1 Introduction
Nanogenerators (NGs) are power generation devices that produce power from waste mechanical energy in the surrounding environment [
1,
2]. Nanogenerator is a type of energy-harvesting technology that harvests mechanical energy from a variety of sources, including human motion (walking, exhaling, running, and pulse rates), vibrations, moving water, rainfall, and air currents. It differs from a traditional electric generator in a manner that it produces electricity without the need for an exhaustible fuel such as oil, coal, or gas [
3,
4]. These are great alternative sources for chemical batteries and have useful applications. In addition, NGs are effective energy sources and operate without any external power supply [
5]. With the advancement of low-power electronics, they have become a realistic option. They do not have competition with conventional sources of energy generators because the amount of power generated in NGs is very limited. The rapid development in NG will lay the groundwork for self-powered systems in the future and perhaps will foster the growth of energy and sensor technologies [
6]. As a sign of the new era, multifunctional electronic devices change how people perceive the world and improve the efficiency of lives of people. A novel method proposed by Suh et al. [
7] for blocking airborne pathogens has been developed using fabrics with opposite triboelectric properties. By harvesting kinetic energy from human motion, the fabric induces triboelectric charges, efficiently repelling microbes and achieving microbial blocking with low-pressure loss [
7]. This self-powered system holds promise for public health protection and addresses the limitations of conventional methods.
In the discipline of materials science and engineering, there are two distinct study areas, triboelectric nanogenerators (TENGs) and self-healing materials. TENGs are energy-harvesting devices that transform mechanical energy into electrical energy, whereas self-healing materials have been researched for their capacity to repair damage on their own [
8]. The current state-of-the-art technologies such as applicability, device design, and system integration are presented in Fig.1 [
9–
11]. Self-healing TENGs, a field that combines these two disciplines, have a lot of promise for use in a variety of contexts. TENGs that can mend themselves have come a long way in recent years [
12]. To increase the durability and lifetime of TENGs, researchers have been experimenting with various methods to include self-healing characteristics into them. One typical tactic is to include self-healing coatings or polymers in the TENG framework. The lifespan of the device can be increased since certain materials have the capacity to heal themselves when broken [
13]. TENGs can tolerate mechanical stress, such as bending, stretching, or scratching, which would generally result in performance deterioration or device failure, by adding self-healing materials [
14].
NGs are mainly classified into four types based on available energy sources and working modes as TENG, piezoelectric nanogenerator (PENG), pyroelectric nanogenerator (PNG), and hybrid nanogenerator (HNG) [
15,
16]. TENG focuses on the concept of triboelectrification which is also termed contact electrification and electrostatic induction. When two dissimilar materials come into touch, electric charges are transferred from one side to the other [
17]. Mechanical force is required for the electrification of charges between two materials via separation and unification. Charges on the conductive substance shift from one spot to another in this manner [
18]. This redistribution of charge creates a potential difference that is measured along the electrodes. The charge distribution between the electrodes is not necessarily reversible. Hence, an extra charge is developed across one end while scarcity is on the other end. Then the separation of electrodes results in the redistribution of the charges. In this manner, an alternating current is produced due to the negative and positive charge polarity [
19,
20]. TENG has various modes of operation such as vertical contact separation approach with double electrode, in-plane sliding approach with double electrode, free-standing approach with double electrode, vertical contact separation approach with single electrode, in-plane sliding approach with single electrode, free-standing approach with single electrode, which are shown in Fig.2. Two materials with distinct triboelectric characteristics are in direct contact in the vertical contact-separation mode before being vertically separated. The triboelectric effect occurs when two materials come into contact, transferring electrons from one to the other [
21]. The potential difference between the materials causes an electric field to form when they are separated, which produces an electric current. This mode is frequently employed in TENGs that employ actions like tapping, pushing, or shaking [
22]. Two materials with differing triboelectric characteristics are sliding continuously against one another in the same plane in the in-plane sliding mode. The materials continually exchange electrons as a result of the relative motion, which results in the creation of an electric current. This mode is appropriate for lateral motion-related applications like sliding or rubbing. A single material with different triboelectric characteristics is employed in single electrode mode [
23]. Due to the triboelectric effect, when the material comes in touch with various surfaces or objects, electrons move between them, creating an electric current. When there is just one material available or when the TENG needs to be integrated into a particular item or surface, this mode is helpful [
24]. Variable portions of the material might have variable triboelectric characteristics. The contact and separation between the various zones produce an electric current as the material moves or vibrates. This mode is frequently used in TENGs created for wind energy collecting or mechanical vibration applications. Each method of operation has special benefits and is appropriate for various situations. TENGs offer a flexible and effective way to transform mechanical energy into electrical energy through the use of these modes for a variety of applications, such as self-powered systems, wearable electronics, and environmental sensors.
Other types of NGs can be fabricated together like PENG and TENG, or other sources like solar cells and TENG/PENG may be fabricated together. They have numerous applications in the world of energy generation. HNGs are cutting-edge devices that harness energy from two or more different sources simultaneously, unlocking a multitude of benefits and possibilities for energy generation [
25]. By integrating various types of energy harvesting technologies, such as PENG and TENG, or combining sources like solar cells with TENG/PENG, HNGs offer an unprecedented versatility and efficiency in producing the desired output. The fabrication of HNGs opens up a whole new realm of possibilities for energy harvesting. For instance, by combining PENG and TENG technologies, different sources of mechanical energy can be tapped into, such as vibrations, movements, or pressure, to generate electricity. This means that even in complex environments where multiple forms of mechanical energy are present, HNGs can effectively capture and convert them into usable power. Furthermore, by integrating solar cells with TENG/PENG, HNGs can leverage both light and mechanical energy, making them highly efficient and adaptable to diverse environmental conditions.
2 TENG
TENG is a green and sustainable power source for small-scale energy harvesting. They are termed as “energy of the new era” and are often termed self-powered devices. It is based on triboelectrification and electrostatic induction principles. Electrostatic induction transforms mechanically induced energy into electricity due to an applied change in electric potential, while contact electrification produces static polarized charges. It uses two material layers acting as electrodes and external mechanical pressure or force stimulates the action of charge transfer between them. It has three principal components, triboelectric layers, electrodes, and a spacer. The electrostatic charge exchange between the triboelectric surfaces constitutes a capacitive energy device. Triboelectrification, also known as contact electrification, generates static polarized charges on material surfaces in interaction, whereas electrostatic induction drives the conversion of mechanical energy to electrical energy by a change in electrical potential caused by physically agitating secession [
26–
28]. A typical TENG in double electrode mode is made up of two distinct components, one being an electrode and the other a triboelectric substance. The electrode is brought into contact with the triboelectric substance [
29]. The electrode is typically formed of a conductive substance like metal or carbon, but the triboelectric material is frequently a polymer-based material with excellent triboelectric characteristics, such as polydimethylsiloxane (PDMS) [
30]. The triboelectric effect causes electrons to begin moving between the electrode and the triboelectric substance when they come into contact. Because various materials have varying affinities for electrons, this phenomenon happens [
31]. The triboelectric material is typically the material with a lower electron affinity, while the electrode is typically the material with a higher electron affinity. The electrode acquires a negative charge (negative triboelectric polarity) as a result of this charge transfer, whereas the triboelectric substance acquires a positive charge (positive triboelectric polarity). The triboelectric substance and the electrode are cut off after the initial contact. Typically, mechanical motion, such as pushing the two components apart, is used to accomplish this separation [
32]. An electric field is produced when they are separated due to the potential difference between the negatively charged electrode and the positively charged triboelectric substance. An electric current is produced as a result of electrons moving from the triboelectric material to the electrode as a result of the electric field caused by the potential difference. As long as there is a potential difference between the triboelectric material and the electrode, this charge induction process continues to take place [
33]. A resistor, LED, or other external loads can be connected to the TENG in order to capture and use the produced electric current. The load uses the electrical energy produced by the TENG for a variety of functional purposes. It is possible to repeat the cycle of triboelectric charging, separation, and charge induction by bringing the electrode and triboelectric substance back into contact after they have separated. As long as there is mechanical motion or vibration to drive the cyclic action, the TENG may constantly create electrical energy. As a result, a conventional TENG that uses two electrodes works by making use of the triboelectric effect, in which the contact and separation of two electrodes with triboelectric materials produce a potential difference and induce an electric current. As a result, the TENG can transform mechanical energy such as that from human motion or vibrations in the environment into useful electrical energy.
