Tunable and Nacre-Mimetic Multifunctional Electronic Skins for Highly Stretchable Contact-Noncontact Sensing.

Electronic skins (e-skins) have attracted great attention for their applications in disease diagnostics, soft robots, and human-machine interaction. The integration of high sensitivity, low detection limit, large stretchability, and multiple stimulus response capacity into a single e-skin remains an enormous challenge. Herein, inspired by the structure of nacre, an ultra-stretchable and multifunctional e-skin with tunable strain detection range based on nacre-mimetic multi-layered silver nanowires /reduced graphene oxide /thermoplastic polyurethane mats is fabricated. The e-skin possesses extraordinary strain response performance with a tunable detection range (50 to 200% strain), an ultralow response limit (0.1% strain), a high sensitivity (gauge factor up to 1902.5), a fast response time (20 ms), and an excellent stability (stretching/releasing test of 11 000 cycles). These excellent response behaviors enable the e-skin to accurately monitor full-range human body motions. Additionally, the e-skin can detect relative humidity quickly and sensitively through a reversible physical adsorption/desorption of water vapor, and the assembled e-skin array exhibits excellent performance in noncontact sensing. The tunable and multifunctional e-skins show promising applications in motion monitoring and contact-noncontact human machine interaction.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications.

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Bimodal Tactile Sensor without Signal Fusion for User-Interactive Applications.

Tactile sensors with multimode sensing ability are cornerstones of artificial skin for applications in humanoid robotics and smart prosthetics. However, the intuitive and interference-free reading of multiple tactile signals without involving complex algorithms and calculations remains a challenge. Herein a pressure-temperature bimodal tactile sensor without any interference is demonstrated by combining the fundamentally different sensing mechanisms of optics and electronics, enabling the simultaneous and independent sensing of pressure and temperature with the elimination of signal separation algorithms and calculations. The bimodal sensor comprises a mechanoluminescent hybrid of ZnS-CaZnOS and a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thermoresistant material, endowing the unambiguous transduction of pressure and temperature into optical and electrical signals, respectively. This device exhibits the highest temperature sensitivity of -0.6% °C-1 in the range of 21-60 °C and visual sensing of the applied forces at a low limitation of 2 N. The interference-free and light-emitting characteristics of this device permit user-interactive applications in robotics for encrypted communication as well as temperature and pressure monitoring, along with wireless signal transmission. This work provides an unexplored solution to signal interference of multimodal tactile sensors, which can be extended to other multifunctional sensing devices.

Bioinspired Multifunctional Photonic-Electronic Smart Skin for Ultrasensitive Health Monitoring, for Visual and Self-Powered Sensing.

Smart skin is highly desired to be ultrasensitive and self-powered as the medium of artificial intelligence. Here, an ultrasensitive self-powered mechanoluminescence smart skin (SPMSS) inspired by the luminescence mechanism of cephalopod skin and the ultrasensitive response of spider-slit-organ is developed. Benefitting from the unique strain-dependent microcrack structure design based on Ti3 C2 Tx (MXene)/carbon nanotube synergistic interaction, SPMSS possesses excellent strain sensing performances including ultralow detection limit (0.001% strain), ultrahigh sensitivity (gauge factor, GF = 3.92 × 107 ), ultrafast response time (5 ms), and superior durability and stability (>45 000 cycles). Synchronously, SPMSS exhibits tunable and highly sensitive mechanoluminescence (ML) features under stretching. A relationship between ML features, strain sensing performances, and the deformation has been established successfully. Importantly, the SPMSS demonstrates excellent properties as triboelectric nanogenerator (4 × 4 cm2 ), including ultrahigh triboelectric output (open-circuit voltage VOC  = 540 V, short-circuit current ISC  = 42 µA, short-circuit charge QSC  = 317 nC) and power density (7.42 W m-2 ), endowing the smart skin with reliable power source supply and self-powered sensing ability. This bioinspired smart skin exhibits multifunctional applications in health monitoring, visual sensing, and self-powered sensing, showing great potential in artificial intelligence.

Motion recognition by a liquid filled tubular triboelectric nanogenerator.

The development of flexible electronics has extended the limit of the intelligent devices, which are highly sensitive, soft and capable of sustaining arbitrary deformation. Here, we report a liquid-polymer tubular triboelectric nanogenerator (L-P TENG) that is filled with liquid for a shape-adaptive sensor in various working modes. The L-P TENG is based on liquid-solid contact electrification with the use of displacement current and excited by the shape change of the tubular structure. The high softness of the device makes it possible to be twisted to any curve and bear extreme strain. It can be used to detect a slight difference in pressure from touch, pressing and stretching and is suitable for a wide-range force recognition with high sensitivity. The independent and multifunctional properties of the L-P TENG extend the potential applications through combinations. Such assembled units with crossings can sense the approaching object. This study provides a new direction for flexible electromechanical sensing and has potential applications in self-powered sensors, wearable electronics, smart human-machine interaction and auxiliary motion correction.

