Energy Harvesters for Wearable and Stretchable Electronics: From Flexibility to Stretchability

Harnessing Dynamic Electrostatic Fields for Energy Generation with Diode Cells

Harvesting energy from distributed mechanical motions has garnered significance in future power sources for small electronics and sensors. Although technologies like triboelectric nanogenerators have shown promising results, their efficacy hinges on the alignment of motion vectors and device architectures. Here, an approach employing stationary diode cells (DiCes) to generate electricity is presented. This approach leverages dynamically changing electrostatic fields to induce potential differences across diode junctions via electrostatic induction, which is verified theoretically and experimentally. DiCes constructed with multiple diodes can directly output DC voltage and current. A 0.02 m2 sized DiCe contains 360 diodes can supply a DC voltage and current of maximum 490 V and 1.08 mA, respectively, which equals a DC power density of 26.5 W·m-2. Capable of functioning in both contact and non-contact modes, DiCes offer versatile applications, from wirelessly powering implanted medical devices to harvesting energy from vehicles and roads.

Self-Powered Switchable Gas-Humidity Difunctional Flexible Chemosensors Based on Smart Adaptable Hydrogel

The development of self-powered, flexible, and multi-function sensors is highly anticipated in wearable electronics, however, it remains a daunting challenge to identify different signals based on a single device with singular sensing material without algorithmic support. Here, a smart adaptable hydrogel is developed by co-introducing two ions with vastly different hydrophilicity for the construction of an electrochemically self-powered, flexible, and reversibly switchable difunctional chemosensor with a metal-air battery structure. The prepared hydrogel can readily switch between water-rich and water-deficient states for crosstalk-free detection of oxygen and humidity respectively, since O2 gas and water molecules can directly participate in the oxygen reduction reaction in the device and act alone as limiting reactants and catalysts to affect the reaction rate under different hydrogel states. The resulting sensor demonstrates breakthrough O2 and humidity sensing performance with sensitivities as high as 4170.5%/% and 380.2%/% RH in water-rich and water-deficient states, respectively, and ultrawide detection ranges. Thanks to these, the devices can be applied for real-time and remote monitoring of ambient oxygen, transcutaneous oxygen pressure changes, respiration, and skin moisture by combining with wireless communication technology, and therefore have important application prospects in the fields of safety, health management, and non-contact human-machine interaction.

Molecular Chain Interpenetration-Enabled High Interfacial Compatibility of Ionic and Electronic Conductors for Stretchable Ionic Devices

Ionic devices find applications such as flexible electronics and biomedicines and function by exploiting hybrid circuits of mobile ions and electrons. However, the poor interfacial compatibility of hard electronic conductors with soft ionic conductors in ionic devices leads to low deformability, sensitivity, electromechanical responses, and stability. Herein, an interpenetrating interface between silicone-modified polyurethane/carbon nanotube electronic conductors and ionoelastomers in an ionic device using in situ polymerization is fabricated. A robust interpenetrating electronic/ionic conductor interface is realized through molecular chain entanglement and molecular forces (such as ion-dipole interactions and H-bonds), effectively enhancing the bonding strength and contact area between the components and resulting in an excellent flexibility, stability, and device performance. The electroadhesive prepared based on this strategy exhibits a superrobust shear strength of 317 kPa under a reduced voltage input of -4 V, and the diode and the transistor can undergo arbitrary deformation while maintaining the semiconductor device characteristics, including rectification and switching. In addition, electromechanical transducers exhibit sensitive electrical responses to various deformation signals. This solution to the interfacial compatibility problems of electronic and ionic conductors holds promise for the development of multifunctional ionic devices.

Enhanced Piezoelectric Performance of Highly-Aligned ZnO Nanorods Embedded in P(VDF-TrFE) Nanofiber Membranes

Flexible and wearable electronics often rely on piezoelectric materials, and Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) membranes are popular for this application. However, their electromechanical performance is limited due to a relatively low piezoelectric coefficient. To address this, this study investigates the incorporation of zinc oxide (ZnO) nanorods (NRs) into a P(VDF-TrFE) nanofiber membrane matrix. ZnO NRs were synthesized and doped into well-aligned P(VDF-TrFE) nanofibers using electrospinning with a high-speed rotating drum. The impact of ZnO NRs' mass fraction on the piezoelectric properties of the membranes was evaluated. Results show that a maximum piezoelectric coefficient (d33) of -62.4 pC/N, 9.5 times higher than neat P(VDF-TrFE), was achieved. These enhanced membranes demonstrated excellent performance in finger-tapping and bending detection, making them promising for large-scale flexible sensor applications in wearable electronics. This approach offers a simple and effective route to improve the performance of piezoelectric materials in flexible devices.

Output Characteristics of Carbon Nanotube Thermoelectric Generator with Slitted Kirigami Structure

The objective of our research is to improve the power generation of a thermoelectric generator (TEG) using a single-walled carbon nanotube (SWCNT) sheet by applying the out-of-plane deformation of a slitted kirigami structure. In order to obtain a large amount of power from a TEG using a thin-film thermoelectric (TE) element such as a SWCNT sheet, it is necessary to generate a large temperature difference in the in-plane direction of the thin-film TE element. However, it is difficult to realize a large temperature difference when the thin-film TE element is in contact with a heat source due to the need for a layer with high heat insulation. In this research, we proposed and fabricated a TEG with the out-of-plane deformation of a kirigami structure with slits using a p-n patterned SWCNT sheet as the thin-film TE material and evaluated the open circuit voltage with respect to the out-of-plane deformation and the number of TE elements. As a result, the output performance of SWCNT TEG was clarified when the out-of-plane deformation and the number of TE element pairs were varied.

