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Electrohydrodynamic (EHD) 3D printing of carbon-based materials in the form of orderly networks can have various applications. In this work, microscale carbon/nickel (C-Ni) composite electrodes with controlled porosity have been utilized in electrochemical energy storage of supercapacitors. Polyacrylonitrile (PAN) was chosen as the basic material for its excellent carbonization performance and EHD printing property. Nickel nitrate (Ni(NO3)2) was incorporated to form Ni nanoparticles which can improve the conductivity and the capacitance performance of the electrode. Well-aligned PAN-Ni(NO3)2 composite structures have been fabricated and carbonized as C-Ni electrodes with the typical diameter of 9.2±2.1 μm. The porosity of the as-prepared C-Ni electrode can be controlled during the EHD process. Electrochemical results show the C-Ni network electrode has achieved a 2.3 times higher areal specific capacitance and 1.7 times higher mass specific capacitance than those of a spin-coated electrode. As such, this process offers a facile and scalable strategy for the fabrication of orderly carbon-based conductive structures for various applications such as energy storage devices and printable electronics.

Existing soft actuators for adaptive microlenses suffer from high required input voltage, optical loss, liquid loss, and the need for assistant systems. In this study, we fabricate a polyvinyl chloride-based gel using a new synergistic plasticization method to achieve simultaneously a high optical transparency and an ultrasoft rubber-like elastic behavior with a large voltage-induced deformation under a weak electric field. By compressing the smooth gel between two sets of annular electrodes, a self-contained biconvex microlens is realized that is capable of considerable shape changes in the optical path. Each surface of the dual-curvature microlens can be independently adjusted to focus or scatter light to capture real or virtual images, yield variable focal lengths (+31.8 to -11.3 mm), and deform to various shapes to improve aberrations. In addition to simple fabrication, our microlens operates silently and consumes low power (0.52 mW), making it superior to existing microlenses.

Additive manufacturing, or more commonly known as 3D printing, has been at the forefront of manufacturing research for the past couple decades. Many advances have occurred in the basic printing techniques, materials, and post-processing schemes. In recent years, there have been many critical developments in the field 3D printing, such as the development of volumetric 3D printing to increase the printing speed, composite 3D printing to create piezoelectric devices, and hydrogel structures highly similar to blood vessels. The spectrum of materials that can be printed has also increased, allowing researchers to print a variety of materials including live cells, modulus changing polymers and even spacecraft-grade metals.

Paper-based pressure/strain sensors could have potential wide applications with wearable features in disposable products. In this study, molybdenum carbide-graphene (MCG) composites with porous and stacking micro-structures are fabricated on top of the paper substrate to act as piezo-resistivity strain/pressure sensors. As a strain sensor, this paper-based device can detect not only the amplitude and frequency of applied strain but also the direction as tensile or compressive deformation. The gauge factors are 73 and 43 for tensile and compressive strain, respectively, with demonstration example in detecting and recognizing human body motions. As a pressure sensor, this MCG-based paper device has high sensitivity to weak pressure signals such as sounds by distinguishing the seven piano notes. Our study provides a simple strategy for developing paper-based electronics with unique properties toward practical applications.

In this work, NiCo2O4@TiO2 electrodes based on micro-region heterojunctions were synthesized by a simple one-step hydrothermal method. The material characteristics of resultant samples were characterized by XPS, XRD, SEM, HRTEM and BET. Compared with NiCo2O4 electrodes prepared with the same hydrothermal method, the NiCo2O4@TiO2 electrode shows higher electrochemical performance, cycling stability and lower charge transfer resistance. Specifically, the mass specific capacitances of NiCo2O4 and NiCo2O4@TiO2 electrodes are 564.3 F · g−1 and 1085.7 F · g−1 at current density of 5 A · g−1 and retained 84.4% and 95.5% after 10,000 cycles, while the Rct of NiCo2O4 and NiCo2O4@TiO2 are 1.72 Ω and 0.18 Ω, respectively. For practical applications of NiCo2O4@TiO2 electrode, a NiCo2O4@TiO2//AC two-electrode system was assembled and the performances were tested to achieve 255.9 F · g−1 at current density of 2.5 A · g−1. Additionally, the corresponding energy density and power density were recorded as 91 wh/kg and 4 kw/kg. The superior electrochemical performance and cycle stability of NiCo2O4@TiO2 electrode might be attributed to the higher specific surface and micro-region heterojunctions for enormous future applications.

It is of great interest to link Raman scattering to the properties of disorders in graphene membranes, which provides an effective characterization method to probe atomic scale defects. The built-in stress effect on the defect densities of substrate-supported monolayer graphene membranes around wells is investigated. First, a modified phenomenological model is developed to depict the relationship between built-in stresses and defect-activated Raman intensities. To validate the rationality of the modified model, Raman spectroscopy is used to characterize stretched graphene membranes on different patterned substrates with micro-scale wells. The experimental data indicate that the intensity ratio of D mode to G mode ID/IG increases with the Raman test point approaching the well edge. According to the modified model, the increase of ID/IG means the rise of defect densities, which originates from the propagation of initial defects in graphene membranes under built-in tension. The underlying mechanism of defect density increasing phenomenon is that the built-in stresses provide the energy for defect propagations in stretched graphene membranes. Theoretical and experimental comparison well validates the rationality of the modified model. The work can provide a theoretical foundation for Raman characterization method of defect propagations in stretched graphene and applications of defective graphene-based nanodevices.

