Patterned Nanoparticle Arrays Fabricated Using Liquid Film Rupture Self-Assembly.

Self-assembly is an important bottom-up fabrication approach based on accurate manipulation of solid-air-liquid interfaces to construct microscale structures using nanoscale materials. This approach plays a substantial role in the fabrication of microsensors, nanosensors, and actuators. Improving the controllability of self-assembly to realize large-scale regular micro/nano patterns is crucial for this approach's further development and wider applications. Herein, we propose a novel strategy for patterning nanoparticle arrays on soft substrates. This strategy is based on a unique process of liquid film rupture self-assembly that is convenient, precise, and cost-efficient for mass manufacturing. This approach involves two key steps. First, suspended liquid films comprising monolayer polystyrene (PS) spheres are realized via liquid-air interface self-assembly over prepatterned microstructures. Second, these suspended liquid films are ruptured in a controlled manner to induce the self-assembly of internal PS spheres around the morphological edges of the underlying microstructures. This nanoparticle array patterning method is comprehensively investigated in terms of the effect of the PS sphere size, morphological effect of the microstructured substrate, key factors influencing liquid film-rupture self-assembly, and optical transmittance of the fabricated samples. A maximum rupture rate of 95.4% was achieved with an optimized geometric and dimensional design. Compared with other nanoparticle-based self-assembly methods used to form patterned arrays, the proposed approach reduces the waste of nanoparticles substantially because all nanoparticles self-assemble around the prepatterned microstructures. More nanoparticles assemble to form prepatterned arrays, which could strengthen the nanoparticle array network without affecting the initial features of prepatterned microstructures.

High-performance hybrid nanogenerator for self-powered wireless multi-sensing microsystems.

Wireless sensor network nodes are widely used in wearable devices, consumer electronics, and industrial electronics and are a crucial component of the Internet of Things (IoT). Recently, advanced power technology with sustainable energy supply and pollution-free characteristics has become a popular research focus. Herein, to realize an unattended and reliable power supply unit suitable for distributed IoT systems, we develop a high-performance triboelectric-electromagnetic hybrid nanogenerator (TEHNG) to harvest mechanical energy. The TEHNG achieves a high load power of 21.8 mW by implementing improvements of material optimization, configuration optimization and pyramid microstructure design. To realize a self-powered integrated microsystem, a power management module, energy storage module, sensing signal processing module, and microcontroller unit are integrated into the TEHNG. Furthermore, an all-in-one wireless multisensing microsystem comprising the TEHNG, the abovementioned integrated functional circuit and three sensors (temperature, pressure, and ultraviolet) is built. The milliwatt microsystem operates continuously with the TEHNG as the only power supply, achieving self-powered operations of sensing environmental variables and transmitting wireless data to a terminal in real time. This shows tremendous application potential in the IoT field.

Integrated hybrid sensing and microenergy for compact active microsystems.

Wearable electronics, as essential components of the Internet of Things (IoT), have attracted widespread attention, and the trend is to configure attractive wearable smart microsystems by integrating sensing, powering, and other functions. Herein, we developed an elastic hybrid triboelectric-electromagnetic microenergy harvester (named EHTE) to realize hybrid sensing and microenergy simultaneously. This EHTE is a highly integrated triboelectric nanogenerator (TENG) and electromagnetic nanogenerator (EMG). Based on the triboelectric-electromagnetic hybrid mechanism, an enhanced electrical output of the EHTE was achieved successfully, which demonstrates the feasibility of the EHTE for microelectronics powering. Moreover, with the merits of the EMG, the developed hybrid microenergy harvester integrated both active frequency sensing and passive inductive sensing capabilities. Specifically, the almost linear correlation of the electromagnetic outputs to the frequencies of the external stimulus endowed the proposed EHTE with an outstanding active frequency sensing ability. In addition, due to the unique structural configuration of the EMG (i.e., a conductive permanent magnet (PM), hybrid deformation layer, and flexible printed circuit board (FPCB) coil), an opportunity was provided for the developed EHTE to serve as a passive inductive sensor based on the eddy current effect (i.e., a form of electromagnetic induction). Therefore, the developed EHTE successfully achieved the integration of hybrid sensing (i.e., active frequency sensing and passive inductive sensing) and microenergy (i.e., the combination of electromagnetic effect and triboelectric effect) within a single device, which demonstrates the potential of this newly developed EHTE for wearable electronic applications, especially in applications of compact active microsystems.

Wearable Multi-Functional Sensing Technology for Healthcare Smart Detection.