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Understanding and Controlling Electrostatic Discharge in Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) have been widely used to harness various forms of mechanical energy for conversion to electrical energy. However, the contentious challenge in characterising TENGs is the lack of standard protocols for assessing mechanical-to-electrical energy conversion processes. Herein, macroscopic signal analysis is used to identify three key charging events within triboelectric signals: charge induction (CI), contact electrification (CE), and electrostatic discharge (ESD). By considering two phases of motion during contact-separation (approach and departure of the contact materials), CI arising from the motion of bound surface charge (varying electric field) between opposing contact materials is shown to dominate the measured displacement current signal, rather than the process of CE itself. Furthermore, the conventional signal (i. e., voltage, current, charge) interpretation of CE and CI during approach and departure phases is re-assessed, to indicate that the sudden spike of current often observed immediately prior to contact (or after separation) arises from polarity inverting electrostatic discharge (ESD). This aspect of the measured triboelectric effect, which is often ignored, is crucial for the design of TENGs and hence, techniques to enhance the understanding and control over the stochastic occurrence of ESDs is explored. The methods proposed for the deconvolution of the macroscopic signal components of TENGs, and mitigation of ESD occurrences, will allow for precise quantification of the associated charging events. The applications of this study will template the design and development of future super-TENGs with optimised energy conversion capabilities.

Separating Charge Centers of Chain Segments in Dielectric Elastomer through Steric Hindrance Engineering

Theoretically, separating the positive and negative charge centers of the chain segments of dielectric elastomers (DEs) is a viable alternative to the conventional decoration of chain backbone with polar handles, since it can dramatically increase the dipole vector and hence the dielectric constant (ε') of the DEs while circumvent the undesired impact of the decorated polar handles on the dielectric loss (tan δ). Herein, a novel and universal method is demonstrated to achieve effective separation of the charge centers of chain segments in homogeneous DEs by steric hindrance engineering, i.e., by incorporating a series of different included angle-containing building blocks into the networks. Both experimental and simulation results have shown that the introduction of these building blocks can create a spatially fixed included angle between two adjacent chain segments, thus separating the charge center of the associated region. Accordingly, incorporating a minimal amount of these building blocks (≈5 mol%) can lead to a considerably sharp increase (≈50%) in the ε' of the DEs while maintaining an extremely low tan δ (≈0.006@1 kHz), indicating that this methodology can substantially optimize the dielectric performance of DEs based on a completely different mechanism from the established methods.

Autonomous Triboelectric Smart Textile Sensor for Vital Sign Monitoring

Wearable smart textile sensors for monitoring vital signs are fast, noninvasive, and highly desirable for personalized health management to diagnose health anomalies such as cardiovascular diseases and respiratory dysfunction. Traditional biosignal sensors, with power consumption issues, constrain the use of wearable medical devices. This study introduces an autonomous triboelectric smart textile sensor (AUTS) made of reduced graphene oxide/manganese dioxide/polydimethylsiloxane (RGO-M-PDMS) and polytetrafluoroethylene (TEFLON)-knitted silver electrode, offering promise for vital sign monitoring with self-powering, flexibility, and wearability. The sensor exhibits impressive output performance, with a sensitivity of 7.8 nA/kPa, response time of ≈40 ms, good stability of >15,000 cycles, stretchability of up to 40%, and machine washability of >20 washes. The AUTS has been integrated to the TriBreath respiratory belt for monitoring respiratory signals and pulse strap for pulse signals concurrently at different body pulse points. These sensors wirelessly transmitted the acquired biosignals to a smartphone, demonstrating the potential of a self-powered and real-time vital sign monitoring system.

Static charge is an ionic molecular fragment

What is static charge? Despite the long history of research, the identity of static charge and mechanism by which static is generated by contact electrification are still unknown. Investigations are challenging due to the complexity of surfaces. This study involves the molecular-scale analysis of contact electrification using highly well-defined surfaces functionalized with a self-assembled monolayer of alkylsilanes. Analyses show the elementary molecular steps of contact electrification: the exact location of heterolytic cleavage of covalent bonds (i.e., Si-C bond), exact charged species generated (i.e., alkyl carbocation), and transfer of molecular fragments. The strong correlation between charge generation and molecular fragments due to their signature odd-even effects further shows that contact electrification is based on cleavage of covalent bonds and transfer of ionic molecular fragments. Static charge is thus an alkyl carbocation; in general, it is an ionic molecular fragment. This mechanism based on cleavage of covalent bonds is applicable to general types of insulating materials, such as covalently bonded polymers. The odd-even effect of charging caused by the difference of only one atom explains the highly sensitive nature of contact electrification.

Contact efficiency optimization for tribovoltaic nanogenerators

Energy harvesters based on the tribovoltaic effect that can convert mechanical energy into electricity offer a potential solution for the energy supply of decentralized sensors. However, a substantial disparity in output current, exceeding 106 times, exists between micro- and macro-contact tribovoltaic nanogenerators (TVNGs). To tackle this challenge, we develop a quantification method to determine the effective contact efficiency of conventional large-scale TVNGs, revealing a mere 0.038% for a TVNG of 1 cm2. Thus, we implement an optimization strategy by contact interface design resulting in a remarkable 65-fold increase in effective contact efficiency, reaching 2.45%. This enhancement leads to a current density of 23 A m-2 and a record-high charge density of 660 mC m-2 for the TVNG based on Cu and p-type silicon. Our study reveals that increasing the effective contact efficiency will not only address the existing disparities but also have the potential to significantly enhance the output current in future advancements of large-scale TVNGs.

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.

Visible-Light-Curable Acrylic Resins toward UV-Light-Blocking Adhesives for Foldable Displays

Current technological advances in the organic light-emitting diode panel design of foldable smartphones demand advanced adhesives with UV-blocking abilities, beyond their conventional roles of bonding objects and relieving deformation stress. However, optically clear adhesives (OCAs) with UV-blocking ability cannot be prepared using conventional UV-curing methods relying on a photoinitiator. Herein, a new acrylic resin that can be efficiently cured using visible light without oxygen removal is presented, which may be used to develop UV-blocking OCAs for use in current flexible displays. A novel photocatalyst and a specific combination of additives facilitate sufficiently rapid curing under visible light in the presence of UV-absorbers. Only a very small amount of the highly active photocatalyst is required to prepare UV-blocking OCA films with very high transparency in the visible region. Using this system, a UV-blocking OCA that nearly meets the specifications of an OCA used in commercialized foldable smartphones is realized. This technology can also be utilized in other applications that require highly efficient visible light curing, such as optically clear resins, dental resins, and 3D/4D-printable materials.

