Fingerprint‐Inspired Conducting Hierarchical Wrinkles for Energy‐Harvesting E‐Skin

In the field of bionics, sophisticated and multifunctional electronic skins with a mechanosensing function that are inspired by nature are developed. Here, an energy‐harvesting electronic skin (energy‐E‐skin), i.e., a pressure sensor with energy‐harvesting functions is demonstrated, based on fingerprint‐inspired conducting hierarchical wrinkles. The conducting hierarchical wrinkles, fabricated via 2D stretching and subsequent Ar plasma treatment, are composed of polydimethylsiloxane (PDMS) wrinkles as the primary microstructure and embedded Ag nanowires (AgNWs) as the secondary nanostructure. The structure and resistance of the conducting hierarchical wrinkles are deterministically controlled by varying the stretching direction, Ar plasma power, and treatment time. This hierarchical‐wrinkle‐based conductor successfully harvests mechanical energy via contact electrification and electrostatic induction and also realizes self‐powered pressure sensing. The energy‐E‐skin delivers an average output power of 3.5 mW with an open‐circuit voltage of 300 V and a short‐circuit current of 35 µA; this power is sufficient to drive commercial light‐emitting diodes and portable electronic devices. The hierarchical‐wrinkle‐based conductor is also utilized as a self‐powered tactile pressure sensor with a sensitivity of 1.187 mV Pa^‐1 in both contact‐separation mode and the single‐electrode mode. The proposed energy‐E‐skin has great potential for use as a next‐generation multifunctional artificial skin, self‐powered human–machine interface, wearable thin‐film power source, and so on.     

Single‐Thread‐Based Wearable and Highly Stretchable Triboelectric Nanogenerators and Their Applications in Cloth‐Based Self‐Powered Human‐Interactive and Biomedical Sensing

The development of wearable and large‐area fabric energy harvester and sensor has received great attention due to their promising applications in next‐generation autonomous and wearable healthcare technologies. Here, a new type of “single” thread‐based triboelectric nanogenerator (TENG) and its uses in elastically textile‐based energy harvesting and sensing have been demonstrated. The energy‐harvesting thread composed by one silicone‐rubber‐coated stainless‐steel thread can extract energy during contact with skin. With sewing the energy‐harvesting thread into a serpentine shape on an elastic textile, a highly stretchable and scalable TENG textile is realized to scavenge various kinds of human‐motion energy. The collected energy is capable to sustainably power a commercial smart watch. Moreover, the simplified single triboelectric thread can be applied in a wide range of thread‐based self‐powered and active sensing uses, including gesture sensing, human‐interactive interfaces, and human physiological signal monitoring. After integration with microcontrollers, more complicated systems, such as wireless wearable keyboards and smart beds, are demonstrated. These results show that the newly designed single‐thread‐based TENG, with the advantage of interactive, responsive, sewable, and conformal features, can meet application needs of a vast variety of fields, ranging from wearable and stretchable energy harvesters to smart cloth‐based articles.     

Ionic Tactile Sensors for Emerging Human‐Interactive Technologies: A Review of Recent Progress

Ionic tactile sensors (ITS) represent a new class of deformable sensory platforms that mimic not only the tactile functions and topological structures but also the mechanotransduction mechanism across the biological ion channels in human skin, which can demonstrate a more advanced biological interface for targeting emerging human‐interactive technologies compared to conventional e‐skin devices. Recently, flexible and even stretchable ITS have been developed using novel structural designs and strategies in materials and devices. These skin‐like tactile sensors can effectively sense pressure, strain, shear, torsion, and other external stimuli with high sensitivity, high reliability, and rapid response beyond those of human perception. In this review, the recent developments of the ITS based on the novel concepts, structural designs, and strategies in materials innovation are entirely highlighted. In particular, biomimetic approaches have led to the development of the ITS that extend beyond the tactile sensory capabilities of human skin such as sensitivity, pressure detection range, and multimodality. Furthermore, the recent progress in self‐powered and self‐healable ITS, which should be strongly required to allow human‐interactive artificial sensory platforms is reviewed. The applications of ITS in human‐interactive technologies including artificial skin, wearable medical devices, and user‐interactive interfaces are highlighted. Last, perspectives on the current challenges and the future directions of this field are presented.     

