Self-powered Wearable Sensor Research Articles

Self-Powered Flexible Sensors: From Fundamental Mechanisms toward Diverse Applications

Abstract Today, human life is inseparable from energy for clothing, food, housing, use and travelling, however, traditional resources can no longer meet our energy needs today. Self-powered sensing devices that can run consistently and sustainably without a source of power from the outside are top choices for new energy sources with their unique advantages. Based on advances in the field of materials and manufacturing techniques, it is able to autonomously extract energy from different sources. In this review, we focus on reviewing the current state of research on self-powered wearable sensors, providing a concise overview of energy harvesting technologies, energy conversion mechanisms, structural or material innovations, energy storage and processing platforms. Then，experimental advances in different energy sources are presented, showing their underlying mechanisms, and the potential for energy acquisition. In addition, we discuss applications of self-powered flexible sensors in medicine, sports and food. Despite considerable achievements in this field, widespread commercialization will necessitate enhanced sensor detection abilities, improved design factors for adaptable devices, and a harmony between sensitivity and standardisation.

In-situ cured gel polymer/ecoflex hierarchical structure-based stretchable and robust TENG for intelligent touch perception and biometric recognition

Gel-based sensors for next generation touch panels have been acknowledged for their exceptional sensitivity and flexibility. However, these sensors typically depend on a metal grid connection, which is susceptible to structural deformation under heavy stress applications and necessitates external power. Here, we report a novel in-situ cured gel polymer electrode-based triboelectric nanogenerator (GPE-TENG) that is stretchable, semi-transparent, and durable, designed to enable a self-powered touch panel for intelligent touch perception. The in-situ curing of the hierarchical structure of the ionic polymer gel encapsulated within the ecoflex ensures robust adhesion of the ionic conductive polymer gel (PEO/LiTFSI) to the ecoflex layers, addressing the issue of delamination in TENG components under mechanical stress. As a result, the GPE-TENG demonstrates high durability, enduring under stretching of approximately 375 % and sustaining heavy mechanical deformations (under folding, twisting, and rolling) over a long period (approximately 2 months) without loss of functionality. Remarkably, the GPE-TENG exhibits outstanding energy harvesting capabilities with a peak power density of 0.36 W m-2. Notably, the GPE-TENG generates electrical signals through simple device stretching, thus serving as a self-powered wearable sensor for human activity monitoring. Moreover, a 9-digital arrayed (3 × 3) flexible, semi-transparent, and self-powered touch panel based on the GPE-TENG shows multifunctionality, including touch track/pattern recognition (i.e. touch and sliding mode) and a highly accurate (∼98 %) deep learning assisted smart biometric system for user identification.

Enhancing self-induced polarization of PVDF-based triboelectric film by P-doped g-C3N4 for ultrasensitive triboelectric pressure sensors

Progress in wearable energy devices has reached the point of popularity over a few decades, as reflected by the rise of numerous sensors and storage devices driven by the urgent need for Internet of Things (IoT) applications. Triboelectric sensor (TES), on the basis of triboelectric nanogenerators (TENGs), exhibits high sensitivity and stability for pressure detection. Maximizing the sensitivity of the TES hinges upon the triboelectric layer selection capable of harnessing triboelectrification and electrostatic induction. Among them, PVDF has gained attention owing to its exceptional flexibility and tribo-negativity. However, the multiphase of pristine PVDF poses challenges, limiting its potential for TES with higher sensitivity. Tackling this issue, herein, we introduced phosphorus-doped graphitic carbon nitride (PCN) into the PVDF films to achieve self-induced polarization to enhance the sensitivity of the triboelectric pressure sensor. Through a controlled annealing temperature, phosphorus atoms are incorporated into the g-C3N4 (CN) matrix, enhancing the electronegativity of PVDF/PCN composite film while inducing a polarized β crystal phase in PVDF, thereby enabling greater attraction of positive charges. Consequently, the triboelectric output with PVDF/PCN composite film exhibited a significant improvement compared to the pristine PVDF film, achieving over a 100 V increment and the ability to power more than 100 LEDs. Furthermore, for the pressure-sensing applications, the TES with flexible PVDF/PCN triboelectric film exhibits a pressure sensitivity of 1.48 V/kPa, which shows a threefold increase in sensitivity compared to the pristine PVDF film (0.51 V/kPa), suggesting their great potential in self-powered wearable pressure sensors.

