Electronic Skin for Closed-Loop Systems

Fibres-threads of intelligence-enable a new generation of wearable systems

Fabrics represent a unique platform for seamlessly integrating electronics into everyday experiences. The advancements in functionalizing fabrics at both the single fibre level and within constructed fabrics have fundamentally altered their utility. The revolution in materials, structures, and functionality at the fibre level enables intimate and imperceptible integration, rapidly transforming fibres and fabrics into next-generation wearable devices and systems. In this review, we explore recent scientific and technological breakthroughs in smart fibre-enabled fabrics. We examine common challenges and bottlenecks in fibre materials, physics, chemistry, fabrication strategies, and applications that shape the future of wearable electronics. We propose a closed-loop smart fibre-enabled fabric ecosystem encompassing proactive sensing, interactive communication, data storage and processing, real-time feedback, and energy storage and harvesting, intended to tackle significant challenges in wearable technology. Finally, we envision computing fabrics as sophisticated wearable platforms with system-level attributes for data management, machine learning, artificial intelligence, and closed-loop intelligent networks.

Skin Comfort Sensation with Mechanical Stimulus from Electronic Skin

The field of electronic skin has received considerable attention due to its extensive potential applications in areas including tactile sensing and health monitoring. With the development of electronic skin devices, electronic skin can be attached to the surface of human skin for long-term health monitoring, which makes comfort an essential factor that cannot be ignored in the design of electronic skin. Therefore, this paper proposes an assessment method for evaluating the comfort of electronic skin based on neurodynamic analysis. The holistic analysis framework encompasses the mechanical model of the skin, the modified Hodgkin-Huxley model for the transduction of stimuli, and the gate control theory for the modulation and perception of pain sensation. The complete process, from mechanical stimulus to the generation of pain perception, is demonstrated. Furthermore, the influence of different factors on pain perception is investigated. Sensation and comfort diagrams are provided to assess the mechanical comfort of electronic skin. The comfort assessment method proposed in this paper provides a theoretical basis when assessing the comfort of electronic skin.

Humidity Adaptive Antifreeze Hydrogel Sensor for Intelligent Control and Human-Computer Interaction

Conductive hydrogels have emerged as ideal candidate materials for strain sensors due to their signal transduction capability and tissue-like flexibility, resembling human tissues. However, due to the presence of water molecules, hydrogels can experience dehydration and low-temperature freezing, which greatly limits the application scope as sensors. In this study, an ionic co-hybrid hydrogel called PBLL is proposed, which utilizes the amphoteric ion betaine hydrochloride (BH) in conjunction with hydrated lithium chloride (LiCl) thereby achieving the function of humidity adaptive. PBLL hydrogel retains water at low humidity (<50%) and absorbs water from air at high humidity (>50%) over the 17 days of testing. Remarkably, the PBLL hydrogel also exhibits strong anti-freezing properties (-80 °C), high conductivity (8.18 S m-1 at room temperature, 1.9 S m-1 at -80 °C), high gauge factor (GF approaching 5.1). Additionally, PBLL hydrogels exhibit strong inhibitory effects against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), as well as biocompatibility. By synergistically integrating PBLL hydrogel with wireless transmission and Internet of Things (IoT) technologies, this study has accomplished real-time human-computer interaction systems for sports training and rehabilitation evaluation. PBLL hydrogel exhibits significant potential in the fields of medical rehabilitation, artificial intelligence (AI), and the Internet of Things (IoT).

Highly Stretchable Double Network Ionogels for Monitoring Physiological Signals and Detecting Sign Language

At the heart of the non-implantable electronic revolution lies ionogels, which are remarkably conductive, thermally stable, and even antimicrobial materials. Yet, their potential has been hindered by poor mechanical properties. Herein, a double network (DN) ionogel crafted from 1-Ethyl-3-methylimidazolium chloride ([Emim]Cl), acrylamide (AM), and polyvinyl alcohol (PVA) was constructed. Tensile strength, fracture elongation, and conductivity can be adjusted across a wide range, enabling researchers to fabricate the material to meet specific needs. With adjustable mechanical properties, such as tensile strength (0.06-5.30 MPa) and fracture elongation (363-1373%), this ionogel possesses both robustness and flexibility. This ionogel exhibits a bi-modal response to temperature and strain, making it an ideal candidate for strain sensor applications. It also functions as a flexible strain sensor that can detect physiological signals in real time, opening doors to personalized health monitoring and disease management. Moreover, these gels' ability to decode the intricate movements of sign language paves the way for improved communication accessibility for the deaf and hard-of-hearing community. This DN ionogel lays the foundation for a future in which e-skins and wearable sensors will seamlessly integrate into our lives, revolutionizing healthcare, human-machine interaction, and beyond.

A Skin-Inspired Self-Adaptive System for Temperature Control During Dynamic Wound Healing

The thermoregulating function of skin that is capable of maintaining body temperature within a thermostatic state is critical. However, patients suffering from skin damage are struggling with the surrounding scene and situational awareness. Here, we report an interactive self-regulation electronic system by mimicking the human thermos-reception system. The skin-inspired self-adaptive system is composed of two highly sensitive thermistors (thermal-response composite materials), and a low-power temperature control unit (Laser-induced graphene array). The biomimetic skin can realize self-adjusting in the range of 35-42 °C, which is around physiological temperature. This thermoregulation system also contributed to skin barrier formation and wound healing. Across wound models, the treatment group healed ~ 10% more rapidly compared with the control group, and showed reduced inflammation, thus enhancing skin tissue regeneration. The skin-inspired self-adaptive system holds substantial promise for next-generation robotic and medical devices.

Electronic Skin Based on Polydopamine-Modified Superelastic Fibers with Superior Conductivity and Durability

Owing to their excellent elasticities and adaptability as sensing materials, ionic hydrogels exhibit significant promise in the field of intelligent wearable devices. Nonetheless, molecular chains within the polymer network of hydrogels are susceptible to damage, leading to crack extension. Hence, we drew inspiration from the composite structure of the human dermis to engineer a composite hydrogel, incorporating dopamine-modified elastic fibers as a reinforcement. This approach mitigates crack expansion and augments sensor sensitivity by fostering intermolecular forces between the dopamine on the fibers, the hydrogel backbone, and water molecules. The design of this composite hydrogel elevates its breaking tensile capacity from 35 KJ to 203 KJ, significantly enhancing the fatigue resistance of the hydrogel. Remarkably, its electrical properties endure stability even after 2000 cycles of testing, and it manifests heightened sensitivity compared to conventional hydrogel configurations. This investigation unveils a novel method for crafting composite-structured hydrogels.