Self‐Healable and Stretchable Organic Thermoelectric Materials: Electrically Percolated Polymer Nanowires Embedded in Thermoplastic Elastomer Matrix

Self‐healable and stretchable energy‐harvesting materials can provide a new avenue for the realization of self‐powered wearable electronics, including electronic skins, whose main materials are required to be robust to and stable under external damage and severe mechanical stresses. However, thermoelectric (TE) materials showing both self‐healing properties and stretchability have not yet been demonstrated despite their great potential to harvest thermal energy in the human body. As most existing TE materials are either mechanically brittle or unrecoverable after being subjected to damage, a novel approach is necessary for designing such materials. Herein, self‐healable and stretchable TE materials based on all‐organic composite system wherein polymer semiconductor nanowires are p‐doped with a molecular dopant and embedded in a thermoplastic elastomer matrix are reported. The polymer nanowires are electrically percolated in the matrix, and the resulting composite materials exhibit good TE performance. The composites also exhibit both excellent self‐healing properties under mild heat and pressure conditions and good stretchability. It is believed that this work can be a cornerstone for the design of self‐healable and stretchable energy‐harvesting materials as it provides useful guidelines for imparting electrical conductivity to insulating thermoplastic elastomers, which typically possess versatile and useful mechanical properties.     

Textile‐Based Triboelectric Nanogenerators for Self‐Powered Wearable Electronics

Wearable smart electronic devices based on wireless systems use batteries as a power source. However, recent miniaturization and various functions have increased energy consumption, resulting in problems such as reduction of use time and frequent charging. These factors hinder the development of wearable electronic devices. In order to solve this energy problem, research studies on triboelectric nanogenerators (TENGs) are conducted based on the coupling of contact‐electrification and electrostatic induction effects for harvesting the vast amounts of biomechanical energy generated from wearer movement. The development of TENGs that use a variety of structures and materials based on the textile platform is reviewed, including the basic components of fibers, yarns, and fabrics made using various weaving and knitting techniques. These textile‐based TENGs are lightweight, flexible, highly stretchable, and wearable, so that they can effectively harvest biomechanical energy without interference with human motion, and can be used as activity sensors to monitor human motion. Also, the main application of wearable self‐powered systems is demonstrated and the directions of future development of textile‐based TENG for harvesting biomechanical energy presented.     

A Highly Stretchable Fiber‐Based Triboelectric Nanogenerator for Self‐Powered Wearable Electronics

The development of flexible and stretchable electronics has attracted intensive attention for their promising applications in next‐generation wearable functional devices. However, these stretchable devices that are made in a conventional planar format have largely hindered their development, especially in highly stretchable conditions. Herein, a novel type of highly stretchable, fiber‐based triboelectric nanogenerator (fiber‐like TENG) for power generation is developed. Owing to the advanced structural designs, including the fiber‐convolving fiber and the stretchable electrodes on elastic silicone rubber fiber, the fiber‐like TENG can be operated at stretching mode with high strains up to 70% and is demonstrated for a broad range of applications such as powering a commercial capacitor, LCD screen, digital watch/calculator, and self‐powered acceleration sensor. This work verifies the promising potential of a novel fiber‐based structure for both power generation and self‐powered sensing.     

Stretchable Self‐Powered Fiber‐Based Strain Sensor

The rapid development of electrical skin and wearable electronics raises the requirement of stretchable strain sensors. In this study, an active fiber‐based strain sensor (AFSS) is fabricated by coiling a fiber‐based generator around a stretchable silicone fiber. The AFSS shows the sensitive and stable performance and has the ability to detect the strain up to 25%, which is also demonstrated to detect finger motion states. It may play an essential role in future self‐powered sensor system.     

Self‐Healing and Stretchable 3D‐Printed Organic Thermoelectrics

With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy‐harvesting solutions that are compatible with such wearables. However, many of the proposed energy‐harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self‐healability is essential to realize self‐powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self‐healing and stretchable properties produced via 3D‐printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self‐healable and solution‐processable composite, 3D‐printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self‐healing. This approach represents a significant step in achieving damage‐free and truly wearable 3D‐printed organic thermoelectrics.     

