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Please note this is a comparison between Version 2 by Xueqing Wang and Version 1 by Xueqing Wang.
智能响应材料可以通过可逆机制对外部刺激做出反应，并可以直接与摩擦电纳米发电机(TENG)结合，以提供各种智能应用，如传感器、执行器、机器人、人工肌肉和受控药物输送。不仅如此，创新材料可逆响应中的机械能可以被提取并转化为可解读的电信号。Smart responsive materials can respond to external stimuli through reversible mechanisms and can be directly combined with triboelectric nanogenerators (teng) to provide a variety of intelligent applications such as sensors, actuators, robotics, artificial muscles, and controlled drug delivery. Not only that, but the mechanical energy in the reversible response of innovative materials can be extracted and converted into an interpretable electrical signal.
- smart materials
- stimuli-responsive
- triboelectric nanogenerator
- energy conversion
- self-powered system
1.介绍Introduction
Currently, the ever-growing need for versatile and efficient intelligent devices stimulates people’s intense research interest in integrating smart response materials with traditional electronics [1,2,3]. The original concept of smart materials stems from natural biological systems that follow a perception-reaction-learning mechanism. Presently, they are defined as materials that can respond to a variety of external stimuli or environmental changes. This is because smart materials can adjust their function and respond accordingly by rearranging their structure at the molecular level. They represent a family of innovative materials for understanding experiences with self-awareness and purposeful responses [4,5,6,7,8].
Historically, applications of functional smart materials with controllable shape or volume changes in response to external stimuli have been investigated in several frontier fields, such as sensors, actuators, optoelectronic devices, information storage, and biomedicine [9,10,11,12,13]. These external stimuli include chemical incentives (e.g., changes in concentration, humidity, pH) [14,15,16], mechanical stimuli (e.g., pressure, strain) [17,18], physical stimuli (e.g., light, sound, temperature, color) [19,20,21,22,23], and electromagnetic stimuli (e.g., electric, magnetic, charge injection) [24,25,26,27]. The unique and excellent responsive characteristics of smart materials enable them to be used specifically and accurately in certain applications, allowing them to perceive changes in the environment and adapt future smart polymers to similar situations and specific behaviors in specific applications [28,29,30].
A triboelectric nanogenerator (TENG) is a newly developed energy harvesting technology based on the coupling effect of contact charge and electrostatics [31,32]. It can convert almost any type of mechanical energy, such as vibration [33], water waves [34], wind [35], raindrops [36], and human motion [37], into electrical energy, and power various sensors. In principle, TENGs have a wide selection of materials, such as organic materials [27,38], polymers [39], hydrogels [40], and elastic rubbers [41,42], which can be directly used to manufacture TENG or serve as active component of TENG systems. Thus, smart materials can also be promising candidates combined with TENG devices and generate many multifunctional applications. The introduction of smart materials not only improves the performance of TENG in terms of material optimization, but also provides various applications, such as sensors [43,44], actuators [45], robots [46], artificial muscles [47], and controlled drug delivery [48]. Moreover, the mechanical energy in the reversible response of smart materials can be cleared and converted into electrical energy, which is highly dependent on the amplitude and frequency of environmental stimuli and can be digitized for the latter. Therefore, a self-powered smart system can be constructed, with broad applications in catalysis [49], sensors [50], drug/gene delivery [51,52], and self-assembly [53,54].
2. Basic Principle and Working Modes of TENGs
TENG is capable of transforming mechanical energy from the surrounding environment into electrical power by utilizing a combination of triboelectrification and electrostatic induction effects [55,56]. Typically, this involves contacting two materials with differing charge affinities, causing electrostatic charges to build up on their surfaces until reaching saturation with repeated contact. These charges generate an electric field that drives the flow of electrons through an external load over time, ultimately converting mechanical energy into electrical energy [57,58]. TENGs can be categorized into four working modes(Figure 1): the vertical contact separation mode, which uses vertical polarization; the lateral sliding mode, utilizing lateral polarization from the relative movement between two media [59]; the single-electrode mode, which can harvest energy from a freely moving object without attaching conductive wires [60]; the independent triboelectric layer mode, which uses electrostatic induction between a pair of electrodes to generate electrical energy [61].
Figure 1. The basic principle of TENG.
