Transparent and Flexible Self-Charging Power Film and Its Application in a Sliding Unlock System in Touchpad Technology

Self-Powered Sensing in Wearable Electronics─A Paradigm Shift Technology

With the advancements in materials science and micro/nanoengineering, the field of wearable electronics has experienced a rapid growth and significantly impacted and transformed various aspects of daily human life. These devices enable individuals to conveniently access health assessments without visiting hospitals and provide continuous, detailed monitoring to create comprehensive health data sets for physicians to analyze and diagnose. Nonetheless, several challenges continue to hinder the practical application of wearable electronics, such as skin compliance, biocompatibility, stability, and power supply. In this review, we address the power supply issue and examine recent innovative self-powered technologies for wearable electronics. Specifically, we explore self-powered sensors and self-powered systems, the two primary strategies employed in this field. The former emphasizes the integration of nanogenerator devices as sensing units, thereby reducing overall system power consumption, while the latter focuses on utilizing nanogenerator devices as power sources to drive the entire sensing system. Finally, we present the future challenges and perspectives for self-powered wearable electronics.

Recent progress in flexible pressure sensors based on multiple microstructures: from design to application

Flexible pressure sensors (FPSs) have been widely studied in the fields of wearable medical monitoring and human-machine interaction due to their high flexibility, light weight, sensitivity, and easy integration. To better meet these application requirements, key sensing properties such as sensitivity, linear sensing range, pressure detection limits, response/recovery time, and durability need to be effectively improved. Therefore, researchers have extensively and profoundly researched and innovated on the structure of sensors, and various microstructures have been designed and applied to effectively improve the sensing performance of sensors. Compared with single microstructures, multiple microstructures (MMSs) (including hierarchical, multi-layered and hybrid microstructures) can improve the sensing performance of sensors to a greater extent. This paper reviews the recent research progress in the design and application of FPSs with MMSs and systematically summarizes the types, sensing mechanisms, and preparation methods of MMSs. In addition, we summarize the applications of FPSs with MMSs in the fields of human motion detection, health monitoring, and human-computer interaction. Finally, we provide an outlook on the prospects and challenges for the development of FPSs.

Flexible Wood-Based Triboelectric Self-Powered Smart Home System

With the rapid development of the Internet of Things, artificial intelligence, and big data, the smart home has played a critical role in human life and smart cities, where large amounts of distributed sensors should be applied. Here, we report a natural wood-based triboelectric self-powered sensor (WTSS) for building the smart home system. Based on an effective and simple treatment strategy for natural wood, the WTSS shows superior sensitivity, flexibility, stability, and thinness. Owing to the extensive use of wood materials in home construction, the WTSS is integrated with household facilities and applied in three real-time human-machine interfaces, including a smart home control system, a smart password gate control system, and a smart floor monitoring system, with advantages of low cost, easy operation, and eco-friendliness. This work promotes the development of wood-based flexible electronics and shows potential applications in the construction of smart homes and future cities.

Self-Powered Smart Arm Training Band Sensor Based on Extremely Stretchable Hydrogel Conductors

The development of elastic electronic technology has promoted the application of triboelectric nanogenerators (TENGs) in flexible wearable electronics. However, most of the flexible electronics cannot achieve the requirements of being extremely stretchable, transparent, and highly conductive at the same time. Herein, we report a TENG constructed using a double-network polymer ionic conductor sodium alginate/zinc sulfate/poly acrylic-acrylamide (SA-Zn) hydrogel, which exhibited outstanding stretchability (>10,000%), high transparency (>95%), and good conductivity (0.34 S·m^-1). The SA-Zn hydrogel TENG (SH-TENG) could harvest energy from typical human movements, such as bending, stretching, and twisting, which could light up 234 green commercial LEDs easily. Additionally, the SH-TENG can be used to prepare a self-powered smart training band sensor for monitoring arm stretching motion. This work may provide an innovative platform for accessing the next generation of sustainable wearable and sports monitoring electronics.