Thermoelectric Hydrogel Electronic Skin for Passive Multimodal Physiological Perception

Electronic skins (e-skins) are being extensively researched for their ability to recognize physiological data and deliver feedback via electrical signals. However, their wide range of applications is frequently restricted by the indispensableness of external power supplies and single sensory function. Here, we report a passive multimodal e-skin for real-time human health assessment based on a thermoelectric hydrogel. The hydrogel network consists of poly(vinyl alcohol)/low acyl gellan gum with [Fe(CN)6]4-/3- as the redox couple. The introduction of glycerol and Li+ furnishes the gel-based e-skin with antidrying and antifreezing properties, a thermopower of 2.04 mV K-1, fast self-healing in less than 10 min, and high conductivity of 2.56 S m-1. As a prospective application, the e-skin can actively perceive multimodal physiological signals without the need for decoupling, including body temperature, pulse rate, and sweat content, in real time by synergistically coupling sensing and transduction. This work offers a scientific basis and designs an approach to develop passive multimodal e-skins and promotes the application of wearable electronics in advanced intelligent medicine.

Flexible Ag2Se Thermoelectric Films Enable the Multifunctional Thermal Perception in Electronic Skins

Skin is critical for shaping our interactions with the environment. The electronic skin (E-skin) has emerged as a promising interface for medical devices to replicate the functions of damaged skin. However, exploration of thermal perception, which is crucial for physiological sensing, has been limited. In this work, a multifunctional E-skin based on flexible thermoelectric Ag2Se films is proposed, which utilizes the Seebeck effect to replicate the sensory functions of natural skin. The E-skin can enable capabilities including temperature perception, tactile perception, contactless perception, and material recognition by analyzing the thermal conduction behaviors of various materials. To further validate the capabilities of constructed E-skins, a wearable device with multiple sensory channels was fabricated and tested for gesture recognition. This work highlights the potential for using flexible thermoelectric materials in advanced biomedical applications including health monitoring and smart prosthetics.

Soft Bioelectronics for Therapeutics

Soft bioelectronics play an increasingly crucial role in high-precision therapeutics due to their softness, biocompatibility, clinical accuracy, long-term stability, and patient-friendliness. In this review, we provide a comprehensive overview of the latest representative therapeutic applications of advanced soft bioelectronics, ranging from wearable therapeutics for skin wounds, diabetes, ophthalmic diseases, muscle disorders, and other diseases to implantable therapeutics against complex diseases, such as cardiac arrhythmias, cancer, neurological diseases, and others. We also highlight key challenges and opportunities for future clinical translation and commercialization of soft therapeutic bioelectronics toward personalized medicine.

Recent progress in construction methods and applications of perovskite photodetector arrays

Metal halide perovskites are considered promising materials for next-generation optoelectronic devices due to their excellent optoelectronic performances and simple solution preparation process. Precise micro/nano-scale patterning techniques enable perovskite materials to be used for array integration of photodetectors. In this review, the device types of perovskite-based photodetectors are introduced and the structural characteristics and corresponding device performances are analyzed. Then, the typical construction methods suitable for the fabrication of perovskite photodetector arrays are highlighted, including surface treatment technology, template-assisted construction, inkjet printing technology, and modified photolithography. Furthermore, the current development trends and their applications in image sensing of perovskite photodetector arrays are summarized. Finally, major challenges are presented to guide the development of perovskite photodetector arrays.

Integrated intelligent tactile system for a humanoid robot

Tactile perception is the basis of human motion. Achieving artificial tactility is one of the challenges in the fields of smart robotics and artificial intelligence (AI), because touch emulation relies on high-performance pressure sensor arrays, signal reading, information processing, and feedback control. In this paper, we report an integrated intelligent tactile system (IITS) that is integrated with a humanoid robot to achieve human-like artificial tactile perception. The IITS is a closed-loop system that includes a multi-channel tactile sensing e-skin, a data acquisition and information processing chip, and a feedback control. With customized preset values of threshold pressures, the IITS-integrated robot can flexibly grasp various objects. The IITS has potential applications in the design of prosthetic hands, space manipulators, deep-sea exploration robots, and human-robot interactions.

Silicone-Based Multifunctional Thin Films with Improved Triboelectric and Sensing Performances via Chemically Interfacial Modification

The development of triboelectric nanogenerators (TENGs) technology has advanced in recent years. However, TENG performance is affected by the screened-out surface charge density owing to the abundant free electrons and physical adhesion at the electrode-tribomaterial interface. Furthermore, the demand for flexible and soft electrodes is higher than that for stiff electrodes for patchable nanogenerators. This study introduces a chemically cross-linked (XL) graphene-based electrode with a silicone elastomer using hydrolyzed 3-aminopropylenetriethoxysilanes. The conductive graphene-based multilayered electrode was successfully assembled on a modified silicone elastomer using a cheap and eco-friendly layer-by-layer assembly method. As a proof-of-concept, the droplet-driven TENG with the chemically XL electrode of silicone elastomer exhibited an output power of approximately 2-fold improvement owing to its higher surface charge density than without XL. This chemically XL electrode of silicone elastomer film demonstrated remarkable stability and resistance to repeated mechanical deformations like bending and stretching. Moreover, due to the chemical XL effects, it was used as a strain sensor to detect subtle motions and exhibited high sensitivity. Thus, this cheap, convenient, and sustainable design approach can provide a platform for future multifunctional wearable electronic devices.

