Electrospun ion gel nanofibers for flexible triboelectric nanogenerator: electrochemical effect on output power

Self-powered high-sensitivity all-in-one vertical tribo-transistor device for multi-sensing-memory-computing

Devices with sensing-memory-computing capability for the detection, recognition and memorization of real time sensory information could simplify data conversion, transmission, storage, and operations between different blocks in conventional chips, which are invaluable and sought-after to offer critical benefits of accomplishing diverse functions, simple design, and efficient computing simultaneously in the internet of things (IOT) era. Here, we develop a self-powered vertical tribo-transistor (VTT) based on MXenes for multi-sensing-memory-computing function and multi-task emotion recognition, which integrates triboelectric nanogenerator (TENG) and transistor in a single device with the simple configuration of vertical organic field effect transistor (VOFET). The tribo-potential is found to be able to tune ionic migration in insulating layer and Schottky barrier height at the MXene/semiconductor interface, and thus modulate the conductive channel between MXene and drain electrode. Meanwhile, the sensing sensitivity can be significantly improved by 711 times over the single TENG device, and the VTT exhibits excellent multi-sensing-memory-computing function. Importantly, based on this function, the multi-sensing integration and multi-model emotion recognition are constructed, which improves the emotion recognition accuracy up to 94.05% with reliability. This simple structure and self-powered VTT device exhibits high sensitivity, high efficiency and high accuracy, which provides application prospects in future human-mechanical interaction, IOT and high-level intelligence.

Transparent, Ultra-Stretching, Tough, Adhesive Carboxyethyl Chitin/Polyacrylamide Hydrogel Toward High-Performance Soft Electronics

To date, hydrogels have gained increasing attentions as a flexible conductive material in fabricating soft electronics. However, it remains a big challenge to integrate multiple functions into one gel that can be used widely under various conditions. Herein, a kind of multifunctional hydrogel with a combination of desirable characteristics, including remarkable transparency, high conductivity, ultra-stretchability, toughness, good fatigue resistance, and strong adhesive ability is presented, which was facilely fabricated through multiple noncovalent crosslinking strategy. The resultant versatile sensors are able to detect both weak and large deformations, which owns a low detection limit of 0.1% strain, high stretchability up to 1586%, ultrahigh sensitivity with a gauge factor up to 18.54, as well as wide pressure sensing range (0-600 kPa). Meanwhile, the fabrication of conductive hydrogel-based sensors is demonstrated for various soft electronic devices, including a flexible human-machine interactive system, the soft tactile switch, an integrated electronic skin for unprecedented nonplanar visualized pressure sensing, and the stretchable triboelectric nanogenerators with excellent biomechanical energy harvesting ability. This work opens up a simple route for multifunctional hydrogel and promises the practical application of soft and self-powered wearable electronics in various complex scenes.

High-Performance Triboelectric Nanogenerators Based on Commercial Textiles: Electrospun Nylon 66 Nanofibers on Silk and PVDF on Polyester

A high-performance textile triboelectric nanogenerator is developed based on the common commercial fabrics silk and polyester (PET). Electrospun nylon 66 nanofibers were used to boost the tribo-positive performance of silk, and a poly(vinylidene difluoride) (PVDF) coating was deployed to increase the tribo-negativity of PET. The modifications confer a very significant boost in performance: output voltage and short-circuit current density increased ∼17 times (5.85 to 100 V) and ∼16 times (1.6 to 24.5 mA/m²), respectively, compared with the Silk/PET baseline. The maximum power density was 280 mW/m² at a 4 MΩ resistance. The performance boost likely results from enhancing the tribo-positivity (and tribo-negativity) of the contact layers and from increased contact area facilitated by the electrospun nanofibers. Excellent stability and durability were demonstrated: the nylon nanofibers and PVDF coating provide high output, while the silk and PET substrate fabrics confer strength and flexibility. Rapid capacitor charging rates of 0.045 V/s (2 μF), 0.031 V/s (10 μF), and 0.011 V/s (22 μF) were demonstrated. Advantages include high output, a fully textile structure with excellent flexibility, and construction based on cost-effective commercial fabrics. The device is ideal as a power source for wearable electronic devices, and the approach can easily be deployed for other textiles.

