Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review

Abstract

The rapid advancement in electronics, electric vehicles, and grid storage has created a growing need for cutting-edge electronic devices. Researchers are actively investigating innovative materials that can withstand mechanical stress, exhibit electrical conductivity, and self-repair to ensure top-tier performance in various scenarios. Traditional polymer materials, though versatile, often lack the necessary qualities to meet the rigorous demands of flexible electronic and energy applications. To address these limitations, a significant breakthrough has emerged with conductive self-healing epoxy composites. These composites incorporate conductive materials into the polymer matrix, providing them with electrical conductivity, making them ideal for use as active components in electronic systems. Moreover, the integration of self-healing mechanisms significantly enhances their durability, countering performance degradation from mechanical stress. This chapter explores the recent developments in electrically self-healing epoxy-based polymer composites, with a focus on intrinsic self-healing mechanisms, evaluation strategies, the design of the conductive network in these composites, and their diverse applications in electronic components. These applications encompass adhesives, anti-corrosion coatings, sensors, EMI shielding, soft actuators, and energy storage/harvesting devices such as supercapacitors, batteries, and nanogenerators. The chapter also addresses the challenges and prospects of advancing flexible devices, paving the way for more reliable and efficient electronic systems in the future.

1. Introduction

In the ever-advancing world of electronics, the demand for materials that combine exceptional mechanical properties with electrical conductivity is ever-increasing. Traditional epoxy materials are renowned for their excellent mechanical strength, chemical resistance, and adhesive properties, making them widely utilized in various applications [1, 2, 3, 4]. However, their inherent insulating nature limits their usefulness in applications where electrical conductivity is required. The introduction of conductive materials, such as metallic particles, metallic nanowires, carbon nanotubes (CNTs), and graphene-based materials, has enabled the incorporation of electrical conductivity into epoxy composites (ECs). The conductive materials need to exhibit excellent conductivity while also demonstrating strong compatibility with the host polymer. Yet, these materials remain vulnerable to damage caused by repeated deformations, which can give rise to the formation of microcracks and fractures. Such structural impairments disrupt electrical pathways, yielding functional deficiencies that ultimately compromise performance and lifespan in electronic applications [5, 6, 7, 8]. A captivating development aimed at addressing this demand is the fusion of self-healing properties with electrical conductivity in epoxy materials. Self-healing ECs endowed with electrical conductivity not only ensure the longevity and dependability of electronic components but also enable the development of advanced electronic systems that boast enhanced resilience and durability. This self-repairing capability is achieved through the incorporation of dynamic chemical bonds within the polymer matrix, including noncovalent bonds (hydrogen bonds [9, 10], metal-ligand coordination [11, 12], etc.) and reversible covalent bonds (hindered urea bonds [13, 14], disulfide bonds [15, 16], Schiff base [17, 18], etc.). Figure 1 visually depicts the utilization of electrically functional epoxy composites in self-healing electronic components. The impetus for exploring the fusion of self-healing attributes and electrical conductivity in epoxy materials arises from the surging demand for robust materials possessing both mechanical vigor and electrical prowess. The amalgamation of these attributes opens avenues to augment the dependability and lifespan of electronic devices, while concurrently bolstering their resilience against mechanical and electrical adversities.

Figure 1.

Explore the electronic applications where epoxy composites with self-healing and electrical conductivity functionalities are utilized.

Within this chapter, we embark on a comprehensive exploration to present a panoramic overview of the substantial progress achieved in the application of epoxy composites (ECs) endowed with both self-healing capabilities and electrical functionality, specifically tailored for versatile and flexible applications. Our primary objective is to delve into their diverse implementation across various domains. These domains encompass adhesives, flexible coatings designed for anticorrosion purposes, sensors, electronic components such as EMI shielding, soft actuators, and energy devices. We place distinct emphasis on meticulously unraveling the foundational differences between the mechanisms and strategies governing the healing of mechanical fractures and electrical breakdowns within ECs.

Furthermore, we endeavor to articulate the core principles, self-healing assessment, and design strategies of conductive networks that underpin self-healing EC-based devices. This highlighting showcases their inherent capacity not only to restore mechanical integrity but also to reinstate functionality and performance, even in the presence of simultaneous mechanical and electrical damage. While acknowledging the frontier that lies ahead, we also shed light on the challenges that await and the promising opportunities for future research. Through these efforts, this chapter aims to provide a comprehensive, up-to-date, and thorough understanding of the ever-evolving realm of conductive self-healing ECs within the landscape of flexible devices.

2. Fundamentals of conductive self-healing ECs

In the interest of providing comprehensive information, this section offers a concise overview of EC’s self-healing strategies and materials design. However, the main emphasis of this chapter revolves around exploring the uses of conductive self-healing ECs in flexible electronic devices.

2.1 Self-healing chemistry strategies

The healing strategies for epoxy materials can be broadly categorized into two groups based on their structures and chemical compositions: intrinsic and extrinsic approaches. These two categories are distinguished by the mechanisms they employ to achieve healing functionality.

Intrinsic self-healing approaches do not rely on external healing agents. Instead, they possess an inherent ability to self-heal, which is facilitated by the thermal diffusion of molecules, reorganization of the polymer matrix, and the reformation of reversible/dynamic bonds or supramolecular interactions at the damaged interfaces. The efficiency of self-healing in epoxy polymers is dependent on critical factors such as the chain mobility, dissociation rate, and association rates. The majority of intrinsic recovery strategies are based on: (1) reversible covalent bonds (RCBs), which include Diels-Alder (DA) reactions [19], disulfide bonds (S∙S) [20], boronated ester bonds [21], and imine bonds [22]. These RCBs offer a particularly favorable approach for designing self-healing epoxy polymers with enhanced mechanical properties, thanks to their robust bonding strength. According to Krishnakumar et al. [23], they demonstrated the potential of graphene oxide in facilitating healing of epoxy vitrimer nanocomposites, achieving approximately 90% restoration of the original strength through disulfide bonds. However, this healing process necessitates heating the cracked region to activate the bond exchange mechanism. At the earliest, Hao et al. [24] have showcased an innovative approach that involves a vanillin-based hyperbranched epoxy resin (VEHBP), integrating the synergistic effects of disulfide and imine dynamic covalent bonds (Figure 2a). This integration results in an epoxy resin that is both recyclable and malleable, while maintaining a notably high glass transition temperature (T_g), along with remarkable improvements in creep resistance and mechanical properties. Remarkably, the inclusion of 5% VEHBP in the dynamic covalent epoxy resin led to substantial enhancements in key characteristics. These enhancements encompassed a remarkable glass transition temperature of 175°C and a creep temperature of 130°C. Notably, the mechanical properties saw significant boosts, including a remarkable 34.1% increase in tensile strength, a 19.7% rise in storage modulus, and an impressive 173.3% surge in tensile toughness when compared to the pristine resin. Importantly, the distinctive hyperbranched architecture of VEHBP, coupled with the dual dynamic bonds, imbued these materials with exceptional self-healing capabilities, the potential for reprocessing, and a positive environmental footprint due to their degradability. These properties mark a significant stride forward in the realm of designing and crafting high-performance epoxy covalent adaptable networks. Liu and co-workers [26] conducted a study involving the synthesis of a range of ester-exchange catalysts, which were then analyzed for their potential in generating dynamic covalent bonds. In their investigation, they introduced metal-ion-free triethanolamine as an ester-exchange catalyst. This catalyst was utilized to develop epoxy resins possessing optimal characteristics suitable for the manufacturing of traditional electrical equipment. Furthermore, they combined 3,3′-dithiodipropionic acid (DTDPA), a compound containing carboxylic acid groups and S∙S bonds, with MHHPA to create an epoxy vitrimer featuring dual dynamic bonds. By manipulating the ratio of the curing agents, they fine-tuned the structure of the dynamic covalent bonds within the crosslinking network. The results highlighted that the repair efficiency for surface scratches exceeded 90%. After fracture repair, the mechanical strength was rejuvenated to over 70%, and even instances of electrical damage exhibited some degree of restoration. Zou et al. [27] also investigated self-healing epoxy coatings utilizing reversible crosslinking network based on the DA reactions, which incorporated MXene flakes responsive to near-infrared (NIR) light. Their observations revealed that the presence of MXene enabled the epoxy to rapidly convert NIR light energy into heat, resulting in a self-healing capability of approximately 15 mins or/and (2) supramolecular chemistries primarily focus on reversible noncovalent interactions (RNBs), which encompass hydrogen bonds [28], metal-ligand coordination [29], ionic interaction [30], and host-guest interactions [31]. Liu et al. [32] developed a room-temperature self-healing epoxy coating with high elasticity in which the damaged coating can be repaired within 5 mins even under aqueous immersion thanks to reassociation of UPy hydrogen bonding units. In a study by Hu et al. [33], they introduced self-healable photocured epoxy acrylate resins that were formulated using photocured blends containing bisphenol-A epoxy diacrylate, a host-guest complex formed by 6-monomethacryl-substituted β-cyclodextrin and acrylamide-azobenzene, as well as butyl acrylate. The researchers demonstrated a unique approach to mending these light-responsive host-guest interactions. They achieved this by subjecting the material to UV light exposure at 365 nm, followed by a subsequent heat treatment at 120°C. This sequential process effectively enabled the damaged films to undergo self-healing. While not as strong as RCBs, these interactions have the advantage of creating mechanically dynamic and resilient systems, making them highly desirable for self-healing polymer designs. Due to their reversible nature, intrinsic self-healing epoxy materials can undergo multiple healing cycles through these interactions or reactions.

