Energy Harvesting and Storage with Soft and Stretchable Materials

Highly Flexible, High-Performance, and Stretchable Piezoelectric Sensor Based on a Hierarchical Droplet-Shaped Ceramics with Enhanced Damage Tolerance

Stretchable self-powered sensors are of significant interest in next-generation wearable electronics. However, current strategies for creating stretchable piezoelectric sensors based on piezoelectric polymers or 0-3 piezoelectric composites face several challenges such as low piezoelectric activity, low sensitivity, and poor durability. In this paper, a biomimetic soft-rigid hybrid strategy is used to construct a new form of highly flexible, high-performance, and stretchable piezoelectric sensor. Inspired by the hinged bivalve Cristaria plicata, hierarchical droplet-shaped ceramics are manufactured and used as rigid components, where computational models indicate that the unique arched curved surface and rounded corners of this bionic structure can alleviate stress concentrations. To ensure electrical connectivity of the piezoelectric phase during stretching, a patterned liquid metal acts as a soft circuit and a silicone polymer with optimized wettability and stretchability serves as a soft component that forms a strong mechanical interlock with the hierarchical ceramics. The novel sensor design exhibits excellent sensitivity and durability, where the open circuit voltage remains stable after 5000 stretching cycles at 60% strain and 5000 twisting cycles at 180°. To demonstrate its potential in heathcare applications, this new stretchable sensor is successfully used for wireless gesture recognition and assessing the progression of knee osteoarthritis.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

Largely Improving the Output Performance of Stretchable Triboelectric Nanogenerators via Thermo-Compressive Technology

Stretchable triboelectric nanogenerators (TENGs) are widely applied in wearable and implantable electronics, smart medical devices, and soft robots. However, it is still a challenge to produce stretchable TENGs with both exceptional elasticity and output performance, which limits their application scope. In this work, high-performance stretchable TENGs are developed through a thermo-compression (TC) fabrication process. In particular, a poly(vinylidene fluoride) film is compactly bound to the elastic thermoplastic polyurethane substrate, which inherits excellent stretchability with a strain of up to 815%. Furthermore, owing to the large surface area, tight contact, and effective vertical transport of tribo-induced charges between the coupled fibrous tribo-layer and soft substrate, the TC composite film-based TENGs exhibit a greater output (2-4 times) than unlaminated film-based TENGs. Additionally, the broad universality of this method is proven using various tribo- and substrate materials. The proposed technology provides a novel and effective approach to conjointly boost the output and stretchability of TENGs, showing encouraging application prospects in self-powered wearable and flexible electronics.

Stretchable Fibers with Highly Conductive Surfaces and Robust Electromechanical Performances for Electronic Textiles

One-dimensional conductive fibers that can simultaneously accommodate multiple deformations are crucial materials to enable next-generation electronic textile technologies for applications in the fields of healthcare, energy harvesting, human-machine interactions, etc. Stretchable conductive fibers (SCFs) with high conductivity on their external structure are important for their direct integration with other electronic components. However, the dilemma to achieve high conductivity and concurrently large stretchability is still quite challenging to resolve among conductive fibers with a conductive surface. Here, a three-layer coaxial conductive fiber, which can provide robust electrical performance under various deformations, is reported. A dual conducting structure with a semisolid metallic layer and a stretchable composite layer was designed in the fibers, providing exceptional conductivity and mechanical stability under mechanical strains. The conductive fiber achieved an initial conductivity of 2291.83 S cm-1 on the entire fiber and could be stretched up to 600% strains. With the excellent electromechanical properties of the SCF, we were able to demonstrate different electronic textile applications including physiological monitoring, neuromuscular electrical stimulation, and energy harvesting.

Harvesting Inertial Energy and Powering Wearable Devices: A Review

Amidst the swift progression of microelectronics and Internet of Things technology, wearable devices are gradually gaining ground in the domains of human health monitoring. Recently, human bioenergy harvesting has emerged as a plausible alternative to batteries. This paper delves into harvesting human inertial energy that stimulates inertial masses through human motion and then transmutes the motion of the inertial masses into electrical energy. The inertial energy harvester is better suited for low-frequency and irregular human motion. This review first identifies the sources of human motion excitation that are compatible with inertial energy harvesters and then provides a summary of the operating principles and the comparisons of the commonly used energy conversion mechanisms, including electromagnetic, piezoelectric, and triboelectric transducers. The review thoroughly summarizes the latest advancements in human inertial energy-harvesting technology that are categorized and grouped based on their excitation sources and mechanical modulation methods. In addition, the review outlines the applications of inertial energy harvesters in powering wearable devices, medical health monitoring, and as mobile power sources. Finally, the challenges faced by inertial energy-harvesting technologies are discussed, and the review provides a perspective on the potential developments in the field.