3. Classifications and Underlying Mechanisms of TENGs That Are Based on Smart Materials
The self-adaptability and self-actuation of smart materials are achieved through the autonomous movement of particles at the nanoscale. When stimulated by external factors, molecules and atoms will aggregate, align, and orient themselves to provide the optimal response to the external stimuli [62,63]. If combined with TENG, the system may deliver various intelligent applications, such as sensors [64], actuators [65], robots [66], artificial muscles [67], and controlled drug delivery [68] with immediate response to stress, electrical current, temperature, magnetic field, or even chemical compounds. This section introduces the combination of different types of smart materials with TENG based on their principles and mechanisms. Table 1 lists the synthesis methods, properties, and application comparison of TENGs combined with different types of smart materials.Table 1. Comparison of synthesis methods, properties and applications of TENGs combined with different types of smart materials.
| Type | Material | Method of Synthesis |
Performance | Application | References |
|---|---|---|---|---|---|
| Magnetic responsive materials | γ-Fe2O3, PVDF | Spin coating, Solution casting |
0.8 µA, 90 V | Scavenging energy from rotary pump vibrations. | [69] |
| Al/Ni fabric, PDMS | Solidified stripping | 30 µA, 205 V | Harvesting wind energy. | [70] | |
| NdFeB magnets | Solution casting | 1.17 mA·cm–2 | Harvesting wind energy. | [71] | |
| Optical responsive materials | ZnO-NPs, PVDF | Solution casting | 28 V | Monitoring of movement. | [72] |
| PEDOT-MeOH | Sol-gel method, Spin coating |
- | Self-powered smart windows | [73] | |
| Hygro-responsive materials | Chitosan-glycerol film | Coating method | 130 V | Sweat sensor, Monitoring of movement. |
[74] |
| PFSA ionomer | Solution casting, Spin coating |
- | Self-powered steam sensor | [75] | |
| SnS2 | Chemical vapor deposition | 7.6 µA | Humidity sensor | [76] | |
| Temperature-responsive material | PCL, fluorinated alumina | Solid–liquid mixing | 85 nA, 6 V | Wetting monitoring and temperature sensing | [77] |
| CNFs, AgNWs, PA, BP | Vacuum filtration | 116 V, 3.8μA, 42 nC, 33.4 μm−2 | Fire warning, Temperature sensing |
[78] | |
| NIPAM, MMA | Free radical polymerization and spin-coating. | 242.72 nA, 0.72 V, 10.78 nC |
A stable warning system for real-time liquid temperature monitoring | [79] | |
| pH-responsive materials | PTFE, PDMS | Spin coating | 26.37 μA, 69.04 V |
Acid rain sensor | [80] |
| Self-Healing materials | acrylic acid,1-vinyl-3-ethyl imidazolium dicyandiamide, ZnO NPs | Solution casting | 3.15 W·m−2 | Monitoring of movement | [81] |
| AAM, KPS, glycerol, NH2-PDMS-NH2 | Solidified stripping | 157 V, 16 µA, 29 nC, 710 mW·m−2. |
Monitoring of movement | [82] | |
| PU | Solution casting | 180 V, 1.3 μA |
Monitoring of movement | [83] | |
| Shape-memory polymers | PCL, n-butyl acrylate |
Solidified stripping | - | Smart wrist splint with an alarm system. | [84] |
| Polyurethane | Electrospinning | 150–320 V, 2.5–4 μA·cm−2 |
Self-powered water temperature sensor. | [85] | |
| PSeDAE | Cross-linking, Laser engraving |
44.5 V, 1 μA, 17.5 nC | Improvised explosive device. | [86] |
3.1. Magnetic Responsive Materials
In an external magnetic field, flexible materials embedded with magnetic particles can produce a series of deformations, such as stretching, deformation, and expansion. A controllable magnetic field can control these deformations of soft magnetic materials because the interaction between electrons creates a solid molecular lot in ferromagnetic materials. Ferromagnetic materials, in the process of spontaneous magnetization, will produce a small magnetization in different directions to reduce the magnetostatic energy, resulting in saturation magnetization. This is called the magnetic domain [87,88].