Figure 2.

(a) Synthetic routes of an epoxy resin incorporating a hyperbranched structure derived from vanillin, featuring disulfide and imine dynamic covalent bonds. Reprinted with permission from [24]. Copyright 2023 American Chemical Society. (b) Diagram illustrating the fabrication process of bilayered microcapsules encapsulating epoxy and (C₂H₅)₂O·BF₃ dual agents through the utilization of MOF-stabilized Pickering emulsion and solvent precipitation techniques. Reprinted with permission from [25]. Copyright 2023 American Chemical Society.

In contrast to intrinsic self-healing, extrinsic self-healing epoxy materials involve the incorporation of dispersed healing agents within an epoxy polymer matrix. These agents consist of reactive fresh precursors and catalysts. When damage occurs, these agents are released and initiate a process of spontaneous polymerization and reconstruction of the cross-linking network through chemical reactions, thereby repairing the affected areas [34]. Epoxy composites containing healing agent microcapsules have been also reported in several studies [35, 36, 37, 38]. For instance, Zhu et al. [39] utilized a solvent evaporation technique to develop self-healing wave-absorption microcapsules, wherein epoxy resin cores were enclosed by hybrid walls composed of carbonyl iron powder and ethyl cellulose. As a result of this novel design, the material exhibited remarkable self-healing properties when subjected to electromagnetic irradiation. This improvement can be attributed to the increased mobility of the epoxy resin, achieved through the conversion of electromagnetic energy into thermal energy. This innovative approach shows significant potential for advancing self-healing epoxy materials, with promising applications across various fields. In a recent investigation, a novel approach was employed to achieve remarkable self-healing properties in epoxy resin. By incorporating 15 wt% microcapsules, the epoxy resin demonstrated an impressive 95% healing efficiency within a mere 10 mins at room temperature [25]. Simultaneously, the composite’s interlaminar shear strength reached an outstanding 92% improvement within just 5 mins. This groundbreaking study marks the first instance of achieving rapid self-healing across diverse damage modes in both epoxy resin and carbon fiber/epoxy resin composites. This achievement was realized through the utilization of a unique self-healing system incorporating single-component double-shell microcapsules (Figure 2b). The innovation primarily revolved around the design of double-shell microcapsules, loaded with both epoxy resin and a catalyst ((C₂H₅)₂O·BF₃). These capsules were ingeniously created by combining the principles of Pickering emulsion templating and solvent precipitation techniques. Noteworthy was the strategic use of metal-organic frameworks (MOFs) with precisely controlled sizes and hydrophilic properties. These MOFs acted as effective stabilizers for the Pickering emulsion, ensuring the stability and size control of the inner microcapsules. Furthermore, their distinctive properties provided an ideal platform for host-guest interactions, enabling the efficient capture, storage, and release of the cationic catalyst.

2.2 Self-healing performance assessment

Self-healing efficiency refers to the capacity of epoxy materials to restore their original functionalities and quality after experiencing damage. Typically, the evaluation of self-healing performance involves subjecting the materials to induced damage followed by conducting a mechanical test to initiate and quantify the self-healing process. This evaluation can be broadly categorized into three aspects: self-healing efficiency, healing rate, and damage volume.

2.2.1 Self-healing efficiency

The self-healing efficiency (η) is a crucial metric used to assess the restorative capabilities of a material following a damage and healing cycle. It quantifies the degree of functionality restoration, and there are several definitions of healing efficiency, with the most widely employed formula being η = P_healed/P_original. The parameter “P” can encompass a range of characteristics based on the research focus, including ultimate tensile strength, elongation, fracture toughness, Young’s modulus values, and other properties that pertain to electrical, thermal, optical, or capacity-related indices. For instance, carbon/epoxy composites with self-healing capabilities, achieved through the incorporation of healing agents, exhibit a notable healing efficiency of 62% [40]. This efficiency is determined based on the changes observed in flexural strength. Benazzo et al. [41] engineered a high-performance epoxy vitrimer for deployment as the matrix phase within fiber-glass composites featuring the integration of Fiber Bragg Grating (FBG) sensors. The investigation showcased that laminates possessing a greater proportion of dynamic crosslinkers manifest enhanced reparative capabilities. Notably, during the initial healing cycle, the average and maximum recorded healing efficiencies, computed with respect to critical fracture toughness, were determined to be 89% and 95%, respectively. A higher healing efficiency enhances the material’s self-healing ability, resulting in an extended service lifetime that aligns with specific application requirements.

2.2.2 Self-healing rate

The rate at which a material experiences damage is influenced by multiple factors, including strain rate, stress amplitude, and loading frequency. The effectiveness of self-healing processes relies on the healing rate exceeding the damage rate. This rate is influenced by various factors, including the concentration of broken bonds, the polymer chain mobility, and activation energies for bond exchange. These elements are intricately connected to the self-healing kinetics, encompassing temperature, mobility, and polymerization rate. To attain a high healing rate in self-healing materials, it is essential to customize the reaction kinetics by adjusting in temperature, repair time, or concentration. By carefully optimizing these parameters, the self-healing material can be designed to facilitate rapid and efficient healing processes. For example, the epoxy composite film, made using graphene-assisted host-guest chemistry, exhibits substantial improvement in healing rate at various time intervals. At 120°C, it achieves a 45.3% recovery after 5 mins and an 85.1% recovery after 30 mins [31]. In addition, raising temperatures typically enhances molecular diffusion, resulting in an increased healing rate and improved healing efficiency. For instance, Vertuccio et al. [42] investigated the impact of the curing temperature on a rubber-toughened bifunctional epoxy resin that was infused with self-healing molecules. This infusion aimed to confer an auto-repair capability to the resin. The study revealed that elevating the functionalization temperature to 160°C proved more effective in facilitating the interaction between the rubber phase and the epoxy precursor during the functionalization process. Notably, at 160°C, a healing efficiency of over 69% was achieved. The most remarkable outcome (88%) was observed in the sample incorporating the self-healing 1.3-dimethylbarbituric acid (DBA) filler. However, when the same sample underwent functionalization at 120°C, its healing efficiency dropped to 52%. Nevertheless, the practical application of thermal treatment is constrained, making room temperature self-healing highly desirable and a focal point of current research.