Self‐Healing,Flexible, and Tailorable Triboelectric Nanogenerators for Self‐Powered Sensors based on Thermal Effect of Infrared Radiation

Self‐healing triboelectric nanogenerators (TENGs) with flexibility, robustness, and conformability are highly desirable for promising flexible and wearable devices, which can serve as a durable, stable, and renewable power supply, as well as a self‐powered sensor. Herein, an entirely self‐healing, flexible, and tailorable TENG is designed as a wearable sensor to monitor human motion, with infrared radiation from skin to promote self‐healing after being broken based on thermal effect of infrared radiation. Human skin is a natural infrared radiation emitter, providing favorable conditions for the device to function efficiently. The reversible imine bonds and quadruple hydrogen bonding (UPy) moieties are introduced into polymer networks to construct self‐healable electrification layer. UPy‐functionalized multiwalled carbon nanotubes are further incorporated into healable polymer to obtain conductive nanocomposite. Driven by the dynamic bonds, the designed and synthesized materials show excellent intrinsic self‐healing and shape‐tailorable features. Moreover, there is a robust interface bonding in the TENG devices due to the similar healable networks between electrification layer and electrode. The output electric performances of the self‐healable TENG devices can almost restore their original state when the damage of the devices occurs. This work presents a novel strategy for flexible devices, contributing to future sustainable energy and wearable electronics.     

Liquid Metal Supercooling for Low‐Temperature Thermoelectric Wearables

Elastomers embedded with droplets of liquid metal (LM) alloy represent an emerging class of soft multifunctional composites that have potentially transformative impact in wearable electronics, biocompatible machines, and soft robotics. However, for these applications it is crucial for LM alloys to remain liquid during the entire service temperature range in order to maintain high mechanical compliance throughout the duration of operation. Here, LM‐based functional composites that do not freeze and remain soft and stretchable at extremely low temperatures are introduced. It is shown that the confinement of LM droplets to micro‐/nanometer length scales significantly suppresses their freezing temperature (down to ?84.1 from ?5.9 °C) and melting point (down to ?25.6 from +17.8 °C) independent of the choice of matrix material and processing conditions. Such a supercooling effect allows the LM inclusions to preserve their fluidic nature at low temperatures and stretch with the surrounding polymer matrix without introducing significant mechanical resistance. These results indicate that LM composites with highly stabilized droplets can operate over a wide temperature range and open up new possibilities for these emerging materials, which are demonstrated with self‐powered wearable thermoelectric devices for bio‐sensing and personal health monitoring at low temperatures.     

Cellular Polypropylene Piezoelectret for Human Body Energy Harvesting and Health Monitoring

Self‐powered and wearable electronics, which are away from the problems of batteries, can provide the sustainable and comfortable interactive service for people. In this work, cellular polypropylene piezoelectret, which is with excellent physical and electrical properties, is utilized to build the human body energy harvesting and self‐powered human health monitoring systems. The cellular polypropylene piezoelectret flexible generator can reach a maximum peak power density of ≈52.8 mW m^?2. Simultaneously, self‐powered human body biological signals detecting sensors are demonstrated to detect the human physiological signals, such as coughing action and arterial pulses. This study strongly indicates the great compatibility and potential applications in human healthy monitoring may pave a new developing way for portable and wearable electronics systems.     

High‐Energy Asymmetric Supercapacitor Yarns for Self‐Charging Power Textiles

Rapid growth of electronic textile increases the demand for textile‐based power sources, which should have comparable lightweight, flexibility, and comfort. In this work, a self‐charging power textile interwoven by all‐yarn‐based energy‐harvesting triboelectric nanogenerators (TENG) and energy‐storing yarn‐type asymmetric supercapacitors (Y‐ASC) is reported. Common polyester yarns with conformal Ni/Cu coating are utilized as 1D current collectors in Y‐ASCs and electrodes in TENGs. The solid‐state Y‐ASC achieves high areal energy density (≈78.1 µWh cm^?2), high power density (14 mW cm^?2), stable cycling performance (82.7% for 5000 cycles), and excellent flexibility (1000 cycles bending for 180°). The TENG yarn can be woven into common fabrics with desired stylish designs to harvest energy from human daily motions at high output (≈60 V open‐circuit voltage and ≈3 µA short‐circuit current). The integrated self‐charging power textile is demonstrated to power an electronic watch without extra recharging by other power sources, suggesting its promising applications in electronic textiles and wearable electronics.     

