Recent Progress of Nature Materials Based Triboelectric Nanogenerators for Electronic Skins and Human–Machine Interaction

The current field of human‐machine interaction (HMI) and electronic skin (e‐skin) is confronting challenges associated with energy supply, sensitivity, and biocompatibility. Traditional HMI systems, though rich in features, are constrained in their applications in the medical and wearable technology sectors due to their rigid structure and reliance on third‐party power inputs. Triboelectric nanogenerators (TENGs) based on natural materials are emerging as a focal point of research in this domain, attributed to their self‐powering capabilities, flexibility, and biocompatibility. These natural material‐based TENGs emulate the mechanical and biological characteristics of human skin, adapt to various body shapes and movements, and offer an efficient and reliable solution for the precise detection and analysis of complex, subtle physiological signals, thereby fostering innovations in wearable devices and robotic technology. This article provides a comprehensive review of recent advancements in natural material‐based TENGs, detailing the materials employed, including proteins, chitosan, and cellulose. It encapsulates their applications in the realms of electronic skin and HMI. Finally, the challenges confronted by natural material‐based TENGs are outlined and future trajectories for development in this burgeoning field are projected.

54][55][56][57][58] Notably, these TENGs efficiently convert mechanical energy from body movements into electrical energy, supporting self-powered devices.65][66] In this review, we summarize the recent advancements in the research of TENGs based on natural materials in the context of e-skin and HMIs.In Figure 1, we categorize the natural materials used for constructing TENGs into proteins, chitosan, and cellulose, and discuss the four fundamental operating modes and several charge transfer mechanisms of TENGs.Additionally, we will explore various applications of TENGs grounded on natural materials in e-skins and HMIs, including limb motion sensing, pulse monitoring, electronic device control, and robotic control.In conclusion, we will evaluate the significant challenges faced by TENGs based on natural materials in the realms of e-skins and HMIs and anticipate future research trends for natural material-based TENGs.

Material Classification of Natural Material-Based TENG
Natural materials, including protein, chitosan, and cellulose, offer significant advantages when employed in the construction of TENGs.Proteins, renowned for their diverse amino acid sequences and three-dimensional structures, provide a biocompatible and flexible basis for e-skin.This versatility enables the tailoring of protein-based layers to specific sensing needs, ensuring it is safe and comfortable for wear.Chitosan boasts biodegradability and robust mechanical strength, attributable to its unique chemical structure, which consists of randomly distributed β-(1-4)-linked D-glucosamine and N-acetyl-D-glucosamine.This enhances the durability and eco-friendliness of TENGs, making them suitable for long-term and environmentally sensitive applications.Cellulose, being abundantly available and easily processable, brings forth its inherent properties of high tensile strength and flexibility due to its β-(1-4) glycosidic bonds and extensive hydrogen bond network.This offers cost-effectiveness and scalability in production.Together, these natural materials contribute to the enhanced sensitivity, adaptability, and efficiency of e-skin based on TENGs, offering promising applications in wearable electronics, sensor networks, and beyond.Their biocompatibility, eco-friendliness, and abundant availability make them a sustainable and efficient choice for extracting mechanical power and changing it into electrical energy.