2.2.3 Damage volume

The capacity of a self-healing system to address damage volume plays a vital role in determining its ability to improve material reliability and lifespan. This capability relies on factors such as loading conditions, geometry, and polymer properties, and it can vary depending on the specific self-healing approach employed. Intrinsic self-healing systems are particularly effective for handling small damage volumes because they require the damaged surfaces to be in close proximity for re-bonding to occur. Wu et al. [43] employed epoxidized soybean oil (ESO) and natural glycyrrhizic acid to fabricate bio-based recyclable vitrimers that possess outstanding mechanical properties and thermal stability. Notably, crack widths measuring 100 microns gradually vanished within a span of 60 mins, highlighting the material’s potential for recycling and chemical degradation. Microcapsule-based systems repair small to moderate damage volumes at limited microcapsule volume fractions. Microvessel-based systems offer the advantage of addressing a broader range of damage volumes since the healing agent can be replenished as required. In another study, it was observed that the microcrack located within the interface region possesses the ability for triple healing, showcasing its most remarkable self-healing performance at an impressive efficiency of 90.60% [44]. This noteworthy achievement was accomplished by developing an innovative interfacial self-healing system that utilizes carbon fiber and epoxy composites, anchored in the principles of the Diels-Alder reaction. Through the strategic integration of post-synthetic modified (PSM) metal-organic frameworks, these frameworks not only undergo in-situ grafting onto the carbon fiber (CF) surface but also serve as nanofillers, contributing to the dispersion of epoxy resin and enabling the fabrication of PSM/epoxy nanocomposites.

2.3 Design of electrically functional self-healing ECs

Combining self-healing capabilities with electrical properties offers a groundbreaking solution for overcoming material damage, enhancing durability, and opening new avenues for multifunctional applications. Within this subsection, we will discuss the design strategies employed for conductive networks in self-healing epoxy-based composites as well as explore the methods used for their manufacturing.

2.3.1 Percolation network by conductive filler reinforcement

Percolation theory states that introducing conductive fillers into a non-conductive matrix above the percolation threshold leads to the formation of a continuous conductive path in the composite. It is a common observation that the conductive filler concentrations (e.g., carbon-based, metal-based fillers, etc.) in the matrix exhibit a conductivity profile that can be represented by an S-shaped curve (Figure 3). This behavior indicates a substantial increase in conductivity within a small range of filler loading. For an insulating elastic matrix, the percolation threshold for conductive nanofillers, such as carbon blacks (CBs) with spherical morphology, typically falls within the range of 10–20% by volume fraction. This range is consistent with the predictions of classical percolation theory [46, 47]. An epoxy hybrid system was created by introducing multi-wall carbon nanotubes (MWCNTs) and graphene nanosheets (GNs) in two specific filler quantities: less than 0.1 wt% and greater than 0.5 wt% [48]. This was done using different ratios of MWCNTs to GNs. These hybrid epoxy systems demonstrated outstanding electrical capabilities, credited to the interactions between the π–π bonds of the multi-wall carbon nanotubes and the dispersed graphene layers within the epoxy resin matrix. When using high loading of stiff nanofillers, challenges arise due to increased viscosities, and inferior mechanical performance resulting from nanofiller aggregation at higher concentrations. Therefore, achieving an effective reduction in the percolation threshold while preserving electrical properties becomes crucial for fabricating high-performance conductive composite materials. To achieve this objective, various parameters of conductive nanofillers can be tailored, including aspect ratio, dimensional size, size distribution, surface wettability, and contact resistance between individual nanofillers. This customization allows for fine-tuning the corresponding electrical properties to suit specific applications.

Figure 3.

The effective electrical conductivity exhibits an S-shaped curve near the percolation threshold f_c. This proximity to the threshold reveals notable phenomena of percolation theory and nonlinear changes in the transport properties of composites. The insets illustrate the geometric phase transition of fillers within the microstructures of the composites precisely at the percolation point. Reproduced from Ref. [45] with permission from the Royal Society of Chemistry.

2.3.2 Segregated conductive filler network

By strategically forming segregated filler networks within an epoxy matrix, conductive epoxy composites can achieve remarkably improved conductivity even at lower nanofiller concentrations. Instead of being dispersed randomly throughout the entire matrix, conductive nanofillers are intentionally positioned at the interfaces of epoxy matrix domains, leading to a significantly lowered percolation threshold compared to conventional percolation networks. The establishment of these segregated conductive networks depends on the fabrication approach used during the construction of the composites [49, 50].

Through the implementation of an interface engineering strategy, branched CNTs were skillfully coated on the surface of catalyst-infused epoxy waste particles. Subsequently, vitrimerized epoxy/CNT composites were created via compression molding [51]. This innovative approach resulted in the development of a segregated filler structure at the interfaces of “vitrimerized” epoxy particles, achieving two important objectives simultaneously. Firstly, it enabled the recycling of epoxy waste, contributing to sustainability efforts. Secondly, it significantly enhanced the mechanical and electrical properties of the recycled epoxy, even with low carbon nanofiller content (below 1 wt%). The successful incorporation of branched CNTs facilitated the fabrication of highly electrically conductive composites, which hold great promise for applications such as electromagnetic interference (EMI) shielding and strain sensing. Zhang et al. [52] successfully accomplished the controlled dispersion of multi-walled carbon nanotubes (MWCNTs) within the distinct network of hexagonal boron nitride (h-BN) in the epoxy vitrimer matrix using compression molding. This method facilitated the creation of interfacial bonds through dynamic transesterification involving small molecules, particularly at elevated temperatures. As a result of this process, the epoxy vitrimer composites demonstrated improved thermal conductivity and electrical insulation even with a low concentration of fillers. This improvement is attributed to the collaborative effect of the well-organized nanofiller network. In a composite containing 1 wt% MWCNTs and 8 wt% h-BN, the recorded thermal conductivity and electrical resistivity values were 0.83 W (m K)⁻¹ and 1.92 × 10¹¹ Ω cm, respectively. Furthermore, by increasing the portion of separated h-BN, it is possible to further enhance the electrical resistivity.

2.3.3 Production of electrically functional self-healing epoxy composites

Various strategies have been extensively explored to address the issue of inadequate electrical conductivity in epoxy composites. One commonly employed approach involves blending conductive single or hybrid fillers to utilize synergistic effects between epoxy polymers and various filler dimensions. For instance, Bian and his colleagues [53] introduced a remarkable self-healing epoxy composite capable of inhibiting electrical tree growth. They achieved this by incorporating SiO₂ particles into the epoxy matrix and creating hydrogen bonds between the SiO₂ particles and epoxy chains through solution blending. The results demonstrated excellent self-healing properties, with cracks healing upon the application of heat, and effective inhibition of electrical tree growth, a common cause of electrical breakdown in high-voltage applications.

Another approach, vacuum-assisted filtration, uses hierarchical filler architecture to prepare epoxy polymer composite films [54]. However, the time-consuming nature of this method has limited its practical application. On the other hand, the electrospinning coating method, known as the core-shell structure, provides a promising and simple means to create ordered composites [55, 56, 57, 58]. Chen et al. [59] reported tough and rapidly self-healing carbon/epoxy composites using electrospun thermoplastic polyamide nanofiber (PAnf). The PAnf not only toughens the composite interlayer but also provides excellent self-healing capabilities. With just 1.2 wt% addition, the interlaminar shear strength and bending properties of composites increased by 17.6% and 14.7%, respectively. The entangled nanofiber structure enhances interlaminar adhesion, effectively suppressing microcrack and delamination propagation. The epoxy composites with PAnf demonstrated the best mechanical properties and healing efficiency after 3 h of electrospinning. Multiple healing cycles showed healing efficiencies of 70.1% for the third time. The inherent characteristic of thermoplastic PAnf allows it to repeatedly and steadily heal interlaminar fractures without depletion. The uniform distribution of porous PAnf promotes resin flow and impregnation without compromising the storage modulus and T_g of the composites, preserving their original properties.

Additionally, a novel approach involves generating a 3D highly electrically conductive network by dispersing the epoxy polymer matrix, ensuring high electrical conductivity [60]. However, complex template preparation and polymer filling have hindered its widespread use. Magnetic field forces have also been used to promote filler orientation or construct electrical conduction networks [61]. Additionally, hot pressing is sometimes employed after processes like filtration, electrospinning, or template methods to reinforce the structure [62]. This innovative technique holds promise for advancing electrical conductivity in epoxy composites.

