The principles and recent advancements in self-powered wearable fiber sensors

Abstract

Wearable sensors are widely used in sports monitoring, health care and daily life health management due to their real-time feedback of personal health status and physiological data. However, traditional wearable sensors require a battery to provide continuous power to the device. Therefore, additional line connections, limited energy supply, and frequent battery replacement hinder the practical application of wearable sensors. Self-powered sensors present an effective solution to these challenges by harvesting energy from the environment to power the device. Among them, fibers and textiles are the preferred choice for wearable sensors because they are light weight, breathable, braided, stretchable, and suitable for everyday activities. This review explores the operational principles of wearable self-powered fiber sensors, detailing the mechanisms of moist-electric, triboelectric, piezoelectric, thermoelectric, and photovoltaic sensors. It also introduces research progress in healthcare monitoring, disease diagnosis, and human–machine interaction, focusing on materials and effects. Finally, the review discusses the challenges and prospects in the field of wearable self-powered fiber sensor research. The potential for these sensors to operate without external power sources opens new possibilities for the future of wearable devices, promoting sustainability and reducing the environmental impact associated with battery usage.

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Ying Chen

Ying Chen obtained her bachelor's degree at the School of Chemical Engineering, Changchun University of Technology in 2021. Her is now pursuing her PhD degree under the guidance of Prof. Guanghui Gao at the School of Chemical Engineering, Changchun University of Technology. Her current research interests are the structure design and preparation of hydrogel fibers and their applications in self-powered sensors.

Tianyu Wang

Tianyu Wang obtained his BS degree at the School of Materials Science and Engineering, Jilin Jianzhu University in 2019. He is now pursuing her PhD degree under the guidance of Prof. Guanghui Gao at the School of Chemical Engineering, Changchun University of Technology. His scientific interests are focused on structure design and preparation of gel and electrospinning nanofiber membrane materials.

Guanghui Gao

Guanghui Gao is a professor of the School of Chemical Engineering and Advanced Institute of Materials Science of Changchun University of Technology. He obtained his bachelor's degree from Shanghai Jiaotong University and PhD from Sungkyunkwan University of Korea. He built the Polymeric and Soft Materials Laboratory at Changchun University of Technology. His research interests include polymer hydrogel and fiber in the field of biomedicine.

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1. Introduction

With the rapid development of modern society and the continuous progress of science and technology, people are paying more and more attention to health and disease prevention. Biological signals, such as variations in skin surface strain, chest and throat vibrations, limb movements, and respiration patterns, offer valuable insights for medical monitoring, disease diagnosis, and human–machine interfaces.^1–11 These signals not only reflect the internal physiological activities of the human body but also reveal the influence of the external environment on it, constituting an important indicator for evaluating health status. Therefore, developing technologies that can accurately collect and analyse these biological signals is essential for proactive healthcare management and personalized well-being.

Traditional sensor devices based on metals or semiconductors can only detect deformations of less than 5%, and are frequently not comfortable to wear due to their inherent rigidity.^12–15 In contrast, flexible wearable sensors are increasingly significant for collecting personal health and physiological data. These sensors are typically light weight and comfortable, capable of continuously monitoring physiological parameters without disrupting daily routines.^16–18 However, most wearable devices rely on traditional batteries, which need regular replacement. This not only increases maintenance costs but also risks data loss due to battery depletion, particularly in long-term monitoring applications. Additionally, discarded batteries pose an environmental threat.¹⁹

To address these limitations, researchers have been exploring self-powered technologies. Self-powered sensors are innovative systems capable of generating the electrical energy needed for their operation without relying on an external power source. These sensors harness energy from the environment, such as mechanical, thermal, light, and moisture, converting it into electrical power to sustain their functions. These sensors find applications in diverse fields including the Internet of Things (IoT), wearable devices, environmental monitoring, and industrial automation. By eliminating the need for batteries or external power supplies, self-powered sensors enhance equipment lifespan, reduce maintenance costs, and boost the reliability and sustainability of systems. Based on the sources of energy they collect and their power generation methods, self-powered sensors can be categorized as follows:

• Piezoelectric self-powered sensors: these sensors utilize piezoelectric materials to convert mechanical energy into electrical energy. They are characterized by high sensitivity and fast response times, making them suitable for low-frequency vibration monitoring applications, such as gait analysis and structural health monitoring of bridges.^20,21

• Thermoelectric self-powered sensors: these sensors convert temperature gradients into electrical energy using thermoelectric materials. They offer high energy density and are ideal for monitoring temperature changes in industrial equipment and human body temperature. However, their effectiveness requires a significant temperature difference.^11,22

• Photovoltaic self-powered sensors: utilizing photovoltaic materials, these sensors convert light energy into electrical energy. They are environmentally friendly and have high energy density, making them suitable for outdoor applications like agricultural greenhouse monitoring and meteorological stations. Their performance, however, is dependent on lighting conditions.^23,24

• Triboelectric self-powered sensors: these sensors harness the triboelectric effect to convert mechanical energy into electrical energy. They are effective for monitoring low-frequency vibrations and human movements, such as in smart bracelets and insoles. Despite their utility, they have lower energy density and are susceptible to contamination.^25,26

• Moist-electric self-powered sensors: these sensors exploit the moisture absorption properties of materials to convert changes in environmental humidity into electrical energy. They are highly sensitive and have quick response times, making them suitable for applications in meteorological monitoring, agricultural greenhouse control, industrial humidity management, and smart home environments.^27,28

These energy collection mechanisms provide a sustainable energy solution for wearable devices, reducing reliance on traditional batteries and enhancing the convenience and environmental friendliness of the devices.

With the ongoing advancements in self-powered technology, wearable devices are becoming more intelligent and multifunctional. These devices will not only monitor basic physiological parameters such as heart rate, blood pressure, and body temperature but also predict diseases by analyzing complex biological signals. For instance, continuous monitoring of electrocardiogram (ECG) signals can detect early symptoms of heart diseases, enabling timely preventive measures.²⁹ In the realm of sports and fitness, self-powered wearable devices show immense potential. Athletes can utilize these devices to acquire real-time sports data, optimize their training schedules, and enhance their performance. Additionally, they can monitor physiological changes during exercise, helping athletes avoid overtraining and injuries.^30,31 Beyond the medical and sports fields, the applications of self-powered wearable devices are becoming increasingly widespread in daily life. For example, smart clothing can monitor the wearer's health through built-in sensors and send alerts when abnormalities are detected.^22,32–34 Smartwatches and masks can record steps and sleep quality, and offer comprehensive health management services by connecting with smartphones.^35–38 In conclusion, such devices not only improve people's quality of life, but also provide powerful support for personalized health management and disease prevention.

Among self-powered technologies, fiber-based sensors have gained special attention due to their unique advantages.^39–42 Fibers are light weight, breathable, weavable, and stretchable, allowing them to adapt to various mechanical deformations of the human body during daily activities. These characteristics make fiber-based sensors highly promising for wearable devices, particularly in applications requiring long-term wear and comfort.^43–45 Recent years have witnessed considerable advancements in self-powered fiber sensors, including piezoelectric, triboelectric, thermoelectric, photovoltaic, and moist-electric effects. The broader field of sensors, as well as the narrower field of self-powered sensors, has seen a rapid increase in published articles. Fig. 1a shows that the number of articles on sensors and self-powered sensors has increased by a factor of two and seven, respectively, from 2014 to 2023. Additionally, our search for the annual number of relevant articles on fibers and fiber sensors from 2014 to 2023, as shown in Fig. 1b, indicates that fiber sensors are receiving increasing attention.

Fig. 1 (a) The aggregate quantity of relevant material produced from 2014 to 2023, obtained by searching for “sensors” and “sensors” in conjunction with “self-powered”. (b) The annual volume of relevant publications from 2013 to 2014, focusing on the keywords “fiber” and “fiber sensors.” Data source: web of science.

The purpose of this review is to explore the working principles of self-powered wearable fiber sensors, assess their effectiveness in monitoring biological signals, and highlight the latest advancements in healthcare, disease diagnosis, and human–machine interfaces. This review will specifically focus on piezoelectric, triboelectric, thermoelectric, photovoltaic, and hydroelectric sensors, which have garnered significant attention for their innovative energy harvesting and signal detection capabilities. A thorough analysis will be conducted on the design, material selection, manufacturing processes, and practical applications of these sensors. By providing a comprehensive perspective, this review aims to support researchers and developers in advancing the development and application of self-powered wearable technology.

2. Mechanism of self-powered wearable fiber sensors

2.1. Moist-electric generators

Water is a natural and valuable resource, abundantly found in glaciers, rivers, lakes, oceans, and soils. It constantly interacts with moist air through processes such as evaporation, condensation, transpiration, and respiration (Fig. 2a). Covering approximately 71% of the Earth's surface, water absorbs more than 35% of solar energy and holds a significant amount of energy within the water cycle.^46,47

Fig. 2 (a) The water cycle on Earth. (b) Phase diagram for water. (c) Transfer of charge from the environment to metallic surfaces. The generation of positive charge on a basic oxide. The formation of a negative charge on an acidic oxide.⁴⁸ Reproduced with permission from the American Chemical Society, copyright 2010. (d) Illustration of proton transport via hydrogen bonds in water wires within liquid water demonstrating the dynamic equilibrium between the establishment and disruption of hydrogen bonds.⁴⁹ Reproduced with permission from Wiley, 2020. (e) The hydrogen bonds existing in the water dimer (at the top) and the water trimer (at the bottom).⁴⁹ Reproduced with permission from Wiley, copyright 2020.

A water molecule comprises one oxygen atom and two hydrogen atoms. The oxygen atom, possessing an electronegativity of 3.5, forms a covalent bond with hydrogen, which has an electronegativity of 2.1, transferring electrons from hydrogen to oxygen. This produces a negatively charged oxygen end and a positively charged hydrogen end. The bond angle between these atoms is 104.5°, creating an uneven charge distribution and significant external polarity. The dipole moment of a water molecule, which represents its polarity, is calculated as the product of the distance between the centers of positive and negative charges and their respective electric charges, approximately 3.336 × 10³⁰ Cm.^50,51 When water molecules approach each other, the positively charged hydrogen atom forms a hydrogen bond with the negatively charged oxygen atom of another molecule. The bond energy of these hydrogen bonds is around 0.16 eV, which is less than the O–H bond energy within individual H₂O molecules (4.76 eV) but greater than van der Waals forces (0.08 eV).⁵² Hydrogen bonds play a crucial role in the phase transitions of water, and these bonds are disrupted upon heating and vaporization. In natural environments, liquid water absorbs heat from its surroundings and vaporizes because the vapor pressure of water in the air is lower than its saturated vapor pressure (Fig. 2b).