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.

Flexible thin film thermocouples: From structure, material, fabrication to application

Flexible thin-film thermocouples (TFTCs) have been garnering interest as temperature sensors due to the advantages of being flexible, ultrathin, and ultralight. Additionally, they have fast response times and enable detection of temperature. These properties have made them suitable for applications such as wearable electronics, healthcare, portable personal devices, and smart detection systems. This review presents the progress in the development of flexible TFTCs. The mechanism, structural design, materials, fabrication methods, and related applications of flexible TFTCs are also elaborated. Finally, future development directions of flexible TFTCs are discussed such as wide-range temperature measurement, multiple sensor integration, and achieving reliable cold-end compensation systems.

Near-Infrared Light-Responsive Hydrogels for Highly Flexible Bionic Photosensors

Soft biological tissues perform various functions. Sensory nerves bring sensations of light, voice, touch, pain, or temperature variation to the central nervous system. Animal senses have inspired tremendous sensors for biomedical applications. Following the same principle as photosensitive nerves, we design flexible ionic hydrogels to achieve a biologic photosensor. The photosensor allows responding to near-infrared light, which is converted into a sensory electric signal that can communicate with nerve cells. Furthermore, with adjustable thermal and/or electrical signal outputs, it provides abundant tools for biological regulation. The tunable photosensitive performances, high flexibility, and low cost endow the photosensor with widespread applications ranging from neural prosthetics to human-machine interfacing systems.

Foldable RF Energy Harvesting System Based on Vertically Layered Metal Electrodes within a Single Sheet of Paper

Radio frequency energy harvesting (RFEH) systems have emerged as a critical component for powering devices and replacing traditional batteries, with paper being one of the most promising substrates for use in flexible RFEH systems. However, previous paper-based electronics with optimized porosity, surface roughness, and hygroscopicity still face limitations in terms of the development of integrated foldable RFEH systems within a single sheet of paper. In the present study, a novel wax-printing control and water-based solution process are used to realize an integrated foldable RFEH system within a single sheet of paper. The proposed paper-based device includes vertically layered foldable metal electrodes, a via-hole, and stable conductive patterns with a sheet resistance of less than 1 Ω sq^-1 . The proposed RFEH system exhibits an RF/DC conversion efficiency of 60% and an operating voltage of 2.1 V in 100 s at a distance of 50 mm and a transmitted power of 50 mW. The integrated RFEH system also demonstrates stable foldability, with RFEH performance maintained up to a folding angle of 150°. The single-sheet paper-based RFEH system thus has the potential for use in practical applications associated with the remote powering of wearable and Internet-of-Things devices and in paper electronics.

High-Performance Stretchable Thermoelectric Generator for Self-Powered Wearable Electronics

Wearable thermoelectric generators (TEGs), which can convert human body heat to electricity, provide a promising solution for self-powered wearable electronics. However, their power densities still need to be improved aiming at broad practical applications. Here, a stretchable TEG that achieves comfortable wearability and outstanding output performance simultaneously is reported. When worn on the forehead at an ambient temperature of 15 °C, the stretchable TEG exhibits excellent power densities with a maximum value of 13.8 µW cm^-2 under the breezeless condition, and even as high as 71.8 µW cm^-2 at an air speed of 2 m s^-1 , being one of the highest values for wearable TEGs. Furthermore, this study demonstrates that this stretchable TEG can effectively power a commercial light-emitting diode and stably drive an electrocardiogram module in real-time without the assistance of any additional power supply. These results highlight the great potential of these stretchable TEGs for power generation applications.

Piezoelectric Enhancement of Piezoceramic Nanoparticle-Doped PVDF/PCL Core-Sheath Fibers

Electrospinning is considered to be an efficient method to prepare piezoelectric thin films because of its ability to transform the phase of the polymers. A core-sheath structure can endow fibers with more functions and properties. In this study, fibers with a core-sheath structure were prepared using polyvinylidene fluoride (PVDF) included with nanoparticles (NPs) as the shell layer and polycaprolactone (PCL) as the core layer. Their mechanical and piezoelectric properties were studied in detail. During the course of the electrospinning process, PVDF was demonstrated to increase the amount of its polar phase, with the help of nanoparticles acting as a nucleating agent to facilitate the change. PCL was chosen as a core material because of its good mechanical properties and its compatibility with PVDF. Transmission electron microscope (TEM) assessments revealed that the fibers have a core-sheath structure, and shell layers were loaded with nanoparticles. Mechanical testing showed that the core layer can significantly improve mechanical properties. The XRD patterns of the core-sheath structure fibers indicated the β phase domain the main component. Piezoelectric testing showed that the doped nanoparticles were able to enhance piezoelectric performances. The increases of mechanical and piezoelectric properties of core-sheath structure fibers provide a feasible application for wearable electronics, which require flexibility and good mechanical properties.

Highly Flexible Triboelectric Nanogenerator Using Porous Carbon Nanotube Composites

The rapid development of portable and wearable electronic devices has led researchers to actively study triboelectric nanogenerators (TENGs) that can provide self-powering capabilities. In this study, we propose a highly flexible and stretchable sponge-type TENG, named flexible conductive sponge triboelectric nanogenerator (FCS-TENG), which consists of a porous structure manufactured by inserting carbon nanotubes (CNTs) into silicon rubber using sugar particles. Nanocomposite fabrication processes, such as template-directed CVD and ice freeze casting methods for fabricating porous structures, are very complex and costly. However, the nanocomposite manufacturing process of flexible conductive sponge triboelectric nanogenerators is simple and inexpensive. In the tribo-negative CNT/silicone rubber nanocomposite, the CNTs act as electrodes, increasing the contact area between the two triboelectric materials, increasing the charge density, and improving charge transfer between the two phases. Measurements of the performance of flexible conductive sponge triboelectric nanogenerators using an oscilloscope and a linear motor, under a driving force of 2-7 N, show that it generates an output voltage of up to 1120 V and a current of 25.6 µA. In addition, by using different weight percentages of carbon nanotubes (CNTs), it is shown that the output power increases with the weight percentage of carbon nanotubes (CNTs). The flexible conductive sponge triboelectric nanogenerator not only exhibits good performance and mechanical robustness but can also be directly used in light-emitting diodes connected in series. Furthermore, its output remains extremely stable even after 1000 bending cycles in an ambient environment. In sum, the results demonstrate that flexible conductive sponge triboelectric nanogenerators can effectively power small electronics and contribute to large-scale energy harvesting.

