fig. 13).

Filling nanoscale conductive rigid materials, such as graphene, carbon nanotubes and silver nanowires, into self-healing polyurethane is one of the important strategies for the preparation of stretchable electrodes^{[122⇓~124,127,128]}. The two-dimensional sp2-hybridized carbon structure in graphene can effectively adsorb polymer chains, thereby improving the conductivity and mechanical robustness of the composite. Therefore, a self-healing and stretchable flexible electrode can be obtained by blending graphene with self-healing polyurethane containing oxime-urethane bonds and hydrogen bonds^[121]. The obtain flexible conductor can have a tensile strain as high as 1000% (about 6 MPa), yet has a very low resistivity (R=47.8Ω^-1). Due to the presence of oxime-urethane bonds and hydrogen bonds in the polyurethane substrate, the prepared flexible electrode can also self-heal at room temperature (10 min, 25 ℃), and the mechanical properties and conductivity can be restored to more than 90% of the original after self-healing.

In addition to the excellent conductivity of rigid materials such as graphene and carbon nanotubes, liquid metal (LM) also has good conductivity. Different from rigid conductive materials, liquid metal (LM) is a special kind of high deformation metal, which exists in liquid form at room temperature and has good fluidity^[129,130]. Considering this point, choosing LM and self-healing polyurethane for overcoating or blending is also one of the important strategies to prepare stretchable electrodes. Inspired by this, we used LM as the conductive ink as the inner core and self-healing polyurethane as the outer shell to prepare a stretchable, self-healing and highly conductive flexible electrode based on the core-shell structure^[29]. Due to the disulfide bond in the polyurethane, the flexible electrode can self-heal at room temperature, and the resistance can be restored to 7. 6 Ω after self-healing. In addition, LM can also be used as a filler to combine with self-healing polyurethane containing six-fold hydrogen bonds, which will not lose the mechanical properties of polyurethane, nor will it hinder the movement of polyurethane molecular segments in the self-healing process and affect the overall self-healing performance. The prepared conductive film can achieve complete self-healing at 75 ℃ for 10 min^[47]. Due to the existence of LM, the resistance of the prepared conductive film is only 1 to 2 ohms, and the conductive film is one of excellent flexible electrode materials.

Packaging is an essential part of the sensing element in order to protect it from the environment. Specifically, the encapsulation layer can protect the sensing element from the following aspects: (1) Mechanical protection. The sensing element is generally small and easily damaged by external force, so the packaging layer can provide physical protection for the sensing element as a buffer layer to prevent damage caused by external force; (2) Environmental protection. As mentioned in the above article, sensing elements generally transmit signals through electrons or ions, which are easily disturbed by environmental factors such as external humidity and temperature. If exposed to harsh environment for a long time, the sensing element is prone to failure. Therefore, the packaging layer can provide moisture-proof, dust-proof and other protection for the sensing element to prolong its service life; (3) Electrical protection. In the process of using the sensing element, the external electromagnetic, electrostatic discharge and other factors will destroy the process of signal transmission and affect its normal work. Therefore, the encapsulation layer can provide anti-electromagnetic interference protection for the sensing element and maintain its normal use. In addition, the encapsulation layer also has the advantages of facilitating the installation and use of the sensing element.

At present, the common packaging materials are ceramics, plastics, epoxy resins and elastomers, etc^[131⇓~133]. Ceramic is a high strength, high temperature resistant packaging material that protects the sensing element from the effects of temperature. Plastics are used in the packaging of sensing elements because of their excellent mechanical properties and flame retardancy. Epoxy resin has excellent chemical resistance and electrical insulation properties, and is also a good packaging material. However, the mechanical strength of these materials is too large and the ductility is too poor, so they can only be used to package conventional devices, and can not meet the growing demand for flexible sensors^[134]. Elastomer materials not only have good high temperature resistance, chemical resistance and electrical insulation, but also have good impact resistance and stretchability, which are most suitable for flexible sensing. Common elastomeric encapsulating materials include polydimethylsiloxane (PDMS), polyurethane, and polyolefin thermoplastic elastomer (POE). Among these elastomers, self-healing polyurethane is considered to be one of the most reliable encapsulating materials due to its outstanding combination of properties. On the one hand, the high stretchability of polyurethane allows the flexible sensor to retain its original stretchability after packaging; On the other hand, polyurethane has high resistivity, which can be used as an encapsulation layer to effectively prevent the sensor from losing its sensing performance due to short circuit. More importantly, compared with ordinary polyurethane, the use of self-healing polyurethane as the encapsulation material can also endow the sensor with overall self-healing performance at the same time, thus improving the service life of the sensor (Fig. 14)^{[35,135⇓ ~137]}.

The self-healing polyurethane can be directly cast in the form of a solution on a sensor containing a 3D binary conductive network of silver nanowires @ sulfated graphene foam (AgNWs @ TGF) to realize the encapsulation of the sensing element^[138]. Thanks to the good elasticity of the FPU, the resulting strain transducer exhibits excellent mechanical properties, including good stretchability (up to 60% strain) and 800 cycle fatigue tests. In addition, the strain sensor also exhibits excellent self-healing properties due to the strong intermolecular hydrogen bonding in the packaging material and the dynamic exchange reaction between aromatic disulfides. In addition to the casting method, the mixture of liquid metal and copper powder Cu/GaInSn can also be used as an ink to be printed on the self-healing polyurethane AL-PU containing dynamic disulfide bonds, and then turned over to coat the ink on the dielectric layer AL-PU^[30]. Due to the excellent mechanical properties of AL-PU, the flexible sensor E-Skin packaged with it also retains its excellent mechanical properties, not only the tensile strain can be as high as 800%, but also the modulus does not decrease significantly after 1000 repetitions at 100% strain, showing excellent fatigue resistance. Moreover, thanks to the dynamic disulfide bond in AL-PU, the damaged encapsulation layer can recover its original mechanical properties after 300 min of self-healing at room temperature, preventing the external environment from interfering with it. In addition, the flexible sensor encapsulated with pure liquid metal and self-healing polyurethane BS-PU containing disulfide bonds also has good stretchability and self-healing performance while retaining excellent sensing performance^[29].