As early as 400 BC, humans constructed waterwheels to utilize the kinetic energy of water flow for agricultural and industrial production. By the late 19th century, hydraulic power generation technology utilized water flow to drive electromagnetic generators, converting mechanical energy into electrical energy. Since the 20th century, scientists such as Volta, Faraday, Kelvin, and Lenard have demonstrated that water exhibits electric properties during phase transitions in both natural environments and under human influence.⁵³ In 2007, Fernando et al. used a Kelvin electrostatic voltmeter to measure charge enrichment in cellulose paper under high humidity and proposed a charge transfer mechanism based on electrostatic potential influences.⁵⁴ Similar charge accumulation has been observed in hydrophilic particles (e.g., SiO₂, AlPO₄), metals (e.g., Al, Cu), and polyethylene at elevated humidity levels.^48,55,56 Water molecules in the atmosphere adsorb onto specific solid surfaces, generating H⁺ and OH⁻ ions through transfer processes, as depicted in the electrostatic potential diagram in Fig. 2c. In liquid water, every molecule establishes a minimum of one hydrogen bond, while ions such as H₃O⁺ and OH⁻ exist at low densities.⁴⁸ Proton transfer occurs through hydrogen bond networks within liquid water and gas-phase hydrogen-bonded water bridges (Fig. 2d).⁴⁹

The generation mechanisms of moist-electric generators, based on the source of transferred charge in the water–solid interaction process, can be classified into two types: ion gradient diffusion and streaming potential.^57,58 Generators that rely on ion gradient diffusion primarily depend on creating a gradient of water or functional groups within the material. In contrast, those based on the streaming potential effect primarily depend on solid–liquid interface interactions. In some cases, both mechanisms may coexist.^59,60 Recently, researchers have proposed novel working mechanisms for these generators, such as active electrode adsorption, redox reactions, and ionic hydration energy.^61–65

2.1.1. Ion gradient diffusion. A moist-electric generator based on ion gradient diffusion consists of a pair of electrodes and a hydrophilic active layer rich in oxygen-containing groups, such as –OH, –COOH, and –SO₃H. When the material absorbs water molecules, it disrupts polar chemical bonds and dissociates functional groups, creating a mobile carrier concentration gradient, like Na⁺ and H⁺. This process drives ion diffusion and migration, causing a potential difference between the electrodes, which produces an output voltage. The key to ion diffusion and migration is the concentration gradient of ions formed by the uneven distribution of hydrophilic groups or asymmetric water absorption. This process depends on both relative humidity and the gradient of functional groups on the material's surface. Therefore, establishing a concentration gradient of oxygen-containing functional groups and creating a relative humidity gradient are two primary methods for generating an ion concentration gradient.

The linear moist-electric generator with a one-dimensional structure can enhance the interaction with water molecules due to its elevated aspect ratio and extensive specific surface area, thereby improving its wet electric performance. As illustrated in Fig. 3a and b, moist-electric generation can be achieved by creating ion concentration gradients in the radial or axial direction of one-dimensional fibers based on the ion gradient diffusion mechanism.^66,67 The one-dimensional fiber-based moist-electric generator with a skin-core structure is typically produced in a continuous manner using core-spun technology and coaxial wet spinning. Graphene oxide (GO) easily assembles into a macroscopic fibrous structure and exhibits strong interaction with water vapor. Shao et al. utilized a simple dip coating technique to coat the GO layer onto a silver wire as the inner electrode (Fig. 3c).⁵⁷ After drying, another silver wire was wound on its surface as the outer electrode to form the fiber-hybrid electric generator (FHEG). When humidity increases, the numerous highly oriented microporous channels in graphene oxide nanosheets within the electric generator rapidly absorb water, release positively charged hydrogen ions (H⁺), and generate an axial concentration gradient diffusion that leads to voltage generation. Conversely, when humidity decreases, the GO layer releases water and hydrogen ions recombine into it, resulting in decreased voltage output. In this manner, FHEG can convert environmental humidity changes into electrical energy that can be utilized for powering wearable electronics through a simple series/parallel connection.

Fig. 3 (a) A schematic illustration of the radial ion gradient diffusion principle for a moisture-driven generator based on a one-dimensional core–shell structure yarn. (b) A schematic illustration of the axial ion gradient diffusion principle for a moisture-driven generator based on a one-dimensional core–shell structure yarn. (c) The fabrication process of the FHEG and the scanning electron microscopy (SEM) images depicting the top and cross-sectional perspectives of the FHEG.⁵⁷ Reproduced from Elsevier, copyright 2018, with permission. (d) Diagram illustrating the development of SS films derived from silkworm cocoons.²⁸ Reproduced with permission from the American Chemical Society, copyright 2024. (e) Cross-section of the SS film by composition.²⁸ Reproduced with permission from the American Chemical Society, copyright 2024. (f) Variations in human respiratory signals.²⁸ Reproduced with permission from the American Chemical Society, copyright 2024. (g) Variations in human motion signals.²⁸ Reproduced with permission from the American Chemical Society, copyright 2024.

A two-dimensional fibric moist-electric generator can be constructed with either a planar asymmetry or symmetrical sandwich structure to establish a vertical ion gradient along the thickness for wet gas power generation.^68,69 As illustrated in Fig. 3d, He and coworkers utilized electrospinning technology to prepare sericin/polythene oxide (SF/PEO) films, introducing a sericin concentration gradient along the thickness through spray grafting.²⁸ As shown in Fig. 3e, fibers with a high sericin content adhere to the SF/PEO fibers in the middle layer. Meanwhile, fibers with a low sericin content attach to the bottom layer, forming a network structure. Sericin's abundant polar side chain amino acids give it remarkable hygroscopicity and moisture retention properties. When exposed to a humid environment, sericin absorbs water molecules, forming hydrogen bonds within the film and trapping the water. These absorbed molecules react with sericin's oxygenating functional groups, generating free protons (H⁺), which establish an ion concentration gradient in the film and facilitate directed proton migration from high to low concentrations. Leveraging SS-MEG's exceptional sensitivity to humidity levels, this device could be integrated into commercial face masks to create intelligent sensors for monitoring human respiration patterns (Fig. 3f). Additionally, it could be designed as a simple electronic bracelet that detects changes in body sweat during exercise, with the electrical signal strength increasing proportionally to movement intensity (Fig. 3g).

2.1.2. Streaming potential. When a solid surface contacts water or other liquids, ionization or dissociation of surface groups creates an electric charge layer on the solid surface. This charge attracts oppositely charged ions in the surrounding liquid, resulting in the formation of an electric double layer (EDL) structure, as shown in Fig. 4a.⁷⁰ The EDL consists of two components: a Stern layer close to the solid surface and a diffusion layer containing mobile counterions. In micron- or nanometre-scale channels, the superposition of EDLs becomes particularly prominent, causing a loosened distribution of counterions within the channel. Under pressure gradient or concentration gradient conditions, these counterions move with the flowing liquid, generating a flow current that accumulates at both ends of the channel, forming a flow potential. The Debye length (D) is a key parameter that describes the characteristic thickness of the EDL. It typically ranges from tens of nanometers and decreases with increasing electrolyte concentration.⁷⁰ When the channel diameter is equal to or smaller than the Debye length, the double layers effectively overlap throughout the entire channel space, a phenomenon known as the Debye screening effect. This effect enables the channels to achieve highly selective ion filtration.

Fig. 4 (a) Schematic representation of the streaming potential mechanism. (b) Schematic illustration of a single FSMEG.⁷¹ Reproduced with permission from Wiley, copyright 2023. (c) Schematic representation of the one-pot synthesis process for the ionic hydrogel.⁷¹ Reproduced with permission from Wiley, copyright 2023. Current response to different (d) weights and (e) bending angles.⁷¹ Reproduced with permission from Wiley, copyright 2023.

Wen et al. developed a fully stretchable hydroelectric generator (FSMEG) with a multilayered structure, consisting of an ionized hydrogel layer and an electrode layer.⁷¹ The unique hygroscopic ionic hydrogel layer was prepared through a straightforward one-pot method utilizing a heat-cooling photopolymerization process, as shown in Fig. 4c. In a humid environment, the FSMEG's ionic hydrogel layer exhibited strong hygroscopic properties, absorbing water from the surroundings (Fig. 4b). This absorption led to the migration of ions. Simultaneously, water molecules evaporated from the hydrogel layer onto the carbon-black-coated cotton knitted fabric layer. The dissociated ions and absorbed water move unidirectionally through the negatively charged nanochannels of the carbon-black-coated cotton knitted fabric, attracting positive ions and creating an electric double layer at the solid–liquid interface. The movement of this double layer due to evaporation generates electricity. Additionally, copper electrodes may react with the hydrogel, releasing copper ions that further enhance the device's performance. With an impressive 400% stretchability and the ability to continuously produce approximately 0.3 V and a significant current of 50 μA, this hydroelectric generator could be seamlessly integrated onto human skin or other curved surfaces. It harnessed water energy generated during physiological activities such as breathing and sweating, enabling self-energy sensing capabilities and applications for wearable electronic devices.

