Full 3D Printing of Stretchable Piezoresistive Sensor with Hierarchical Porosity and Multimodulus Architecture

A soft piezoresistive sensor with its unique characteristics, such as human skin, light weight, and multiple functions, yields a variety of possible practical applications to skin‐attachable electronics, human–machine interfaces, and electronic skins. However, conventional filler‐matrix piezoresistive sensors often suffer from unsatisfactory sensitivity or insufficient measurement range, as well as significant cross‐correlation between out‐of‐plane pressure and in‐plane extension. Here, a stretchable piezoresistive sensor (SPS) is realized by combining a hierarchically porous sensing element with a multimodulus device architecture via a full 3D printing process. As a result, the sensor exhibits high sensitivity (5.54 kPa^?1), large measurement range (from 10 Pa to 800 kPa), limited cross‐correlation, and excellent durability. Meanwhile, benefiting from the porous structure and mechanical mismatch design, which efficiently distributes the stress away from the sensing element, the device experiences only 7% resistance change at 50% stretching. This approach is employed to rapidly program and readily manufacture stylish, all‐in‐one, functional devices for various applications, demonstrating that the technique is promising for customized stretchable electronics.     

1D/2D Nanomaterials Synergistic,Compressible, and Response Rapidly 3D Graphene Aerogel for Piezoresistive Sensor

Graphene‐based aerogels have been widely studied for their high porosity, good compressibility, and electrical conductivity as piezoresistive sensors. However, the fabrication of graphene aerogel sensors with good mechanical properties and excellent sensing properties simultaneously remains a challenge. Therefore, in this study, a novel nanofiber reinforced graphene aerogel (aPANF/GA) which has a 3D interconnected hierarchical microstructure with surface‐treated PAN nanofiber as a support scaffold throughout the entire graphene network is designed. This 3D interconnected microporous aPANF/GA aerogel combines an excellent compressive stress of 43.50 kPa and a high piezoresistive sensitivity of 28.62 kPa^?1 as well as a wide range (0–14 kPa) linear sensitivity. When aPANF/GA is used as a piezoresistive sensor, the compression resilience is excellent, the response time is fast at about 37 ms at 3 Pa, and the structural stability and sensing durability are good after 2600 cycles. Indeed, the current signal value is 91.57% of the initial signal value at 20% compressive strain. Furthermore, the assembled sensors can monitor the real time movement of throat, wrist pulse, fingers, wrist, and knee joints of the human body at good sensitivity. These excellent features enable potential applications in health detection.     

Microchannel‐Confined MXene Based Flexible Piezoresistive Multifunctional Micro‐Force Sensor

Multifunctional micro‐force sensing in one device is an urgent need for the higher integration of the smaller flexible electronic device toward wearable health‐monitoring equipment, intelligent robotics, and efficient human–machine interface. Herein, a novel microchannel‐confined MXene‐based flexible piezoresistive sensor is demonstrated to simultaneously achieve multi‐types micro‐force sensing of pressure, sound, and acceleration. Benefiting from the synergistically confined effect of the fingerprint‐microstructured channel and the accordion‐microstructured MXene materials, the as‐designed sensor remarkably endows a low detection limit of 9 Pa, a high sensitivity of 99.5 kPa^?1, and a fast response time of 4 ms, as well as non‐attenuating durability over 10 000 cycles. Moreover, the fabricated sensor is multifunctionally capable of sensing sounds, micromotion, and acceleration in one device. Evidently, such a multifunctional sensing characteristic can highlight the bright prospect of the microchannel‐confined MXene‐based micro‐force sensor for the higher integration of flexible electronics.     

Multiscale Wrinkled Microstructures for Piezoresistive Fibers

Even though flexible piezoresistive materials are widely researched in recent years, it is full of challenges to simultaneously satisfy high conductivity, stretchability, and sensitivity. In this study, core–shell conductive fibers with high conductivity (10^?4–10^?5 Ω cm), stretchability (400%), and durability (more than 1200 s ultrasonic treatment) are presented and multiscale wrinkled microstructures (about 1.7 μm in height and 2.6 μm in length) are built on the surfaces of fibers via simply writing silver nanowires ink on prestrained commercial polyurethane fibers. The as prepared core–shell elastic fibers are twisted to construct flexible piezoresistive fibers, which show desirable sensitivity to pressure and bending deformations (0.12 kPa^?1 and 0.012 Rad^?1), fast response and relaxation time (35 and 15 ms), very low detection limit (10 mg), and excellent working stability (>4000 loading/unloading cycles). The wrinkled microstructures to overcome the viscoelastic delay of polymer composites are observed, making substantial contributions to improve the responsiveness. The investigations to the sensing mechanism indicate that increasing the contact points inner the flexible piezoresistive fibers will significantly improve the sensitivity. Finally, the potential applications of the flexible piezoresistive fibers as wearable devices and smart fabrics are demonstrated.     