It is desirable and still challenging to develop flexible, breathable and anti-corrosive wearable strain sensors with high stretchability and sensitivity that can realize full range body motions. Here, a superhydrophobic and conductive nanofiber composites (SCNCs) with a hierarchical SiO2/graphene shell and polyurethane (PU) nanofiber core microstructure were fabricated by assembling graphene on PU nanofibers under the assistance of ultrasonication, followed by stretching-induced SiO2 nanoparticles decoration onto the graphene shell. The introduction of graphene and SiO2 nanoparticles improves both Young’s modulus, tensile strength and the elongation at break of PU nanofibrous membrane. The superhydrophobicity and conductivity can be almost maintained after the SCNCs are subject to cyclic stretching or abrasion or even exposed to harsh conditions. When used as strain sensors, the SCNCs show high stretchability, reliability and good durability and can be used in harsh environment including acid and salt conditions. The SCNCs are then assembled to monitor full range body motions including both subtle and large body movements, making it a promising candidate in wearable electronics.

Two-dimensional materials are incipient as an innovative class of multifunctional elements with promising attributes for energy and environmental applications. Herein, a novel nanohybrid consisting of Ag-MoS2 loaded mesoporous g-C3N4 (herein ‘m-CN’) with ordered structure was synthesized by a nanocasting method using cubic and ordered mesoporous silica (KIT-6) as a hard template. Due to its unique 3D mesoporous architecture, the as-synthesized Ag-MoS2@m-CN nanohybrid revealed excellent visible-light absorption for enhanced charge separation of photo-induced e−–h+ pairs to improve the degradation performance, high stability, and reusability with respect to Rhodamine B (RhB). The measured very high degradation efficiency is explained by synergistic effects from catalytically active Ag and MoS2 nanoparticles cumulatively decorated on the cubic mesoporous m-CN in form 3D mesoporous architecture. Additionally, the Ag-MoS2@m-CN nanohybrid exhibits high response and selectivity toward n-butanol gas at low-operating temperatures for reliable detection of volatile organic compounds (VOCs). The photocatalysis behavior and VOC sensing response of Ag-MoS2@m-CN nanohybrid material are discussed in detail for possible various application avenues toward environmental purifications and monitoring.

For many industrial applications of the methane sensor in a wireless node, the primary requirements are low power consumption and wide detection range. These requirements are considerably challenging as compared to the requirements related with conventional methane sensors. To meet such requirements of Internet of Things (IoT) development for methane monitoring in underground coal mines, we propose a full concentration range methane detector using a micro silicon device with low power consumption. The microheaters are fabricated with a CMOS-compatible SOI (silicon-on-insulator) MEMS (microelectromechanical system) process. Joule heating is utilized to enable micro-local high temperature for methane sensing. To obtain signals with high sensitivity and low heating power under an appropriate supplied current, a working point is determined by means of the maximum voltage variations from the voltage-current characteristics of the microheater. In general, the methane sensitivity of the micro heater increases and the power consumption decreases, as the length of the heater increases. The device has an exponential-decay response in the entire methane concentration range. A typically low power consumption around 27 mW yields an average sensitivity of approximately 20 mV/% CH4 for the methane concentration ranging from 0 to 17%.

Flexible humidity sensors play important roles in wearable devices and consuming electronics which provide a convenient way between digital and physical worlds. This work presents an easy fabricated method for flexible humidity sensors all based on carbon material including electrodes and functional layers. The interdigital electrodes are made by direct laser writing on commercial Kapton tapes and the transferring to flexible Polydimethylsiloxane (PDMS) substrates. The humidity sensing material is reduced graphene oxide (rGO) in nanometer thickness by electrospray. The rGO flakes covered the micro-size laser induced graphite (LIG), forming rGO-graphite balls, dramatically increase surface areas to interact with water molecules. The results show high precision sensitivity and fast response time for adsorption (0.9 s) and desorption (4.5 s). This method provides a novel method for fabricating cost-effective flexible humidity sensors.

Black silicon is a kind of micro-/nanostructure formed on the surface of silicon, which can greatly reduce light reflection. In this review, five main fabrication techniques of black silicon are introduced and these techniques can change the morphology of the surface of crystalline silicon to a certain extent, thus making the preparation of black silicon more feasible and convenient. The utilization of black silicon is then reviewed. Black silicon has originally been used as a bactericidal to eradicate bacteria such as Pseudomonas aeruginosa. It is bound to have more novel applications in the field of sterilization. Black silicon is like a sponge that absorbs light. It can capture almost all sunlight, both visible and infrared light. It also reduces the amount of silicon used in optical sensors, making products cheaper, smaller, and lighter. Due to the above characteristics, black silicon is widely used in Surface Enhanced Raman Scattering detection, biomedical sensing, and infrared device manufacturing. It also has the potential to improve the performance of solar cells. This review is intended to serve as a useful introduction to this novel material and its properties, and provide a general overview of recent progress in its main applications.