2.1.3. Other mechanisms. Active metal electrodes, such as zinc (Zn) and aluminum (Al), can spontaneously adsorb ionized OH⁻ or H⁺ from water through acid–base sites when moisture contacts the generator, leading to counter-ion accumulation on the electrode surface.^48,72 By combining appropriate electrodes, the released mobile ions from the electrolyte are absorbed by counter-ions on the electrode surface and then diffuse directionally, generating an electrical output. Inspired by capacitors, a moist-electric generator could be created using electrodes with different charges and electrospun nanofiber films filled with electrolytes (Fig. 5a).⁶³ Anionic surfactants like sodium dodecyl benzene sulfonate (SDBS) and cationic surfactants like dodecyl trimethyl ammonium chloride (DTAB) were employed to prepare nanofiber membranes containing electrolytes through electrospinning technology. When moisture contacts the device, the electrospun nanofiber membrane trapped water, releasing free ions such as Na⁺ and R-SO₃⁻ from SDBS. The zinc electrode adsorbed OH⁻ ions at its acidic site, causing H⁺ ions to accumulate on its surface, making it positively charged. This created a potential difference between the differently charged electrodes, similar to an electric double layer in a capacitor, leading to charge separation and current generation. The generator could continuously produce electricity without requiring a gradient, relying instead on charged electrodes for ion adsorption. The capacitor-inspired moisture electricity generator (CMEG) responded to moisture produced by breathing, allowing for the real-time display of breathing signals under different conditions when integrated into a mask. Additionally, this device was sensitive to finger touch, enhancing its applicability in various wearable electronic devices (Fig. 5b).

Fig. 5 (a) Diagram showing a standard electrical double-layer capacitor on the left, and CMEG on the right.⁶³ Reproduced with permission from the Royal Society of Chemistry, copyright 2022. (b) Current output signal of the device due to (i) a human breath, (ii) normal breath, fast breath, and deep breath and (iii) being touched by a finger.⁶³ Reproduced with permission from the Royal Society of Chemistry, copyright 2022.

2.2. Triboelectric generator

Electrostatic charging occurs when charges are generated on the surfaces of two distinct materials upon contact. This phenomenon, extensively utilized since antiquity, is particularly evident with insulating materials that retain transferred charges for extended periods. However, it has also led to numerous adverse effects in industrial manufacturing, transportation, and aviation. Triboelectric nanogenerators (TENG) operate through the combination of triboelectric and electrostatic induction effects.⁷³ When two materials with opposite frictional polarities come into contact and slide relative to each other, equal and opposite frictional charges are generated on their surfaces. Upon separation, induced charges are produced on the back electrodes of both materials, causing electrons to flow in a closed circuit due to the electric potential difference between the electrodes, leading to the formation of a current. The first electrostatic generator was invented by British inventor James Wimshurst between 1880 and 1883. Electrostatic induction is defined as the generation of power when electrons transfer from one electrode to another through an external load to equilibrate their potential differential. In 2012, Wang Zhonglin's research group introduced the triboelectric nanogenerator (TENG), successfully harnessing friction current and efficiently converting mechanical energy into electrical energy.⁷⁴ This breakthrough offers a promising opportunity for capturing and reusing wasted energy.

Triboelectric materials are categorized into a series known as the triboelectric series, depending on their ability to acquire or lose electrons. Commonly used materials in this context include silk, wool, nylon, and organic polymers like PDMS, PVDF, and PTFE.⁷⁵ The phenomenon of frictional electrification is prevalent in many materials we encounter in daily life. As a result, triboelectric nanogenerators (TENG) can be utilized as a source of energy for portable or implantable electronic devices. Additionally, they can function as self-powered sensors, playing a crucial role in the advancement of the future Internet of Things (IoT).⁷⁶

Triboelectric nanogenerators (TENGs) can be categorized into four main primary operational modes according to their structures and principles, as presented in Fig. 6a: vertical contact-separation (CS) mode, single-electrode (SE) mode, lateral-sliding (LS) mode and freestanding triboelectric-layer (FT) mode.⁷⁷ The CS and SE modes have relatively simple structures, making them easy to integrate into wearable devices. These modes are highly sensitive to small mechanical movements and can quickly respond to contact and separation actions, making them ideal for detecting subtle changes in human body movements, such as breathing and pulse. In contrast, the LS and FT modes typically require larger movements or displacements to generate significant electrical signals and are less sensitive to subtle physiological signal changes. Therefore, this review focuses on the applications of the CS and SE modes, as well as their hybrid mode in human sensing.

Fig. 6 (a) Four primary operational modes of TENG: (i) vertical contact-separation mode, (ii) lateral-sliding mode, (iii) single-electrode mode, and (iv) freestanding triboelectric-layer mode.⁷⁷ Reproduced with permission from Wiley, copyright 2019. (b) Diagram showing the structure of the HFSS with helical design.⁷⁸ Reproduced with permission from the American Chemical Society, copyright 2022. (c) Production process for core–shell fibers.⁷⁸ Reproduced with permission from the American Chemical Society, copyright 2022. (d) The voltage signals were recorded during normal, rapid, and deep breathing states, with enlarged curves shown in the insets.⁷⁸ Reproduced with permission from the American Chemical Society, copyright 2022. (e) Schematic diagram of the core–shell CIC@HFP NFs and its application.⁴³ Reproduced with permission from the American Chemical Society, copyright 2024. (f) Generated voltage and signal response of wrist bending, elbow bending, and walking.⁴³ Reproduced with permission from the American Chemical Society, copyright 2024.

2.2.1. Vertical contact-separation (CS) mode. This mode is one of the earliest and most extensively researched methods in this field. When two triboelectric materials come into contact, charge transfer occurs due to their differing electron affinities. As a result, one material acquires a positive charge while the other becomes negatively charged. When these materials separate, the disparity in surface charges generates a static electric field. This field drives free electrons to flow from one pole to another to balance the charges, producing an alternating current pulse output throughout the circuit. This repetitive process of contact and separation facilitates the back-and-forth flow of free electrons between the two poles, generating an alternating current pulse signal.^79,80 Due to its simple structure and adaptability to human movement, the CS mode is highly suitable for collecting small-scale mechanical energy, such as human motion, weak vibrations, or touches. This makes it an ideal choice for applications in wearable technology and human sensing, where capturing subtle and continuous movements is crucial.

Ning et al. used a multi-axis fiber winding machine to encase silver-plated fibers (core fibers) with PTFE or nylon fibers (shell fibers), intertwining them within a fastening sleeve to create a core–shell structured fiber.⁷⁸ The PTFE/Ag braided fiber and nylon/Ag braided fiber were then alternately wound around the base fiber, forming the final helical fiber strain sensor (HFSS). This HFSS excelled in capturing respiratory and cardiac signals.

For example, if a respiratory pause exceeds 6 seconds, the system will automatically contact a designated cell phone for assistance. This underscored its prospective uses in personal health monitoring, intelligent wearable medical devices, and other domains. Zhi et al. developed a one-step electrospinning-assisted self-assembly method to create core–shell structured nanofibers (CIC@HFP NFs) with enhanced β phase and self-aligned nanocrystals.⁴³ Experimental determinations and molecular dynamics investigations disclosed that the hydrogen bond between Cs₂InCl₅(H₂O) and PVDF-HFP prompted automatic dipole alignment and stabilizes the β phase. The TENG based on CIC@HFP NFs and polynon-6,6 NFs demonstrated significantly improved output voltage, output current, and peak power density of 681 V, 53.1 μA and 6.94 W m⁻², respectively. This showcased its energy harvesting capabilities and self-actuated monitoring capacity through human motion, enabling efficient charging and operation of electronic devices such as commercial LEDs, stopwatches, and calculators.

2.2.2. Single electrode (SE) mode. Both the vertical contact-separation mode and the lateral sliding mode require the connection of two electrodes to the load, forming a closed loop for electron flow. This significantly limits the ability of TENGs based on these modes to harvest energy from freely moving objects. The introduction of the single-electrode mode addresses this issue, particularly when part of the TENG consists of a moving object that cannot be connected to the load. In the single-electrode mode, the bottom electrode of the TENG is grounded. When the friction layer comes into contact, an equal and opposite charge is generated on the contact surface due to differences in electron affinity between materials. As the friction layer separates from the electrode, electrons move from the ground to the metal electrode, neutralizing the positive charges on its surface, generating current in an external circuit. When the negatively charged friction layer approaches the electrode again, electrons on the electrode flow back to the ground to neutralize the negative charges, resulting in current flowing in the opposite direction within the external circuit.

Zhi et al. developed a bio-inspired dual-mode water and energy system (DMWES) using a heterogeneous fiber membrane structure and a conductive MXene/CNTs electrospun coating (Fig. 7a).⁸¹ By integrating a hydrophobic layer (C-PVDF) and a hydrophilic layer (PAN) through the electrospun MXene/CNTs conductive layer, they achieved unidirectional water transfer driven by a surface energy gradient and push–pull effect. This design facilitated the immediate absorption of perspiration from the skin, ensuring consistent biological electrical signals. The DMWES not only exhibited outstanding pressure sensing performance and high sensitivity but also functions as a single-electrode triboelectric nanogenerator (Fig. 7b). This enabled comprehensive health monitoring, including pulse monitoring, voice recognition, and gait recognition (Fig. 7c). The adhesion between the hydrophilic and hydrophobic layers in Janus textiles significantly influences the stability and functionality of the friction generator structure. Cheng et al. fabricated a Janus textile with dual-gradient pore size and wettability by emulating the internal structure of plants through continuous electrospinning and electro spraying technology (Fig. 7d).⁸²

Fig. 7 (a) Schematic diagram showing the production and use of the DMWES membrane.⁸¹ Reproduced with permission from Springer Nature, copyright 2023. (b) Schematic working principle of the DMWES under triboelectric sensing.⁸¹ Reproduced with permission from Springer Nature, copyright 2023. (c) Signal of (i) the different words and (ii) wrist pulse.⁸¹ Reproduced with permission from Springer Nature, copyright 2023. (d) The structural characteristics and applications of Janus textile.⁸² Reproduced with permission from Elsevier, copyright 2023. (e) Signals of the bending motion in the knuckle, wrist, elbow, and respiration.⁸² Reproduced with permission from Elsevier, copyright 2023.

This approach allows for controlled transport of sweat from the skin to the external environment while maintaining skin dryness. To address the disparity in elastic modulus between the HPAN/BNNS layer and the TPU layer, they applied MA adhesive onto the PAN/BNNS nanofiber mat using electrostatic spraying technology, creating connecting nodes that prevent delamination and ensure structural integrity. As an energy harvester, the TPU layer acts as a frictional electric layer, demonstrating exceptional open-circuit voltage, short-circuit current, and power density. Additionally, the Janus textile electronic skin could securely adhere to human skin, enabling the monitoring of movement and physiological signals (Fig. 7e).

