Self‐Healing Stress Sensors: Coupling Stress‐Sensing Performance with Dynamic Chemistry

Flexible stress sensors that can convert different forces into electrical signals show great potential for applications in wearable electronics and electronic skin. The integration of self‐healing capacity into stress‐sensing materials can boost their reliability, stability, and long‐term performance. There have been many reviews describing the progress of research on self‐healing stress sensors. However, most have typically investigated the self‐healing design separately from the sensing mechanism. The review focuses on how the latest advances in coupling stress‐sensing performance with dynamic chemistry balance self‐healing and sensing performance. Then, based on sensing structures and self‐healing dynamic bonds, the design strategies, and preparation methods of self‐healing sensors and their advantages and disadvantages in recent years are summarized in detail. It is anticipated that this work will provide insights and guidance for the future development of sensors with synergistically enhanced self‐healing and sensing properties and their application designs.

Nowadays, some of the existing reviews associated with selfhealing sensors are describing how self-healing components can be integrated into the sensors to improve their service life. [67][68][69] In terms of execution mechanisms, self-healing stress sensors are a combination of the fields of self-healing and force-electric sensing. However, in a multi-component sensing device, the individual self-healing capability of each component does not ensure that the entire device can properly recover from damage. Compatibility between different parts of the device is also needed to ensure the functional repair capability of each component as well as the entire sensing system. It is also important to take into account the structural integrity and functional responsiveness aspects of the recovery self-healing capability at the same time. It is worth noting that the self-healing structure design usually affects the electrical signal transmission, leaving the sensing structure and the self-healing structure mutually constrained. Therefore, it is important but still a challenge to integrate and even synergistically improve both self-healing and sensing performance.
Here, our progress report provides an up-to-date status of advanced materials for soft self-healing stress sensors. We in-www.advancedsciencenews.com www.advsensorres.com troduce typical types of polymers or composites under different force-electrical sensing mechanisms and how to integrate and even synergistically enhance self-healing and sensing properties. Then scale-up processing methods for self-healing stress sensors are presented and discussed for future applications of artificial electronic skin and wearable electronics. The main purpose of this paper is to provide a general summary of a contradictory domain of research and by highlighting specific examples to facilitate an intuitive and physical understanding for the reader.
Finally, standardized approaches, perspectives and future directions are discussed, as well as some new issues that have not been addressed before.

Integrated Design of Dynamic Bonds and Stress-Sensing Structure
Ionic conductors are functional materials, and although their resistance is greater than that of conventional electron-conducting materials due to the relatively low migration of ions compared to electrons, they remain a prospective solution for numerous sensing applications. [72,73] Ionic conductors are intrinsically extensible and transparent, and the resistance of ionic conductors varies much less than that of conventional electronic conductors at large strains. [74] Thus, ionic conductors have been used to prepare for strain sensors allowing the measurement of strains of up to 1000% or even greater. [75][76][77] Ionic conductors are mainly limited to liquid and solid phases. [78,79] There has been an increasing interest in solid-type ionic conductors predicated on polymer networks solventized in electrolyte solutions and ionic liquids. Several types of autonomous and nonautonomous self-healing ion conductors were developed and employed for sensing applications, including hydrogels, organic hydrogels, ionic gels/polyelectrolytes, ionomers, and encapsulated ionic electrolytes. [80] It is worth emphasizing that the dynamic bonding introduced by the self-healing design may affect the migration of ions, leading to instability of the material conductivity, which is detrimental to the stable output of electrical signals. The stable combination of sensing and design of self-healing structures under ion-conducting mechanisms is of wide attention.

Self-Healing Hydrogel Sensors
A very promising sensing material for ion conductors is hydrogels, where the movement of ions in aqueous, causes ionic con-ductivity and the polymer network acts as a substrate of solids to constrain both ions and solvents. Ionic conductive gels with the introduction of dynamic bonding in the system, freely migrating salt ions are not stable in conductivity in an aqueous environment and are not favorable for application as sensor devices. [81][82][83] How to design interactions to maintain or even enhance conductivity in self-healing conductive gels is still a matter that researchers are working on.