Continuous fabrication of triboelectric/piezoelectric hybrid yarns with high output for self-powered wearable sensors

Yarn-based nanogenerators exhibit potential for use in self-powered wearable sensors. However, their current limitations include intricate manufacturing procedures, low electrical output performance, and a restricted level of reliability. Here, we present a continuous fabrication approach for fabricating triboelectric/piezoelectric hybrid nanogenerators utilizing yarns with integrated exceptional performance. The continuous production process exhibits the attributes of one-step forming, wherein piezoelectric nanofibers and triboelectric silicone rubber covering are simultaneously and effectively attached to a single yarn to create yarn-based hybrid nanogenerators. The hybrid yarn demonstrates exceptional electrical output performance by harnessing the advantages of both piezoelectric and triboelectric effects, resulting in a power density of 26.72 μW/m2 at 17 MΩ. Demonstrating high sensitivity, stability, and mechanical resilience, yarn-based hybrid nanogenerators are suitable for self-powered wearable sensors, enabling applications in Morse code recognition and gait detection. This innovation in yarn-based nanogenerators broadens their scope, heralding new possibilities in the energy-harvesting device and wearable technology.

Flexible Textile Structures Composed of Various Functional Threads and Managing Circuitry for Self-Powered Wearable Sensor System

Flexible Textile Structures Composed of Various Functional Threads and Managing Circuitry for Self-Powered Wearable Sensor System

High-performance and flexible sodium ion-based electrochemical devices toward self-powered wearable volatile organic compounds sensing system

Self-powered wearable electronic systems are considered the ideal solution to realize ubiquitous Internet-of-Things (IoT), personalized healthcare, and point-to-care diagnosis. Here, we demonstrate the integration of a high-performance and ultra-stable sodium ion-based micro-supercapacitor (MSC) and a high-sensitive sensor for self-powered wearable Volatile Organic Compounds (VOC) sensing systems. It is found that a sodium-based solid electrolyte exhibited high ionic conductivity, reaching 2.4 mS cm−1 at 70 °C. Further, the inclusion of TiO2 NPs additive in the solid electrolyte can reduce the crystallinity and expand the electrode/electrolyte interface, which increases about 2.25 times in the capacitance of sodium ion-based MSC. PDMS-encapsulated MSC showed ultra-stability of capacitance retention (approximately 98.8 %) after 50,000 cycles while also achieving a high flexibility of 1000 cycles with a minimal capacitance change. As a proof-of-concept of a self-powered wearable sensor, it is demonstrated that an integrated polymer electrolyte-based VOC sensor powered by a charged-MSC can continuously operate for more than 4000 s (over 1.1 h) in the ethanol/nitrogen gas chamber. Therefore, we believe that our integration of sodium ion-based electrochemical storage and VOC sensor capabilities will pave a way to revolutionize the approach of wearable electronic applications toward self-powered systems.

Self-Powered Wearable Pressure Sensor System for Smart Healthcare Applications

Wearable sensors mark a groundbreaking advancement in the realm of health monitoring technology. However, traditional wearable devices lack comprehensive solutions for tracking sports exercise behaviors and providing feedback on rehabilitation for those undergoing home-based recovery. To bridge this gap, a self-powered triboelectric nanogenerator (TENG)-based wearable sensing system has been developed with in-house monitoring capabilities. The wearable sensor system is composed of triboelectric pressure sensors with an electro-conductive silicone sheet and patterned Ecoflex film as the contact layers. Highly sensitive TENG sensors have been successfully embedded into a bicycle saddle and human shoe insole for real-time pressure monitoring. Experimental results demonstrate that biomechanical energy, originating from both the pelvic-saddle interactions on bicycle and human gait movements, has been successfully converted into electrical energy. The generated sensor voltage remains stable under frequency variations and external environmental conditions, such as temperature and humidity variations. Utilizing distinctive information contained in individual body mass imbalances and gait characteristics, a smart healthcare monitoring system is envisioned to extract essential biomechanical insights via artificial intelligence, facilitating a pivotal role in in-house monitoring and tracking of sports rehabilitation exercises. The as-designed low-cost, highly scalable self-powered wearable sensor system offers an accessible, user-friendly point-of-care solution for gait detection and rehabilitation monitoring for individuals with various physical impairments, with considerable potential for commercialization. Figure 1