An Overview of Flexible Sensors: Development, Application, and Challenges

The emergence and advancement of flexible electronics have great potential to lead development trends in many fields, such as "smart electronic skin" and wearable electronics. By acting as intermediates to detect a variety of external stimuli or physiological parameters, flexible sensors are regarded as a core component of flexible electronic systems and have been extensively studied. Unlike conventional rigid sensors requiring costly instruments and complicated fabrication processes, flexible sensors can be manufactured by simple procedures with excellent production efficiency, reliable output performance, and superior adaptability to the irregular surface of the surroundings where they are applied. Here, recent studies on flexible sensors for sensing humidity and strain/pressure are outlined, emphasizing their sensory materials, working mechanisms, structures, fabrication methods, and particular applications. Furthermore, a conclusion, including future perspectives and a short overview of the market share in this field, is given for further advancing this field of research.

Study of a Mixed Conductive Layer Fabricated by Ion Implantation and Distribution Theory

Electrodes are essential parts of capacitors that can consist of a variety of materials depending on the application. In dielectric elastomer transducers (DETs)-a type of special variable capacitor-the electrode needs to deform with a soft base. However, the current carbon-based electrodes are not stable, and the metal-based ones are not flexible for use in DETs. Thus, the need to fabricate an electrode which can meet both the stability and flexibility requirements is extremely important. In this work, silver ions with energy levels of 40 keV were implanted into the surface of polydimethylsiloxane (PDMS) to explore the effect of ion implantation on surface conductivity. The experimental results showed that the surface resistivity of PDMS reached 251.85 kΩ per square and dropped by 10 orders of magnitude after ion implantation. This indicates that the surface conductivity was significantly improved. EDS characterization results showed that the maximum penetration depth that ions could reach was about 2.5 μm. The surface resistivity of the sample coated with carbon black was further reduced by an order of magnitude after ion implantation and changed more stably with time. A quasi-melting-collision model was established to investigate the distribution of carbon black particles. The concentration of carbon black particles at a distance from the PDMS surface followed a Gaussian-like distribution.

Intelligent wearable devices based on nanomaterials and nanostructures for healthcare

Emerging classes of flexible electronic sensors as alternatives to conventional rigid sensors offer a powerful set of capabilities for detecting and quantifying physiological and physical signals from human skin in personal healthcare. Unfortunately, the practical applications and commercialization of flexible sensors are generally limited by certain unsatisfactory aspects of their performance, such as biocompatibility, low sensing range, power supply, or single sensory function. This review intends to provide up-to-date literature on wearable devices for smart healthcare. A systematic review is provided, from sensors based on nanomaterials and nanostructures, algorithms, to multifunctional integrated devices with stretchability, self-powered performance, and biocompatibility. Typical electromechanical sensors are investigated with a specific focus on the strategies for constructing high-performance sensors based on nanomaterials and nanostructures. Then, the review emphasizes the importance of tailoring the fabrication techniques in order to improve stretchability, biocompatibility, and self-powered performance. The construction of wearable devices with high integration, high performance, and multi-functionalization for multiparameter healthcare is discussed in depth. Integrating wearable devices with appropriate machine learning algorithms is summarized. After interpretation of the algorithms, intelligent predictions are produced to give instructions or predictions for smart implementations. It is desired that this review will offer guidance for future excellence in flexible wearable sensing technologies and provide insight into commercial wearable sensors.

Assembling nanocelluloses into fibrous materials and their emerging applications

Nanocelluloses, derived from various plants or specific bacteria, represent the renewable and sophisticated nano building blocks for emerging functional materials. Especially, the assembly of nanocelluloses as fibrous materials can mimic the structural organization of their natural counterparts to integrate various functions, thus holding great promise for potential applications in various fields, such as electrical device, fire retardance, sensing, medical antibiosis, and drug release. Due to the advantages of nanocelluloses, a variety of fibrous materials have been fabricated with the assistance of advanced techniques, and their applications have attracted great interest in the past decade. This review begins with an overview of nanocellulose properties followed by the historical development of assembling processes. There will be a focus on assembling techniques, including traditional methods (wet spinning, dry spinning, and electrostatic spinning) and advanced methods (self-assembly, microfluidic, and 3D printing). In particular, the design rules and various influencing factors of assembling processes related to the structure and function of fibrous materials are introduced and discussed in detail. Then, the emerging applications of these nanocellulose-based fibrous materials are highlighted. Finally, some perspectives, key opportunities, and critical challenges on future research trends within this field are proposed.

Stretchable One-Dimensional Conductors for Wearable Applications

Continuous, one-dimensional (1D) stretchable conductors have attracted significant attention for the development of wearables and soft-matter electronics. Through the use of advanced spinning, printing, and textile technologies, 1D stretchable conductors in the forms of fibers, wires, and yarns can be designed and engineered to meet the demanding requirements for different wearable applications. Several crucial parameters, such as microarchitecture, conductivity, stretchability, and scalability, play essential roles in designing and developing wearable devices and intelligent textiles. Methodologies and fabrication processes have successfully realized 1D conductors that are highly conductive, strong, lightweight, stretchable, and conformable and can be readily integrated with common fabrics and soft matter. This review summarizes the latest advances in continuous, 1D stretchable conductors and emphasizes recent developments in materials, methodologies, fabrication processes, and strategies geared toward applications in electrical interconnects, mechanical sensors, actuators, and heaters. This review classifies 1D conductors into three categories on the basis of their electrical responses: (1) rigid 1D conductors, (2) piezoresistive 1D conductors, and (3) resistance-stable 1D conductors. This review also evaluates the present challenges in these areas and presents perspectives for improving the performance of stretchable 1D conductors for wearable textile and flexible electronic applications.