A Dual-Functional MXene-Based Bioanode for Wearable Self-Charging Biosupercapacitors

As a reliable energy-supply platform for wearable electronics, biosupercapacitors combine the characteristics of biofuel cells and supercapacitors to harvest and store the energy from human's sweat. However, the bulky preparation process and deep embedding of enzyme active sites in bioelectrodes usually limit the energy-harvesting process, retarding the practical power-supply sceneries especially during the complicated in vivo motion. Herein, a MXene/single-walled carbon nanotube/lactate oxidase hierarchical structure as the dual-functional bioanode is designed, which can not only provide a superior 3D catalytic microenvironment for enzyme accommodation to harvest energy from sweat, but also offers sufficient capacitance to store energy via the electrical double-layer capacitor. A wearable biosupercapacitor is fabricated in the "island-bridge" structure with a composite bioanode, active carbon/Pt cathode, polyacrylamide hydrogel substrate, and liquid metal conductor. The device exhibits an open-circuit voltage of 0.48 V and the high power density of 220.9 µW cm-2 at 0.5 mA cm-2 . The compact conformal adhesion with skin is successfully maintained under stretching/bending conditions. After repeatedly stretching the devices, there is no significant power attenuation in pulsed output. The unique bioelectrode structure and attractive energy harvesting/storing properties demonstrate the promising potential of this biosupercapacitor as a micro self-powered platform of wearable electronics.

A Cross-Plane Design for Wearable Thermoelectric Generators with High Stretchability and Output Performance

Stretchable wearable thermoelectric (TE) generators (WTEGs) without compromising output performance for real wearables have attracted much attention recently. Herein, a 3D thermoelectric generator with biaxial stretchability is constructed on the device level. Ultraflexible inorganic Ag/Ag2 Se strips are sewn into the soft purl-knit fabric, in which the thermoelectric legs are aligned in the direction of vertical heat flux. A stable and sufficient temperature gradient of 5.2 °C across the WTEG is therefore achieved when contacted with the wrist at a room temperature of 26.3 °C. The prepared TEG generates a high power density of 10.02 W m-2 at a vertical temperature gradient of 40 K. Meanwhile, the reliable energy harvesting promises a variation of less than 10% under the biaxial stretching up to 70% strain via leveraging the combined effects of the stretchability of knit fabric and geometry of TE strips. The knit fabric-supported TEG enables a seamless conformation to the skin as well as efficient body heat harvesting, which can provide sustainable energy to low power consumption wearable electronics.

Perspectives on recent advancements in energy harvesting, sensing and bio-medical applications of piezoelectric gels

The development of next-generation bioelectronics, as well as the powering of consumer and medical devices, require power sources that are soft, flexible, extensible, and even biocompatible. Traditional energy storage devices (typically, batteries and supercapacitors) are rigid, unrecyclable, offer short-lifetime, contain hazardous chemicals and possess poor biocompatibility, hindering their utilization in wearable electronics. Therefore, there is a genuine unmet need for a new generation of innovative energy-harvesting materials that are soft, flexible, bio-compatible, and bio-degradable. Piezoelectric gels or PiezoGels are a smart crystalline form of gels with polar ordered structures that belongs to the broader family of piezoelectric material, which generate electricity in response to mechanical stress or deformation. Given that PiezoGels are structurally similar to hydrogels, they offer several advantages including intrinsic chirality, crystallinity, degree of ordered structures, mechanical flexibility, biocompatibility, and biodegradability, emphasizing their potential applications ranging from power generation to bio-medical applications. Herein, we describe recent examples of new functional PiezoGel materials employed for energy harvesting, sensing, and wound dressing applications. First, this review focuses on the principles of piezoelectric generators (PEGs) and the advantages of using hydrogels as PiezoGels in energy and biomedical applications. Next, we provide a detailed discussion on the preparation, functionalization, and fabrication of PiezoGel-PEGs (P-PEGs) for the applications of energy harvesting, sensing and wound healing/dressing. Finally, this review concludes with a discussion of the current challenges and future directions of P-PEGs.