3. Conductive self-healing ECs for flexible applications

Long-term operation of flexible electronic devices can lead to fatigue, corrosion, and damage, which ultimately culminate in significant device failure. To tackle this issue, researchers are actively investigating the potential of self-healing electronic devices. These devices possess the remarkable ability to automatically restore their electrical and mechanical performance in the face of mechanical damage, whether with or without external assistance. Integrating conductivity and self-healing functionality in flexible electronic devices is predicted to significantly enhance both their durability and performance. In this section, we emphasize the deployment of epoxy-based self-healing polymer composites in various components and devices of flexible electronics, encompassing adhesives, coatings, sensors, electromagnetic interference (EMI) shielding, soft actuators, and energy devices.

3.1 Adhesives

Epoxy resins find common applications as adhesives, with electrically conductive adhesives (ECAs) presenting a versatile solution in electronics due to their unique dual functionality: enabling electric current conductivity while mechanically bonding diverse substrates. Typically, conductive fillers like Ag, Au, Ni, carbon materials, or MXene are integrated into acrylic, silicone, or epoxy resins to create ECAs. Notably, epoxy resin-based ECAs have gained significant traction in electronics, serving purposes such as die attachment and solderless interconnection. This popularity stems from their exceptional attributes: a blend of chemical and thermal resistance, robust mechanical qualities, strong adhesion, and compatibility with various substrates and additives.

Of remarkable significance are epoxy-based ECAs with self-healing capabilities, which can autonomously repair minor damage in the conductive pathways of the adhesive matrix. These properties hold great promise for industries like flexible electronics, wearables, and automotive electronics, offering advantages through self-healing conductive adhesives. Their pliable nature makes them suitable for scenarios where traditional rigid soldering techniques are impractical. An example by Zhang et al. [63] involves a novel epoxy resin synthesized using the curing agent 1,4,5-oxadithiepane-2,7-dione (DSAA), containing reversible disulfide bonds. This epoxy resin demonstrated self-healing and malleability, showing potential as a reusable adhesive. However, effective self-healing required a relatively high level of DSAA (22.7 wt%).

Advancements in “green chemistry” aim to develop non-petroleum-based matrices for self-healing fiber-reinforced composites. Hu’s group [64] obtained fully biobased vitrimers from camphoric acid (CPA) and epoxidized soybean oil (ESO), using them as matrices for a carbon fiber-reinforced composite (Figure 4). Network rearrangements via transesterification reactions (TERs) provided CF/CPA/ESO composites with reprocessing, self-adhesion, and repair capabilities at elevated temperatures (200°C). Carbon fibers could be fully recycled by degrading CF/CPA/ESO composites with ethylene glycol. In another study, Xu and colleagues [65] developed carbon fiber reinforced composites utilizing a degradable bio-based epoxidized menthane diamine-adipic acid (EMDA-AA) matrix. Notably, the EMDA-AA vitrimer exhibited topological rearrangement through dynamic transesterification reactions, facilitated by autocatalysis of tertiary amines, even in the absence of a catalyst. These developed vitrimers displayed self-healing behavior at 180°C for 30 mins (healing efficiency = 92.9%) and shape memory at 100°C, applicable to both EMDA-AA and EMDA-AA-CF systems. The reported epoxy vitrimer-based composite showcased commendable adhering properties (lap shear strength = 3.8 MPa) when two EMDA-AA-CF composites were bonded under a hot press at 180°C and 15 MPa.

Figure 4.

Depictions showcasing the dynamic crosslinking rearrangements within CPA/ESO networks, and the creation of a composite laminate involving carbon fiber-reinforced composite. Reproduced from Ref. [64] with permission from the Royal Society of Chemistry.

Zhang et al. [66] introduced a fully bio-based epoxy vitrimer for recoverable adhesives, utilizing ozonated lignin and sebacic acid-derived epoxy cured with a zinc catalyst (Zn(acac)₂). This adhesive achieved a lap shear strength of 6.5 MPa when applied to coarse aluminum sheets. Moreover, separated halves of aluminum plates could be rebonded using catalyst-accelerated transesterification at 190°C for 1 h, a feat not achievable with traditional adhesives. This indicates potential for vitrimers as self-healing and recoverable adhesives from bio-based materials. Beyond practical advantages, these adhesives align with eco-friendly principles and cost-saving potential, positioning them as sustainable choices for future electronics. As research and development progress, self-healing conductive adhesives hold exciting promise for revolutionizing electronics manufacturing and maintenance paradigms.

3.2 Coatings

Coating represents one of the most prevalent applications for epoxy resins. Epoxy materials are extensively employed in coatings to safeguard surfaces and provide decorative attributes, owing to their exceptional mechanical properties and chemical resistance. However, when a coating becomes damaged, its intended functions are compromised. The removal of such coatings from substrates is challenging due to their robust interfacial bonding. To mitigate waste and extend the service life, the optimal approach likely involves repairing the coating and reinstating it for use. Yet, repairing epoxy coatings is intricate due to their permanent cross-linked structure, differing from conventional thermoplastics. A recent advancement by Han and colleagues involved the development of a catalyst-free epoxy vitrimer coating, exhibiting impressive self-healing characteristics [67]. This innovative coating was formulated by curing hyperbranched epoxy (HBE) prepolymers with succinic anhydride (SA). The abundance of hydroxyl groups in HBE facilitated rapid stress relaxation and efficient self-healing at elevated temperatures (>120°C). When applied as a vitrimer layer onto metal plates (with a thickness of approximately 80 μm), this coating not only demonstrated excellent adhesion and hardness but also displayed efficient self-healing at 150°C for 1 h. Furthermore, the healed coating effectively safeguarded the metal plates from electrolyte erosion, confirming its potent self-healing capability. Incorporating DGEBA (DER 331) into this system resulted in a repairable vitrimer coating that provided robust protection against salt and electrochemical corrosion. This type of epoxy coating holds potential for prolonging the lifespan of thermosets and reducing waste. Raspberry-like polydopamine@ polypyrrole (PDA@PPy) nanoparticles were synthesized to serve as a photothermal agent and subsequently incorporated into an epoxy resin, creating a self-healing coating [68]. Following 808 nm near-infrared (NIR) irradiation for 10 mins, the impact of varying amounts of PDA@PPy on scratch closure efficiency was explored. Elevated PDA@PPy content correlated with improved healing results, although excessive quantities hindered the repair process due to restrictions on the movement of epoxy resin molecular chains. Hence, precise incorporation of photothermal fillers is pivotal for optimizing the self-healing capacity of coatings. When evaluating corrosion resistance, the linear sweep voltammetry curve illustrated that the corrosion current of a healed coating diminished comparably to an intact coating, underscoring the effective safeguarding of X70 steel.

In a research endeavor undertaken by Hao et al. [69], they carried out a study where they modified Kraft lignin using esterification alongside 4-methylcylohexane-1,2-dicarboxylic anhydride. This chemical process resulted in a novel form of Kraft lignin referred to as L-COOH. The innovative aspect of this modification was its increased solubility in ethanol. By subsequently subjecting it to a reaction with poly(ethylene glycol) diglycidyl ether, utilizing a zinc catalyst, they successfully produced a vitrimer with a notable lignin content exceeding 47 wt%. The vitrimer’s dynamic exchange mechanism, primarily involving ester bonds, was triggered at elevated temperatures beyond 140°C. This temperature-induced activation facilitated the efficient repair of coatings within a brief duration of 15 mins at 190°C, with the assistance of glycol. Notably, this coating also exhibited a distinctive swelling capability when exposed to alkaline aqueous solutions. This behavior was attributed to the conversion of phenol and carboxyl groups into sodium phenolate or sodium carboxylate, which caused a transformation in the chemical nature of the coating. As a result, the bonding between the coating and the substrate was weakened, leading to easier coating removal. Liu et al. [70] introduced an example of free radical polymerization based on a dynamic transesterification network. They synthesized a UV-curable oligomer (TMG) from tung oil using microwave technology, which was subsequently photopolymerized alongside a biobased-reactive diluent derived from malic acid (MA). This strategy yielded UV-curable coatings or materials featuring multiple hydroxyl and ester groups. These materials exhibited the capacity for repair and reshaping through dynamic transesterification reactions, activated at higher temperatures with the aid of a zinc catalyst. Importantly, the tensile strengths of welded (12.2 MPa) or reshaped materials (28.7 MPa) exhibited significant enhancement compared to the original material (7.1 MPa). This enhancement was attributed to an elevated crosslinking density resulting from C〓C polymerization and esterification during high-temperature heating.