Electrospun P3HT/PVDF-HFP semiconductive nanofibers for triboelectric nanogenerators

This paper describes a simple electrospinning approach for fabricating poly(3-hexylthiophene) (P3HT)/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) semiconductive nanofiber mat triboelectric nanogenerators (TENGs). Measurements of the electrical properties of the P3HT/PVDF-HFP semiconductive nanofiber TENGs revealed that the output voltage could be enhanced up to 78 V with an output current of 7 μA. The output power of the device reached 0.55 mW, sufficient to power 500 red light-emitting diodes instantaneously, as well as a digital watch. The P3HT/PVDF-HFP semiconductive nanofiber TENG could be used not only as a self-powered device but also as a sensor for monitoring human action. Furthermore, it displayed good durability when subjected to 20,000 cycles of an external force test.

Recent Development of Flexible Tactile Sensors and Their Applications

With the rapid development of society in recent decades, the wearable sensor has attracted attention for motion-based health care and artificial applications. However, there are still many limitations to applying them in real life, particularly the inconvenience that comes from their large size and non-flexible systems. To solve these problems, flexible small-sized sensors that use body motion as a stimulus are studied to directly collect more accurate and diverse signals. In particular, tactile sensors are applied directly on the skin and provide input signals of motion change for the flexible reading device. This review provides information about different types of tactile sensors and their working mechanisms that are piezoresistive, piezocapacitive, piezoelectric, and triboelectric. Moreover, this review presents not only the applications of the tactile sensor in motion sensing and health care monitoring, but also their contributions in the field of artificial intelligence in recent years. Other applications, such as human behavior studies, are also suggested.

Electron-Ion Coupling Mechanism to Construct Stable Output Performance Nanogenerator

Recently, triboelectric nanogenerators (TENGs) have been promoted as an effective technique for ambient energy harvesting, given their large power density and high energy conversion efficiency. However, traditional TENGs based on the combination of triboelectrification effect and electrostatic induction have proven susceptible to environmental influence, which intensively restricts their application range. Herein, a new coupling mechanism based on electrostatic induction and ion conduction is proposed to construct flexible stable output performance TENGs (SOP-TENGs). The calcium chloride doped-cellulose nanofibril (CaCl₂-CNF) film made of natural carrots was successfully introduced to realize this coupling, resulting from its intrinsic properties as natural nanofibril hydrogel serving as both triboelectric layer and electrode. The coupling of two conductive mechanisms of SOP-TENG was comprehensively investigated through electrical measurements, including the effects of moisture content, relative humidity, and electrode size. In contrast to the conventional hydrogel ionotronic TENGs that require moisture as the carrier for ion transfer and use a hydrogel layer as the electrode, the use of a CaCl₂-CNF film (i.e., ion-doped natural hydrogel layer) as a friction layer in the proposed SOP-TENG effectively realizes a superstable electrical output under varying moisture contents and relative humidity due to the compound transfer mechanism of ions and electrons. This new working principle based on the coupling of electrostatic induction and ion conduction opens a wider range of applications for the hydrogel ionotronic TENGs, as the superstable electrical output enables them to be more widely applied in various complex environments to supply energy for low-power electronic devices.