Thus, when a soft magnetic material is affected by the strength of an external magnetic field, the functional magnetic domains will align in the direction of the area. Magnetic soft materials are considered ideal alternative materials for specific space components due to their responsiveness, ease of handling, and self-assembly. At present, the research field and application range of intelligent magnetic response devices is less diverse than other types of response materials. In most cases, they are combined with flexible polymer materials and magnetically driven, employing unidirectional magnetization. Further investigation shows that the magnetic directions of the domains are randomly distributed due to the irregularity of the thermal motion, which causes the magnetic moments between the disciplines to cancel each other due to the opposite directions. Thus, in an external magnetic field with a strength of 0, the entire ferromagnet does not show magnetization. Once the external magnetic field drives the magnetic moment, it tends to coincide with the direction of the external magnetic field. In this dynamic process, an induced magnetic field is formed, and a magnetized current is generated. Li et al. applied magnetized wind generated by a magnetic field to ferromagnets, which improved the electrical output performance and energy conversion efficiency of TENG and opened up a breakthrough direction for studying the output characteristics of TENG [89].
The magnetic nanocomposites used to create the Magnetic Response Triboelectric Nanogenerator (TENG) offer a unique benefit beyond the additional driving force provided by the magnetizing current. Specifically, the solid magnetic response of the material enables the external magnetic field to facilitate the contact and separation of the TENG’s layers, without requiring any additional mechanical force. Therefore, it can work in non-contact mode to prevent the device from coming into direct contact with external mechanical shocks that could damage the electrodes and triboelectric layer. Bushara et al. synthesized γ-Fe2O3/PVDF transparent nanocomposites as friction layer materials for TENG by spinning coating method, thus producing TENG devices with flexibility and stretchability advantages and stability [69]. In the preparation process, PET material was used as the positive triboelectric layer material, and on one of the substrates, the spun γ-Fe2O3/PVDF was used as the negative triboelectric material. The two are connected face to face to a curved mylar film, and the spacing between them can be periodically changed by externally applied forces and create a potential difference. The TENG moves vertically in response to the attraction of the magnet. When the magnet is near the device, the magnetic nanocomposite is near the PET’s upper surface. When the magnet is removed from above the device, the PET layer and the nanocomposite layer separate due to gravity. The electrical output performance of the magnetic drive TENG in non-contact mode is 0.8 μA and 90 V, respectively. In addition, the output characteristics can be further optimized by changing the nanocomposite’s magnetized flux density or the NP embedment’s mass fraction.
In the past few years, TENG has been studied to capture wind and blue energy. Environmental factors for the outdoor energy harvesting process of the TENG in an open environment, such as gas composition, especially humidity, have a significant impact on the performance of the TENG. To adapt to wind energy and blue energy harvesting in high-modesty environments, Huang et al. proposed a magnetic-assisted non-contact TENG in which the device consists of two parts: a magnetic response layer (PDMS/FeCoNi composite/PET/Al/PDMS) and a non-magnetic response layer (Al/Ni fabric/PET) [70]. Wind or water flow can drive the rotation of the entire device, directly leading to contact separation of the magnetically assisted non-contact TENG. As the magnet rotates toward the device, the magnetic response layer is absorbed, causing the PDMS film to come into contact with the Al/Ni fabric. In contrast, the PDMS film and Al/Ni fabric separate as the magnet rotates away from the device. The blue energy can be harvested and converted into electrical energy by a contact separation between the magnetic and non-magnetic responsive layers. Obviously, the higher wind speed makes the panel spin faster, resulting in a higher frequency of contact separation between the magnetic and non-magnetic auxiliary layers.
Recently, Zhao et al. developed a magnetic elastomer generator that can change the magnetic field in a soft system through wind-induced mechanical deformation. Additionally, it can convert wind energy into electrical energy through electromagnetic induction in any direction [71]. The magnetic-elastic generator is composed of magnetic particles and a polymer matrix with Young’s modulus of 980.9 kPa, which has suitable mechanical tensile properties. The results indicate that a high current density of 1.17 mA cm−2 and an extremely low internal impedance of 68 Ω are obtained at 266 r/min and driven by ambient rain-blown wind. The system can decompose water to produce hydrogen at a rate of 7.5 × 102 mL h−1 at a wind speed of 20 m s−1. In other words, this work brings a whole new platform technology to the wind harvesting community.