Hybridized TENG is being used by researchers for better results. Hybrid TENG is fabricated using different modes to replace the most out of TENG [
34]. All the modes of interaction measure current and voltage which is directly proportional to the magnitude of charge created in the triboelectric process. The energy generated is proportional to the square of the charge density, and the electrostatic induction in TENG is likewise proportional to the dielectric constant of material. Both factors must always be enhanced to achieve the best results [
35]. Electrons or ions are involved in this process. Which one is dominant depends upon the material properties. Reviewing the outcomes of experiments reveals whether ions or electrons predominate. By looking at the temporal charge density distribution of the surface, the charge distribution can be investigated. The electron-ion transport mechanism must be investigated to have a better understanding. The electron-cloud-potential-well model may be able to explain the contact-electrification process for all sorts of materials [
36]. The results show that the surface charge density decreases as temperature increases. With the exception of the first few minutes, the degradation is exponential with time at higher temperatures. Electrode material must be extremely efficient if TENG is to be more effective [
37]. The energizing process uses a finite number of electrode materials. Instead of a solid–liquid or liquid–liquid interface, a solid-solid interface produced a feasible result [
38]. Tab.1 lists a few materials that can be used to create TENGs with self-healing capabilities. For greater performance, nanostructured materials like carbon nanotubes are frequently employed. They have not only a large contact surface area compared to any other material, but also enough active contact sites available for interaction. They also have an effective area of contact and intimate sites for a better electrification process. They have robust mechanical durability and stability. Composites give an advantage of fabricating materials composed of efficient properties. The materials and structures such as dielectric polymers, metallic electrodes, double-layer electrodes, and spring-based structures can be used. TENG is standardized by quantifying the performance of the triboelectric process in terms of charge density. It is measured by taking reference to any material. Then the result is normalized [
28]. The reference material is more prominently liquid because it has a more intimate and smooth surface interaction. Modifications in chemical compositions of materials, enhancement in the functional contact area, and changes in ambient circumstances all affect the charge density. When triboelectric charges are on the surface, they stay there for hours or even days without dispersing [
39]. This indicates that the mechanical contacts are only required for the first ten cycles of operation and that the output power is unlikely to be significantly affected. Subsequent cycles of operation do not require direct contact between the two surfaces until eventually [
40]. In this instance, no heat, waste, or resistance would exist or be generated. The lifespan of TENG could be effectively emphasized. As a result, mechanical “switchable” structures must be built that allow for minimal contact between the two surfaces while maintaining operation. Extremely configurable TENGs that can swiftly cure both fractures and attrition at room temperature are desperately required. Synthesis of electrode material with the self-curative ability and shape remembrance property plays a vital role in obtaining an effective TENG. In some paths, artificially induced charges are also being used that produce an effectual electrostatic process [
17,
19].
2.1 Mechanism of TENG
The central tenet of TENG is triboelectrification, which is the transfer of charge from one material to another when the two materials come into contact. A charge is transferred between two materials, one of which acts as an anode and the other as a cathode. The charge amenities and molecule or material structures differ, which causes the transfer. Therefore, objects can be selected based on their cathodic and anodic behavior. When the surface makes contact with others, they experience forces (attractive, equilibrium, and repulsive) [
41]. The various methods for improving the reliability and effectiveness of TENG devices are discussed in Fig.3, and their mechanism characteristic is discussed in Ref. [
22].
The charge transfer occurred leads to stress on the material contact region or may occur without even contact. It is important to embrace the micro/nano-structural properties of the contact material. In addition, only the contact area does not have to be responsible for triboelectrification. Even contact pressure affects the charge collection [
42]. There are three ways to transfer charge, electron transfer, ion transfer, and material transfer [
43]. The work function and the interfacial barrier both play important roles in determining the charge or triboelectric properties depending upon the cause of transfer (electron, ion, or material transfer), a tabular representation of which is discussed in Fig.4. Hence, some points can be listed that affect the triboelectrification: contact area, contact pressure, stress or deformation during contact, the work function of the surface material in contact, material transfer, and intermediate energy level lying in the forbidden energy gap.
Whenever there is contact between a metal and a polymer, an amount of polymer gets transferred to the material surface and vice versa as shown in Fig.5. During contact electrification, this occurs when the number of atoms transported per unit area exceeds the number of electrons per unit area. Charge transfer happens when a charge-carrying substance is transferred. After the first contact transfer, the amount transferred decreases, and so on. By oxidizing polymers (polyethene), contact electrification can be enhanced [
44].
As seen in Fig.6, contact electrification can also be caused by ion transfer. The net internal charge in insulators is caused by charged imperfections in the crystal lattice [
44]. When two surfaces with differing affinities come into touch with each other, ion transfer will occur. The instance of metals makes it clear. In ionic polymers, one ion is fixed to the matrix of the polymer while another ion with the opposite charge is mobile. Electrostatic contact is mostly brought on by the movement of mobile ions. The best way to illustrate ion transport processes is with pyroelectric insulators [
45]. Moreover, it is less often that the charge transfer in the insulators is done by surface ions (studied in the case of pyroelectric insulators) and can be due to internal structure properties [
46]. Water plays a vital role in the creation of OH
− and O
+ ions in non-ionic polymers, and the nature of the extent of -OH
− adsorption causes charge separation when non-ionic polymers are brought into contact.
Charges can also be transferred by a transfer of electrons (Fig.7), which is based on differences in the work functions. Experiments reveal that the Fermi level and the work functions of the material define their charge densities or triboelectric properties. In the case of insulators, the bandgap is large, thus they require high energy to overcome the potential barrier [
47]. There is a mediate state in the forbidden region which reduces the barrier for charge transfer due to the approach of the surface atoms in the case of metal–insulator contact surfaces. Thus, the interface barrier is a prominent factor in contact electrification [
41]. There is also evidence of photo-emission and thermionic emission on the surface during the contact [
48]. Reportedly, there are some reactions in the case of triggered pairs of polymer insulators which lead to the conclusion that these are only performed by electrons. Additionally, other experimental shreds of evidence reveal that there are other parameters present during the contact and thus, which result in the conclusion that electrons are dominant during charge transfer.
