Transparent, low-power pressure sensor matrix based on coplanar-gate graphene transistors

Controlled Growth of Single-Crystal Graphene Wafers on Twin-Boundary-Free Cu(111) Substrates

Single-crystal graphene (SCG) wafers are needed to enable mass-electronics and optoelectronics owing to their excellent properties and compatibility with silicon-based technology. Controlled synthesis of high-quality SCG wafers can be done exploiting single-crystal Cu(111) substrates as epitaxial growth substrates recently. However, current Cu(111) films prepared by magnetron sputtering on single-crystal sapphire wafers still suffer from in-plane twin boundaries, which degrade the SCG chemical vapor deposition. Here, it is shown how to eliminate twin boundaries on Cu and achieve 4 in. Cu(111) wafers with ≈95% crystallinity. The introduction of a temperature gradient on Cu films with designed texture during annealing drives abnormal grain growth across the whole Cu wafer. In-plane twin boundaries are eliminated via migration of out-of-plane grain boundaries. SCG wafers grown on the resulting single-crystal Cu(111) substrates exhibit improved crystallinity with >97% aligned graphene domains. As-synthesized SCG wafers exhibit an average carrier mobility up to 7284 cm2 V-1 s-1 at room temperature from 103 devices and a uniform sheet resistance with only 5% deviation in 4 in. region.

A Digital-Analog Bimodal Memristor Based on CsPbBr3 for Tactile Sensory Neuromorphic Computing

Memristor with digital and analog bipolar bimodal resistive switching offers a promising opportunity for the information-processing component. However, it still remains a huge challenge that the memristor enables bimodal digital and analog types and fabrication of artificial sensory neural network system. Here, a proposed CsPbBr3 -based memristor demonstrates a high ON/OFF ratio (>103 ), long retention (>104 s), stable endurance (100 cycles), and multilevel resistance memory, which acts as an artificial synapse to realize fundamental biological synaptic functions and neuromorphic computing based on controllable resistance modulation. Moreover, a 5 × 5 spinosum-structured piezoresistive sensor array (sensitivity of 22.4 kPa-1 , durability of 1.5 × 104 cycles, and fast response time of 2.43 ms) is constructed as a tactile sensory receptor to transform mechanical stimuli into electrical signals, which can be further processed by the CsPbBr3 -based memristor with synaptic plasticity. More importantly, this artificial sensory neural network system combined the artificial synapse with 5 × 5 tactile sensing array based on piezoresistive sensors can recognize the handwritten patterns of different letters with high accuracy of 94.44% under assistance of supervised learning. Consequently, the digital-analog bimodal memristor would demonstrate potential application in human-machine interaction, prosthetics, and artificial intelligence.

Carbon-Based Flexible Devices for Comprehensive Health Monitoring

Traditional public health systems suffer from incomprehensive, delayed, and inefficient medical services. Convenient and comprehensive health monitoring has been highly sought after recently. Flexible and wearable devices are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Using carbon materials with overall superiorities can facilitate the development of wearable and flexible devices with various functions and excellent performance, which can comprehensively and real-time monitor human health status and prevent diseases. Herein, the latest advances in the rational design and controlled fabrication of carbon materials for applications in health-related flexible and wearable electronics are reviewed. The fabrication strategies, working mechanism, performance, and applications in health monitoring of carbon-based flexible devices, including electromechanical sensors, temperature/humidity sensors, chemical sensors, and flexible conductive wires/electrodes, are reviewed. Furthermore, integrating multiple carbon-based devices into multifunctional wearable systems is discussed. Finally, the existing challenges and future opportunities in this field are also proposed.

Highly Stretchable Graphene Scrolls Transistors for Self-Powered Tribotronic Non-Mechanosensation Application

Owing to highly desired requirements in advanced disease diagnosis, therapy, and health monitoring, noncontact mechanosensation active matrix has drawn considerable attention. To satisfy the practical demands of high energy efficiency, in this report, combining the advantage of multiparameter monitoring, high sensitivity, and high resolution of active matrix field-effect transistor (FET) with triboelectric nanogenerators (TENG), we successfully developed the tribotronic mechanosensation active matrix based on tribotronic ion gel graphene scrolls field-effect transistors (GSFET). The tribopotential produced by TENG served as a gate voltage to modulate carrier transport along the semiconductor channel and realized self-powered ability with considerable decreased energy consumption. To achieve high spatial utilization and more pronounced responsivity of the dielectric of this transistor, ion gel was used to act as a triboelectric layer to conduct friction and contact electrification with external materials directly to produce triboelectric charges to power GFET. This tribopotential-driving device has excellent tactile sensing properties with high sensitivity (1.125 mm^-1), rapid response time (~16 ms), and a durability operation of thousands of cycles. Furthermore, the device was transparent and flexible with the capability of spatially mapping touch stimuli and monitoring real-time temperature. Due to all these unique characteristics, this novel noncontact mechanosensation GSFET active matrix provided a new method for self-powered E-skin with promising potential for self-powered wearable devices and intelligent robots.

Photoresponse Analysis of All-Inkjet-Printed Single-Walled Carbon Nanotube Thin-Film Transistors for Flexible Light-Insensitive Transparent Circuit Applications

We report daylight-stable, transparent, and flexible single-walled carbon nanotube thin-film transistors (SWCNT TFTs) using an all-inkjet printing process. Although most of the previous reports classified SWCNT TFTs as photodetectors, we demonstrated that SWCNT films actually show two different types of photoresponses depending on the power levels of light sources. The electrical characteristics of SWCNT TFTs show no significant change under daily illumination conditions such as halogen lamps and sunlight, while under high-power laser illumination, they change as reported in the previous results. In addition, the low-temperature solution process of the SWCNT with its one-dimensional nature allows us to realize highly transparent and flexible TFTs and logic circuits on plastic substrates. Our result will provide new insights into utilizing SWCNT TFTs for light-insensitive transparent and flexible electronic applications.

Large-Area 3D Printable Soft Electronic Skin for Biomedical Applications

Soft electronic skin (soft-e-skin) capable of sensing touch and pressure similar to human skin is essential in many applications, including robotics, healthcare, and augmented reality. However, most of the research effort on soft-e-skin was confined to the lab-scale demonstration. Several hurdles remain challenging, such as highly complex and expensive fabrication processes, instability in long-term use, and difficulty producing large areas and mass production. Here, we present a robust 3D printable large-area electronic skin made of a soft and resilient polymer capable of detecting touch and load, and bending with extreme sensitivity (up to 150 kPa^-1) to touch and load, 750 times higher than earlier work. The soft-e-skin shows excellent long-term stability and consistent performance up to almost a year. In addition, we describe a fabrication process capable of producing large areas and in large numbers, yet is cost-effective. The soft-e-skin consists of a uniquely designed optical waveguide and a layer of a soft membrane with an array of soft structures which work as passive sensing nodes. The use of a soft structure gives the liberty of stretching to the soft-e-skin without considering the disjoints among the sensing nodes. We have shown the functioning of the soft-e-skin under various conditions.

