In current era, the rapid growth of the internet of things (IoTs) represented by wearable electronic devices and wireless sensor networks has changed every aspect of our daily life, such as health monitoring, medical treatment, communication, entertainment, transportation, environmental protection, infrastructure monitoring, and security ^[1][2][3][4]. However, in order to ensure reliable and continuous operation of the sensing nodes, so as to realize the full power of IoTs, there arises an urgent demand for prosperous and sustainable energy harvesting devices which can scavenge energies from the ambient environment or our body and charge energy storage devices in a continuous way ^{[5][6][7][8][9]}. Meanwhile, there is also a rising dependence of advanced health monitoring and therapeutic technologies on biocompatibility or biodegradability, where biodegradable electronic components can be degraded and disintegrated into monomeric and oligomeric building blocks, thereby reducing their environmental and physiological impacts ^{[10][11][12][13]}.

Since the first report of Wang’s team in 2012, TENG has been widely used in green electronic systems, not only for self-powered green electronic systems, but also as an active electronic sensor to detect various chemical and physical inputs ^[14][15]. By harvesting mechanical energy from the surrounding environment and organisms, it can generate green power for real-time monitoring, such as human/animal body, joint movement, all kinds of vibration, deformation, sound wave, liquid/air flow, wind flow, and even electronic health care monitoring ^{[16][17][18][19][20][21][22]}. Since then, many studies have been conducted to improve its electrical performance, sustainability, quality, and adaptability ^[23][24][25]. Due to the fact that the operation of TNEG is based on the coupled effects of triboelectrification and electrostatic induction, polymers such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), nylon, and other organic–inorganic composites have been quite legitimately explored as triboelectric materials due to their position in the triboelectric polarity series ^{[1][11][26][27][28][29][30]}. On the one hand, the wide option of triboelectric materials brings tremendous advantages such as low cost, high efficiency, extensive use, and the possibility of continued ascension. On the other hand, like other non-degradable electronics, TENG also causes environmental problems at the end of its useful life, and will eventually be thrown into a landfill or abandoned in an unprotected dump site ^{[31][32][33][34]}.

Meanwhile, BD-TENG represents a trend in developing green electronic devices. It can be degraded naturally in our environment or resorbed in the body after completing its work cycle without any adverse long-term effects or secondary surgical removal, thus avoiding environmental pollution and improving our health ^[35][36]. Presently, tunable electrical output capabilities and degradation features can be achieved by using different biodegradable materials. As a result, BD-TENGs with the advantages of biocompatibility, controllability, and biodegradability are fabricated and will find wide applications in in vivo biomechanical energy harvesting and self-powered sensors ^{[37][38][39][40][41][42]}.

At present, numerous environmentally friendly biodegradable materials, including cellulose, chitin, and silk fibroin, have been found to have biocompatibility, biodegradability, and triboelectric effects ^[43][44]. For instance, in 2015, Luca Valentini and colleagues took the initiative to employ biodegradable polymer materials into TENG ^[45]. The solvent casting approach created a TENG based on a biodegradable sodium alginate/graphene oxide nanocomposite sheet. Zheng et al. reported an implantable BD-TENG in 2016 ^[46]. Due to the layered structure formed by biodegradable polymers (BDPs) and absorbable metal, the biodegradable polymer materials used are poly (lactic acid glycolic acid) copolymer (PLGA), poly (caprolactone) (PCL), polyvinyl alcohol (PVA), poly (hydroxybutyrate valerate) (PHB/V). These polymers are characterized by low cost, commercial purchase, simple processing, etc. At the same time, the component materials can be selected according to the different ability of the triboelectric layer to gain and lose electrons. As a result, the adjustable output voltage of 10~40 V in vitro can be achieved ^{[40][41][42][43]}. BD-TENG can be degraded and reabsorbed after completing its work cycle in animals. What’s more, according to the different degradation rates in vivo, the packaging materials can be changed to make the degradation rate of the whole device controllable. Accordingly, Hyun Jun Kim et al. reported a silk-based Bio-TENG in the same year ^[47]. Since 2017, many studies on BD-TENG from plants have steadily been carried out. So far, BD-TENG has been studied from multiple perspectives including energy harvesting, signal detection, medical care, and other procedures, among which more BDPs have been utilized ^[31].