Conductive self-healing hydrogels based on self-healing mechanisms can be classified as external stimuli (heat, self-healing agents) or autonomous interactions of the material itself (dynamic chemical bonding, non-covalent interactions). [84][85][86][87] Recently reported mechanisms of conductive self-healing hydrogels are mainly based on autonomous self-healing. These include mainly dynamic covalent bonds (Schiff base reactions, disulfide bonds, Diels-Alder reactions, and borate ester bonds), and noncovalent bonds (hydrogen bonds, ionic interactions, hydrophobic interactions, and host-guest interactions). Ren et al. [88] proposed a self-healing conductive hydrogel with good morphology and properties based on an alginate-gelatin network and polypyrrole (Figure 2a). The alginate/gelatin network consists of a wide range of reversible Schiff base units that act as dynamic crosslinking agents for the self-healing property of the hydrogel. The hydrogel is mechanically responsive to bending or compression (Figure 2b), so it can monitor hand movements and demonstrate its application in repairable circuits and mechanical sensors ( Figure 2c). Fang et al. [89] developed a novel stretchable, sensitive, self-healing, and recyclable hydrogel using dynamic bonding based on Diels-Alder chemistry between a polyurethane substrate and a polyaniline conducting polymer (PU-DA-1/1-PANI). The PU-DA-1/1-PANI hydrogel has favorable mechanistic and electrical conductivity properties with extension at break. Hydrogels with dynamic covalent bonding as the self-healing mechanism have superior mechanical properties, but longer selfhealing time.
Non-covalent linkages are another prospective mechanism for repairing impairment through physical interactions, including hydrogen bonding, ionic interactions, hydrophobic interactions, and host-guest interactions. In general, these hydrogels have well-established self-healing capabilities. However, the mechanistic strength of hydrogel restoration is restricted on account of weak non-covalent bonding. To enhance the mechanical strength, the synergistic effect of multiple non-covalent bonds is usually present in a single hydrogel. Ionic hydrogel nanocomposites were prepared by integrating dynamic metal-ligand bonds in a loose cross-linked network of ionic gel nanocomposites doped with Fe 3 O 4 nanoparticles. [90] It has excellent self-healing properties (>95% healing efficiency), strong adhesion (347.3 N m −1 ), and high tensile properties (2000%) (Figure 2e,f). And as shown in Figure 2g,h, a proof-of-concept based ionic gel strain sensor with high sensitivity was demonstrated by varying the resistance with specific deformations, such as balloon expansion with arbitrary bending and moving surfaces, and typical deformations (e.g., extensive stretching and finger bending).
Electric carriers dominate in conductive hydrogels and polymer networks are present at low concentrations. [76] This property ensures the continuity of the conductive phase during the deformation process. However, too much fluid electrolyte cannot be effectively bound through the polymer network, which  Reproduced with permission. [88] Copyright 2019, Royal Society of Chemistry. b) Resistance curve of hydrogel during compression and bending progress. Reproduced with permission. [88] Copyright 2019, Royal Society of Chemistry. c) Demonstration of applications of self-healing conductive hydrogel for repairing the disconnected circuit and serving as one component to fabricate reconfigurable circuit and light up the LEDs. Reproduced with permission. [88] Copyright 2019, Royal Society of Chemistry. d) Schematic illustration of preparation procedures of the ionogel nanocomposites and chemical structures of the reagents. Reproduced with permission. [90] Copyright 2019, Wiley-VCH. e) healing time and f) healing temperature on the tensile stress and healing efficiency of the nanocomposites. Reproduced with permission. [90] Copyright 2019, Wiley-VCH. g) The dependence of area strain (left) and resistance (right) of a self-healed 2.5Fe 3 O 4 @PAA/20PAA-based strain sensor attached to an air inflating balloon. Reproduced with permission. [90] Copyright 2019, Wiley-VCH. h) The 1.5Fe 3 O 4 @PAA/20PAA-based strain sensor fixed on forefinger of a human finger to monitor bending motions, via the change of corresponding resistance. Bending forefinger gradually changed to 90°from standard position, and the resistance of sensor also corresponded to increase. Reproduced with permission. [90] Copyright 2019, Wiley-VCH.