Bending-Insensitive Intrinsically Flexible Ultraviolet Encoding Devices Based on Piezoelectric Nanogenerator-Supplied Liquid Crystalline Polymer Fabrics

It is significantly challenging for state-of-the-art wearable electronics to stably monitor physicochemical signals under dynamic motions. Herein, a bending-insensitive, self-powered, and intrinsically flexible UV detector has been realized based on well-designed oriented composite fabrics, consisting of ionic liquid (IL)-containing liquid crystalline polymers (ILCPs) and piezoelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanogenerators. The novel composite fabrics establish effective UV illuminance-internal stress-electric signal conversion by coupling resistive and piezoelectric effects, with a fast response time of 190 ms. Particularly, benefiting from the intrinsic flexibility of composite fabrics, the ILCP/P(VDF-TrFE) device can maintain stable performance under dynamic bending even if the frequency is up to 2.5 Hz, with a bending insensitivity of less than 1% performance variation under 1.0 mW cm^-2 UV light. Combined with the Internet of Things and the American Standard Code for Information Interchange (ASCII), wearable encoding electronics have been successfully implemented with a printing speed of 3.2 s per character under dynamic bending.

Artificial Synapses Based on WSe2 Homojunction via Vacancy Migration

Artificial synapses based on two-dimensional (2D) transition metal dichalcogenides (TMDs) materials have attracted wide attention to boost the development of neuromorphic computing in recent years. Various structures have been adopted to build 2D-material-based artificial synapses. In lateral- and vertical-structures, the realization of synaptic function mainly results from the migration of the defects and vacancies, which requires the strong ion diffusion ability. Here, we successfully demonstrate an artificial synapse based on lateral WSe₂ homojunction. The migration of Se vacancies from the thin region to the thick region has been promoted by applying negative gate voltage, resulting in n-type doping in the thick region due to the accumulation of Se vacancies, which would diminish the barrier width of the metal-semiconductor junctions in the thick region. Consequently, the transformation from a high-resistance state (HRS) to a low-resistance state (LRS) is achieved. Significantly, our device can efficiently emulate the biological synaptic functions with a large synaptic weight change. Additionally, the transition from short-term memory (STM) to long-term memory (LTM) can be accomplished with a simpler structure, which would be beneficial to realizing the large-scale integration of transistor-based artificial synapses.

Low energy consumption fiber-type memristor array with integrated sensing-memory

The increasing growth of electronic information science and technology has triggered the renaissance of the artificial sensory nervous system (SNS), which can emulate the response of organisms towards external stimuli with high efficiency. In traditional SNS, the sensor units and the memory units are separated, and therefore difficult to miniaturize and integrate. Here, we have incorporated the sensor unit and the memory unit into one system, taking advantage of the unique properties of the ion-gel system. Meanwhile, the weaving-type memory array presents paramount advantages of integration and miniaturization and conformal lamination to curved surfaces. It is worth noting that the electrical double layer (EDL) within the ion gel endow the device with a low operation voltage (<1 V) to achieve low energy consumption. Finally, according to the relationship of pressure stimuli and electrical behavior, the integrated responsiveness-storage external stimuli ability is achieved. Our work offers a new platform for designing cutting edge SNS.

Spatiotemporal Measurement of Arterial Pulse Waves Enabled by Wearable Active-Matrix Pressure Sensor Arrays

Wearable pressure sensors have demonstrated great potential in detecting pulse pressure waves on the skin for the noninvasive and continuous diagnosis of cardiac conditions. However, difficulties lie in positioning conventional single-point sensors on an invisible arterial line, thereby preventing the detection of adequate signal amplitude for accurate pulse wave analysis. Herein, we introduce the spatiotemporal measurements of arterial pulse waves using wearable active-matrix pressure sensors to obtain optimal pulse waveforms. We fabricate thin-film transistor (TFT) arrays with high yield and uniformity using inkjet printing where array sizes can be customizable and integrate them with highly sensitive piezoresistive sheets. We maximize the pressure sensitivity (16.8 kPa^-1) and achieve low power consumption (10¹ nW) simultaneously by strategically modulating the TFT operation voltage. The sensor array creates a spatiotemporal pulse wave map on the wrist. The map presents the positional dependence of pulse amplitudes, which allows the positioning of the arterial line to accurately extract the augmentation index, a parameter for assessing arterial stiffness. The device overcomes the positional inaccuracy of conventional single-point sensors, and therefore, it can be used for medical applications such as arterial catheter injection or the diagnosis of cardiovascular disease in daily life.

Sensitive Micro-Breathing Sensing and Highly-Effective Photothermal Antibacterial Cinnamomum camphora Bark Micro-Structural Cotton Fabric via Electrostatic Self-Assembly of MXene/HACC

Natural fabrics are gradually becoming the ideal substrate for flexible smart wearable devices due to their excellent moisture absorption, softness, and skin-friendliness. However, the bonding fastness of the conductive layer and the corresponding durability during service have not yet been well satisfied. In this report, we successfully prepared a smart wearable multifunctional protective cotton fabric with microbreathing monitoring and rapid-photothermal antibacterial abilities of Cinnamomum camphora bark microstructure, by combining chitosan quaternary ammonium salt (HACC) with MXene nanosheets through electrostatic self-assembly. Impressively, MXene nanosheets and HACC established a strong interaction using the electrostatic attraction, endowing the fiber surface with ordered nanosheets. Meanwhile, the fabric decorated with MXene/HACC retains its original characteristics of outstanding breathability and softness, and its conductivity exhibits noticeable stability in terms of resistances to oxidation, washing, various solvents, and long-term bending cycles. On the basis of the principle of adsorption and release of water molecules in the MXene multilayer structures, the MXene/HACC fabric could accurately monitor the physiological health activities of users according to their breathing frequency and depth. Benefiting from the local surface plasmon resonance (LSPR) effect, the MXene/HACC shows encouraging photothermal conversion ability, photothermal stability under long time irradiation, washing resistance, and cycle stability. In addition, the fabric achieved an antibacterial efficiency of nearly 100% against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus within 5 min under an irradiation intensity of 400 mW/cm². More importantly, after 10 washes, the antibacterial efficiency against the two bacteria could still reach 99.975% and 99.98%, respectively. This multifunctional protective MXene/HACC cotton fabric is expected to play a unique role in the new generation of smart wearable microbreathing sensing and against to bacterial attack, and shows a broad application prospect.