Entirely,Intrinsically, and Autonomously Self‐Healable,Highly Transparent,and Superstretchable Triboelectric Nanogenerator for Personal Power Sources and Self‐Powered Electronic Skins

Power and electronic components that are self‐healable, deformable, transparent, and self‐powered are highly desirable for next‐generation energy/electronic/robotic applications. Here, an energy‐harvesting triboelectric nanogenerator (TENG) that combines the above features is demonstrated, which can serve not only as a power source but also as self‐powered electronic skin. This is the first time that both of the triboelectric‐charged layer and electrode of the TENG are intrinsically and autonomously self‐healable at ambient conditions. Additionally, comparing with previous partially healable TENGs, its fast healing time (30 min, 100% efficiency at 900% strain), high transparency (88.6%), and inherent superstretchability (>900%) are much more favorable. It consists of a metal‐coordinated polymer as the triboelectrically charged layer and hydrogen‐bonded ionic gel as the electrode. Even after 500 cutting‐and‐healing cycles or under extreme 900%‐strain, the TENG retains its functionality. The generated electricity can be used directly or stored to power commercial electronics. The TENG is further used as self‐powered tactile‐sensing skin in diverse human–machine interfaces including smart glass, an epidermal controller, and phone panel. This TENG with merits including fast ambient‐condition self‐healing, high transparency, intrinsic stretchability, and energy‐extraction and actively‐sensing abilities, can meet wide application needs ranging from deformable/portable/transparent electronics, smart interfaces, to artificial skins.     

Self‐Powered Iontophoretic Transdermal Drug Delivery System Driven and Regulated by Biomechanical Motions

Transdermal drug delivery (TDD) systems with feedback control have attracted extensive research and clinical interest owing to their unique advantages of convenience, self‐administration, and safety. Here, a self‐powered wearable iontophoretic TDD system that can be driven and regulated by the energy harvested from biomechanical motions is proposed for closed‐loop motion detection and therapy. A wearable triboelectric nanogenerator (TENG) is used as the motion sensor and energy harvester that can convert biomechanical motions into electricity for iontophoresis without stored‐energy power sources, while a hydrogel‐based soft patch with side‐by‐side electrodes is designed to enable noninvasive iontophoretic TDD. Proof‐of‐concept experiments on pig skin with dyes as model drugs successfully demonstrate the feasibility of the proposed system. This work not only extends the application of TENG in the biomedical field, but may also provide a cost‐effective solution for noninvasive, electrically assisted TDD with closed‐loop sensing and treatment.     

An Amphiphobic Hydraulic Triboelectric Nanogenerator for a Self‐Cleaning and Self‐Charging Power System

Raindrop falling, which is one kind of water motions, contains large amount of mechanical energy. However, harvesting energy from the falling raindrop to drive electronics continuously is not commonly investigated. Therefore, a self‐cleaning/charging power system (SPS) is reported, which can be exploited to convert and store energy from falling raindrop directly for providing a stable and durable output. The SPS consists of a hydraulic triboelectric nanogenerator (H‐TENG) and several embedded fiber supercapacitors. The surface of H‐TENG is amphiphobic, enabling the SPS self‐cleaning. The fiber supercapacitor which uses α‐Fe₂O₃/reduced graphene oxide composite possesses remarkable specific capacitance, excellent electrical stability, and high flexibility. These properties of the fiber supercapacitor make it suitable for a wearable power system. A power raincoat based on the SPS is demonstrated as application. After showering by water flow, which simulates falling raindrops, for 100 s, the power raincoat achieves an open‐circuit voltage of 4 V and lights a light‐emitting diode for more than 300 s. With features of low cost, easy installation, and good flexibility, the SPS harvesting energy from the falling raindrop renders as a promising sustainable power source for wearable and portable electronics.     

A High‐Reliability Kevlar Fiber‐ZnO Nanowires Hybrid Nanogenerator and its Application on Self‐Powered UV Detection

A microfiber‐nanowire hybrid structure is the fundamental component for a wearable piezoelectric nanogenerator (PENG) for harvesting body motion energy. Here, a novel approach combining surface coating and plasma etching techniques is reported to enhance the mechanical reliability of Kevlar microfiber‐ZnO nanowires (NWs) hybrid structure that is used for PENG. After treatment, the hybrid structure has dramatically improved high flexibility, robustness, and durability. On the basis of the coupled piezoelectric and semiconducting properties of ZnO, the processed Kevlar fibers covered with ZnO NWs are utilized to fabricate a 2D nanogenerator (2DNG). The open‐circuit voltage and short‐circuit current of the 2DNG are 1.8 mV and 4.8 pA, respectively. Furthermore, the 2DNG is successfully employed to quantitatively detect UV intensity from 0.2 to 1 mW cm^?2 as a self‐powered system.     

