Self-powered electro-tactile system for virtual tactile experiences

Wearable Pressure Sensors for Pulse Wave Monitoring

Cardiovascular diseases remain the leading cause of death worldwide. The rapid development of flexible sensing technologies and wearable pressure sensors have attracted keen research interest and have been widely used for long-term and real-time cardiovascular status monitoring. Owing to compelling characteristics, including light weight, wearing comfort, and high sensitivity to pulse pressures, physiological pulse waveforms can be precisely and continuously monitored by flexible pressure sensors for wearable health monitoring. Herein, an overview of wearable pressure sensors for human pulse wave monitoring is presented, with a focus on the transduction mechanism, microengineering structures, and related applications in pulse wave monitoring and cardiovascular condition assessment. The conceptualizations and methods for the acquisition of physiological and pathological information related to the cardiovascular system are outlined. The biomechanics of arterial pulse waves and the working mechanism of various wearable pressure sensors, including triboelectric, piezoelectric, magnetoelastic, piezoresistive, capacitive, and optical sensors, are also subject to systematic debate. Exemple applications of pulse wave measurement based on microengineering structured devices are then summarized. Finally, a discussion of the opportunities and challenges that wearable pressure sensors face, as well as their potential as a wearable intelligent system for personalized healthcare is given in conclusion.

Recent Progress Regarding Materials and Structures of Triboelectric Nanogenerators for AR and VR

With the continuous advancement in technology, electronic products used in augmented reality (AR) and virtual reality (VR) have gradually entered the public eye. As a result, the power supplies of these electronic devices have attracted more attention from scientists. Compared to traditional power sources, triboelectric nanogenerators (TENGs) are gradually being used for energy harvesting in self-powered sensing technology such as wearable flexible electronics, including AR and VR devices due to their small size, high conversion efficiency, and low energy consumption. As a result, TENGs are the most popular power supplies for AR and VR products. This article first summarizes the working mode and basic theory of TENGs, then reviews the TENG modules used in AR and VR devices, and finally summarizes the material selection and design methods used for TENG preparation. The friction layer of the TENG can be made of a variety of materials such as polymers, metals, and inorganic materials, and among these, polytetrafluoroethylene (PTFE) and polydimethylsiloxane (PDMS) are the most popular materials. To improve TENG performance, the friction layer material must be suitable. Therefore, for different application scenarios, the design methods of the TENG play an important role in its performance, and a reasonable selection of preparation materials and design methods can greatly improve the work efficiency of the TENG. Lastly, we summarize the current research status of nanogenerators, analyze and suggest future application fields, and summarize the main points of material selection.

Flexible Electronics and Devices as Human-Machine Interfaces for Medical Robotics

Medical robots are invaluable players in non-pharmaceutical treatment of disabilities. Particularly, using prosthetic and rehabilitation devices with human-machine interfaces can greatly improve the quality of life for impaired patients. In recent years, flexible electronic interfaces and soft robotics have attracted tremendous attention in this field due to their high biocompatibility, functionality, conformability, and low-cost. Flexible human-machine interfaces on soft robotics will make a promising alternative to conventional rigid devices, which can potentially revolutionize the paradigm and future direction of medical robotics in terms of rehabilitation feedback and user experience. In this review, the fundamental components of the materials, structures, and mechanisms in flexible human-machine interfaces are summarized by recent and renowned applications in five primary areas: physical and chemical sensing, physiological recording, information processing and communication, soft robotic actuation, and feedback stimulation. This review further concludes by discussing the outlook and current challenges of these technologies as a human-machine interface in medical robotics.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators

Flexible triboelectric nanogenerators (TENGs) have attracted increasing interest since their advent in 2012. In comparison with other flexible electrodes, hydrogels possess transparency, stretchability, biocompatibility, and tunable ionic conductivity, which together provide great potential as current collectors in TENGs for wearable applications. The development of hydrogel-based TENGs (H-TENGs) is currently a burgeoning field but research efforts have lagged behind those of other common flexible TENGs. In order to spur research and development of this important area, a comprehensive review that summarizes recent advances and challenges of H-TENGs will be very useful to researchers and engineers in this emerging field. Herein, the advantages and types of hydrogels as soft ionic conductors in TENGs are presented, followed by detailed descriptions of the advanced functions, enhanced output performance, as well as flexible and wearable applications of H-TENGs. Finally, the challenges and prospects of H-TENGs are discussed.