For convenience, BDPs are divided into three categories according to different sources: plant-based BDPs, animal-based BDPs, and synthetic BDPs. Plant-based BDPs are degradable materials that are extracted from plant sources, such as leaves, wood and cellulose molecules, alginate, rice paper, etc. Similarly, animal-based degradable materials refer to BDPs that are from animal sources, such as chitosan, silk fibroin, and gelatin. Synthetic BDPs are biodegradable substances processed in industry, such as PLA (polylactic acid), PHB/V (polyhydroxy butyrate valerate), PCL (polycaprolactone), etc. With the utilization of these green energy materials, energy harvesting technologies have significantly changed in vitro/in vivo biomedical science and made breakthroughs in the pollution-free reuse of biological wastes (Figure 1).

2. Basic Principle and Working Modes of TENG

TENG was invented for harvesting mechanical energy on the base of the coupling of friction electrification and electrostatic induction ^[48][49]. In the triboelectric series, the relative position of two triboelectric layers determines the polarity of their surface electrostatic charges. TENG is divided into four different types according to the operation mode. There are four different modes: (1) vertical-contact separation mode, (2) lateral sliding mode, (3) single-electrode mode, and (4) freestanding triboelectric layer mode. (Figure 2) The electrodes and triboelectric layers of the four modes are arranged differently. The contact and separation of the triboelectric layer will change to generate electrostatic surface charges ^[50]. The first mode of TENG development is vertical contact separation (Figure 2a). It consists of two different materials with different triboelectric surface potentials. The triboelectric layers come in touch with one another when an external mechanical force is applied, and this causes the triboelectric layer to produce a surface charge. Potential differences occur due to the separation of the two triboelectric surfaces when the mechanical force is released. Electrostatic induction makes an electric charge on the metal electrode attached to the triboelectric layer’s outer surface. To shield the potential difference created by the free charge carrier, the carrier passes over the electrode, producing a pulse of current ^[50][51]. The harvester is positioned on the triboelectric layer’s outer surface, and it is made up of two triboelectric layers that are vertically separated. This model has been investigated the most because of its straightforward structure ^[52][53]. The second is the lateral sliding mode (Figure 2b), which is due to the relative sliding of two triboelectric layers and generates triboelectric surface charges. Sliding can be caused by various mechanical motions, including disk and cylinder rotation, plane motion, and more. Different grating structures have been created for transverse sliding mode TENG with outstanding energy harvesting capability. TENG can also produce energy during stretching and be employed successfully in this mode ^[54][55]. The third is a single electrode mode consisting of a metal or a triboelectric layer grounded through a load resistor (Figure 2c). This model is independent and unrestricted, where any object in contact with triboelectric materials, including human hands can generate triboelectric surface charges. Besides, the two triboelectric layers can be separated without the use of a spacer. Thanks to the single dielectric layer and electrode-only structure, the deformable or self-repairing TENG is simple to realize ^[56]. The fourth mode is the freestanding triboelectric layer mode, which is composed of freestanding dielectric layers and symmetrical electrodes (Figure 2d). Currents are caused by the contact and subsequent separation of the separate triboelectric layer from the electrode, which results in a potential difference between the two electrodes. The independent TENG can also be divided into contact separation type and sliding type according to the contact mode of the triboelectric layers. Each mode of TENG has its specific benefits and uses ^[57][58].