Protein
Proteins, as integral components in the development of e-skins based on TENGs, are classified based on their structural and functional properties.Functional proteins like enzymes and antibodies are employed for their specificity and sensitivity, enhancing the e-skin's ability to detect and respond to environmental stimuli.Structural proteins such as collagen and keratin contribute to the mechanical strength, flexibility, and durability of the e-skin.In application, protein-incorporated TENGs excel in biocompatibility, ensuring the e-skin is non-toxic and comfortable for long-term wear.The proteins also facilitate enhanced sensitivity and responsiveness, making the e-skin adept at real-time monitoring of physiological signals and environmental changes.The integration of proteins ensures that the e-skins are not only robust and efficient but also adaptable for various applications, including health monitoring, HMI, and wearable electronics, marking a significant stride toward personalized and precision electronics.
The design of e-skin faces several challenges, including electrode lightweight, low cost, skin-friendly properties, skin conformability, and long-term skin adhesion.Long-term wear necessitates a focus on both biocompatibility and skin comfort.Liu et al. successfully prepared a soft and ultra-thin silk fibroin membrane, which has excellent biocompatibility, breathability, water permeability, and compliance, and is suitable as a triboelectrode for long-term skin contact. [67]The silk fibroin membrane, made from a natural biomaterial, converts the mechanical energy generated by finger tapping into electrical signals (Figure 2a).The membrane is prepared through a series of steps including degumming, dissolution, dialysis, and evaporation of the solution.To enhance the function of the electrode, they evenly coated silver nanowires on the silk fibroin film and encapsulated and protected it with another layer of silk fibroin film.Silver nanowires are not only light-transmissive but also enhance the aesthetics of e-skin (Figure 2b).The thickness of the device is only ≈38 μm, and the effective size is 1 Â 1 cm 2 , adapting to the needs of fingers.To optimize the skin's shape retention, the researchers mixed silk with reinforcing agents and prepared a modified silk fibroin film through natural evaporation, which has better tensile properties than pure silk fibroin film.
The utilization of hydrogel is pivotal in advancing e-skin due to its myriad benefits.The material demonstrates remarkable biocompatibility and can simulate the touch and elasticity of real skin, enabling a natural and comfortable wearing experience.The high water content of the hydrogel also gives it excellent conductive and sensitive properties, enabling precise detection and response to external physical cues like pressure, temperature, and humidity by the e-skin.The ease of processing and fabrication of hydrogels allows for low-cost and large-scale production.In addition, its self-healing ability also ensures that the e-skin can restore its original performance after damage and extend its service life.He et al. innovated by mixing silk fibroin (SF), poly (3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS), graphene oxide (GO), and polyacrylamide (PAM) (Figure 2c). [68]A soft conductive hydrogel was designed.The first step involved preparing a silk fibroin solution using a standardized approach.The solution primarily consisted of three secondary structural elements: β-sheets, α-helices, and random coils.They added silk fibroin, GO, and PEDOT: PSS to PAM in a stepwise fashion and subsequently fabricated a PAM/SF/ GO/PEDOT: PSS (PSGP) composite hydrogel (Figure 2d).The PSGP hydrogel boasts exceptional elasticity and resilience, making it adaptable to a wide range of shapes.Incorporating PEDOT: PSS enhanced both the tensile strength and electrical conductivity of the hydrogel (Figure 2e).This new hydrogel exhibits outstanding tensile and compressive characteristics and can be utilized to create strain and pressure sensors with a broad sensing range (strain, 2-600%; pressure, 0.5-119.4kPa) and dependable performance (Figure 2f ).Sensors can precisely capture and monitor a wide range of physical signals from the human body, such as joint movements, facial expressions, pulse, and respiration, among others.Since the hydrogel and SF-based sensor exhibits excellent biocompatibility, it does not cause allergic reactions on human skin.In tribe-nanogenerator applications, this conductive hydrogel demonstrates a favorable response performance.
The intrinsic presence of collagen, known for its exceptional biocompatibility, positions it as an optimal material for designing e-skin.Collagen-based hydrogels, renowned for their biodegradability and skin-friendly attributes, offer a means to emulate the mechanical characteristics and tactile sensation of human skin.Despite their manifold advantages, challenges pertaining to mechanical stability, self-healing, adaptability to environmental conditions, and multifunctionality persist.In addressing these issues, Song et al. harnessed oxidized hyaluronic acid (Col) extracted from Simmental cattle Achilles tendons using an acidase extraction method, leading to the creation of a novel collagenbased conductive hydrogel. [69]They incorporated acrylic acid (AA) and Black wattle bark tannin (BWT)@AgNPs solutions into a previously prepared Zr(SO 4 ) 2 solution to generate organic hydrogels.The preparation of the pregel solution involved dissolving oxidized hyaluronic acid (OHA) and Col in the solution while introducing ethylene glycol (EG) and ammonium persulfate (APS) (Figure 2g).The synergy of collagen, oxidized hyaluronic acid, acrylic acid, and Zr 4þ ions result in a dual network structure via multiple dynamic covalent cross-linking, effectively balancing mechanical stability and self-healing properties.The incorporation of ethylene glycol and silver nanoparticles further enhances the mechanical characteristics and self-healing capabilities of the collagen-based conductive organic hydrogel.Additionally, the new hydrogel boasts impressive attributes, including environmental stability, transparency, antibacterial features, and biocompatibility.Notably, this organic hydrogel offers a unique trifecta of monitoring capabilities, encompassing strain resistance, bioelectrodes, and TENG self-powered sensing.This multifaceted monitoring capacity substantially enhances the precision and reliability of human biological signal capture and analysis, thus expanding the horizons of e-skin's functionalities and applications.
Protein-based e-skin shows significant potential in various fields, including seamlessHMI, tactile sensors, and wearable bioelectronics, due to its affordability, straightforward production, biocompatibility, flexibility, and transparency.Augmenting protein-based e-skin with conductive properties offers several benefits, such as improved sensor sensitivity and response speed, precise biological signal capture and transmission, and enhanced compatibility with electronic devices.Nonetheless, achieving this objective entails surmounting challenges, notably maintaining protein biocompatibility and mechanical elasticity while ensuring enduring conductivity.Gogurla and team achieved the successful development of an exceptionally flexible and water-resistant free-standing silk membrane by incorporating plasticizers into silk protein and promoting the formation of a secondary β-sheet structure. [70]This silk membrane serves as the basis for fabricating a protein-based strain sensor and a single-electrode bio-TENG (Figure 2h).Embedded silver nanowire (AgNW) electrodes within the free-standing silk membrane maintain stable sheet resistance even during mechanical bending.This resistance stability under strain conditions facilitates the integration of optoelectronic components as strain sensors.The silk protein-based device harnesses electrical energy from mechanical vibrations on the human skin, showcasing the ability to generate an open circuit voltage of up to 90 V and a short circuit current of 0.06 μA, with a maximum power density of 2 mW m À2 , which is adequate to power five commercial LEDs.Exceptional transparency enables silk protein-based TENGs to function as superior touch sensors on electronic devices, such as mobile phones and computer mice, while harvesting biomechanical energy up to 1 μW (Figure 2i,j).The strain sensor and bio-TENG can be seamlessly integrated into a single silk chip, which can be attached to the skin or fabric to Reproduced with permission. [67]Copyright 2022, Elsevier.c-f ) Multifunctional silk fibroin hydrogels for wearable strain/pressure sensors.Reproduced with permission. [68]Copyright 2020, American Chemical Society.g) Ultra-stable self-repairing coordinated collagen-based multifunctional dual-network organic hydrogel e-skin.Reproduced with permission. [69]Copyright 2023, Elsevier.h-j) Skin contact-driven single-electrode protein TENGs.Reproduced with permission. [70]Copyright 2019, Elsevier.
enable concurrent strain monitoring and biomechanical energy harvesting.