A flexible silicone tube arrangement structure triboelectric nanogenerator for tennis training monitoring

As the Internet of Things (IoTs) rapidly gain popularity, the demand for self-powered flexible electronic devices is continuously rising, particularly in the intelligent sports field. Hence, we introduced a silicone tube-based triboelectric nanogenerator (ST-TENG) designed for mechanical energy harvesting and tennis training monitoring. The ST-TENG, with its innovative tubular structure, effectively harvests low-frequency mechanical energy and converts it into electrical energy. At a working frequency of 6 Hz, the ST-TENG achieved an open-circuit voltage (Voc) of 122.51 V, a short-circuit current (Isc) of 15.05 µA, and a transfer charge (Qsc) of 33.74 nC. The ST-TENG demonstrates high sensitivity and accuracy in capturing subtle motion details, providing comprehensive data on various aspects of an athlete’s performance. The ST-TENG demonstrated excellent responsiveness to pressure and bending, making it suitable for real-time motion monitoring in tennis. Integrating the ST-TENG into the clothing and equipment of tennis players effectively monitored wrist, waist, and foot movements, providing detailed motion data. This research paves the way for developing highly efficient, self-powered wearable sensors that can significantly enhance the accuracy and sustainability of real-time athletic training monitoring.

Recent Advances in Self-Powered Wearable Flexible Sensors for Human Gaits Analysis.

The rapid progress of flexible electronics has met the growing need for detecting human movement information in exoskeleton auxiliary equipment. This study provides a review of recent advancements in the design and fabrication of flexible electronics used for human motion detection. Firstly, a comprehensive introduction is provided on various self-powered wearable flexible sensors employed in detecting human movement information. Subsequently, the algorithms utilized to provide feedback on human movement are presented, followed by a thorough discussion of their methods and effectiveness. Finally, the review concludes with perspectives on the current challenges and opportunities in implementing self-powered wearable flexible sensors in exoskeleton technology.

High-performance and robust biomimetic triboelectric nanogenerators for energy harvesting and self-powered wearable tactile sensing

High-performance and robust Triboelectric nanogenerators (TENGs) with biomimetic micro-structured PDMS and nanostructured aluminum as main tribo-layers have been proposed. Leaf of Calathea Zebrina plant is used as a mold to prepare PVA template which is further used to obtain micro-structured PDMS layer whereas nanostructures on aluminum surface have been created by water-assisted oxidation process. Corresponding TENG assembled in vertical contact-separation mode produces VOC ∼430 V and ISC ∼45 μA with high power density of 8.89 W m−2. Excellent performance stability observed at least over 20,000 cycles and up to 120 days is another remarkable feature of the proposed TENG. Further, these TENGs are found highly viable as self-powered wearable tactile sensors for several hand and foot-based tactile activities, which can precisely record spatial and temporal details of the activity. Such TENG development strategies, which involve mimicking of natural micro-patterns have huge performance potential and may play an integral role to augment commercial prospects of TENGs in their wide application spectrum.

Electromechanical Performance and Figure of Merit Optimization for Flexible Lead-Free PDMS-BaTiO 3 Piezocomposites.

In the modern era of the Internet of Things, the potential role of flexible piezoelectric generators (PEG) reflects the rapid increase in self-powered devices and wearable technologies. In this study, a casting process to elaborate the polydimethylsiloxane (PDMS)/barium titanate (BaTiO 3) composite has been presented. The addition of 15 wt % BaTiO 3 microparticles into the PDMS polymer greatly enhances the piezoelectric coefficient (d 31 = 24 pC N-1), leading to an increased output voltage of approximately 4 V under finger tapping force. The proposed flexible microgenerator yielded an excellent piezoelectric figure of merit (FoM 31 = 13.1 × 10-12 m2 N-1), significantly enhancing successfully the energy-harvesting performance (power density of 35 nW/cm2). Furthermore, the fabricated lead-free PEG exhibited an excellent flexibility figure of merit (fFoM) due to the low young modulus values (Maximum E = 3.4 MPa). These results indicate efficient energy conversion and demonstrate a favorable balance between the flexibility and piezoelectric properties of the composite, highlighting its potential for a wide range of applications in self-powered wearable sensors able to collect different human motions in applications such as gesture tracking and finger motion detection.