Advances in Biodegradable Electronic Skin: Material Progress and Recent Applications in Sensing, Robotics, and Human-Machine Interfaces

The rapid growth of the electronics industry and proliferation of electronic materials and telecommunications technologies has led to the release of a massive amount of untreated electronic waste (e-waste) into the environment. Consequently, catastrophic environmental damage at the microbiome level and serious human health diseases threaten the natural fate of the planet. Currently, the demand for wearable electronics for applications in personalized medicine, electronic skins (e-skins), and health monitoring is substantial and growing. Therefore, "green" characteristics such as biodegradability, self-healing, and biocompatibility ensure the future application of wearable electronics and e-skins in biomedical engineering and bioanalytical sciences. Leveraging the biodegradability, sustainability, and biocompatibility of natural materials will dramatically influence the fabrication of environmentally friendly e-skins and wearable electronics. Here, the molecular and structural characteristics of biological skins and artificial e-skins are discussed. The focus then turns to the biodegradable materials, including natural and synthetic-polymer-based materials, and their recent applications in the development of biodegradable e-skin in wearable sensors, robotics, and human-machine interfaces (HMIs). Finally, the main challenges and outlook regarding the preparation and application of biodegradable e-skins are critically discussed in a near-future scenario, which is expected to lead to the next generation of biodegradable e-skins.

From Materials to Devices: Graphene toward Practical Applications

Graphene, as an emerging 2D material, has been playing an important role in flexible electronics since its discovery in 2004. The representative fabrication methods of graphene include mechanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, redox reaction, etc. Based on its excellent mechanical, electrical, thermo-acoustical, optical, and other properties, graphene has made a great progress in the development of mechanical sensors, microphone, sound source, electrophysiological detection, solar cells, synaptic transistors, light-emitting devices, and so on. In different application fields, large-scale, low-cost, high-quality, and excellent performance are important factors that limit the industrialization development of graphene. Therefore, laser scribing technology, roll-to-roll technology is used to reduce the cost. High-quality graphene can be obtained through chemical vapor deposition processes. The performance can be improved through the design of structure of the devices, and the homogeneity and stability of devices can be achieved by mechanized machining means. In total, graphene devices show promising prospect for the practical fields of sports monitoring, health detection, voice recognition, energy, etc. There is a hot issue for industry to create and maintain the market competitiveness of graphene products through increasing its versatility and killer application fields.

Vanadium Dioxide Nanosheets Supported on Carbonized Cotton Fabric as Bifunctional Textiles for Flexible Pressure Sensors and Zinc-Ion Batteries

Flexible pressure sensors and aqueous batteries have been widely used in the rapid development of wearable electronics. The synergistic functionalities of versatile materials with multidimensional architectures are recognized to have a significant impact on the performance of flexible electronics. Herein, a facile hydrothermal strategy was demonstrated to conformally grow vanadium dioxide nanosheets on carbonized cotton fabrics (VO₂/CCotton), which is a candidate material used in flexible piezoresistive sensors. As a result, the VO₂/CCotton-based pressure sensor behaved with high sensitivity (S = 7.12 kPa^-1 in the pressure range of 0-2.0 kPa) and a stable sensing ability in a wide pressure scale of 0-120 kPa. Further practical applications were performed in monitoring delicate physiological signals as well, such as twisting, blowing, and voice vibration recognitions. In addition, another application for energy storage was investigated as well. A quasi-solid-state aqueous zinc-ion battery was assembled with VO₂/CCotton as the cathode and a film of Zn nanosheets/carbon nanotube as the anode. A capacity as high as 301.5 mAh g^-1 and remarkable durability of 88.7% capacity retention after 5000 cycles at 10 A g^-1 were found. These exceptional outcomes are attributed to the unique three-dimensional architecture and the prominent synergetic effects of CCotton and VO₂ and allow for the proposal of novel guidelines for next-generation multifunctional flexible electronics.

Fully Fibrous Large-Area Tailorable Triboelectric Nanogenerator Based on Solution Blow Spinning Technology for Energy Harvesting and Self-Powered Sensing

An all-fibrous large-area (20 × 50 cm² ) tailorable triboelectric nanogenerator (LT-TENG) is prepared using a one-step solution blow spinning technology, which has the advantages of easy operation, scale-up in the area, and high production efficiency. The prepared LT-TENG is composed of polyvinylidene fluoride (PVDF)/MXene (Ti₃ C₂ T_x ) nanofibers (NFs) and conductive textile. Benefiting from the fibrous materials and large-area properties, the LT-TENG possesses the merits of good tailorability, breathability, hydrophobicity, and washability. When optimized by mixing the MXene into PVDF NFs, the LT-TENG has a preferable output and sensing property, with a detection range over 16 kPa and a relatively high sensitivity of 12.33 V KPa^-1 . At maximum applied pressure, the voltage, current, and charge are 108 V, 38 µA, and 35 nC, respectively. This LT-TENG can serve as a biomechanical energy harvester when used as wearable devices with an output power density of 12.6 mW m^-2 at an external load resistance of 500 MΩ, and it also has the ability of self-powered tactile sensing for pressure mapping and slide sensing. Thus, this LT-TENG exhibits great potential prospects in wearable devices, intelligent robots, and human-machine interaction.

Breathable Electronic Skins for Daily Physiological Signal Monitoring

With the aging of society and the increase in people's concern for personal health, long-term physiological signal monitoring in daily life is in demand. In recent years, electronic skin (e-skin) for daily health monitoring applications has achieved rapid development due to its advantages in high-quality physiological signals monitoring and suitability for system integrations. Among them, the breathable e-skin has developed rapidly in recent years because it adapts to the long-term and high-comfort wear requirements of monitoring physiological signals in daily life. In this review, the recent achievements of breathable e-skins for daily physiological monitoring are systematically introduced and discussed. By dividing them into breathable e-skin electrodes, breathable e-skin sensors, and breathable e-skin systems, we sort out their design ideas, manufacturing processes, performances, and applications and show their advantages in long-term physiological signal monitoring in daily life. In addition, the development directions and challenges of the breathable e-skin are discussed and prospected.