Emerging iongel materials towards applications in energy and bioelectronics

In the past two decades, ionic liquids (ILs) have blossomed as versatile task-specific materials with a unique combination of properties, which can be beneficial for a plethora of different applications. The additional need of incorporating ILs into solid devices led to the development of a new class of ionic soft-solid materials, named here iongels. Nowadays, iongels cover a wide range of materials mostly composed of an IL component immobilized within different matrices such as polymers, inorganic networks, biopolymers or inorganic nanoparticles. This review aims at presenting an integrated perspective on the recent progress and advances in this emerging type of material. We provide an analysis of the main families of iongels and highlight the emerging types of these ionic soft materials offering additional properties, such as thermoresponsiveness, self-healing, mixed ionic/electronic properties, and (photo)luminescence, among others. Next, recent trends in additive manufacturing (3D printing) of iongels are presented. Finally, their new applications in the areas of energy, gas separation and (bio)electronics are detailed and discussed in terms of performance, underpinning it to the structural features and processing of iongel materials.

A fluorescent triboelectric nanogenerator manufactured with a flexible janus nanobelt array concurrently acting as a charge-generating layer and charge-trapping layer

Triboelectric nanogenerators (TENGs) have opened a new direction in the field of flexible devices. Here, a fluorescent TENG-JNA constructed with a flexible Janus nanobelt array (JNA) and PVDF/PVP nanofibers membrane by electro-spinning is reported for the first time. The building unit of JNA is the [PANI/CNTs/PMMA]//[Tb(BA)₃phen/PMMA] Janus nanobelts, which demonstrate green fluorescence and electrical conduction bi-function, where two independent partitions are microscopically realized in the Janus nanobelts. In TENG-JNA, JNA concurrently gains excellent charge-trapping ability and charge-generating capability by optimizing the PANI content; therefore, JNA serves as both a charge-generating layer and charge-trapping layer. The interface between TB(BA)₃phen and PMMA, the existence of aromatic ring structures in the PANI main chain and the interface between PANI and PMMA are conducive to trap a large number of triboelectric charges in time to prevent the triboelectric charges from combining with induced charges, which can significantly improve the output performance of TENG-JNA. The maximum output current and voltage of TENG-JNA are 6.20 μA and 155 V, respectively. The introduction of Tb(BA)₃phen ensures the strong fluorescence of TENF-JNA, and this fluorescence can be used to judge whether TENF-JNA works normally or is out of order in a dark environment. TENG-JNA possesses other compelling features, such as prominent flexibility, good hydrophobicity, durability and light weight, which provides the premise for TENG-JNA to be used as a flexible device in a wet environment or for warning functions. The Janus nanobelt was firstly used to assemble a TENG, which provides theoretical, material and technical support for the development of new building units of TENGs and paves a pathway for designing and assembling new TENGs.

Multifunctional Mechanical Sensing Electronic Device Based on Triboelectric Anisotropic Crumpled Nanofibrous Mats

Integrating multiple mechanical sensing capabilities in one device is highly desired to mimic the amazing functions of human skin, and it demonstrates promising applications in the human-machine interface and wearable robotic exoskeletons. Yet, challenges remain in how to couple the multisensations using as few modules as possible to increase the compactness and conformality of the electronics. Herein, we report a self-powered multiple mechanical sensing electronic device capable of sensing multiple motion modes of strain ratios, strain direction, and pressure. The self-powered property derives from the triboelectric working mechanism of the sensor. The multiple mechanical sensing is realized by utilizing an anisotropic crumpled nanofibrous membrane as the triboelectric layer and ionic conductor as the electrode layer. For strain ratios and pressure sensing, the output voltages of the sensor changed with the changes of these external stimulus with a comparable sensitivity. More importantly, contributed by the anisotropic structure of the designed crumples, the directional strain sensing is realized by the anisotropic sensitivity in three stretched directions.