Chitosan
Chitosan, a naturally occurring linear biopolymer derived from chitin, has been extensively utilized in the realm of e-skin, especially within the context of TENGs.It is typically categorized based on its molecular weight or degree of deacetylation, each category serving distinct roles in electronic applications.In the domain of e-skin, chitosan's prominence is attributed to its biocompatibility, biodegradability, and excellent film-forming capability.These characteristics render it an ideal material for flexible, eco-friendly, and wearable electronic skin devices.Chitosan's inherent properties have been harnessed to enhance the sensitivity and durability of TENGs, leading to more accurate and reliable sensory data collection and energy harvesting from ambient mechanical movements.It acts as a functional layer, playing a pivotal role in charge generation, transfer, and storage, thereby augmenting the performance and application scope of TENGs in wearable technology, sensory augmentation, and HMIs.Its ability to be easily modified and tailored adds to the diversity of its applications, making chitosan-based TENGs a focal point in the evolving narrative of self-powered, adaptable electronic skin systems.
In the manufacturing process of multifunctional e-skin, a persistent challenge is reconciling the inherent conflict between combining functional materials and maintaining a simple structural design.Simultaneously achieving properties like flexibility, stretchability, transparency, comfort, biodegradability, and selfpowering capabilities is a complex undertaking.To address this challenge, Peng et al. introduced a multifunctional e-skin characterized by outstanding biocompatibility, biodegradability, and electron energy supply capabilities through the integration of chitosan (CS) with a TENG. [71]The preparation process involved dissolving CS powder in acetic acid and subsequently purifying the solution via centrifugation, resulting in a stable and homogeneous CS solution (Figure 3a).This solution was then transformed into a CS film through a solution casting process.The acid neutralization and water evaporation steps induced a microcrack structure in the CS film, granting it superior sweat permeability and controlled biodegradability.To enhance flexibility, hydrophobicity, and water vapor permeability, glycerol was introduced.Ultrathin, transparent, flexible, stretchable, breathable, and high-resolution patterned Au NFs electrodes were created, capitalizing on the substantial specific surface area and numerous micro/nanopore structures within the interwoven nanofiber network (Figure 3b).The use of electrospinning, magnetron sputtering, and photolithography technologies ensured that the e-skin not only complied with environmental standards but also displayed exceptional transparency and sweat permeability, thereby paving the way for innovative approaches to designing multifunctional e-skins.
In the development of wearable e-skins, a significant challenge lies in achieving a harmonious blend of high flexibility, breathability, and antimicrobial properties.Conventional e-skins typically incorporate rubber, organic semiconductors, or other dense elastomers.While these materials offer notable elasticity and flexibility, they are associated with drawbacks such as limited breathability and potential bacterial proliferation, leading to discomfort during prolonged wear and the risk of health issues like itching and inflammation.To address this issue, Shi et al. proposed an innovative approach for crafting an e-skin based on TENGs (TENG) characterized by flexibility, breathability, and antibacterial attributes. [72]The e-skin is constructed with AgNW electrodes sandwiched between a thermoplastic polyurethane (TPU) sensing layer and a polyvinyl alcohol/chitosan (PVA/CS) substrate.This design integrates environmentally friendly and biocompatible PVA, as well as biodegradable and antibacterial CS composite materials, serving as the flexible base layer, ensuring a comfortable fit on the skin (Figure 3c).The hydrophilic characteristics of the PVA/CS layer enable swift absorption and transfer of sweat and heat from the skin, thereby enhancing thermal and moisture comfort.The combination of CS and AgNW acts as an antibacterial agent, which can effectively inhibit bacterial growth and provide the wearer with a healthy and comfortable wearing environment.Therefore, this design not only solves the breathability problem but also successfully prevents bacterial growth by utilizing the antibacterial properties of CS and AgNW, especially demonstrating significant antibacterial effects against E. coli and Staphylococcus aureus.This comprehensive design strategy provides an effective way to solve the comfort and health issues of wearable e-skin when worn for long periods.
Traditional ultraviolet and hypochlorous acid disinfection methods cannot fully meet the needs of antibacterial and sterilization.TENG has attracted much attention due to their wear resistance, portability, and versatility.However, research on TENG's bactericidal and mite removal aspects is relatively lacking.Lu et al. explored the potential of natural biomaterials in this regard, focusing specifically on pectin because the cell walls of apples and kiwis are rich in pectin, which has anti-inflammatory and antioxidant properties, is rich in raw materials, is simple to prepare, and has lower cost. [73]They successfully synthesized chitosan-hydroxyethylcellulose-pectin (CHP) film through a hydrothermal method and utilized it as a triboelectric material.In addition, considering the high conductivity and excellent environmental stability of polypyrrole (PPy), they selected PPy as the electrode material.By combining PPy with a CHP film with a sandwich structure, a single-electrode CHP-PPy-CHP TENG (CPC-TENG) was constructed.It is worth noting that when 11 wt% chitosan is added, the sterilization rate of CPC-TENG can be as high as 92.86%, and its voltage and current output also increase by 162% and 175% respectively (Figure 3d).This innovation not only showcases the potential of triboelectric antibacterial mechanisms but also offers a new strategy and direction for the development of next-generation medical and intelligent monitoring systems (Figure 3e).This research expands the application field of TENG to the fields of antibacterial and sterilization, laying a foundation for future research and applications.
As a natural polysaccharide extracted from brown algae, sodium alginate is widely used in biomedicine, food, and pharmaceutical fields due to its biocompatibility, biodegradability, and gel-forming ability.In the field of e-skin, this material also shows potential value due to its flexibility, breathability, and comfort.It can naturally fit the human skin and become an ideal interface for wearable devices.However, its application is not without challenges, and sodium alginate's limitations in mechanical strength and electrical conductivity may affect its performance and durability in e-skins.To improve this limitation, scientists are exploring the use of composite materials and nanotechnology to enhance sodium alginate-based e-skin.Zhao and his team went in the other direction. [74]They used oxide sodium alginate, aminated gelatin, acrylic acid, and AlCl 3 in a 1,3-propanediol/water binary solvent system to prepare transparent, Multifunctional organic hydrogel (PAOAM-PDO) with antifreeze properties as electrodes for TENG and strain sensors (Figure 3f ).The hydrogel demonstrated over 90% transparency, self-healing ability, adhesion, antibacterial properties, and longterm environmental stability, and its electrical conductivity reached 1.13 S m À1 .The addition of PDO makes PAOAM-PDO have anti-freeze properties and can maintain performance in low-temperature environments of À60 °C.It is suitable as a skin protective barrier to resist low-temperature frostbite (Figure 3g).The hydrogel can also be fashioned into a strain sensor with exceptional strain sensitivity, enabling real-time monitoring of human movement.This paves the way for a novel practical approach to enhance the environmental adaptability and functional versatility of multifunctional organic hydrogels.