Self-Healing Polymerizable Deep Eutectic Solvent-Based Sensors:
Recently, due to the advantages of simple, solvent-free preparation and high ionic conductivity, the newly emerging type of polymerizable deep eutectic solvent (PDES) has attracted the attention of functional materials. [92,93] PDES are usually obtained by the complexion of hydrogen bond acceptors, mainly quaternary ammonium salts, with hydrogen bond donors. Abundant hydrogen bonding sites and ionic conductivity endow it with potential applications in self-healing or sensing materials. Its excellent transparency allows it to be used as a substrate for luminescence and color-changing response, leading to the development of multimodal sensors. For example, Li et al. [94] developed skin-like elastomers by embedding cellulose nanocrystal (CNC) liquid crystal backbones into PDES through in situ polymerization (Figure 3a). Thanks to the elastic ion-conducting PDES matrix and dynamic interfacial hydrogen bonding, the resulting elastomer exhibits strain sensing capability and strain-induced wide range of dynamic structural coloration and excellent self-healing ability (78.9-90.7%) over a large strain range, which is ideal for the development of flexible sensors with wide applications and high reliability. Cui et al. [95] carefully constructed a fully bio-based self-healing sensor based on PDES ionic elastomer for skin-contact multifunctional e-skin applications ( Figure 3b). Using the synergistic modulation of physical and hydrogen bonding networks constructed by polydopamine-encapsulated CNCs, the bio-derived PDES breaks the limitations of general bio-based materials in terms of strength and toughness, while providing excellent self-healing properties.
While these mechanical properties are adapted for skin-like wearable electronic devices for compliance, repairability, and self-protection, artificial ionic skins that mimic the full range of sensory, self-healing, and strain-hardening properties of natural skin are still rarely reported. Zhang et al. [96] demonstrated a robust plasmonic conducting ionic skin by introducing an entropically driven re-combinable network of supramolecular amphiphiles in a hydrogen-bonded polycarboxylic acid network design. This design permits two energetic networks with different interaction strengths to bond successively by stretching, thereby eliminating the conflict between elasticity, self-healing, and strain hardening. As shown in Figure 3c-j, their reported representative polyacrylic acid/betaine PDES elastomers exhibit  [94] Copyright 2022, Wiley-VCH. b) Schematic illustration of the structure, biomass source, and bimodal monitoring of human motion and sweat visualization analysis. Reproduced with permission. [95] Copyright 2022, American Chemical Society. c) Successive tensile loading-unloading curves of PAA/betaine elastomer as stretched to different strains. The inset is the overlapped curves with single tensile curve to break. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. d) Corresponding strain-dependent dissipation energy and dissipation ratios. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. e) Cyclic tensile curves of PAA/betaine elastomer at a fixed maximum strain of 1000% for uninterrupted 100 cycles (tensile speed = 100 mm min −1 ). Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. f) Humidity-dependent water content changes of PAA/betaine elastomer at 25°C. The inset is the water content changes when exposed to RH 60% for 7 days. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. g,h) Mechanical properties and ionic conductivities of PAA/betaine elastomers equilibrated at different humidities. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. i) DMA tensile curves of PAA/betaine elastomers at different temperatures. The inset picture shows that PAA/betaine elastomer remains elastic at −40°C. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group. j) DSC heating and cooling curves of PAA/betaine elastomers equilibrated at RH 70%, 80%, and 90%. Reproduced with permission. [96] Copyright 2021, Nature Publishing Group.
Innovative strategies for combining ion-conductive materials with conducting groups and flexible groups with reversible bonds provide ionic conductivity and self-healing properties. [97] It is analogous to composites in which fillers are covalently attached to a matrix or repairable copolymers containing conjugated and supramolecular parts. [98] These polymers are advantageous be-cause a mismatch and rejection existing in different phases of the compounding method can be abolished by them.

Self-Healing Electron-Conductive Sensors
The main strategy used to obtain self-healing electronic conductors is based on the development of composites of self-healing polymers with electronically conductive materials. Electronically conductive materials usually have high modulus and the introduction of a large number of dynamic bonds in the polymer required for self-healing properties leads to low modulus of the polymer, which is difficult to match when compounded with conductive materials, leading to phase separation and thus unstable electrical signal transmission. To address these issues, we present an overview of the different processing and preparation methods from electronically conductive composites classified.