Flexoelectric enhanced film for an ultrahigh tunable piezoelectric-like effect

Recent advancements in electromechanical coupling effects enable electromechanical materials in soft and stretchable formats, offering unique opportunities for biomimetic applications. However, high electromechanical performance and mechanical elasticity hardly coexist in soft materials. Flexoelectricity, an electromechanical coupling between strain gradient and electric polarization, possesses great potential of strain gradient engineering and material design in soft elastomeric materials. In this work, we report a flexoelectric enhanced elastomer-based film (FEEF) with both high electromechanical capability and stretchability. The integrated strategies with biaxial pre-stretch, crosslinking density of the elastomer along with nanoparticle size, particle filling ratio and electric field charging lead to an enhanced flexoelectricity by two orders of magnitude. Furthermore, this FEEF reveals an ultrahigh electromechanical performance by flexoelectric enhancement with its mechanical design. As a representative demonstration, an ultrahigh piezoelectric-like sensing array is fabricated for multifunctional sensing applications in strain, force and vibration, verifying an equivalent piezoelectric coefficient d₃₃ value as high as 1.42 × 10⁴ pC N^-1, and an average d₃₃ value of 4.23 × 10³ pC N^-1 at a large-scale deformation range. This proposed ultra-high piezoelectric-like effect with its approach is anticipated to provide a possibility for highly tunable piezoelectric-like effect by enhanced flexoelectricity and mechanical design in elastomeric materials.

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.

Manipulating Single-Walled Carbon Nanotube Arrays for Flexible Photothermoelectric Devices

Flexible photothermoelectric (PTE) devices possess great application prospects in the field of light energy and thermoelectric energy harvesting which are some of the cornerstones of modern green renewable energy power generation. However, the low efficiency of PTE materials and lack of suitable manufacturing processes remain an impediment to restrict its rapid development. Here, we designed a flexible PTE device by printing a highly integrated single-walled carbon nanotubes (SWCNTs) array at intervals that were surface-functionalized with poly(acrylic acid) and poly(ethylene imine) as p-n heterofilms. After the introduction of a mask to give a selective light illumination and taking advantage of the photothermal effect of SWCNTs, a remarkable temperature gradient along the printed SWCNTs and a considerable power density of 1.3 μW/cm² can be achieved. Meanwhile, both experimental data and COMSOL theoretical simulations were adopted to optimize the performance of our device, showing new opportunities for new generation flexible PTE devices.

Enhancement and Function of the Piezoelectric Effect in Polymer Nanofibers

The realization of intelligent, self-powered components and devices exploiting the piezoelectric effect at large scale might greatly contribute to improve our efficiency in using resources, albeit a profound redesign of the materials and architectures used in current electronic systems would be necessary. Piezoelectricity is a property of certain materials to generate an electrical bias in response to a mechanical deformation. This effect enables energy to be harvested from strain and vibration modes, and to sustain the power of actuators, transducers, and sensors in integrated networks, such as those necessary for the Internet of Thing. Polymers, combining structural flexibility with lightweight construction and ease of processing, have been largely used in this framework. In particular, the poly(vinylidene fluoride) [PVDF, (CH₂CF₂) _n ] and its copolymers exhibit strong piezoelectric response, are biocompatibile, can endure large strains and can be easily shaped in the form of nanomaterials. Confined geometries, improving crystal orientation and enhancing piezoelectricity enable the fabrication of piezoelectric nanogenerators, which satisfy many important technological requirements, such as conformability, cheap fabrication, self-powering, and operation with low-frequency mechanical inputs (Hz scale). This account reports on piezoelectric polymer nanofibers made by electrospinning. This technique enables the formation of high-aspect-ratio filaments, such as nanowires and nanofibers, through the application of high electric fields (i.e., on the order of hundreds of kV/m) and stretching forces to a polymeric solution. The solution might be charged with functional, organic or inorganic, fillers or dopants. The solution is then fed at a controlled flow rate through a metallic spinneret or forms a bath volume, from which nanofibers are delivered. Fibers are then collected onto metallic surfaces, and upon a change of the collecting geometry, they can form nonwovens, controlled arrays, or isolated features. Nanofibers show unique features, which include their versatility in terms of achievable chemical composition and chemico-physical properties. In addition, electrospinning can be up-scaled for industrial production. Insight into the energy generation mechanism and how the interaction among fibers can be used to enhance the piezoelectric performance are given in this paper, followed by an overview of fiber networks as the active layer in different device geometries for sensing, monitoring, and signal recognition. The use of biodegradable polymers, both natural and synthetic, as critically important building blocks of the roadmap for next-generation piezoelectric devices, is also discussed, with some representative examples. In particular, biodegradable materials have been utilized for applications related to life science, such as the realization of active scaffolds and of electronic devices to be placed in intimate contact with living tissues and organs. Overall, these materials show many relevant properties that can be of very high importance for building next-generation, sustainable energy harvesting, self-rechargeable devices and electronic components, for use in several different fields.

Silicone Rubber Based-Conductive Composites for Stretchable "All-in-One" Microsystems

Wearable electronics with development trends such as miniaturization, multifunction, and smart integration have become an important part of the Internet of Things (IoT) and have penetrated various sectors of modern society. To meet the increasing demands of wearable electronics in terms of deformability and conformability, many efforts have been devoted to overcoming the nonstretchable and poor conformal properties of traditional functional materials and endowing devices with outstanding mechanical properties. One of the promising approaches is composite engineering in which traditional functional materials are incorporated into the various polymer matrices to develop different kinds of functional composites and construct different functions of stretchable electronics. Herein, we focus on the approach of composite engineering and the polymer matrix of silicone rubber (SR), and we summarize the state-of-the-art details of silicone rubber-based conductive composites (SRCCs), including a summary of their conductivity mechanisms and synthesis methods and SRCC applications for stretchable electronics. For conductivity mechanisms, two conductivity mechanisms of SRCC are emphasized: percolation theory and the quantum tunneling mechanism. For synthesis methods of SRCCs, four typical approaches to synthesize different kinds of SRCCs are investigated: mixing/blending, infiltration, ion implantation, and in situ formation. For SRCC applications, different functions of stretchable electronics based on SRCCs for interconnecting, sensing, powering, actuating, and transmitting are summarized, including stretchable interconnects, sensors, nanogenerators, antennas, and transistors. These functions reveal the feasibility of constructing a stretchable all-in-one self-powered microsystem based on SRCC-based stretchable electronics. As a prospect, this microsystem is expected to integrate the functional sensing modulus, the energy harvesting modulus, and the process and response modulus together to sense and respond to environmental stimulations and human physiological signals.