3. Current Progress in BD-TENGs

3.1. BD-TENGs Based Plant Polymers

Recently, the fabrication of TENG has been combined with many plant-based BDPs for environmental protection and biodegradability. Currently, six types of plant degradable materials are used in TENG: paper-based cellulose, bacterial cellulose, leaves, wood, rice paper, plant straw and alginate, etc. ^[59][60]. Cellulose is one of the most common and abundant macromolecular polysaccharides in nature, accounting for more than half of the carbon in the plant kingdom. In plants, it is usually combined with hemicellulose, pectin, and lignin. Cellulose is one of the bottom materials in the triboelectric series, which has better electron loss capacity. Although these materials deteriorate in the presence of an acid, alkali, and specialized enzymes, the rate of deterioration depends on the material’s particular structure, and the specifically altered molecules can provide cellulose with improved physical and chemical characteristics, as well as greater capacity to transmit electrons ^[61][62]. For instance, alginate is a byproduct of iodine and mannitol extraction from kelp or Sargassum of brown algae ^[63], which is typically mixed with other components to generate a composite membrane by cross-linking. Furthermore, alginate materials are polysaccharide derivatives with good biocompatible and simple degradation; the presence of ions makes it easy to dissolve in water or other solutions. These features allow TENG’s manageable degeneration. In addition to the above, rice paper is also an excellent plant-based biodegradable material commonly utilized in TENG, one which can be easily degraded under natural conditions ^[64]. Starch, a popular edible polysaccharide derived from rice, corn, and other crops, is also the main raw material of BD-TENG. The main components of leaves are cellulose, a small amount of polysaccharide, and protein ^[65].

3.1.1. BD-TENGs Based on Paper Cellulose and Starch

Paper-based cellulose is an excellent candidate material for environmentally friendly BD-TENG production because of its good biocompatibility, biodegradability, and recyclability. Biocompatibility is critical for implantation of wearable TENG to prevent rejection, pain, skin irritation, or inflammation ^[12][61]. Since TENG is biodegradable, it can be dissolved in the human body or in the environment with fewer side effects ^[66]. In addition, cellulose nanofibers (CNF) have special properties of transparency and high mechanical strength after special treatment, which provides a variety of possibilities for the design of BD-TENG ^[67]. Up to date, numerous cellulose paper-based TENGs have been produced on the basis of the material’s outstanding flexibility, portability, biodegradability, and reusability, where cellulose paper can be used as an electrode, triboelectric layer, substrate, or perhaps all three materials of TENG at the same time. By modifying cellulose paper, for example, increasing surface roughness and introducing functional groups, the output performance of paper based TENG can be significantly improved. By integrating cellulose paper-based TENG with other types of energy collectors (such as electromagnetic generators and solar panels), the hybrid nano generators can simultaneously collect multiple types of energy. In practical applications, cellulose paper-based TENG can be used as a sustainable power source for self-powered anti-corrosion and antifouling, self-powered electrochemical reaction and droplet controllable operation. It can also be used as an active sensor for pressure sensing, motion monitoring, sound recording and human-computer interaction. For example, Chen et al. developed a paper-based TENG (PTENG) using widely available cellulose derivatives ^[68]. The substrate of PTENG is made of printing paper, while the triboelectric layer of PTENG is made of crepe cellulose paper (CCP) and nitrocellulose membrane (NCM) (Figure 3a). This simple and economical method is essential to fabricate environmentally friendly electrical equipment. Wu et al. used a simple and scalable manufacturing process to obtain a PTENG with low cost and high mechanical energy harvesting capability (Figure 3b) ^[40]. These PTENGs are of great significance for developing green and portable energy-harvesting equipment. Starch is a widely obtained simple, renewable, eatable, and easily-degrading polymer. Its significant presence of amorphous regions in the form of films and a large number of -OH groups can provide a suitable matrix for cation and ion dissolution, resulting in enhanced dielectric properties in TENGs. Recently, rice paper has been considered as a viable component for constructing biodegradable substrates because it is inexpensive and generally commercially available, compared to other alternatives (such as polymers). Developing high-performance TENG using recyclable rice paper’s has been widely researched. For example, Chi et al. suggested a new BD-TENG that operates on a single electrode ^[64]. As shown in Figure 3d, Rice paper and transparent conductive ink, both biodegradable materials, are employed as triboelectric materials and conductive electrodes, respectively.

3.1.2. BD-TENGs Based on Micro/Nano Cellulose

Microcrystalline cellulose (MCC), cellulose nanofibers (CNF), and cellulose nanocrystals (CNC) are all types of micro/nano cellulose. The functional materials derived from cellulose have outstanding mechanical characteristics, perfect micro/nano surface roughness, and are lightweight ^[72]. For instance, Wang et al. developed a water-soluble TENG (WS-TENG) utilizing biodegradable recycled paper and a water-soluble graphite electrode ^[69]. CNC and methylcellulose (MC) are combined to form CNC/MC films, which are utilized as positive triboelectric materials, and regular MC films are as negative triboelectric materials (Figure 3c). This low-cost, lightweight, BD-WS-TENG can be directly washed away by water and developed as a bandage sensor.