Extraordinarily Sensitive and Low‐Voltage Operational Cloth‐Based Electronic Skin for Wearable Sensing and Multifunctional Integration Uses: A Tactile‐Induced Insulating‐to‐Conducting Transition

Electronic skin sensing devices are an emerging technology and have substantial demand in vast practical fields including wearable sensing, robotics, and user‐interactive interfaces. In order to imitate or even outperform the capabilities of natural skin, the keen exploration of materials, device structures, and new functions is desired. However, the very high resistance and the inadequate current switching and sensitivity of reported electronic skins hinder to further develop and explore the promising uses of the emerging sensing devices. Here, a novel resistive cloth‐based skin‐like sensor device is reported that possesses unprecedented features including ultrahigh current‐switching behavior of ≈10⁷ and giant high sensitivity of 1.04 × 10⁴–6.57 × 10⁶ kPa^?1 in a low‐pressure region of <3 kPa. Notably, both superior features can be achieved by a very low working voltage of 0.1 V. Taking these remarkable traits, the device not only exhibits excellent sensing abilities to various mechanical forces, meeting various applications required for skin‐like sensors, but also demonstrates a unique competence to facile integration with other functional devices for various purposes with ultrasensitive capabilities. Therefore, the new methodologies presented here enable to greatly enlarge and advance the development of versatile electronic skin applications.     

A Monocharged Electret Nanogenerator‐Based Self‐Powered Device for Pressure and Tactile Sensor Applications

This study reports a self‐powered pressure sensor based on a monocharged electret nanogenerator (MENG). The sensor exhibits great advantages in terms of high reliability, ease of fabrication, and relatively high sensitivity. The working mechanism of the MENG sensor is studied by both theoretical derivations and finite element analyses to determine the electric potential distribution during the device operation. The MENG sensor exhibits a stable open circuit voltage ≈10 V at a 30.8 kPa pressure and a corresponding sensitivity of 325 mV kPa^?1. The stability testing result shows that the device has only ≈5% attenuation after 10 000 cycles of repeated testing at 30.8 kPa pressure. Furthermore, it is found that the MENG sensor responds not only to a dynamic force but also a static force. Finally, a sensor array consisting of nine MENG sensor elements is fabricated. The testing results from the sensor array also reveal that a single touch of the sensor element can immediately light up an LED light at the corresponding position. This device holds great promise for use in future tactile sensors and artificial skin applications.     

Flexible Normal‐Tangential Force Sensor with Opposite Resistance Responding for Highly Sensitive Artificial Skin

An electronic skin (e‐skin) that can detect both normal and tangential forces with a differentiable signals output is essential for wearable electronics. A flexible, stretchable, and highly sensitive tactile sensor is presented that enables the detection of both normal and tangential forces, with specific opposite and thus easily being differentiated resistance changing outputs. The e‐skin, which is based on two‐sublayered carbon nanotubes (CNTs)/graphene oxide (GO) hybrid 3D conductive networks, that are anchored on a thin porous polydimethylsiloxane (PDMS) layer, is synthesized via a porogen (GO wrapped NaCl) assisted self‐assembling process. The fabricated CNTs/GO@PDMS‐based e‐skin shows superior sensitivity (gauge factor of 2.26 under a pressure loading of 1 kPa) to tangential force, moderate sensitivity (?0.31 kPa^?1 at 0.05–3.8 kPa, and ?0.03 kPa^?1 at 3.8–6.3 kPa, respectively) to normal force, and a high‐reproducible response over 5000 loading cycles including stretching, bending, and shearing. For applications, the e‐skin can not only detect wrist pulsing, discriminating different roughness of surfaces, but also produce an obvious responding to an extremely slight ticking (<20 mg) from a feather, and even can real‐timely monitor human's breath and music in rhythm.     

Bioinspired Interlocked and Hierarchical Design of ZnO Nanowire Arrays for Static and Dynamic Pressure‐Sensitive Electronic Skins

The development of electronic skin (e‐skin) is of great importance in human‐like robotics, healthcare, wearable electronics, and medical applications. In this paper, a bioinspired e‐skin design of hierarchical micro‐ and nano‐structured ZnO nanowire (NW) arrays in an interlocked geometry is suggested for the sensitive detection of both static and dynamic tactile stimuli through piezoresistive and piezoelectric transduction modes, respectively. The interlocked hierarchical structures enable a stress‐sensitive variation in the contact area between the interlocked ZnO NWs and also the efficient bending of ZnO NWs, which allow the sensitive detection of both static and dynamic tactile stimuli. The flexible e‐skin in a piezoresistive mode shows a high pressure sensitivity (?6.8 kPa^?1) and an ultrafast response time (<5 ms), which enables the detection of minute static pressure (0.6 Pa), vibration level (0.1 m s^?2), and sound pressure (≈57 dB). The flexible e‐skin in a piezoelectric mode is also demonstrated to be able to detect fast dynamic stimuli such as high frequency vibrations (≈250 Hz). The flexible e‐skins with both piezoresistive and piezoelectric sensing capabilities may find applications requiring both static and dynamic tactile perceptions such as robotic hands for dexterous manipulations and various healthcare monitoring devices.     