2.2.3. Freestanding triboelectric-layer (FT) mode. In the independent triboelectric layer mode, an autonomous friction layer moves freely between two stationary electrodes without direct contact. As this charged layer oscillates between the electrodes, it induces a change in potential difference, facilitating the flow of electrons through an external circuit load and generating power output. This operational mode is particularly well-suited for monitoring the motion of dynamic objects.^83–86

2.2.4. Lateral sliding (LS) mode. The horizontal sliding mode closely resembles the vertical touch separation mode. When two different dielectric films come into contact, they develop surface charges with opposite signs. As these surfaces undergo relative sliding in the horizontal direction due to external forces, friction charges are generated. This horizontal polarization compels electrons to migrate between the higher and lower electrodes to equilibrate the static electric field generated by the frictional charges. An alternating current output is produced by regularly separating and rejoining. This sliding can occur in various forms, including plane sliding, cylinder sliding, and disc sliding.^87–90

2.2.5. Mixed model. The effectiveness of the multi-mode in enhancing the energy output of fiber-based triboelectric nanogenerators (F-TENG) has been well demonstrated.⁹¹ Xu et al. introduced a novel F-TENG with a tubular structure resembling a vascular stent, using commercial copper wire and silver-coated polyethylene terephthalate (Ag@PET) as inner and outer electrodes, along with polydimethylsiloxane (PDMS) as the dielectric material (Fig. 8a).⁹² The hollow tubular framework was created by weaving PET monofilaments and heat-treating them to enhance stability and elasticity. Ag@PET electrodes were formed by coating silver ink on the surface of PET, while PDMS served as the dielectric and protective layer wrapped around the Ag@PET framework. Nylon yarn was woven around the copper wire to form the inner electrode. This integrated structure enabled synergistic operation in the single-electrode mode and contact-separation mode through precise control of linear motor movement position. The F-TENG demonstrated an impressive voltage generation of 9 V cm⁻¹ at a frequency of 1 Hz, which is three times that of the single electrode mode and twice that of the vertical contact-separation mode. Furthermore, even after 10 000 cycles, the F-TENG exhibited exceptional stability, proving suitable for applications in smart insoles for monitoring human walking activities.

Fig. 8 (a) Fabrication schematic of the vascular stent-like F-TENG.⁹² Reproduced with permission from Elsevier, copyright 2023. (b) The working mechanism of the F-TENG in practical application situations.⁹² Reproduced with permission from Elsevier, copyright 2023. (c) Evaluation of output efficiency under various operating modes at 1 Hz.⁹² Reproduced with permission from Elsevier, copyright 2023. (d) Collection of data from the F-TENG at different frequencies for analysis.⁹² Reproduced with permission from Elsevier, copyright 2023. (e) Structure design of the FF-WTNG to harness mechanical energy generated by human activities.⁹³ Reproduced with permission from the American Chemical Society, copyright 2020. (f) The operational mechanism of FF-WTNGs activated by lateral sliding mode. A schematic diagram of the FF-WTNG affixed to a fabric for harvesting mechanical energy from a reciprocating arm, accompanied by the resultant output current. Inset: Image of the FF-TENG generating power for six LEDs from arm movements.⁹³ Reproduced with permission from the American Chemical Society, copyright 2020. (g) The operating mechanism of the FF-WTNG driven by the contact-separation mode. A schematic diagram of FFWTNGs sewn onto a cloth for harvesting mechanical energy from an arm swinging up and down. Inset: Photograph of an FF-WTNG sewn onto cloth and the corresponding output current.⁹³ Reproduced with permission from the American Chemical Society, copyright 2020.

Zhang et al. employed reactive ion etching technology to fabricate nanowire arrays on the surface of nylon and polyester fabrics (Fig. 8e).⁹³ These fabrics were then sewn with conductive tape electrodes onto a cotton base to form a series circuit. This circuit effectively harvested mechanical energy from human movement through both contact-separation and lateral sliding modes, converting it into electrical energy. The instantaneous output power can reach 0.37 milliwatts in the contact-separation mode and 2.1 milliwatts in the lateral sliding mode. This fabrication method not only enhances output performance but also simplifies the FF-WTNG structure, facilitating its integration with clothing.

2.3. Piezoelectric generator

The piezoelectric effect, discovered by Pierre and Jacques Curie in 1880, describes an asymmetric shift of charges or ions in a piezoelectric material when subjected to mechanical strain (Fig. 9a).^94,95 When external forces are applied to the surface of the material, it deforms and generates charges on its surface. The intensity of these charges is precisely proportional to the exerted force, creating an electric field within the material. Upon removal of the force, both the deformation and the electric field cease, and the material returns to its uncharged state. This is referred to as the positive piezoelectric effect. Conversely, the inverse piezoelectric effect occurs when an electric field causes internal and surface charge transfer within the material, resulting in deformation proportional to the electric field intensity. Once the electric field is removed, the material reverts to its original state. Mechanical energy, the most abundant environmental energy source, can be harnessed and converted into useful electrical energy through this effect.^96–98 The positive piezoelectric effect, which converts mechanical energy into electrical energy, is widely applied in energy harvesting and self-powered sensors. On the other hand, the inverse piezoelectric effect is suitable for applications such as sound emitters, dampers, and actuators. Flexible piezoelectric sensors, upon being exposed to external forces, give rise to strain along with positive and negative charges, thereby generating electrical energy.

Fig. 9 (a) (i) Piezoelectric materials. When subjected to an applied tensile force (ii) or pressure (iii), piezoelectric materials generate a voltage. This voltage is then applied to the electrodes located at both ends of the piezoelectric material, resulting in either mechanical extension deformation (iv) or mechanical compression deformation (v). (i) Piezoelectric materials. When subjected to an applied tensile force (ii) or pressure (iii), piezoelectric materials generate a voltage. This voltage is then applied to the electrodes located at both ends of the piezoelectric material, resulting in either mechanical extension deformation (iv) or mechanical compression deformation (v).⁹⁵ Reproduced with permission from the American Chemical Society, copyright 2023. (b) Operation modes of the piezoelectric transducer.⁹⁹ Reproduced with permission from Elsevier, copyright 2021. (c) Comparisons between unmodified and modified electrospun fibers.¹⁰⁰ Reproduced with permission from Wiley, copyright 2021. (d) The output waveforms and current of MFP textiles worn by diverse subjects and under diverse sports conditions.¹⁰⁰ Reproduced with permission from Wiley, copyright 2021. (e) Output voltage during wrist joint rotating, wrist joint rotating and shallow squat.¹⁰¹ Reproduced with permission from Wiley, copyright 2020.

Since its discovery, the piezoelectric effect has seen rapid development in self-powered sensing. A piezoelectric sensor typically consists of an insulating material with two electrodes wrapped around its surfaces. It operates in two modes: d33 and d31. The d33 mode refers to the situation where the electric field generated in the piezoelectric material is parallel to the direction of the applied stress.⁹⁹ When pressure is applied, the material's volume decreases, causing the positive and negative charge centers to separate and form an electric dipole, creating a potential difference between the electrodes. In contrast, the d31 mode generates an electric field perpendicular to the stress direction. This pattern often requires a specific structural design that causes the material to bend or deform under stress, creating an electric charge in a direction perpendicular to the stress. Piezoelectric sensors operating in the d33 mode are widely used due to their simple structure and easy integration.

However, d31 mode sensors have attracted attention because they can theoretically provide higher sensitivity, despite practical challenges such as the need for more complex structural designs to achieve efficient force transfer and charge collection. For human sensing applications, the d33 mode is generally more suitable for monitoring changes in pressure and force perpendicular to the sensor surface, such as heartbeats, breathing detection, and plantar pressure detection. Meanwhile, the d31 mode is better suited for monitoring small movements or shear forces parallel to the sensor surface, such as muscle vibrations. For wearable devices, it may be necessary to detect forces and pressures in multiple directions simultaneously, so a combination of d31 and d33 modes may be needed to provide more comprehensive data.

Piezoelectric materials are categorized into inorganic and polymer materials. Inorganic materials include piezoelectric crystals such as quart and lithium tantalate, and piezoelectric ceramics like barium titanate (BT), lead zirconate titanate (PZT), lead barium niobate (PBN) and lead metaniobate.^102–111 Polymer materials include polyvinylidene difluoride (PVDF) and its copolymers polyacrylonitrile (PAN), polypropylene (PP) polyurethane (PU), and poly-L-lactic acid (PLLA).^112–123 Recent advancements have introduced silicon carbide as a new piezoelectric material, noted for its simplicity, low cost, and excellent performance even at high temperatures.¹²⁴

2.3.1. Inorganic piezoelectric materials. Piezoelectric crystals have an asymmetric crystal structure, which eliminates the need for a polarization process. These crystals exhibit excellent mechanical properties, a simple structure, and high stability. However, compared to piezoelectric ceramics, they are more expensive and have higher production costs, as well as lower piezoelectric coefficients. As a result, they are primarily used in sensors with standard or stringent requirements.

PDA@BTO (barium titanate)/PVDF nanofibers were fabricated using electrospinning technology, inspired by the interaction between human muscle fibers and surrounding connective tissue (Fig. 9c).¹⁰⁰ Polydopamine (PDA) was used as a surface modifier to enhance the interfacial adhesion between inorganic piezoelectric ceramics and organic polymer matrices. A non-woven piezoelectric textile, inspired by muscle fibers, was developed with remarkable attributes such as high sensitivity (3.95 V N⁻¹) and long-term stability. This piezoelectric textile has potential for diverse physiological monitoring applications, including pulse wave monitoring, human movement tracking, and active speech recognition.

The piezoelectric sensor developed by Hou et al. uses a PZT film composed of PZT nanofibers and PDMS, coated with a copper (Cu) electrode layer.¹⁰¹ This design optimized the size of the PZT film, leveraging its piezoelectric effect and the softness of PDMS to efficiently convert mechanical forces into electrical signals. The sensor exhibited excellent linearity (pressure–voltage correlation of 0.9909) and good repeatability (over 2 000 cycles) across a wide linear range (1.25 kPa to 250 kPa). It can detect various human movements, such as joint bending, wrist deformation, and common motor actions. Utilizing these advantages, a 3 × 3 sensor array was designed to enable multi-touch stimulation, showcasing trajectory tracking and personal identification through surface pressure (Fig. 9e).