3.1.3. BD-TENGs Based on Crop Wastes and Edible Materials

As a renewable and readily biodegradable environmental protection material, plant straw shows excellent potential in replacing the traditional triboelectric layer. Wheat straw is commonly utilized as natural green manure because of its excellent degradability. For instance, Ma et al. structured a wheat straw natural WS-TENG that can switch from mechanical energy to electrical energy (Figure 3e) ^[70]. Similarly, peanut shell is a common crop waste generally treated as garbage, and occasionally simply burned in production. As a biodegradable material, however, it has certain advantages in developing BD-TENG due to its rich cellulose and lignin (Figure 3f) ^[71]. Meanwhile, using edible food materials to develop BD-TENG is also a meaningful research direction. For example, Khandelwal et al. designed a light edible TENG (E-TENG) ^[73]. The active layer is laver covered with edible silver leaves, the base is rice flakes, and the electrode is edible silver leaves (usually used for candy and oriental food) (Figure 4a). It can be quickly digested in phosphate-buffered saline (PBS) and gastric juice. It is expected to promote the development of edible energy and other electronic equipment such as edible and digestible functional equipment in the future.

3.1.4. BD-TENGs Based on Leaves

Building an intelligent interface between plants and the environment is important for real-time monitoring of plant health while harvesting energy. Harvesting wind energy and raindrops in the environment through plants can be used for various sensors, making agricultural automatic monitoring possible. The leaf is a naturally degradable material utilized in the manufacturing of BD-TENG. For example, Wu et al. reported a green TENG with blade cuticle and internal conductive tissue as the friction material and electrode, and water droplets as the corresponding substance (Figure 4b) ^[74]. This technology can be combined with urban greening, family gardens, woodlands, and islands for energy harvesting and sensing.

3.1.5. BD-TENGs Based on Bacterial Cellulose

Bacterial cellulose is a porous reticular nanoscale biopolymer synthesized by microbial fermentation, and it is also a polymer with good degradation performance. It has unique biological affinity, biocompatibility, biodegradability, biological adaptability, and no allergic reaction. Therefore, bacterial cellulose shows excellent potential in biodegradable and environmentally friendly triboelectric active materials. For example, Kim et al. were the first to develop a bio-TENG based on bacterial nano-cellulose ^[75]. The triboelectric layer was made of a transparent, bendable bacterial nano-cellulose sheet. As a positive triboelectric material, the strong electrical feeding capabilities of copper film allows the bacterial nano-cellulose film to generate 4.8 mW m⁻² of output power (Figure 4c). Zhang et al. built an environmentally friendly and renewable all-cellulose energy harvesting and interaction equipment using bacterial cellulose (BC) and conductive BC ^[79]. Under the condition of the cellulolytic enzyme, the active substance can be completely degraded within 8 h.

3.1.6. BD-TENGs Based on Lignocellulose

Lignocellulose is an organic flocculent fiber material made from naturally regenerated wood through chemical treatment and mechanical processing ^[80]. For self-powered equipment such as TNEGs for environmental monitoring and medical rehabilitation, biodegradable environmental protection materials are frequently employed. For example, An et al. developed a BD-TENG based on nanofibers (NF) by using an industrially scalable solution blow-molding process to prepare NF from solutions containing soybean protein and lignin. This TENG based on biopolymers has excellent efficiency in biomedical applications, which makes it a piece of sustainable and environmentally friendly self-powered equipment (Figure 4d) ^[76]. In addition to being a positive triboelectric material for TENG, lignocellulose has also been developed as a negative triboelectric by adding strong electron absorption groups through modification. For instance, Bang et al. explored an all-wood green TENG ^[77]. Flexible porous wood triboelectric layers were utilized as positive and negative materials to increase the triboelectric output of TENG. The positive triboelectric layer was treated with trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane. In contrast, the negative triboelectric layer was treated with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (Figure 4e). The output voltage of chemically treated TENG was about 20.5 times than that of solemn TENG. Similarly, Yao et al. proposed a straightforward technique, including pretreatment and post-modification to produce flexible wood triboelectric layer materials with excellent mechanical properties ^[78]. They attached the positively charged quaternary ammonium group using the cationic modifier 3-chloro-2-hydroxypropyl trimethylammonium chloride (CHPTAC) to the free hydroxyl group on the cellulose backbone. The modified veneer displayed a high surface potential, demonstrating a significant energy difference between the positive and negative triboelectric layers (Figure 4f).