Cellulose
Cellulose is an organic polymer that is natural, renewable, biodegradable, and the primary constituent of plant cell walls.Cellulose is widely used in various fields, including textiles,  [71] Copyright 2022, Wiley.c) Chitosan for e-skin sterilization.Reproduced with permission. [72]Copyright 2021, American Chemical Society.d,e) Chitosan-based antibacterial and mite-removing TENG Reproduced with permission. [73]Copyright 2023, Wiley.f,g) Skin contact-driven single-electrode protein TENGs.Reproduced with permission. [74]Copyright 2023, American Chemical Society.
paper, and biomedicine, due to its abundant source, biodegradability, and biocompatibility.Cellulose-based e-skin exploits these advantages and exhibits excellent biocompatibility, sustainability, and environmental friendliness.Cellulose-based materials also excel in flexibility, breathability, and comfort, making them a favorable choice for making wearable sensors and e-skins for the human body.However, cellulose-based e-skins also face some challenges.The first is its inherent electrically insulating properties, which limits its applications in electrical conduction and electronic sensing.To overcome this problem, complex chemical or physical modification processes are often required, such as doping conductive nanomaterials to give cellulose conductive properties, but this may also affect its original biocompatibility and mechanical properties.Secondly, the water sensitivity of cellulose is also a problem that needs attention.Excessive water sensitivity may lead to reduced stability and durability in humid environments.Hence, in order to facilitate the broad utilization of cellulose in the e-skin field, it is imperative to continue researching and addressing these challenges.
In order to address the limitations of conventional materials like glass, plastic, and rubber, maintaining folds to create a threedimensional (3D) structure after bending is a challenging task.The transparent cellulose-based multilayer film (CMLF) developed by Xu et al. provides an innovative strategy. [75]MLF is constructed by stepwise deposition of a triboelectric layer made of ethylcellulose (EC) or wood nanofibrillated cellulose (NFC), a silver nanowire electrode layer, and a protective cellulose layer.This technology utilizes the abundant hydroxyl groups of cellulose to chemically modify or combine with active ingredients to improve the performance of TENGs (Figure 4a).Because each layer is composed of nanofibers/nanowires or molecular-level polymers, with plasticizers added to the protective layer, it gives the film excellent flexibility and foldability.These characteristics enable CMLF to be readily cut or folded into a wide range of intricate spatial configurations.These unique properties indicate that cellulose-based transparent TENGs may become an effective approach to simple, low-cost manufacturing of high-performance thin film electronics in the future.
Polymers and metals are frequently employed as the primary materials in the friction layer of wearable TENGs, owing to their outstanding electrical output characteristics.However, the difficulty of recycling and degrading these materials has led to problems of resource waste and environmental pollution.To solve this challenge, Wang and colleagues introduced a water-soluble TENG (WS-TENG) using repurposed waste printing paper, enabling real-time monitoring of human physiological signals. [76]The charged layer consists of an MC film (negative electrode) and a CNC/MC composite membrane (positive electrode).CNC is obtained from waste printing paper using a sulfuric acid hydrolysis process and is dispersed within the MC film as nanofillers.This innovative WS-TENG design is not only low-cost, lightweight, and biodegradable, it is also completely watersoluble (Figure 4b).The WS-TENG can be utilized as a disposable bandage sensor for tracking human respiration and  [75] Copyright 2022, Elsevier.b) Fully biodegradable water-soluble TENG based on cellulose.Reproduced with permission. [76]Copyright 2022, Elsevier.c) Bacterial cellulose for eco-friendly, recyclable TENGs.Reproduced with permission. [77]Copyright 2021, Elsevier.d) High-performance paper-based TENG loaded with branched polyethyleneimine.Reproduced with permission. [78]Copyright 2022, Elsevier.other physiological indicators.When in use, graphite powder adheres to the water-soluble tape, creating a water-soluble electrode with exceptional electrical conductivity.After the WS-TENG has finished its monitoring operation, the entire device can be directly rinsed with water, during which the MC, CNC/MC films, and graphite powder completely dissolve in the water.Environmentally friendly, biodegradable WS-TENG provides a solution with significant potential, aiming to overcome the environmental and recycling issues of traditional TENGs composed of polymers and metals, opening up new avenues for medical monitoring and human physiological signal monitoring.
Bacterial cellulose (BC) is a natural cellulose produced by bacteria, characterized by its unique 3D porous network and nanoscale fibril structure with a diameter ranging from 10 to 100 nm.BC boasts exceptional attributes, including superior purity, crystallinity, porosity, and permeability to both liquids and gases when compared to plant cellulose.At the same time, its biocompatibility and mechanical strength are also better.Zhang et al. leveraged the benefits of BC and devised an all-cellulose energy harvesting and interaction device employing a BC-CNT-PPy/BC/ BC-CNT-PPy sandwich structure TENG. [77]In this setup, a highpurity BC film produced by Gluconacetobacter hansenii was utilized as one of the friction layers, whereas a conductive BC film doped with CNT/PPy was used as the other friction layer (Figure 4c).This all-cellulose TENG generated a maximum open-circuit voltage of 29 V, a short-circuit current of 0.6 μA, an output power of 3 μW, and had a paired resistance of 25 MΩ.This all-cellulose TENG is not only environmentally friendly but also recyclable.BC and BC-CNT-PPy membranes can undergo complete degradation within 8 h, and the remaining CNTs can be recycled and repurposed.This technological path utilizing BC materials provides a new approach to the environmentally friendly development of TENGs and potential selfpowered biointerfaces, demonstrating a practical and sustainable energy harvesting method.
Cellulose, the most abundant biopolymer on Earth and the primary component of paper, naturally develops microstructures due to the interwoven network of its fibers.This biopolymer offers numerous advantages, including high flexibility, affordability, lightweight nature, biodegradability, and ease of modification.Polyethylenimine (PEI) is a polymer rich in amine groups that interacts strongly with cellulose fibers through hydrogen bonding and electrostatic adhesion.In view of this characteristic, Wu and his team used PEI as a liquid filler and combined it with cellulose paper to create a PEI paper composite material. [78]This structure effectively enhances the triboelectric polarity of cellulose paper.This composite material inherits the advantages of cellulose and PEI and has excellent flexibility, lightweight, high strength, and low cost (Figure 4d).When the PEI paper composite material is applied to the friction-positive layer of the paperbased TENG, the mechanical and output properties of the TENG are significantly improved.It is worth noting that PEI not only enhances the electrical properties of cellulose paper but also gives the composite material antibacterial properties.This improvement does not affect the rapid disintegration performance of cellulose paper in water, thereby maintaining its biodegradable and environmentally friendly qualities.This groundbreaking composite material, which combines cellulose and PEI, offers an efficient means to enhance the performance of paper-based TENGs while preserving their eco-friendly and biodegradable characteristics.