Mostly, the triboelectric materials are non-conducting like insulators carrying ambient charge density. However, contact electrification can be achieved via metal–metal, metal–insulator, or insulator–insulator contact modes [
47]. There are double-electrode configurations of the single electrode and double electrode in the operating modes (contact-separation mode, sliding mode, and freestanding mode). As the fundamental principle comes to an end, the amount of charge or triboelectrification is required to generate a certain amount of power. Because many parameters affect this process, such as mechanical stress, strain, charging time, material surface, roughness, and friction rate, it is difficult to choose the right material. These all are experimental parameters. Hence to combat all the problems, the experimental setup of a TENG is made based on theoretical research in a controlled manner [
49].
The main feature of the TENG device is the contact surface. As the contact surface increases, the triboelectrification increases. If there are “
n” contact surfaces and the duration of each contact is “
t” seconds and if a single contact site has a contact duration of “
Nt” seconds, then the former one will have more charge generation. The contact surface can be metal–metal, metal–insulator, or insulator–insulator. The charge transfer is difficult on a metal–metal contact surface because the charge collected is large and can be lost via the ground if not set up properly. Additionally, they are not completely necessary for charge buildup. The charge that has built up on the metal may be calculated using a variety of techniques. It must be free of any outside influences that could interfere with the setup in order to achieve optimum results. The most often used concepts in metal–metal contact electrification are surface potential, work function, and electrochemical potential [
50]. Contact potential difference
V0 and contact capacitance
C0 are two parameters that influence the output. The degree of abrasion of the surfaces in contact determines the contact capacitance in most cases [
47].
In metal–insulator contact surface, the results depend upon the nature of the insulator, material, type of contact, and duration of contact. In insulators, the charge accumulated by metal contact is spread along with the contact site or track. The charge accumulated is much more than in the case of normal electrification. To achieve the minimum electrostatic energy, the charge collected far away from the contact site in the insulator flows back to the metal. The virtuous insulators do not guest this flow back of charge. The charge density and the area of contact are the main points. It is reported that most of the charge densities of the insulators are organic [
47]. The calculation of the charge densities depends upon the position of energy bands [
43]. Further, the charge is linearly proportional to the work function of the metal when there are multiple contact sites due to deformations resulting in more area of contact [
51]. In this contact alignment, the interface dangling bonds improve the charge transfer, and the electron transfer could be determined by the Fermi level of metals and the unoccupied condition of the insulator [
52]. It is still unclear how charges are generated or transmitted between insulators. However, since they lack any free electrons in their valence band that are available for conduction, it can be stated that they occur via surface ion exchange. The chemical and molecular composition, surface flaws, and electron location of an insulator play a role in determining an energy of electron [
53]. Surface impurities can be taken as the main account as a medium of ion exchange. Weak forces due to intermolecular interactions help the ions reside on the surface of an insulator. Many insulators often have a minimal layer of water to produce H
+ and OH
– ions, making them available for ion exchange. It can be said that the adsorption energy acts as a work function of the insulator for studying the triboelectric effect [
53]. It would be more generous to understand and perform experiments to gather information about the charge exchange phenomena on the insulator–insulator contact surfaces. Teflon surfaces can be taken as an accountable example of charge surfaces rather than other polymers to exhibit better charge densities [
54].
3 Self-healable materials
Nature shows prominent examples of self-healing. There is a drive to develop a new synthesis process of the materials relevant to let them rebuild themselves. The chemistry concentrates on the H–H bonds and π–π bonds, which can break and connect easily to make the process easy. Weighing their building process, they are two ways of describing self-healing. These ways are categorized into autonomous self-healing and non-autonomous self-healing. Another way relies on the competency of a material to self-heal which is intrinsic self-healing and extrinsic self-healing. Self-healing can be autonomous or done mechanically or by chemically treated processes. The self-healing process involves changes in chemical properties, which may be intrinsic or extrinsic, but it only applies to a specific quality or a function of the material. Hence all the other properties are not regenerated and only specific property is studied and monitored [
55]. The dominant aspect relies on the ability to heal itself without making significant modifications to the original. Healing is quantified, which is referred to as healing efficiency, as a result of recovery.
3.1 Need for self-healable TENGs
There is an extensive need for devices with a long-lasting durability. Any device in use may possess some damage. There is a need to search for a more prolonged approach to design devices with a better resistance to damage or it is better to look for self-healable materials. Materials with self-healing properties (or self-repairing properties) can repair any damage automatically without external influence [
56]. Potential new focus areas that could direct the future of TENG devices are presented in Fig.8. Materials that can self-heal are of excellent use and rely on an inclusive field of interest in different expertise. They improve the functionality of the materials and other devices and are of long-term use. They increase the sustainability and durability of the device or material in use by self-restoring the degraded properties. The design and preparation of such a material focus on better structural and functional properties to self-heal. The damage itself leads to the initiation of self-healing to ensue. The design strategies represent various examples like paint coatings, back covers of devices, metal alloys, composites, nano-tubes, polymers, etc. After repairing the damages, they all tend to get back to their original form. In the process, the material releases the requisite healing agent at the target cracks or damages that occur during the development and manufacturing of the material or any external random or accidental disruption. Self-healing properties play a vital role in enhancing the durability and longevity of TENGs, making them an important aspect of their design and functionality. TENGs are often subjected to mechanical stress, such as repeated bending or stretching, during operation. Over time, these mechanical strains can lead to material degradation, resulting in reduced performance or even failure of the device [
57]. By incorporating self-healing properties, TENGs can recover from minor damages and restore their functionality, ensuring long-term reliability and robustness. The ability to self-heal allows TENGs to recover from wear and tear, thereby extending their lifespan [
58]. As TENGs are envisioned for a wide range of applications, including wearable electronics, self-powered sensors, and flexible devices, their longevity is of utmost importance. Self-healing properties enable TENGs to withstand harsh operating conditions and maintain their performance over an extended period, reducing the need for frequent repairs or replacements [
59]. In TENGs, the generation of electricity relies on the efficient interaction between two triboelectric materials, typically through contact and separation. Any surface damage or degradation of these materials can significantly impact their triboelectric performance and subsequently decrease the overall power output. By incorporating self-healing properties, TENGs can effectively repair surface defects or scratches, thus ensuring consistent and optimized performance. The ability to self-heal can potentially reduce the cost associated with the maintenance and repair of TENGs. Rather than replacing the entire device in the event of damage, self-healing properties allow for localized repair, minimizing downtime and the need for costly replacements [
60]. This becomes particularly relevant in large-scale applications or deployments where maintenance efforts need to be minimized. In summary, self-healing properties are indeed important for TENGs. By providing durability, extending the lifespan, maintaining efficiency, and reducing maintenance costs, self-healing capabilities contribute significantly to the advancement and practicality of the TENG technology.
3.2 Approach for synthesis of self-healing materials
The primary goal in designing the self-healing material (SHM) is attained by sustaining its physical properties during the restoration process. The tensile strength of material and elasticity are both lower in practice [
62]. To quantify the physical property, better healing efficiency is required. The performance depends upon the healing speed and the scale of damage caused to the material. SHMs must have the capability to form multiple bonds around the damaged area. Considering all these factors, two approaches can be counted on, one bing the embedded self-curative approach, the other being the intrinsic self-curative approach.