3.1.7. BD-TENGs Based on Alginate and Agar

Sodium alginate is a by-product of extracting iodine and mannitol from kelp or sargassum of brown algae. It is a polysaccharide with good biodegradability and biocompatibility, and also a promising candidate material for BD-TENG ^[63]. Carrageenan (ϰC) and agar are natural, low-cost edible polysaccharides extracted from red algae and are also used to develop BD-TENG. Kang et al. developed a natural derivative-Carrageenan agar (ϰC-Agar) composite using as high-performance triboelectric material for BD-TENG ^[81].

3.1.8. Potential Applications of Plant-Based BD-TENGs

Plant-Based BD-TENGs for Energy Harvesting

Since the first invention of TENG, obtaining energy from the environment and organisms has always been the most important goal of TENG. The emergence of BD-TENG has brought dawn for providing new environmental protection and degradable energy. Yang et al. built a human-computer interaction and environmentally friendly energy collection system. Cellulose filter paper was used as the triboelectric layer ^[82]. The unique three-dimensional porous network structure and high crystallinity of cellulose filter paper enable the TENG to have good output performance. Their degradation experiments showed that the prepared CFP-based TENG dispersed and dissolved the composite membrane materials in ultrapure water within 30 min through ultrasonic treatment, which could fully demonstrate its environmentally friendly nature. (Figure 5a). They also deeply investigated the output characteristics of TENGs based on CFP, with a maximum output voltage of 192 V and output current of 9.3 μA. The output power was 736.7 mW m⁻². The TENG based on CFP can be conveniently used to power commercial electronic products and control computer programs as a wearable interface. This green TENG is of great significance for realizing environmental protection of electronic equipment, promoting energy conservation and emission reduction, and realizing carbon neutralization. Cellulose paper can be used as an electrode along with a substrate and triboelectric layers. Shi et al. created portable and long-lasting SCPs by combining paper-based TENG with paper-based SC ^[83]. Cellulose paper showed excellent conductivity after coating with polypyrrole (PPy), The cellulose paper/PPy composite material was then employed as an electrode, cellulose paper was used as the positive triboelectric layer, and nitrocellulose membrane NCM as the negative triboelectric layer (Figure 5b). P-TENG exhibited good triboelectric characteristics (with a V_oc of 60 V and a power density of 0.83 W m⁻²). In addition to this, the technology was also developed for applications in self-powered devices. As a flexible, ultralight, and renewable power source, the created SCP can be used to power various electronic devices, demonstrating its potential in flexible and compact green technologies. This research provides essential inspiration for developing degradable TENG made of cellulose paper. Considering portability, Wu et al. have demonstrated a cutting P-TENG to collect mechanical energy ^[40]. P-TENG has the advantages of excellent flexibility, lightweight, high conductivity and low cost, and can work normally after being cut into any shape. It can obtain mechanical energy, body movement energy, sound energy and wind energy. By measuring the effectiveness of electronic transmission, the TENG can distinguish many materials, including glass, cotton and wood (Figure 5c). CNF is a renewable, biodegradable and rich material, and Yao et al. have studied its application in TENG structures, ^[84]. The flexible transparent CNF film with its nano-structure was used as positive triboelectric material (Figure 5d). When it is trampled by normal people, it can generate up to ~30 V and ~90 μA current to illuminate the 35 LEDs integrated with a power fiberboard which has strong recyclability and excellent mechanical integrity. When it is trampled by normal people, it will generate up to ~30 V and ~90 μA current to illuminate the 35 LEDs integrated with the power fiberboard. This proves the great potential of producing friction electric Bakelite/fiberboard or flooring on a wide range of eco-friendly scales. It can be predicted that CNF and other woods will play an important role in the dynamic development of flooring, packaging and auxiliary infrastructure, as well as the unique ability to efficiently obtain mechanical energy from the environment. Ma et al. has structured a wheat straw natural WS-TENG that can switch from mechanical energy to electrical energy ^[70]. WS-TENG performs well in terms of output and it can charge not just capacitors but also several portable electronic devices such as hygrometers and electronic watches (Figure 6a). More intriguingly, the researcher built WS-TENG into a windmill and bionic lawn, which can be used as a wind speed sensor. This BD-TENG offers a novel answer for advanced science and technology employed in agricultural production and self-powered sensors used for energy harvesting and application. Jie et al. presented a green and environmentally friendly innovative blade-assembled Leaf-TENG that can efficiently harvest environmentally and mechanical energy using natural leaves ^[85]. Under optimal circumstances, the peak values of V_oc, I_sc, and maximum output power of natural Leaf-TENG in single electrode mode can reach around 230 V, 9.5 µA, and 45 mW m⁻² (Figure 6b). Natural Leaf-TENG has been indicated to be competent for powering LEDs and charging capacitors, indicating that it has excellent mechanical energy harvesting capability. The natural Leaf-TENG was constructed autonomously into a tree-shaped energy harvester to gather mechanical energy from the environment over a vast region. Similarly, Feng et al. constructed a TENG using biodegradable natural plant leaves and dry leaf powder to harvest wind energy and obtained high V_oc (430, 560 V) and I_sc (15, 25 µA) accordingly ^[86]. To tackle the problem of delicate contact and make maximum use of the leaves, dry leaves are ground into powder (Figure 6c). Poly-L-lysine is utilized to optimize the outputting capability of TENG. Because fresh leaves contain complimentary water, which lowers the electronic generation of natural plant leaves, the performance of dry powder leaf based TENG is better than that of plant leaves. As is shown, the TENG tree consists of both living and artificial foliage, which has promising application potential in distant areas and can be utilized for early warning and indicator lighting. Consequently, plentiful biodegradable materials can be employed to develop future energy. BD-TENG has also been reported in the application of blue energy; for example, Pang et al. designed a BD-TENG using alginate membrane ^[63]. It can be used to collect water wave energy. This biodegradable and ecologically responsible TENG based on marine plants have broadened the application field of blue energy.