In recent years, considerable research efforts have been devoted to the development of wearable multi-functional sensing technology to fulfill the requirements of healthcare smart detection, and much progress has been achieved. Due to the appealing characteristics of flexibility, stretchability and long-term stability, the sensors have been used in a wide range of applications, such as respiration monitoring, pulse wave detection, gait pattern analysis, etc. Wearable sensors based on single mechanisms are usually capable of sensing only one physiological or motion signal. In order to measure, record and analyze comprehensive physical conditions, it is indispensable to explore the wearable sensors based on hybrid mechanisms and realize the integration of multiple smart functions. Herein, we have summarized various working mechanisms (resistive, capacitive, triboelectric, piezoelectric, thermo-electric, pyroelectric) and hybrid mechanisms that are incorporated into wearable sensors. More importantly, to make wearable sensors work persistently, it is meaningful to combine flexible power units and wearable sensors and form a self-powered system. This article also emphasizes the utility of self-powered wearable sensors from the perspective of mechanisms, and gives applications. Furthermore, we discuss the emerging materials and structures that are applied to achieve high sensitivity. In the end, we present perspectives on the outlooks of wearable multi-functional sensing technology.

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.

Electron-Ion Coupling Mechanism to Construct Stable Output Performance Nanogenerator.

Recently, triboelectric nanogenerators (TENGs) have been promoted as an effective technique for ambient energy harvesting, given their large power density and high energy conversion efficiency. However, traditional TENGs based on the combination of triboelectrification effect and electrostatic induction have proven susceptible to environmental influence, which intensively restricts their application range. Herein, a new coupling mechanism based on electrostatic induction and ion conduction is proposed to construct flexible stable output performance TENGs (SOP-TENGs). The calcium chloride doped-cellulose nanofibril (CaCl2-CNF) film made of natural carrots was successfully introduced to realize this coupling, resulting from its intrinsic properties as natural nanofibril hydrogel serving as both triboelectric layer and electrode. The coupling of two conductive mechanisms of SOP-TENG was comprehensively investigated through electrical measurements, including the effects of moisture content, relative humidity, and electrode size. In contrast to the conventional hydrogel ionotronic TENGs that require moisture as the carrier for ion transfer and use a hydrogel layer as the electrode, the use of a CaCl2-CNF film (i.e., ion-doped natural hydrogel layer) as a friction layer in the proposed SOP-TENG effectively realizes a superstable electrical output under varying moisture contents and relative humidity due to the compound transfer mechanism of ions and electrons. This new working principle based on the coupling of electrostatic induction and ion conduction opens a wider range of applications for the hydrogel ionotronic TENGs, as the superstable electrical output enables them to be more widely applied in various complex environments to supply energy for low-power electronic devices.

Flexible Hybrid Photo-Thermoelectric Generator Based on Single Thermoelectric Effect for Simultaneously Harvesting Thermal and Radiation Energies.

Wearable electronic devices have great potential in the fields of the Internet of Things (IoT), sports and entertainment, and healthcare, and they are essential in advancing the development of next-generation electronic information technology. However, conventional lithium batteries, which are currently the main power supply of wearable electronic devices, have some critical issues, such as frequent charging, environmental pollution, and no surface adaptability, which limit the further development of wearable electronic devices. To address these challenges, we present a flexible hybrid photothermoelectric generator (PTEG) with a simple structure composed of a thermoelectric generator (TEG) and a light-to-thermal conversion layer to simultaneously harvest thermal and radiation energies based on a single working mechanism. The mature mass-fabrication technology of screen printing was applied to successively prepare n-type (i.e., Bi2Te2.7Se0.3) and p-type (i.e., Sb2Te3) thermoelectric inks atop a polyimide substrate to form the TEG with a serpentine thermocouple chain, which was further covered by a light-to-thermal conversion layer to constitute the PTEG. The resulting PTEG with five pairs of thermocouples generated a direct-current output of 82.4 mV at a temperature difference of 50 °C and a direct-current output of 41.2 mV under 20 mW/cm2 infrared radiation. Meanwhile, the remarkable mechanical reliability and output stability were experimentally demonstrated through a systematic test, which indicated the feasibility and potential of the developed PTEG as a reliable power source. In addition, as desirable application prototypes, the fabricated PTEGs have been successfully demonstrated to harvest biothermal energy and infrared radiation to drive portable electronic devices (e.g., a calculator and a clock). Hybrid energy harvesting technology based on a simple structure may provide a new solution to current power supply issues of wearable electronic device.

Textile-Based Triboelectric Nanogenerators for Wearable Self-Powered Microsystems.