3.2. Optical Responsive Materials
Light stimulation is a fundamental and straightforward approach for investigating single stimulus responses, offering fast response times, high driving performance change rates, and excellent stability [90,91,92]. In particular, photochromic molecules are crucial components of light-responsive materials, capable of capturing optical signals and converting them into valuable properties that can drive changes in shape and structure, as well as exhibit macroscopic characteristics [93]. Integrating photochromic materials into TENG systems enables the expansion of self-powered sensing applications and propels the development of TENG towards higher precision [94].
Santanu Jana et al. have produced a wearable polymer composite NG (PCNG), which facilitates the formation of γ crystalline phases by adding ZNO-NPs to PVDF films [72]. PCNG delivers an open-circuit voltage of up to 28 V and a short-circuit current of 450 nA by simple repetitive human finger application (at a pressure amplitude of 8.43 kPa). More importantly, when a perceptive response is generated by PCNG, where ZNO-NPs can be used as an optical probe, near-blue emission is observed from the photoluminescence spectrum.
The original model of solar-driven photo electrochromic devices (PECDs) is a hybrid system of dye-sensitized solar cells (DSSCs) and electrochromic devices (ECDs), which is a potential application for convenient self-powered applications. According to the structure design, PECDs can be divided into two classical types: separated photochromic devices (S-PECD) and combined photochromic devices (C-PECD). In order to further improve the optical contrast and reduce the response time, a novel hybrid photochromic device (H-PECD) was proposed by Cheng et al. [73]. The novel H-PECD structure comprises a photoelectrode coated with an electro-chromic conducting polymer layer, which operates independently and exhibits exceptional speed and optical variation during coloring and bleaching under diverse weather conditions. This performance is comparable to conventional ECDs while circumventing the necessity for an additional power supply or intricate wiring.
3.3. Hygro-Responsive Materials
When the external environment’s relative humidity (RH) changes, the humidity-sensitive material can produce reversible expansion and shrinkage phenomena. Meanwhile, it can convert chemical potential energy in water into mechanical energy. Therefore, changing humidity conditions can provide the driving force for the TENG to switch to the extreme environment. The moisture-sensitive TENG can effectively capture energy from wind and raindrops. On the one hand, the device can optimize the working mode and automatically switch the working structure without outside intervention, forming a wholly self-powered intelligent system. On the other hand, the particular advantages of TENGs, such as fast response speed and high output voltage, can be fully utilized. Therefore, introducing hygro-responsive materials into the TENG research route could generate some attractive diversified applications in personal health monitoring or meteorology.
Jao et al. have developed a triboelectric nanogenerator based on chitosan-glycerol thin films (C-TENG), which have resistance, environmental stability, and shape adaptability [74]. The output characteristics of the manufactured C-TENG are very stable under various humidity conditions. More importantly, the C-TENG can be combined with functional textiles to enable a self-powered sensor to detect humidity, sweat, and gait phases. With the increased NaCl concentration, the output voltage of the sweat sensor decreases from 26 V to 12.5 V. Because of this output tendency, NaCl sensors can be implemented. Since the concentration of NaCl is an essential indicator for people to monitor their physical condition during exercise, especially for high-intensity or long-term exercise, C-TENG can be used to detect the concentration in sweat.
Humidity sensing is a well-studied field of smart response. The vapor-absorbing materials used to make moisture-responsive actuators usually have porous or fibrous structures designed to allow better passage of water molecules, and the change in water molecule content will produce internal stress, thus causing spontaneous deformation. To achieve a self-actuated TENG, a steam-driven actuator is an ideal device for coupling with the TENG. Ren et al. demonstrated a wet-responsive TENG for vapor-driven materials based on perfluorinated sulfonic acid (PFSA) ionomers [75]. At relatively high (or low) concentrations of PFSA membranes, water molecules can be absorbed (or desorbed) by these nanochannels, which gradually expand (or contract) to generate internal stresses and drive TENG leaf deformation. This self-powered sensor can be used to detect vapor concentration changes and monitor vapor leakage.