3.2.1 Embedded self-curative approach
An embedded self-curative approach for self-healing materials synthesis is a strategy that involves incorporating healing agents or functionalities directly into the material structure at the time of synthesis [
63]. This approach enables the material to autonomously repair damage or restore its functionality without the need for external intervention. The embedded self-curative approach focuses on designing materials with inherent healing capabilities by integrating healing agents, such as microcapsules filled with healing agents or vascular networks containing healing fluids, during the material synthesis process. These healing agents are evenly distributed throughout the material matrix, ensuring that they are readily available to react and repair any damage that occurs [
64]. The embedded self-curative approach offers several advantages over conventional repair methods. It allows for the continuous healing of the material, even for multiple damage events, and can significantly extend the lifespan of the material. Additionally, the healing process can occur at ambient conditions, making it convenient and cost-effective. However, there are challenges associated with this approach, including the selection and incorporation of suitable healing agents, optimization of their distribution within the material, and ensuring compatibility between the healing agents and the material matrix. Researchers are actively investigating various types of healing agents, such as encapsulated chemicals, micro vascular networks, and reversible polymers, to enhance the healing efficiency and durability of self-healing materials [
65,
66]. The embedded self-curative approach for self-healing materials synthesis involves incorporating healing agents or functionalities into the material structure during synthesis, enabling the material to autonomously repair the damage. This approach holds a great promise for creating advanced materials with an extended lifespan and improved reliability in a wide range of applications.
3.2.2 Intrinsic self-curative approach
The intrinsic self-curative approach for self-healing materials synthesis focuses on developing materials with inherent healing capabilities without the need for external additives or agents. In this approach, the healing functionality is embedded directly into the molecular structure during synthesis, enabling autonomous healing without the reliance on external triggers or interventions [
67]. The material is designed to have dynamic chemical bonds or reversible interactions at the molecular level. These reversible bonds can undergo spontaneous rearrangement when the material is subjected to damage, allowing it to heal itself. Examples of reversible bonds include disulfide bonds, hydrogen bonds, metal-ligand interactions, and reversible covalent bonds. The intrinsic self-curative approach offers several advantages over other healing strategies. It eliminates the need for external healing agents or triggers, simplifying the material design and reducing dependency on external factors. Additionally, the healing process can occur repeatedly and over multiple damage events, making the material more durable and reliable [
68]. Designing materials with the appropriate reversible bonds and optimizing their distribution and density within the material matrix is a complex task. Achieving a balance between mechanical strength and healing efficiency is also a challenge, as materials with high healing capabilities may sacrifice other desirable properties. Researchers are exploring various strategies to enhance the intrinsic self-healing capabilities of materials, such as the incorporation of self-assembling polymers, shape memory polymers, or supramolecular structures [
69]. These approaches aim to improve the healing efficiency of materials, control the healing kinetics, and extend the range of damage that can be repaired. The intrinsic self-curative approach for self-healing material synthesis involves designing materials with inherent healing capabilities by incorporating reversible bonds or interactions at the molecular level. This approach enables autonomous healing without the need for external additives or agents. While challenges exist, ongoing research aims to advance this approach and develop materials with improved healing efficiency and durability [
62]. There is another approach that concerns the ability of the material to be remolded. It will be very helpful to develop a polymeric network that can remember its novel shape. This self-healing property will ensure to get the material back into its former shape and morphology after the distortion. These types of polymeric materials are known as shape memory polymers and provide better healing efficiency.
3.3 Shape memory polymers
Shape memory polymers are dual-shape materials that can return to their original shape after being deformed, making them a preferable option for self-healing. As the name implies, they have the “memory” of their shape [
70]. They can get back to their original shape after deformation when a particular stimulus is applied. To bring the material back to its original shape, different stimuli can be used viz. heat (above a particular transition temperature), light, irradiation with infrared light, immersion in water, and application of electric or magnetic fields. Shape memory polymers have a semi-crystalline structure. The design includes flexible and mobile polymeric chains and concerns the molecular structure rather than the chemical structure. At glass transition temperature (
Tg), the polymer changes from one state to the other (from crystalline to amorphous). The shape-memory polymer has two stages of this temperature. Initially, the segments of the material are frozen or fixed at their place but when heated the state begins to change attaining more flexibility [
71]. The amorphous stage is achieved due to easier rotation of the segment bonds at the raised temperatures. The development of these materials is a fiery topic of research. Some extensively used materials are polytetrafluorethylene (PTFE), polylactide (PLA), ethylene-vinyl acetate (EVA), etc.
3.4 Types of self-healing materials
Self-healing materials that have a great influence in developing durable and sustainable devices can be used in electronic devices, storage devices, etc. Based upon their usage, self-healing materials can be divided into the following categories such as conductors, insulators, polymers (also shape memory polymers), metallo-polymers, and composites. When it comes to metallic shape memory polymers, it is not easy to design due to its high melting point. The material created often reacts with oxygen or other gases when the healing occurs, which often hinders the new multiple bonding near the crack or damage. Desirable solid-state techniques can be counted on like precipitation healing or semisolid-state methods like shape memory healing, the vascular approach, and electroplating [
72].
3.5 Uses of self-healing materials
Self-restoring materials and shape memory polymers have vivid applications such as those presented in Fig.9. Due to their better engagement in curating the degradation of the devices, they are extensively used in NGs and self-powered devices [
73]. NGs have high potential in energy harvesting and are mainly known for their robustness and endurance. They have a high impact on the field of renewable sources of energy. They exhibit greater environmental awareness and are put to good use. To increase productivity and improve efficiency, new methods are being developed. The first thing that must be done is to research their lifetime and durability, which is a serious matter. The majority of NGs experience deterioration after prolonged use [
74]. Conventional energy sources used inorganic materials such as aluminum, copper, iron, silicon, and others which lack flexibility and stretchability. In microelectronic devices, where deformation is required, the typical methods have drawbacks [
75]. To ensure their durability, they must be enhanced with materials that have self-healing and shape-memory properties.
4 Working of self-healing materials in TENG
The viable application of electronic devices relies on durability and efficiency during triboelectrification. The materials used must be flexible, easily deformable (shape-memory), and self-healable [
76]. The material chosen must possess a better functionality concerning the power supply. In TENG, distortion mainly occurs due to the friction produced when the material surface is made in contact or when tapped, bent, or stretched during energy harvesting. It thus reduces the enhancement and durability, which reduces the efficiency time by time. Therefore, choosing a self-healable and shape-memory material will be a better choice. Studies show that nanoscale materials effectively achieve optimum surface utilization and generate more electrostatic charges during triboelectrification. The material must have soft solid–solid interface contact. The charge generated is maximum when the two contact surfaces are rubbed rather than just touched. The reason for this is that in rubbing, the area of contact increases. In addition, there exist some velocity in rubbing too. Some paper reveal that the charge generated increases with velocity but also decreases to some extent, due to heating and back-flow of charges. This shows that there is some particular speed at which the electrification is maximum. Thus the variation of charge with speed occurs due to different mechanisms. Moreover, rubbing cannot be regarded as the prominent one [
55].