Surface-Deposited Conductive Layer: Electronically conductive flexible sensing materials are generally deposited with conductive layers on the surface of flexible substrates, including thermo deposition, magnetic sputtering, 3D printing, ink-jet printing, silk-screen printing, spinning, spraying, and dip coating. To provide simultaneously high sensitivity and high stretchability, prestretching is the latest common and most valid option. [99][100][101] This pre-stretching of the flexible film substrate prior to deposition of the conductive nanomaterials can produce micron-or nanoscale wrinkles and cracks during the stretching process, which results in the desired sensitivity and wide strain range. [102] The stability of the nanoscale conductive layer needs to be further improved due to its vulnerability to damage. Jin and his co-workers [103] incorporated self-healing polymer matrices with functionalized Au nanoparticle films to derive transducer arrays capable of rapid self-healing (<3 h) and high healing efficiency of both the matrices and the sensing films. Even if some strategies are designed to enable the flexible substrates to be self-healing after suffering damage, the conductive layer is usually some nanocarbon material, metal, or metal oxide leading to difficult repair of the conductive layer and functional failure. [104] Huynh et al. [72] synthesized and assembled self-healing disulfide cross-linked polyurethane (sh-crl-PU) and polyurethane/silver particle ( Ag) composites, where each part is self-healing (Figure 4). This enables flexible and extensible self-healing chemists for pressure/strain, temperature, and VOC sensing. Although the conductive layer is selfhealing even after cutting, showing no decrease in sensitivity to pressure detection, this layered design, which tends to delaminate under long-term dynamic loading, is a significant limitation to the stable operation of the device.
Blend Conductive Fillers: Another important approach is the preparation of conductive co-blended polymer nanocomposites, which is a viable alternative to surface-deposited conductive layer sensor technology. [105] However, the interface between the conductive filler and the polymer substrate is weak, and it is necessary to start from the interface design to develop self-healing sensors that can operate stably in the long term. Meanwhile, Young's modulus imbalance between rigid conductive fillers and soft matrix materials constitutes a significant challenge for the application of stretchable electronics at large strains. Fan et al. [106] explored a self-healing elastomer in the form of epoxidized natural rubber (ENR) and carboxylated oxidized CNC. The carboxyl group reacts with the epoxide group to form interface ester bonds that confer self-healing ability to the elastomer. In addition, higher healing efficiency can be obtained by forming more -hydroxy ester bonds through ester exchange reactions. The use of CN as a reinforcing filler leads to a considerable increase in both mechanical properties and healing efficiency for the modification of the elastomer. Recently, our team proposed a 3D self-assembly strategy based on nano-conductive fillers embedded in Pickering emulsion-stabilized latex of natural rubber to form highly interconnected 3D conductive networks (Figure  5a). [107,108] CNCs with amphiphilic are instrumental within this methodology. Amphiphilic CNC with plentiful hydroxyl groups can facilitate the assembly or aggregation of nano-conductive fillers on rubber emulsions. In addition, CNC can be immobilized at the interface of latex and water to form emulsion with homogeneous dispersion and stability. [109,110] On the basis of this 3D self-assembly strategy, a range of conductive nanofillers, involving synthetic polyaniline, [111] carbon nanotubes, [112] carbon black, [113] graphene, [114] and silver nanoparticles, [115] are already employed in the construction of conductive composites with tunable permeable network structure of 3D connections for conductivity, susceptibility and mechanics properties. High-sensitivity stress sensors coupled with self-healing capabilities are proven for enduring human-computer interaction applications thanks to effective 3D conductive pathways and interfacial supramolecular interactions (Figure 5b). [116] Some molecules with multiple hydrogen bonding sites, such as amino acids and polyphenols, can also anchor some nanoconductive fillers to form similar 3D networks (Figure 5c-f). [117] In the presence of enriched interactions between conductive fillers and self-healing substrates, the movement of molecular chains toward the damaged area during surface rearrangement forces the conductive fillers to move and thus conductive pathways are reconstructed. However, terrible transparency prevents their use in touchscreen as well as seamless electronic induction platforms. Expensive conductive nanomaterials and relatively complex and time-consuming fabrication processes are also important constraints faced by these stretchable conductors.