Advances in Smart Sensing and Medical Electronics by Self-Powered Sensors Based on Triboelectric Nanogenerators

With the rapid progress of artificial intelligence, humans are moving toward the era of the intelligent connection of all things. Therefore, the demand for sensors is drastically increasing with developing intelligent social applications. Traditional sensors must be triggered by an external power source and the energy consumption is high for equipment that is widely distributed and working intermittently, which is not conducive to developing sustainable green and healthy applications. However, self-powered sensors based on triboelectric nanogenerators (TENG) can autonomously harvest energy from the surrounding environment and convert this energy into electrical energy for storage. Sensors can also be self-powered without an external power supply, which is vital for smart cities, smart homes, smart transportation, environmental monitoring, wearable devices, and bio-medicine. This review mainly summarizes the working mechanism of TENG and the research progress of self-powered sensors based on TENG about the Internet of Things (IoT), robotics, human–computer interaction, and intelligent medical fields in recent years.

Manipulating Electrical Properties of Silica-Based Materials via Atomic Oxygen Irradiation

Regulated triboelectrification has attracted considerable research attention due to its potential applications in harvesting energy and importance in antistatic protection. Irradiation is an effective and stable modification method due to its adjustable and uniform irradiation parameters. Moreover, atomic oxygen (AO) irradiation is an important component in the low earth orbit, which is a considerable factor for promoting triboelectric nanogenerators (TENGs) in the outer space. AO irradiation was utilized to manipulate the surface structure and chemical composition to regulate electrical properties. AO irradiation can increase electron-donating groups and enhance electrical positivity of polydimethylsiloxane (PDMS) films due to the transition from Si-C bonds to Si-O bonds. Therefore, different trends of polytetrafluoroethylene (PTFE) and polystyrene (PS) were caused by their TENG composition with irradiated PDMS. Tribocharge cross-over polarity and charge generation were prevented completely in PS- and PDMS-based TENGs by adjusting the irradiation time to 4.1 h. Short-circuit current enhanced from 5 to 22 μA and the output voltage increased from 160 to 760 V when PDMS films in PTFE- and PDMS-based TENGs were subjected to AO irradiation for 6 h. This study demonstrated that AO irradiation can manipulate triboelectric properties of silica-based materials, which are potential components for harvesting energy and preventing electrostatic hazard in the outer space.

The Triboelectric Nanogenerator as an Innovative Technology toward Intelligent Sports

In the new era of the Internet-of-Things, athletic big data collection and analysis based on widely distributed sensing networks are particularly important in the development of intelligent sports. Conventional sensors usually require an external power supply, with limitations such as limited lifetime and high maintenance cost. As a newly developed mechanical energy harvesting and self-powered sensing technology, the triboelectric nanogenerator (TENG) shows great potential to overcome these limitations. Most importantly, TENGs can be fabricated using wood, paper, fibers, and polymers, which are the most frequently used materials for sports. Recent progress on the development of TENGs for the field of intelligent sports is summarized. First, the working mechanism of TENG and its association with athletic big data are introduced. Subsequently, the development of TENG-based sports sensing systems, including smart sports facilities and wearable equipment is highlighted. At last, the remaining challenges and open opportunities are also discussed.

Triboelectric nanogenerators for human-health care

Since the world's first triboelectric nanogenerator (TENG) was proposed in 2012, numerous TENG-based devices and equipment have sprung up in various fields. In particular, TENG has great potential in the field of human-health care due to its small size, self-powered and low cost. With the continuous deepening of TENG research, its structure, function and technical concept are becoming more and more abundant. In order to summarize the progress and development status of TENG in health care, based on the different types of applications subdirection, this paper reviews the TENG-based research work of this field in recent eight years. The characteristics of various types of TENG-based applications and their corresponding technologies are introduced and analyzed, under the comparison of their structure and performance. This review is dedicated to provide reference and inspiration for the future development and innovation of TENG for health care.

An acid and alkali-resistant triboelectric nanogenerator

With the development of technology, environmental problems have become more and more acute and the use of electronic devices in harsh environments has gradually attracted attention. For example, the friction layer of triboelectric nanogenerators (TENGs) may be contaminated and corroded in harsh environments (such as acidic, alkaline or oily environments), resulting in damage or destruction of the TENGs. In this study, we use electrospinning followed by a sintering process to prepare a super-hydrophobic sintered polyvinyl alcohol-polytetrafluoroethylene (S-PVA-PTFE) composite membrane and general industrial oil-absorbing paper to construct a TENG. The maximum power density of the TENG is 261 mW m-2, it can light up 100 blue LEDs, and can power a variety of small electronic devices. Moreover, after 72 h of soaking the friction layer in a strong acid solution followed by a strong alkali solution, the performance of the TENG has no obvious change. The TENG can work stably in an oily working environment. The TENG provides a novel approach for self-powered sensors that work in harsh environments.