The selection of the device depends upon what kind of TENG is being used. The healing properties of the triboelectric-charged layer and electrode are first examined before the device is deployed. The first TENG was developed in 2012, and since then various types of research have been conducted, which confirmed that the most common type of artificial SHM is polymer-based. Weighing their building process, they are of two categories, inherent self-healing polymers having reversible bonds that require several heating-cooling cycles to aid healing [
77], and depletion of healing substances for self-healing [
62]. Some researchers have produced ionic conductors to improve elastic properties and conductivity in addition to the approach of doping conductive particles into polymers to fill the gaps, which are elastomer-based ultra-stretchable TENG (US-TENG) with inner-healing capacity and dual reversible bonds [
72]. The self-healing mechanisms of TENGs could be induced by thermal or NIR irradiation of carbon nanotubes or silver nanowires implanted in a disulfide bond-based elastomer [
78]. By combining magnetic-assisted electrodes well with PDMS-PU polymer, a sturdy TENG device with full thermal-healing capability was created [
78,
79]. When compared to the traditional copper foil electrode, the hydrogel can substantially improve the output performance of TENG. The PDA-PAM hydrogel segment can be used as the electrode in a basic single-electrode mode TENG with a layered arrangement (PP-TENG). Eventually, this PDA-PAM hydrogel is a good choice for flexible TENG electrodes [
80]. Microencapsulated restorative agents, dynamic non-covalent supermolecular congregation (H-bonding, metal-ligand coordination, etc.) and reversible covalent bonding are all examples of artificial healing (the Diels-Alder reaction, S–S bonds, imine, etc.) [
81].
4.1 Mechanism of self-healing materials
Materials that can restore themselves whenever any external obstruction occurs are known as self-restoring or self-healing materials. Some of the basic mechanisms of self-healing materials are shown in Fig.10, in which it is observed clearly how self-healing occurs in the material via an intrinsic approach. They can regenerate their damaged sites by interacting with external stimuli. The regeneration is done in various ways, which can be intrinsic, extrinsic, optical, thermal, etc.
The toughness and durability of TENGs are significantly harmed by repeated and inevitable mechanical damage, such as fractures and finally rupture, which can fail. Modern devices which are handy, easy to wear, and flexible are prone to such harms. As a response, developing self-healing TENGs to extend life expectancy is widely preferred [
82]. The rate of self-healing is very important and must occur at room temperature. To quantify the healing process, the efficiency of the self-healing materials is measured which are given by 0−
f0 × 100, where 0 is the healing rate of the original material, and
f is the healing rate of the self-healed material. The rate of harm versus the rate of healing balance regulates the efficiency of healing. External factors such as loading frequency, strain rate, and stress amplitude determine the rate of material damage. The self-healing ability to heal is influenced by a variety of factors, including damage volume, impact rate, healing rate, healing temperature, and the bonding strength between the recovered material and the matrix material. However, the rate of healing or restoration may be altered or changed for various injury types by changing the reaction kinetics via species concentration or temperature. Self-purpose healing is to establish material stabilization by balancing the rate of restoration with the rate of damage [
83]. There are two approaches to self-healing, the intrinsic approach and the extrinsic approach (capsule driven and vascular). The intrinsic approach offers healing on a small scale only, whereas the extrinsic approach is viable for large damages. When the weak dynamic bonds break during damage and new bonds are formed, they have a high healing ability as compared to the prior one.
4.1.1 Encapsulation self-healing
The healing agent is produced and the self-healing process begins when these capsules are burst by the deformation generated by the materials. A limited number of capsules can be inserted into the materials. Analysis, design, incorporation, physical characterization, activating, and healing evaluation comprise the design cycle. Fig.11 shows the schematic of the processes involved in self-healing via encapsulation.
The first concern is selecting the most appropriate way to sequester the healing agent and polymerizer. Encapsulation of phase separation can be used. Dissolution rate, sensitivity, fluidity, volatility, and pH are the most relevant factors for confined materials. The core of the capsule is oil-soluble and has a low viscosity. The assimilation of capsule into the substance is the next phase. The forces generated due to mixing, processing temperature, and reactivity may vary and thus size of the capsule gets varied. The manufacturing of PU, UF, and MF/MUF-based capsules are available in multiple scale sizes [
83]. The mechanical qualities, initiating mechanism, and process of healing are all characterized after the integration procedure. The size, volume percentage, binding strength, and toughness of the capsule determine the mechanical characteristics of the self-healing material. The triggering process initiated in response to external stimuli or force is studied and observed under SEM, IR, and EDS spectroscopy of the cracked or deformed plane. After this, the testing of the self-material is done depending on the intended application. The materials used in the structural matrix such as polymers and composites are assessed by the mechanical properties such as fracture toughness or stiffness.
4.1.2 Vascular self-healing
Vascular self-healing works on a principle similar to that of encapsulation. The process involves material characterization, triggering phenomena, and healing performance. Vascular healing involves hollow glass fibers (HGFs) filled with a suitable healing agent. For application in composites, these fibers are incorporated into the glass and carbon fiber piles. The linkage of chains of vascular tubes has various advantages as the multiple linkages of fiber tubes offer easy refilling of the healing agent.
4.1.3 Intrinsic self-healing
The synthesis of low molecular weight spices and the breakdown of weak dynamic bonds following injury or under the impact of an external stimulus are two mechanisms of intrinsic healing. They have a significant healing capacity, which helps the body repair more quickly. The initial structure of polymer and functional qualities are recovered in the final stage. The path of self-recovery might occur multiple times. This can be done by the use of thermally reversible processes, H-bonding, ionomeric coupling, a distributed meltable thermoplastic phase, or molecular diffusion. Development, material characterization, triggering, and healing evaluation are the four stages of the synthesis. It eliminates several issues that can occur with vascular and encapsulation procedures and does not call for any healing agent to be inserted in the matrix. Reversible chain reactions, distributed thermoplastic polymers, molecular diffusion, ionomeric self-healing materials, and supermolecular self-healing materials are some of the methods that can be used [
82].
4.1.4 Self-healing in polymers
The self-restoring phenomena in polymers involve (a) surface rearrangement (surface topography and roughness, chain-end distribution, and molecular-weight distribution), (b) surface approach, (c) randomization, (d) diffusion, and (e) wetting [
84]. Non-covalent intrinsic self-healing mechanisms are presented in Fig.12. After considering surface rearrangements when two surfaces approach each other to initiate self-healing, they form an interface and wet each other before the process of diffusion. The interfacial region created after mechanical damage is important and the mobility and diffusion rate particulate to that area is important. There are three approaches to performing healing, physical approaches, chemical approaches, and physico-chemical approaches.
Physical approaches: To achieve self-healing characteristics in polymers, a number of physical methods have been devised. In a physical technique called microencapsulation, healing chemicals are enclosed in tiny capsules and spread throughout the matrix of self-healing polymers. When a material is damaged, the capsules burst, releasing the healing agents. These substances then flow into the holes or cracks and solidify to repair the material. Numerous types of polymers, such as epoxy resins, polyurethanes, and polymeric coatings, have been employed for this strategy. Another physical method utilized in self-healing polymers is reversible crosslinking. In this method, damage can be repaired by breaking and reforming crosslinking sites within the polymer matrix. To promote self-healing, this enables the polymer to experience reversible changes in its physical characteristics, such as viscosity or modulus. Dynamic covalent bonds, supramolecular interactions, and physical entanglements are a few examples of reversible crosslinking mechanisms utilized in self-healing polymers [
51].