In addition, materials with two-dimensional (2D) structures have significant advantages in many areas of sensors. SnS2 has the most exciting design among two-dimensional (2D) transition metal disulfide (TMD) materials, consisting of a tightly packed sheet of tin atoms sandwiched between two sheets of sulfur atoms. It is worth noting that SnS2 material has high carrier mobility compared with other TMD material structures. The essential method to obtain SnS2 with different morphologies is chemical vapor deposition (CVD). Through this method, SnS2 can acquire a large number of available active sites, which is the critical parameter to realize gas or humidity sensing. Leyla et al. made a humidity sensor based on FTO/Kapton CS-TENG using SnS2 [76].
3.4. Temperature-Responsive Material
An important property of temperature-responsive materials is the low critical solution temperature (LCST). LCSTS are mainly found in polymer-based materials, and the polymer will dissolve into micelles in various proportions at temperatures below the LCST. In contrast, at temperatures above the LCTS, it will form separate phases [95,96]. Intelligent temperature response TENG is vital for energy harvesting and application expansion. The current working environment of electronic devices is mostly room temperature. In fact, temperature also has a non-negligible influence on the output performance of TENG, which limits their applicability significantly. Combining the temperature-sensitive material with the TENG can not only directly and accurately judge the real-time temperature through the output signal but also serve as a probe to reflect the ambient state at different temperatures.
Li et al. utilized the intelligent temperature-regulating behavior of tribological properties to propose a poly(ε-caprolactone) (PCL)-based liquid–solid TENG. The PCL material comprises a hydrophobic backbone [-(CH2)5], hydrophilic end groups (-COOH, -OH), and amphiphilic ester groups, rendering it thermally responsive [77]. As a heterogeneous polymer, the surface of PCL will recombine when the temperature changes, even when the polymer is in a glassy state. Under relatively low temperatures (20 °C), the PCL chain migrates slowly, resembling the “frozen state”, which is the thermodynamically unstable state. As the temperature increases (40 °C), the PCL chain transitions to an “active state”, and most of the hydrophobic groups at the interface are eventually replaced by hydrophilic groups , reaching a state of thermodynamic equilibrium.
Recently, most TENGs based on biologically derived natural materials have been reported. Nanocellulose is a rich raw biomolecular material, which is renewable, bio-degradable, and highly biocompatible. Wang et al. developed a flexible and biodegradable flame retardant friction nanogenerator (FR-TENG) using cellulose nanofibrils (CNF) and phosphorous flame retardants for fire monitoring and warning purposes [78]. Tannic acid (TA) modified BPNS (TA-BPNs), and PA were added to CNF as non-toxic co-flame retardants as triboelectric layers (CNF-BP-PA films) and silver nanowires (AgNWs) as conductive layers. Compared to pure CNF films, CNF-BP-PA/AgNWs films exhibited excellent fire resistance and flame retardation, with a thermal index of 3779.16 K, indicating the film’s sensitivity to temperature. The BPNS of composite CNF-BP-PA films can be used as the flame-retardant mechanism of condensing and gas phases to achieve effective flame-retardant performance. At the initial stage of combustion, the layered structure of TA-BPNS can act as a physical barrier to prevent the transfer of pyrolytic products to the flame and simultaneously inhibit continuous combustion by generating heat. Subsequently, for the condensation phase flame retardant mechanism, BP will be oxidized to P2O5, promoting the dehydration and carbonization of the polymer, forming a protective carbon layer. In addition, P2O5 traps water molecules and includes a series of phosphoric acids that coated the surface of the material, blocking oxygen and heat from the outside world. The feasibility of FR-TENG in temperature sensing and its application in fire alarms are proven.
N-isopropyl acrylamide (NIPAM) has a thermal phase transition behavior and intelligent adjustable properties as a biologically compatible and temperature-sensitive material. Poly (n-isopropyl acrylamide) (PNIPAM) is water soluble at room temperature and has a low critical solution temperature (LCST) of 32 °C [97,98]. By changing the external temperature, it can undergo a phase transition from curl to the ball. Feng et al. prepared the temperature-sensitive NIPAM by free radical polymerization and spin coating and introduced it into L-S TENGs [79]. Due to its thermal properties, the main groups and surface wettability vary with temperature. On this basis, in order to demonstrate the practicality of the PNM-based L-S TENG, an alarm device was designed to monitor fluid pipe temperatures exceeding 30 °C. In the temperature sensor design, the tribological energy generated by water droplets flowing over the surface of PNM at different temperatures is the key to triggering the alarm.