Light-Boosting Highly Sensitive and Ultrafast Piezoelectric Sensor Based on Composite Membrane of Copper Phthalocyanine and Graphene Oxide.

Self-powered wearable pressure sensors based on flexible electronics have emerged as a new trend due to the increasing demand for intelligent and portable devices. Improvements in pressure-sensing performance, including in the output voltage, sensitivity and response time, can greatly expand their related applications; however, this remains challenging. Here, we report on a highly sensitive piezoelectric sensor with novel light-boosting pressure-sensing performance, based on a composite membrane of copper phthalocyanine (CuPC) and graphene oxide (GO) (CuPC@GO). Under light illumination, the CuPC@GO piezoelectric sensor demonstrates a remarkable increase in output voltage (381.17 mV, 50 kPa) and sensitivity (116.80 mV/kPa, <5 kPa), which are approximately twice and three times of that the sensor without light illumination, respectively. Furthermore, light exposure significantly improves the response speed of the sensor with a response time of 38.04 µs and recovery time of 58.48 µs, while maintaining excellent mechanical stability even after 2000 cycles. Density functional theory calculations reveal that increased electron transfer from graphene to CuPC can occur when the CuPC is in the excited state, which indicates that the light illumination promotes the electron excitation of CuPC, and thus brings about the high polarization of the sensor. Importantly, these sensors exhibit universal spatial non-contact adjustability, highlighting their versatility and applicability in various settings.

3D printed electret as self-powered wearable sensor and futuristic implementation for on-spot bone injury

The importance of wearable pressure sensors extends to the progress of modern society and personal healthcare. The utility of currently available self-powered sensors is restricted in complex areas due to their limited geometrical configurations. In this study, an electret composed of polylactic acid (PLA) has been developed by 3D printing technology, where the pressure sensors with the desired complicated structure are possible to design. We demonstrate that the 3D-printed electret-based pressure sensor (3D-EPS) exhibits outstanding sensitivity (∼178 mV/kPa) with ultrafast response time (6 ms). It enables the capture of the biomechanical signals from different parts of bone-joint motions, which could be utterly suitable for monitoring injury recovery. We show that if the electret structure is printed as a shoe sole shape, then it generates 65 V of open circuit voltage and 12 μA of short circuit current during the human running action, indicating to use as an effective wearable energy harvester. It is also capable of detecting different motion and foot pressure distributions, which makes it a potential candidate as a wearable biomedical sensor. To demonstrate its futuristic potential applications in patient-specific bone damage recovery monitoring, the feasibilities of electrets with commonly desired structures have been shown to print.

Exploiting flow manipulation to engineer the electroactive phase for improved piezo response in size tunable PVDF microspheres via microfluidic technology

Polyvinylidene fluoride (PVDF), a semi-crystalline polymer with remarkable piezo-, ferro-, and pyroelectric properties in its electroactive phase (EA), holds significant potential. Microfluidic-based techniques, particularly droplet-microfluidics, have become a preferred method for producing PVDF microspheres, offering advantages like size and shape adjustability, simplicity, efficiency, and effective heat dissipation. This article focuses on employing a microfluidic flow focussing device (MFFD) with thermal initiated off-chip polymerization to process PVDF microspheres. The MFFD disperses uniform droplets into an oil bath, initiating polymerization and yielding microspheres ranging from 126 to 754 µm. Fine-tuning flow rates achieves precise control over microparticle uniformity, monodispersity, size, and spherical shape. Optimization of reaction temperature (Toil) significantly enhances the EA phase, reaching 82.05 % at Toil = 60 °C. The resulting microspheres are comprehensively characterized for structure, morphology, composition, phase, hydrophobicity, biocompatibility, thermal, and piezoelectric properties. Additionally, machine learning analysis is employed to predict the mean diameter as well as the EA and β-phase of PVDF microspheres. A flexible piezoelectric device made from PVDF microspheres, demonstrating an open circuit voltage of ∼23.5 V in response to finger and foot tapping movements, showcases potential for self-powered wearable sensors and devices. This study underscores the synergy of microfluidics, polymer science, and artificial intelligence in advancing smart materials.