Ultrathin transition metal carbides with high capacity, high surface area, and high conductivity are a promising family of materials for applications from energy storage to catalysis. However, large-scale, cost-effective, and precursor-free methods to prepare ultrathin carbides are lacking. Here, we demonstrate a direct pattern method to manufacture ultrathin carbides (MoCx, WCx, and CoCx) on versatile substrates using a CO2 laser. The laser-sculptured polycrystalline carbides (macroporous, ~10–20 nm wall thickness, ~10 nm crystallinity) show high energy storage capability, hierarchical porous structure, and higher thermal resilience than MXenes and other laser-ablated carbon materials. A flexible supercapacitor made of MoCx demonstrates a wide temperature range (−50 to 300 °C). Furthermore, the sculptured microstructures endow the carbide network with enhanced visible light absorption, providing high solar energy harvesting efficiency (~72 %) for steam generation. The laser-based, scalable, resilient, and low-cost manufacturing process presents an approach for construction of carbides and their subsequent applications., Nature Communications, 10, ISSN:2041-1723

Flexible and wearable devices with the capabilities of both detecting and generating mechanical stimulations are critical for applications in human-machine interfaces, such as augmented reality (AR) and virtual reality (VR). Herein, a flexible patch based on a sandwiched piezoelectret structure is demonstrated to have a high equivalent piezoelectric coefficient of d33 at 4050 pC/N to selectively perform either the actuating or sensing function. As an actuator, mechanical vibrations with a peak output force of more than 20 mN have been produced, similar to those from the vibration mode of a modern cell phone, and can be easily sensed by human skin. As a sensor, both the pressure detection limit of 1.84 Pa for sensing resolution and excellent stability of less than 1% variations in 6000 cycles have been achieved. The design principle together with the sensing and driving characteristics can be further developed and extended to other soft matters and flexible devices.

Quantum-dot (QD) white light-emitting diodes (LEDs) are promising for illumination and display applications due to their excellent color quality. Although they have a high quantum yield close to unity, the reabsorption of QD light leads to high conversion loss, significantly reducing the luminous efficacy and stability of QD white LEDs. In this report, SBA-15 mesoporous particles (MPs) with two-dimensional hexagonal pore structures (2D-HPS) are utilized to largely enhance the luminous efficacy and color-conversion efficiency of QD white LEDs in excess of 50%. The reduction in conversion loss also helps QD white LEDs to achieve a lifetime 1.9 times longer than that of LEDs using QD-only composites at harsh aging conditions. Simulation and testing results suggest that the waveguide effect of 2D-HPS helps in reducing the reabsorption loss by constraining the QD light inside the wall of 2D-HPS, decreasing the probability of being captured by QDs inside the hole of 2D-HPS. As such, materials and mechanisms like SBA-15 MPs with 2D-HPS could provide a new path to improve the photon management of QD light, comprehensively enhancing the performances of QD white LEDs.

External tactile force/pressure sensing, as a vital function of electronic skins, is very important to robots in non-structured environments, intelligent prosthesis of the disabilities for baby care or daily life and medical cares, etc. Despite of great advance of pressure sensors for medium pressure (10–100 kPa), to achieve high sensitivity, good pressure resolution and fast response in electronic skins for applications in biomedicine, artificial prosthetics, personal health monitoring and robotics remains challengeable. Here we direct-wrote graphene nanoplatelets (GNPs) using Weissenberg effect on a PDMS substrate to fabricate a flexible pressure sensor with high pressure sensitivities about 6.56 MPa−1 and 0.335 kPa−1 respectively under the pressure less than 65 kPa and around 100 kPa. Minimum pressure change of ca. 14 Pa to the placement or removal of a tiny item can be detected. It is characterized to be of faster response/recovery speed (ca. 171 ms/110 ms), better repeatability and cycling stability, compared with most reported flexible medium-pressure sensor. The pressure electronic skin demonstrates good performance in its applications including pulse sensing on wrist and fingertip grabbing, indicating its greatly potential sensor candidate for electronic skins.

A piezoelectret pulse sensor combined with the approximate entropy (ApEn) analysis is utilized to detect human pulses to reveal health conditions. Inspired by the traditional Chinese medicine (TCM) with a foundation of more than 2500 years, human pulses are helpful in the diagnostics of illness as the body's vital energy circulates through blood vessels with branches connected to organs. A flexible cellular polypropylene (PP) piezoelectret film with a wooden cylinder substrate is used to emulate the pulse taking process in TCM to record the pulse characteristics. A group of 26 volunteers have been successfully diagnosed by the wearable sensing system and the approximate entropy analysis has been implemented to analyze the data. Results show a threshold approximate entropy value of 0.1 as the separation point between the volunteers of normal and abnormal health conditions.

Conductive polymer composite based strain sensors have promising applications in the fields of artificial skin, wearable health-care device, etc. However, fabrication of strain sensors with good stretchability, anti-corrosion, excellent durability and reliability remains challenging. In this work, a superhydrophobic strain sensor based on conductive thermoplastic polyurethane/carbon nanotubes/polydimethylsiloxane (TPU/CNTs/PDMS) was prepared by ultrasonication induced CNTs decoration onto the electrospun TPU nanofiber surface, followed by the PDMS modification. Uniformly dispersed CNTs on the nanofiber surface with a hierarchical structure construct the conductive network. The PDMS layer with a low surface energy endows the nanofiber composite with superhydrophobicity thus anti-corrosion property. The introduction of CNTs/PDMS improves both the Young's modulus, tensile strength and the elongation at break. The superhydrophobicity and conductivity can be maintained after the cyclic stretching-releasing test, displaying excellent durability. When used as a wearable strain sensor, the nanofiber composite is capable of detecting body motion and could work even under harsh conditions (moisture, acid and alkaline environment), showing promising application in wearable electronics.