Current state and future prospects of sensors for evaluating polymer biodegradability and sensors made from biodegradable polymers: A review

Since the invention of fully synthetic plastic in the 1900s, plastics have been extensively applied in various fields and represent a significant market due to their satisfactory properties. However, the non-biodegradable nature of most plastics has contributed to the accumulation of plastic waste, which poses a threat to both the environment and living beings. Given this, biodegradable polymers have emerged as eco-friendly substitutes for non-biodegradable polymers, and standard test methods have been established to evaluate polymer biodegradability. Technological advancement and the weaknesses of conventional test methods drive the invention of sensors that enable real-time monitoring of biodegradability. Besides, biodegradable polymers have been utilized to make sensors with different functionalities. Given this, the current paper is the first to compare and contrast sensors capable of identifying biodegradable polymers. The detection using sensors represents an innovative perspective for real-time monitoring of biodegradability. Besides, sensors made from biodegradable polymers are included, and these sensors are of different types and show various applications. Finally, the challenges associated with developing these sensors are described to advance future research.

Layered MXene/Aramid Composite Film for a Soft and Sensitive Pressure Sensor

In recent years, the two-dimensional material MXene has shown great advantages in the field of wearable electronics and pressure sensors. Toward advanced applications, achieving a conformal pressure sensor with ultrathin thickness and great flexibility through a simple preparation principle, while maintaining its high sensitivity and wide detection range, is still a key challenge for the development of high-performance pressure sensors. Herein, we proposed an optimized mild LiF/HCl etching scheme and successfully achieved a high-concentration (>25 mg/mL) preparation of few-layer Ti₃C₂T_x MXene. Combining the prepared MXene with an aramid nanofiber (ANF), we designed an ultrathin layered pressure sensor based on an MXene/ANF composite through layer-by-layer suction filtration. The mechanical strength is greatly enhanced by composition with the ANF, while the pure MXene film is fragile. The sensor achieves a high sensitivity of 16.7 kPa^-1, wide detection range (>100 kPa), only 10 μm thickness, great flexibility, and up to 10% stretchability, which are greatly beneficial to practical sensors. We demonstrated the wide application perspective of the sensor in human motion monitoring and human-machine interfaces from low pressure (human pulse) to high pressure (push-up).

Research Progress on Hydrogel-Elastomer Adhesion

Hydrophilic hydrogels exhibit good mechanical properties and biocompatibility, whereas hydrophobic elastomers show excellent stability, mechanical firmness, and waterproofing in various environments. Hydrogel-elastomer hybrid material devices show varied application prospects in the field of bioelectronics. In this paper, the research progress in hydrogel-elastomer adhesion in recent years, including the hydrogel-elastomer adhesion mechanism, adhesion method, and applications in the bioelectronics field, is reviewed. Finally, the research status of adhesion between hydrogels and elastomers is presented.

Self-Powered Multifunction Ionic Skins Based on Gradient Polyelectrolyte Hydrogels

Human skin is the largest organ, and it can transform multiple external stimuli into the biopotential signals by virtue of ions as information carriers. Ionic skins (i-skins) that can mimic human skin have been extensively explored; however, the limited sensing capacities as well as the need of an extra power supply significantly restrict their broad applications. Herein, we develop self-powered humanlike i-skins based on gradient polyelectrolyte membranes (GPMs) that can directly and accurately perceive multiple stimuli. Prepared by a hydrogel-assisted reaction-diffusion method, the GPMs exhibit gradient-distributed charged groups across polymer networks, enabling one to generate a thickness-dependent and thermoresponsive self-induced potential in a hydrated situation and in a humidity-sensitive self-induced potential in a dehydrated/dried situation, respectively. Consequently, the GPM-based i-skins can precisely detect pressure, temperature, and humidity in a self-powered manner. The coupling of mechano-electric and thermo-electric effects inherent in GPMs provides a general strategy for developing innovative self-powered ion-based perception systems.

Bimodal Tactile Sensor without Signal Fusion for User-Interactive Applications

Tactile sensors with multimode sensing ability are cornerstones of artificial skin for applications in humanoid robotics and smart prosthetics. However, the intuitive and interference-free reading of multiple tactile signals without involving complex algorithms and calculations remains a challenge. Herein a pressure-temperature bimodal tactile sensor without any interference is demonstrated by combining the fundamentally different sensing mechanisms of optics and electronics, enabling the simultaneous and independent sensing of pressure and temperature with the elimination of signal separation algorithms and calculations. The bimodal sensor comprises a mechanoluminescent hybrid of ZnS-CaZnOS and a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) thermoresistant material, endowing the unambiguous transduction of pressure and temperature into optical and electrical signals, respectively. This device exhibits the highest temperature sensitivity of -0.6% °C^-1 in the range of 21-60 °C and visual sensing of the applied forces at a low limitation of 2 N. The interference-free and light-emitting characteristics of this device permit user-interactive applications in robotics for encrypted communication as well as temperature and pressure monitoring, along with wireless signal transmission. This work provides an unexplored solution to signal interference of multimodal tactile sensors, which can be extended to other multifunctional sensing devices.

Biodegradable, Breathable Leaf Vein-Based Tactile Sensors with Tunable Sensitivity and Sensing Range

Resistive pressure sensors have been widely studied for application in flexible wearable devices due to their outstanding pressure-sensitive characteristics. In addition to the outstanding electrical performance, environmental friendliness, breathability, and wearable comfortability also deserve more attention. Here, a biodegradable, breathable multilayer pressure sensor based piezoresistive effect is presented. This pressure sensor is designed with all biodegradable materials, which show excellent biodegradability and breathability with a three-dimensional porous hierarchical structure. Moreover, due to the multilayer structure, the contact area of the pressure sensitive layers is greatly increased and the loading pressure can be distributed to each layer, so the pressure sensor shows excellent pressure-sensitive characteristics over a wide pressure sensing range (0.03-11.60 kPa) with a high sensitivity (6.33 kPa^-1 ). Furthermore, the sensor is used as a human health monitoring equipment to monitor the human physiological signals and main joint movements, as well as be developed to detect different levels of pressure and further integrated into arrays for pressure imaging and a flexible musical keyboard. Considering the simple manufacturing process, the low cost, and the excellent performance, leaf vein-based pressure sensors provide a good concept for environmentally friendly wearable devices.