3.5. pH-Responsive Materials
pH-responsive materials have attracted much attention from academia and researchers in the past few decades. pH-responsive polymers are materials that respond sensitively to the pH of solution through changes in structural properties such as solubility, surface activity, configuration, and chain structure formation [99,100]. Polymer-based materials were widely used as pH response materials during their development because of their structurally weak acid/primary groups. Significant changes in pH value can lead to swelling/deswelling of polymers, dissolution/precipitation of polymer chains, or the effect of hydrophobic/hydrophilic surface characteristics on changes in surface properties.
3.6. Self-Healing Materials
During the process of mechanical energy collection, the TENG may suffer from long-term physical impact, mechanical stiffness, and chemical corrosion, which may cause equipment failure [101,102]. The sustainability and safety of the equipment are greatly affected. In conclusion, the development of self-repairing materials can effectively improve the durability of TENG and enhance the functionalization of application scenarios. Self-healing materials mimic living organisms with self-healing properties after damage and injury. Through the reconstruction of chemical bonds, self-healing materials essentially repair losses and restore the original performance of devices. Most of the self-healing polymers studied so far are in gel form. Hydrates of hydrogels or organic gels are synthesized by fixing water or organic solvents in a self-healing polymer grid. Their structure is similar to the three-dimensional structure, which makes them complement the natural organization, with excellent adaptability and flexibility.
Li et al. proposed a transparent self-healing ion gel-based TENG (SI-TENG) [81]. The self-healing gel is synthesized by in situ polymerization of acrylic acid (AA) in 1-vinyl-3-ethyl imidazolium dicyanamide ([Emim] [OAc]) with zinc acetate dihydrate and ZnO nanoparticles (NPs). SI-TENG was prepared by assembling PAA-Zn50/ZnO20/IL60 ionic gel and two 3 M 9495 tape pieces into a sandwich structure. When the cut SI-TENG was rejoined, the coordination bonds on the fractured surface gradually re-formed. The self-healing ability of PAA-Zn50/ZnO20/IL60 enables SI-TENG to maintain its original electrical output after multiple cutting/healing cycles.
The self-healing characteristic provides the TENG with excellent electrical output recovery capability, which can quickly return to its original state over a wide temperature range. The novel PAAM-Clay organic hydrogel-based TENG designed by Huang et al. can operate in a wide temperature range from −30 to 80 °C [82]. Organic hydrogels, and IU-PDMS can repair themselves and form a segment that cannot be separated by mechanical stretching. There is no significant change in mechanical properties after self-healing compared with before damage. The output properties of the new TENG are 157 V, 16μA, and 29 nC, respectively.
However, most current studies focus on preparing self-healing materials for mechanical damage, ignoring the high risk of damage caused by an electrical breakdown in TENG. Wu et al. proposed a TENG for a repairable elastomer based on a double dynamic network of hydrogen and coordination bonds [83]. After the breakdown of multiple electric fields, the mechanical self-healing efficiency of TENG is still 96%, and the electrical self-healing efficiency is almost 100%. It provides an excellent idea for TENG to avoid mechanical or electrical breakdown damage.
3.7. Shape-Memory Polymers Materials
Shape-memory polymers (SMPs) are a class of responsive materials that can change their temporary shape and recover their original shape intact in response to external stimuli. This process is due to the dissociation–association process that occurs within the dynamic interaction of the polymer [103,104]. Due to their dielectric properties, wide electron affinity range, flexibility, and barrier properties, SMPs can be used as contact materials for TENG, providing considerable benefits to the device [105]. For instance, polyurethane SMPs fabricated through photolithography have been used to prepare pyramid-patterned TENGs, which can improve the device’s lifespan [106]. This approach may represent a promising direction to enhance the durability and functional diversification of TENG devices.