The influences of temperature, humidity, and O2 to the gas sensing characteristics of graphene field effect transistors (FETs) have been studied as these environmental factors are often encountered in practical gas sensing applications. Both empirical results and theoretical analyses are characterized for heated graphene FET gas sensors from room temperature to 100 °C under a wide range of applied gate voltages. It is found that at a constant applied gate voltage of −20 V with respect to the gate voltage at the neutrality point, the sensitivity of the device to humidity decreases; while the sensitivity to O2 decreases first, and increases afterwards as the operation temperature increases. These phenomena are explained by using the physisorption and chemisorption models between gases and the graphene surface. Furthermore, devices operate in the hole regime (the majority carrier is hole in the prototype devices) result in lower sensitivity to humidity and O2 as compared to those results of gas sensors operating in the electron regime due to the p-type doping effects of moisture and O2. As such, this work provides good foundations for graphene-based FET gas sensors in practical application environments under the influences of ambient air, temperature, and humidity.

Metal-based materials have been widely used for the electromagnetic interference (EMI) shielding due to their excellent intrinsic conductivity. However, their high density, poor corrosion resistance, and poor flexibility limit their further application in aerospace and flexible electronics. Here, we reported a facile means to prepare lightweight, mechanically durable, superhydrophobic and conductive polymer fabric composites (CPFCs) with excellent electromagnetic shielding performance. The CPFC could be fabricated by three steps: (1) the polypropylene (PP) fabric was coated by a polydopamine (PDA) layer; (2) PP/PDA adsorbed the Ag precursor that was then chemically reduced to Ag nanoparticles (AgNPs); (3) PP/PDA/AgNPs fabrics were modified by one layer of polydimethylsiloxane (PDMS). The contact angle (CA) of the CPFCs could reach ∼152.3° while the sliding angle (SA) was as low as ∼1.5°, endowing the materials with excellent self-cleaning performance. Thanks to the extremely high conductivity of 81.2 S/cm and the unique porous structure of the fabric, the CPFC possessed outstanding EMI shielding performance with the maximum shielding effectiveness (SE) of 71.2 dB and the specific shielding effectiveness (SSE) of 270.7 dB cm

Graphene quantum dots (GQDs) have been explored in recent years for electrochemical applications with considerable potentials. Here, we present GQD-doped CuCo2S4 nanocomposites through two-step hydrothermal process for supercapacitor electrodes. The surface of CuCo2S4 nanosheets changes from smooth to particles-accumulative shape, which assists the electrochemical cycling processes as well as the ion diffusion and charge transfer kinetics for improved supercapacitor performances. As a result, GQD/CuCo2S4 electrodes demonstrate a specific capacitance of 1725 F g−1 under a current density of 0.5 A g−1 and a cycling life of 10,000 cycles by retaining 90% of the energy storage capability. As such, this work extends the potential of GQDs in electrochemical applications by means of morphology change of CuCo2S4 nanosheets.

Fabricating side by side (SBS) nanofibers with two distinct materials using dual spinnerets is challenging because of the formation of bulk heterojunctions, which limits the application of these nanofibers. In the present work, we demonstrate the synthesis of SBS nanofibers with fully exposed surfaces of different polymers. The SBS nanofibers were fabricated by a simple electrospinning method composed of oppositely charge spinnerets with a metal sheet to form a 2D auxiliary electrode plate. Furthermore, SBS TiO2 and CoO nanofibres were explored for photocatalytic rhodamine B degradation by a photochemical method. Due to the combined action of the heterojunction of TiO2 and CoO, the as-synthesized SBS fibres exhibit the highest photocatalytic activity compared with the single composite fibres, which is supported by their superior morphology, and textural and optical properties.

Due to the influence of temperature, the traditional quartz crystal microbalance (QCM) dew point sensors require an independent study on the temperature frequency characteristics (TFC) for calibrations as the frequency response is susceptible to external interferences. QCM dew point sensors with either montmorillonite (MTT) or molybdenum disulfide (MoS2) coatings together with a dew point recognition algorithm are demonstrated for dew point recognitions. The frequency signals output from three kinds of QCM sensors are processed by the proposed recognition algorithm and compared with the standard dew point data provided by MICHELL S4000. The results show QCM sensors with MTT coating have the best accuracy and repeatability test for 5 days in range of 3 ℃ DP–15 ℃ DP environment has shown the maximum relative error as compared with the standard dew-point values is less than ±0.3 ℃ DP. As such, this work shows the feasibility of utilizing QCM humidity sensors without the influence of temperature on frequency measurements with good repeatability.

The dense ZrO2-8 wt.% Y2O3 ceramics with tetragonal (t') phase were prepared by spark plasma sintering (SPS). The micro-structure modification in poly-crystalline t' phase YSZ ceramics irradiated with 60 KeV He ions over a wide fluence range were studied. It is found that, the t' → t + c phase transformation was detected for the samples irradiated at the fluence above 2 × 1017 cm−2. In addition, a three-steep model for damage stages has been proposed and discussed for interpreting the evolution of mechanical properties of t'-YSZ ceramics after irradiated, mainly revealed by the analysis from HRTEM results. Firstly, as the He+ fluences range from 0.5 × 1017 cm−2 to 1 × 1017 cm−2, the hardening effects were confirmed in t'-YSZ owing to the irradiation induced defects serve as obstacles to resist slipping of dislocations. As the fluence was increased to 2 × 1017 cm−2, the degradation of mechanical properties was evidenced, which could be ascribed to the deterioration effect of aggregated He bubbles inside t' YSZ grains. While the fluence increased to 10 × 1017 cm−2, a seriously damaged region was approximately located at 250 nm in depth, at which a great number of He bubbles with different shape and dislocation loops were evidenced, leading to a further reduction of hardness and elastic modulus. No micro-cracks emerge even for He+ fluences as high as 10 × 1017 cm−2, indicating high irradiation stability and resistance to micro-crack propagation in t'-YSZ.