High Energy Harvesting Performances Silicone Elastomer via Filling Soft Dielectric with Stretching Deformability

Dielectric elastomer generators (DEGs) with high generated energy density and high conversion efficiency are of great interest. Among several dielectric elastomers (DEs), silicone elastomer filled with ceramic fillers have been extensively studied for their high elasticity, insulation, and permittivity. However, the stretched breakdown strength (E_bs ) of such composites decreases significantly under large strain, thus sharply reduces its energy harvesting performances. In this study, a polar rubber-based dielectric (GNBR) is synthetized and creatively used as "soft filler" for silicone elastomer. Benefiting from the deformability under stretching and its inherent strong interface bonding with silicone elastomer, this soft filler effectively avoids the formation of weak interface under large strain and reduces the local field strength of interface area. As expected, the composite filled with soft filler (GNBR/PMVS) shows enhanced E_bs of 2.8 times that of composite with traditional hard filler (TiO₂ /PMVS) under equibiaxial strain of 200%. As a result, GNBR/PMVS composite exhibits maximum energy density of 130.5 mJ g^-1 with up-to-date highest power conversion efficiency of reported DEG (44.5%). The findings will provide new insights in the rational design of DE composites characterized by high stretched breakdown strength for advanced energy harvesting system.

Nanopolyhybrids: Materials, Engineering Designs, and Advances in Thermal Management

The fundamental requirements for thermal comfort along with the unbalanced growth in the energy demand and consumption worldwide have triggered the development and innovation of advanced materials for high thermal-management capabilities. However, continuous development remains a significant challenge in designing thermally robust materials for the efficient thermal management of industrial devices and manufacturing technologies. The notable achievements thus far in nanopolyhybrid design technologies include multiresponsive energy harvesting/conversion (e.g., light, magnetic, and electric), thermoregulation (including microclimate), energy saving in construction, as well as the miniaturization, integration, and intelligentization of electronic systems. These are achieved by integrating nanomaterials and polymers with desired engineering strategies. Herein, fundamental design approaches that consider diverse nanomaterials and the properties of nanopolyhybrids are introduced, and the emerging applications of hybrid composites such as personal and electronic thermal management and advanced medical applications are highlighted. Finally, current challenges and outlook for future trends and prospects are summarized to develop nanopolyhybrid materials.

Shaping a Soft Future: Patterning Liquid Metals

This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non-spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e-skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Flexible-to-Stretchable Mechanical and Electrical Interconnects

Stretchable electronic devices that maintain electrical function when subjected to stress or strain are useful for enabling new applications for electronics, such as wearable devices, human-machine interfaces, and components for soft robotics. Powering and communicating with these devices is a challenge. NFC (near-field communication) coils solve this challenge but only work efficiently when they are in close proximity to the device. Alternatively, electrical signals and power can arrive via physical connections between the stretchable device and an external source, such as a battery. The ability to create a robust physical and electrical connection between mechanically disparate components may enable new types of hybrid devices in which at least a portion is stretchable or deformable, such as hinges. This paper presents a simple method to make mechanical and electrical connections between elastomeric conductors and flexible (or rigid) conductors. The adhesion at the interface between these disparate materials arises from surface chemistry that forms strong covalent bonds. The utilization of liquid metals as the conductor provides stretchable interconnects between stretchable and non-stretchable electrical traces. The liquid metal can be printed or injected into vias to create interconnects. We characterized the mechanical and electrical properties of these hybrid devices to demonstrate the concept and identify geometric design criteria to maximize mechanical strength. The work here provides a simple and general strategy for creating mechanical and electrical connections that may find use in a variety of stretchable and soft electronic devices.

A Full-Device Autonomous Self-Healing Stretchable Soft Battery from Self-Bonded Eutectogels

Next-generation energy storage devices should be soft, stretchable, and self-healable. Previously reported self-healable batteries mostly possess limited stretchability and rely on healable electrodes or electrolytes rather than achieving full-device self-healability. Herein, an all-component self-bonding strategy is reported to obtain an all-eutectogel soft battery (AESB) that simultaneously achieves full-cell autonomous self-healability and omnidirectional intrinsic stretchability (>1000% areal strain) over a broad temperature range (-20~60 °C). Without requiring any external stimulus, the five-layered soft battery can efficiently recover both its mechanical and electrochemical performance at full-cell level. The developed AESB can be easily configured into various 3D architectures with highly interfacial compatible eutectogel electrodes, electrolyte, and substrate, presenting an excellent opportunity for the development of embodied energy technologies. The present work provides a general and user-friendly soft electronic material platform for fabricating a variety of intrinsic self-healing stretchable multi-layered electronics, which are promising beyond the field of energy storage, such as displays, sensors, circuits, and soft robots.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