Conductive Polymer Composites: The performance of the device could be tuned through bottom-up conductive hybrid polymer composites synthesis and manufacture. The architectural design of compound sensing devices can be accomplished by various material processing processes. Conductive polymers are inherently flexible and soluble and are compatible with widearea settlement processing methods including spin coating, inkjet and screen printing. These distinct advantages of conductive polymer compounds offer feasibility for susceptibility engineering applications. However, a significant drawback of most conductive polymers is their highly rigid conjugated structure, a property that is mutually exclusive with self-healing. Through mixing self-healing flexible polymers and hard conductive polymers, there is potential to attain simultaneous selfhealing and conductivity within a single material. It is also possible to incorporate additive materials such as small molecules into such systems, which can enhance the conductivity and can act as plasticizers to adjust the softness of the composite. Wang et al. [118] reported ternary polymer composites composed of polyaniline, polyacrylic acid, and phytic acid with high tensile properties (≈500%), excellent self-healing properties, and showing an electrical conductivity of 0.12 S cm −1 (Figure 6). Upon breakage, both conductivity and mechanical properties can be recovered within 24 h, which is achieved through dynamic bonding, molecular entanglement, and electrostatic interactions. The research further shows that these composites are sensitive to both strains and pressures, and consequently can be utilized to fabricate strain and pressure sensors. Jiang et al. [119] proposed a sliding strategy to systematically introduce polyrotaxanes into soft conductive films made from the conductive polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (Figure  7a,b). The cyclodextrins allow for sliding back and forward along the chain, and thereby inhibiting PEG crystallization, delivering better elongation, and exhibiting enhanced electrical conductivity ( Figure 7c-i). The resulting composites overcome the stiff nature of the conducting polymers and allow direct formation of densely stretchable formations, which enable high-precision bioelectronics applications in complex sites (Figure 7j-m).
Based on the above different processing strategies for electronically conductive sensors, the preparation of surface-deposited conductive layers and co-mingled conductive fillers generally re-sult in opaque flexible devices, and the preparation of conductive polymer composites allows the preparation of transparent flexible devices. The surface-deposited conductive layer approach to the preparation of the sensor conductivity is high but usually difficult to heal, the stability of the device needs to be addressed. The hybrid conductive fillers need to construct conductive pathways inside the composites and require micro and nanostructure de- Figure 5. a) Proposed supramolecular multiple hydrogen bonding network combined with nanostructured CNT conductive network in ENR latex microspheres. Reproduced with permission. [133] Copyright 2017, Wiley-VCH. b) Robot control system by the facial expression (left) and electronic larynx (right) are fabricated based on the self-healing strain sensors. c) Interfacial supramolecular hydrogen bonds combined with the 3D segregated conductive networks design in NMSE. Reproduced with permission. [117] Copyright 2020, American Chemical Society. d) TEM images of the well-organized MXene nanostructure in NMSE at different magnifications. Reproduced with permission. [117] Copyright 2020, American Chemical Society. e) Electric conductivity variation as a function of different MXene content in MXenes/ENR (black line), S-MXenes/ENR (red line), and MXenes/ENR with randomly distributed conductive network (blue line) samples. Reproduced with permission. [117] Copyright 2020, American Chemical Society. f) Comparison of healed mechanical properties among different self-healing elastomers. Reproduced with permission. [117] Copyright 2020, American Chemical Society. sign to enhance the sensing performance. The conductive polymer approach is able to produce transparent devices but usually has poor conductivity.

Self-Healing Capacitors
Capacitors for sensors change their capacitance according to the physical force applied. [120,121] The parallel plate structure is the most commonly used structure for capacitive sensors and consists of a dielectric polymer layer interposed between two conductive poles. [122,123] The separation distance and the region of overlap between the two electrodes of the capacitor respond to mechanical forces and can consequently be applied as a haptic sensor. Capacitive pressure sensors are known to have no temper-ature drift, high sensitivity and robust construction and are less sensitive to lateral stress and other environmental influences. Pei and his co-workers manufactured a repairable capacitive sensor featuring great transparency and high force sensitivity. [124] The repairable complex sensing film was created by laying AgNWs upon a D-A polymer substrate surface. After the sensing film was scratched with a knife, it was heated at 80°C for 30 s to restore its initial capacitance value.
While self-healing electrodes as well as self-healing node coatings are available to compensate for degraded sensor output performance, total restoration of capacitor damage is difficult. For the self-healing electrode layer of a capacitor, the electrical properties can be quickly restored when the broken capacitors contact each other after self-healing, but the dielectric layer cannot selfheal. As a result, self-healing capacitors may have reduced me- chanical performance, and electrical performance and mechanical strength may not recover simultaneously. On the contrary, the self-healing dielectric layer of the capacitor can facilitate the electrodes in the capacitor to recover themselves, as the thin conductive layer adhering to the surface of the dielectric layer can drive the close contact of the damaged electrode layers to restore electrical conductivity. Based on the combination of dynamic metalligand bonds ( -diketone-europium interactions) and hydrogen bonds in a multiphase segregation network, Zhang et al. [125] designed a self-healing elastic substrate material (Figure 8a,b). The obtained polymer network developed a slightly phase-separated structure and displayed high fracture stress (≈1.8 MPa) and high fracture strain (≈900%). A biodynamic bonded polymer system was used to fabricate stretchable and self-healing dielectric layers, and a polymer was used as a matrix for silver composites to create self-healing conductive layers. These materials were employed to fabricate capacitive sensors to exhibit extensible and completely self-healing touch panels (Figure 8c-e).