Laser-induced graphene (LIG) has both good electrical conductivity and three-dimensional porous structures. Here, porous graphene interdigital electrodes (IDE) were constructed as a capacitive sensor from commercial polymer films by the laser ablation process and transferred to the polydimethylsiloxane (PDMS) substrate. The graphene oxide (GO) adsorption layer was electrosprayed as a humidity sensing structure, and a Peltier device was used to control the temperature to produce the condensation of water vapors. The dew point was identified by the equilibrium state of the capacitor when the adsorption layer and the surface air reached the saturation equilibrium. The performances of the hydrophilic dew point sensing system under different environmental conditions were investigated. The results show that the precision of the carbon-based dew point sensor of ≤±0.8 °C DP with good stability and repeatability is better than those of other dew point instrument based on electrical sensing parameters at ±1.0 °C DP.

This paper reports the use of machine learning in accelerating the MEMS design process. Candidate designs are represented by pixelated binary 2D images. Instead of common computational tools like FEA, we use trained neural network for quickly obtaining physical properties of interest for each candidate design. Circular disk resonators are used as an example to demonstrate the capability of our method. After sufficient training with 9000 images, the resulting neural network can serve as a high-speed, high-accuracy analyzer: it can identify four vibrational modes of interest and calculate the corresponding frequencies 4000 times faster than commonly used FEA software, with remarkable accuracy (~98%).

This work presents the sense of touch via non-contact ultrasonic waves by a dual-electrode bimorph piezoelectric micromachined transducer (pMUT) array. The prototype device has 12×12 elements with circular diaphragms of 415μm in radius made of 2 μm-thick AlN. They are fabricated by a CMOS compatible micromachining process resulting a resonant frequency at 109.4 kHz. Experimentally, a best haptic sensation on human fingers is found when emitting high frequency ultrasonic waves to emulate 100 Hz signals by means of pulse width modulation with a 50% duty cycle. Strong haptic sensations are reported by volunteers under an AC peak-to-peak amplitude of 12 V up to 10 cm away from the transducers. Applications as the human-machine interfaces in biomedical, gaming, and AR/VR are a few of the MEMS uses; we present results by transmitting a series of Morse codes remotely to the skins of volunteers.

Electrospinning setup with a sharp tip guiding electrode has been designed for high-aspect-ratio three-dimensional (3D) electrospinning by enhancing the electrical field strength to guide nanofibers toward designated paths layer by layer. This setup helps to deposit nanofibers under a steady charge transfer process and overcomes the electrical interferences. The polymer jet via the electrohydrodynamic process has been used to direct-write ordered, 3D structures with the aspect-ratio (height/width) up to 25 for various patterns, such as rectangular grids and five-point stars. Experimental results show the deposited nanofibers can form 3D high-aspect-ratio structures with uniform nanofiber diameter and good position accuracy higher than 1.5 μm.

p-Type and n-type thermoelectric semiconductor materials with compatible performance are key components for thermoelectric devices. Great improvement in thermoelectric performance has been achieved in p-type PbTe, whereas the n-type counterpart still shows much inferior thermoelectric performance compared to that of the p-type PbTe. This inspires many strategies focused on advancing n-type PbTe thermoelectrics. Herein, not only effective mass engineering, resonance states, point defects, and nanostructures but also newly developed concepts including dynamic doping for stabilizing the optimal carrier concentration and introducing dislocations for reducing lattice thermal conductivity are summarized. In addition, the synergistic effects for further enhancing the thermoelectric performance are outlined, together with a discussion and outlook for boosting the advancement in n-type PbTe thermoelectric materials. Strategies discussed here are expected to be applicable to other thermoelectric materials.

Agility and trajectory control are two desirable features for robotics, but they become very challenging for soft robots without rigid structures to support rapid manipulations. Here, a curved piezoelectric thin film driven at its structural resonant frequency is used as the main body of an insect-scale soft robot for its fast translational movements, and two electrostatic footpads are used for its swift rotational motions. These two schemes are simultaneously executed during operations through a simple two-wire connection arrangement. A high relative centripetal acceleration of 28 body length per square second compared with existing robots is realized on a 65-milligram tethered prototype, which is better than those of common insects, including the cockroach. The trajectory manipulation demonstration is accomplished by navigating the robot to pass through a 120-centimeter-long track in a maze within 5.6 seconds. One potential application is presented by carrying a 180-milligram on-board sensor to record a gas concentration route map and to identify the location of the leakage source. The radically simplified analog motion adjustment technique enables the scale-up construction of a 240-milligram untethered robot. Equipped with a payload of 1660 milligrams to include the control circuit, a battery, and photoresistors, the untethered prototype can follow a designated, 27.9-centimeter-long "S"-shaped path in 36.9 seconds. These results validate key performance attributes in achieving both high mobility and agility to emulate living agile insects for the advancements of soft robots.