Emerging Iontronic Sensing: Materials, Mechanisms, and Applications

Iontronic sensors represent a novel class of soft electronics which not only replicate the biomimetic structures and perception functions of human skin but also simulate the mechanical sensing mechanism. Relying on the similar mechanism with skin perception, the iontronic sensors can achieve ion migration/redistribution in response to external stimuli, promising iontronic sensing to establish more intelligent sensing interface for human-robotic interaction. Here, a comprehensive review on advanced technologies and diversified applications for the exploitation of iontronic sensors toward ionic skins and artificial intelligence is provided. By virtue of the excellent stretchability, high transparency, ultrahigh sensitivity, and mechanical conformality, numerous attempts have been made to explore various novel ionic materials to fabricate iontronic sensors with skin-like perceptive properties, such as self-healing and multimodal sensing. Moreover, to achieve multifunctional artificial skins and intelligent devices, various mechanisms based on iontronics have been investigated to satisfy multiple functions and human interactive experiences. Benefiting from the unique material property, diverse sensing mechanisms, and elaborate device structure, iontronic sensors have demonstrated a variety of applications toward ionic skins and artificial intelligence.

Transistor-Based Biomolecule Sensors: Recent Technological Advancements and Future Prospects

Transistor-based sensors have been widely recognized to be highly sensitive and reliable for point-of-care/bed-side diagnosis. In this line, a range of cutting-edge technologies has been generated to elevate the role of transistors for biomolecule detection. Detection of a wide range of clinical biomarkers has been reported using various configurations of transistors. The inordinate sensitivity of transistors to the field-effect imparts high sensitivity toward wide range of biomolecules. This overview has gleaned the present achievements with the technological advancements using high performance transistor-based sensors. This review encloses transistors incorporated with a variety of functional nanomaterials and organic elements for their excellence in selectivity and sensitivity. In addition, the technological advancements in fabrication of these microdevices or nanodevices and functionalization of the sensing elements have also been discussed. The technological gap in the realization of sensors in transistor platforms and the resulted scope for research has been discussed. Finally, foreseen technological advancements and future research perspectives are described.

Self-powered bifunctional sensor based on tribotronic planar graphene transistors

With the development of material science, micro-nano-fabrication and microelectronics, the higher level requirements are posed on the electronic skins (E-skin). The lower energy consumption and multiple functions are the imperative requirements to spurred scientists and mechanists to make joint efforts to meet. To achieve lower energy consumption, a promising energy-harvesting style of triboelectric nanogenerators (TENG) is incorporated into the field effect transistors (FETs) to play the important role for sensor. For bifunctional sensor, to harness the difficult for reflecting the magnitude of frequency, we resorted to synaptic transistors to achieve more intelligentization. Furthermore, with regards to the configuration of FET, we continued previous work: using the electrolyte gate dielectrics of FET-ion gel as the electrification layer to achieve high efficient, compact and extensively adoption for mechanosensation. The working principle of the GFET was based on the coupling effects of the FET and the TENG. This newly emerged self-powered sensor would offer a new platform for lower power consumption sensor for human-machine interface and intelligent robot.

2D Materials for Skin-Mountable Electronic Devices

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Highly Sensitive, Flexible, Stable, and Hydrophobic Biofoam Based on Wheat Flour for Multifunctional Sensor and Adjustable EMI Shielding Applications

Biofoam materials are attractive alternatives for petroleum-based foams to be used to solve environmental problems. Inspired by steamed bread, we report herein a novel utilization of wheat flour (WF) with the introduction of carbon nanotubes (CNTs) to form an environmentally friendly WF/CNT composite foam. This foam displayed a high elasticity (nearly 100% shape recovery), recyclable (5000 cycles), fast (100 ms), and superstability pressure-sensing response. It could serve as a new pressure sensor to detect the tiny pressure (1.76 Pa) and acoustic vibrations from piano notes. As an acoustic sensor, WF/CNT foam detected and recognized different volumes and frequencies of piano sounds. As an electromagnetic interference (EMI) shielding switch, the EMI shielding effectiveness (SE) of the foam could be easily regulated under self-fixable compression-recovery cycles. In addition, the WF/CNT foam could be converted into the WF/CNT film by a hot-compress process. This flexible film was applied as a multifunctional sensing device for detecting various motions. Therefore, wheat flour as a renewable resource could be designed into various WF-based biofoams with new functionalities and outstanding mechanical properties through a simple process.

Ionically Connected Floating Electrodes for Long-Distance (>1 mm) Coplanar-Gating Graphene Transistors

Exploiting the long-range polarizability of an electrolyte based on ion migration, electric double-layer transistors (EDLTs) can be constructed in an unconventional configuration; here, the gate electrode is placed coplanarly with the device channel. In this paper, we demonstrate the influence of the distance factors of the electrolyte layer on the operation of EDLTs with a coplanar gate. As the promptness of the electric double-layer formation depends on the distance between the channel and the gate, the dynamic characteristics of a remote-gated transistor degrade with long distances. To suppress this degradation, we suggest using multiple coplanar floating gates bridged through ionic dielectric layers. Unlike remotely gated EDLTs that utilize a single extended electrolyte layer, the devices with multiple segmented electrolyte layers operate effectively even when they are gated from a distance longer than 1 mm.

Paper-based triboelectric nanogenerators and their applications: a review

The development of industry and of the Internet of Things (IoTs) have brought energy issues and huge challenges to the environment. The emergence of triboelectric nanogenerators (TENGs) has attracted wide attention due to their advantages, such as self-powering, lightweight, and facile fabrication. Similarly to paper and other fiber-based materials, which are biocompatible, biodegradable, environmentally friendly, and are everywhere in daily life, paper-based TENGs (P-TENGs) have shown great potential for various energy harvesting and interactive applications. Here, a detailed summary of P-TENGs with two-dimensional patterns and three-dimensional structures is reported. P-TENGs have the potential to be used in many practical applications, including self-powered sensing devices, human-machine interaction, electrochemistry, and highly efficient energy harvesting devices. This leads to a simple yet effective way for the next generation of energy devices and paper electronics.

Graphene, an epoch-making material in RFID technology: a detailed overview

Many researchers claim that graphene, a specially studied carbon allotrope, has a single layer of atoms organized in a two-dimensional honeycomb lattice.