A flexible electrostatic nanogenerator and self-powered capacitive sensor based on electrospun polystyrene mats and graphene oxide films

Electrostatic nanogenerators or capacitive sensors that leverage electrostatic induction for power generation or sensing, has attracted significant interests due to their simple structure, ease of fabrication, and high device stability. However, in order for such devices to work, an additional power source or a post-charging process is necessary to activate the electrostatic effect. In this work, an electrostatic nanogenerator is fabricated using electrospun polystyrene (PS) mats and dip-coated graphene oxide (GO) films as the self-charged components. The electret performances of the PS mats and GO films are characterized via the electrostatic force microscopy phase shift and surface potential measurements. With a multilayer device structure that consists of top electrodes/GO films/spacer/electrospun PS mats/bottom electrodes, the resultant device acts as an electrostatic generator that operates in the noncontact mode. The nanogenerator can output a peak voltage of ca. 6.41 V and a peak current of ca. 6.57 nA at a rate of 1 Hz of mechanical compression, and with no attenuation of electrical outputs even after 50 000 cycles over a 13 h period. Furthermore, this as-prepared device is also capable of serving as a self-powered capacitive sensor for detection of tiny mechanical impacts and measurement of human finger bending. This results of this work provides a new avenue to easily fabricate electrostatic nanogenerators with high durability and self-powered capacitive sensors for the detection of small impacts.

Single-Layer Triboelectric Nanogenerators Based on Ion-Doped Natural Nanofibrils

As emerging ambient energy harvesting technology, triboelectric nanogenerators (TENGs) have proven to be a robust power source and have demonstrated the unique ability to power micro-nano electronics autonomously to form self-powered devices. Although four working modes of TENGs have been developed to promote the feasibility of self-powered micro-nano systems, the relatively complicated structure composed of multilayer and movable components limits the practical applications of TENGs. Herein, we propose a single-layer triboelectric nanogenerator (SL-TENG) based on ion-doped natural nanofibrils. Compared with the simplest mode of currently existing TENGs, i.e., the single-electrode type, this novel single-electrode TENG further simplifies the configuration by the removal of the dielectric layer. The underlying mechanism of the proposed SL-TENG is comprehensively investigated through electrical measurements and the analysis of the effect of ion species at different concentrations. In contrast to conventional TENGs that require electrodes to realize charge transfer, it is revealed that the ions doped into natural nanofibrils effectively realize charge transfer due to the separation and migration of cations and anions. This new working principle based on the combination of electrons and ions enables TENGs to show greater potential for applications since the ultrasimple single-layer configuration enables them to be more easily integrated with other electronic components; additionally, the whole device of the proposed SL-TENG is biodegradable because the natural nanofibrils are completely extracted from carrots.

A Highly Porous Nonwoven Thermoplastic Polyurethane/Polypropylene-Based Triboelectric Nanogenerator for Energy Harvesting by Human Walking

A highly porous nonwoven thermoplastic polyurethane (TPU)/Polypropylene (PP) triboelectric nanogenerator (N-TENG) was developed. To fabricate the triboelectric layers, the TPU nanofiber was directly electrospun onto the nonwoven PP at different basis weights (15, 30, and 50 g/m2). The surface morphologies and porosities of the nonwoven PP and TPU nanofiber mats were characterized by field-emission scanning electron microscopy and porosimetry. The triboelectric performance of the nonwoven TPU/PP based TENG was found to improve with an increase in the basis weight of nonwoven PP. The maximum output voltage and current of the TPU/PP N-TENG with 50% PP basis weight reached 110.18 ± 6.06 V and 7.28 ± 0.67 µA, respectively, due to high air volume of nonwoven without spacers. In order to demonstrate its practical application as a generator, a TPU/PP N-TENG-attached insole for footwear was fabricated. The N-TENG was used as a power source to turn on 57 light-emitting diodes through human-walking, without any charging system. Thus, owing to its excellent energy-conversion performance, simple fabrication process, and low cost, the breathable and wearable nonwoven fiber-based TENG is suitable for large-scale production, to be used in wearable devices.