Compared to resistive sensors, it is easier to replace electrodes by ionic conductors made of physically cross-linked supramolecular hydrogels, which can make them highly transparent and biocompatible. Capacitive pressure sensors can be used to detect pressure in human tissues such as blood vessels, bladder, and skin, and to monitor health conditions in the field of artificial intelligence systems and wearable healthcare devices. However, this capacitive variation can only detect the touch of specific objects, including human skin, and not many other objects. Therefore, when seeking pressure sensing for touch panels, surface capacitive sensors may provide a suitable technology.

Self-Healing Self-Powered Sensors
Another sensing mechanism is to construct some self-powered devices with force field response, such as piezoelectric and triboelectric sensors. In the complex composition of sensor devices, we can use the self-healing material as one of the electrode materials of the device. Therefore, not all parts of the whole device are self-healing. In addition, self-powered devices usually exhibit poor interfacial compatibility between the self-healing layer and the electrode. The difference in Young's modulus between the self-healing layer and the electrode material of the other pole results in poor interfacial adhesion between them, which leads to Figure 7. Reproduced with permission. [119] Copyright 2021, American Association for the Advancement of Science. a) Chemical structure of PR-PEGMA and individual roles of each building block. b) Schematic diagram illustrating the interaction between PR and PEDOT:PSS for enhanced conductivity. c,d) Stretching tests showing that the PR-blended PEDOT:PSS film could substantially enhance stretchability compared with other control samples. Films were deposited on styrene-ethylene-butylene-styrene (SEBS) elastomers with thicknesses of ≈200 nm. R, resistance. e) AFM height image and corresponding surface profile of a photopatterned TopoE array with 2-m width. f) Resistance change as a function of strain for TopoE films with different PR over PEDOT:PSS dry mass weight ratios. All films in (f) to (i) were UV cross-linked after blending followed by washing in water and blow drying. g) Statistical comparison of Young's moduli measured by nanoindentation for different TopoE films indicating that PR can reduce the overall film stiffness. h) Four-point probe measurements showing enhanced film conductivity with higher PR content. i) XPS profiles indicating reduced PSS content as the PR over PEDOT:PSS dry mass ratio increases in the film. a.u., arbitrary units. Photographic images showing the conformal interface between stretchable PEDOT:PSS devices and underlying tissues, including j) the brainstem, k) the wrist, l) the finger, and m) the back of the hand. Figure 8. Reproduced with permission. [125] Copyright 2018, Wiley-VCH. a) Molecular structure of curcumin (Cur)-embedded polymer. b) A schematic depiction of Eu (III) coordinated P-Cur material, highlighting the Eu-curcumin coordination bonds and the hydrogen bond derived from the urethane groups. c) Capacitance before and after scratching and healing. d) Cutting the device by a razor blade along the middle of the three pixels and then realigning the two cut pieces back together and healing at room temperature for 24 h. The front and back image after healing was shown. e) After stretching the healing device at 85% strain along the vertical direction to the cutting line, the green light can then be turned on after the correct passcodes were input. them being separated or even disengaged in prolonged combat, ultimately resulting in combat failure.