Working Mode and Charge Transfer Mechanism of TENG
TENGs operate in four distinct modes: vertical contact separation, horizontal sliding, single electrode, and independent layer. [79]Each of these modes offers unique characteristics and advantages, enabling the harnessing of diverse forms of mechanical energy.In the vertical contact separation mode, two dielectric films with back electrodes are positioned vertically, and their vertical separation generates periodic electrical signals.This mode is adept at harvesting energy from vibrations, impacts, and sound, serving as the simplest and most widely adopted structure among TENGs.The horizontal sliding mode closely resembles the vertical contact separation mode, differing in the close contact between friction material films, resulting in an alternating current signal through continuous contact and separation of the dielectric films.This mode is well-suited for capturing rotational energy from sources like water flow and wind, maximizing mechanical energy collection with no gaps between friction electrodes.The single-electrode mode incorporates freely moving and electrode-bearing friction layers connected to the ground through a load, creating a periodic current through frictional electrification and electrostatic induction.This mode is versatile and mobile for energy collection and finds applications in human movement, water droplet energy harvesting, intelligent sensing, and human-computer interaction.The independent layer mode comprises an autonomous friction layer and fixed electrodes.Reciprocating motion of the dielectric film between the electrodes generates a periodic potential difference and current in the external circuit.This mode excels without direct contact, effectively capturing kinetic and vibration energy from activities like walking and vehicular movement, while minimizing damage to friction materials and energy loss.Notably, this mode eliminates the need for connecting wires between the moving parts, enhancing flexibility for capturing diverse forms of mechanical energy.In summary, the four TENG working modes each offer unique advantages and are suitable for distinct application ranges.
From a macroscopic perspective, the electron transport model can be used to illustrate the working principle of TENGs (Figure 5a). [80]Two materials, polytetrafluoroethylene (PTFE) and copper, are employed as examples to elucidate this model.In the initial state, a specific separation is maintained between the two triboelectric materials, polytetrafluoroethylene (PTFE) and copper.When these two materials come into surface contact under the influence of external force, the electrons from copper are transferred to PTFE because of the stronger electronegativity of PTFE.Consequently, PTFE becomes negatively charged due to electron gain, while copper becomes positively charged owing to electron loss.At this time, PTFE becomes negatively charged because it gains electrons, while copper becomes positively charged because it loses electrons.Since PTFE and copper are in contact with each other, there is no potential difference when the two are in contact.When the contacts separate, a potential difference develops between the two oppositely charged bodies.This potential difference will push free electrons to flow through the external circuit, and the transfer of electrons will generate current until the two friction layers form a new electrostatic equilibrium state.As the external force brings the two friction layers back into proximity, a reverse current flow is induced in the external circuit to equalize the change in potential difference between them.This alternating current flow in both directions produces an alternating current signal.The electron transport model is generally used to describe macroscopic triboelectricity.From the microscopic level, researchers have proposed a surface state model to clarify the basic charge transfer mechanism during the contact electrification process between metals and semiconductors.Although the surface state theory can be used to explain the contact electrification mechanism related to semiconductors, it is difficult to explain the contact electrification mechanism related to polymers.This is because the surface state theory is derived from the energy band theory, and polymers It is an amorphous material and does not have a strict crystal structure, so the surface state does not apply to this type of material.In this case, Academician Wang proposed the electron cloud potential well model based on the electron cloud overlap between materials.When two solids come into contact under the influence of external force, if the distance between atoms is less than the bond length, the electron distribution adheres to the energy levels and is confined by the electron orbit, and, at this point, no charge transfer takes place.However, as the distance between materials decreases, mechanical forces cause the electron clouds to overlap, lowering the barrier to electron transfer.Following the formation of a covalent or ionic bond between the atoms of two objects, electrons transition from a higher atomic potential well to a relatively lower atomic potential well, leading to contact electrification between the two objects.This phenomenon is called Wang's formula transition model.Therefore, after triboelectric charging, a charged surface is formed when the two solids separate.Due to the characteristics of liquid flow and dispersion, the adsorption process of ions or molecules on the solid surface makes the solid-liquid contact electrification mechanism more complex and uncertain.To explore the contact electrification phenomenon between solid and liquid, researchers proposed a double electric layer model (Figure 5b).First, when a liquid comes into contact with the initial solid surface, molecules and ions (such as water molecules, cations, anions, etc.) make contact with the solid surface due to their thermal motion and the pressure applied by the liquid.During this impact process, the electron clouds of solid atoms and water molecules overlap, leading to the transfer of electrons between them.Simultaneously, ionization reactions may occur on the solid surface, resulting in the generation of electrons and ions on the surface.For instance, Figure 5. a) Four working modes of TENG.Reproduced with permission. [79]Copyright 2022, Wiley.b) Potential electricity generation modes of TENG.Reproduced with permission. [80]Copyright 2021, Springer Nature.
in the charging of SiO2 in contact with deionized water, electron transfer plays a predominant role.Secondly, the counterions in the liquid are attracted to the charged surface through electrostatic interactions and migrate to the charged surface, forming an electric double layer.These are the four-electron transfer mechanisms currently used in TENGs.

Application of TENG Based on Natural Materials in Electronic Skin and Human-Machine Interaction
TENGs based on natural materials have shown excellent performance in e-skin and human-computer interaction due to their biocompatibility, sensitivity, and environmentally friendly properties.In limb activity monitoring, it can provide high-resolution motion capture and achieve accurate recording and analysis of human body movements.In terms of pulse monitoring, TENG is non-invasive and highly sensitive and can monitor heart rate and other physiological signals.In the field of electronic device control, it serves as a responsive, intuitive interface that allows users to control devices through simple touch and gesture operations.For robot control, TENGs based on natural materials can be incorporated into the robot's skin to enable precise operation and control of the robot, thus facilitating natural and efficient human-machine collaboration.

Body Movement Sensing
Nature materials-based TENGs are revolutionizing body movement sensing, utilizing the triboelectric effect to convert mechanical energy from human motions into electrical signals.These biocompatible, flexible TENGs, often incorporating natural polymers like chitosan, have broadened applications in wearable technologies for health monitoring and HMIs.For instance, joint motion sensors crafted from chitosan-based TENGs provide precise monitoring of body kinematics, essential for medical diagnostics and sports performance analysis.Additionally, their integration with artificial intelligence enables innovative applications such as gesture recognition, enhancing interactive technologies.The unique combination of eco-friendly materials and efficient energy conversion positions these TENGs at the forefront of non-invasive, real-time motion tracking, and analysis.
Through the water heat method, Lu and others successfully prepared the chitosan-hydroxyl ethyl cellulose-pectin (CHP) film and found its potential application in friction electrical materials (Figure 6a). [73]They further explored the polypyle (PPY) a kind of electrode material known for its high conductivity and excellent environmental stability, and integrated it with CHP film to form a compact single electrode CHP-PPY-CHP Teng (CPC-TENG).The compact, portable, and durable features of CPC-TENG make it a perfect self-powered intelligent sensor for monitoring human body joint motion.When the human body performs bending, the CPC-TENG friction electrical layer is in contact with the skin, and curvature deformation occurs, which proves that it is suitable for monitoring human daily and clinical movements.In addition, CPC-TENG can also be applied to the finger recognition of the artificial intelligence system.By connecting it to ten fingers, real-time voltage signal monitoring of the finger movement is achieved.This research has opened up an innovation path for the next generation of medical and intelligent monitoring systems.
Shi et al. based on their innovative flexible, breathable, antibacterial TENG e-skin, developed a self-power volleyball monitoring system (Figure 6b). [72]The e-skin is created through the electrostatic spinning of PVA/CS and TPUs, and AGNW is applied using spraying technology to form a 2 Â 3 integrated e-skin arrays.In the experiment, three sensing units were incorporated into each arm of the athletes as receiving sensor pixels.During the process of receiving a volleyball in volleyball play, when the volleyball impacts the two pixels at the same location on the arms, the e-skin produces a consistent output signal.These signals can be measured by multi-channel output voltage measurement and subsequent signal processing, which can evaluate the speed and effect of volleyball in real-time.By conducting a procedural analysis, researchers can gather statistical data and analytical results related to volleyball, thereby demonstrating the feasibility and potential applications of the self-powered sensing system in terms of motion detection, location tracking, and distribution statistics.
Kim and others have developed a biocontrolled skin attachment sensor integrated motion sensor based on chitosan-diatom (CD) membrane and stainless-steel mesh (Figure 6c). [81]The sensor can accurately monitor and analyze the movements and movements of joints.The CD membrane is designed to completely cover the stainless-steel mesh, which enhances its flexibility and prevents rupture, thereby adapting to complex and extensive joint movements.The sensor can easily attach to the wrist, elbow, knee, and ankle to keep it stable in a wide range of exercises.In practical applications, when joints are curved and stretched, the output voltage generated by CD-TENG can identify different motion statuses.The operation time of the joint can be accurately captured and recorded by the sensor from the beginning to the end.The amplitude of the output voltage is positively correlated with the movement speed, compression force, and the range of motion of the joint, which provides detailed insight into the joint dynamics.The skin attachment sensor can not only capture and repeat the voltage signal generated by joint motion multiple times, but also successfully demonstrate its practicality and potential applications in the biocompatible power supply watch.
Wang et al. successfully developed a medical sensor that can monitor respiratory frequencies in real-time (Figure 6d). [76]The sensor consists of an MC membrane and CNC/MC membrane, which can be pasted directly on the abdominal skin to achieve continuous monitoring of human health.When an individual inhales, the decreased diaphragm and abdominal expansion cause MC and CNC/MC membranes; when exhaling, abdominal contraction separates two layers of membranes.The WS-TENG sensor shows a stable voltage change of 0.3-0.8V under different breathing conditions.In the experiment, the respiratory frequency data of the two volunteers was highly the same as the output signal of WS-TENG, which proved the high sensitivity and accuracy of the sensor.Different breathing modes, such as slow breathing and deep breathing, can cause corresponding changes in the output voltage, providing accurate electrical signs for the frequency and mode of breathing.The importance of this study lies in the potential applications of WS-TENG, especially in monitoring the value of respiratory diseases.By analyzing the output signal of WS-TENG, doctors can obtain detailed information about the patient's respiratory mode and frequency to further diagnose and treat respiratory disease-related diseases.The introduction of this technology provides a novel and effective method for real-time and non-invasive health monitoring.