Graphene Based Low Voltage Field Effect Transistor Coupled with Biodegradable Piezoelectric Material Based Dynamic Pressure Sensor

Pressure sensors form the basic building block for realization of an electronic or tactile skin used in prothesis, robotics, and other similar applications. This paper presents a device consisting of biodegradable piezoelectric material based dynamic pressure sensor coupled with a graphene field-effect-transistor (GFET) operated at very low voltage (50 mV). The device has a biodegradable β-glycine/chitosan composite based metal-insulator-metal (MIM) structure connected with GFET in an extended gate configuration. The developed device shows a sensitivity of 2.70 × 10^-4 kPa^-1 for a pressure range of 5-20 kPa and 7.56 × 10^-4 kPa^-1 for a pressure range between 20 and 35 kPa. A distinctive feature of the presented device is its very low operation voltage, which offers a significant advantage toward the development of energy efficient large-area electronic skin. Further, the biodegradability of piezoelectric material makes the presented sensors useful in terms of reduced electronic waste, which is currently another growing area of interest.

Memristors Based on 2D Materials as an Artificial Synapse for Neuromorphic Electronics

The memristor, a composite word of memory and resistor, has become one of the most important electronic components for brain-inspired neuromorphic computing in recent years. This device has the ability to control resistance with multiple states by memorizing the history of previous electrical inputs, enabling it to mimic a biological synapse in the neural network of the human brain. Among many candidates for memristive materials, including metal oxides, organic materials, and low-dimensional nanomaterials, 2D layered materials have been widely investigated owing to their outstanding physical properties and electrical tunability, low-power-switching capability, and hetero-integration compatibility. Hence, a large number of experimental demonstrations on 2D material-based memristors have been reported showing their unique memristive characteristics and novel synaptic functionalities, distinct from traditional bulk-material-based systems. Herein, an overview of the latest advances in the structures, mechanisms, and memristive characteristics of 2D material-based memristors is presented. Additionally, novel strategies to modulate and enhance the synaptic functionalities of 2D-memristor-based artificial synapses are summarized. Finally, as a foreseeing perspective, the potentials and challenges of these emerging materials for future neuromorphic electronics are also discussed.

Facile Approach to Fabricating Stretchable Organic Transistors with Laser-Patterned Ag Nanowire Electrodes

Stretchable electronics are poised to revolutionize personal healthcare and robotics, where they enable distributed and conformal sensors. Transistors are fundamental building blocks of electronics, and there is a need to produce stretchable transistors using low-cost and scalable fabrication techniques. Here, we introduce a facile fabrication approach using laser patterning and transfer printing to achieve high-performance, solution-processed intrinsically stretchable organic thin-film transistors (OTFTs). The device consists of Ag nanowire (NW) electrodes, where the source and drain electrodes are patterned using laser ablation. The Ag NWs are then partially embedded in a poly(dimethylsiloxane) (PDMS) matrix. The electrodes are combined with a PDMS dielectric and polymer semiconductor, where the layers are individually transfer printed to complete the OTFT. Two polymer semiconductors, DPP-DTT and DPP-4T, are considered and show stable operation under the cyclic strain of 20 and 40%, respectively. The OTFTs maintain electrical performance by adopting a buckled structure after the first stretch-release cycle. The conformability and stretchability of the OTFT is also demonstrated by operating the transistor while adhered to a finger being flexed. The ability to pattern highly conductive Ag NW networks using laser ablation to pattern electrodes as well as interconnects provides a simple strategy to produce complex stretchable OTFT-based circuits.

Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications

In recent years, innovative research has been widely conducted on flexible devices for wearable electronics applications. Many examples of wearable electronics, such as smartwatches and glasses, are already available to consumers. However, strictly speaking, the sensors used in these devices are not flexible. Many studies are underway to address a wider range of wearable electronics and the development of related fields is progressing very rapidly. In particular, there is intense interest in the research field of flexible pressure sensors because they can collect and use information regarding a wide variety of sources. Through the combination of novel materials and fabrication methods, human-machine interfaces, biomedical sensors, and motion detection techniques, it is now possible to produce sensors with a superior level of performance to meet the demands of wearable electronics. In addition, more compact and human-friendly sensors have been invented in recent years, as biodegradable and self-powered sensor systems have been studied. In this review, a comprehensive description of flexible pressure sensors will be covered, and design strategies that meet the needs for applications in wearable electronics will be presented. Moreover, we will cover several fabrication methods to implement these technologies and the corresponding real-world applications.

Motion Detection Using Tactile Sensors Based on Pressure-Sensitive Transistor Arrays

In recent years, to develop more spontaneous and instant interfaces between a system and users, technology has evolved toward designing efficient and simple gesture recognition (GR) techniques. As a tool for acquiring human motion, a tactile sensor system, which converts the human touch signal into a single datum and executes a command by translating a bundle of data into a text language or triggering a preset sequence as a haptic motion, has been developed. The tactile sensor aims to collect comprehensive data on various motions, from the touch of a fingertip to large body movements. The sensor devices have different characteristics that are important for target applications. Furthermore, devices can be fabricated using various principles, and include piezoelectric, capacitive, piezoresistive, and field-effect transistor types, depending on the parameters to be achieved. Here, we introduce tactile sensors consisting of field-effect transistors (FETs). GR requires a process involving the acquisition of a large amount of data in an array rather than a single sensor, suggesting the importance of fabricating a tactile sensor as an array. In this case, an FET-type pressure sensor can exploit the advantages of active-matrix sensor arrays that allow high-array uniformity, high spatial contrast, and facile integration with electrical circuitry. We envision that tactile sensors based on FETs will be beneficial for GR as well as future applications, and these sensors will provide substantial opportunities for next-generation motion sensing systems.

Electron-Induced Perpendicular Graphene Sheets Embedded Porous Carbon Film for Flexible Touch Sensors

Graphene-based materials on wearable electronics and bendable displays have received considerable attention for the mechanical flexibility, superior electrical conductivity, and high surface area, which are proved to be one of the most promising candidates of stretching and wearable sensors. However, polarized electric charges need to overcome the barrier of graphene sheets to cross over flakes to penetrate into the electrode, as the graphene planes are usually parallel to the electrode surface. By introducing electron-induced perpendicular graphene (EIPG) electrodes incorporated with a stretchable dielectric layer, a flexible and stretchable touch sensor with "in-sheet-charges-transportation" is developed to lower the resistance of carrier movement. The electrode was fabricated with porous nanostructured architecture design to enable wider variety of dielectric constants of only 50-μm-thick Ecoflex layer, leading to fast response time of only 66 ms, as well as high sensitivities of 0.13 kPa-1 below 0.1 kPa and 4.41 MPa-1 above 10 kPa, respectively. Moreover, the capacitance-decrease phenomenon of capacitive sensor is explored to exhibit an object recognition function in one pixel without any other integrated sensor. This not only suggests promising applications of the EIPG electrode in flexible touch sensors but also provides a strategy for internet of things security functions.