Fiber-Based Triboelectric Nanogenerator for Mechanical Energy Harvesting and Its Application to a Human-Machine Interface

Mechanical energy harvesters including piezoelectric nanogenerators, electromagnetic generators and triboelectric nanogenerators (TENG) used to convert the mechanical motion into electricity are more and more important in the recent decades. Specifically, the fiber-based TENG (FTENG) has gained considerable favors due to its flexibility, light weight, and high environmental tolerance for the wearable devices. The traditional FTENGs made of Teflon result in better performance but are not suitable for long-term wear in person. Here, we propose a novel FTENG using a flexible micro-needle-structured polydimethylsiloxane (MN-PDMS) together with the comfortable commercially available 2D-polyester fibers, and electroless nickel-plated cotton cloth of which two are widely used in human daily life. The MN-PDMS is formed by a laser engraved mold for improving its output performance of FTENG compared to the flat-PDMS. The open-circuit voltage (Voc) and the short-circuit current (Isc) of MN-FTENG increased to 73.6 V and 36 μA, respectively, which are 34% and 37% higher than the flat-FTENG. In terms of power, the performance of MN-FTENG reaches 1.296 mW which is 89% higher than that of flat-TENG and it can also light up 90 LEDs. For application, human motion at the joints can be detected and collected with various signals that are used for the human-machine interface (HMI) through the cooperation of components for the Internet of Things (IoT). It can light up the LED bulb through MN-FTENG to potentially develop IoT HMI systems for human motion control of robot in the future.

Improved performance of stretchable piezoelectric energy harvester based on stress rearrangement

With the development of wearable devices and soft electronics, the demand for stretchable piezoelectric energy harvesters (SPEHs) has increased. Energy harvesting can provide energy when large batteries or power sources cannot be employed, and stretchability provides a user-friendly experience. However, the performance of SPEHs remains low, which limits their application. In this study, a wearable SPEH is developed by adopting a kirigami structure on a polyvinylidene fluoride film. The performance of the SPEH is improved by rearranging the stress distribution throughout the film. This is conducted using two approaches: topological depolarization, which eliminates the opposite charge generation by thermal treatment, and optimization of the neutral axis, which maximizes the stress applied at the surface of the piezoelectric film. The SPEH performance is experimentally measured and compared with that of existing SPEHs. Using these two approaches, the stress was rearranged in both the x-y plane and z-direction, and the output voltage increased by 21.57% compared with that of the original film with the same stretching motion. The generated energy harvester was successfully applied to smart transmittance-changing contact lenses.

A Flexible Multifunctional PAN Piezoelectric Fiber with Hydrophobicity, Energy Storage, and Fluorescence

Lightweight, flexible, and hydrophobic multifunctional piezoelectric sensors have increasingly important research value in contemporary society. They can generate electrical signals under the action of pressure and can be applied in various complex scenarios. In this study, we prepared a polyacrylonitrile (PAN) composite fiber doped with imidazolium type ionic liquids (ILs) and europium nitrate hexahydrate (Eu (NO₃)₃·6H₂O) by a facile method. The results show that the PAN composite fibers had excellent mechanical properties (the elongation at break was 114% and the elastic modulus was 2.98 MPa), hydrophobic self-cleaning ability (water contact angle reached 127.99°), and can also emit light under UV light irradiation red fluorescence. In addition, thanks to the induction of the piezoelectric phase of PAN by the dual fillers, the composite fibers exhibited efficient energy storage capacity and excellent sensitivity. The energy density of PAN@Eu-6ILs reached a maximum of 44.02 mJ/cm³ and had an energy storage efficiency of 80%. More importantly, under low pressure detection, the sensitivity of the composite fiber was 0.69 kPa⁻¹. The research results show that this PAN composite fiber has the potential to act as wearable piezoelectric devices, energy storage devices, and other electronic devices.