In recent years, wearable electronic devices have made considerable progress thanks to the rapid development of the Internet of Things. However, even though some of them have preliminarily achieved miniaturization and wearability, the drawbacks of frequent charging and physical rigidity of conventional lithium batteries, which are currently the most commonly used power source of wearable electronic devices, have become technical bottlenecks that need to be broken through urgently. In order to address the above challenges, the technology based on triboelectric effect, i.e., triboelectric nanogenerator (TENG), is proposed to harvest energy from ambient environment and considered as one of the most promising methods to integrate with functional electronic devices to form wearable self-powered microsystems. Benefited from excellent flexibility, high output performance, no materials limitation, and a quantitative relationship between environmental stimulation inputs and corresponding electrical outputs, TENGs present great advantages in wearable energy harvesting, active sensing, and driving actuators. Furthermore, combined with the superiorities of TENGs and fabrics, textile-based TENGs (T-TENGs) possess remarkable breathability and better non-planar surface adaptability, which are more conducive to the integrated wearable electronic devices and attract considerable attention. Herein, for the purpose of advancing the development of wearable electronic devices, this article reviews the recent development in materials for the construction of T-TENGs and methods for the enhancement of electrical output performance. More importantly, this article mainly focuses on the recent representative work, in which T-TENGs-based active sensors, T-TENGs-based self-driven actuators, and T-TENGs-based self-powered microsystems are studied. In addition, this paper summarizes the critical challenges and future opportunities of T-TENG-based wearable integrated microsystems.

Recent progress in silk fibroin-based flexible electronics.

With the rapid development of the Internet of Things (IoT) and the emergence of 5G, traditional silicon-based electronics no longer fully meet market demands such as nonplanar application scenarios due to mechanical mismatch. This provides unprecedented opportunities for flexible electronics that bypass the physical rigidity through the introduction of flexible materials. In recent decades, biological materials with outstanding biocompatibility and biodegradability, which are considered some of the most promising candidates for next-generation flexible electronics, have received increasing attention, e.g., silk fibroin, cellulose, pectin, chitosan, and melanin. Among them, silk fibroin presents greater superiorities in biocompatibility and biodegradability, and moreover, it also possesses a variety of attractive properties, such as adjustable water solubility, remarkable optical transmittance, high mechanical robustness, light weight, and ease of processing, which are partially or even completely lacking in other biological materials. Therefore, silk fibroin has been widely used as fundamental components for the construction of biocompatible flexible electronics, particularly for wearable and implantable devices. Furthermore, in recent years, more attention has been paid to the investigation of the functional characteristics of silk fibroin, such as the dielectric properties, piezoelectric properties, strong ability to lose electrons, and sensitivity to environmental variables. Here, this paper not only reviews the preparation technologies for various forms of silk fibroin and the recent progress in the use of silk fibroin as a fundamental material but also focuses on the recent advanced works in which silk fibroin serves as functional components. Additionally, the challenges and future development of silk fibroin-based flexible electronics are summarized. (1) This review focuses on silk fibroin serving as active functional components to construct flexible electronics. (2) Recent representative reports on flexible electronic devices that applied silk fibroin as fundamental supporting components are summarized. (3) This review summarizes the current typical silk fibroin-based materials and the corresponding advanced preparation technologies. (4) The current challenges and future development of silk fibroin-based flexible electronic devices are analyzed.

Wearable multi-sensing double-chain thermoelectric generator.

Wearable electronics play a crucial role in advancing the rapid development of artificial intelligence, and as an attractive future vision, all-in-one wearable microsystems integrating powering, sensing, actuating and other functional components on a single chip have become an appealing tendency. Herein, we propose a wearable thermoelectric generator (ThEG) with a novel double-chain configuration to simultaneously realize sustainable energy harvesting and multi-functional sensing. In contrast to traditional single-chain ThEGs with the sole function of thermal energy harvesting, each individual chain of the developed double-chain thermoelectric generator (DC-ThEG) can be utilized to scavenge heat energy, and moreover, the combination of the two chains can be employed as functional sensing electrodes at the same time. The mature mass-fabrication technology of screen printing was successfully introduced to print n-type and p-type thermoelectric inks atop a polymeric substrate to form thermocouples to construct two independent chains, which makes this DC-ThEG flexible, high-performance and cost-efficient. The emerging material of silk fibroin was employed to cover the gap of the fabricated two chains to serve as a functional layer for sensing the existence of liquid water molecules in the air and the temperature. The powering and sensing functions of the developed DC-ThEG and their interactions were systematically studied via experimental measurements, which proved the DC-ThEG to be a robust multi-functional power source with a 151 mV open-circuit voltage. In addition, it was successfully demonstrated that this DC-ThEG can convert heat energy to achieve a 3.3 V output, matching common power demands of wearable electronics, and harvest biothermal energy to drive commercial electronics (i.e., a calculator). The integration approach of powering and multi-functional sensing based on this new double-chain configuration might open a new chapter in advanced thermoelectric generators, especially in the applications of all-in-one self-powered microsystems.