Anisotropic Carrier Mobility from 2H WSe2

Transition metal dichalcogenides (TMDCs) with 2H phase are expected to be building blocks in next-generation electronics; however, they suffer from electrical anisotropy, which is the basics for multi-terminal artificial synaptic devices, digital inverters, and anisotropic memtransistors, which are highly desired in neuromorphic computing. Herein, the anisotropic carrier mobility from 2H WSe₂ is reported, where the anisotropic degree of carrier mobility spans from 0.16 to 0.95 for various WSe₂ field-effect transistors under a gate voltage of -60 V. Phonon scattering, impurity ions scattering, and defect scattering are excluded for anisotropic mobility. An intrinsic screening layer is proposed and confirmed by Z-contrast scanning transmission electron microscopy (STEM) imaging to respond to the electrical anisotropy. Seven types of intrinsic screening layers are created and calculated by density functional theory to evaluate the modulated electronic structures, effective masses, and scattering intensities, resulting in anisotropic mobility. The discovery of anisotropic carrier mobility from 2H WSe₂ provides a degree of freedom for adjusting the physical properties of 2H TMDCs and fertile ground for exploring and integrating TMDC electronic transistors with better performance along the direction of high mobility.

Skin-Driven Ultrasensitive Mechanoluminescence Sensor Inspired by Spider Leg Joint Slits

Inspired by the spider's slit organ embedded in the leg joint exoskeleton and its ultrasensitive stress perception, we propose to fix the conflict between the stress concentration requirement for bright mechanoluminescence (ML) and the stress dispersion effect of soft material via integrating slit microstructures into flexible films. The designed slits focus weak stresses onto the corner to achieve high sensitivity, leading to 10-30 times ML intensity improvement at weak strain (<10% stretch) application. Slit morphology and various patterns were well investigated to address the stress distribution regularity. The slit-based ML film offers a facile light-luminescent artificial skin for visualizable stress presentations or detections without electricity power source. It is a practical endeavor of photonic skin for visible vocalization and a significant contribution to dysaudia auxiliary or luminescence augmented expressions for human social interactions, similar to jellyfish or squids.

Anisotropic conductive networks for multidimensional sensing

In the past decade, flexible physical sensors have attracted great attention due to their wide applications in many emerging areas including health-monitoring, human-machine interfaces, smart robots, and entertainment. However, conventional sensors are typically designed to respond to a specific stimulus or a deformation along only one single axis, while directional tracking and accurate monitoring of complex multi-axis stimuli is more critical in practical applications. Multidimensional sensors with distinguishable signals for simultaneous detection of complex postures and movements in multiple directions are highly demanded for the development of wearable electronics. Recently, many efforts have been devoted to the design and fabrication of multidimensional sensors that are capable of distinguishing stimuli from different directions accurately. Benefiting from their unique decoupling mechanisms, anisotropic architectures have been proved to be promising structures for multidimensional sensing. This review summarizes the present state and advances of the design and preparation strategies for fabricating multidimensional sensors based on anisotropic conducting networks. The fabrication strategies of different anisotropic structures, the working mechanism of various types of multidimensional sensing and their corresponding unique applications are presented and discussed. The potential challenges faced by multidimensional sensors are revealed to provide an insightful outlook for the future development.

Flexible, anti-damage, and non-contact sensing electronic skin implanted with MWCNT to block public pathogens contact infection

If a person comes into contact with pathogens on public facilities, there is a threat of contact (skin/wound) infections. More urgently, there are also reports about COVID-19 coronavirus contact infection, which once again reminds that contact infection is a very easily overlooked disease exposure route. Herein, we propose an innovative implantation strategy to fabricate a multi-walled carbon nanotube/polyvinyl alcohol (MWCNT/PVA, MCP) interpenetrating interface to achieve flexibility, anti-damage, and non-contact sensing electronic skin (E-skin). Interestingly, the MCP E-skin had a fascinating non-contact sensing function, which can respond to the finger approaching 0-20 mm through the spatial weak field. This non-contact sensing can be applied urgently to human-machine interactions in public facilities to block pathogen. The scratches of the fruit knife did not damage the MCP E-skin, and can resist chemical corrosion after hydrophobic treatment. In addition, the MCP E-skin was developed to real-time monitor the respiratory and cough for exercise detection and disease diagnosis. Notably, the MCP E-skin has great potential for emergency applications in times of infectious disease pandemics.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (fabrication of MCP E-skin, laser confocal tomography, parameter optimization, mechanical property characterization, finite element simulation, sensing mechanism, signal processing) is available in the online version of this article at 10.1007/s12274-021-3831-z.

Ultrasensitive strain sensor based on superhydrophobic microcracked conductive Ti3C2Tx MXene/paper for human-motion monitoring and E-skin

With the rapid development of wearable intelligent devices, low-cost wearable strain sensors with high sensitivity and low detection limit are urgently demanded. Meanwhile, sensing stability of sensor in wet or corrosive environments should also be considered in practical applications. Here, superhydrophobic microcracked conductive paper-based strain sensor was fabricated by coating conductive Ti₃C₂T_x MXene on printing paper via dip-coating process and followed by depositing superhydrophobic candle soot layer on its surface. Owing to the ultrasensitive microcrack structure in the conductive coating layer induced by the mismatch of elastic modulus and thermal expansion coefficient between conductive coating layer and paper substrate during the drying process, the prepared paper-based strain sensor exhibited a high sensitivity (gauge factor, GF = 17.4) in the strain range of 0-0.6%, ultralow detection limit (0.1% strain) and good fatigue resistance over 1000 cycles towards bending deformation. Interestingly, it was also applicable for torsion deformation detection, showing excellent torsion angle dependent, repeatable and stable sensing performances. Meanwhile, it displayed brilliant waterproof, self-cleaning and corrosion-resistant properties due to the existence of micro/nano-structured and the low surface energy candle soot layer. As a result, the prepared paper-based strain sensor can effectively monitor a series of large-scale and small-scale human motions even under water environment, showing the great promising in practical harsh outdoor environments. Importantly, it also demonstrated good applicability for spatial strain distribution detection of skin upon body movement when assembled into electronic-skin (E-skin). This study will provide great guidance for the design of next generation wearable strain sensor.