Controllable Preparation and Strengthening Strategies towards High-Strength Carbon Nanotube Fibers

Carbon nanotubes (CNTs) with superior mechanical properties are expected to play a role in the next generation of critical engineering mechanical materials. Crucial advances have been made in CNTs, as it has been reported that the tensile strength of defect-free CNTs and carbon nanotube bundles can approach the theoretical limit. However, the tensile strength of macro carbon nanotube fibers (CNTFs) is far lower than the theoretical level. Although some reviews have summarized the development of such fiber materials, few of them have focused on the controllable preparation and performance optimization of high-strength CNTFs at different scales. Therefore, in this review, we will analyze the characteristics and latest challenges of multiscale CNTFs in preparation and strength optimization. First, the structure and preparation of CNTs are introduced. Then, the preparation methods and tensile strength characteristics of CNTFs at different scales are discussed. Based on the analysis of tensile fracture, we summarize some typical strategies for optimizing tensile performance around defect and tube-tube interaction control. Finally, we introduce some emerging applications for CNTFs in mechanics. This review aims to provide insights and prospects for the controllable preparation of CNTFs with ultra-high tensile strength for emerging cutting-edge applications.

Logic-enabled textiles

Textiles hold great promise as a soft yet durable material for building comfortable robotic wearables and assistive devices at low cost. Nevertheless, the development of smart wearables composed entirely of textiles has been hindered by the lack of a viable sheet-based logic architecture that can be implemented using conventional fabric materials and textile manufacturing processes. Here, we develop a fully textile platform for embedding pneumatic digital logic in wearable devices. Our logic-enabled textiles support combinational and sequential logic functions, onboard memory storage, user interaction, and direct interfacing with pneumatic actuators. In addition, they are designed to be lightweight, easily integrable into regular clothing, made using scalable fabrication techniques, and durable enough to withstand everyday use. We demonstrate a textile computer capable of input-driven digital logic for controlling untethered wearable robots that assist users with functional limitations. Our logic platform will facilitate the emergence of future wearables powered by embedded fluidic logic that fully leverage the innate advantages of their textile construction.

Self-Sustainable Biomedical Devices Powered by RF Energy: A Review

Wearable and implantable medical devices (IMDs) have come a long way in the past few decades and have contributed to the development of many personalized health monitoring and therapeutic applications. Sustaining these devices with reliable and long-term power supply is still an ongoing challenge. This review discusses the challenges and milestones in energizing wearable and IMDs using the RF energy harvesting (RFEH) technique. The review highlights the main integrating frontend blocks such as the wearable and implantable antenna design, matching network, and rectifier topologies. The advantages and bottlenecks of adopting RFEH technology in wearable and IMDs are reviewed, along with the system elements and characteristics that enable these devices to operate in an optimized manner. The applications of RFEH in wearable and IMDs medical devices are elaborated in the final section of this review. This article summarizes the recent developments in RFEH, highlights the gaps, and explores future research opportunities.

More Powerful Twistron Carbon Nanotube Yarn Mechanical Energy Harvesters

Stretching a coiled carbon nanotube (CNT) yarn can provide large, reversible electrochemical capacitance changes, which convert mechanical energy to electricity. Here, it is shown that the performance of these "twistron" harvesters can be increased by optimizing the alignment of precursor CNT forests, plastically stretching the precursor twisted yarn, applying much higher tensile loads during precoiling twist than for coiling, using electrothermal pulse annealing under tension, and incorporating reduced graphene oxide nanoplates. The peak output power for a 1 and a 30 Hz sinusoidal deformation are 0.73 and 3.19 kW kg-1 , respectively, which are 24- and 13-fold that of previous twistron harvesters at these respective frequencies. This performance at 30 Hz is over 12-fold that of other prior-art mechanical energy harvesters for frequencies between 0.1 and 600 Hz. The maximum energy conversion efficiency is 7.2-fold that for previous twistrons. Twistron anode and cathode yarn arrays are stretched 180° out-of-phase by locating them in the negative and positive compressibility directions of hinged wine-rack frames, thereby doubling the output voltage and reducing the input mechanical energy.

Voltage Improvement of a Swing-Magnet-Type Generator for Harvesting Bicycle Vibrations

This paper proposes a swing-magnet-type generator that utilizes environment vibration for energy harvesting applications. This device consisted of a liquid, a swing magnet with a float, and a coil, and it was expected to generate electricity using the minute vibration of a bicycle. The vibration of the wide frequency band of the bicycle was converted into a vibration of a low-frequency mover. The yoke size of the permanent magnet affected the linkage flux and swing characteristics. Therefore, we verified the effect of the mover characteristics on the swing moment by structural simulations and vibration experiments using a linear motor. The yoke size changed the torque, which affected the resonant frequency of the swing. The magnetic-field analysis revealed the effect on the flux linkage in the yoke. The output voltage of the generator in the bicycle was 2.1 V, which could power a light-emitting diode.