Triboelectric Nanogenerator-Enabled Dendrite-Free Lithium Metal Batteries.

Lithium metal batteries (LMBs) are prominent among next-generation energy-storage systems because of their high energy density. Unfortunately, the commercial application of LMBs is hindered by the dendrite growth issue during the charging process. Herein, we report that the triboelectric nanogenerator (TENG)-based pulse output with a novel waveform and frequency has restrained the formation of dendrites in LMBs. The waveform and operation frequency of TENG can be regulated by TENG-designed and smart power management circuits. By regulating the waveform and frequency of the TENG-based pulse output, the pulse duration becomes shorter than the lithium dendrite formation time at any current of pulse waveform, and lithium ions can replenish in the entire electrode surface during rest periods, eliminating concentration polarization. Therefore, the optimized TENG-based charging strategy can improve the Coulombic efficiency of lithium plating/stripping and realize homogeneous lithium plating in LMBs. This TENG-based charging technology provides an innovative strategy to address the Li dendrite growth issues in LMBs, and accelerates the application of TENG-based energy collection systems.

Improved Triboelectric Nanogenerator Output Performance through Polymer Nanocomposites Filled with Core-shell-Structured Particles.

Core-shell-structured BaTiO3-poly( tert-butyl acrylate) (P tBA) nanoparticles are successfully prepared by in situ atom transfer radical polymerization of tert-butyl acrylate ( tBA) on BaTiO3 nanoparticle surface. The thickness of the P tBA shell layer could be controlled by adjusting the feed ratio of tBA to BaTiO3. The BaTiO3-P tBA nanoparticles are introduced into poly(vinylidene fluoride) (PVDF) matrix to form a BaTiO3-P tBA/PVDF nanocomposite. The nanocomposites keep the flexibility of the PVDF matrix with enhanced dielectric constant (∼15@100 Hz) because of the high permittivity of inorganic particles and the ester functional groups in the P tBA. Furthermore, the BaTiO3-P tBA/PVDF nanocomposites demonstrate the inherent small dielectric loss of the PVDF matrix in the tested frequency range. The high electric field dielectric constant of the nanocomposite film was investigated by polarization hysteresis loops. The high electric field effective dielectric constant of the nanocomposite is 26.5 at 150 MV/m. The output current density of the nanocomposite-based triboelectric nanogenerator (TENG) is 2.1 µA/cm2, which is above 2.5 times higher than the corresponding pure PVDF-based TENG.

Triboelectric-Based Transparent Secret Code.

Private and security information for personal identification requires an encrypted tool to extend communication channels between human and machine through a convenient and secure method. Here, a triboelectric-based transparent secret code (TSC) that enables self-powered sensing and information identification simultaneously in a rapid process method is reported. The transparent and hydrophobic TSC can be conformed to any cambered surface due to its high flexibility, which extends the application scenarios greatly. Independent of the power source, the TSC can induce obvious electric signals only by surface contact. This TSC is velocity-dependent and capable of achieving a peak voltage of ≈4 V at a resistance load of 10 MΩ and a sliding speed of 0.1 m s-1, according to a 2 mm × 20 mm rectangular stripe. The fabricated TSC can maintain its performance after reciprocating rolling for about 5000 times. The applications of TSC as a self-powered code device are demonstrated, and the ordered signals can be recognized through the height of the electric peaks, which can be further transferred into specific information by the processing program. The designed TSC has great potential in personal identification, commodity circulation, valuables management, and security defense applications.

Screen-Printed Washable Electronic Textiles as Self-Powered Touch/Gesture Tribo-Sensors for Intelligent Human-Machine Interaction.

Multifunctional electronic textiles (E-textiles) with embedded electric circuits hold great application prospects for future wearable electronics. However, most E-textiles still have critical challenges, including air permeability, satisfactory washability, and mass fabrication. In this work, we fabricate a washable E-textile that addresses all of the concerns and shows its application as a self-powered triboelectric gesture textile for intelligent human-machine interfacing. Utilizing conductive carbon nanotubes (CNTs) and screen-printing technology, this kind of E-textile embraces high conductivity (0.2 kΩ/sq), high air permeability (88.2 mm/s), and can be manufactured on common fabric at large scales. Due to the advantage of the interaction between the CNTs and the fabrics, the electrode shows excellent stability under harsh mechanical deformation and even after being washed. Moreover, based on a single-electrode mode triboelectric nanogenerator and electrode pattern design, our E-textile exhibits highly sensitive touch/gesture sensing performance and has potential applications for human-machine interfacing.