This work presents theoretical analysis and finite element simulations of a novel piezoelectric micromachined ultrasonic transducer (pMUT) design. A pMUT with free boundary condition is introduced for the first time and compared with conventional pMUTs of fixed boundary. For the same transducer dimensions, the proposed design has more than 2.5x electromechanical coupling coefficient compared to that from traditional clamped boundary design due to increased mechanical capacitance by the free edge. The cavity underneath the disc behaves as an acoustic resonator. By matching the frequencies of mechanical and acoustical resonances, the fractional bandwidth of the free boundary pMUT has 13x more fractional bandwidth compared to that of the fixed boundary design. In addition, the internal stresses created during microfabrication are expected to be released with the free edges. These results indicate that free pMUTs have the potential for high performance ultrasonic transducer applications.

The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

Isolation of circulating tumor cells (CTCs) from blood samples has important prognostic and therapeutic implications for cancer treatments, but the process is very challenging due to the low concentration of CTCs. In this study, we report a novel 3D printed microfluidic device functionalized with anti-EpCAM (epithelial cell adhesion molecule) antibodies to isolate CTCs from human blood samples. A 3D printing technology was utilized with specially designed interior structures to fabricate a microfluidic device with high surface area and fluid flow manipulation, increasing capture efficiency of tumor cells. These devices with the optimal flow rate (1 mL/h) and channel length (2 cm) were demonstrated to test three kinds of EpCAM positive cancer cell lines (MCF-7 breast cancer, SW480 colon cancer, and PC3 prostate cancer), and one kind of EpCAM negative cancer cell line (293T kidney cancer). Experimentally, the capture efficiency higher than 90% has been achieved, and the isolation of MCF-7 tumor cells from spiked human blood samples has also been demonstrated. Combined with DNA-based detection (e.g. polymerase chain reaction or DNA sequencing), the detection and analysis of released DNAs from captured tumor cells could be another future direction for clinical diagnosis and cancer treatment.

A 3.7 MHz piezoelectric micromachined ultrasonic transducer (pMUT) system based on the piezoelectric aluminum nitride (AlN) thin films has been developed to construct pulsed wave Doppler (PWD) ultrasound for wearable blood flow measurements. In the blood emulation experiments, Nylon powder, surfactant, glycerinum and deionized (DI) water combined blood-mimicking fluid has been prepared and the PWD transceiver circuits are built for the signal processing. The Doppler effect has shown that both the average Doppler frequency and Doppler power spectrum density (PSD) shift linearly with respect to the blood flow rate. As such, this system could be further implemented as wearable ergonomics devices for real-time blood flow measurements.

We made an efficient electrocatalyst for hydrogen harvest, with an overpotential as low as 0.131V to achieve a current density of 10 mA/cm2. The morphology of the material was extremely delicate: branches on the nanowire, and smaller twigs on the branches, such that the specific surface area was greatly enlarged. The two-step hydrothermal process is easily accessible. The improved performance of GQD/NiCo 2 S 4 should mainly be ascribed to graphene quantum dots (GQDs), which induce morphology change of NiCo 2 S 4 nanowires. The delicate morphology serves as a buffer for volume change, and also a reservoir for electrolytes to transport protons [1]. In the meaning time, GQDs inherit excellent properties from both graphene and quantum dots, improving conductivity. Moreover, the addition of GQDs creates numerous defects in both the basal and edge planes for the diffusion of protons and thus help overcome the sluggish redox kinetics of the electrode, leading to improved overall performances [2].

We report a flexible, hydrogel-based electrolyte material for microsupercapacitors with: 1) self-healing property in ambient environment, 2) high stretchability (elongation> 1000%), 3) 280 times increase in ion-conductivity as compared to that of conventional polyvinyl alcohol (PVA) based acidic electrolytes, and 4) high transparency. A transfer-printing-based patterning process was developed to allow high-resolution pattering on hydrogel. Prototype self-healable micro-supercapacitors (SHMS) have been fabricated with three key demonstrations: 1) working as a power supply for commercial LEDs, 2) retention of same performances before/after a 180-degree folding process, and 3) restoration of performances even after being cut through by a razor blade and after the self-healing process.

Microfluidic concentration gradient generators (µ-CGGs) have been utilized to identify optimal drug compositions through antimicrobial susceptibility testing (AST) for the treatment of antimicrobial-resistant (AMR) infections. Conventional µ-CGGs fabricated via photolithography-based micromachining processes, however, are fundamentally limited to two-dimensional fluidic routing, such that only two distinct antimicrobial drugs can be tested at once. This work addresses this limitation by employing Multijet-3D-printed microchannel networks capable of fluidic routing in three dimensions to generate symmetric multidrug concentration gradients. The three-fluid gradient generation characteristics of the fabricated 3D µ-CGG prototype were quantified through both theoretical simulations and experimental validations. Furthermore, the antimicrobial effects of three highly clinically relevant antibiotic drugs, tetracycline, ciprofloxacin, and amikacin, were evaluated via experimental single-antibiotic minimum inhibitory concentration (MIC) and pairwise and three-way antibiotic combination drug screening (CDS) studies against model antibiotic-resistant Escherichia coli bacteria. As such, this 3D µ-CGG platform has great potential to enable expedited combination AST screening for various biomedical and diagnostic applications.

We present the synthesis and characterizations of jellyfish inspired biomimetic hydrogels for potential applications in transparent, self-healing, and ultra-stretchable sensors and actuators. Compared to the state-of-art, this work achieved five distinctive advancements: (1) highly transparent with ∼85% transmittance within the visible light range; (2) ultra-stretchable without mechanical failure under more than 1100% applied strain; (3) excellent self-healing capability with broken pieces fully amended visually in 24 hours and piezoresistive gauge factor increased from 1.4 to 2.3; (4) highly adhesive with the self-bonding property, and (5) demonstrations of strain sensing and mechanical actuating for various potential applications.