Improving Wastewater Treatment by Triboelectric-Photo/Electric Coupling Effect

The ability to meet higher effluent quality requirements and the reduction of energy consumption are the biggest challenges in wastewater treatment worldwide. A large proportion of the energy generated during wastewater treatment processes is neglected and lost in traditional wastewater treatment plants. As a type of energy harvesting system, triboelectric nanogenerators (TENGs) can extensively harvest the microscale energies generated from wastewater treatment procedures and auxiliary devices. This harvested energy can be utilized to improve the removal efficiency of pollutants through photo/electric catalysis, which has considerable potential application value in wastewater treatment plants. This paper gives an overall review of the generated potential energies (e.g., water wave energy, wind energy, and acoustic energy) that can be harvested at various stages of the wastewater treatment process and introduces the application of TENG devices for the collection of these neglected energies during wastewater treatment. Furthermore, the mechanisms and catalytic performances of TENGs coupled with photo/electric catalysis (e.g., electrocatalysis, photoelectric catalysis) are discussed to realize higher pollutant removal efficiencies and lower energy consumption. Then, a thorough, detailed investigation of TENG devices, electrode materials, and their coupled applications is summarized. Finally, the intimate coupling of self-powered photoelectric catalysis and biodegradation is proposed to further improve removal efficiencies in wastewater treatment. This concept is conducive to improving knowledge about the underlying mechanisms and extending applications of TENGs in wastewater treatment to better solve the problems of energy demand in the future.

Monitoring the Degree of Comfort of Shoes In-Motion Using Triboelectric Pressure Sensors with an Ultrawide Detection Range

Shoes play an important role in sports and human daily life. Here, an in-shoe sensor pad (ISSP) attached to the vamp lining is based on a triboelectric nanogenerator (TENG) for monitoring the real-time stress distribution on the top side of a foot. Each sensor unit on this ISSP is an air-capsule TENG (AC-TENG) consisting of activated carbon/polyurethane (AC/PU) and microsphere array electrodes. The detection range of each AC-TENG reaches 7.27 MPa, which is enough for monitoring the pressure change during different sports. This multifunctional ISSP can realize many typical functions of conventional smart shoes, including step counting and human-machine interaction. Moreover, it can also reveal special information, including the fitness of shoes, the stress concentration on toes, and the in-motion comfort degree. The signal processing and data transmission modules in the system have a hybrid power supply with wireless power transfer, while the real-time information about feet can be observed on a cell phone. Hence, this ISSP provides a potential approach to study the feet motion and comfort degree of shoes in long-term operations, which can guide both athlete training and the customized design of shoes.

A Self-Powered Triboelectric Hybrid Coder for Human-Machine Interaction

Human-machine interfaces have penetrated various academia and industry fields such as smartphones, robotic, virtual reality, and wearable electronics, due to their abundant functional sensors and information interaction methods. Nevertheless, most sensors' complex structural design, monotonous parameter detection capability, and single information coding communication hinder their rapid development. As the frontier of self-powered sensors, the triboelectric nanogenerator (TENG) has multiple working modes and high structural adaptability, which is a potential solution for multi-parameter sensing and miniaturizing of traditional interactive electronic devices. Herein, a self-powered hybrid coder (SHC) based on TENG is reported to encode two action parameters of touch and press, which can be used as a smart interface for human-machine interaction. The top-down hollow structure of the SHC, not only constructs a compositing mode to generate stable touch and press signals but also builds a hybrid coding platform for generating action codes in synergy mode. When a finger touches or presses the SHC, Morse code and Gray code can be transmitted for text information or remote control of electric devices. This self-powered coder is of reference value for designing an alternative human-machine interface and having the potential to contribute to the next generation of highly integrated portable smart electronics.

Recent progress in self-powered multifunctional e-skin for advanced applications

Electronic skin (e-skin), new generation of flexible wearable electronic devices, has characteristics including flexibility, thinness, biocompatibility with broad application prospects, and a crucial place in future wearable electronics. With the increasing demand for wearable sensor systems, the realization of multifunctional e-skin with low power consumption or even autonomous energy is urgently needed. The latest progress of multifunctional self-powered e-skin for applications in physiological health, human-machine interaction (HMI), virtual reality (VR), and artificial intelligence (AI) is presented here. Various energy conversion effects for the driving energy problem of multifunctional e-skin are summarized. An overview of various types of self-powered e-skins, including single-effect e-skins and multifunctional coupling-effects e-skin systems is provided, where the aspects of material preparation, device assembly, and output signal analysis of the self-powered multifunctional e-skin are described. In the end, the existing problems and prospects in this field are also discussed.

Smart Face Mask Based on an Ultrathin Pressure Sensor for Wireless Monitoring of Breath Conditions

A smart face mask that can conveniently monitor breath information is beneficial for maintaining personal health and preventing the spread of diseases. However, some challenges still need to be addressed before such devices can be of practical use. One key challenge is to develop a pressure sensor that is easily triggered by low pressure and has excellent stability as well as electrical and mechanical properties. In this study, a wireless smart face mask is designed by integrating an ultrathin self-powered pressure sensor and a compact readout circuit with a normal face mask. The pressure sensor is the thinnest (totally compressed thickness of ≈5.5 µm) and lightest (total weight of ≈4.5 mg) electrostatic pressure sensor capable of achieving a peak open-circuit voltage of up to ≈10 V when stimulated by airflow, which endows the sensor with the advantage of readout circuit miniaturization and makes the breath-monitoring system portable and wearable. To demonstrate the capabilities of the smart face mask, it is used to wirelessly measure and analyze the various breath conditions of multiple testers.