Magnetized Micropillar-Enabled Wearable Sensors for Touchless and Intelligent Information Communication

The wearable sensors have recently attracted considerable attentions as communication interfaces through the information perception, decoding, and conveying process. However, it is still challenging to obtain a sensor that can convert detectable signals into multiple outputs for convenient, efficient, cryptic, and high-capacity information transmission. Herein, we present a capacitive sensor of magnetic field based on a tilted flexible micromagnet array (t-FMA) as the proposed interaction interface. With the bidirectional bending capability of t-FMA actuated by magnetic torque, the sensor can recognize both the magnitude and orientation of magnetic field in real time with non-overlapping capacitance signals. The optimized sensor exhibits the high sensitivity of over 1.3 T^-1 and detection limit down to 1 mT with excellent durability. As a proof of concept, the sensor has been successfully demonstrated for convenient, efficient, and programmable interaction systems, e.g., touchless Morse code and Braille communication. The distinguishable recognition of the magnetic field orientation and magnitude further enables the sensor unit as a high-capacity transmitter for cryptic information interaction (e.g., encoded ID recognition) and multi-control instruction outputting. We believe that the proposed magnetic field sensor can open up a potential avenue for future applications including information communication, virtual reality device, and interactive robotics.

Spider-Web and Ant-Tentacle Doubly Bio-Inspired Multifunctional Self-Powered Electronic Skin with Hierarchical Nanostructure

For the practical applications of wearable electronic skin (e-skin), the multifunctional, self-powered, biodegradable, biocompatible, and breathable materials are needed to be assessed and tailored simultaneously. Integration of these features in flexible e-skin is highly desirable; however, it is challenging to construct an e-skin to meet the requirements of practical applications. Herein, a bio-inspired multifunctional e-skin with a multilayer nanostructure based on spider web and ant tentacle is constructed, which can collect biological energy through a triboelectric nanogenerator for the simultaneous detection of pressure, humidity, and temperature. Owing to the poly(vinyl alcohol)/poly(vinylidene fluoride) nanofibers spider web structure, internal bead-chain structure, and the collagen aggregate nanofibers based positive friction material, e-skin exhibits the highest pressure sensitivity (0.48 V kPa-1 ) and high detection range (0-135 kPa). Synchronously, the nanofibers imitating the antennae of ants provide e-skin with short response and recovery time (16 and 25 s, respectively) to a wide humidity range (25-85% RH). The e-skin is demonstrated to exhibit temperature coefficient of resistance (TCR = 0.0075 °C-1 ) in a range of the surrounding temperature (27-55 °C). Moreover, the natural collagen aggregate and the all-nanofibers structure ensure the biodegradability, biocompatibility, and breathability of the e-skin, showing great promise for practicability.

A Self-Powered Photodetector Based on MAPbI3 Single-Crystal Film/n-Si Heterojunction with Broadband Response Enhanced by Pyro-Phototronic and Piezo-Phototronic Effects

Pyro-phototronic and piezo-phototronic effect have shown their important roles for high performance heterojunction-based photodetectors (PDs). Here, a coupling effect of pyro-phototronic and piezo-phototronic effect is utilized to fabricate a self-powered and broadband PD based on the MAPbI₃ single-crystal film/n-Si heterojunction. First, by using the pyro-phototronic effect derived from MAPbI₃ , the maximum photoresponsivity of the self-powered PD is 1.5 mA W^-1 for 780 nm illumination, which is enhanced by more than 20 times in consideration of the relative peak-to-peak output current. Light-induced temperature change in MAPbI₃ film will create pyro-charges distributed at heterojunction interface, resulting in a downward bending of the energy band, facilitating the transport of photon-generated electrons and holes, and generating spike-like output currents. Second, piezo-phototronic effect is further introduced by applying vertical pressures onto the PD. With a vertical pressure of 155 kPa, the responsivity can be improved by more than 120% compared to the condition with no pressure. The overall enhancement is due to the piezo-phototronic and pyro-phototronic coupling effects which utilize the polarization charges to modulate the band structure of heterojunction. These results provide a promising approach to develop high-performance self-powered and broadband perovskite-based PDs by coupling pyro-phototronic and piezo-phototronic effect.

Advanced Flexible Skin-Like Pressure and Strain Sensors for Human Health Monitoring

Recently, owing to their excellent flexibility and adaptability, skin-like pressure and strain sensors integrated with the human body have the potential for great prospects in healthcare. This review mainly focuses on the representative advances of the flexible pressure and strain sensors for health monitoring in recent years. The review consists of five sections. Firstly, we give a brief introduction of flexible skin-like sensors and their primary demands, and we comprehensively outline the two categories of design strategies for flexible sensors. Secondly, combining the typical sensor structures and their applications in human body monitoring, we summarize the recent development of flexible pressure sensors based on perceptual mechanism, the sensing component, elastic substrate, sensitivity and detection range. Thirdly, the main structure principles and performance characteristic parameters of noteworthy flexible strain sensors are summed up, namely the sensing mechanism, sensitive element, substrate, gauge factor, stretchability, and representative applications for human monitoring. Furthermore, the representations of flexible sensors with the favorable biocompatibility and self-driven properties are introduced. Finally, in conclusion, besides continuously researching how to enhance the flexibility and sensitivity of flexible sensors, their biocompatibility, versatility and durability should also be given sufficient attention, especially for implantable bioelectronics. In addition, the discussion emphasizes the challenges and opportunities of the above highlighted characteristics of novel flexible skin-like sensors.

Biomimetic Hairy Whiskers for Robotic Skin Tactility

Touch sensing is among the most important sensing capabilities of a human, and the same is true for smart robotics. Current research on tactile sensors is mainly concentrated on electronic skin (e-skin), but e-skin is prone to be easily dirtied, damaged, and disturbed after repeated usage, which greatly limits its practical applications in robotics. Here, by mimicking the way that animals explore the environment using hair-based sensors, a bendable biomimetic whisker mechanoreceptor (BWMR) is designed for robotic tactile sensing. Owing to the advantages of triboelectric nanogenerator technology, the BWMR can convert external mechanical stimuli into electrical signals without a power supply, which is conducive to its widespread applications in robots. Because of the leverage effect of the whisker, the BWMR can distinguish an exciting force of 1.129 μN by amplifying external weak signals, which can be further improved by increasing the whisker length. Real-time sensing is demonstrated using a BWMR, exhibiting its potential for robotic tactile systems.