Flexible hybrid nanogenerator coupling of triboelectric and photovoltaic effects based on fluoride dielectric regulation for energy harvesting

Recently, constructing hybrid nanogenerators (NGs) that can simultaneously harvest frictional and solar energies has attracted a growing interest, among which halide perovskite is considered as one of the most suitable active materials. However, current perovskite-based hybrid NGs are low in performance and limited to rigid substrates. Strategies to minimize performance loss caused by deep defects in perovskite functional layer are still lacking. In this work, we proposed a fluoride additive passivation strategy to fabricate a new kind of flexible, efficient perovskite-based hybrid NGs. The fluoride with high electronegativity can effectively passivate defect density of perovskite film and further enhance its dielectric constant to minimize the ability of defects to capture charges. Hence, more efficient charge transport and suppressed recombination loss are achieved, which maximizes the triboelectric-photovoltaic coupling effect. The maximum current of the hybrid NG is up to 9.7 μA under illumination, which is about 25 times higher than that of the control device. The hybrid NG could light 50 LEDs and charge capacitors. Moreover, it exhibited excellent flexibility to act as a self-powered wearable sensor to detect human motions. Our work proposed a new strategy for flexible triboelectric-photovoltaic hybrid NGs with high-output performance for smart wearable applications.

Microelectronic printed chitosan/chondroitin sulfate/ZnO flexible and environmentally friendly triboelectric nanogenerator

The triboelectric nanogenerator (TENG) of natural biomaterials is a new type of energy harvesting device and can be used as a self-powered sensor, which has received extensive research and attention. In this paper, based on the biocompatibility of chitosan and chondroitin sulfate, ZnO-modified chitosan/chondroitin sulfate/ZnO TENG was prepared for research on wearable devices and sustainable power supply devices. This study employs molecular dynamics to compute the interaction energy between chitosan and ZnO molecules. Theoretical calculations have unequivocally substantiated the occurrence of a binding interaction between these two molecular entities. The effect of ZnO on chitosan/chondroitin sulfate morphology was investigated by atomic force microscopy. The chitosan/chondroitin sulfate/ZnO TENG has high flexibility and electrical output performance. It can reach 105 V and 3.3 µA of open-circuit voltage and short-circuit current. Chitosan/chondroitin Sulfate/ZnO TENG successfully converts the mechanical energy of human motion into electrical energy. Strong electrical signals are exhibited when making fists and waving fingers and wrists. The TENG is a self-powered source and lights up 70 blue light-emitting diodes (LEDs). The chitosan/chondroitin sulfate/ZnO TENG has demonstrated its capabilities in energy harvesting and wearable self-powered sensors.

Electrospun PAN/BaTiO3/MXene nanofibrous membrane with significantly improved piezoelectric property for self-powered wearable sensor

The power supply system is an important part of modern electronic equipment. Traditional batteries have disadvantages such as large volume and poor flexibility. Nanogenerators possess characteristics such as miniaturization, sustainable power supply, and independence from external energy sources. In this work, a piezo-triboelectric hybridized nanogenerator is developed. Barium titanate (BaTiO3) and MXene (Ti3C2) are incorporated into the electrospinning process of polyacrylonitrile (PAN), resulting in an electrospun PAN/BaTiO3/MXene nanofibrous membrane with significantly enhanced piezoelectric properties in comparison with the pure PAN fibrous membrane. Meanwhile, the electronegative properties of the PAN/BaTiO3/MXene nanofibrous membrane are augmented as the negative layer during contact and separation friction. The piezoelectric output voltage of the PAN/BaTiO3/MXene nanofibrous membrane reaches up to 6.4 V. When a nickel-copper conductive fabric is used as the positive layer, the piezo-triboelectric hybridized nanogenerator produces an output voltage of 11.3 V, an output current of 300nA, and a maximum output power density of 3.4 mW·m−2 over a contact-separation area of only 2 × 2 cm2. In addition, given that MXene itself exhibits excellent photothermal conversion ability, the PAN/BaTiO3/MXene nanofibrous membrane can be quickly heated up to about 54℃ under near-infrared irradiation at a power of 0.8 W·cm−2, highlighting its potential in photothermal-therapy applications.