Feng and collaborators designed tung oil-loaded polyurethane (PU) microcapsules [71], with conductive polyaniline (PANI) impregnated into the microcapsule wall structure. Corrosion tests highlighted the exceptional anti-corrosion properties of the self-healing epoxy coating incorporating 10 wt% tung oil-loaded PU/PANI microcapsules. These microcapsules, with a tung oil core and PANI wall, acted as corrosion protectors, forming a self-healing film and a passivation layer. In a recent study by Alias et al. [72], linseed oil was encapsulated for corrosion protection of magnesium (Mg). Coatings containing microcapsules were applied to a bare Mg substrate and subjected to scratching to evaluate their self-healing capacity. Electrochemical measurements in a 3.5 wt% NaCl solution demonstrated that the epoxy coating combining linseed oil and urea-formaldehyde substantially reduced the corrosion current density (icorr) on the Mg sheet (1.552 A cm⁻²) compared to the bare Mg sheet (109.8 μA cm⁻²). Consequently, these self-healing coatings exhibited notable recovery in both self-healing and corrosion resistance on Mg alloys.

In another investigation, Haddadi et al. [73] reported the creation of amino-functionalized MXene (Ti₃C₂) nanosheets through an etching technique and a modification process involving 3-aminopropyltriethoxysilane. MXene’s favorable interface interaction and stability with organic/polymeric materials position it as a promising anticorrosion nanofiller for enhancing the passive barrier properties of organic coatings [74]. It also serves as a suitable host for loading corrosion inhibitors [75]. Utilizing cerium (Ce³⁺) cations as corrosion inhibitors, MXene nanosheets encapsulating these cations were employed to produce self-healing epoxy composite coatings, exhibiting robust corrosion protection performance (Figure 5). In a study by Niu et al. [76], a robust omniphobic slippery coating was employed to create self-healing coatings on ultrathin MXene multilayers. Upon exposure to just 10 mins of solar irradiation, the surface temperature of the coating rapidly rose to approximately 50°C. The successful fabrication of MXene multilayers on diverse metal substrates suggests the potential for developing a range of self-healing coatings. Consequently, the utilization of photothermal activation for self-healing emerges as a pivotal element within light-responsive coating technology. Wang and his colleagues [77] introduced a novel Ti₃C₂T_x/layered double hydroxide (LDH) pigmented epoxy composite hybrid coating that demonstrated remarkable corrosion resistance and self-healing capabilities on AZ31 magnesium alloy. The incorporation of environmentally friendly organic acid anions from cysteine acid (Cys) as corrosion inhibitors and the growth of Ti₃C₂T_x/MgAl-LDH (TML) heterostructure nanosheets via in-situ assembly led to the self-healing function. The TML heterostructure displayed excellent dispersion and compatibility with epoxy resin. The composite coating exhibited exceptional corrosion resistance, with a corrosion current density of 1.4 × 10⁻⁹ A cm⁻² and an impedance value of 1.66 × 10⁷ Ω cm⁻² at low frequency. Furthermore, the self-healing efficiency of the epoxy-TML composite coating reached 56.17% after 6 days of immersion in a corrosive environment.

Figure 5.

Diagram illustrating the inhibition mechanism within the coating phase involving Ti₃C₂ MXene-Ce³⁺. In the MXene-Ce³⁺@EP coating containing a defect, Ce³⁺ ions have the potential to migrate out from Ti₃C₂ MXene-Ce³⁺ nanosheets. These ions subsequently get deposited onto the surface of the mild steel plate within the scribed zone, effectively obstructing cathodic sites. Reprinted with permission from [73]. Copyright 2021 American Chemical Society.

Lorwanishpaisarn et al. [78] conducted a study with the goal of developing a self-healing epoxy vitrimer/CNT nanocomposite for use as a coating material. They employed two bio-based curing agents, namely cashew nut shell liquid (CNSL) and citric acid (CA), to create adaptable networks held together by covalent bonds. Within the epoxy/CNSL/CA matrix, they incorporated CNTs using probe sonication and agitation, spanning concentrations from 0 to 0.5 wt% (referred to as V-CNT0-0.5). The research outcomes indicated that the thermomechanical properties of the V-CNT nanocomposites improved as the CNT content increased. The study also found that a bond exchange reaction involving esterification could be triggered by near-infrared (NIR) light. Among the tested compositions, V-CNT0.5 exhibited the highest self-healing efficiency, as evidenced by a Shore D hardness of 97.34%. To evaluate corrosion resistance, coated steel samples containing V-CNT0 and V-CNT0.5 were immersed in a 3.5 wt% NaCl solution for a duration of 7 days. In this context, the corrosion rate of steel coated with V-CNT0.5 significantly decreased from 9.53 × 10² MPY to 3.12 × 10⁻⁵ MPY. Additionally, the protection efficiency surged by 99.99%. Capitalizing on these remarkable self-healing and anti-corrosion attributes, it is evident that V-CNT0.5 holds promise as an appealing material for organic anti-corrosion coatings. In a novel achievement, a thermally robust and high-performance pH-responsive anti-corrosive nanoreservoir system was developed [79]. This system is built upon a double-ligand zinc phosphate framework (ZPF) that decorates a graphene oxide skeletal template (G-ZPF) using the ligand exchange theory. The outcomes revealed that the G-ZPF nanoreservoirs loaded with structural inhibitors offer a combination of active (self-healing) and passive (ion-water barrier) attributes within the organic coating. As a result, the epoxy-polyamide coating embedded with G-ZPF nanoreservoirs demonstrated remarkable corrosion prevention capabilities and exhibited exceptional thermal and mechanical performance.

3.3 Sensors

A sensor is a sophisticated apparatus designed to identify and perceive external chemical or physical signals and subsequently translate the gathered data into the desired format. It plays a pivotal role in the advancement of wearable and adaptable intelligent electronic devices, which have vast applications in displays, artificial intelligence, and healthcare [80]. However, sensors equipped with multiple flexible functionalities are prone to encountering diverse damages during their operational lifespan, thereby impacting the overall longevity of these devices. Consequently, there has been a significant surge of interest in the development of self-healing sensors. The primary focus behind this research is to bolster the durability and sustainability of these electronic devices, ensuring they remain functional for an extended period.