Highly Stretchable, Adhesive, and Self-Healing Silk Fibroin-Dopted Hydrogels for Wearable Sensors

In recent years, the preparations of flexible electronic devices have attracted great attention. Here, a simple one-pot method of thermal polymerization is introduced to fabricate silk fibroin-dopted hydrogels (SFHs), which are both chemically and physically cross-linked by acrylamide (AM), acrylic acid (AA), and silk fibroin (SF). The addition of SF can effectively enhance the mechanical property of the SFH_12% by 59% compared with SFH_0% . Taking the advantage of its wide working range of stress (about 0.455-568.9 kPa), the SFH can work as a resistance-type pressure sensor to monitor different human motions. What is more, the excellent adhesion, about 75.17 N m^-1 of SFH_46% enables it to fit tightly to other objects during the testing, which significantly reduces the loss of small signals due to poor fit. In addition, the SFH demonstrates excellent self-healing property without requiring external excitation and a sensitive temperature response in the range of -10 to 60 °C. The SFH is expected to be applied in the field of electronic skin, soft robots, and other flexible electronic products as well as speech recognition.

Nanofibrous Grids Assembled Orthogonally from Direct-Written Piezoelectric Fibers as Self-Powered Tactile Sensors

Tactile sensors are indispensable to wearable electronics, but still lack self-powering, high resolution, and flexibility. Herein, we present direct-written piezoelectric poly(vinylidene difluoride) fibers that are orthogonally assembled into nanofibrous grids (NFGs) as self-powered tactile sensors. Five nanofibrous strips (NFSs) are written on a polyurethane film via a uniform-field electrospinning (UFES) process, and two polyurethane films are orthogonally assembled into 5 × 5 NFGs with 25 pixels. Benefited from the mechanical flexibility and helical architecture of UFES fibers, stable piezoelectric outputs have been detected according to different locations or different pressures on an NFS, and a sensitivity of 7.1 mV/kPa is detected from the slope of voltage-pressure curves. In the orthogonally assembled NFGs, the pressure on a pixel of an NFS causes corresponding deformations of neighboring NFSs. The piezoelectric outputs vary with the distance from the pressing point, enabling us to position the pressing points and track the pressing trajectory in real time. Through judging piezoelectric outputs of all NFSs, precise locations of any pressed pixel with a resolution of 1 mm are presented vividly via luminous light-emitting diodes (LED), and the mapping profiles are displayed by pressing metal letters (S, W, J, T, and U) on multiple pixels. Furthermore, the coordinates of pressure either on an NFS or between NFSs with a resolution of 0.5 mm are reported digitally on a liquid crystal display (LCD). Thus, we developed novel self-powered tactile sensors with orthogonal NFGs to achieve real-time motion tracking, accurate spatial sensing, and location identification with high resolutions, which provide potential applications in electronic skin, robotics, and interface of artificial intelligence.

Ultrathin and Conformable Lead Halide Perovskite Photodetector Arrays for Potential Application in Retina-Like Vision Sensing

Solution-processed lead halide perovskites are considered one of the promising materials for flexible optoelectronics. However, the array integration of ultrathin flexible perovskite photodetectors (PDs) remains a significant challenge limited by the incompatibility of perovskite materials with manufacturing techniques involving polar liquids. Here, an ultrathin (2.4 µm) and conformable perovskite-based PD array (10 × 10 pixels) with ultralight weight (3.12 g m^-2 ) and excellent flexibility, is reported. Patterned all-inorganic CsPbBr₃ perovskite films with precise pixel position, controllable morphology, and homogenous dimension, are synthesized by a vacuum-assisted drop-casting patterning process as the active layer. The use of waterproof parylene-C film as substrate and encapsulation layer effectively protects the perovskite films against penetration of polar liquids during the peeling-off process. Benefitting from the encapsulation and ultrathin property, the device exhibits long-term stability in the ambient environment, and robust mechanical stability under bending or 50% compressive strain. More importantly, the ultrathin flexible PD arrays conforming to hemispherical support realize imaging of light distribution, indicating the potential applications in retina-like vision sensing.

Strain-Discriminable Pressure/Proximity Sensing of Transparent Stretchable Electronic Skin Based on PEDOT:PSS/SWCNT Electrodes

Pressure/proximity sensing as the essential function of electronic skin (e-skin) has become an emerging technological goal for new-generation electronic devices in a wide variety of application fields, for example, smart electronics, human-machine interaction, and prosthetics. However, the current research lacks pressure/proximity detection of the stretched e-skin, which ignores the key elastic characteristic of skin and hinders the development of e-skin. Here, the pressure/proximity detection of the transparent e-skin in the stretching state is demonstrated based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/single-walled carbon nanotube (SWCNT). The high transparency of the e-skin realizes the visual imperception for wearable electronic systems. The perfect combination of stretchable SWCNT and highly conductive PEDOT:PSS endows the sensors with high stretchability and high discrimination capability toward strain, providing an effective way to overcome the interference of strain to realize accurate pressure/proximity detection of stretched e-skin at different strains.