Self-Healing Triboelectric Nanogenerators
Triboelectric nanogenerators (TENGs) are of great interest due to their extensive resource availability, diverse manufacturability, cost-effectiveness, and ease of fabrication. TENGs can be utilized effectively for harvesting mechanical energy to create electricity and signals due to the synergistic effect of contact initiation and electrostatic sensing between different frictional electric materials. The mechanically triggered generated electricity supports not only new energy suppliers but also self-powered sensing applications. However, the metal electrodes of most TENGs are unable to achieve self-healing capability. As a result, research has begun to seek solutions to give these devices natural capabilities, including flexibility, stretchability, and self-healing capabilities. [28,[126][127][128] Parida et al. [128] presented a mucus-based ionic conductor for TENG collectors, which greatly enhances their energy generation, extensibility, transparency, and confers self-healing properties. This is the first demonstration of a www.advancedsciencenews.com www.advsensorres.com TENG-based autonomous self-healing sensor. However, in such triboelectric sensors, only the electrodes for collecting the current are self-healing, while the remainder of the device is composed of non-repairable materials. This leads to a possible reduction in the mechanical performance of the self-healing TENG, and the simultaneous recovery of both electrical performance and mechanical strength is not possible. Deng et al. [129] embedded silver nanowire networks in disulfide bonds containing vitrimer elastomers to present the first self-repairing, flexible, and stretchable mechanical energy harvester and haptic sensing element (Figure 9a,b). Interactive dynamic disulfide exchange reactions incorporated within self-healing elastomers enable rapid structural and functional recovery and provide active conformation or reconfigurability (Figure 9c-i). Such self-healing and conformal adaptive TENGs are available for mechanical energy harvesters and self-powered haptic/pressure sensors, offering longer life and outstanding design versatility. Xun et al. [130] designed a selfhealing PU based on disulfide bonding and hydrogen bonding as a friction layer with conductive self-healing PU doped with conductive carbon black as an electrode for TENG (Figure 9j,k). The similar modulus and homogeneous structure between the friction layer of pristine self-healing PU and the electrode coating of conductive self-healing PU ameliorate the boundary compatibility and cyclic tensile weariness between them, and TENG exhibits a consistent mechanical induction capability at 50% tensile transformation (Figure 9m,n). The two layers have completely organic homeostructures with identical molecular chains, disulfide bonds and hydrogen bonding, and the completely cleaved TENGs achieve complete self-healing from the tribological layer to the electrode layer. The homogeneous structure of the conducting and tribological layers can be developed to significantly improve the robustness of TENGs and address the problem of interfacial adhesion or detachment caused by heterogeneous materials.

Self-Healing Piezoelectric Nanogenerators
Piezoelectricity is another commonly used pressure self-powered sensor. The piezoelectric sensing system is extensively used in wearable electronic skin owing to its great sensitivity and remarkably fast response time. Hou et al. [131] demonstrated the first self-healing, mechanically strong, stretchable, self-activating nanostructure of a pressure sensor device. This structure is composed of piezoelectric and conductive layers with a convalescent substrate (Figure 10a-c). 3D PVA/rGO films with mutually permeable PDMAA were employed as robust and extensible electrode films, and electrospun piezoelectric polyvinylidene fluoride (PVDF) nanofibers were used between PDMAA-PVA/rGO to prepare self-healing piezosensitive hybrid films. Combining the excellent mechanical performance and self-healing ability of the hybrid films, the PVDF layer stops working after damage but still generates an electrical charge and the electrode will actually heal. The parts of it that are essentially irreparable can recover their structure. In certain kinds of devices, such recovery is adequate to resume full functionalities. Despite the good self-healing properties, the voltage obtained (1 < V) is usually lower than the voltage obtained in recent devices based on frictional electricity (<100 V). Wang et al. [132] designed a new concept of piezoelectric generator by synthesizing poly(butylene gly-col/lactide/sebacate/scarboxylate) based elastomers with excellent mechanical properties and piezoelectric output (Figure 10d). The low elastic modulus at ambient temperature is accomplished by suppressing inhibition of crystallinity and lowering the glass transition temperature, therefore, the elastomer could withstand stretching deformation up to 4.39 times. 19.75 A cm −2 of high voltage electrical short-circuit current density is generated by large lattice distortion (Figure 10e). And the elastomer exhibits rapid self-healing behavior, allowing recovery from scratches or cuts within 15 min at body temperature (Figure 10f). Even after five cut-healing cycles, its electrical and mechanical properties maintained 77.38% and 44.98% of their initial values, respectively.
Although a range of self-healing self-powered sensors have been developed using different approaches to cope with different injuries and to improve the durability and stability of selfpowered sensors, the development and utilization of self-healing self-powered sensors are still mainly limited to small devices and single electrode modes. In addition, energy harvesting is mainly focused on collecting human energy and self-powered sensors in wearable devices, which limits the applicable promotion. Therefore, it is imperative to innovatively design large-size self-healing self-powered sensor devices and expand their application areas.