Nano- And Microfiber-Based Fully Fabric Triboelectric Nanogenerator For Wearable Devices

The combination of the triboelectric effect and static electricity as a triboelectric nanogenerator (TENG) has been extensively studied. TENGs using nanofibers have advantages such as high surface roughness, porous structure, and ease of production by electrospinning; however, their shortcomings include high-cost, limited yield, and poor mechanical properties. Microfibers are produced on mass scale at low cost; they are solvent-free, their thickness can be easily controlled, and they have relatively better mechanical properties than nanofiber webs. Herein, a nano- and micro-fiber-based TENG (NMF-TENG) was fabricated using a nylon 6 nanofiber mat and melt blown nonwoven polypropylene (PP) as triboelectric layers. Hence, the advantages of nanofibers and microfibers are maintained and mutually complemented. The NMF-TENG was manufactured by electrospinning nylon 6 on the nonwoven PP, and then attaching Ni coated fabric electrodes on the top and bottom of the triboelectric layers. The morphology, porosity, pore size distribution, and fiber diameters of the triboelectric layers were investigated. The triboelectric output performances were confirmed by controlling the pressure area and basis weight of the nonwoven PP. This study proposes a low-cost fabrication process of NMF-TENGs with high air-permeability, durability, and productivity, which makes them applicable to a variety of wearable electronics.

Amphiphobic triboelectric nanogenerators based on silica enhanced thermoplastic polymeric nanofiber membranes

It is necessary to construct an amphiphobic triboelectric nanogenerator (TENG) since water, oil and other antistatic agents will have a great impact on its electrical output performance during practical applications. Herein, we put forward a high-performance TENG based on silica enhanced thermoplastic polymeric nanofiber membranes that possess outstanding droplet-repellency after being modified with fluorine-containing polymers. With in situ polycondensation of SiO2 nanoparticles on the surface of the raw materials of garment-thermoplastic nanofiber membranes fabricated by the melt-blending extrusion method, the electrical output performance of the prepared TENGs enhanced a lot, corresponding to an excellent peak power density of 2.14 W m-2 that was enough to supply several green LEDs. For improving its ability to resist moisture and antistatic agents existing in daily life, FAS/PTFE was dip-coated on the above modified membranes to achieve remarkable amphiphobicity that gave it another ability for self-cleaning. Considering the good stability of amphiphobicity and the excellent compatibility between the thermoplastic polymeric nanofiber membranes and garment, the developed TENG is believed to be the most suitable candidate for powering wearable electronics in the near future.

Pyroelectric Energy Conversion and Its Applications-Flexible Energy Harvesters and Sensors

Among the various forms of natural energies, heat is the most prevalent and least harvested energy. Scavenging and detecting stray thermal energy for conversion into electrical energy can provide a cost-effective and reliable energy source for modern electrical appliances and sensor applications. Along with this, flexible devices have attracted considerable attention in scientific and industrial communities as wearable and implantable harvesters in addition to traditional thermal sensor applications. This review mainly discusses thermal energy conversion through pyroelectric phenomena in various lead-free as well as lead-based ceramics and polymers for flexible pyroelectric energy harvesting and sensor applications. The corresponding thermodynamic heat cycles and figures of merit of the pyroelectric materials for energy harvesting and heat sensing applications are also briefly discussed. Moreover, this study provides guidance on designing pyroelectric materials for flexible pyroelectric and hybrid energy harvesting.

Cost-Effective Copper⁻Nickel-Based Triboelectric Nanogenerator for Corrosion-Resistant and High-Output Self-Powered Wearable Electronic Systems

Recent years, triboelectric nanogenerators (TENGs) have attracted increased attention from researchers worldwide. Owing to their conductivity and triboelectric characteristics, metal materials can be made as both triboelectric materials and conductive electrodes. However, the surface of typical metals (such as copper, aluminum, and iron) is likely to be corroded when the sweat generated by human-body movement drops on the surface of TENGs, as this corrosion is detrimental to the output performance of TENGs. In this work, we proposed a novel corrosion-resistant copper-nickel based TENG (CN-TENG). Copper-nickel alloy conductive tape and polytetrafluoroethylene (PTFE) tape played the role of the triboelectric materials, and polymethyl methacrylate (PMMA) was utilized as the supporting part. The conductive copper-nickel alloy tape also served as a conductive electrode. The open-circuit voltage (VOC) and short-circuit current (ISC) can arrive at 196.8 V and 6 μA, respectively. Furthermore, peak power density values of 45 μW/cm2 were realized for the CN-TENG. A series of experiments confirmed its corrosion-resistant property. The approximate value of VOC for the fabricated TENG integrated into the shoe reached 1500 V, which is capable of driving at least 172 high-power LEDs in series. The results of this research provide a workable method for supporting corrosion-resistant self-powered wearable electronics.