2.3.2. Piezoelectric polymer materials. In piezoelectric polymers, changes in molecular structure and orientation facilitate the manifestation of the piezoelectric effect. Polyvinylidene fluoride (PVDF) is widely used in sensing and detection fields due to its flexible structure, ease of processing, commendable chemical resistance, exceptional biocompatibility, and robust mechanical strength. The piezoelectric properties of PVDF primarily arise from its transition from the α phase to the β phase.

Pan et al. synthesized cellulose nanocrystals (CNCs) from recycled cellulose slurry obtained from waste cotton textiles.¹²⁵ Using wet spinning technology and incorporating CNCs, PVDF was stretched to achieve a high content of the piezoelectric active β phase (Fig. 10a). Remarkably, without requiring additional polarization processing, it exhibited exceptional piezoelectric properties with a piezoelectric coefficient reaching 26.2 pC N⁻¹ (d33), capable of generating an open-circuit voltage up to 12 V and a short-circuit current of 100 nA. With its stable output signal and high sensitivity, this material was well-suited for monitoring both subtle and extensive human movements, as well as sound sensing applications (Fig. 10b).

Fig. 10 (a) The mechanism of β-phase formation in PVDF.¹²⁵ Reproduced with permission from Elsevier, copyright 2024. (b) Human motion monitoring of finger bending, knee bending, blowing and the electrical signals produced by ringtones.¹²⁵ Reproduced with permission from Elsevier, copyright 2024. (c) The preparation and assembly process of TPENGs and the model representing the conformational transition of PVDF and PAN subsequent to electrospinning and external compression.¹²⁶ Reproduced with permission from the American Chemical Society, copyright 2024. (d) The LED light is lit by different degrees of arm bending.¹²⁶ Reproduced with permission from the American Chemical Society, copyright 2024. (e) Digital photo of the alarm circuit board when it works.¹²⁶ Reproduced with permission from the American Chemical Society, copyright 2024.

Polyacrylonitrile (PAN), another commonly used piezoelectric polymer, has a smaller molecular size that leads to a more uniform lattice structure, exhibiting superior piezoelectric properties. Huang et al. incorporated multi-walled carbon nanotubes (CNTs) into PVDF and PAN, respectively, to fabricate self-powered smart fibers with two interpenetrating nanocomposite structures (Fig. 10c).¹²⁶ These fibers demonstrated enhanced β-phase PVDF and PAN planar serrated conformation, utilizing both piezoelectric and triboelectric conversion mechanisms. The synergistic effect of these mechanisms enabled the smart fibers to exhibit excellent electrical output (187 V, 8.0 μA, and 1.52 W m⁻²), capable of simultaneously illuminating up to 50 commercial LEDs. Consequently, these smart fibers could function not only as self-powered sensors for monitoring human movement but also as alarm systems in medical, fire safety, and surveillance applications.

2.3.3. Mixed materials. Combining inorganic piezoelectric materials with polymer materials often results in a generator with superior piezoelectric performance. Xu et al. fabricated a directional electrospun PLLA/ZnO composite fiber film, which exhibited enhanced piezoelectric properties due to an increased number of active sites and improved crystallization of PLLA molecular chains facilitated by the addition of ZnO nanoparticles (Fig. 11a).¹²⁷ The incorporation of ZnO also increased the β crystal content. This PLLA/ZnO-based generator demonstrated a significant and stable piezoelectric signal without requiring polarization or stretch post-processing, achieving an open-circuit voltage of 7.9 V, nearly 4.6 times higher than that observed for random nanofiber membranes. This wearable self-energy sensor showed promise for distinguishing human movement and monitoring human health (Fig. 11b). Huang et al. employed PAN nanofibers as a flexible substrate, combined MXene and PDA-modified ZnO as dual fillers, and fabricated PAN/MXene/PDA@ZnO composite nanofiber membranes by means of electrospinning technology.¹²⁸ The addition of PDA enhanced the dispersibility and compatibility of ZnO in the PAN substrate, mitigated agglomeration, and facilitated the improvement of the charge separation efficiency of the material when subjected to mechanical stress, thereby enhancing the piezoelectric output. The PMPO piezoelectric sensor exhibited outstanding piezoelectric sensing capability, exhibiting exceptional mechanical stability and endurance exceeding 3000 loading-unloading cycles, together with a sensitivity of 28.56 V N⁻¹, and the ability to monitor subtle physiological activities in the human body (Fig. 11d–g).

Fig. 11 (a) A diagrammatic representation of the manufacturing process for electrospun OPLLA/ZnO fiber-based piezoelectric generators.¹²⁷ Reproduced with permission from Elsevier, copyright 2023,. (b) The OPLLA/10ZnO sample and EDS mapping illustrating the distribution of the Zn element from a chosen area in the related FESEM image.¹²⁷ Reproduced with permission from Elsevier, copyright 2023. (c) (i–iv) Applications of the OPLLA/10ZnO generator for sensing human motion as a wearable self-sustaining sensor: (i) in reaction to walking and running, (ii) in response to wrist pulse detection, (iii) during finger bending and relaxing movements, and (iv) in relation to vocal cord vibrations.¹²⁷ Reproduced with permission from Elsevier, copyright 2023. (d) Top: Operational principle of the PMPO piezoelectric sensor. Bottom: Principles output in compressing mode of unmodified PAN/MXene/ZnO composite film sensors and PDA-modified PMPO piezoelectric sensors.¹²⁸ Reproduced with permission from Elsevier, copyright 2024. (e) Voltage-force linear fitting curve.¹²⁸ Reproduced with permission from Elsevier, copyright 2024. (f) Stability testing.¹²⁸ Reproduced with permission from Elsevier, copyright 2024. (g) PMPO piezoelectric sensor for measuring minor bodily signals: (i) respiration, (ii) frowning, (iii) cheek bulging, and (iv) smiling.¹²⁸ Reproduced with permission from Elsevier, copyright 2024.

2.4. Thermoelectric generator

The thermoelectric effect refers to the phenomenon in which electrons (or holes) migrate from a thermally elevated region to a thermally diminished region along the thermal gradient, resulting in the generation of an electric current or charge accumulation. Thermoelectric materials primarily exploit three fundamental thermoelectric effects for energy conversion: the Seebeck effect, the Peltier effect, and the Thomson effect (Fig. 12a).¹²⁹ These effects elucidate the relationship between temperature difference and electric current.

Fig. 12 (a) Diagrammatic representation of thermoelectric (TE) effects. (i) The Seebeck phenomenon in thermoelectric generators (TEGs), (ii) the Peltier phenomenon in thermoelectric coolers, and (iii) the Thomson effect related to reversible heating or cooling processes.¹²⁹ Reproduced with permission from Elsevier, copyright 2024. (b) Schematic representation of micro–nano thermoelectric fibers fabricated using the thermal drawing technique with a glass fiber template.¹³⁰ Reproduced with permission from Wiley, copyright 2022. (c) The simplified manufacturing process of TE string and the optical images of ∼1.2-m long THC-TES.¹³¹ Reproduced with permission from the Royal Society of Chemistry, copyright 2022. (d) Illustrations of the thermoelectric device (top: general perspective; bottom: lateral view, showcasing the alternating vertical configuration of p-type and n-type segments).¹³¹ Reproduced with permission from the Royal Society of Chemistry, copyright 2022,. (e) Output power density of TET as a function of external resistance at different ΔT.¹³¹ Reproduced with permission from the Royal Society of Chemistry, copyright 2022. (f) The exhibition of heat regulation in the human body.¹³¹ Reproduced with permission from the Royal Society of Chemistry, copyright 2022. (g) A schematic diagram of the preparation process of CNTFs.¹³² Reproduced with permission from Elsevier, copyright 2021. (h) Schematic representation of the thermoelectric device architecture alongside an optical image of film and carbon nanotube film-based thermoelectric generators.¹³² Reproduced with permission from Elsevier, copyright 2021. (i) Alteration in resistance following numerous scrubs or prolonged exposure to tap water.¹³² Reproduced with permission from Elsevier, copyright 2021.

The Seebeck effect refers to the phenomenon of converting thermal energy into electrical energy, which is also the principle underlying the power generation by thermoelectric materials. This phenomenon was discovered by Thomas Johann Seebeck in 1821. When one end of two dissimilar metals or alloy materials is heated while the other end remains at a lower temperature, a voltage will be generated between the contact points of these two materials. The essence lies in the fact that when two different conductive materials are connected and there is a temperature difference, it leads to changes in charge carrier distribution (i.e., electrons or holes) within the conductor, resulting in a phenomenon known as the thermoelectric potential (or thermal electromotive force) within the conductor. It should be noted that the Seebeck effect can also occur in a single material when a temperature gradient exists between its two extremities, causing migration of charge carriers. For P-type materials dominated by holes, this results in the formation of cold positive and hot negative thermoelectric potential as holes migrate towards the positive electrode from the cold end; conversely, for N-type materials dominated by electrons, this leads to the formation of cold negative and hot positive thermoelectric potential as electrons migrate towards the negative electrode from the cold end. In accordance with the Seebeck effect, when two different conductive materials are connected and a temperature gradient exists across their junctions, an electromotive force, denoted as V, is generated within the closed circuit. The Seebeck coefficient (S), a thermoelectric characteristic of a material, represents the EMF produced per unit temperature difference. The value of this coefficient depends on the material's electronic structure, resulting in significant variations in thermoelectric performance among different materials.

The Peltier effect, first discovered by French physicist Jean Charles Athanase Peltier in 1834, is the inverse effect of the Seebeck effect. This phenomenon elucidates the absorption or release of heat at the junction among dissimilar conductive or semiconductive materials when an electric current traverses a circuit composed of said materials, resulting in a decrease in temperature at one end (cold) and an increase in temperature at the other end (hot). The foundation of this effect lies upon the physical principle that electric current arises from charge carriers' motion within conductors. As charge carriers exist at distinct energy levels across different materials, they emit surplus energy while transitioning from higher to lower energy levels, thereby releasing heat. Conversely, transitioning from lower to higher energy levels necessitates external energy absorption and leads to heat assimilation. This exchange of energy manifests as thermal transfer occurring at the interface between these two materials; moreover, it is noteworthy that this effect exhibits reversibility—altering the direction of electric current also modifies where heat is absorbed or released. The Peltier effect arises from the difference in electron energy levels across various materials. When an electric current flows through the junction, it alters the energy states of the electrons, resulting in either the absorption or emission of thermal energy. The degree of this thermal change is measured by the material's Peltier coefficient (π).