Biometrics-protected optical communication enabled by deep learning-enhanced triboelectric/photonic synergistic interface

Security is a prevailing concern in communication as conventional encryption methods are challenged by progressively powerful supercomputers. Here, we show that biometrics-protected optical communication can be constructed by synergizing triboelectric and nanophotonic technology. The synergy enables the loading of biometric information into the optical domain and the multiplexing of digital and biometric information at zero power consumption. The multiplexing process seals digital signals with a biometric envelope to avoid disrupting the original high-speed digital information and enhance the complexity of transmitted information. The system can perform demultiplexing, recover high-speed digital information, and implement deep learning to identify 15 users with around 95% accuracy, irrespective of biometric information data types (electrical, optical, or demultiplexed optical). Secure communication between users and the cloud is established after user identification for document exchange and smart home control. Through integrating triboelectric and photonics technology, our system provides a low-cost, easy-to-access, and ubiquitous solution for secure communication.

Electronic skin as wireless human-machine interfaces for robotic VR

The coronavirus pandemic has highlighted the importance of developing intelligent robotics to prevent infectious disease spread. Human-machine interfaces (HMIs) give a chance of interactions between users and robotics, which play a significant role in teleoperating robotics. Conventional HMIs are based on bulky, rigid, and expensive machines, which mainly focus on robots/machines control, but lack of adequate feedbacks to users, which limit their applications in conducting complicated tasks. Therefore, developing closed-loop HMIs with both accurate sensing and feedback functions is extremely important. Here, we present a closed-loop HMI system based on skin-integrated electronics, whose electronics compliantly interface with the whole body for wireless motion capturing and haptic feedback via Bluetooth, Wireless Fidelity (Wi-Fi), and Internet. The integration of visual and haptic VR via skin-integrated electronics together into a closed-loop HMI for robotic VR demonstrates great potentials in noncontact collection of bio samples, nursing infectious disease patients and many others.

Study of Contact Electrification at Liquid-Gas Interface

It is known that the suspended liquid droplets in clouds can generate electrostatic charges, which finally results in the lightning. However, the detailed mechanism related to the contact-electrification process on the liquid-gas (L-G) interfaces is still poorly understood. Here, by introducing an acoustic levitation method for levitating a liquid droplet, we have studied the electrification mechanism at the L-G interface. The tribo-motion between water droplets and air induced by the ultrasound wave leads to the generation of positive charges on the surface of the droplets, and the charge amount of water droplets (20 μL) gradually reaches saturation within 30 s. The mixed solid particles in droplets can increase the amount of transferred charge, whereas the increase of ion concentration in the droplet can suppress the charge generation. This charge transfer phenomenon at L-G interfaces and the related analysis can be a guidance for the study in many fields, including anti-static, harvesting rainy energy, micro/nano fluidics, triboelectric power generator, surface engineering, and so on. Moreover, the surface charge generation due to L-G electrification is an inevitable effect during ultrasonic levitation, and thus, this study can also work for the applications of the ultrasonic technique.

Recent Progress in the Energy Harvesting Technology-From Self-Powered Sensors to Self-Sustained IoT, and New Applications

With the fast development of energy harvesting technology, micro-nano or scale-up energy harvesters have been proposed to allow sensors or internet of things (IoT) applications with self-powered or self-sustained capabilities. Facilitation within smart homes, manipulators in industries and monitoring systems in natural settings are all moving toward intellectually adaptable and energy-saving advances by converting distributed energies across diverse situations. The updated developments of major applications powered by improved energy harvesters are highlighted in this review. To begin, we study the evolution of energy harvesting technologies from fundamentals to various materials. Secondly, self-powered sensors and self-sustained IoT applications are discussed regarding current strategies for energy harvesting and sensing. Third, subdivided classifications investigate typical and new applications for smart homes, gas sensing, human monitoring, robotics, transportation, blue energy, aircraft, and aerospace. Lastly, the prospects of smart cities in the 5G era are discussed and summarized, along with research and application directions that have emerged.