Emerging Development of Auto-Charging Sensors for Respiration Monitoring

In recent years, the development of biomedical monitoring systems, including respiration monitoring systems, has been accelerated. Wearable and implantable medical devices are becoming increasingly important in the diagnosis and management of disease and illness. Respiration can be monitored using a variety of biosensors and systems. Auto-charged sensors have a number of advantages, including low cost, ease of preparation, design flexibility, and a wide range of applications. It is possible to use the auto-charged sensors to directly convert mechanical energy from the airflow into electricity. The ability to monitor and diagnose one's own health is a major goal of auto-charged sensors and systems. Respiratory disease model output signals have not been thoroughly investigated and clearly understood. As a result, figuring out their exact interrelationship is a difficult and important research question. This review summarized recent developments in auto-charged respiratory sensors and systems in terms of their device principle, output property, detecting index, and so on. Researchers with an interest in auto-charged sensors can use the information presented here to better understand the difficulties and opportunities that lie ahead.

Eu3+-Doped Electrospun Polyvinylidene Fluoride-Hexafluoropropylene/Graphene Oxide Multilayer Composite Nanofiber for the Fabrication of Flexible Pressure Sensors

The development of flexible materials with higher piezoelectric properties and electrostrictive response is of great significance in many applications such as wearable functional devices, flexible sensors, and actuators. In this study, we report an efficient fabrication strategy to construct a highly sensitive (0.72 kPa^-1), red light-emitting flexible pressure sensor using electrospun Eu³⁺-doped polyvinylidene fluoride-hexafluoropropylene/graphene oxide composite nanofibers using a layer-by-layer technology. The high β-phase concentration (96.3%) was achieved from the Eu³⁺-doped P(VDF-HFP)/GO nanofibers, leading to a high piezoelectricity of the composite nanofibers. We observed that a pressure sensor is enabled to generate an output voltage of 4.5 V. Furthermore, Eu³⁺-doped P(VDF-HFP)/GO composite nanofiber-based pressure sensors can also be used as an actuator as it has a good electrostrictive effect. At the same time, the nanofiber membrane has excellent ferroelectric properties and good fluorescence properties. These results indicate that this material has great application potential in the fields of photoluminescent fabrics, flexible sensors, soft actuators, and energy storage devices.

Serpentine-inspired Strain Sensor with Predictable Cracks for Remote Bio-Mechanical Signal Monitoring

The flexible strain sensors have attracted intense interests due to their application as intelligent wearable electronic devices. However, it is still a huge challenge to achieve the flexible sensor with simultaneous high sensitivity, excellent durability and wide sensing region. In this work, a crack-based strain sensor with paired-serpentine conductive network is fabricated onto flexible film by screen printing. The innovative conductive network exhibits a controlled crack morphology during stretching, which endows the prepared sensor with outstanding sensing characteristics, including the high sensitivity (gauge factor up to 2391.5), wide detection (rang up to 132%), low strain detection limit, fast response time (about 40 ms), as well as excellent durability (more than 2000 stretching/releasing cycles). Benefiting from these excellent performances, full-range human body motions including subtle physiological signals and large motions are accurately detected by the prepared sensor. Besides, wearable electronic equipment integrated with wireless transmitter and the prepared strain sensor shows great potential for remote motion monitoring and intelligent mobile diagnosis for humans. This work provides an effective strategy for the fabrication of the novel strain sensors with highly comprehensive performance. This article is protected by copyright. All rights reserved.

Recent Advances in Materials for Wearable Thermoelectric Generators and Biosensing Devices

Recently, self-powered health monitoring systems using a wearable thermoelectric generator (WTEG) have been rapidly developed since no battery is needed for continuous signal monitoring, and there is no need to worry about battery leakage. However, the existing materials and devices have limitations in rigid form factors and small-scale manufacturing. Moreover, the conventional bulky WTEG is not compatible with soft and deformable tissues, including human skins or internal organs. These limitations restrict the WTEG from stabilizing the thermoelectric gradient that is necessary to harvest the maximum body heat and generate valuable electrical energy. This paper summarizes recent advances in soft, flexible materials and device designs to overcome the existing challenges. Specifically, we discuss various organic and inorganic thermoelectric materials with their properties for manufacturing flexible devices. In addition, this review discusses energy budgets required for effective integration of WTEGs with wearable biomedical systems, which is the main contribution of this article compared to previous articles. Lastly, the key challenges of the existing WTEGs are discussed, followed by describing future perspectives for self-powered health monitoring systems.

Stretchable sodium-ion capacitors based on coaxial CNT supported Na2Ti3O7 with high capacitance contribution

Stretchable sodium-ion capacitors (SSICs) are promising energy storage devices for wearable electronic devices, and the development bottleneck is the realization of stretchable battery-type electrodes with desirable electrochemical properties during dynamic deformation. Herein, we find that electrostatic modification of acidified carbon nanotubes with polyamines can introduce active sites and modulate the surface pH microenvironment, thereby developing a route to realize the in situ coaxial nanometerization of sodium titanate (nCNT@NTO). The nCNT@NTO anode material has a fast Na⁺ transport and the high capacitive contribution, which can deliver a high specific capacity (206.5 mA h g^-1 at 0.1 A g^-1) and high rate performance (maintain 51% capacity at 10 A g^-1), and the ideal cycle stability (∼93% capacity retention after 1000 cycles at 5 A g^-1). In addition, acrylate-rubber with high stickiness and stretchability are served as the elastic matrix both of the stretchable electrodes and quasi-solid-state electrolytes, which endows strong adhesion between electrodes and electrolytes. Thus, the accordingly assembled SSIC delivers high energy density of 8.8 mW h cm^-3 (at a power density of 0.024 W cm^-3), and excellent deformation stability (89% capacitance retention after 500 stretching cycles under 100% strain).