Self-Powered Multifunctional Electronic Skin for a Smart Anti-Counterfeiting Signature System

Self-powered electronic skin is a promising field for human-machine interfaces to the next generation of intelligent and interactive products due to its capability of including multiple physical parameters for sensing without additional energy supply. This paper reports a novel active multifunctional electronic skin capable of independently detecting contact trajectory, acceleration, velocity, and pressure based on the synchronized triboelectrification and piezoelectric effect. Motion trajectories in the full plane can be identified by using a net-cross electrodes configuration design. Under this electrode special structure design, the motion information such as velocity and acceleration can be accurately obtained by the time difference between the peak values of the triboelectric signal. Real-time detection of dynamic pressure with only two electrodes is achieved by a spacer-grid design and a high quality piezoelectric nanofiber film. By virtue of its high sensitivity and precision, a smart anti-counterfeiting signature system (SASS) can be achieved by this self-powered multifunctional electronic skin with the capability of recognizing the writing habits of people within a 100 ms error for security. It is also a promising candidate in terms of human-machine interaction, cyber security, and so on.

Disruptive, Soft, Wearable Sensors

The wearable industry is on the rise, with a myriad of technical applications ranging from real-time health monitoring, the Internet of Things, and robotics, to name but a few. However, there is a saying "wearable is not wearable" because the current market-available wearable sensors are largely bulky and rigid, leading to uncomfortable wearing experience, motion artefacts, and poor data accuracy. This has aroused a world-wide intensive research quest for novel materials, with the aim of fabricating next-generation ultra-lightweight and soft wearable devices. Such disruptive second-skin-like biosensing technologies may enable a paradigm shift from current wearable 1.0 to future wearable 2.0 products. Here, the state-of-the-art progress made in the key phases for future wearable technology, namely, wear → sense → communicate → analyze → interpret → decide, is summarized. Without a doubt, materials innovation is the key, which is the main focus of the discussion. In addition, emphasis is also given to wearable energy, multicomponent integration, and wireless communication.

Molybdenum Disulfide Nanosheets Aligned Vertically on Carbonized Silk Fabric as Smart Textile for Wearable Pressure-Sensing and Energy Devices

Flexible electronics have gained considerable research concern due to their wide prospect for health monitoring, soft robotics, and artificial intelligence, wherein flexible pressure sensors are necessary components of wearable devices. It is well known that the synergistic functions and multiscale structures of hybrid materials exert tremendous effects on the performance of flexible devices. Herein, inspired by the unique structure of the faceplate of sunflowers, we construct a hierarchical structure by in situ grown vertically aligned molybdenum disulfide (MoS₂) nanosheets on carbonized silk fabric (MoS₂/CSilk), which is applied as the sensing material in flexible pressure sensors. The MoS₂/CSilk sensor displayed high sensitivity and good stability. We demonstrated its applications in monitoring subtle physiology signals, such as pulse wave and voice vibrations. In addition, it served as electrodes in lithium-ion batteries. The MoS₂/CSilk electrode delivered ultrahigh first-cycle discharge and charge capacities of 2895 and 1594 mA h g^-1, respectively. The MoS₂/CSilk electrode exhibited a high capacity of 810 mA h g^-1 with a CE close to 100% even after 300 cycles, suggesting good stability. The excellent overall performances are ascribed to the unique structure of the MoS₂/CSilk and the synergistic effect of CSilk and MoS₂. The concept and strategy of this work can be extended to the design and fabrication of other multifunctional devices.

Mechanoluminescent, Air-Dielectric MoS2 Transistors as Active-Matrix Pressure Sensors for Wide Detection Ranges from Footsteps to Cellular Motions

Tactile pressure sensors as flexible bioelectronic devices have been regarded as the key component for recently emerging applications in electronic skins, health-monitoring devices, or human-machine interfaces. However, their narrow range of sensible pressure and their difficulty in forming high integrations represent major limitations for various potential applications. Herein, we report fully integrated, active-matrix arrays of pressure-sensitive MoS₂ transistors with mechanoluminescent layers and air dielectrics for wide detectable range from footsteps to cellular motions. The inclusion of mechanoluminescent materials as well as air spaces can increase the sensitivity significantly over entire pressure regimes. In addition, the high integration capability of these active-matrix sensory circuitries can enhance their spatial resolution to the level sufficient to analyze the pressure distribution in a single cardiomyocyte. We envision that these wide-range pressure sensors will provide a new strategy toward next-generation electronics at biomachine interfaces to monitor various mechanical and biological phenomena at single-cell resolution.

Fully Transparent Gas Sensor Based on Carbon Nanotubes

In this paper, we demonstrate the feasibility of realization of transparent gas sensors based on carbon nanotubes (CNTs). Both sensing layer and electrodes consist of CNTs deposited by spray deposition. The transparent sensor—with a transmittance higher than 60% in both sensing layer and electrodes—is characterized towards NH₃ and CO₂ and compared with a reference sensor with the same active layer but evaporated Au electrodes. In particular, the sensitivity towards NH₃ is virtually identical for both reference and transparent sensors, whereas the transparent device exhibits higher sensitivity to CO₂ than the reference electrode. The effect of the spacing among consecutive electrodes is also studied, demonstrating that a wider spacing in fully CNT based sensors results in a higher sensitivity because of the higher sensing resistance, whereas this effect was not observed in gold electrodes, as their resistance can be neglected with respect to the resistance of the CNT sensing layer. Overall, the transparent sensors show performance comparable—if not superior—to the traditionally realized ones, opening the way for seamlessly integrated sensors, which do not compromise on quality.

Flexible Pressure-Sensitive Contact Transistors Operating in the Subthreshold Regime

Organic thin-film transistor (TFT)-based pressure sensors have received huge attention for wearable electronic applications such as health monitoring and smart robotics. However, there still remains a challenge to achieve low power consumption and high sensitivity at the same time for the realization of truly wearable sensor systems where minimizing power consumption is significant because of limited battery run time. Here, we introduce a flexible pressure-sensitive contact transistor (PCT), a new type of pressure-sensing device based on organic TFTs for next-generation wearable electronic skin devices. The PCT consists of deformable S/D electrodes integrated on a staggered TFT. The deformable S/D electrodes were fabricated by embedding conducting single-walled carbon nanotubes on the surface of microstructured polydimethylsiloxane. Under pressure loads, the deformation of the electrodes on an organic semiconductor layer leads modulation of drain current from variation in both the channel geometry and contact resistance. By strategic subthreshold operation to minimize power consumption and increase the dominance of contact resistance because of gated Schottky contact, the PCT achieves both ultralow power consumption (order of 10¹ nW) and high sensitivity (18.96 kPa^-1). Finally, we demonstrate a 5 × 5 active matrix PCT array on a 3 μm-thick parylene substrate. The device with ultralow power consumption and high sensitivity on a biocompatible flexible substrate makes the PCT promising candidate for next-generation wearable electronic skin devices.