In the earliest investigation conducted by Wang’s research group [81], they developed a self-healing graphene/rubber-based supramolecular elastomer (GRSE) that capitalized on the synergistic interplay between dynamic boroxines and interfacial hydrogen bonds. The incorporation of graphene nanosheets not only elevated the conductivity and sensitivity of the sensors but also played a pivotal role in enhancing the overall performance. To further enhance the amino groups and augment adhesion among the graphene sheets and elastomers, the surface of the graphene nanosheet was modified through the absorption of 1-pyrenamine (PA) via π∙π conjugation. This strategic modification effectively improved the sensor’s properties. This wearable sensor exhibited remarkable attributes, including high electrical conductivity (0.0029 S m⁻¹), rapid response time (250 ms), and a low detection threshold (1%). Additionally, the engineered sensor initiated its healing process under room temperature conditions, displaying impressive mechanical robustness (3.46 MPa) and a healing efficiency (η) of 91.1%. Leveraging its stress-sensing capabilities, this sensor holds promise for applications in human motion detection. Kai et al. [82] introduced photonic vitrimer-based electronics (PVBEs) as an innovative class of flexible and stretchable electronics that amalgamate the advantageous attributes of photonic crystals (PCs) and piezoresistive carbon textiles (CCT) with the intrinsic self-healing properties inherent in vitrimers. By addressing the limitations inherent in current flexible electronics, such as susceptibility to damage and constrained electrical output, the research employs synthesized poly(urethane-urea) vitrimer elastomers derived from specific constituents, encompassing polytetramethylene ether glycol (PTMG), poly(1,4-butanediol) bis(4-aminobenzoate) (PBDAB), isophorone diisocyanate (IPDI), and glycerin (GLY). These vitrimers exhibit robust mechanical characteristics, resilience, and a remarkable 93% self-healing efficiency attributed to dynamic covalent networks. The integration of photonic crystals and carbonized cotton textiles within the vitrimer matrix gives rise to PVBEs that manifest synchronized color alterations and electromechanical responses under conditions of mechanical stress or stretching. PVBEs showcase exceptional strain sensing capabilities, including swift synchronous electrical and optical responses (0.25 s), heightened sensitivity (with a gauge factor of 10.3), exceptional endurance (exceeding 10,000 cycles), mechanochromism, potential for self-healing in optical functionalities through dynamic covalent networks, stable electromechanical performance facilitated by a fully integrated structure, and the capacity for wireless transmission, enabling real-time monitoring of human movements with dual-signal feedback. This study pioneers a transformative platform in the realm of flexible electronics, promising applications in domains such as wearable devices, soft robotics, and the rapidly expanding landscape of the Internet of Things.

Yang et al. [83] also successfully developed a series of functional epoxy elastomer/carboxylated carbon nanotube composites, combining self-healing capabilities and degradability to create flexible and stretchable strain sensors (see Figure 6a). The epoxy elastomer was synthesized using carboxyl-terminated poly(ethylene glycol), 2,2′-dithiodibenzoic acid, and 1,4-butanediol diglycidyl ether as monomers, along with a bio-based epoxidized soybean oil as the crosslinker. These composites showcased exceptional mechanical properties, boasting a high tensile stress of 5.07 MPa and impressive stretchability, with a capacity to stretch up to 477%. The composite’s remarkable self-healing performance of 92.5% at 80°C for 24 h (Figure 6b) was a result of the synergistic healing effect derived from the hydrogen bonds and disulfide bonds present in the epoxy matrix. Moreover, the strain sensor based on the elastomer composite with microstructure displayed a notably high gauge factor (GF) sensitivity of 176.7, along with rapid response and relaxation times of 60 ms and 100 ms, respectively. Additionally, it demonstrated exceptional repeatability withstanding 1000 cycles. Its versatile applications included successful detection of human motions and recognition of objects with various shapes, as depicted in Figure 6c.

Figure 6.

Schematic illustrations of (a) the preparation process of the elastomer composites-based strain sensor. (b) Healing behavior after 24 h and mechanical deformations of epoxy elastomers. (c) The epoxy elastomer composite-based strain sensor finds practical use in detecting human motion and recognizing objects. Reprinted with permission from [83]. Copyright 2022 American Chemical Society.

Enhancing the stability of strain sensors is of utmost importance. However, the presence of scratches, mechanical damage, and harsh environments can significantly curtail their lifespan. To address this challenge, Lu and colleagues [84] have devised a remarkable solution—self-healing electronic sensors that boast exceptional stability and reproducibility. These sensors rely on the principles of metal-ligand coordination and possess a well-organized hierarchical structure. The preparation method for these self-healing sensors is quite straightforward: they employ polydopamine (PDA) to modify epoxidized natural rubber, which is then cross-linked using Fe³⁺. Additionally, the incorporation of CNTs into the self-healing matrix further enhances their performance. As a result, the manufactured strain sensor exhibits outstanding mechanical properties and retains its high sensitivity even after undergoing extensive bending (over 50,000 times) or repetitive washing. A key highlight of these self-healing strain sensors is their remarkable ability to heal themselves at room temperature, making them stand out from conventional approaches. Given these exceptional attributes, these strain sensors are ideally suited for utilization in smart electronic devices, opening up new possibilities for advanced electronic applications.

A remarkable advancement in polymer technology lies in the development of bio-based epoxy vitrimers derived from soybean oil and tung oil. These novel polymer networks possess inherent advantages, including self-healing capabilities, repeatable processing, and recyclability. Additionally, when combined with carbon materials, these epoxy vitrimers can be utilized to create versatile multifunctional sensors, serving as energy converters, temperature warning sensors, and fire warning sensors. This breakthrough offers exciting new possibilities for a wide range of construction applications and sensor technologies [85, 86, 87, 88]. Jia et al. [89] successfully developed bio-based polyschiff base vitrimer/graphene oxide composites with remarkable self-healing and reprocessability attributes, making them highly efficient for applications as temperature warning sensors and fire warning sensors. These polyschiff vitrimers were skillfully crafted using vanillin and tung oil as sustainable raw materials, resulting in a commendable tensile strength range of 1.20 MPa to 1.91 MPa. These vitrimer-based composites exhibited the ability to self-heal, with cracks effectively mending after being subjected to a temperature of 120°C in an oven for 120 mins. This self-healing phenomenon was attributed to a combination of sensitively dynamic imine covalent bonds and structural compatibility with the flexible aliphatic hydrocarbon chain of tung oil. Furthermore, the reprocessed composites displayed tensile strengths ranging from 1.21 MPa to 1.90 MPa, which closely resembled the tensile strengths of the original samples. This reprocessability feature adds to the practicality and durability of the bio-based vitrimer composites.

Cao et al. [90] utilized bio-derived carboxyl cellulose nanocrystals to create a novel nanostructured supramolecular sensor. This sensor was designed to interact with chitosan-decorated epoxy natural rubber latex, forming multiple H-bonding interactions. The results were impressive, as the sensor exhibited ultrafast self-healing (within 15 s) with remarkable repeatability. The healing efficiency of the sensor was exceptional, reaching 93% after the third healing cycle. Additionally, the team developed a highly sensitive strain sensor using a layer-by-layer technique, incorporating H-bonding between chitosan solutions and nanocomposite-assisted carbon nanotubes. This strain sensor demonstrated an impressively low strain detection limit of 0.2%. Even after undergoing cutting-healing cycles and being subjected to more than 20,000 bends, the sensor maintained its stability and provided reliable and repeatable response signals. Furthermore, the researchers took it a step further by integrating these sensitive and self-healable, flexible sensors into a human-machine interaction system. This system proved to be versatile, serving as a facial expression control system and an electronic larynx. The potential applications of this innovative technology are vast, and it opens up exciting possibilities in the field of flexible and interactive electronic devices.

3.4 Electromagnetic interference (EMI) shielding

As electronic technology and telecommunication-related industries continue to advance rapidly, the issue of electromagnetic radiation pollution has become increasingly prominent. Consequently, there is a growing need to explore new materials for electromagnetic interference (EMI) shielding. One class of high-performance EMI shielding materials that has gained significant attention is carbon-based materials, owing to their lightweight nature and ease of fabrication [91]. Among these materials, CF stands out as an appealing choice in electronic apparatus due to its exceptional mechanical and electrical properties [92]. To enhance the shielding effectiveness even further, CF is often coated with metal nickel, imparting magnetic properties to the material [93]. Yu and his coworkers [94] introduced a novel self-healing composite for EMI shielding, utilizing the Diels–Alder (DA) thermoreversible reaction system. This dynamic covalent bonding network was created by crosslinking furan-modified epoxy resin (FM-EP) with a bismaleimide at 60°C, resulting in an impressive self-healing efficiency of 92.5%. Within this self-healing thermoreversible epoxy resin matrix, nickel-coated carbon fiber (Ni/CF) was uniformly dispersed as a conductive filler to attenuate electromagnetic waves. The researchers achieved an EMI shielding effectiveness (EMI SE) of 40.5 dB by increasing the volume percentage of the conductive filler to 18%, while still maintaining excellent healing performance.