High-sensitivity wearable multi-signal sensor based on self-powered MXene hydrogels

Developing high-performance wearable sensors with multi-sensing capabilities for different signals such as temperature, human motion and physiological signals remains a challenge. In this report, a novel catechol functionalization of PVA-CA/Poly(N-acryloyl glycinamide)/MXene hydrogels(PcNA-M) was fabricated by combining the thermosensitivity of Poly(N-acryloyl glycinamide)(PNAGA) hydrogel and the self-powered properties of the catechol functionalization of PVA(Pc)/MXene hydrogels. The mixing ratio of PNAGA and Pc was adjusted to impart the resulting PcNA-M with good tensile, compression, rebound properties, rapid self-healing and temperature sensing properties. In addition, the conductivity of the hydrogel was found to have contributions from ionic and electrical conductivity. To produce different forms of electrical signals for different signal sources, the PcNA-M sensor was utilized to monitor the resistive and voltage signals during body movements, switching temperature and handwriting motion. The PcNA-M sensor exhibits high sensitivity, identifiability, and reliability in applications of self-powered wearable sensors, and demonstrates great prospects in the field of low-frequency mechanical energy harvesting. This study opens a new possibility for wearable self-powered multi-signal sensors to monitor different signals related to human motion.

Boosting the Piezoelectric Response and Interfacial Compatibility in Flexible Piezoelectric Composites via DET-Doping BT Nanoparticles.

With the advent of the Internet of Things, self-powered wearable sensors have become increasingly prevalent in our daily lives. The utilization of piezoelectric composites to harness and sense surrounding mechanical vibrations has been extensively investigated during the last decades. However, the poor interface compatibility between ceramics nanofillers and polymers matrix, as well as low piezoelectric performance, still serves as a critical challenge. In this work, we employed Di(dioctylpyrophosphato) ethylene titanate (DET) as the coupling agent for modifying barium titanate (BTO) nanofillers. Compared to the BTO/PVDF counterpart, the DET-BTO/PVDF nanofibers exhibit an augmented content of piezoelectric β phase (~85.7%) and significantly enhanced stress transfer capability. The piezoelectric coefficient (d33) is up to ~40 pC/N, which is the highest value among reported BTO/PVDF composites. The piezoelectric energy harvesters (PEHs) present benign durability and attain a high instantaneous power density of 276.7 nW/cm2 at a matched load of 120 MΩ. Furthermore, the PEHs could sense various human activities, with the sensitivity as high as 0.817 V/N ranging from 0.05-0.1 N. This work proposes a new strategy to boosting the piezoelectric performance of PVDF-based composites via DET-doping ceramics nanoparticles, and in turn show significantly improved energy harvesting and sensing capability.

Transformation of ZnS microspheres to non-centrosymmetric nanoplates for the fabrication of flexible piezoelectric nanogenerators: Electrode surface area modification for boosting the output performance

We report the transformation of ZnS particles from microsphere to 2D nanoplate having a non-centrosymmetric phase by optimizing growth parameters of hydrothermal method, and fabrication of flexible piezoelectric nanogenerators (PENGs). ZnS nanoplates-based PENG showed maximum open circuit voltage and short circuit current of ∼ 3.8 V and 1.8 μA, respectively. Thereafter, surface of aluminium (Al) electrode was modified by making parallel and cross-parallel lines using a cost-effective technique. Surface-modified Al-electrode-based PENG device generated ∼ 6.6 and 2.5 times higher open circuit voltage and short circuit current, respectively, with a power density of ∼ 41.5 μWcm−2. This device was successfully used as portable electronics by flashing 15 red LEDs after charging a 1 μF capacitor up to 7.1 V in 75 s. Furthermore, the device exhibited noticeable electric output signals under various kinds of human body motions, which makes it a promising candidate for self-powered wearable sensor applications.