Co4N-WNx composite for efficient piezocatalytic hydrogen evolution

A dual-phase transition metal nitride (TMN) based Co₄N-WN_x system has been fabricated using nitridation of CoWO₄. The interface between centrosymmetric Co₄N and non-centrosymmetric WN_x promotes charge carrier separation. This system also shows piezoelectric behavior. The piezoelectric property has been proved using piezoelectric force microscopy (PFM) measurements. In addition, modulating the non-centrosymmetric structure of Co₄N-WN_x allows a hydrogen production rate of about 262.7 μmol g^-1 h^-1 in pure water. We also show that the piezocatalytic hydrogen evolution efficiency is satisfactory. Co₄N-WN_x can also help achieve simultaneous piezocatalytic hydrogen production and RhB degradation. This work provides a novel strategy for designing efficient piezocatalytic materials.

Electrical mapping of thermoelectric power factor in WO3 thin film

With growing environmental awareness and considerable research investment in energy saving, the concept of energy harvesting has become a central topic in the field of materials science. The thermoelectric energy conversion, which is a classic physical phenomenon, has emerged as an indispensable thermal management technology. In addition to conventional experimental investigations of thermoelectric materials, seeking promising materials or structures using computer-based approaches such as machine learning has been considered to accelerate research in recent years. However, the tremendous experimental efforts required to evaluate materials may hinder us from reaping the benefits of the fast-developing computer technology. In this study, an electrical mapping of the thermoelectric power factor is performed in a wide temperature-carrier density regime. An ionic gating technique is applied to an oxide semiconductor WO3, systematically controlling the carrier density to induce a transition from an insulating to a metallic state. Upon electrically scanning the thermoelectric properties, it is demonstrated that the thermoelectric performance of WO3 is optimized at a highly degenerate metallic state. This approach is convenient and applicable to a variety of materials, thus prompting the development of novel functional materials with desirable thermoelectric properties.

MXenes for Energy Harvesting

Energy harvesting modules play an increasingly important role in the development of autonomous self-powered microelectronic devices. MXenes (i.e., 2D transition metal carbide/nitride) have recently emerged as promising candidates for energy applications due to their excellent electronic conductivity, large specific surface area, and tunable properties. Herein, a perspective on using MXenes to harvest energy from various sources in the environment is presented. First, the characteristics of MXenes that facilitate energy capturing are systematically introduced and the preparation strategies of MXenes and their derived nanostructures tailored toward such applications are summarized. Subsequently, the harvesting mechanism of different energy sources (e.g., solar energy, thermoelectric energy, triboelectric energy, piezoelectric energy, salinity-gradient energy, electrokinetic energy, ultrasound energy, and humidity energy) are discussed. Then, the recent progress of MXene-based nanostructures in energy harvesting, as well as their applications, is introduced. Finally, opinions on the existing challenges and future directions of MXene-based nanostructure for energy harvesting are presented.

Triboresistive Touch Sensing: Grid-Free Touch-Point Recognition Based on Monolayered Ionic Power Generators

Recent growing pursuit of skin-mountable devices has been impeded by the complicated structures of most sensing systems, containing electrode grids, stacked multilayers, and even external power sources. Here, a type of touch sensing, termed "triboresistive touch sensing", is introduced for gridless touch recognition based on monolayered ionic power generators. A homogeneous monolayer, i.e., ionic poly(dimethylsiloxane) (PDMS), generates electricity based on the electric field generated by touch. Voltages generated at each corner of the ionic PDMS rely on resistance between touch points and each corner, ensuring recognition of the touch positions without the need for electrode grid layers and external power sources. With notable advantages of high transparency (96.5%), stretchability (539.1%), and resilience (99.0%) of the ionic PDMS, epidermal triboresistive sensing is demonstrated to express touch position and readily play a musical instrument. A gridless system of triboresistive sensing allows rearrangement of the touch sections according to a given situation without any physical modification, and thus easily completes consecutive missions of controlling position, orientation, and gripping functions of a robot.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Towards enduring autonomous robots via embodied energy

Autonomous robots comprise actuation, energy, sensory and control systems built from materials and structures that are not necessarily designed and integrated for multifunctionality. Yet, animals and other organisms that robots strive to emulate contain highly sophisticated and interconnected systems at all organizational levels, which allow multiple functions to be performed simultaneously. Herein, we examine how system integration and multifunctionality in nature inspires a new paradigm for autonomous robots that we call Embodied Energy. Whereas most untethered robots use batteries to store energy and power their operation, recent advancements in energy-storage techniques enable chemical or electrical energy sources to be embodied directly within the structures and materials used to create robots, rather than requiring separate battery packs. This perspective highlights emerging examples of Embodied Energy in the context of developing autonomous robots.