Self-charging power system for distributed energy: beyond the energy storage unit

Power devices for the smart sensor networks of Internet of things (IoT) are required with minimum or even no maintenance due to their enormous quantities and widespread distribution. Self-charging power systems (SCPSs) refer to integrated energy devices with simultaneous energy harvesting, power management and effective energy storage capabilities, which may need no extra battery recharging and can sustainably drive sensors. Herein, we focus on the progress made in the field of nanogenerator-based SCPSs, which harvest mechanical energy using the Maxwell displacement current arising from the variation in the surface-polarized-charge-induced electrical field. Prototypes of different nanogenerator-based SCPSs will be overviewed. Finally, challenges and prospects in this field will be discussed.

Transparent and flexible high-power supercapacitors based on carbon nanotube fibre aerogels

In this work, we report the fabrication of continuous transparent and flexible supercapacitors by depositing a CNT network onto a polymer electrolyte membrane directly from an aerogel of ultra-long CNTs produced floating in the gas phase. The supercapacitors show a combination of a power density of 1370 kW kg-1 at high transmittance (ca. 70%), and high electrochemical stability during extended cycling (>94% capacitance retention over 20 000 cycles) and against repeated 180° flexural deformation. They represent a significant enhancement of 1-3 orders of magnitude compared to prior state-of-the-art transparent supercapacitors based on graphene, CNTs, and rGO. These features mainly arise from the exceptionally long length of CNTs, which makes the material behave as a bulk conductor instead of an aspect ratio-limited percolating network, even for electrodes with >90% transparency. The electrical and capacitive figures-of-merit for the transparent conductor are FoMe = 2.7, and FoMc = 0.46 F S-1 cm-2 respectively.

Three-Dimensional SiC/Holey-Graphene/Holey-MnO2 Architectures for Flexible Energy Storage with Superior Power and Energy Densities

Although nanostructured materials have recently enabled a dramatic improvement of the current energy-storage units in portable electronics with enhanced functionality, it is still challenging to provide a cost-efficient solution to attain the ultrahigh energy and power densities of supercapacitors (SCs) since nearly arbitrary electrodes are limited to the thinner porous structure with de facto rather low mass loading (∼1 mg cm^-2) because of the huge limitations of pronounced impaired ion transport in subnanometer pores in thicker compact electrodes. In this contribution, we report the fabrication of a macro/mesoporous hybrid hierarchical nanocomposite SiC/holey-graphene/holey-MnO₂ (SiC/HG/h-MnO₂) with tailored porosity by knitting together the quasi-aligned single-crystalline doped 3C-SiC nanowire array and in situ surface-reduced holey graphene framework into a three-dimensional quasi-ordered structure, which enables the mass growth of ultrathin h-MnO₂ nanosheets at approximately practical levels of mass loading. The produced synergistically favorable interconnected porous architecture allows for the highly efficient electron transfer and rapid ion transport up to interior surfaces of the network. Remarkably, the all-solid-state flexible asymmetric supercapacitors (ASCs) made with SiC/HG/h-MnO₂ and SiC/graphitic carbon (GC) nanoarrays are mechanically robust and show a high areal capacity (0.32 mWh cm^-2) and a high rate capability (280 mW cm^-2) at ultrahigh mass loading (6.5 mg cm^-2), much higher than most of previous superior SCs in aqueous or gelled electrolytes and thus offer an entirely new prototype of textile-based ASCs, which represents a critical step toward practical applications for various portable electronics.