High-k polymeric gate insulators for organic field-effect transistors

Gate insulators play a role as important as that of the semiconductor in high performance OFETs, with a high on/off current ratio, low hysteresis, and device stability. The essential requirements for gate dielectrics include high capacitance, high dielectric breakdown strength, solution-processibility, and flexibility. In this paper we review progress in recent years in developing high-k gate polymeric insulators for modern organic electronic applications. After a general introduction to OFETs, three types of high-k polymeric gate insulating materials are enumerated in achieving high-quality OFETs, including polymer gate insulators, polymer-inorganic gate composites or bilayers, and ion gel electrolytes. Especially, we emphasize the significance, implementation and development of high-k polymeric gate insulators used in OFETs for future low voltage operated and flexible electronics. Finally, a brief summary and outlook are presented.

Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications

Recently, graphene has been extensively researched in fundamental science and engineering fields and has been developed for various electronic applications in emerging technologies owing to its outstanding material properties, including superior electronic, thermal, optical and mechanical properties. Thus, graphene has enabled substantial progress in the development of the current electronic systems. Here, we introduce the most important electronic and thermal properties of graphene, including its high conductivity, quantum Hall effect, Dirac fermions, high Seebeck coefficient and thermoelectric effects. We also present up-to-date graphene-based applications: optical devices, electronic and thermal sensors, and energy management systems. These applications pave the way for advanced biomedical engineering, reliable human therapy, and environmental protection. In this review, we show that the development of graphene suggests substantial improvements in current electronic technologies and applications in healthcare systems.

High Sensitivity Flexible Electronic Skin Based on Graphene Film

Electronic skin with high sensitivity, rapid response, and long-term stability has great value in robotics, biomedicine, and in other fields. However, electronic skin still has challenges in terms of sensitivity and response time. In order to solve this problem, flexible electronic skin with high sensitivity and the fast response was proposed, based on piezoresistive graphene films. The electronic skin was a pressure sensor array, composed of a 4 × 4 tactile sensing unit. Each sensing unit contained three layers: The underlying substrate (polyimide substrate), the middle layer (graphene/polyethylene terephthalate film), and the upper substrate bump (polydimethylsiloxane). The results of the measurement and analysis experiments, designed in this paper, indicated that the flexible electronic skin achieved a positive resistance characteristic in the range of 0 kPa⁻600 kPa, a sensitivity of 10.80 Ω /kPa in the range of 0 kPa⁻4 kPa, a loading response time of 10 ms, and a spatial resolution of 5 mm. In addition, the electronic skin realized shape detection on a regular-shaped object, based on the change in the resistance value of each unit. The high sensitivity flexible electronic skin designed in this paper has important application prospects in medical diagnosis, artificial intelligence, and other fields.

Triboiontronic Transistor of MoS2

Electric double layers (EDLs) formed in electrolyte-gated field-effect transistors (FETs) induce an extremely large local electric field that gives a highly efficient charge carrier control in the semiconductor channel. To achieve highly efficient triboelectric potential gating on the FET and explore diversified applications of electric double layer FETs (EDL-FETs), a triboiontronic transistor is proposed to bridge triboelectric potential modulation and ion-controlled semiconductor devices. Utilizing the triboelectric potential instead of applying an external gate voltage, the triboiontronic MoS₂ transistor is efficiently operated owing to the formation of EDLs in the ion-gel dielectric layer. The operation mechanism of the triboiontronic transistor is proposed, and high current on/off ratio over 10⁷ , low threshold value (75 μm), and steep switching properties (20 µm dec^-1 ) are achieved. A triboiontronic logic inverter with desirable gain (8.3 V mm^-1 ), low power consumption, and high stability is also demonstrated. This work presents a low-power-consuming, active, and a general approach to efficiently modulate semiconductor devices through mechanical instructions, which has great potential in human-machine interaction, electronic skin, and intelligent wearable devices. The proposed triboiontronics utilize ion migration and arrangement triggered by mechanical stimuli to control electronic properties, which are ready to deliver new interdisciplinary research directions.

The Semiconductor/Conductor Interface Piezoresistive Effect in an Organic Transistor for Highly Sensitive Pressure Sensors

The piezoresistive pressure sensor, a kind of widely investigated artificial device to transfer force stimuli to electrical signals, generally consists of one or more kinds of conducting materials. Here, a highly sensitive pressure sensor based on the semiconductor/conductor interface piezoresistive effect is successfully demonstrated by using organic transistor geometry. Because of the efficient combination of the piezoresistive effect and field-effect modulation in a single sensor, this pressure sensor shows excellent performance, such as high sensitivity (514 kPa^-1 ), low limit of detection, short response and recovery time, and robust stability. More importantly, the unique gate modulation effect in the transistor endows the sensor with an unparalleled ability-tunable sensitivity via bias conditions in a single sensor, which is of great significance for applications in complex pressure environments. The novel working principle and high performance represent significant progress in the field of pressure sensors.

Touchpoint-Tailored Ultrasensitive Piezoresistive Pressure Sensors with a Broad Dynamic Response Range and Low Detection Limit

Wearable pressure sensors with high sensitivity, broad dynamic response range, and low detection limit are highly desirable to enable the applications in electronic skins and soft robotics. In this work, we report a high-performance wearable pressure sensor based on microstructured polydimethylsiloxane (PDMS)/Ag and rough polyimide/Au interdigital electrodes. By tailoring the touchpoints, the resulting pressure sensors show ultrahigh sensitivity (259.32 kPa^-1 in the range of 0-2.5 kPa), broad dynamic response range (0-54 kPa), fast response (∼200 μs), and low detection limit (0.36 Pa). Furthermore, the effect of different sensor structural configurations, PDMS geometrical feature, and Ag thickness on the performance of the pressure sensors are systematically investigated. Thanks to these merits, the fabricated pressure sensor is capable of real-time monitoring pulse wave and can act as artificial skin for robot hand to detect weak pressure changes, leading to the great application promise in the fields of biomedical, real-time health monitoring, and intelligent robot.