Freestanding Ion Gels for Flexible, Printed, Multifunctional Microsupercapacitors

Freestanding ion gels (FIGs) provide unique opportunities for scalable, low-cost fabrication of flexible microsupercapacitors (MSCs). While conventional MSCs employ a distinct electrolyte and substrate, FIGs perform both functions, offering new possibilities for device integration and multifunctionality while maintaining high performance. Here, a capillarity-driven printing method is demonstrated to manufacture high-precision graphene electrodes on FIGs for MSCs. This method achieves excellent self-alignment and resolution (width: 50 μm, interdigitated electrode footprint: <1 mm²) and 100% fabrication yield (48/48 devices) and is readily generalized to alternative electrode materials including multiwalled carbon nanotubes (MWCNTs). The devices demonstrate good performance, including high specific capacitance (graphene: 0.600 mF cm^-2; MWCNT: 6.64 mF cm^-2) and excellent stability against bending, folding, and electrical cycling. Moreover, this strategy offers unique opportunities for device design and integration, including a bifacial electrode structure with enhanced capacitance (graphene: 0.673 mF cm^-2; MWCNT: 7.53 mF cm^-2) and improved rate performance, print-and-place versatility for integration on diverse substrates, and multifunctionality for light emission and transistor gating. These compelling results demonstrate the potential of FIGs for scalable, low-cost fabrication of flexible, printed, and multifunctional energy storage devices.

Increased Interfacial Area between Dielectric Layer and Electrode of Triboelectric Nanogenerator toward Robustness and Boosted Energy Output

Given the operation conditions wherein mechanical wear is inevitable, modifying bulk properties of the dielectric layer of a triboelectric nanogenerator (TENG) has been highlighted to boost its energy output. However, several concerns still remain in regards to the modification due to high-cost materials and cumbersome processes being required. Herein, we report TENG with a microstructured Al electrode (TENG_ME) as a new approach to modifying bulk properties of the dielectric layer. The microstructured Al electrode is utilized as a component of TENG to increase the interfacial area between the dielectric layer and electrode. Compared to the TENG with a flat Al electrode (TENG_F), the capacitance of TENG_ME is about 1.15 times higher than that of TENG_F, and the corresponding energy outputs of a TENG_ME are 117 μA and 71 V, each of which is over 1.2 times higher than that of the TENG_F. The robustness of TENG_ME is also confirmed in the measurement of energy outputs changing after sandpaper abrasion tests, repetitive contact, and separation (more than 10⁵ cycles). The results imply that the robustness and long-lasting performance of the TENG_ME could be enough to apply in reliable auxiliary power sources for electronic devices.

Natural Biopolymer-Based Triboelectric Nanogenerators via Fast, Facile, Scalable Solution Blowing

Here, we fabricated nanofiber (NF)-based triboelectric nanogenerators (TENGs) from natural biopolymers using the industrially scalable solution blowing. This technique eliminates severe restrictions on solutions to be used and allows one to achieve biocompatible devices. Here, solutions of soy protein and lignin were blown into continuous monolithic NFs of hundreds of nanometers in diameter. The technique we employed yields large-area NF mats within tens of minutes and has never been employed to form TENGs. Furthermore, in contrast to electrospun and meltblown fiber mats, solution-blown NF mats are much fluffier/porous, which is beneficial for achieving higher voltages by means of triboelectricity. In particular, triboelectricity generated by our biopolymer-based TENGs revealed that they hold great promise as sustainable and environmentally friendly self-powered devices for biomedical applications with the highest efficiency in their class. Moreover, these are the first nanotextured plant-derived biopolymer-made TENGs.