Environment-friendly degradable zinc-ion battery based on guar gum-cellulose aerogel electrolyte

With the vigorous development of electronics and the increasingly prominent problem of environmental pollution, it is particularly important to exploit environmentally friendly electronic devices. Transient electronics represent a kind of device that once the specified functions have completed can completely or partially disappear through physical or chemical actions. In this work, we introduce a novel guar gum-cellulose aerogel (GCA) membrane based on natural biomaterials and successfully use it as an electrolyte film to fabricate a degradable zinc-ion battery (DZIB). All components of the prepared DZIBs can be successfully degraded or disintegrate in phosphate-buffered saline (PBS) containing a solution of proteinase K after approximately 40 days. This electrolyte film has a high ionic conductivity of approximately 4.73 × 10^-2 S cm^-1 and a good mechanical stress property. When applied to DZIB, the production of zinc dendrites can be restrained, leading to the battery showing excellent electrochemical performance. The battery exhibits a specific capacity of 309.1 mA h g^-1 at a current density of 308 mA g^-1 after 100 cycles and a steady cycling ability (100% capacity retention after 200 cycles). More importantly, the electrochemical performance of DZIB is better than that of transient batteries reported in the past, taking a solid step in the field of transient electronics in the initial stage.

Powering smart contact lenses for continuous health monitoring: Recent advancements and future challenges

As the tear is noninvasively and continuously available, it has been turned into a convenient biological interface as a wearable medical device for out-of-hospital and self-monitoring applications. Recent progress in integrated circuits (ICs) and biosensors coupled with wireless data communication techniques have led to the implementation of smart contact lenses that can continuously sample tear fluid, analyze physiological conditions, and wirelessly transmit data to an electronic device such as smartphone, which can send data to relevant healthcare units. Continuous analyte monitoring is one of the significant characteristics of wearable biosensors. However, despite several advantages over other on-skin wearable medical devices, batteries cannot be incorporated on smart contact lenses for continuous electrical power supply due to the limited area. Herein, we review the progress of power delivery techniques of smart contact lenses for the first time. Different approaches, including wireless power transmission (WPT), biofuel cells, supercapacitors, flexible batteries, wired connections, and hybrid methods, are thoroughly discussed to understand the principles of self-sustainable contact lens biosensors comprehensively. Additionally, recent progress in contact lens biosensors is reviewed in detail, thereby providing the prospects for further developments of smart contact lenses as a common biosensing platform for various disease monitoring and diagnostic applications.

Applications of liquid metals in nanotechnology

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.

Temperature sensing using junctions between mobile ions and mobile electrons

We develop temperature sensors on the basis of charges accumulated at the electrolyte/dielectric interface and dielectric/electrode interface. The accumulated charges make the temperature sensors self-powered, which simplifies circuit design and enables portable sensing. The sensors are stretchable, but deformation does not affect temperature sensing. The sensors have high sensitivity and fast response. They can be made small and transparent. Such temperature sensors open new possibilities to create human–machine interfaces and soft robots in healthcare and engineering.

Sensing technology is under intense development to enable the Internet of everything and everyone in new and useful ways. Here we demonstrate a method of stretchable and self-powered temperature sensing. The basic sensing element consists of three layers: an electrolyte, a dielectric, and an electrode. The electrolyte/dielectric interface accumulates ions, and the dielectric/electrode interface accumulates electrons (in either excess or deficiency). The ions and electrons at the two interfaces are usually not charge-neutral, and this charge imbalance sets up an ionic cloud in the electrolyte. The design functions as a charged temperature-sensitive capacitor. When temperature changes, the ionic cloud changes thickness, and the electrode changes open-circuit voltage. We demonstrate high sensitivity (∼1 mV/K) and fast response (∼10 ms). Such temperature sensors can be made small, stable, and transparent. Depending on the arrangement of the electrolyte, dielectric, and electrode, we develop four designs for the temperature sensor. In addition, the temperature sensor has good linearity in the range of tens of Kelvin. We further show that the temperature sensors can be integrated into stretchable electronics and soft robots.