This paper reports the non-contact surface temperature measurements using a single piezoelectric micromachined ultrasonic transducers (pMUTs) array based on time-of-flight (ToF) for the first time. Compared with the state-of-art technologies, four distinctive advancements have been achieved: (1) non-contact surface temperature sensing via pMUTs by the pulse-echo mode to detect the changes in ToF; (2) independence on surface material properties, e.g. emissivity; (3) integration with an ultrasonic analog frontend for both transmitting and receiving; and (4) recorded sensitivity of 143 ns/°C in ToF. As such, the proposed sensing scheme and results could open up a new class of non-contact temperature sensing in various media (air, liquid, tissue …) for industrial and biomedical applications.

The emerging wearable electronic devices requires power source available as anytime as possible, and the piezoelectric polymer based nanogenerators attract research interests as a candidate. Herein, we demonstrate a shoepad nanogenerator based on electrospun PVDF nanofibers harvesting energy during walking or running. We first compared three popular processes of electrospinning and find a best one which can produce maximum β-phase content in PVDF nanofibers. Another comparative experiment shows for nanofabric mats the sandwiched electrodes are better for outputting more energy than parallel electrodes. Finally a nanogenerator is designed and fabricated utilizing the far field electrospun PVDF nanofabric mat with the sandwiched electrodes. It has the optimal output power of about 6.45 μW with load resistance of 5.5 MΩ.

A green and efficient approach for efficient nanohybrid photocatalysts in extending the light response to the visible spectrum is a hot research topic in sustainable energy technologies. In this work, novel Au–TiO2@m-CN nanocomposite was synthesized using hard template of cubic ordered mesoporous KIT-6 via the nanocasting process. The as-prepared Au–TiO2@m-CN nanohybrids exhibit enhanced photocatalytic activities with improved stability and reusability using methyl orange dye. The enhanced photocatalytic performance is a result of the conjugated effect of catalytic active Au and TiO2 nanoparticles supported on highly efficient visible light sensitizer, graphitic carbon nitride (m-CN or g-C3N4), and ordered mesoporous morphology. Besides, the sensing performance of Au–TiO2@m-CN nanohybrids was also tested for the detection of amine gases, wherein a significant response was reported for triethylamine at low operating temperatures. This study reveals a simple and scalable methodology to design and develop ne...

Yttria-stabilized zirconia (YSZ) films with tetragonal (t′) phase was prepared on China Low Activation Martensitic steel (CLAMs) substrates by supersonic atmospheric plasma spraying (SAPS) with ZrO2-8 wt% Y2O3 nano-powders as feedstock. This study deals with the subject of the structural modifications and phase transformations in t′ phase YSZ coatings induced by implantation of low-energy (60 keV) He ions. It is found that the phase transformation in t′ phase YSZ coatings during helium ion-irradiation as a function of fluence can be divided into a two-stage process based on the microstructural evolution as follows: (i) In the case of He ions at 0.5 × 1017 cm2 (∼1.5 dpa), no obvious volume swelling can be detected, suggesting the irradiation-induced defects is primarily associated with a positive relaxation volume. Therefore, only t′ → t phase transformation was observed for the samples implanted at a fluence of 0.5 × 1017 cm−2. (ii) While increasing the fluence from 0.5 × 1017 cm−2 to 2 × 1017 cm−2, an increase in the defect concentrations can be observed, leading to an exponential growth in volume swelling from 3.2% to 9.7% observed. These high- density extended defects can induce a plastic deformation in the lattice structure, resulting significant lateral compressive stress. Correspondingly, the t′ → t + c phase transformation was detected for the samples implanted at the fluence of 2 × 1017 cm−2. At a fluence of 10 × 1017 cm−2, the formation of the He bubbles, and ellipsoidal cavities can be observed by TEM, inducing the further volume swelling. Meanwhile, the intensity of (4 0 0)c peak increases while fluence increase from 2 × 1017 cm−2 to 10 × 1017 cm−2. Based on the experimental results, a reasonable mechanism for the phase transformation of t′ phase YSZ samples during He ions irradiation is proposed.

Appropriate wing rotational movements in flapping wing flight are essential to generate aerodynamic lift force. This paper presents the usage of a pair of “pivot-spar” brackets to guide rotational movements of an electrostatic flapping-wing actuator for enhanced lift force. The wing rotation pattern is guided by the bracket design with the help of numerical simulations. Wing movements with desired rotation pattern are recorded by a high speed camera and the instantaneous lift force generated by the flapping-wing actuator is measured by a test platform using a lever and a high-precision force sensor. Experimentally, the flapping-wing actuator using the “pivot-spar” brackets with a diverging angle of 40° can achieve a maximum wing rotation angle of 29° and generate an aerodynamic lift force at 79.2 μN. Besides, this work also uses advanced micro fabrication methods based on laser cutting technique and the mass of the flapping-wing actuator can be reduced to 50 mg. With 2.5 times higher lift force at 79.2 μN and 20 times lower overall system mass at 50 mg as compared with the earlier report, 50 times higher efficiency is achieved by calculating the total lift force/weight ratio for the presented electrostatic flapping-wing actuator with “pivot-spar” design. This result advances the field of electrostatic flapping-wing actuator toward its possible takeoff in the future.