The Progress of PVDF as a Functional Material for Triboelectric Nanogenerators and Self-Powered Sensors

Ever since a new energy harvesting technology, known as a triboelectric nanogenerator (TENG), was reported in 2012, the rapid development of device fabrication techniques and mechanical system designs have considerably made the instantaneous output power increase up to several tens of mW/cm². With this innovative technology, a lot of researchers experimentally demonstrated that various portable/wearable devices could be operated without any external power. This article provides a comprehensive review of polyvinylidene fluoride (PVDF)-based polymers as effective dielectrics in TENGs for further increase of the output power to speed up commercialization of the TENGs, as well as the fundamental issues regarding the materials. In the end, we will also review PVDF-based sensors based on the triboelectric and piezoelectric effects of the PVDF polymers.

Mechanosensation-Active Matrix Based on Direct-Contact Tribotronic Planar Graphene Transistor Array

Mechanosensitive electronics aims at replicating the multifunctions of human skin to realize quantitative conversion of external stimuli into electronic signals and provide corresponding feedback instructions. Here, we report a mechanosensation-active matrix based on a direct-contact tribotronic planar graphene transistor array. Ion gel is utilized as both the dielectric in the graphene transistor and the friction layer for triboelectric potential coupling to achieve highly efficient gating and sensation properties. Different contact distances between the ion gel and other friction materials produce different triboelectric potentials, which are directly coupled to the graphene channel and lead to different output signals through modulating the Fermi level of graphene. Based on this mechanism, the tribotronic graphene transistor is capable of sensing approaching distances, recognizing the category of different materials, and even distinguishing voices. It possesses excellent sensing properties, including high sensitivity (0.16 mm^-1), fast response time (∼15 ms), and excellent durability (over 1000 cycles). Furthermore, the fabricated mechanosensation-active matrix is demonstrated to sense spatial contact distances and visualize a 2D color mapping of the target object. The tribotronic active matrix with ion gel as dielectric/friction layer provides a route for efficient and low-power-consuming mechanosensation in a noninvasive fashion. It is of great significance in multifunction sensory systems, wearable human-machine interactive interfaces, artificial electronic skin, and future telemedicine for patient surveillance.

Ferroelectric nanoparticle-embedded sponge structure triboelectric generators

We report high-performance triboelectric nanogenerators (TENGs) employing ferroelectric nanoparticles (NPs) embedded in a sponge structure. The ferroelectric BaTiO₃ NPs inside the sponge structure play an important role in increasing surface charge density by polarized spontaneous dipoles, enabling the packaging of TENGs even with a minimal separation gap. Since the friction surfaces are encapsulated in the packaged device structure, it suffers negligible performance degradation even at a high relative humidity of 80%. The TENGs also demonstrated excellent mechanical durability due to the elasticity and flexibility of the sponge structure. Consequently, the TENGs can reliably harvest energy even under harsh conditions. The approach introduced here is a simple, effective, and reliable way to fabricate compact and packaged TENGs for potential applications in wearable energy-harvesting devices.

Triboelectric-Nanogenerator-Based Soft Energy-Harvesting Skin Enabled by Toughly Bonded Elastomer/Hydrogel Hybrids

A major challenge accompanying the booming next-generation soft electronics is providing correspondingly soft and sustainable power sources for driving such devices. Here, we report stretchable triboelectric nanogenerators (TENG) with dual working modes based on the soft hydrogel-elastomer hybrid as energy skins for harvesting biomechanical energies. The tough interfacial bonding between the hydrophilic hydrogel and hydrophobic elastomer, achieved by the interface modification, ensures the stable mechanical and electrical performances of the TENGs. Furthermore, the dehydration of this toughly bonded hydrogel-elastomer hybrid is significantly inhibited (the average dehydration decreases by over 73%). With PDMS as the electrification layer and hydrogel as the electrode, a stretchable, transparent (90% transmittance), and ultrathin (380 μm) single-electrode TENG was fabricated to conformally attach on human skin and deform as the body moves. The two-electrode mode TENG is capable of harvesting energy from arbitrary human motions (press, stretch, bend, and twist) to drive the self-powered electronics. This work provides a feasible technology to design soft power sources, which could potentially solve the energy issues of soft electronics.