With advancements in thermodynamics, British scientist W. Thomson made a groundbreaking discovery known as the Thomson effect through experimental investigations. As charge carriers, such as electrons, traverse within a conductor, they experience alterations in energy due to temperature gradients present within the conductor. When electrons move from a region of higher temperature to one of lower temperature, they release energy which is absorbed by the conductor in the form of heat. Conversely, when electrons migrate from a lower temperature region to a higher one, they absorb energy from the conductor. Unlike other thermoelectric effects, the Thomson effect solely applies to an individual conductor rather than being applicable to a closed circuit composed of two distinct conductors. The Thomson effect, though typically less significant than the Seebeck and Peltier effects, cannot be overlooked in high-precision thermoelectric systems. The Thomson coefficient (τ) is a material-specific parameter that quantifies the heat change per unit temperature gradient and per unit current in a conductor. This coefficient's value is influenced by the material's electronic structure and temperature gradient, making it an essential parameter for analyzing thermoelectric properties in research.

Precisely due to these three distinctive thermoelectric effects, thermoelectric materials play an indispensable role in the development of numerous modern industries. Thermoelectric materials can be classified into three categories: inorganic thermoelectric materials, organic thermoelectric materials, and hybrid thermoelectric materials.

2.4.1. Inorganic thermoelectric materials. Including carbon-based materials, Bi₂Te₃,^133–135 PbTe, MXenes, SiGe, half-Heusler compounds, and filled skutterudite antimonides are commonly assessed based on their figure of merit for thermoelectric efficiency (ZT value).^{129,133–149} To achieve a high ZT value, the material should have a high Seebeck coefficient, enhanced electrical conductivity, and minimal thermal conductivity.

Sun et al. employed a two-step hot stretching process to fabricate micro-nano polycrystalline thermoelectric fibers of Bi₂Te₃ matrix material within a glass fiber template, showcasing exceptional thermoelectric performance and remarkable flexibility (Fig. 12b).¹³⁰ The ZT value of the 4 μm diameter fibers at 300 K surpasses that of the corresponding bulk material by 40%, reaching approximately 1.4. Moreover, these fibers exhibited mechanical stability under reversible bending with a radius as small as 50 μm, approaching the theoretical elastic limit of Bi₂Te₃. These findings suggested the potential applications for these fibers as self-powered sensors' energy sources, harnessing environmental heat to generate essential electrical energy for sensor operation. Zheng et al.¹³¹ developed and fabricated a novel three-layer coaxial thermoelectric string (THC-TES) in the form of bead-like structures (Fig. 12c). This string consisted of interconnected liquid metal p-type Bi_0.4Sb_1.3Te₃ (BST) and n-type Bi₂Te_3.3Se_0.2 (BTS) segments encapsulated within a PDMS elastomer material. By employing a semi-automatic weaving machine, they successfully integrated THC-TES into flexible and stretchable thermoelectric textiles (TETs). These TETs exhibited exceptional washability, enduring more than 20 cycles, while attaining a high power density of as much as 0.58 W m⁻² at a temperature difference of 25 K. In practical applications, these TETs could be conveniently worn on the human arm even under ambient temperatures around 8 °C to effectively power various wearable electronic devices for monitoring environmental conditions as well as human physiological signals and activities.

Inorganic thermoelectric materials, based on carbon materials, exhibit high electrical conductivity and the capability to adjust the Seebeck coefficient. Typical carbon materials include multi-walled carbon nanotubes (MWNTs), single-walled carbon nanotubes (SWNTs), and graphene. The majority of CNTs and graphene demonstrate positive Seebeck coefficients due to oxygen doping. Consequently, current research in the field of carbon thermoelectric materials primarily focuses on reducing thermal conductivity and developing chemically stable n-type materials. Jin et al. employed electrostatic spray technology for the fabrication of high-performance carbon nanotube fibers (CNTFs), which could enhance electron concentration and exhibit n-type semiconductor characteristics by doping with polyethyleneimine (PEI) as an alkaline substance (Fig. 12g).¹³² This facile process readily converted the material into an n-type material, displaying a Seebeck coefficient of −34.5 μV K⁻¹. Utilizing the prepared p-type and n-type CNTFs, a π-type thermoelectric generator (TEG) was fabricated, demonstrating minimal degradation in resistance even after repeated washing and deformation.

2.4.2. Organic thermoelectric materials. These materials are composed of conductive polymers and small molecules. Conductive polymers possess a conjugated large π bond structure, which enables electrical conduction upon doping. The conduction mechanism involves the alternation of single and double bonds, resulting in π conjugation within the polymer's main chain. This leads to effective delocalization of π electrons along the chain, facilitating charge transport through unpaired π electrons in carbon atoms and exhibiting semiconductor behavior. Commonly utilized organic materials include polyaniline (PANI), polyacetylene (PA), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives. These organic thermoelectric materials share common characteristics such as a lightweight nature, excellent flexibility, high ductility, and low intrinsic thermal conductivity.^150–158

The wet spinning technique was employed by Wu et al. to fabricate fiber bundles of PEDOT:PSS, which were doped with dimethyl sulfoxide (Fig. 13a).¹⁵⁹ Subsequent treatment with concentrated sulfuric acid effectively eliminated excess PSS, resulting in a remarkable enhancement of electrical conductivity up to 4464 S cm⁻¹. Furthermore, the parallel bundle structure significantly improved the power factor to 80.8 μW m⁻¹ K⁻², thereby demonstrating the exceptional potential of these wearable fiber bundles in reducing resistance while exhibiting high tensile strength and stability. Ruan et al. developed a novel hollow-structured thermoelectric fiber based on PEDOT:PSS (Fig. 13c), which exhibited unique characteristics that contribute to its enhanced performance.¹⁶⁰ The incorporation of a hollow structure provided an enlarged surface area compared to solid fibers of the same diameter, thereby improving heat exchange efficiency with the surrounding environment by maximizing contact with external heat sources. Additionally, this design facilitated the removal of insulating PSS and enhanced the electrical conductivity of the fiber, consequently strengthening its thermoelectric properties. As a result, this fabricated sensor demonstrated rapid voltage response upon contact and could be effectively utilized as a self-powered flexible sensor for finger touch detection applications.

Fig. 13 (a) An illustration depicting the process of creating dimethyl sulfoxide (DMSO)-infused PEDOT:PSS fiber bundles through wet-spinning, followed by a careful post-treatment using concentrated sulfuric acid (98% H₂SO₄).¹⁵⁹ Reproduced with permission from Springer Nature, copyright 2024. (b) Images showcasing the produced PEDOT:PSS fiber bundles along with their characteristics such as high tensile strength, electrical conductivity, and suitability for wearable applications.¹⁵⁹ Reproduced with permission from Springer Nature, copyright 2024. (c) Fabrication scheme for producing the thermoelectric (TE) fiber sensor.¹⁶⁰ Reproduced with permission from the Multidisciplinary Digital Publishing Institute, copyright 2020. (d) SEM image of the PEDOT:PSS fiber.¹⁶⁰ Reproduced with permission from the Multidisciplinary Digital Publishing Institute, copyright 2020. (e) Schematic diagram of the selector and (f) thermographic images of (i) the PMDS layer before touching the PDMS surface, (ii) the finger pressing onto the surface of the layer for 10 s, and (iii) the PDMS layer after leaving the surface.¹⁶⁰ Reproduced with permission from the Multidisciplinary Digital Publishing Institute, copyright 2020.

2.4.3. Hybrid thermoelectric materials. Composite materials incorporating conductive polymers and other components such as carbon nanotubes, graphene, inorganics, and metals represent a highly promising strategy for achieving a synergistic balance between enhanced thermoelectric performance and exceptional flexibility. The inherent low thermal conductivity of the conductive polymer component further contributes to optimizing the ZT value of these composite materials. Li et al. fabricated a series of composite fibers comprising PEDOT:PSS/PC-Te NWs through wet spinning and subsequent post-treatment (Fig. 14a).¹⁶¹ The incorporation of an EDOT:PSS nanolayer effectively prevented the aggregation of tellurium nanowires, enhancing compatibility with the PEDOT:PSS matrix and enabling a Te NW content of up to 70 wt% without compromising fiber flexibility. Due to the high aspect ratio of PC-Te NWs and internal stress during the spinning process, these nanowires exhibited an orderly arrangement along the composite fibers. By optimizing the content of PC-Te NWs and implementing appropriate post-treatment techniques, they achieved a remarkable power factor value of 385.4 μW m⁻¹ K⁻², which was approximately 4.9 times higher than previously reported values for PEDOT: PSS/Te NW composite fibers. These findings demonstrated exceptional output performance and considerable promise for utilization in wearable thermoelectric devices. Zhu et al. employed flexible substrates composed of polyethylene terephthalate (PET) fibers with core-sheath structures featuring distinct melting points for the integration of PEDOT:PSS and SWCNTs onto the sheath surfaces through heat treatment (Fig. 14c).¹⁶² The resulting TE fabric demonstrated exceptional compression strain response, as well as precise temperature detection capability (achieving a minimum detection temperature of 0.17 K). These attributes rendered it highly suitable for applications in human motion recognition, encompassing pulse monitoring, sign language expression, and joint movement detection.