Pulse Monitoring
TENGs based on natural materials enable advanced pulse monitoring by converting mechanical heartbeats into electrical signals.These TENGs are made from biocompatible materials such as chitosan and cellulose to ensure user comfort and safety.When worn on the skin, TENGs can sensitively detect pulse waves, providing real-time, non-invasive heart rate monitoring.The lightweight and flexible properties of TENGs make them ideal for wearable health devices, with great potential in personalized healthcare and cardiovascular abnormality detection.Chen et al. employed a combination of facile regeneration and hot pressing with a sieve template to fabricate microstructured cellulose thin films (M-CF) (Figure 7a). [82]Following a straightforward carbonization process, M-CF was further transformed into microstructured carbonized cellulose films (M-CCF), which exhibited excellent electrical conductivity while retaining their original array of surface pits.The pressure sensor based on M-CCF demonstrated remarkable flexibility, high sensitivity (2.67 kPa À1 ), and a wide operating range (0-20 kPa).This sensor showcased applications in detecting pressure variations induced by finger presses, wrist flexion, wrist pulses, and sound stimuli.Moreover, the M-CCF pressure sensor was employed for monitoring wrist pulses in the human body, effectively capturing changes in the pulse signal before and after physical activity.As evident from the resulting pulse frequency curves, the subjects exhibited pulse frequencies of 90 and 126 min À1 before and after exercise, respectively (Figure 7b).The sensor was securely affixed above a Bluetooth speaker that intermittently played music.The electrical current variations accurately distinguished the onset and cessation of music playback: stable signals Figure 6.The application of natural material-based e-skin in physical movement monitoring.a) CPC-TENG serves as a self-powered smart sensor to monitor the motion status of human joints.Reproduced with permission. [73]Copyright 2023, Wiley.b) Volleyball reception statistical analysis system based on chitosan TENG.Reproduced with permission. [72]Copyright 2021, American Chemical Society.c) Application of chitosan-diatom in wearable devices through human movement operations.Reproduced with permission. [81]Copyright 2020, Elsevier.d) The cellulose-based TENG attached to the abdomen is used for real-time respiratory monitoring.Reproduced with permission. [76]Copyright 2022, Elsevier.
were received when the music was paused, while noise signals were detected upon music resumption.This phenomenon was attributed to the generation of air disturbances during music playback, inducing subtle vibrations in the sensor and thus generating corresponding signals.
Niu et al. have suggested the development of a silk nanoribbon-based bio-triboelectric nanogenerator (SNR Bio-TENG) utilizing freshly derived SNR films (SNRF) and regenerated silk fibroin protein films (RSFF) (Figure 7c). [83]To retain the inherent mesoscale/nanoscale architecture of silk, subnanometer-thick silk nanoribbons (SNRs) with a thickness of 0.38 nm were directly extracted from natural silk.Ribbons of regenerated silk fibroin (RSFF) and SNRs exhibit distinct microstructures and work functions.The SNR Bio-TENG demonstrates remarkable output performance, achieving a maximum voltage, current, and power density (PD) of 41.64 V, 0.5 μA, and 86.7 mW m À2 , respectively.When worn on the wrist, this TENG effectively discerns heartbeats or pulses.As a result, the SNRF/RSFF TENG can seamlessly monitor subtle human movements without requiring an external power source.Thanks to its high sensitivity and the capability to generate electricity solely through human pulses, the all-silk Bio-TENG shows promise as an attractive power source for implantable self-powered electronic devices, such as pacemakers and implanted sensors.
Lin and others proposed a TENG based on PEO/CCP-4, which can provide an innovative method for cardiac health monitoring (Figure 7d). [84]This device can be pasted in the human body's aorta position to monitor heart activity in real-time.In an experiment, TENG successfully recorded a stable heartbeat rate of 99 times per minute squared.This data is faster than the normal value, revealing the increase in the heart rate.This device can not only monitor the heart rate, but also evaluate the overall health status of the individual by determining the pulse of the human body.The experimental results show that in a static state, the pulse frequency of the test object is 92 times per minute.This data fully meets the physiological standards of healthy adults.After 5 min of exercise, its heartbeat frequency rose to 120 times per minute.It is worth noting that the device can also accurately measure the veins of the wrist in a heartbeat cycle.By measuring the time interval between P peaks and the first inflection point peak (called ΔTEV1 in this study), the individual's arterial hardness can be evaluated.The experimental results show that the ΔTEV1 value of the tester is about 252 ms, which meets the standards of healthy adults.TENG developed by Lin et al. not only can accurately measure heart rate and pulse frequency, but also evaluate arterial hardness, showing excellent life signs detection capabilities (Figure 7e).This discovery provides new possibilities for economic and efficient medical diagnosis and early analysis of diseases.Reproduced with permission. [82]Copyright 2022, Elsevier.c) Silk fibroin-based bio-TENG.Reproduced with permission. [83]Copyright 2020, Elsevier.d,e) PEO/cellulose composite paper group testing is used for ECG and pulse testing.Reproduced with permission. [84]Copyright 2023, Elsevier.