This paper proposes a fast-converged reduced-order model for small vibrations of electrostatically actuated rectangular microplates around their deformed states. On this foundation, one-mode analysis method and thus closed-form analytical expressions are developed for the fundamental resonant frequency analysis. Numerical multi-mode analysis is conducted to investigate the convergence. It is concluded that the one-mode analysis method can give a converged solution, which demonstrates that the proposed model has a faster convergence than previous models where multi modes are needed. The directly calculated resonant frequencies by the closed-form expressions are in good agreement with the numerical results by FEM simulations with less than 5% variation before pull-in. Parametric study shows the closed-form expressions are applicable to the cases in which the thickness is less than 1/20 of the length and width under a nominal length to width ratio of 1∼2.5, and the gap distance is less than or equal to the thickness of the microplate. Additionally, a kind of electrostatic configurations based on multilayer microplates, capacitive micromachined ultrasonic transducers (CMUTs), were used to experimentally validate the closed-form expressions and good agreement was observed.

Stretchable strain sensors are important elements in flexible and skin-mountable electronics typically fabricated using semiconductor materials in cleanroom-based manufacturing processes. This work demonstrates piezoresistive strain sensors with both strain and pressure sensing capabilities by a cost-effective and versatile process utilizing a laser patterning, graphite conversion, and polymeric transfer process. The resulting sensing systems exhibit high gauge factor of 37 and pressure sensitivity of 0.088kPa-1 with high sustainable strain up to 70%. These exceptional performances are explained and observed by deforming the sensor under an in-situ SEM to show self-healing characteristics of films under large deformations. The highly sensitive strain sensors have been shown in human interface demonstrations, such as measuring the physiological signal of the human pulses, finger pressure and bending of fingers as well as assisting a robotic arm for gripping and releasing operations.

A dew point (DP) sensor structure was proposed with active temperature control using the thermally conductive electrodes on the quartz crystal microbalance (QCM). Two copper wires were bonded on both sides of the QCM via the conductive adhesive as the main heat transfer components. The transmittal of the temperature and the fixation of sensitive cell were realized by the connection of electrode pins. Finite element analysis was carried out to theoretically validate the high-temperature transfer efficiency and good vibration characteristics of the structure with optimal dimensions. Gases with different DP were measured to evaluate the performance of the sensor. The accuracy of the DP sensor was found to be ±0.22 °C DP in the range of 5 °C DP ~16 °C DP by referencing to a commercial MICHELL S4000 DP meter.

Organic-inorganic hybrid layer-by-layer (LBL) composite structures can not only increase the strength and ductility of materials but also well disperse nanomaterials for better-conducting pathways. Here, we discovered the self-assembly process of an organic and silver (Ag) LBL hybrid structure having excellent sustainability during the long-term bending cycle. During the assembly process, the organic and Ag hybrid structure can be self-assembled into a layered structure. Unlike other conventional LBL fabrication processes, we applied the hydrogel scaffold of a biological polymer, which can spontaneously phase separate into an LBL structure in a water/alcohol solvent system. This new hydrogel-based Ag LBL patterns can successfully be printed on a flexible polyimide film without nozzle-clogging problem. Although these Ag LBL patterns cracked during the bending cycle, carbonized organic compounds between the Ag layers help to self-heal within few minutes at a low temperature (

Flexible piezoelectric devices are attractive for energy conversion and strain sensing applications, including in the area of wearable electronics and self-powered sensors. Here, we report a piezoelectric nanogenerator/strain sensor based on lead (II) iodide (PbI2) nanosheets with the shape of two-dimensional (2D) structures, of which the piezoelectricity is not affected by the number of layers. A typical 2D piezoelectric device fabricated with 3 layers PbI2 nanosheets has recorded peak values of open-circuit voltage, short-circuit current and loading power as 29.4 mV, 20 pA, and 0.12 pW, respectively, and also possess good charging and integration capabilities. Moreover, these devices can be applied as self-powered strain sensors, with high sensitivity and excellent stability. As such, this study provides new knowledge and strategy for flexible energy harvesting or strain sensing devices based on 2D structures.

We present thiopene-based devices fabricated via spin-coating and electropolymerization (EP) for usage in solar-powered, photocatalytic hydrogen gas (H2) harvesting. Two innovative claims are achieved in this work: (1) demonstration of electropolymerized photoelectrochemical (PEC) devices for water splitting, and (2) drastically improved performance of EP-PEC devices over spin-coated PEC hydrogen harvesters, achieving >0.5 V improvement in onset voltage (Von, bias voltage needed to produce photocurrent), with Von of 0 V vs. Ag/AgCl. As such, this work points to new opportunities for material and device fabrication for cheaper and efficient PEC hydrogen-harvesting systems.

This work utilizes an AC phase sensing approach of chemical vapors to achieve minimal baseline drift and fast recovery on graphene-based field effect transistors (FETs). Phase lag signals between channel resistance and gate voltage are detected with ultrafast recovery speed (∼10 s) on defect-rich FETs made of chemical vapor deposition (CVD) graphene as the channel materials without surface functionalization at room temperature. The responses of the phase change upon exposure to water, methanol and ethanol vapors show at least ten times faster recovery speed than those of the conventional DC resistance measurements with minimal baseline drift, large dynamic range, and good stability. The effects of relative humidity on methanol and ethanol gas response properties are also studied. As such, the AC phase sensing scheme could open up a new class of research in gas sensors for improved sensing speed and baseline stability.