Gate-Free Hydrogel-Graphene Transistors as Underwater Microphones

A perfect impedance match from water-rich hydrogels to an oceanic background makes hydrogel microphones ideal for long-distance, underwater acoustic reception with zero reflection. A novel hydrogel-graphene transistor is thus designed to work under a gate-free mode, in which a sheet of graphene directly converts mechanical vibrations from a microstructured hydrogel into electrical current. This work shows that the quantum capacitance of graphene plays an important role in determining the shift of the Fermi level in graphene and subsequently the amplitude of the current signal. Once employed underwater, this device provides a response to sound waves with high stability, low noise, and high sensitivity in a much-needed low-frequency domain.

Paper-Based Supercapacitive Mechanical Sensors

Paper has been pursued as an interesting substrate material for sensors in applications such as microfluidics, bio-sensing of analytes and printed microelectronics. It offers advantages of being inexpensive, lightweight, environmentally friendly and easy to use. However, currently available paper-based mechanical sensors suffer from inadequate range and accuracy. Here, using the principle of supercapacitive sensing, we fabricate force sensors from paper with ultra-high sensitivity and unprecedented configurability. The high sensitivity comes from the sensitive dependence of a supercapacitor's response on the contact area between a deformable electrolyte and a pair of electrodes. As a key component, we develop highly deformable electrolytes by coating ionic gel on paper substrates which can be cut and shaped into complex three-dimensional geometries. Paper dissolves in the ionic gel after determining the shape of the electrolytes, leaving behind transparent electrolytes with micro-structured fissures responsible for their high deformability. Exploiting this simple paper-based fabrication process, we construct diverse sensors of different configurations that can measure not just force but also its normal and shear components. The new sensors have range and sensitivity several orders of magnitude higher than traditional MEMS capacitive sensors, in spite of their being easily fabricated from paper with no cleanroom facilities.

A Flexible Wearable Pressure Sensor with Bioinspired Microcrack and Interlocking for Full-Range Human-Machine Interfacing

Flexible wearable pressure sensors have drawn tremendous interest for various applications in wearable healthcare monitoring, disease diagnostics, and human-machine interaction. However, the limited sensing range (<10%), low sensing sensitivity at small strains, limited mechanical stability at high strains, and complicated fabrication process restrict the extensive applications of these sensors for ultrasensitive full-range healthcare monitoring. Herein, a flexible wearable pressure sensor is presented with a hierarchically microstructured framework combining microcrack and interlocking, bioinspired by the crack-shaped mechanosensory systems of spiders and the wing-locking sensing systems of beetles. The sensor exhibits wide full-range healthcare monitoring under strain deformations of 0.2-80%, fast response/recovery time (22 ms/20 ms), high sensitivity, the ultrasensitive loading sensing of a feather (25 mg), the potential to predict the health of patients with early-stage Parkinson's disease with the imitated static tremor, and excellent reproducibility over 10 000 cycles. Meanwhile, the sensor can be assembled as smart artificial electronic skins (E-skins) for simultaneously mapping the pressure distribution and shape of touching sensing. Furthermore, it can be attached onto the legs of a smart robot and coupled to a wireless transmitter for wirelessly monitoring human-motion interactivities.

Flexible and Highly Sensitive Resistive Pressure Sensor Based on Carbonized Crepe Paper with Corrugated Structure

Recently, cellulose paper based materials have emerged for applications in wearable "green" electronics due to their earth abundance, low cost, light weight, flexibility, and sustainability. Herein, for the first time, we develop an almost all cellulose paper based pressure sensor through a facile, cost-effective, scalable, and environment-friendly approach. The screen-printed interdigital electrodes on the flat printing paper and the carbonized crepe paper (CCP) with good conductivity are integrated into a flexible pressure sensor as substrates and active materials, respectively. The porous and corrugated structure of the CCP endows the pressure sensor with high sensitivity (2.56-5.67 kPa^-1 in the range of 0-2.53 kPa), wide workable pressure range (0-20 kPa), fast response time (<30 ms), low detection limit (∼0.9 Pa), and good durability (>3000 cycles). Additionally, we demonstrate the practical applications of the CCP pressure sensor in detection of finger touching, wrist pulse, respiration, phonation, acoustic vibration, etc., and real-time monitoring of spatial pressure distribution. The proposed CCP pressure sensor has great potentials in various applications as wearable electronics. Moreover, the subtle fabrication of the desired materials based on commercially available products provides new insights into the development of green electronics.

Mechanosensation-Active Matrix Based on Direct-Contact Tribotronic Planar Graphene Transistor Array

Mechanosensitive electronics aims at replicating the multifunctions of human skin to realize quantitative conversion of external stimuli into electronic signals and provide corresponding feedback instructions. Here, we report a mechanosensation-active matrix based on a direct-contact tribotronic planar graphene transistor array. Ion gel is utilized as both the dielectric in the graphene transistor and the friction layer for triboelectric potential coupling to achieve highly efficient gating and sensation properties. Different contact distances between the ion gel and other friction materials produce different triboelectric potentials, which are directly coupled to the graphene channel and lead to different output signals through modulating the Fermi level of graphene. Based on this mechanism, the tribotronic graphene transistor is capable of sensing approaching distances, recognizing the category of different materials, and even distinguishing voices. It possesses excellent sensing properties, including high sensitivity (0.16 mm^-1), fast response time (∼15 ms), and excellent durability (over 1000 cycles). Furthermore, the fabricated mechanosensation-active matrix is demonstrated to sense spatial contact distances and visualize a 2D color mapping of the target object. The tribotronic active matrix with ion gel as dielectric/friction layer provides a route for efficient and low-power-consuming mechanosensation in a noninvasive fashion. It is of great significance in multifunction sensory systems, wearable human-machine interactive interfaces, artificial electronic skin, and future telemedicine for patient surveillance.

Ultrasensitive, Low-Power Oxide Transistor-Based Mechanotransducer with Microstructured, Deformable Ionic Dielectrics

The development of a highly sensitive artificial mechanotransducer that mimics the tactile sensing features of human skin has been a big challenge in electronic skin research. Here, we demonstrate an ultrasensitive, low-power oxide transistor-based mechanotransducer modulated by microstructured, deformable ionic dielectrics, which is consistently sensitive to a wide range of pressures from 1 to 50 kPa. To this end, we designed a viscoporoelastic and ionic thermoplastic polyurethane (i-TPU) with micropyramidal feature as a pressure-sensitive gate dielectric for the indium-gallium-zinc-oxide (IGZO) transistor-based mechanotransducer, which leads to an unprecedented sensitivity of 43.6 kPa^-1, which is 23 times higher than that of a capacitive mechanotransducer. This is because the pressure-induced ion accumulation at the interface of the i-TPU dielectric and IGZO semiconductor effectively modulates the conducting channel, which contributed to the enhanced current level under pressure. We believe that the ionic transistor-type mechanotransducer suggested by us will be an effective way to perceive external tactile stimuli over a wide pressure range even under low power (<4 V), which might be one of the candidates to directly emulate the tactile sensing capability of human skin.