Highly heterogeneous epitaxy of flexoelectric BaTiO3-δ membrane on Ge

The integration of complex oxides with a wide spectrum of functionalities on Si, Ge and flexible substrates is highly demanded for functional devices in information technology. We demonstrate the remote epitaxy of BaTiO₃ (BTO) on Ge using a graphene intermediate layer, which forms a prototype of highly heterogeneous epitaxial systems. The Ge surface orientation dictates the outcome of remote epitaxy. Single crystalline epitaxial BTO_3-δ films were grown on graphene/Ge (011), whereas graphene/Ge (001) led to textured films. The graphene plays an important role in surface passivation. The remote epitaxial deposition of BTO_3-δ follows the Volmer-Weber growth mode, with the strain being partially relaxed at the very beginning of the growth. Such BTO_3-δ films can be easily exfoliated and transferred to arbitrary substrates like Si and flexible polyimide. The transferred BTO_3-δ films possess enhanced flexoelectric properties with a gauge factor of as high as 1127. These results not only expand the understanding of heteroepitaxy, but also open a pathway for the applications of devices based on complex oxides.

The integration of epitaxial complex oxides on semiconductor and flexible substrates is required but challenging. Here, the authors report the highly heterogeneous epitaxy of transferrable BaTiO_3-δ membrane with enhanced flexoelectricity on Ge (011).

High-Power-Density Wearable Thermoelectric Generators for Human Body Heat Harvesting

Wearable thermoelectrics has attracted significant interest in recent years. Among them, rigid-structure thermoelectric generators (TEGs) were seldomly employed for wearable applications, although those exhibit significant advantages of high device output performance and impact resistance. Here, we report a type of rigid wearable TEGs (w-TEGs) without ceramic substrates made using a simple cutting-and-bonding method. Owing to the small contact area, the w-TEGs comprising 48-n/p-pairs can be well attached to the human body. The lack of ceramic substrates leaves more space in the height direction, which benefits the wearability in practical applications and high power density. We demonstrated that increasing the height of w-TEGs from 1.38 to 3.14 mm significantly improves the power density by a factor of 10. As a result, the maximum power densities of 7.9 μW cm^-2 and 43.6 μW cm^-2 for the w-TEGs were realized under the breezeless condition and a wind speed for normal walking, respectively. This work provides a feasible design solution for rigid-structure free-substrate w-TEGs with very high power density, which will speed up the research of wearable thermoelectrics.

Ionic Gelatin-Based Flexible Thermoelectric Generator with Scalability for Human Body Heat Harvesting

The prosperity of intelligent wearables brings an increasingly critical problem of power supply. Regular rechargeable lithium or disposable button batteries have some problems, such as limited capacity, frequent replacement, environmental pollution, etc. Wearable energy harvester (WEH) can fundamentally solve these problems. Among WEHs, thermoelectric generator (TEG) is a promising option due to its independence of light condition or the motion of the wearer, and thermoelectric conversion (TEC) has the characteristics of quietness and continuity. Therefore, TEG has become a suitable choice for harvesting low-grade heat energy such as human body heat. Ionic thermoelectric gel (iTEG) has the advantages of a large Seebeck coefficient, freely defined shape and size, low processing cost, wide material sources, easy encapsulation, etc. In this paper, the gelatin-based iTEG is regulated and optimized by silica nanoparticles (SiO2 NPs). The optimal compound quantity of SiO2 NPs is determined, and the optimization mechanism is discussed through a series of characterization tests. Based on the iTEG, a kind of scalable flexible TEGs is proposed, and its preparation method is described in detail. A small wristband TEG (STEG) was made, and its Seebeck coefficient is 74.5 mV/K. Its bendability and stretchability were verified, and the impedance matching experiment was carried out. By charging a capacitor, the STEG successfully lights up an LED at a temperature difference (ΔT) of ~15.5 K. Subsequently, a large extended oversleeve TEG (LTEG) was prepared, and a set of heat sinks was added at the cooling end of the LTEG. Being worn on a volunteer’s forearm, the LTEG output a voltage of more than 3 V at ~20 °C. Through storing the converted energy in a capacitor, the LTEG directly drove a calculator without a DC–DC booster. The proposed iTEG and TEGs in this paper have the prospect of mass production, extending to people’s clothes, harvesting human body heat and directly powering wearable electronics.

Mechanical Behaviors of the Origami-Inspired Horseshoe-Shaped Solar Arrays

The importance of flexibility has been widely noticed and concerned in the design and application of space solar arrays. Inspired by origami structures, we introduce an approach to realizing stretchable and bendable solar arrays via horseshoe-shaped substrate design. The structure has the ability to combine rigid solar cells and soft substrates skillfully, which can prevent damage during deformations. The finite deformation theory is adapted to find the analytic model of the horseshoe-shaped structure via simplified beam theory. In order to solve the mechanical model, the shooting method, a numerical method to solve ordinary differential equation (ODE) is employed. Finite element analyses (FEA) are also performed to verify the developed theoretical model. The influences of the geometric parameters on deformations and forces are analyzed to achieve the optimal design of the structures. The stretching tests of horseshoe-shaped samples manufactured by three-dimensional (3D) printing are implemented, whose results shows a good agreement with those from theoretical predictions. The developed models can serve as the guidelines for the design of flexible solar arrays in spacecraft.