Motion Detection Using Tactile Sensors Based on Pressure-Sensitive Transistor Arrays

In recent years, to develop more spontaneous and instant interfaces between a system and users, technology has evolved toward designing efficient and simple gesture recognition (GR) techniques. As a tool for acquiring human motion, a tactile sensor system, which converts the human touch signal into a single datum and executes a command by translating a bundle of data into a text language or triggering a preset sequence as a haptic motion, has been developed. The tactile sensor aims to collect comprehensive data on various motions, from the touch of a fingertip to large body movements. The sensor devices have different characteristics that are important for target applications. Furthermore, devices can be fabricated using various principles, and include piezoelectric, capacitive, piezoresistive, and field-effect transistor types, depending on the parameters to be achieved. Here, we introduce tactile sensors consisting of field-effect transistors (FETs). GR requires a process involving the acquisition of a large amount of data in an array rather than a single sensor, suggesting the importance of fabricating a tactile sensor as an array. In this case, an FET-type pressure sensor can exploit the advantages of active-matrix sensor arrays that allow high-array uniformity, high spatial contrast, and facile integration with electrical circuitry. We envision that tactile sensors based on FETs will be beneficial for GR as well as future applications, and these sensors will provide substantial opportunities for next-generation motion sensing systems.

Flexible and durable wood-based triboelectric nanogenerators for self-powered sensing in athletic big data analytics

In the new era of internet of things, big data collection and analysis based on widely distributed intelligent sensing technology is particularly important. Here, we report a flexible and durable wood-based triboelectric nanogenerator for self-powered sensing in athletic big data analytics. Based on a simple and effective strategy, natural wood can be converted into a high-performance triboelectric material with excellent mechanical properties, such as 7.5-fold enhancement in strength, superior flexibility, wear resistance and processability. The electrical output performance is also enhanced by more than 70% compared with natural wood. A self-powered falling point distribution statistical system and an edge ball judgement system are further developed to provide training guidance and real-time competition assistance for both athletes and referees. This work can not only expand the application area of the self-powered system to smart sport monitoring and assisting, but also promote the development of big data analytics in intelligent sports industry.

Intelligent sensing technologies gain interest for the internet of things and applications that require collection and analysis of big data. Here the authors report a flexible and durable wood-based triboelectric nanogenerator for self-powered sensing in athletic big data analytics.

Application of Displacement-Current-Governed Triboelectric Nanogenerator in an Electrostatic Discharge Protection System for the Next-Generation Green Tire

Electrostatic discharge (ESD), a universal phenomenon derived from tribocharging and electrostatic induction, has always been regarded as a negative effect because it may cause various types of damage, such as gas explosions, wildfires, failure of integrated circuits, and so on. Normally, ESD is avoided by conducting those harmful charges through the surface or the whole bulk, by means of improving the conductivity of dielectrics or directly using conductive materials. However, the first approach compromises other performances at the same time, whereas the second one can be applied in only a few circumstances. In this Article, we analyzed the working principle of the triboelectric nanogenerator from the perspective of the time variation of the surface-charge-introduced polarization density ∂Ps∂t , the second term of Maxwell's displacement current. Then, we demonstrated an electrostatic protective system by implanting a conductive layer under the tribocharging surface to form a triboelectric nanogenerator (TENG). Theoretical derivation, finite element analysis, and experimental results prove that this system can efficiently prevent ESD without sacrificing any other performance. Finally, we applied it to the next generation of the green tire, which can save >10% of fuel but still cannot be commercialized due to the potential ESD risk. This research work reveals a way to prevent ESD and shows great potential in the field of engineering.

Nanogenerator-Based Self-Charging Energy Storage Devices

One significant challenge for electronic devices is that the energy storage devices are unable to provide sufficient energy for continuous and long-time operation, leading to frequent recharging or inconvenient battery replacement. To satisfy the needs of next-generation electronic devices for sustainable working, conspicuous progress has been achieved regarding the development for nanogenerator-based self-charging energy storage devices. Herein, the development of the self-charging energy storage devices is summarized. Focus will be on preparation of nanomaterials for Li-ion batteries and supercapacitors, structural design of the nanogenerator-based self-charging energy storage devices, performance testing, and potential applications. Moreover, the challenges and perspectives regarding self-charging energy storage devices are also discussed.