Plant-Based BD-TENGs for Self-Powered Sensing System

TENG can not only be used as an energy harvesting device, but also an excellent sensor in mechanical sensing due to its high sensitivity and short response time. From the perspective of active sensors, TENG can convert mechanical movement into changing electrical signals; the information from a trigger source (amplitude, frequency, position, trajectory, etc.) can be retrieved by analyzing these output signals (voltage, current, frequency, etc.). Environmentally friendly, biocompatible and biodegradable materials also make TENG more unique in the application of transient electronic equipment. Zhang et al. designed a page marking and anti-theft sensor using commercial printing paper ^[51]. The printing paper was used as the substrate of the TENG, and then the film made of PET and ITO was placed on the friction surface. This self-powered anti-theft sensor can be inserted into a book. When it contacts the book, the I_sc increases from 30 nA to 300 nA with the increase of the grid. When someone gently touches the book, the TENG output voltage of the TENG can light the LED alarm bulb as a warning. More interestingly, TENG can identify matching pages according to different signal amplitudes, accurately determine the number of pages in the book, and record the number of pages turned. These studies are a good starting point for creating simple, portable, flexible and green paper made equipment, as well as the progress of self-powered paper made equipment. Stephan et al. has developed a CFP-based TENG by dip coating ^[87]. The CFP-based TENG displayed the V_oc and current density of ≈42 V and ≈1 µA cm⁻², and a power density of ≈25 µW cm⁻². It can light 40 LED bulbs and charge 0.22 µF capacitors to 8 V within 5 s. The TENG can detect simple human movements, finger tapping, finger friction, and trampling. These results prove that cellulose paper can be employed as the base material for manufacturing superior-behavior BD-TENG. With the development of intelligent electronic devices, human-machine interaction (HMI) technology plays an increasingly important role in daily life. TENG based on cellulose paper can also be employed as an active sensor for HMI because its electrical signal can be utilized as the input signal for operating the machine. Chen et al. reported that a V_oc of paper-based PTEN was 196.8 V, I_sc was 31.5 µA, power density was 16.1 W m⁻², and duration exceeded 10,000 times ^[68]. This PTENG was used as a keyboard for designing and manufacturing paper pianos to achieve automatic human-computer interaction. PTENG can detect human movement, such as finger contact. PTENG array keyboard can realize self-powered real-time interaction with computer (Figure 7a). These studies show the potential of using widely available low-cost cellulose fiber derivatives to produce high-performance paper-based TENG. They also show great potential in HMI, which is critical to creating green and sustainable devices. Real-time monitoring of human activities and vital signs is critical for health diagnosis and treatment. He et al. presented a cellulose fiber-based self-powered CF-TENG by converting one-dimensional environmentally friendly CMFs/CNFs into two-dimensional CMFs/CNFs/Ag hierarchical nanostructures ^[88]. The CF-TENG system exhibited an efficiency of 98.83% in removing PM2.5 and can monitor respiratory health without an external power source because of its perfect porous nanostructure and extraordinary power-generating properties (Figure 7b). Recently, Wang et al. developed a BD-TENG to monitor human physiological signals using CNC and MC ^[69]. This low-cost, light weight, biodegradable WS-TENG bandage sensor can be directly rinsed with water. According to the change of output voltage, it can accurately distinguish various respiratory states (Figure 7c). This kind of real-time physiological signal monitoring sensor further expands the application of TENG in the medical monitoring field. Zhu et al. has demonstrated a piece of disposable TENG equipment using environmentally friendly biodegradable starch paper ^[65]. The materials utilized are both inexpensive and widely available. S-TENG can be made with a simple procedure that combines starch paper and metal wire. This TENG can function as a self-powering human sweat sensor, indicating its potential application in wearable electronic products. To create a flexible, robust, and high-performance TENG for self-powering sensing in motion big data analysis, Luo et al. used a straightforward and efficient two-step method to prepare wood films with outstanding physical and triboelectric characteristics as the triboelectric materials ^[89]. Adaptable and long-lasting W-TENG can produce a charge density of 36 C m⁻², more than 70% greater than natural wood-based TENG (Figure 7d). W-TENG was also employed as an active sensor for intelligent table tennis tables with a speed-sensing system for motion tracking. Recently, Wang et al. produced a flexible, biodegradable, and flame-resistant FR-TENG using biodegradable black phosphorus and phytic acid as flame retardants. Tannic acid-modified BPNS (TA-BPNS), and PA were added to CNF as non-toxic synergistic flame retardants as the triboelectric layer, and AgNWs were used as the conductive layer. The FR-TENG can be utilized for temperature sensing and as a fire alarm ^[90]. As a wearable micro/nano power source, FR-TENG can also harness the mechanical energy of human movement to power small electronic gadgets. Being a humidity sensor, the FR-TENG’s output voltage responds linearly to the surrounding humidity. In brief, FR-TENG has a wide range of potential applications in temperature monitoring, fire alarms, and multifunctional wearable electronic devices.

Plant-Based BD-TENGs for Implantable Medical Devices

Implantable electronic devices for medical care and health monitoring have advanced quickly. TENG-based biodegradable power supplies have advantages over batteries in preventing issues like secondary medical surgery. For instance, based on biocompatible medical 317L stainless steel (317L SS) plate and ethyl cellulose (EC) membrane, Wang et al. proposed a healthcare TENG. TENG’s I_sc and V_oc can reach 245 V and 50 µA under ideal circumstances ^[91]. After being submerged in simulated bodily fluid for a period of time, the capabilities of the device had not altered appreciably, indicating that it was well-suited for use as a biomedical power source and had good biocompatibility. This shows that cellulose-based BD-TENG has a bright prospect in the application of implantable medical devices.