Fig. 14 (a) Illustration of the wet-spinning process of the PEDOT:PSS/PC-Te NWs composite fibers.¹⁶¹ Reproduced with permission from Wiley, copyright 2024. (b) TE performance of the P/PTF-65 fibers before and after different post-treatments at RT.¹⁶¹ Reproduced with permission from Wiley, copyright 2024. (c) Illustration of compression sensing and wireless transmission applications of TE fabric.¹⁶² Reproduced with permission from the American Chemical society, copyright 2024. (d) Schematic illustration of the effect of ethanol on the structure of the polymer network and the effect of NaTFSI on the ion diffusion.¹⁶³ Reproduced with permission from Elsevier, copyright 2024. (e) Schematic illustration of the working principle of the ITEC.¹⁶³ Reproduced with permission from Elsevier, copyright 2024. (f) The thermovoltage generated when wearing the ionogel fiber-based wristband.¹⁶³ Reproduced with permission from Elsevier, copyright 2024.

Li et al. enhanced the ion migration rate in a ionic gel by incorporating ethanol and sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), thereby augmenting the diffusion difference between cations and anions (Fig. 14d).¹⁶³ The modified ion gel exhibited a significantly improved ion conductivity of 17.5 mS cm⁻¹ at 70% relative humidity (RH), an ion Seebeck coefficient of 22.9 mV K⁻¹, and an ion power factor of 870.26 μW m⁻¹ K⁻², surpassing that of the original PH/ED ionic gel considerably. Wearable ion gel fiber wristbands with synergistically enhanced thermoelectric performance, mechanical properties, and wearing comfort were fabricated using weaving technology. When worn at room temperature (ΔT = 2.7 K), the i-TE wristband (5 legs) generated a remarkably high thermoelectric voltage of 119.133 mV, demonstrating its immense potential as a wearable energy supply device.

2.5. Photoelectric generator

Solar energy, also referred to as photovoltaic energy, is a widely utilized renewable energy source. In the 19th century, French scientist Becquerel made the discovery that light can induce a potential difference in semiconductor materials. Semiconductors consist of two primary regions: the valence band and the conduction band. Electrons within the valence band are tightly bound to atomic nuclei, while those within the conduction band possess relatively higher mobility and can actively participate in electrical conduction. When photons interact with a semiconductor surface, their energy can be transferred to an electron in the valence band if it meets or exceeds the width of the band gap. This energy transfer facilitates the transition of the electron from the valence band to the conduction band, resulting in its liberation as a free electron. Holes can be considered as positively charged charge carriers and are also capable of participating in conduction within semiconductors. Both free electrons and holes exhibit mobility under the influence of an electric field, thereby giving rise to the photovoltaic effect (Fig. 15).¹⁶⁴ Wearable sensor systems could effectively utilize various fiber-shaped solar cell technologies, such as dye-sensitized solar cells (DSSC), organic solar cells (OSC), and perovskite solar cells (PSC), to capture and convert solar energy through the photovoltaic effect.

Fig. 15 The working principle of p–n junction solar cells.¹⁶⁴ Reproduced with permission from the Royal Society of Chemistry, copyright 2014.

2.5.1. Dye-sensitized solar cells (DSSCs). Fibrous dye-sensitized solar cells (FDSSCs) and traditional planar DSSCs share a common composition, including a photoanode, dye sensitizer, redox electrolyte, and counter electrode; however, the key distinction lies in their conductive substrates. This technology, pioneered by Swiss scientist Michael Grätzel in the early 1990s and commonly referred to as Grätzel cells, employs dye molecules to facilitate light absorption. The operational mechanism of DSSCs involves a series of sequential steps: (i) light absorption: dye molecules within the cell capture photons from sunlight, inducing electron excitation from the ground state to an excited state. (ii) Electron injection: excited-state electrons rapidly (typically within picoseconds to nanoseconds) undergo injection into the conduction band of a semiconductor material, such as titanium dioxide, thereby returning the dye molecules to their ground state. (iii) Electron transfer: injected electrons in the semiconductor are efficiently transported through a well-connected network to an external circuit, thereby facilitating the generation of electric current. (iv) Dye regeneration: dye molecules in their ground state can be regenerated through reduction by a redox electrolyte containing a reducing agent, typically iodide/iodine. This process completes the circuit and enables continuous generation of current. A key advantage of dye-sensitized solar cells (DSSCs) lies in the ability to tailor dye molecules for selective absorption of specific wavelengths, thereby enabling more efficient utilization of the solar spectrum compared to conventional crystalline silicon-based photovoltaic devices.

Researchers have devoted significant efforts to the innovation of fiber electrode materials, design of device structures, and optimization of active layer interfaces. However, challenges inherent in achieving high-quality films on curved fibers and establishing stable interfaces have resulted in a relatively low power conversion efficiency (PCE) for fiber solar cells.

Song et al. achieved efficient control over light fields and maximized energy utilization by integrating an aluminum oxide/polyurethane film as a light diffusion layer on the outer encapsulation tube, and a phosphor/titanium dioxide/poly(vinylidene fluoride-co-hexafluoropropylene) film as a light conversion layer on the inner counter electrode surface for FDSSCs (Fig. 16a).¹⁶⁵ This innovative design significantly enhanced light collection efficiency across different colors while maintaining a high power conversion efficiency (PCE) of up to 13.11%. Furthermore, the introduction of small amounts of colorants into the light diffusion layers allowed for versatile appearances, greatly enhancing design flexibility and practical compatibility for various applications such as integration with textile-based fiber batteries as next-generation power systems.

Fig. 16 (a) Schematic diagram of FDSSCs integrated with light diffusion and conversion layers.¹⁶⁵ Reproduced with permission from Wiley, copyright 2023. (b) Real-time heart rate measurements indicate the user's physiological condition during various physical activities, providing specific notifications through visual displays.¹⁶⁵ Reproduced with permission from Wiley, copyright 2023. (c) Photographs of colorful photovoltaic textiles composed of color-tunable FDSSCs and the theoretically estimated power outputs of various photovoltaic textiles.¹⁶⁵ Reproduced with permission from Wiley, copyright 2023. (d) Schematic diagram of FGDSC with interlaced interfaces.¹⁶⁶ Reproduced with permission from Wiley, copyright 2024. (e) The stability of the FGDSC after being disconnected. The open-circuit voltage demonstrated nearly no variation. Inset: Photographs of the FGDSC powering a hygrothermograph before and after disconnection.¹⁶⁶ Reproduced with permission from Wiley, copyright 2024. (f) An image depicting everyday clothing that incorporates FGDSCs and fiber lithium-ion batteries, which are used to charge a walkie-talkie and a smart bracelet.¹⁶⁶ Reproduced with permission from Wiley, copyright 2024.

In the field of dye-sensitized solar cell (DSSC) research, ensuring interface stability is crucial as it directly impacts battery performance and lifespan. Kang et al. successfully addressed this challenge by incorporating polymer gel electrolytes into aligned channels within fiber electrodes and gaps between photoanodes and counter electrodes in their design of fiber-based DSSCs (FGDSCs) (Fig. 16d).¹⁶⁶ This innovative staggered structure significantly enhanced interface stability, facilitating rapid ion diffusion and efficient charge transfer at interfaces, ultimately resulting in an impressive power conversion efficiency (PCE) of up to 7.95%. Remarkably, even after undergoing more than five thousand bending cycles, these FGDSCs maintained an efficiency above ninety percent. Furthermore, the integration of FGDSCs with fiber batteries enables them to function as self-charging power systems, offering a promising solution for wearable technology applications.

2.5.2. Organic solar cells (OSCs). OSCs are photovoltaic devices that employ organic materials, such as polymers or small molecules, as the light-absorbing layer. In contrast to dye-sensitized solar cells (DSSCs), OSCs consist of a light-active organic semiconductor layer sandwiched between two electrodes, wherein the active layer comprises donor and acceptor materials capable of forming planar or bulk heterojunctions.^167,168 Due to the inherently narrow absorption spectra of organic materials, they exhibit limited capacity for absorbing specific wavelengths of light and thus cannot efficiently harness the entire solar spectrum. Moreover, the charge carrier mobility in organic materials is generally inferior to that observed in inorganic counterparts, leading to reduced efficiency in OSCs. Lv et al. achieved a power conversion efficiency of over 9% by incorporating non-fullerene acceptor (NFA)-based organic semiconductors as light-absorbing materials and utilizing a self-developed programmable slot-die coating system to enhance the uniformity and repeatability of functional layers in fiber organic solar cells (FOSCs) (Fig. 17a).¹⁶⁹ This demonstration underscored the potential utilization of FOSCs as an energy source in wearable electronic devices and smart textiles. Subsequently, to circumvent the performance degradation of the fiber battery during irregular body motions, Lv et al. fabricated stretchable fiber electrodes by uniformly applying silver nanowires and ITO NPs on thermoplastic polyurethane fibers for attaining superior intrinsic stretchability (Fig. 17d).¹⁷⁰ The fabricated FOSCs exhibited an initial efficiency exceeding 90%, which remained above 90% even after a 30% stretching strain. The high flexibility and stretchability empower the device to stably adapt to body movements.

Fig. 17 (a) Illustration of the fiber-shaped organic solar cell (FOSC) and cross-sectional SEM image of the multi-layer film coated on primary electrode.¹⁶⁹ Reproduced with permission from Nature Partner Journals, copyright 2022. (b) The statistical power conversion efficiency (PCE) of the FOSCs measured under various room humidity levels. Chloroform and chlorobenzene serve as solvents for the organic photovoltaic materials.¹⁶⁹ Reproduced with permission from Nature Partner Journals, copyright 2022. (c) The watchband weaved with the FOSCs used to charge the smartwatch.¹⁶⁹ Reproduced with permission from Nature Partner Journals, copyright 2022. (d) A depiction of the process for creating intrinsically stretchable fiber electrodes and fiber-shaped organic solar cells (FOSCs), along with the approach of applying pre-strain during the fabrication of the stretchable FOSCs.¹⁷⁰ Reproduced with permission from Wiley, copyright 2023. (e) The evolution of J–V curves for the FOSCs utilizing a CNT yarn counter electrode as the stretch strain is progressively increased from 0% to 60%.¹⁷⁰ Reproduced with permission from Wiley, copyright 2023.