Triboelectric Nanogenerator Driven Self-Charging and Self-Healing Flexible Asymmetric Supercapacitor Power Cell for Direct Power Generation

The expeditious growth of portable electronics has endorsed the researchers to develop self-powered devices that synchronically harvest and store energy. However, it is quite challenging to integrate two distinct phenomena in a single portable device. Here, we emphasize the fabrication of a triboelectric driven self-charging and self-healing asymmetric supercapacitor (SCSHASC) power cell composed of magnetic cobalt ferrite grown on a stainless steel (SS) fabric (CoFe₂O₄@SS) as positive and iron oxides decorated reduced graphene oxide grown on a SS fabric (Fe-RGO@SS) as negative electrodes separated by a KOH-soaked self-healing polymer hydrogel electrolyte membrane. The membrane contains Fe³⁺ cross-linked polyacrylic acid, whereas self-healing carboxylated polyurethane was utilized for encapsulation. SS fabric and poly(vinylidene fluoride- co-hexafluoropropylene) (PVDF-HFP)/SS-impregnated micropatterned PDMS composite film-strip were employed as positive and negative triboelectric friction layers, respectively. During mechanical deformation, the SCSHASC harvests electrical energy and subsequently stores it as electrochemical energy for sustainable power supply. The sandwich-type SCSHASC power cell (a supercapacitor unit sandwiched between two parallelly connected high-performance triboelectric nanogenerators) was charged up to ∼1.6 V within ∼31 min under periodic compression/stress ( F ≈ 17.6 N, f ≈ 3.80 Hz). Furthermore, the SCSHASC# (with two supercapacitor units in series) can instantly power-up several portable electronic appliances on periodic compression and release. Thus, the SCSHASC with unique design will be extremely beneficial for self-powered electronics.

Crystal-confined freestanding ionic liquids for reconfigurable and repairable electronics

Liquid sensors composed of ionic liquids are rising as alternatives to solid semiconductors for flexible and self-healing electronics. However, the fluidic nature may give rise to leakage problems in cases of accidental damages. Here, we proposed a liquid sensor based on a binary ionic liquid system, in which a flowing ionic liquid [OMIm]PF₆ is confined by another azobenzene-containing ionic liquid crystalline [OMIm]AzoO. Those crystal components provide sufficient pinning capillary force to immobilize fluidic components, leading to a freestanding liquid-like product without the possibility of leakage. In addition to owning ultra-high temperature sensitivity, crystal-confined ionic liquids also combine the performances of both liquid and solid so that it can be stretched, bent, self-healed, and remolded. With respect to the reconfigurable property, this particular class of ionic liquids is exploited as dynamic circuits which can be spatially reorganized or automatically repaired.

Tailoring the energy band in flexible photodetector based on transferred ITO/Si heterojunction via interface engineering

Interface engineering is an important method to modulate electronic structures for improving the physical properties of semiconductors as well as designing novel devices. Recently, development of flexible electronic devices based on inorganic thin films on flexible substrates, which provides solutions to meet the emerging technological demands, may also expend the methodology of interface engineering. Herein, a semitransparent photodetector based on an indium-tin oxide (ITO)-on-silicon (Si) heterojunction was fabricated on a flexible substrate and investigated under mechanical bending strains. It is found that the barrier height of the heterojunction can be tailored continuously and reversibly from 0.23 eV to 0 eV, corresponding to the Schottky and Ohmic junctions respectively. Meanwhile, the turn-on voltage and the response time of the as-prepared photodetector can be obviously reduced under bending strain, which can be attributed to the modulation of the Si bandgap and hole mobility. Our experimental studies not only shed new light on the strain modulation mechanism of the heterojunction interface, but also pave a prominent way to integrated high-performance flexible photodetectors.

Toward Wearable Self-Charging Power Systems: The Integration of Energy-Harvesting and Storage Devices

One major challenge for wearable electronics is that the state-of-the-art batteries are inadequate to provide sufficient energy for long-term operations, leading to inconvenient battery replacement or frequent recharging. Other than the pursuit of high energy density of secondary batteries, an alternative approach recently drawing intensive attention from the research community, is to integrate energy-generation and energy-storage devices into self-charging power systems (SCPSs), so that the scavenged energy can be simultaneously stored for sustainable power supply. This paper reviews recent developments in SCPSs with the integration of various energy-harvesting devices (including piezoelectric nanogenerators, triboelectric nanogenerators, solar cells, and thermoelectric nanogenerators) and energy-storage devices, such as batteries and supercapacitors. SCPSs with multiple energy-harvesting devices are also included. Emphasis is placed on integrated flexible or wearable SCPSs. Remaining challenges and perspectives are also examined to suggest how to bring the appealing SCPSs into practical applications in the near future.