Ultra-high drivability, high-mobility, low-voltage and high-integration intrinsically stretchable transistors

Realizing intrinsically stretchable transistors with high current drivability, high mobility, small feature size, low power and the potential for mass production is essential for advancing stretchable electronics a critical step forward. However, it is challenging to realize these requirements simultaneously due to the limitations of the existing fabrication technologies when integrating intrinsically stretchable materials into transistors. Here, we propose a removal-transfer-photolithography method (RTPM), combined with adopting poly(urea-urethane) (PUU) as a dielectric, to realize integratable intrinsically stretchable carbon nanotube thin-film transistors (IIS-CNT-TFTs). The realized IIS-CNT-TFTs achieve excellent electrical and mechanical properties simultaneously, showing high field-effect-mobility up to 221 cm2 V-1 s-1 and high current density up to 810 μA mm-1 at a low driving voltage of -1 V, which are both the highest values for intrinsically stretchable transistors today to the best of our knowledge. At the same time, the transistors can survive 2000 cycles of repeated stretching by 50%, indicating their promising applicability to stretchable circuits, displays, and wearable electronics. The achieved intrinsically stretchable thin-film transistors show higher electrical performance, higher stretching durability, and smaller feature size simultaneously compared with the state-of-the-art works, providing a novel solution to integratable intrinsically stretchable electronics. Besides, the proposed RTPM involves adopting removable sacrificial layers to protect the PDMS substrate and PUU dielectric during the photolithography and patterning steps, and finally removing the sacrificial layers to improve the electrical and mechanical performance. This method is generally applicable to further enhance the performance of the existing transistors and devices with a similar structure in soft electronics.

Real-time pressure mapping smart insole system based on a controllable vertical pore dielectric layer

Real-time monitoring of plantar pressure has significant applications in wearable biosensors, sports injury detection, and early diagnostics. Herein, an all-in-one insole composed of 24 capacitive pressure sensors (CPSs) with vertical pores in an elastic dielectric layer is fabricated by laser cutting. Optimized CPSs with a hexagonal configuration and a pore size of 600 μm possess good linearity over a wide detection range of 0-200 kPa with a sensitivity of 12 × 10-3 kPa-1. Then, a smart system including the all-in-one insole with the 24 CPS array, a data acquisition system with a wireless transmitter and a PC terminal with a wireless receiver is established for real-time monitoring to realize static and dynamic plantar pressure mapping. Based on this smart insole system, various standing and yoga postures can be distinguished, and variations in the center of gravity during walking can be recognized. This intelligent insole system provides great feasible supervision for health surveillance, injury prevention, and athlete training.

Optimizing the Piezoelectric Strain in ZrO2- and HfO2-Based Incipient Ferroelectrics for Thin-Film Applications: An Ab Initio Dopant Screening Study

HfO₂ and ZrO₂ have increasingly drawn the interest of researchers as lead-free and silicon technology-compatible materials for ferroelectric, pyroelectric, and piezoelectric applications in thin films such as ferroelectric field-effect transistors, ferroelectric random access memories, nanoscale sensors, and energy harvesters. Owing to the environmental regulations against lead-containing electronic components, HfO₂ and ZrO₂ offer, along with AlN, (K,Na)NbO₃- and (Bi_0.5Na_0.5)TiO₃-based materials, an alternative to Pb(Zr_xTi_1-x)O₃-based materials, which are the overwhelmingly used ceramics in industry. HfO₂ and ZrO₂ thin films may show field-induced phase transformation from the paraelectric tetragonal to the ferroelectric orthorhombic phase, leading to a change in crystal volume and thus strain. These field-induced strains have already been measured experimentally in pure and doped systems; however, no systematic optimization of the piezoelectric activity was performed, either experimentally or theoretically. In this screening study, we calculate the ultimate size of this effect for 58 dopants depending on the oxygen supply and the defect incorporation type: substitutional or interstitial. The largest piezoelectric strain values are achieved with Yb, Li, and Na in ZrO₂ and exceed 40 pm V^-1 or 0.8% maximal strain, which exceeds the best experimental findings by a factor of 2. Furthermore, we discovered that Mo, W, and Hg make the polar-orthorhombic phase in the ZrO₂ bulk stable under certain circumstances, which would count in favor of these systems for the ceramic crystallization process. Our work guides the development of the performance of a promising material system by rational design of the essential mechanisms so as to apply it to unforeseen applications.

High-sensitivity, fast-response flexible pressure sensor for electronic skin using direct writing printing

Bionic electronic skin with human sensory capabilities has attracted extensive research interest, which has been applied in the fields of medical health diagnosis, wearable electronics, human–computer interaction, and bionic prosthetics. Electronic skin tactile pressure sensing required high sensitivity, good resolution and fast response for sensing different pressure stimuli. In particular, there were still great challenges in the detection of wide pressure and the preparation of sensitive unit microstructures. Here, the direct-write printing of Weissenberg principle to fabricate GNPs/MWCNT filled conductive composite flexible pressure sensors on PDMS substrates was proposed. The effects of platform moving speed, microneedle rotation speed and the number of direct-write times on the line width of the pressure sensitive structure were investigated based on orthogonal experiments, and the optimal direct-write printing parameters were obtained. The performance of the S-shaped polyline pressure sensor was tested, in which the sensitivity could reached 0.164 kPa⁻¹, and the response/recovery time was 100 ms and 100 ms respectively. The capture cases of objects of different quality and objects with flat/curved surfaces were successively demonstrated to exhibit its excellent sensitivity, stability and fast response performance. This work may paved the road for future integration of high-performance electronic skin in smart robotics and prosthetic solutions.

Bionic electronic skin with human sensory capabilities has attracted extensive research interest, which has been applied in the fields of medical health diagnosis, wearable electronics, human–computer interaction, and bionic prosthetics.

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.

Multimodal Capacitive and Piezoresistive Sensor for Simultaneous Measurement of Multiple Forces

Quantitative information on the magnitudes and directions of multiple contacting forces is crucial for a wide range of applications including human-robot interaction, prosthetics, and bionic hands. Herein we report a highly stretchable sensor integrating capacitive and piezoresistive mechanisms that can simultaneously determine multiple forces. The sensor consists of three layers in a sandwich design. The two facesheets serve as both piezoresistive sensors and electrodes for the capacitive sensor, with the core being a porous structure made by using a simple sugar particle template technique to give them high stretchability. The two facesheets contain segregated conductive networks of silver nanowires (AgNWs) and carbon nanofibers (CNFs). By measuring the changes in the electrical resistance of the facesheets and the capacitance between the facesheets, three separate mechanical stimuli can be determined, including normal pressure, in-plane stretch, and transverse shear force. The newly developed multidirectional sensor offers a significant opportunity for the next generation of wearable sensors for human health monitoring and bionic skin for robots.