Self-Healing and Elastic Triboelectric Nanogenerators for Muscle Motion Monitoring and Photothermal Treatment

Owing to wearing and unpredictable damage, the working lifetime of triboelectric nanogenerators (TENGs) is largely limited. In this work, we prepared a single-electrode multifunctional TENG (MF-TENG) that exhibits fast self-healing, human health monitoring capability, and photothermal properties. The device consists of a thin self-healing poly(vinyl alcohol)-based hydrogel sandwiched between two self-healing silicone elastomer films. The MF-TENG exhibits a short-circuit current, short-circuit transfer charge, and open-circuit voltage of 7.98 μA, 78.34 nC, and 38.57 V, respectively. Furthermore, owing to the repairable networks of the dynamic imine bonds in the charged layer and the borate ester bonds in the electrodes, the prepared device could recover its original state after mechanical damage within 10 min at room temperature. The MF-TENG can be attached to different human joints for self-powered monitoring of personal health information. Additionally, the MF-TENG under near-infrared laser irradiation can provide a photothermal therapy for assisting the recovery of human joints motion. It is envisaged that the proposed MF-TENG can be applied to the fields of wearable electronics and health-monitoring devices.

AI enabled sign language recognition and VR space bidirectional communication using triboelectric smart glove

Sign language recognition, especially the sentence recognition, is of great significance for lowering the communication barrier between the hearing/speech impaired and the non-signers. The general glove solutions, which are employed to detect motions of our dexterous hands, only achieve recognizing discrete single gestures (i.e., numbers, letters, or words) instead of sentences, far from satisfying the meet of the signers' daily communication. Here, we propose an artificial intelligence enabled sign language recognition and communication system comprising sensing gloves, deep learning block, and virtual reality interface. Non-segmentation and segmentation assisted deep learning model achieves the recognition of 50 words and 20 sentences. Significantly, the segmentation approach splits entire sentence signals into word units. Then the deep learning model recognizes all word elements and reversely reconstructs and recognizes sentences. Furthermore, new/never-seen sentences created by new-order word elements recombination can be recognized with an average correct rate of 86.67%. Finally, the sign language recognition results are projected into virtual space and translated into text and audio, allowing the remote and bidirectional communication between signers and non-signers.

Recent Advances in Flexible Tactile Sensors for Intelligent Systems

Tactile sensors are an important medium for artificial intelligence systems to perceive their external environment. With the rapid development of smart robots, wearable devices, and human-computer interaction interfaces, flexible tactile sensing has attracted extensive attention. An overview of the recent development in high-performance tactile sensors used for smart systems is introduced. The main transduction mechanisms of flexible tactile sensors including piezoresistive, capacitive, piezoelectric, and triboelectric sensors are discussed in detail. The development status of flexible tactile sensors with high resolution, high sensitive, self-powered, and visual capabilities are focused on. Then, for intelligent systems, the wide application prospects of flexible tactile sensors in the fields of wearable electronics, intelligent robots, human-computer interaction interfaces, and implantable electronics are systematically discussed. Finally, the future prospects of flexible tactile sensors for intelligent systems are proposed.

Power Management and Reaction Optimization for a Self-Powered Electrochemical System Driven by a Triboelectric Nanogenerator

Harvesting distributed and low-quality mechanical energies by triboelectric nanogenerators to power electrochemical reactions is beneficial to electric energy saving and certain applications. However, the conventional self-powered electrochemical process is awkward about the reaction rate, energy conversion efficiency, high-operation frequency, and mismatched impedance. Here we demonstrate an advanced self-powered electrochemical system. In comparison with the conventional system that is inert in activity, the superior power management and electrochemical reaction regulation in tandem make the novel system outstanding for hydrogen peroxide production. In addition to the visible product, an internal current efficiency of 24.6% in the system was achieved. The developed system provides an optimization strategy toward electric energy saving for electrochemical reactions as well as enabling their applications in remote areas by converting environmental mechanical vibrational energy for ecological improvement or recyclable chemical fuel generation.

Effect of Photo-Excitation on Contact Electrification at Liquid-Solid Interface

Liquid-solid triboelectric nanogenerator (L-S TENG) is one of the major techniques to collect energy from tiny liquids, while the saturated charge density at the L-S interface is the key element to decide its performance. Here, we found that the saturated charge density of L-S contact electrification (CE) can be further increased under the illumination of an ultraviolet (UV) light. The fluorine-containing polymers and SiO₂ are chosen as the electrification materials and with and without UV illumination on the L-S TENG. A series of experiments have been done to rule out the possible influences of anion generation, chemical change of solid surface, ionization of water, and so on. Therefore, we proposed that electrons belonging to water molecules can be excited to high energy states under UV illumination, which then transfer to solid surface and captured by the solid surface. Finally, a photoexcited electron transfer model is proposed to explain the enhancement of CE under the UV illumination. This work not only helps to further understand CE at L-S interface, but also offers an approach to further enhance the performance of L-S TENG, which can promote the TENG applications in the field of microfluidic systems, liquid energy harvesting, and droplet sensory.