An alginate film-based degradable triboelectric nanogenerator

Alginate, as a natural linear polysaccharide derived from brown sea algae, has the advantage of low toxicity, good biocompatibility, and biodegradability, which has aroused wide interests in recent years. In this study, a degradable triboelectric generator based on an alginate film is presented. The calcium alginate film, which is prepared by a simple freeze-drying method and a crosslinking reaction, has a form of porous structures that are beneficial for triboelectric power generation. The fabricated TENG has a stable output performance with a maximum voltage, current, and power of 33 V, 150 nA, and 9.5 μW, respectively. The performances of the TENG were investigated at different thicknesses of the calcium alginate film and various concentrations of the sodium alginate solution, as well as the degradability of the film with different thicknesses and temperatures. In addition, the TENG was designed for harvesting water wave energy in a low-frequency range from 1 to 4 Hz. This study is promising to provide new insights to develop degradable and eco-friendly TENG based on ocean plants and expand the application range in blue energy.

A degradable triboelectric generator based on an alginate film has been proposed, which can be used to harvest water wave energy.

Flexible Organic Tribotronic Transistor for Pressure and Magnetic Sensing

Flexible electronics has attracted enormous interest in wearable electronics and human-machine interfacing. Here, a flexible organic tribotronic transistor (FOTT) without a top gate electrode has been demonstrated. The FOTT is fabricated on a flexible polyethylene terephthalate film using the p-type pentacene and poly(methyl methacrylate)/Cytop composites as the conductive channel and dielectric layer, respectively. The charge carriers can be modulated by the contact electrification between the dielectric layer and a mobile triboelectric layer. Based on the fabricated FOTT, pressure and magnetic sensors have been developed, respectively, that exhibit great sensitivity, fast response time, and excellent stability. The FOTT in this simple structure shows bright potentials of tribotronics in human-machine interaction, electronic skins, wearable electronics, intelligent sensing, and so on.

Transparent and Flexible Triboelectric Sensing Array for Touch Security Applications

Tactile sensors with large-scale array and high sensitivity is essential for human-machine interaction, smart wearable devices, and mobile networks. Here, a transparent and flexible triboelectric sensing array (TSA) with fingertip-sized pixels is demonstrated by integrating ITO electrodes, FEP film, and signal transmission circuits on an undivided palm-sized polyethylene terephthalate substrate. The sensing pixels can be triggered by the corresponding external contact to induce the electrostatic potential in the transparent electrodes without power consumption, which is individually recognized by the sensor. By testing the response of the pixels, the electrical characterization is systematically investigated. The proposed TSA exhibits excellent durability, independence, and synchronicity, which is able to realize real-time touch sensing, spatial mapping, and motion monitoring. The integrated TSA has great potential for an active tactile system, human-machine interface, wearable electronics, private communication, and advanced security identification.

Gold nanoparticle-embedded silk protein-ZnO nanorod hybrids for flexible bio-photonic devices

Silk protein has been used as a biopolymer substrate for flexible photonic devices. Here, we demonstrate ZnO nanorod array hybrid photodetectors on Au nanoparticle-embedded silk protein for flexible optoelectronics. Hybrid samples exhibit optical absorption at the band edge of ZnO as well as plasmonic energy due to Au nanoparticles, making them attractive for selective UV and visible wavelength detection. The device prepared on Au-silk protein shows a much lower dark current and a higher photo to dark-current ratio of ∼10⁵ as compared to the control sample without Au nanoparticles. The hybrid device also exhibits a higher specific detectivity due to higher responsivity arising from the photo-generated hole trapping by Au nanoparticles. Sharp pulses in the transient photocurrent have been observed in devices prepared on glass and Au-silk protein substrates due to the light induced pyroelectric effect of ZnO, enabling the demonstration of self-powered photodetectors at zero bias. Flexible hybrid detectors have been demonstrated on Au-silk/polyethylene terephthalate substrates, exhibiting characteristics similar to those fabricated on rigid glass substrates. A study of the performance of photodetectors with different bending angles indicates very good mechanical stability of silk protein based flexible devices. This novel concept of ZnO nanorod array photodetectors on a natural silk protein platform provides an opportunity to realize integrated flexible and self-powered bio-photonic devices for medical applications in near future.