Stretchable and Shape-Adaptable Triboelectric Nanogenerator Based on Biocompatible Liquid Electrolyte for Biomechanical Energy Harvesting and Wearable Human–Machine Interaction

The significant demand of sustainable power sources has been triggered by the development of wearable electronics (e.g., electronic skin, human health monitors, and intelligent robotics). However, tensile strain limitation and low conformability of existing power sources cannot match their development. Herein, a stretchable and shape-adaptable liquid-based single-electrode triboelectric nanogenerator (LS-TENG) based on potassium iodide and glycerol (KI-Gly) liquid electrolyte as work electrode is developed for harvesting human motion energy to power wearable electronics. The LS-TENG demonstrates high output performances (open-circuit voltage of 300 V, short-circuit current density of 17.5 mA m^–2, and maximum output power of 2.0 W m^–2) and maintains the stable output performances without deterioration under 250% tension stretching and after 10 000 cycles of repeated contact-separation motion. Moreover, the LS-TENG can harvest biomechanical energy, including arm shaking, human walking, and hand tapping, to power commercial electronics without extra power sources. The LS-TENG attached on different joints of body enables to work as self-powered human motion monitor. Furthermore, a flexible touch panel based on the LS-TENG combined with a microcontroller is explored for human–machine interactions. Consequently, the stretchable and shape-adaptable LS-TENG based on KI-Gly electrolyte would act as an exciting platform for biomechanical energy harvesting and wearable human–machine interaction.     

Stretchable and Flexible Buckypaper‐Based Lactate Biofuel Cell for Wearable Electronics

This work demonstrates a stretchable and flexible lactate/O₂ biofuel cell (BFC) using buckypaper (BP) composed of multi‐walled carbon nanotubes as the electrode material. Free‐standing BP, functionalized with a pyrene‐polynorbornene homopolymer, is fabricated as the immobilization matrix for lactate oxidase (LOx) at the anode and bilirubin oxidase at the cathode. This biofuel cell delivers an open circuit voltage of 0.74 V and a high‐power density of 520 µW cm^?2. The functionalized BP electrodes are assembled onto a stretchable screen‐printed current collector with an “island–bridge” configuration, which ensures conformal contact between the wearable BFC and the human body and endows the BFC with excellent performance stability under stretching condition. When applied to the arm of the volunteer, the BFC can generate a maximum power of 450 µW. When connected with a voltage booster, the on‐body BFC is able to power a light emitting diode under both pulse discharge and continuous discharge modes during exercise. This demonstrates the promising potential of the flexible BP‐based BFC as a self‐sustained power source for next‐generation wearable electronics.     

Self‐Powered Tactile Sensor Array Systems Based on the Triboelectric Effect

With the arrival of intelligent terminals, tactile sensors which are capable of sensing various external physical stimuli are considered among the most vital devices for the next generation of smart electronics. To create a self‐powered tactile sensor system that can function sustainably and continuously without an external power source is of crucial significance. An overview of the development in self‐powered tactile sensor array system based on the triboelectric effect is systematically presented. The combination of multi‐functionalization and high performance of tactile sensors aimed at achieving highly comprehensive performance is presented. For the tactile sensor unit, a development is summarized based on the two primary modes which are vertical contact–separation and single‐electrode. For the pressure mapping array, the resolution is significantly enhanced by the novel cross‐type configuration based on the single‐electrode mode. Integrated with other mechanisms, the performance will be further elevated by broadening of the detect range and realizing of visualization of pressure imaging. Then, two main applications of human–machine interaction (HMI) and trajectory monitoring are comprehensively summarized. Finally, the future perspectives of self‐powered tactile sensor system based on triboelectric effect are discussed.     

Hybrid Smart Fiber with Spontaneous Self‐Charging Mechanism for Sustainable Wearable Electronics

Smart wearable electronics that are fabricated on light‐weight fabrics or flexible substrates are considered to be of next‐generation and portable electronic device systems. Ideal wearable and portable applications not only require the device to be integrated into various fiber form factors, but also desire self‐powered system in such a way that the devices can be continuously supplied with power as well as simultaneously save the acquired energy for their portability and sustainability. Nevertheless, most of all self‐powered wearable electronics requiring both the generation of the electricity and storing of the harvested energy, which have been developed so far, have employed externally connected individual energy generation and storage fiber devices using external circuits. In this work, for the first time, a hybrid smart fiber that exhibits a spontaneous energy generation and storage process within a single fiber device that does not need any external electric circuit/connection is introduced. This is achieved through the employment of asymmetry coaxial structure in an electrolyte system of the supercapacitor that creates potential difference upon the creation of the triboelectric charges. This development in the self‐charging technology provides great opportunities to establish a new device platform in fiber/textile‐based electronics.