Mechano-Triboelectric Analysis of Surface Charge Generation on Replica-Molded Elastomeric Nanodomes

Replica molding-based triboelectrification has emerged as a new and facile technique to generate nanopatterned tribocharge on elastomer surfaces. The “mechano-triboelectric charging model” has been developed to explain the mechanism of the charge formation and patterning process. However, this model has not been validated to cover the full variety of nanotexture shapes. Moreover, the experimental estimation of the tribocharge’s surface density is still challenging due to the thick and insulating nature of the elastomeric substrate. In this work, we perform experiments in combination with numerical analysis to complete the mechano-triboelectrification charging model. By utilizing Kelvin probe force microscopy (KPFM) and finite element analysis, we reveal that the mechano-triboelectric charging model works for replica molding of both recessed and protruding nanotextures. In addition, by combining KPFM with numerical electrostatic modeling, we improve the accuracy of the surface charge density estimation and cross-calibrate the result against that of electrostatic force microscopy. Overall, the regions which underwent strong interfacial friction during the replica molding exhibited high surface potential and charge density, while those suffering from weak interfacial friction exhibited low values on both. These multi-physical approaches provide useful and important tools for comprehensive analysis of triboelectrification and generation of nanopatterned tribocharge. The results will widen our fundamental understanding of nanoscale triboelectricity and advance the nanopatterned charge generation process for future applications.

A Soft Variable-Area Electrical-Double-Layer Energy Harvester

The technological promise of soft devices-wearable electronics, implantables, soft robotics, sensors-has accelerated the demand for deformable energy sources. Devices that can convert mechanical energy to electrical energy can enable self-powered, tetherless, and sustainable devices. This work presents a completely soft and stretchable (>400% strain) energy harvester based on variable-area electrical-double-layer (EDL) capacitors (≈40 µF cm^-2 ). Mechanically varying the EDL area, and thus the capacitance, disrupts equilibrium and generates a driving force for charge movement through an external circuit. Prior EDL capacitors varied the contact area by depressing water droplets between rigid electrodes. In contrast, here, the harvester consists of liquid-metal electrodes encased in a hydrogel. Deforming the device by ≈25% strain generates a power density ≈0.5 mW m^-2 . This unconventional approach is attractive because: (1) it does not need an external voltage supply to provide charge; (2) the electrodes themselves deform; and (3) it can work under various modes of deformation such as pressing, stretching, bending, and twisting. The unique ability of the harvester to operate underwater shows promising applications in wearables that contact sweat, underwater sensing, and blue energy harvesting.

A Highly Robust Ionotronic Fiber with Unprecedented Mechanomodulation of Ionic Conduction

Stretchable ionic conductors are appealing for tissue-like soft electronics, yet suffer from a tardy mechanoelectric response due to their poor modulation of ionic conduction arising from intrinsic homogeneous soft chain network. Here, a highly robust ionotronic fiber is designed by synergizing ionic liquid and liquid crystal elastomer with alternate rigid mesogen units and soft chain spacers, which shows an unprecedented strain-induced ionic conductivity boost (≈10³ times enhanced as stretched to 2000% strain). Such a surprisingly high enhancement is attributed to the formation of microphase-separated low-tortuosity ion-conducting nanochannels guided by strain-induced emergence of aligned smectic mesophases, thus allowing for ultrafast ion transport that resembles the role of "swimming lanes." Intriguingly, the boosting conductivity even reverses Pouillet's Law-dictated resistance increase at certain strains, leading to unique waveform-discernible strain sensing. Moreover, the fiber retains thermal actuation properties with a maximum of 70% strain changes upon heating, and enables integrated self-perception and actuation. The findings offer a promising molecular engineering route to mechanically modulate the ion transport behavior of ionic conductors toward advanced ionotronic applications.

Solid-State Electrochemical Energy Storage Based on Soluble Melanin

Biocompatible and biodegradable powering materials are appealing systems for biomedical and electronic devices. Melanin is a natural and multifunctional material with redox capability, which is of great interest in electrochemical energy storage functionalities. In our work, we explored the use of soluble melanin derivatives as active materials for symmetric solid-state supercapacitors operating in the dark and under illumination. We observed that our devices were photo-pseudocapacitive. Additionally, under illumination, our best device showed a specific capacitance of 57.7 mFg−1 at a scan rate of 0.01 Vs−1, with a decrease of 53% in resistance compared to that in the dark. Our outcome suggests that soluble melanin is a promising material for solid-state powering elements in wearable and environmentally friendly devices.