Chemical approaches: In self-healing polymers, chemical techniques are used to accelerate the mending of damaged materials through chemical reactions or interactions. A well-known chemical technique utilized in self-healing polymers is the Diels-Alder reaction. These reversible reactions can be brought about by heat, light, or other stimuli. In a self-healing polymer, Diels-Alder moieties are inserted into the polymer matrix, and when damage occurs, the Diels-Alder bonds break, allowing the polymer to flow and heal the material. The Diels-Alder reaction can be stopped by adding heat or other stimuli, allowing the polymer to repair the bonds and revert to its initial properties. Another chemical strategy utilized in self-healing polymers is covalent bond exchange. In this method, when damage happens, the reversible covalent bonds of polymer might go through exchange reactions. Examples of dynamic covalent bonds that can be integrated into the polymer matrix include disulfide bonds. The disulfide bonds break when damage occurs, allowing the polymer to flow and repair the substance. The disulfide bonds can rebuild by the application of heat or other stimuli, allowing the polymer to restore its original characteristics.
Physico-chemical approaches: To achieve self-healing characteristics in polymers, physico-chemical techniques combine physical and chemical mechanisms. A physical technique called microencapsulation encapsulates therapeutic chemicals in tiny capsules that are scattered throughout the polymer matrix. Chemical agents, such as monomers or catalysts that can conduct chemical reactions to repair the damaged polymer can also be used as healing agents in physicochemical techniques. When a material is injured, the capsules burst, releasing the healing agents, which react with the broken down polymer to create new bonds and repair the material. Another physico-chemical method utilized in self-healing polymers is dual-cure systems. In these systems, self-healing is accomplished through the use of two distinct types of reactions, often a physical and a chemical reaction. The polymer may, for instance, include reversible physical interactions like hydrogen bonding that can promote transient healing. Crosslinking reactions, for example, can be started by outside stimuli at the same moment to permanently repair the damaged polymer. The cohesive energy density (CED) is increased as a result of the stabilizing effects of weak forces on nearby copolymers [
51].
The molecular arrangement of the polymers makes them easy to use rather than metals and ceramics. The temperature range in which they can work is also easy to attain, as they have low gas transition temperatures. When the polymers are stretched, the surface energy is changed into strain energy and thus into mechanical work. Some of the self-healing methods and the comparison are given in Fig.13 and various M-L ligands used as self-healing materials are given in Fig.14 [
54]. This change in energy gives rise to crack closure. Self-healing starts from the bottom of the damaged site toward the surface. The reversible sense of the polymeric chain during the chemical reaction is also very important [
52]. Self-recovery is initiated by molecular rearrangement or re-assembling due to unstable bonds; they reconstruct the structure and the result is a different molecular structure. Hydrogen bonds appear to provide a better molecular assembly for self-healing materials. Most of the polymers (about 60%) have mixed amorphous and crystalline structures [
52]. Polymers are among the most researched and explored types of materials for self-healing due to a variety of factors including ease of chemical functionalization, amorphous morphological characteristics that enable high movement of molecules (and polymer chains) at low temperatures, and good solubility that allows for processing in a variety of sizes and shapes [
53].
Metallopolymers (MPs) are polymeric composites with metallic centers containing polymer chains [
86]. The polymeric structure and metal location can be changed to achieve the desired results. Supramolecular MPs are fascinating materials that combine the technological benefits of polymers (workability, mechanical characteristics, solubility, and so on) with the utility (electrochemical, optical, and magnetic properties) of metals. As a result, these compounds have a lot of promise in a variety of advanced sustainable applications [
87]. Many weak bonds, the synergistic effect of strong and weak bonds, stress dispersion along nanofibers, the multiphase of hard and soft domains are presented in Fig.15. Several supramolecular architectures with flexible and even switchable M–L connections, allowing for structure manipulation for their beneficial activities. In the simplest scenario, physical interactions result in intrinsic self-healing, i.e., heating of the materials causes diffusion of the polymer chains, and leads to the production of new entanglements, which closes the fracture. In the event of ballistic injury, enough heat is produced to allow self-healing mechanisms to take place via secondary chemical bonding. In addition, by using Diels-Alder reactions and Michael additions, functional side groups of polymers are used for reversible heat crosslinking. Weaker interactions, such as hydrogen bonding, appear to offer great prospects for the development of advanced intrinsic self-recovering materials, assuming the spatial density of the bonds is sufficient.
Metallopolymer healing approaches are based on two interacting actions. SHPs function by providing a defined flow that can repair a hollow or disturbed area. The notions of free volume and vacancies can be used to express this, which is essentially concerned with mobility. To date, most SHPs have involved heating the affected portions well above glass transition temperature and afterwards applying effort to accomplish polymer chain interaction, re-entanglement, wetting, and diffusion. The healing process is usually lethargic and ineffective in this instance, and it is rarely used in practice. The presence of reversible chemical bonds causes the second action, which restores mechanical and functional integrity. In self-healing MPs, reversible interactions include undirected ionic interactions between positively charged metal compounds and negatively charged counterions, as well as controlled M–L interactions. The restoring process is generally lethargic and ineffective in this instance, and it is rarely used in practice. If weak and dynamic metal complexes are used, decomplexing and rearranging metal complexes can produce the necessary motion. Ionic metal complexes can result in the formation of ionic clusters. The necessary dynamism may also be achieved by reconfiguring these clusters (again, without opening the complexes). Other metal-ligand combinations might be used to change the characteristics of metallopolymers. Self-restoring metallopolymers have been developed in substantial numbers to date, and the number is growing every year. In materials like polymers and composites, damage and fatigue occur spontaneously in various damage-causing conditions. The concept of reducing this degradation through a self-healing procedure holds the promise of increased lives and enduring strength. Many other restorative agents have been postulated and explored as a result of the simple premise. According to recent numerical research, neither the spherical capsules nor the hollow fiber structure is optimal for achieving high healing efficacy.
4.2 Fabrication approach
The important aspect is the material selection and approach of fabrication [
86]. It is necessary for fabricating high-performance TENG and can be done by several methods.
4.2.1 Functional group grafting (self-assembled monolayer, ultraviolet ozone irradiation, electrospinning)
TENGs can be benefited from functional group grafting in terms of improving their self-healing capabilities. TENG surfaces can produce self-assembled monolayers (SAMs) that act as a protective and self-healing layer [
89]. Through SAM creation, self-healing functional groups, such as encapsulated healing agents or reversible bonds, can be bonded to the surface. The self-healing functional groups can activate and restore the surface of the TENG after injury, regaining its functionality. The strength of TENG and lifetime of device are increased by this method. The surface of TENG materials can be cleaned and activated by ultraviolet ozone irradiation prior to grafting functional groups [
90]. The adherence and performance of the grafted functional groups can be improved by eliminating impurities and activating the surface chemistry. In addition, UV ozone treatment can encourage the development of reactive sites on the surface, facilitating the grafting of self-healing functional groups later [
91]. Self-healing nanofibers mats or coatings can be created by electrospinning and applied to the TENG surfaces [
92]. To produce a fibrous structure, self-healing polymers, such as those with encapsulated healing agents or reversible linkages, can be electrospun. The TENG surface may then be covered with these nanofibers, creating a shield that can automatically fix damage [
93]. The nanofibers can release healing chemicals or go through reversible bond rearrangements to restore the integrity of the surface when the TENG is mechanically damaged. These functional group grafting methods can be applied to the design of TENGs to enhance their capacity for self-healing. This increases the durability and dependability of TENGs in a variety of applications, including energy harvesting, wearable electronics, and self-powered sensors. It also enables the autonomous healing of surface damage.