Through the ingenious utilization of associative dynamic bonding rearrangement within the epoxy vitrimer, Fang et al. [95] have achieved a remarkable breakthrough in developing segregated multiwalled-CNTs/epoxy composites. This innovative approach creates distinct pathways for conductive filler networks, allowing for outstanding performance across a broad range of compression temperatures and pressure conditions. These segregated composites exhibit an exceptionally low conductive percolation threshold of just 0.066 wt% and boast an impressive EMI SE of 22 dB, even with a mere 2 wt% loading of multiwalled-CNTs. Additionally, the incorporation of volatile ethylene glycol contributes to the mechanical robustness by enabling the decomposition and repolymerization of β-hydroxyl ester at the interface, even in the presence of numerous multiwalled-CNTs. Another significant advantage of this advancement is the preservation of the segregation structure and macroscopic properties after reprocessing. Moreover, the nanofillers can be effortlessly recovered using excessive EG at elevated temperatures, further demonstrating the versatility and practicality of this approach. With its exceptional combination of excellent conductive properties, high EMI shielding effectiveness, and mechanical resilience, this novel technique holds enormous promise for diverse applications in various fields.

3.5 Soft actuators

Soft actuators that can exhibit pre-set shapes and respond to various external stimuli such as heat, light, solvent, and electricity are attracting significant interest in diverse fields like soft robotics, energy generation, motors, and fluid propellers. The key attributes sought after in these soft actuators are multi-stimuli responsiveness and multi-stimuli-triggered self-healing capability, which are pivotal for their extensive applications. For practical utility in various scenarios, the integration of multi-stimuli responsiveness and multichannel self-healing ability is highly desirable. This feature would enable the soft actuators to be easily actuated or self-healed using a simple and convenient tool whenever required.

A notable contribution in this area is from Yang et al., who presented a groundbreaking advancement in the form of multifunctional, recyclable, thermosetting, and vitrimer-based soft actuators [96]. These unique actuators were developed through the hot-pressing of carbonized silk fabric (CSF) onto vitrimers, resulting in the formation of composite CSF-vitrimers. These actuators exhibit an impressive five-stimuli-triggered self-healing ability, including heat, light, electricity, electromagnetic waves, and solvent. Moreover, they demonstrate four-stimuli-triggered responsiveness to light, electricity, heat, and solvent. Additionally, the composites boast improved mechanical properties, prevent delaminations, and can be recycled and reprocessed due to their vitrimer feature. Furthermore, this strategy holds promise for expanding to other types of vitrimers (such as polyimine, polyurethane) and other fiber fabrics (such as cotton fabric, man-made-fiber fabrics), suggesting even broader industrial applications for this composite. Chen et al. [97] also developed a remarkable actuator utilizing a vitrimer liquid crystal elastomer equipped with exchangeable dynamic ester bonds. This actuator exhibited self-healing capabilities, enabling it to break old bonds and reform new covalent bonds even under challenging manipulative conditions. Additionally, the actuator displayed shape memory behavior, deforming appropriately in response to various stimulation conditions.

The integration of disulfide bonds in vitrimer actuating materials has proven highly effective and convenient for achieving self-healing properties under mild conditions, presenting a significant advantage over transesterification reactions that necessitate harsh conditions. Tang and colleagues [98] have devised a groundbreaking and accessible method to create multiple elastic-plastic shape-memory cycle soft actuators, capable of intricate movements. This remarkable achievement harnesses the potential of vitrimer liquid crystal elastomer (V-LCE) in combination with dynamic disulfide bonds and azobenzene chromophore functional groups. By incorporating these key components into the main chain through a curing reaction between epoxy and thiol compounds, the V-LCE actuators with dynamic disulfide bonds showcased an extraordinary ability to self-repair damaged areas under heating conditions. Additionally, the motion of the polymer chain was facilitated by changes in the configuration of the azobenzene chromophore moieties, leading to macroscopic shape morphing when exposed to UV light. The outstanding elastic-plastic shape-memory behavior and mechanical properties of V-LCE allowed for enhanced design flexibility, enabling the creation of diverse and complex 2D and 3D shape actuators. These soft “bionic” devices exhibited impressive functionalities such as grasping, transferring, and releasing objects, thanks to the exceptional mechanical robustness, photothermal response performance, and exceptional programability and reconfigurability of the covalent cross-linking system. The simple and efficient fabrication process of V-LCE, combined with its photothermal-responsive and extraordinary capabilities, opens up exciting possibilities for addressing complex tasks and provides a promising avenue for designing intelligent devices and bionic robotics with exceptional performance and versatility.

3.6 Energy devices

3.6.1 Energy storage devices

Supercapacitors (SCs) are devices used for energy storage, combining features found in traditional capacitors and rechargeable batteries. They possess various advantages, including high energy density, fast charge and discharge rates, reliable safety, long-lasting life cycles, low maintenance requirements, and stable performance [99]. In comparison to conventional capacitors, SCs can provide greater power delivery and energy storage capabilities. Nonetheless, their durability may be compromised by mechanical harm. As a result, SCs with self-healing properties have sparked significant interest in both mechanical and electrical research fields.

Typically, a SC device consists of an electrolyte placed between two electrodes. In the quest to develop a self-healing supercapacitor, it becomes essential to design electrodes or electrolytes that possess self-healing capabilities. One promising candidate for this purpose is the solid polymer electrolyte (SPE) known for its versatile properties, combining robust ionic conductivity and mechanical strength. To make it suitable for energy storage applications, the SPE is formed by blending a durable, cross-linked epoxy-based matrix with a fast-diffusing lithium salt/ionic liquid electrolyte. This blending process is carried out using a convenient one-pot curing method. In a successful endeavor, Kwon et al. [100] managed to create epoxy-based cross-linked SPEs by incorporating plasticized lithium salt, ionic liquid, and an inorganic slurry containing Al₂O₃ nanowires, all prepared in a single-pot process (illustrated in Figure 7a). According to their findings, when these SPEs were utilized alongside activated carbon electrodes, they exhibited excellent supercapacitor performance, demonstrating notably high-power density and energy density values. Specifically, the results showed a power density of 9.3 kW at 44 Wh kg⁻¹ and an energy density of 75 Wh kg⁻¹ at 382 W kg⁻¹.

Figure 7.

(a) Chemical structures of epoxy-based solid polymer electrolytes containing an ionic liquid, a lithium salt, and inorganic Al₂O₃ nanowires. Reprinted with permission from [100]. Copyright 2018 American Chemical Society. (b) Fabrication of aligned CNT sheet-reinforced LCEN composite. (c) Flexible LCEN composite supercapacitor with self-healing behavior. Reprinted with permission from [101]. Copyright 2020 American Chemical Society.

Recently, Wang et al. [101] successfully fabricated liquid crystalline epoxy network (LCEN) composites with aligned CNT sheets, demonstrating electrically controlled actuation behavior (Figure 7b). The incorporation of CNTs into the vitrimer network directly promoted the photothermal conversion effect, resulting in higher thermal energy that activated transesterifications. This process facilitated Joule heating and played a crucial role in introducing structural anisotropy to the composite film. As a result, the composite film exhibited spontaneous bending behavior when powered by electricity, eliminating the need to induce macroscopic liquid crystalline orientation. Furthermore, the film displayed self-healing properties, which allowed it to recover not only through exposure to light but also by using electricity after being broken under a voltage of 1.18 V mm⁻¹. Additionally, the authors showcased the potential application of the conductive LCEN composite as self-healing supercapacitors (Figure 7c).