Polymer nanocomposite-enabled high-performance triboelectric nanogenerator with self-healing capability.

Triboelectric nanogenerators (TENG) have been proven to be effective for the collection of low-frequency vibrational energy in the environment. However, most polymer materials as friction layers are highly susceptible to mechanical damage during operation, which reduces the performance and lifetime of TENG. Herein, we report a high-performance, flexible triboelectric nanogenerator with reproducible self-healing electronic characteristics. Based on its soft and flexible polymers, the self-healing triboelectric nanogenerator (SH-TENG) can achieve a peak power of 2.5 W m-2 and triboelectric charge density of about 100 µC m-2. High-conductance Ag nanowires (AgNWs) are semi-embedded in the polymer to fabricate all-in-one friction layers and for an enhanced self-healing process. Both the output voltage and current of the healed device can reach up to about 99% of their original values even after five cutting/healing cycles. The fabricated SH-TENG has excellent stability and flexibility, which presents a significant step towards the fabrication of reliable triboelectric nanogenerators with recoverability and low maintenance costs.

Rotating-Sleeve Triboelectric-Electromagnetic Hybrid Nanogenerator for High Efficiency of Harvesting Mechanical Energy.

Currently, a triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) have been hybridized to effectively scavenge mechanical energy. However, one critical issue of the hybrid device is the limited output power due to the mismatched output impedance between the two generators. In this work, impedance matching between the TENG and EMG is achieved facilely through commercial transformers, and we put forward a highly integrated hybrid device. The rotating-sleeve triboelectric-electromagnetic hybrid nanogenerator (RSHG) is designed by simulating the structure of a common EMG, which ensures a high efficiency in transferring ambient mechanical energy into electric power. The RSHG presents an excellent performance with a short-circuit current of 1 mA and open-circuit voltage of 48 V at a rotation speed of 250 rpm. Systematic measurements demonstrate that the hybrid nanogenerator can deliver the largest output power of 13 mW at a loading resistance of 8 kΩ. Moreover, it is demonstrated that a wind-driven RSHG can light dozens of light-emitting diodes and power an electric watch. The distinctive structure and high output performance promise the practical application of this rotating-sleeve structured hybrid nanogenerator for large-scale energy conversion.

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.

Self-Powered Electrospinning System Driven by a Triboelectric Nanogenerator.

Broadening the application area of the triboelectric nanogenerators (TENGs) is one of the research emphases in the study of the TENGs, whose output characteristic is high voltage with low current. Here we design a self-powered electrospinning system, which is composed of a rotating-disk TENG (R-TENG), a voltage-doubling rectifying circuit (VDRC), and a simple spinneret. The R-TENG can generate an alternating voltage up to 1400 V. By using a voltage-doubling rectifying circuit, a maximum constant direct voltage of 8.0 kV can be obtained under the optimal configuration and is able to power the electrospinning system for fabricating various polymer nanofibers, such as polyethylene terephthalate (PET), polyamide-6 (PA6), polyacrylonitrile (PAN), polyvinylidene difluoride (PVDF), and thermoplastic polyurethanes (TPU). The system demonstrates the capability of a TENG for high-voltage applications, such as manufacturing nanofibers by electrospinning.

Progress in piezo-phototronic effect modulated photovoltaics.

Wurtzite structured materials, like ZnO, GaN, CdS, and InN, simultaneously possess semiconductor and piezoelectric properties. The inner-crystal piezopotential induced by external strain can effectively tune/control the carrier generation, transport and separation/combination processes at the metal-semiconductor contact or p-n junction, which is called the piezo-phototronic effect. This effect can efficiently enhance the performance of photovoltaic devices based on piezoelectric semiconductor materials by utilizing the piezo-polarization charges at the junction induced by straining, which can modulate the energy band of the piezoelectric material and then accelerate or prevent the separation process of the photon-generated electrons and vacancies. This paper introduces the fundamental physics principles of the piezo-phototronic effect, and reviews recent progress in piezo-phototronic effect enhanced solar cells, including solar cells based on semiconductor nanowire, organic/inorganic materials, quantum dots, and perovskite. The piezo-phototronic effect is suggested as a suitable basis for the development of an innovative method to enhance the performance of solar cells based on piezoelectric semiconductors by applied extrinsic strains, which might be appropriate for fundamental research and potential applications in various areas of optoelectronics.