Emulsion Electrospinning of Polytetrafluoroethylene (PTFE) Nanofibrous Membranes for High-Performance Triboelectric Nanogenerators

Electrospinning is a simple, versatile technique for fabricating fibrous nanomaterials with the desirable features of extremely high porosities and large surface areas. Using emulsion electrospinning, polytetrafluoroethylene/polyethene oxide (PTFE/PEO) membranes were fabricated, followed by a sintering process to obtain pure PTFE fibrous membranes, which were further utilized against a polyamide 6 (PA6) membrane for vertical contact-mode triboelectric nanogenerators (TENGs). Electrostatic force microscopy (EFM) measurements of the sintered electrospun PTFE membranes revealed the presence of both positive and negative surface charges owing to the transfer of positive charge from PEO which was further corroborated by FTIR measurements. To enhance the ensuing triboelectric surface charge, a facile negative charge-injection process was carried out onto the electrospun (ES) PTFE subsequently. The fabricated TENG gave a stabilized peak-to-peak open-circuit voltage (V_oc) of up to ∼900 V, a short-circuit current density (J_sc) of ∼20 mA m^-2, and a corresponding charge density of ∼149 μC m^-2, which are ∼12, 14, and 11 times higher than the corresponding values prior to the ion-injection treatment. This increase in the surface charge density is caused by the inversion of positive surface charges with the simultaneous increase in the negative surface charge on the PTFE surface, which was confirmed by using EFM measurements. The negative charge injection led to an enhanced power output density of ∼9 W m^-2 with high stability as confirmed from the continuous operation of the ion-injected PTFE/PA6 TENG for 30 000 operation cycles, without any significant reduction in the output. The work thus introduces a relatively simple, cost-effective, and environmentally friendly technique for fabricating fibrous fluoropolymer polymer membranes with high thermal/chemical resistance in TENG field and a direct ion-injection method which is able to dramatically improve the surface negative charge density of the PTFE fibrous membranes.

Highly Transparent, Stretchable, and Self-Healing Ionic-Skin Triboelectric Nanogenerators for Energy Harvesting and Touch Applications

Recently developed triboelectric nanogenerators (TENGs) act as a promising power source for self-powered electronic devices. However, the majority of TENGs are fabricated using metallic electrodes and cannot achieve high stretchability and transparency, simultaneously. Here, slime-based ionic conductors are used as transparent current-collecting layers of TENG, thus significantly enhancing their energy generation, stretchability, transparency, and instilling self-healing characteristics. This is the first demonstration of using an ionic conductor as the current collector in a mechanical energy harvester. The resulting ionic-skin TENG (IS-TENG) has a transparency of 92% transmittance, and its energy-harvesting performance is 12 times higher than that of the silver-based electronic current collectors. In addition, they are capable of enduring a uniaxial strain up to 700%, giving the highest performance compared to all other transparent and stretchable mechanical-energy harvesters. Additionally, this is the first demonstration of an autonomously self-healing TENG that can recover its performance even after 300 times of complete bifurcation. The IS-TENG represents the first prototype of a highly deformable and transparent power source that is able to autonomously self-heal quickly and repeatedly at room temperature, and thus can be used as a power supply for digital watches, touch sensors, artificial intelligence, and biointegrated electronics.

Improvement of heat transfer by promoting dropwise condensation using electrospun polytetrafluoroethylene thin films

Homogeneous superhydrophobic PTFE thin films showed stable dropwise condensation and much higher heat transfer. They contribute to energy-efficient transfer.