Additionally, there is a growing interest in epoxy composite-based rechargeable batteries with self-healing capabilities. Sun et al. have achieved significant breakthroughs in the field of highly conductive self-healing SPEs [102]. Their innovative approach involved incorporating disulfide dynamic covalent bonds into an epoxy matrix to create a unique and recyclable electrolyte named RFSPE-3. To fabricate this self-healing and recyclable electrolyte, the researchers strategically combined diglycidyl ether of bisphenol A (DGBE) and poly(ethylene glycol) diglycidyl ether (PEGDGE) as the matrix components. The crosslinker used was 2-aminophenyl disulfide (2-AFD), which provided the disulfide bonds crucial for the exceptional self-healing ability and recyclability of the electrolyte. The resulting RFSPE-3 exhibited impressive characteristics. The inclusion of epoxy resin endowed the electrolyte with superior mechanical strength, boasting a tensile strength exceeding 20 MPa. Meanwhile, the dynamic exchange of disulfide bonds in 2-AFD contributed to enhanced self-healing capabilities, achieving a remarkable healing efficiency of over 95% within just 2 h at room temperature. Moreover, RFSPE-3 demonstrated exceptional ion conductivity, reaching 10⁻³ S cm⁻¹, and even after multiple healing processes, there were no changes observed in its ionic conductivity. Furthermore, this electrolyte exhibited an inhibitory effect on the growth of lithium dendrites, ensuring excellent cycling stability for up to 1800 h at 1 mAh cm⁻². These promising results pave the way for the development of advanced and long-term rechargeable batteries, with potential applications in various fields requiring durable and efficient energy storage solutions. The combination of high mechanical strength, excellent self-healing properties, and outstanding ion conductivity positions RFSPE-3 as a promising candidate for next-generation energy storage devices.

Sun and Wu [103] adopted a similar strategy by incorporating disulfide dynamic networks into an epoxy matrix. Their approach involved introducing the disulfide bond through a disulfide-containing aliphatic polyamine as the epoxy-curing agent. The resulting SPEs exhibited remarkable properties, being optically transparent and capable of self-healing above the T_g. An intriguing observation was the substantial decrease in curing time, from 102 to just 1.4 mins, achieved by raising the temperature from 40 to 100°C. Among various formulations tested, the SPE with 15% LiTFSI content demonstrated the highest ion conductivity. Specifically, it exhibited values of 3.35 × 10⁻⁶ S cm⁻¹ at 80°C and 8.31 × 10⁻⁶ S cm⁻¹ at 100°C. Although this conductivity was slightly lower compared to other self-healing SPEs, it still marked a significant advancement in the field. By successfully incorporating disulfide dynamic networks into the epoxy matrix, Sun and Wu’s work represents a notable step forward in the development of solid polymer electrolytes with enhanced properties, showing potential for various applications in the realm of materials science and beyond.

3.6.2 Energy-harvesting devices

Conductive self-healing ECs find another crucial application in the realm of energy harvesting devices. The advancement of energy harvesting technologies represents a significant stride towards achieving an environmentally friendly society. Among these technologies, triboelectric nanogenerators (TENGs) have emerged as a highly regarded and eco-conscious power source. They offer numerous benefits, including cost-effectiveness, simple production processes, abundant material choices, impressive conversion efficiency, and the ability to design them in various versatile configurations [104, 105, 106, 107, 108]. These advantages have drawn considerable attention to TENGs as a sustainable and green solution. These TENGs capitalize on the phenomena of triboelectrification and electrostatic induction, making them a highly promising option for efficiently converting ambient mechanical energies into electricity, particularly in situations where low frequencies are involved [109]. To enhance the output power of conventional TENGs, researchers have employed patterned triboelectric layers with micro-nanostructures or self-assembled monolayers (SAMs) with different functional groups, effectively increasing the contact surface area [110, 111]. Nevertheless, enduring challenges persist, such as the decline in overall performance and the restricted lifespan of TENG devices, primarily attributable to interface friction between dissimilar materials and frequent mechanical deformation, including twisting, stretching, bending, and compressing. A viable approach to address these challenges involves incorporating a “self-healing” capability into the TENGs. This self-healing feature enables the repair of broken micro-nanostructures in the triboelectric layer, thereby restoring the overall performance of the nanogenerators [105, 112].

Ye et al. [113] developed a self-healable vitrimer-based triboelectric nanogenerator (VITENG) by employing a rapid thiol-Michael reaction with diacrylate poly(dimethylsiloxane) (AA-PDMS) elastomer. They introduced 2,3-dihydroxypropyl methacrylate (DHPMA) to generate hydroxy end dangling side chains within the matrix, which enhanced the flexibility and transesterification efficiency of the PDMS network. This innovation resulted in an impressive healing efficiency of 84% within a mere 15 mins for the developed vitrimer elastomer. The research findings revealed that the dangling hydroxy end chains not only enhanced the material’s flexibility and stretchability but also significantly improved its self-healing ability. The low T_g system facilitated efficient collisions during the dynamic bond exchange process, contributing to this enhancement. An important outcome of their work was that VITENG exhibited a remarkable healing effectiveness of 100% and demonstrated a stretchability of 125%. Moreover, the VITENG showcased excellent output performance, producing 135 V of energy at low frequencies and applied forces. This remarkable performance makes the developed VITENG a highly efficient energy harvester with great potential for powering a wide array of soft devices.

4. Conclusions and outlooks

In this chapter, we delve into the fundamental mechanisms and healing strategies of epoxy composites, highlighting their crucial distinctions. Next, we explore the key principles underpinning self-healing epoxy composites-based devices, which enable them to restore mechanical integrity and regain functionality even when faced with mechanical and electrical damage. This exploration is complemented by a comprehensive overview of recent research progress in epoxy composites, providing a broader understanding of their applications. Subsequently, we present a thorough examination of the latest advancements in the use of conductive self-healing epoxy composites within the realm of flexible electronic components. These applications span from sensors, EMI shielding, and soft actuators to energy storage and harvesting electronic devices. While remarkable achievements have been made, we also recognize the existence of challenges and research gaps that necessitate ongoing and dedicated efforts for resolution.

Firstly, the demand for self-healing devices calls for the design of epoxy composite materials capable of repetitive rapid healing while maintaining stable filler conductivity post-damage. Secondly, current self-healing conditions often necessitate high temperatures or prolonged light induction, making it imperative to explore milder experimental conditions that involve simple manipulation. Thirdly, improving the self-healing efficiency becomes crucial to ensure the restoration of the original performance of the materials. Fourthly, the healing efficiency of electrical conductivity and mechanical behavior in conductive self-healing epoxy composites presents a contradiction, necessitating future research to achieve synergistic characteristics of high conductivity and excellent mechanical properties.

Despite significant progress in developing epoxy composites with favorable self-healing and mechanical properties, their applicability in energy electronics remains limited. One of the challenges lies in the mutual influence of different components in some composite functional materials. For instance, in self-healing electrodes, the introduction of self-healing polymers may replace some of the active material, leading to a reduction in electrode capacity. Additionally, conductive fillers can impact the original interface compatibility, mechanical properties, transparency, and other critical factors. To address these limitations, it is crucial to focus on creating self-healing conductive or semiconducting epoxy-based polymers that possess inherent conductivity. Moreover, there is a compelling need to explore the potential research direction of developing epoxy-based materials capable of automatically adapting various device properties based on specific demands. By pursuing these avenues, we can pave the way for advanced and versatile materials that overcome current constraints and unlock new opportunities in energy electronics.

Beyond these challenges, multifunctional electronic and energy systems with diverse interfaces, such as those between devices and substrates or between layers, can encounter delamination due to differences in surface chemistry and mechanical properties. Addressing this issue, self-healing polymer networks can be utilized to enhance mechanical robustness, facilitating seamless integration and self-bonding at layer interfaces, thereby enabling the efficient fabrication of three-dimensional electronic devices. Observing the wide range of applications for electrically conductive self-healing epoxy composites in adhesives, coatings, sensors, electronic components, and energy storage/harvesting, it is clear that these materials will play an increasingly vital role in shaping the future of various industries. Of utmost importance, these self-healable materials possess the capability to significantly diminish electronic waste by employing their autonomic repair or semi-autonomic functions. As a result, they play a pivotal role in propelling our interconnected world towards enhanced sustainability and, potentially, facilitating the growth of space exploration endeavors.

Conflict of interest

The authors declare no conflict of interest.