2.5.3. Perovskite solar cells (PSCs). In comparison to OSCs, PSCs exhibit a fully solid-state structure, along with a thinner electron absorption layer and higher photovoltaic conversion efficiency, rendering them as ideal energy supply devices for wearable electronic applications. Leveraging their distinctive structural advantages, fiber-shaped PSCs demonstrate the capability of achieving multi-angle three-dimensional light absorption, thereby enhancing photon capture. Zhao et al. have developed a flexible self-powered wristband system that integrates zinc ion batteries with defect-rich MnO_2−x nanosheets and perovskite solar cells (Fig. 18a).¹⁷¹ The interlayer spacing and oxygen vacancies of MnO_2−x@CC were significantly optimized through a simple lithium treatment method, leading to substantial enhancement in the electrochemical performance of the positive electrode material for ZIBs. This improvement encompassed exceptional specific capacity under high mass loading, enhanced rate performance, and prolonged cycling stability. Moreover, the wristband system exhibited remarkable flexibility and energy density (5.11 mW h cm⁻²), making it suitable for all-weather operation. While traditional perovskite solar cells exhibit high efficiency, they contain lead and pose significant environmental and health risks. Balilonda et al. developed a novel lead-free perovskite solar fiber by electrospinning methylammonium tin mixed halide PVP composite nanofibers doped with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) directly onto multi-strand carbon fiber yarn coated with poly(3-hexylthiophene-2,5-diyl) (P3HT) (Fig. 18d).¹⁷² The resulting fibers were twisted together with commercial silver yarn as the positive electrode after removing the electron transport layer. Through material and structural optimization, this solar fiber achieved an impressive power conversion efficiency of up to 7.49%. Furthermore, due to their exceptional flexibility and stability, these fibers could be seamlessly integrated into solar textiles, opening up new possibilities for wearable technology and smart textile applications.

Fig. 18 (a) Schematic fabrication process of the MnO₂@CC and MnO_2–x@CC.¹⁷¹ Reproduced with permission from the American Chemical Society, copyright 2021. (b) Digital images illustrating the process of integrating the self-powered smart bracelet.¹⁷¹ Reproduced with permission from the American Chemical Society, copyright 2021. (c) Dynamic sport information was obtained from the self-powered smart bracelet during outdoor running and indoor biking.¹⁷¹ Reproduced with permission from the American Chemical Society, copyright 2021. (d) Diagrams illustrating the procedures for preparing cathode yarn and depicting the overall structure of a lead-free, compact layer-free solar yarn.¹⁷² Reproduced with permission from Elsevier, copyright 2021. (e) The change in power conversion efficiency (PCE) as a function of the length of twisted yarn. (Inset: Image of a fully assembled solar yarn).¹⁷² Reproduced with permission from Elsevier, copyright 2021. (f) The relationship between power conversion efficiency (PCE) and the length of the twisted yarn. (Inset shows a photograph of a single twist unit length).¹⁷² Reproduced with permission from Elsevier, copyright 2021. (g) Trends in the variations of power conversion efficiency (PCE) and power density of the knitted solar fabric as strain levels change. (Inset: A photographic image of the knitted solar fabric).¹⁷² Reproduced with permission from Elsevier, copyright 2021.

3. Conclusions and future outlook

This review provides a comprehensive and detailed overview of the working mechanisms of five self-powered fiber sensors, and systematically introduces the important literature related to them. Each of these fibers have unique and distinctive performance characteristics, which have accumulated a wealth of valuable experience for subsequent related research. The performance indicators of the different sensors (such as sensitivity, linearity, response time, and durability) and their advantages and disadvantages are summarized in Table 1. Although significant progress has been made in both the theoretical research at the laboratory scale and the application verification at the commercial level in the field of self-powered fiber sensors, it must be admitted that there is still a long and challenging road to explore and cross before the field can truly achieve its commercialization and industrialization goals. A key factor in transitioning from laboratory prototypes to commercial products is the choice of manufacturing technique. Several methods that can support mass production are as follows:

Table 1 Comparison of different self-powered sensors

Self-powered sensors	Advantages	Disadvantages	Sensitivity	Linearity	Response time	Durability
Piezoelectric	High sensitivity to mechanical stress, wide range of applications	Performance degrades under high strain, requires mechanical input	High (10–100 μW cm^−2)	Excellent	Fast (ms range)	High, stable under repeated deformation
Triboelectric	Simple structure, high output voltage	Requires continuous contact/separation, wear and tear issues	Moderate to high (10–500 μW cm^−2)	Good	Fast (ms range)	Moderate, sensitive to abrasion and humidity
Thermoelectric	Converts waste heat into usable energy, solid-state and reliable	Low conversion efficiency, dependent on temperature gradient	Low to moderate (1–10 μW cm^−2)	Good	Moderate (s range)	Good, affected by thermal cycling
Photovoltaic	Direct conversion of light to electricity, renewable energy source	Efficiency drops in low light conditions, requires light exposure	Very high (> 1000 μW cm^−2)	Good	Fast (ms range)	High, dependent on light exposure
Moist-electric	Harvests energy from moisture, suitable for humidity sensing	Limited energy output, depends on environmental moisture	Low (1–5 μW cm^−2)	Moderate	Moderate (s range)	Variable, depends on environmental moisture

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• Solution spinning: this method is cost-effective and well-suited for producing continuous fibers with uniform properties. However, the achievable production rate is moderate, and the process may require careful control of parameters to ensure consistency.

• Electrospinning: electrospinning allows for the production of nanofibers with high surface area and fine control over fiber diameter. It is particularly useful for creating sensors with high sensitivity. The main challenge lies in scaling up the process, as electrospinning can be slow and requires specialist equipment.

• Weaving: traditional weaving techniques can be adapted to incorporate conductive fibers and sensors into textiles. This method offers high production rates and is compatible with existing textile manufacturing infrastructure. However, the integration of complex sensor architectures may require additional steps and careful design.

Beyond initial performance, the long-term stability and durability of self-powered sensors are critical for their practical application. The following need to be considered:

• Repeated washing: exposure to water and detergents during washing can degrade sensor materials and affect performance. Encapsulation strategies and protective coatings are essential to enhance durability.

• Sweat exposure: sensors worn close to the skin must withstand exposure to sweat, which can introduce moisture and salts that may corrode materials. Biocompatible and moisture-resistant coatings can mitigate these effects.

• UV light: prolonged exposure to UV light can cause photodegradation of sensor materials. UV-stable materials and protective coatings are necessary to ensure long-term stability.

• Mechanical strain: wearable sensors are subject to continuous mechanical strain due to body movements. Flexible and stretchable materials, as well as robust sensor designs, are crucial for maintaining performance under mechanical stress.

The challenges among the various self-powered sensors are as follows:

(1) Cost control. One of the primary challenges in the development of self-powered sensors is the high cost associated with advanced materials and sophisticated fabrication processes. To make these sensors commercially viable, it is essential to find ways to reduce costs. This can be achieved by:

• Exploring alternative materials: using less expensive yet efficient materials that can provide similar properties to high-cost alternatives.

• Scalable manufacturing techniques: adopting scalable manufacturing methods such as roll-to-roll printing, 3D printing, and inkjet printing, which can produce large quantities of sensors at a lower cost.

• Optimizing material usage: minimizing waste during the manufacturing process by optimizing the use of materials.

(2) Optimization of production processes. Scaling up the production of self-powered sensors while maintaining their performance and quality presents a significant challenge. Key areas of focus include:

• Process uniformity: ensuring that the manufacturing process is uniform and reproducible, which is crucial for maintaining consistent sensor quality.

• Integration of functionalities: developing techniques to integrate multiple functionalities into a single sensor without compromising performance.

• Advanced fabrication techniques: utilizing advanced fabrication methods such as laser patterning, micro/nano imprinting, and flexible electronics to improve the production process.

(3) Market acceptance. Gaining market acceptance is critical for the successful commercialization of wearable self-powered sensors. This involves:

• User-friendly design: ensuring that the sensors are easy to use and comfortable to wear, which can be achieved through ergonomic design and lightweight materials.

• Durability and reliability: designing sensors that can withstand everyday wear and tear and provide reliable performance over time.

• Affordability: balancing cost and functionality to make the sensors affordable for a wide range of consumers.

(4) Energy harvesting efficiency. The efficiency of energy harvesting is a critical factor that determines the practical usability of self-powered sensors. Efforts to improve energy harvesting efficiency include:

• Material innovation: developing new materials with higher energy conversion efficiencies, such as advanced piezoelectric, triboelectric, and thermoelectric materials.

• Design optimization: optimizing the sensor design to maximize the surface area exposed to energy sources and enhance energy capture.

• Hybrid systems: combining multiple energy harvesting mechanisms (e.g., integrating both piezoelectric and photovoltaic elements) to increase the overall energy output.

(5) Integration and miniaturization. The integration of multiple sensing and energy harvesting functions into a compact and efficient device is a complex challenge. Strategies to address this include:

• Multi-functional materials: developing materials that can perform multiple functions, such as sensing and energy harvesting, simultaneously.

• Advanced packaging: utilizing advanced packaging techniques to integrate various components into a single, miniaturized device while maintaining performance.

• Flexible and stretchable electronics: creating flexible and stretchable electronics that can conform to different shapes and surfaces, making the sensors more versatile and adaptable.

• Structural design: the structural design of the integrated system plays a crucial role in its performance. The architecture must facilitate efficient collection and transfer of electrical signals generated by the power generation elements.

Furthermore, the development of wearable self-powered fiber sensors has significant potential for various applications, including:

• Healthcare monitoring: continuous and real-time monitoring of vital signs such as heart rate, body temperature, and movement, which can improve disease diagnosis and management.

• Environmental sensing: monitoring environmental parameters such as temperature, humidity, and air quality, which can contribute to better environmental management and pollution control.

• Human–machine interfaces: enhancing the interaction between humans and electronic devices through touch-sensitive and motion-sensitive interfaces, improving the user experience.

• Sports and fitness: providing athletes with real-time data on their performance, helping them optimize their training and prevent injuries.

• Wearable electronics: integrating self-powered sensors into everyday wearable devices, such as smart clothing and accessories, to provide continuous monitoring and feedback.

The future of wearable self-powered sensors is promising, with ongoing research focused on overcoming current challenges and exploring innovative solutions. As advancements in materials science, fabrication techniques, and sensor design continue to evolve, these sensors are expected to become more efficient, cost-effective, and widely adopted in various fields.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Science and Technology Department of Jilin Province (No. 20230101353JC).

Notes and references

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