Electronic Device Control
Nature materials-based TENGs are increasingly utilized for controlling electronic devices, harnessing the triboelectric effect for user interface applications.These TENGs, often fabricated from eco-friendly materials like chitosan and cellulose, convert mechanical interactions, such as touch and gestures, into electrical signals.This capability enables their integration into interactive systems, where they function as self-powered sensors for device control.The biocompatibility and flexibility of these materials facilitate their use in wearable interfaces, allowing for seamless and intuitive control of electronics.Thus, TENGs are becoming pivotal in the development of sustainable, user-friendly technologies for enhanced HMI.
To overcome the crispy and chemical stability of pure silk protein film.Xu et al. using the doping of recycled silk protein, has significantly enhanced its mechanical flexibility by promoting the transformation of the secondary structure (Figure 8a). [85]Based on this improved silk protein, a TENG is designed with a flexible, stretched, and complete biological degradable tee, named SF-TENG.SF-TENG has the ability of self-power supply and can be powered by mechanical energy generated by finger movement.Its signal can be transmitted wirelessly to the receiver by partial pressure treatment and rectification, thereby controlling the transmission rate of the electrode-to-color rearview mirror, and effectively reducing the interference of the driver of the driver of the rear vehicle high beam.Experimental verification, the SF-TENG controller can effectively control electricalchanging car radios in the real car environment, and can quickly control through the driver or passenger's fingers.It is worth mentioning that SF-TENG has the characteristics of breathable and inflammatory reactions so that it can directly fit the human skin and be used for long-term wireless control.This feature makes SF-TENG have huge application potential in skin patches or implanted intelligent devices.
Zhao and others have built a transparent, frozen polyfunctional organic water gel (Paoam-PDO).TENGPAOAM-PDO organic water gel has the sensitivity and signal stability are successfully transformed into flexible sensors for real-time monitoring of human movement, including large and small movements (Figure 8b). [74]When the sensor degenerates due to external pressure, its real-time resistance changes, which in turn causes changes in the current in the circuit.Electrochemical workstations can sensitively record these current changes and output electrical signals.The sensor can be attached to the different joints of the human body, such as wrists, fingers, and knees, thereby accurately monitoring related movements.The width and height analysis of the peak signal can clearly distinguish between different types of joint movements, which proves the sensitivity of the Paoam-PDO sensor.This sensor can not only monitor large movements but also capture tiny movements, such as smiles and writing behaviors.In the writing test, different letters produce different signal peaks, and the same letter presents similar peak shapes, which verifies the recovery and stability of Figure 8. TENG based on natural materials control electronic equipment.a) Silk fibroin-based friction nano-generator control electrical deformation car reached mirror system.Reproduced with permission. [85]Copyright 2021, American Chemical Society.b) Multifunctional organic hydrogel is used for motion monitoring.Reproduced with permission. [74]Copyright 2023, American Chemical Society.c) The all-cellulose TENG controls the "greedy snake".Reproduced with permission. [77]Copyright 2021, Elsevier.d) Real-time fall alert based on TPNG.Reproduced with permission. [86]Copyright 2018, Elsevier.
the Paoam-PDO sensor.Therefore, this organic hydrogel sensor has extensive application potential in real-time monitoring of human movement, analysis of motion mode, and identifying tiny movements.
Zhang et al. developed BC-TENG based on the sandwich structure is an environmentally friendly and recyclable all-cellular energy collection and interactive equipment, which is proven to apply to control various software-level applications (Figure 8c). [77]A practical example is to use this technology to control the online version of the "greedy snake" game.In this setting, the pure BC film is fixed on the thumb, and the BC-CNT-PPY film is installed on the other four fingers.These different membranes are used to control the upper, lower, left, and right of the "greedy snake" game by touching the electrical signal.The single-chip machine receives the electrical signal generated by the finger and transforms into a platform for the mobile instructions of the "greedy snake" in the game.The test results show that all collected signal peaks remain relatively stable, and they are within the voltage threshold range (0.005-5 V) that can be read by Arduino.The practicality and efficiency of this whole cellulose Teng technology indicate its widespread application in driving commercial electronic products and as wearable interfaces.Its environmental protection and biodegradable characteristics make it an ideal choice for applications with these needs.
Guo et al. employed conductive fabrics as a base to craft a fullfiber hybrid voltage-enhancing nanogenerator (TPNG) using silk protein and polyvinylidene fluoride (PVDF) nanofiber electrospinning (Figure 8d). [86]TPNG's flexible design permits easy customization in terms of size and shape, facilitating seamless integration into the wearer's clothing or gloves.In the event of an accidental fall, TPNG detects the impact force from the ground and generates an electrical pulse through the electrostatic field established between the micro-cantilever and TPNG's bottom electrode.When this static field surpasses the threshold voltage of the microcellular arms, the micro-cantilever is drawn toward the bottom electrode, assuming a guiding state.This mechanism activates a pre-configured message-sending function, transmitting the message to a designated mobile phone via the Internet.This innovative technology opens up possibilities for multifunctional, self-powered wearable electronic devices, supporting activities monitoring and personal healthcare.