Liu et al. reported an SMP-based TENG (STENG) to capture biomechanical energy and detect biomechanical motion [84]. The SMP was synthesized by incorporating a semicrystalline thermoplastic polymer in a chemically cross-linked elastomer. The shape-memory properties of semi-IPN elastomers are qualitatively demonstrated using thin films. Flat films deform into different shapes with higher temperatures. Shape recovery is triggered by changing the room temperature. By utilizing the macroscopic shape-memory ability of SMP, a wrist splint capable of shape adaptation for mechanic sensation was developed using a blend of photocurable elastomer and polycaprolactone SMP film. Additionally, this SMP film was utilized as an energy harvester for body motion. Although this is only a rough model, it has a simple manufacturing process and broad feasibility.
In addition, SMPs also can be implemented under non-thermal stimuli such as water/humidity, photoactivity, electroactivity, and magnetic activity. Xiong et al. broke through the research gap of water-activated microarchitecture shape-memory TENG with water energy harvesting and water temperature sensing capabilities. They proposed a multifunctional TENGs electro-spun microstructure pad with self-recovery capability [85]. Three scalable microfibers (MF), microspheres (MS), and microsphere-nanofiber (MSNF) pads were fabricated by electrospinning. The types of micro-structured shape-memory TENG provide triboelectric sound output (~150–320 V, ~2.5–4 μA cm−2). The shape-memory capability of the display can be adapted to a variety of temporary shapes, and the recovery rate of 100% can be achieved quickly in 10 s. The results show that electrospinning is an effective method for constructing layered microstructures without losing shape memory.
In a dynamic interaction, SMPs store only one temporal shape per shape-memory loop. Therefore, the dual shape-memory effects of heat-sensitivity, light-sensitivity, and chemical sensitivity can be achieved by using reversible dynamic bonds. Further, SMPs with triple or multiple shape-memory effects are created using noninterfering reversible interactions of two or more responses to different stimuli and combined into a system. Xuan et al. synthesized a high-permittivity poly(sebacoyl diglyceride-co-4,4′-azo dibenzoyl diglyceride) elastomer (PSeDAE) with light-to-thermal conversion and triple-shape-memory [86]. PSeDAE as a triboelectric layer constructs an Information Encoding Device (IED) based on TENG. TENG can produce tri-shape-memory effects, inducing both microscopic and macroscopic functions. The micro-scale shape-memory effect enhances the electrical memory properties of IED, enabling program editing capabilities. Meanwhile, it enables the on-demand transformation of wearable applications through macro-scale shape memory.
3.8. Multiple-Stimulus-Responsive Materials
The TENG based on stimulus-response materials is a simple and efficient device that can be accurately controlled and easily prepared. However, in real-world environments, the stimulus sources are diverse, and the application scenarios are complex and variable. Therefore, to meet the diverse requirements of such environments and to enhance the intelligent response of TENG output, a single stimulus-response material is often insufficient. To address this issue, research is currently focusing on developing updated response conditions and diversified response methods. One promising approach is the use of multi-stimulus response smart materials, which incorporate multiple functional stimulus-response groups into polymer materials. Through precise molecular structure design and effective preparation methods, these materials can achieve multiple reaction properties and explore the practicality of smart response TENG functions.
Furthermore, certain polymers exhibit sensitivity to humidity as well as other stimuli, allowing for a diverse range of design functionalities. Li et al. have demonstrated the design of two flexible actuators and grippers that can respond to dual stimuli of ethanol steam and electrostatic power. These devices are capable of completing a series of tasks such as loading, driving, transporting, and unloading objects without requiring an external power supply [65]. They can complete a series of motion tasks such as loading, driving, transportation and unloading objects without an external power supply. Xiang et al. proposed a self-powered triboelectric mechanical luminescent electronic skin (STMES) capable of detecting and discriminating between multiple stimuli [107]. An electronic skin sensor with an electro-optical dual-signal sensing mode is designed by combining the TENG and mechanical luminescence (ML). STMES operates under the synergy of TENG and ML. The electrical signal senses normal pressure or bending stress, while turning or transverse tensioning stimulates the optical signal of the mechanical luminescence spacer. Different responses in the form of electrical/optical signals distinguish mechanical stimuli.
TENG provides a straightforward and effective means to drive many electrically responsive materials and devices, leveraging its unique output characteristics. The output voltage and transmitted charge of the TENG can be precisely controlled by selecting specific operating parameters, enabling the drive and operation of these systems. To effectively navigate and control a range of TENG-based functional systems, it is crucial to maintain high output voltage levels.