Highly Conductive, Photolithographically Patternable Ionogels for Flexible and Stretchable Electrochemical Devices

An ionic conducting membrane is an essential part in various electrochemical devices including ionic actuators. To miniaturize these devices, micropatterns of ionic conducting membrane are desired. Here, we present a novel type of ionogel that can be patterned using standard photolithography and soft imprinting lithography. The ionogel is prepared in situ by UV-initiated free-radical polymerization of thiol acrylate precursors in the presence of ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The resultant ionogel is very flexible with a low Young's modulus (as low as 0.23 MPa) and shows a very high ionic conductivity (up to 2.4 × 10^-3 S/cm with 75 wt % ionic liquid incorporated) and has a reactive surface due to the excess thiol groups. Micropatterns of ionogel are obtained by using the thiol acrylate ionogel solution as an ionic conducting photoresist with standard photolithography. Water, a solvent immiscible with ionic liquid, is used as the photoresist developer to avoid complete removal of ionic liquid from thin micropatterns of the ionogel. By taking advantage of the reactive surface of ionogels and the photopatternability, ionogels with complex three-dimensional microstructure are developed. The surface of the ionogels can also be easily patterned using UV-assisted soft imprinting lithography. This new type of ionogels may open up for building high-performance flexible electrochemical microdevices.

Flexible hemispheric microarrays of highly pressure-sensitive sensors based on breath figure method

Recently, flexible pressure sensors featuring high sensitivity, broad sensing range and real-time detection have aroused great attention owing to their crucial role in the development of artificial intelligent devices and healthcare systems. Herein, highly sensitive pressure sensors based on hemisphere-microarray flexible substrates are fabricated via inversely templating honeycomb structures deriving from a facile and static breath figure process. The interlocked and subtle microstructures greatly improve the sensing characteristics and compressibility of the as-prepared pressure sensor, endowing it a sensitivity as high as 196 kPa-1 and a wide pressure sensing range (0-100 kPa), as well as other superior performance, including a lower detection limit of 0.5 Pa, fast response time (<26 ms) and high reversibility (>10 000 cycles). Based on the outstanding sensing performance, the potential capability of our pressure sensor in capturing physiological information and recognizing speech signals has been demonstrated, indicating promising application in wearable and intelligent electronics.

Silver Nanoflower Decorated Graphene Oxide Sponges for Highly Sensitive Variable Stiffness Stress Sensors

Soft conductive materials should enable large deformation while keeping high electrical conductivity and elasticity. The graphene oxide (GO)-based sponge is a potential candidate to endow large deformation. However, it typically exhibits low conductivity and elasticity. Here, the highly conductive and elastic sponge composed of GO, flower-shaped silver nanoparticles (AgNFs), and polyimide (GO-AgNF-PI sponge) are demonstrated. The average pore size and porosity are 114 µm and 94.7%, respectively. Ag NFs have thin petals (8-20 nm) protruding out of the surface of a spherical bud (300-350 nm) significantly enhancing the specific surface area (2.83 m² g^-1 ). The electrical conductivity (0.306 S m^-1 at 0% strain) of the GO-AgNF-PI sponge is increased by more than an order of magnitude with the addition of Ag NFs. A nearly perfect elasticity is obtained over a wide compressive strain range (0-90%). The strain-dependent, nonlinear variation of Young's modulus of the sponge provides a unique opportunity as a variable stiffness stress sensor that operates over a wide stress range (0-10 kPa) with a high maximum sensitivity (0.572 kPa^-1 ). It allows grasping of a soft rose and a hard bottle, with the minimal object deformation, when attached on the finger of a robot gripper.

Stretchable Triboelectric-Photonic Smart Skin for Tactile and Gesture Sensing

Smart skin is expected to be stretchable and tactile for bionic robots as the medium with the ambient environment. Here, a stretchable triboelectric-photonic smart skin (STPS) is reported that enables multidimensional tactile and gesture sensing for a robotic hand. With a grating-structured metal film as the bioinspired skin stripe, the STPS exhibits a tunable aggregation-induced emission in a lateral tensile range of 0-160%. Moreover, the STPS can be used as a triboelectric nanogenerator for vertical pressure sensing with a maximum sensitivity of 34 mV Pa^-1 . The pressure sensing characteristics can remain stable in different stretching conditions, which demonstrates a synchronous and independent sensing property for external stimuli with great durability. By integrating on a robotic hand as a conformal covering, the STPS shows multidimensional mechanical sensing abilities for external touch and different gestures with joints bending. This work has first demonstrated a triboelectric-photonic coupled multifunctional sensing terminal, which may have great applications in human-machine interaction, soft robots, and artificial intelligence.

Flexible Highly Sensitive Pressure Sensor Based on Ionic Liquid Gel Film

Flexible, semitransparent ionic liquid gel (ionogels) film was first fabricated by in situ polymerization. The optimized ionogels exhibited excellent mechanical properties, high conductivity, and force sensing characteristics. The multifunctional sensor based on the ionogel film was constructed and provided the high sensitivity of 15.4 kPa^-1 and wide detection range sensing from 5 Pa to 5 kPa. Moreover, the aforementioned sensor demonstrated excellent mechanical stability against repeated external deformations (for 3000 cycles under 90° bending). Importantly, the sensor showed advantages in detection of environmental changes to the external stimulus of subtle signals, including a rubber blower blowing the sensor, gently touching, torsion, and bending.

Direct Printing of Stretchable Elastomers for Highly Sensitive Capillary Pressure Sensors

We demonstrate the successful fabrication of highly sensitive capillary pressure sensors using an innovative 3D printing method. Unlike conventional capacitive pressure sensors where the capacitance changes were due to the pressure-induced interspace variations between the parallel plate electrodes, in our capillary sensors the capacitance was determined by the extrusion and extraction of liquid medium and consequent changes of dielectric constants. Significant pressure sensitivity advances up to 547.9 KPa⁻¹ were achieved. Moreover, we suggest that our innovative capillary pressure sensors can adopt a wide range of liquid mediums, such as ethanol, deionized water, and their mixtures. The devices also showed stable performances upon repeated pressing cycles. The direct and versatile printing method combined with the significant performance advances are expected to find important applications in future stretchable and wearable electronics.