Robot Control
In the realm of robotics, nature materials-based TENGs are revolutionizing control mechanisms.These TENGs, crafted from environmentally friendly materials like chitosan and cellulose, convert mechanical movements into electrical signals.When integrated into robotic systems, these signals enable precise control over robotic actions.The inherent flexibility and biocompatibility of these materials make TENGs especially suitable for wearable robot interfaces, allowing for intuitive, gesture-based control.This technology not only enhances the interaction between humans and robots but also paves the way for more sustainable and efficient robotic control systems, vital in various applications ranging from industrial automation to assistive technologies.
Chang et al. engineered an innovative egg white liquid characterized by a sol-gel-sol (EW-EWH-EWL) phase transition process (Figure 9a). [87]Under alkaline conditions, not only is the physical equilibrium realigned to generate solid EWH, but a simultaneous self-clearing process is also instigated to yield EWL.This EWL has been instrumental in the introduction of a somatosensory sensor, a development demonstrated for the first time via a car model and a PC program, showcasing the viability of a gesture-controlled console.Incorporating EWL as an electrode, the team further crafted a TENG marked by its notable compressibility and transparency.They embarked on the creation of software programming and device control grounded in gesture recognition, ingeniously integrated with an EWL switch to morph the remote control unit of a standard remote-controlled car.The distinct systems of the powertrain (PT, responsible for forward and backward movements) and steering system (SS, governing left and right directions) are isolated, each connected to switches stationed on separate hands.The vehicle accelerates either forward or backward when only PT is activated with SS on standby, prompted by the inclination of one hand to the opposite side.Contrarily, the isolated activation of SS instigates wheel turning without initiating movement (Figure 9b).The concerted tilting of both switches unveils four additional operational modes.
Chen et al. have created a self-powered, flexible triboelectric sensor (SFTS) patch designed for finger trajectory sensing.It is constructed from environmentally friendly and flexible materials, including starch-based hydrogel, polydimethylsiloxane (PDMS), and silicone rubber.This sensor patch shows promise in applications related to robot control, touch screens, and e-skin due to its straightforward design and cost-effectiveness (Figure 9c). [88]The sensor patches are divided into two categories: the two-dimensional (2D) SFTS for controlling in-plane robot movements and the one-dimensional (1D) SFTS for outof-plane motion control.The 2D-SFTS features a grid structure on its sensing surface and incorporates four peripheral starchbased hydrogel PDMS elastic body electrodes to capture continuous sliding information from fingertips, including trajectories, speed, and acceleration.Through the integration of 2D-SFTS with 1D-SFTS, the voltage curves generated by the four electrodes corresponding to the sensed fingertip letter trajectories facilitate 3D motion control of a robotic arm (Figure 9d).Consequently, the SFTS patch serves as a 3D motion control interface for robotic manipulators, enabling various multifunctional operations such as handling, welding, and spraying.The research team demonstrated the real-time generation of 3D spatial information and its application in controlling the 3D motion of robotic manipulators.
Dai and his team unveiled a humanoid robotic hand based on anionic e-skin, marking a significant stride in tactile perception and object identification (Figure 9e). [89]The e-skin is constructed from a cell-free skeletal filamentary network, offering mechanical properties, including softness, toughness, and resistance to fatigue fracture, that is strikingly akin to human skin.Equipped with the features of a friction nanogenerator, the SL-TENG generates distinctive frictional electrical signals upon touch, serving as indices for tactile sensing.The peak profiles of the signals are consistent, although there are slight variations when the SL-TENG interacts with different materials.These variations may be attributed to factors such as the ratio of positive to negative peaks, the number of peaks during each process, and the intervals between the peaks (Figure 9f ).These nuanced differences are often elusive to human detection.However, integration with machine learning and IoT technologies elevates the robotic hand's precision in identifying material types among various spherical objects and relocating them as directed.The team employed a machine learning model rooted in recurrent neural networks, training it on the output voltage signals generated by interactions with different dielectric objects, including soft rubber, ping pong balls, PTFE, hard rubber, and wood.After 700 training iterations, the model converged with a signal classification accuracy of 100%, devoid of overfitting.In a series of 600 tests on the identification of five categories of spherical objects, the robotic hand boasted a sorting success rate of 97.2% (Figure 9g).Such precision, combined with remarkable mechanical and environmental resilience, positions this robotic hand as a valuable asset for intelligent sorting, automated operation and assembly in unmanned factories, and the categorization of waste and hazardous materials.The frictional electrical signals produced upon touch and tapping by the fingers of the robotic hand, enhanced by machine learning algorithms, ensure precise object recognition and handling, heralding a new era in automated material identification and management.

Conclusion and Perspectives
TENGs based on natural materials have demonstrated significant potential and innovation in the fields of HMI and e-skin.Their self-powering capability, biocompatibility, and flexibility offer broad application prospects in areas like limb motion monitoring, intelligent robots, and wearable technology.This review comprehensively explores the research advancements and challenges of natural material-based TENGs in these applications.Although these TENGs exhibit immense potential,  [87] Copyright 2020, Wiley.c,d) Intelligent identification and sorting application.Reproduced with permission. [88]Copyright 2018, American Chemical Society.e-g) Intelligent identification and sorting application.Reproduced with permission. [89]Copyright 2023, American Chemical Society.
addressing certain core issues is imperative for their widespread application.
Firstly, large-scale production remains a significant hurdle.Currently, the technology for natural material-based TENGs has not achieved mass, efficient, and cost-effective manufacturing.Attaining this requires novel production techniques and processes, alongside in-depth research and optimization of natural materials to ensure they retain efficient and reliable performance during large-scale production.Secondly, long-term stability is crucial for any HMI and e-skin systems.While natural materials boast biocompatibility and eco-friendliness, maintaining stability and performance under varying environmental and usage conditions remains a challenge.Addressing this issue calls for improvements and enhancements in materials' durability, anti-aging, and environmental resistance.Additionally, the safety of natural materials to the human body is paramount, especially given their close and prolonged contact with skin in wearable applications.It necessitates rigorous testing and validation to ensure these materials are hypoallergenic, non-toxic, and free from adverse effects, thereby safeguarding user health in diverse environmental contexts.Thirdly, sensitivity is one of the key parameters in evaluating TENG performance.At present, enhancing the sensitivity of natural material-based TENGs to accurately and reliably detect and respond to subtle and complex physiological and environmental signals is a core task.This calls for integrated innovation and optimization from materials, design, and manufacturing perspectives.Lastly, structural design plays a pivotal role in optimizing TENG performance and expanding its applications.Future research needs to explore more advanced and multifunctional structural designs to boost the energy conversion efficiency, sensitivity, and adaptability of TENGs, meeting the specific needs of diverse applications and environments.It is foreseeable that natural material-based TENGs will continue to unlock more possibilities in the realms of HMI and e-skin.With advancements in materials science, nanotechnology, and bioengineering, more efficient, reliable, and safer natural material-based TENGs will propel HMI and e-skin technologies to new heights, offering human beings a more convenient, intelligent, and secure living experience.

Figure 7 .
Figure 7. Natural material e-skin is used for pulse monitoring.a,b) Cellulose-based TENGs are used for pressure sensing and pulse monitoring.Reproduced with permission.[82]Copyright 2022, Elsevier.c) Silk fibroin-based bio-TENG.Reproduced with permission.[83]Copyright 2020, Elsevier.d,e) PEO/cellulose composite paper group testing is used for ECG and pulse testing.Reproduced with permission.[84]Copyright 2023, Elsevier.

Figure 9 .
Figure 9. TENG based on natural materials for robot control.a,b) Gesture control demonstration.Reproduced with permission.[87]Copyright 2020, Wiley.c,d) Intelligent identification and sorting application.Reproduced with permission.[88]Copyright 2018, American Chemical Society.e-g) Intelligent identification and sorting application.Reproduced with permission.[89]Copyright 2023, American Chemical Society.