Perspective
In this review, we focus on the research progress of self-healing stress-sensing materials in recent years. According to the different sensing mechanisms of force-electric response materials, stress sensors can be divided into resistive, capacitive, triboelectric, and piezoelectric sensors. Based on the appealing idea, we summarize the latest advances in dynamic bonding are combined with conductive sensing designs to enhance self-healing and sensing performance under different types of stress sensors. Usually, damaged materials can spontaneously self-heal based on intrinsic interactions (e.g., hydrogen bonding, ionic interactions, host-guest interactions, dynamic covalent bonding). In addition, self-healing stress sensors not only offer excellent sensitivity and a wide range of applications but can also repair fatigue and accidental damage that occurs during operation, thereby extending their lifetime and improving stability. Although the development of self-healing force-electric sensors has removed some application limitations, wearable electronic devices based on forceelectric response still have a long way to go and face many challenges before they become commercial products. 1) Mutual exclusivity between strong mechanical properties and high selfhealing efficiency: Although there have been many recent advances in research on self-healing sensing materials, in contrast, the mechanical properties of these materials are still not sufficient for the stable operation of stress sensors under long-term dynamic loading. Therefore, achieving higher mechanical properties and self-healing capability at the same time remains a challenge. 2) Design of micro-nano-structures and supramolecular interactions: The difference in elastic modulus between conductive fillers and polymer matrix is usually very large, which affects the effective contact area between them, and this in turn affects the output performance. It is also a challenge to enhance the output performance of the sensor by constructing effective micro-nano structures, such as 3D network structures, and de- Figure 9. a) Chemical structures of starting materials used to prepare vitrimer elastomer. 1 is bisphenol A diglycidyl etherepoxy oligomer; 2 is 2-(ethylenedioxy) diethanethiol used as a spacer; 3 is polysulfide oligomer as a crosslinker, and 4 is 2,4,6-tris-dimethylaminomethyl phenol used as a curing catalyst. Reproduced with permission. [129] Copyright 2018, Wiley-VCH. b) Tension stress-strain behavior of the vitrimer elastomer for five repeated cycles. c) Space diagram of the square ruler-like VTENG. Reproduced with permission. [129] Copyright 2018, Wiley-VCH. d) Strategy for break and recovery. Reproduced with permission. [129] Copyright 2018, Wiley-VCH. e) Resistance between two copper electrodes of original (left) and repaired (right) VTENG. f,g) Photos and h,i) images from optical microscope and scanning electron microscope of broken and recovered VTENG, respectively. Reproduced with permission. [129] Copyright 2018, Wiley-VCH. j) Components of the insulated PUE film, conductive PUE film, and HRSE-skin. Reproduced with permission. [130] Copyright 2020, American Chemical Society. k) Optical image of the device section. Reproduced with permission. [130] Copyright 2020, American Chemical Society. l) HRSE-skin was rolled at a finger joint. Reproduced with permission. [130] Copyright 2020, American Chemical Society. m) Uniaxial tensile tension-deformation curve of the insulated PUE film, conductive PUE film, and HRSE-skin. Reproduced with permission. [130] Copyright 2020, American Chemical Society. n) Tension-deformation curves of the HRSE-skin for 20 repeated cyclic tensile tests. Reproduced with permission. [130] Copyright 2020, American Chemical Society. Figure 10. Reproduced with permission. [130] Copyright 2022, Elsevier. a) Schematic of the 5S film and its pressure sensitivity. Pressures are induced upon the film and this system detects the total force on the film. Cross-sectional FESEM images of the 5S film, showing microscopic structures of the boundary between b) the two functional layers and c) the electrode layer alone. d) Schematics of the LBPE design concept with original shape and under stretched tension. e) Piezoelectric short-circuit current behaviors of the 16-cm 2 LBPE-40 under different heights of free-fall dropping. f) Self-healing process of the LBPE-40 from blade scratches and self-healing process of the LBPE-40 from cutting damage. www.advancedsciencenews.com www.advsensorres.com signing supramolecular interactions to increase the interfacial layer to prevent debonding of the filler and rupture of the substrate. 3) Diversity of self-healing sensing platforms: Although a series of self-healing force-responsive sensors have been developed using different approaches to cope with different damages and improve the durability and stability of the sensors, the development and utilization of self-healing sensors are still mainly limited to small devices and single force-response modes, which limit the application promotion. Therefore, it is imperative to innovate the design of large-size self-healing multimodal response sensor devices and expand their application areas.
Through integrating self-healing materials into the sensors, self-healing force-electric sensor devices have been developed, which greatly improve the durability and long-term operational stability of force-electric sensors. Although there are still some challenges, it is expected that more excellent self-healing forceelectric response sensors will be developed in the future by designing and tuning the performance of self-healing materials to promote the practical applications of wearable electronic devices.