4.2.2 Ion implantation and injection (ion injection, ion irradiation, ion absorption)
Through a number of techniques, ion implantation, ion injection, ion irradiation, and ion absorption can be used to improve the self-healing capabilities of TENGs. Ion injection entails carefully introducing ions into the TENG substance. Ions can be injected into particular TENG layers or areas to accelerate the healing process in the context of self-healing [
94]. For instance, materials with self-healing capabilities or healing agents can be encapsulated into ions and injected into the damaged regions of TENG. The TENG surface can then be repaired as a result of these injected ions, which can release healing agents or activate self-healing processes. Ion irradiation is the term for the high-energy ions that are used to attack the TENG material [
95]. Ions can have a variety of impacts when they strike on surface, promoting self-healing. For instance, ion irradiation can produce material flaws that can serve as the starting point for healing processes. Additionally, it can alter the surface chemistry and raise the reactivity to promote self-healing processes. The therapeutic capabilities of the TENG can be improved by meticulously regulating the ion energy and dosage. Ion absorption is the process by which ions are taken up by the TENG material, usually through its surface ions with therapeutic characteristics can be absorbed by the material to start repair processes in the context of self-healing [
64]. For instance, ion-absorbing sites or functional groups that can only absorb healing ions can be built into the TENG material. The functioning of the TENG is then restored as a result of these ions participation in self-healing processes. The self-healing capacities of TENGs may be further enhanced by utilizing ion implantation, ion injection, ion irradiation, and ion absorption procedures. To encourage autonomous repair of TENG surfaces, these techniques allow the introduction of healing agents, activation of healing processes, and alteration of material characteristics. This improves the performance, lifespan, and dependability of TENG devices in a variety of settings where self-healing qualities are desired.
4.2.3 Property engineering (sequential infiltration synthesis, high dielectric nanoparticles doping)
Use of property engineering approaches like sequential infiltration synthesis (SIS) and high dielectric nanoparticles doping to improve their self-healing capabilities may be advantagneous to TENGs [
96]. Inorganic compounds are deposited in conformal and homogenous thin films into porous substrates using the SIS method. SIS can be used in the context of TENGs to develop self-healing coatings or layers on the TENG surfaces. In the SIS process, precursor molecules are sequentially introduced into the porous structure followed by a chemical reaction that results in the formation of a solid, uniform film. The film may possess self-healing abilities, such as the capacity to mend cracks or regain functioning after being harmed. Surface damage can be reduced and the performance can be restored on its own by covering the TENG surfaces with self-healing SIS. Another method to improve the self-healing capabilities of TENG materials is to dope them with high dielectric nanoparticles. Ferroelectric or piezoelectric materials, for example, are high dielectric constant nanoparticles that may be included into the TENG matrix [
97]. These nanoparticles raise the total dielectric constant, which can increase its ability to store energy and its potential for self-healing. The high dielectric nanoparticles enable the redistribution of electric charges during self-repairs, hence restoring the electrical function of the TENG after damage. High dielectric nanoparticle concentrations can also increase mechanical stability, making the TENG more resilient to bending or breaking [
98]. The self-healing properties of the devices may be considerably improved by using sequential infiltration synthesis to make self-healing coatings and adding high dielectric nanoparticles to the TENG materials. These methods allow for the autonomous healing of surface damage, the improvement of mechanical stability, and the restoration of electrical performance. In the end, these property engineering techniques help TENGs last longer, be more dependable, and sustain themselves in a variety of applications where self-healing qualities are essential.
4.2.4 Functional sub-layer insertion (electrons capture layer insertion, electrons trapping layer insertion, multiple layered structure)
The quality of TENGs can be improved from the self-healing features of functional sub-layer insertion techniques such electron capture layer insertion, electron trapping layer insertion, and multiple layered structure. To enhance the capture and transport of photo-generated electrons, an electron capture layer can be added to the TENG device construction [
99]. Usually, this layer is positioned between the electrode and the active layer. Due to the greater electron affinity of the electron capture layer, electrons may be effectively captured and transported to the electrode. The TENG can boost charge collection and enhance overall device performance by including an electron capture layer. The electron capture layer can help the TENG regain its functioning once damage occurs in the context of self-healing. To improve the trapping and confinement of electrons, an electron trapping layer can be added to the TENG structure [
100]. Normally, this layer is positioned between the active layer and the electrode or electron capture layer. The presence of sites or materials in the electron trapping layer can efficiently capture and immobilize electrons, limiting their recombination and electrical energy loss. The TENG can boost device performance by decreasing electron leakage, improving charge separation, and adding an electron trapping layer [
101]. By preventing performance loss and maintaining electron confinement in the case of damage, the electron trapping layer can aid in the self-repair procedure. To increase the capacity of TENG for self-healing, a multilayer structure can be devised and built. This entails incorporating many functional layers with distinct characteristics and functions. The TENG structure, for instance, may have layers for self-healing, capturing electrons, trapping them, and other pertinent layers. Each layer has a distinct function, such as mechanical stability, charge confinement, charge collecting, and surface repair. The TENG can autonomously heal damage, restore functionality, and sustain dependable performance for lengthy durations by carefully mixing numerous layers [
102]. The self-healing abilities of TENGs may be greatly improved by using functional sub-layers insertion techniques, such as electron capture layer insertion, electron trapping layer insertion, and constructing multiple layered architectures. These methods make it possible for electrons to be efficiently trapped, transported, and collected, which improves charge collection, slows down performance deterioration, and increases device dependability. In different applications where self-healing qualities are desired, the insertion of functional sub-layers helps TENGs be self-sustaining and last a long time.
5 Literature review
In this section, the literature survey of self-healable polymers, which are used in NGs, were discussed. Poly (methyl methacrylate) (PMMA) and PMMA–poly(methoxyethyl acrylate) (PMEA) copolymers were explored by Jud and Kausch [
103], who examined the impact of molecular weight and copolymerization degree. The healing test method was clamping and heating the damaged surfaces after being bought together. Experiments were performed and resulted that a temperature of 5 °C greater than
Tg was required to heal the surface and healing time was around 1 min. It was seen that it provided better results as compared to normal surface adhesive methods. Wang et al. [
104] studied the crack healing of PMMA with ethanol and methanol, the results were the same approximately in both the cases. In the case of metallopolymer, Al-PET and Al-Kapton were studied. The main components behind the charge transfer phenomena were carboxyl (double bond O) in PET and imide (less in case of ether) in Kapton. π orbital is more to be an electron-gain orbital than a σ orbital. That is the reason for the fact that the electron-gain ability in the case of double-bonded oxygen atoms is better than the single-bonded O atoms [
53]. As long as they keep their temporary stability, reactive CH and CH
2 free radicals acquire extremely selective sp and sp
2 hybridizations, which favor the self-healing process. Thus, it can be expected that the reactive radicals can be part of the surface on one side [
105]. A feather-based TENG (F-STENG) [
106] was also developed, composed of dropped feathers collected from the animal body. Cu layer was fabricated as a metal electrode over the feather surface by magnetic sputtering. It is a single electrode-based TENG. When the feather surface is charged by external media, the triboelectric effect is induced, and a closed-loop current can be formed by grounding the metal electrode. This type of TENG has a special feature for self-healing by not involving chemical methods. The feather has an inbuilt ability to self-heal due to its microstructure. The output result of the self-healing and triboelectric effect is 80.6%. F-STENG is biodegradable, light, and thin, and can be employed in self-powered sensing applications. It can monitor temperature, moisture, and wind with a large sensing area and fast reaction as a self-powered detector with a sensitivity of 0.50 V/°C, −0.98 V/RH, and 1.67 × 10
−3 A/(m·s). High flexibility, stretchability, lightweight, comfort, and other projecting factors attracted TENG to the field of the textile industry. Nanopatterned (PDMS nanostructure) wearable TENG (WTENG) [
58] have been developed, and are seeking much attention. The demonstration output current and voltage of about 120 V and 65 μA, were detected for nanostructured PDMS-based WTENG.