Applications of nanotechnology in smart textile industry: A critical review

Background

In recent years, nanotechnology has been playing an important role in designing smart fabrics. Nanomaterials have been employed to introduce in a sustainable manner, antimicrobial, ultraviolet resistant, electrically conductive, optical, hydrophobic and flame-retardant properties into textiles and garments. Nanomaterial based smart devices are now also being integrated with the textiles so as to perform various functions such as energy harvesting and storage, sensing, drug release and optics. These advancements have found wide applications in the fashion industry and are being developed for wider use in defence, healthcare and on-body energy harnessing applications.

Aim of review

The objective of this work is to provide an insight into the current trends of using nanotechnology in the modern textile industries and to inspire and anticipate further research in this field. This review provides an overview of the most current advances concerning on-body electronics research and the wonders which could be realized by nanomaterials in modern textiles in terms of total energy reliance on our clothes.

Key scientific concepts of review

The work underlines the various methods and techniques for the functionalization of nanomaterials and their integration into textiles with an emphasis on cost-effectiveness, comfort, wearability, energy conversion efficiency and eco-sustainability. The most recent trends of developing various nanogenerators, supercapacitors and photoelectronic devices on the fabric are highlighted, with special emphasis on the efficiency and wearability of the textile. The potential nanotoxicity associated with the processed textiles due to the tendency of these nanomaterials to leach into the environment along with possible remediation measures are also discussed. Finally, the future outlook regarding progress in the integration of smart nano-devices on textile fabrics is provided.

Compression Induced Deformation Twinning Evolution in Liquid-Like Cu2Se

For practical applications of copper selenide (Cu₂Se) thermoelectric (TE) materials with liquid-like behavior, it is essential to determine the structure-property relations as a function of temperature. Here, we investigate β-Cu₂Se structure evolution during uniaxial compression over the temperature range of 400-1000 K using molecular dynamics simulations. We find that at temperatures above 800 K, Cu₂Se exhibits poor stability with breaking order that is described as a liquid-like or hybrid structure comprising a rigid Se sublattice and mobile Cu ions. A uniaxial load causes accumulated structural heterogeneity that is alleviated by diffusion-induced accommodation of local deformations. With increasing strain, the deformation mode changes into a combination of compression and shear, accompanied by restructuring in terms of twinning. Interestingly, in addition to a plastic behavior rarely found in inorganic semiconductors, we find that higher temperature promotes deformation twinning in liquid-like Cu₂Se, showing the role of thermal instability, including Cu diffusion, in structural adaptation and mechanical modulation. These findings reveal the micromechanism of hybrid structural evolution as well as performance tuning through twinning, which provides a theoretical guide toward advanced Cu₂Se TE materials design.

Interface Engineering for Efficient Raindrop Solar Cell

A raindrop solar cell can work either on rainy days to collect mechanical energy of the raindrops or on sunny days to harvest solar energy, which achieves high energy conversion efficiency in various energy environments. However, the low efficiency of raindrop energy harvesting is a dominating barrier to the raindrop solar cells in practical applications. In this work, a MoO₃/top electrode-based triboelectric nanogenerator (MT-TENG) with high rain droplet energy conversion efficiency, integrated with a perovskite solar cell through shared electrodes, has been proposed. The interface electrons between the triboelectric layer and electrode can be blocked by the MoO₃ layer with high permittivity and wide bandgap, and the MoO₃-based TENG (M-TENG) therefore increases the surface charge density. Thus, the top electrode structure in the solid-liquid interface can greatly increase the output charge by 101.1 times in total. By adjusting the water droplet parameters of tap water to simulate the actual application scenario, the raindrop output power and mechanical energy conversion efficiency can reach 0.68 mW and 12.49%, respectively. In addition, due to the high transmittance of the MT-TENG, the perovskite solar cell can still sustain a high photovoltaic power conversion efficiency of 19.38%. By virtue of the shared electrode circuit design, the raindrop solar cell can continue to purvey electric power on rainy and sunny days, and it only takes about 175 s to charge a 2.2 μF capacitor to 5 V.

Objective evaluation of wearable thermoelectric generator: From platform building to performance verification

Wearable thermoelectric generators can harvest heat from the human body to power an intelligent electronic device, which plays an important role in wearable electronics. However, due to the complexity of human skin, there is still no unified standard for performance testing of wearable thermoelectric generators under wearable conditions. Herein, a test platform suitable for a wearable thermoelectric generator was designed and built by simulating the structure of the arm. Based on the biological body temperature regulation function, water flow and water temperature substitute blood flow and blood temperature, the silicone gel with some thickness simulates the skin layer of the human arm, thus achieving the goal of adjusting the thermal resistance of human skin. Meanwhile, the weight is used as the contact pressure to further ensure the reliability and accuracy of the test data. In addition, the environment regulatory system is set up to simulate the outdoor day. Actually, the maximum deviation of the performance of the thermoelectric generator worn on the test platform and human arm is ∼5.2%, indicating the accuracy of objective evaluation.

Printable Self-Activated Liquid Metal Stretchable Conductors from Polyvinylpyrrolidone-Functionalized Eutectic Gallium Indium Composites

Stretchable electronic circuits are critical in a variety of next-generation electronics applications, including soft robots, wearable technologies, and biomedical applications. To date, printable composite conductors comprising various types of conductive fillers have been suggested to achieve high electrical conductance and excellent stretchability. Among them, liquid metal particles have been considered as a viable candidate filler that can meet the necessary prerequisites. However, a mechanical activation process is needed to generate interconnected liquid channels inside elastomeric polymers. In this study, we have developed a chemical strategy of surface-functionalizing liquid metal particles to eliminate the necessity of additional mechanical activation processes. We found that the characteristic conformations of the polyvinylpyrrolidone surrounding eutectic gallium indium particles are highly dependent on the molecular weight of polyvinylpyrrolidone. By virtue of the specific chemical roles of polyvinylpyrrolidone, the as-printed composite layers are highly conductive and stretchable, exhibiting an electrical conductivity approaching 8372 S/cm at 100% strain and an invariant resistance change of 0.92 even at 75% strain after a 60,000 cycle test. The results demonstrate that the self-activated liquid metal-based composite conductors are applicable to traditional stretchable electronics, healable stretchable electronics, and shape-morphable applications.