**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(urethaneurea) 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% selfhealing 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**.

*Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

#### **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 biobased 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 [85–88]. 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

### *Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

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 selfhealing 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 shapememory 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.