At the same mechanical compressive force of 10 kg, the nano-patterned PDMS-based WTNG produced a high output voltage and output current of around 120 V and 65 μA, respectively, while the non-nanopatterned flat PDMS-based WTNG produced an output voltage and output current of 30 V and 20 μA. Furthermore, with the same normal compressive stress, the four-layer-stacked WTNG produced exceptionally high voltage and current outputs with average values of 170 V and 120 μA, respectively. At an external resistance of roughly 1 MΩ, the output power peaked at around 1.1 mW. Throughout 12000 cycles, there were no significant variations in the voltage levels observed from the multilayer-stacked WTNG. An environment-resisted TENG [
107] is also developed with outburst properties, for power generation and self-powered sensors, hydrophobic, ice-phobic, and rapid self-healing materials with non-drying and non-freezing qualities needed. This TENG presented a wide temperature range and outstanding healing ability as compared to any other hydrogel method. As in conventional hydrogel methods, the device freezes at a low temperature and dries at a higher temperature. In addition, the electrical properties of the device get restored without much delay. Ti
3C
2T
x MXene for self-motorized wearable integrated circuit technology has a great ambience in making extremely flexible, effectual electromagnetic interference shielding, and a self-restorable TENG. A biomimetic approach using a soft self-healing polyurea material is presented by Sun et al. [
108] as presented in Fig.16, to overcome the limitations of poor crack resistance. Fig.16(a) presents the capacitive self-charging strain sensor based of the self-healing material whereas Fig.16(b) and 16(c) show the dielectric permittivity with respect to frequency and Young’s modulus. Fig.16(d) depicts the cyclic tensile measurement and Fig.16(e) relative capacitive change. The material achieves improved crack-resistant strain and fracture toughness by adding Galinstan micro-droplets by metal-coordinated assembly, inspired by vascular smooth muscles. The fracture toughness is higher than that of alloys made of aluminum and zinc, reaching 111.16 and 8.76 kJ/m
2, respectively. The material is also suited for sensitive and self-healing capacitive strain-sensors in the presence of potential fractures due to its quick self-healing kinetics (1 min) and high dielectric constants (14.57). Some of the experimental analysis for the materials selected as self-healing materials for the TENG device are summarized in Tab.2.
6 Environmental assessment
TENGs are used to prepare shape memory and self-healing materials, and their environmental evaluation concerns the possible environmental effects of their manufacturing, usage, and disposal [
125]. The materials utilized for the shape memory and self-healing properties of TENGs determine their environmental effect. Raw material extraction and processing, such as that of metals, polymers, and composites, may have an impact on energy use, habitat damage, and resource depletion. To reduce negative environmental effects, it is crucial to take sustainability and ethical source of these materials into account. Environmental effects may result from the TENGs production procedures. Energy-intensive procedures that produce greenhouse gas emissions, including high-temperature synthesis or sophisticated production methods, may be a factor in climate change. To minimize the effects on the environment, it is vital to optimize industrial efficiency, minimize energy usage, and embrace cleaner production techniques. Environmental effects may result from the fabrication of shape memory and self-healing materials using chemicals like solvents, adhesives, or coatings [
126]. When produced or disposed of, harmful compounds have the potential to discharge harmful substances into the environment, endangering ecosystems and human health. Minimizing these effects requires using safe, non-toxic alternatives and implementing effective waste management procedures. Similar to every other electronic item, TENGs have a lifespan that concludes with disposal. TENGs should be disposed of properly to prevent the release of potentially harmful chemicals into the environment, which would increase pollution and pose health hazards. The recovery of valuable materials from garbage and the implementation of efficient recycling and waste management techniques can reduce negative environmental effects and encourage a circular economy. It is essential to provide ecologically friendly preparation techniques and procedures for materials with shape memory and self-healing properties. The environmental impact of TENG preparation can be diminished by the use of green chemical advancements, effective synthesis methods, and renewable resources. Considering ways to reduce resource depletion, energy use, emissions, chemical risks, and adequate waste management are all part of an environmental assessment of TENGs for the creation of shape memory and self-healing materials. TENGs may be created and used in a way that is more ecologically conscious and sustainable by including environmental protection into the stages of design, production, and end-of-life.
7 Conclusions and future scope
Future research can concentrate on investigating new materials with improved mechanical stability, durability, and triboelectric properties because the development of self-healing nanostructures is a topic that is quickly growing. To maximize the performance of TENGs, this may entail improvements in material synthesis, fabrication procedures, and characterization techniques. By investigating techniques including nanocomposites, nanostructures, and surface changes to improve the triboelectric charging efficiency, output power, and long-term stability of the devices, additional study can focus on maximizing the performance of self-healable TENGs. Therefore, underlying mechanisms of self-healing and shape memory qualities on the electrical and mechanical performance of TENGs may need to be further investigated. Beyond wearable electronics, IoT devices, biological implants, and environmental monitoring systems, self-healing TENGs have the potential to find use in a variety of industries. Novel applications in fields including robotics, smart fabrics, smart packaging, and human-machine interfaces, among others, can be explored in the future. To take advantage of the special capacities of self-healing TENGs in a variety of sectors, interdisciplinary cooperation and developments may be needed. Advanced applications may be made possible by combining self-healing TENGs with other cutting-edge technologies including flexible electronics, energy storage, and wireless communication. Future studies could concentrate on creating integrated systems that take advantage of the interactions between self-healing TENGs and other technologies to produce self-powered, autonomous, and intelligent systems, with the use of eco-friendly and biodegradable materials, as well as strategies for waste reduction and recycling, as sustainability is becoming a more important issue. The creation of more environmentally and economically sustainable and cost-effective technologies can be influenced by life cycle assessment and economic analysis, which can shed light on the sustainability of self-healing TENGs.
In summary, the research presented in this review shows how self-healing TENGs have made considerable strides and have great promise to become reliable energy sources for autonomous and self-powered systems. Future developments in self-healing TENGs may include performance optimization, application growth, and integration with new technologies, environmental considerations, and standardization, among other things. Future prospects for sustainable and self-powered systems will be made possible by additional research and development in this area, which is predicted to spur innovation in sensor and energy technology.
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