Ionic Skin with Biomimetic Dielectric Layer Templated from Calathea Zebrine Leaf

Flexible electronic skins (e‐skins) with high sensitivity and broad‐range pressure sensing are highly desired in artificial intelligence, and human–machine interaction. Capacitive‐type e‐skins have a simple configuration, but the change in dimensions of the dielectric layer is often quite limited, although introducing surface microstructures might improve the sensitivity in some extent. Moreover, such surface structures typically require costly microfabrication methods to fabricate. Here, a low‐cost microstructured ionic gel (MIG) with uniform cone‐like surface microstructures for high‐performance capacitive e‐skins is reported. The MIG film is templated from a Calathea zebrine leaf using soft lithography, and sandwiched by two flexible electrodes. The device exhibits a low limit of detection down to 0.1 Pa, a ultrahigh sensitivity of 54.31 kPa^?1 in the low pressure regime (<0.5 kPa), and the sensitivity keeps larger than 1 kPa^?1 over a broad‐range pressure from 0.1 Pa to 115 kPa. Electric double layers (EDL) form on both the top and bottom interfaces, and the area of EDL of the rough interface increases as the cones are compressed. Such ionic skins with biomimetic gel templated Calathea zebrine leaf allow for sensitive tactile sensing in the applications of human–machine interaction.     

Stretchable,Transparent, and Self‐Patterned Hydrogel‐Based Pressure Sensor for Human Motions Detection

In this study, a binary networked conductive hydrogel is prepared using acrylamide and polyvinyl alcohol. Based on the obtained hydrogel, an ultrastretchable pressure sensor with biocompatibility and transparency is fabricated cost effectively. The hydrogel exhibits impressive stretchability (>500%) and superior transparency (>90%). Furthermore, the self‐patterned microarchitecture on the hydrogel surface is beneficial to achieve high sensitivity (0.05 kPa^?1 for 0–3.27 kPa). The hydrogel‐based pressure sensor can precisely monitor dynamic pressures (3.33, 5.02, and 6.67 kPa) with frequency‐dependent behavior. It also shows fast response (150 ms), durable stability (500 dynamic cycles), and negligible current variation (6%). Moreover, the sensor can instantly detect both tiny (phonation, airflowing, and saliva swallowing) and robust (finger and limb motions) physiological activities. This work presents insights into preparing multifunctional hydrogels for mechanosensory electronics.     

All‐Fiber Structured Electronic Skin with High Elasticity and Breathability

With the rapid advancement in artificial intelligence, wearable electronic skins have attracted substantial attention. However, the fabrication of such devices with high elasticity and breathability is still a challenge and highly desired. Here, a route to develop an all‐fiber structured electronic skin with a scalable electrospinning fabrication technique is reported. The fabricated electronic skin is demonstrated to exhibit high pressure sensing with a sensitivity of 0.18 V kPa^?1 in the detection range of 0–175 kPa. This wearable device could maintain prominent sensing performance and mechanical stability in the presence of large deformation, even when the elastic deformation is up to 50%. The electronic skin is easily conformable on different desired objects for real‐time spatial mapping and long‐term tactile sensing. Besides, it possesses high gas permeability with a water vapor transmittance rate of 10.26 kg m^?2 d^?1. More importantly, the electronic skin is capable of working in a self‐powered manner and even serves as a reliable power source to effectively drive small electronics. Possessing several compelling features, such as high sensitivity, high elasticity, high breathability as well as being self‐powered and scalable in fabrication, the presented device paves a pathway for smart electronic skins.     

Flexible pressure sensor with high sensitivity and fast response for electronic skin using near-field electrohydrodynamic direct writing

Electronic skin imitates the function of human skin. The flexible pressure sensor is an important sensor of electronic skin. Although the flexible pressure sensor has made some progress, electronic skin is still a challenging subject with good pressure resolution, high sensitivity, and fast response ability in biomedical, human motion detection, personal health monitoring, and other fields. The PEDOT:PSS/GR/SWCNTs multicomponent solution was directly written on the flexible PDMS substrate by the near-field electrohydrodynamic direct-writing method, a serpentine shaped pressure sensitive unit was prepared, which was encapsulated with the PDMS thin film, and the flexible pressure sensor was fabricated. The sensitivity of the flexible pressure sensor is about 0.39 kPa⁻¹ at 0–0.5 kPa and 1.04 kPa⁻¹ at 0.5–2.4 kPa, and the response/recovery speed is 75 ms/150 ms, respectively. The fabricated flexible pressure sensor can detect a small pressure of about 6.4 Pa. The experimental results show that the fabricated flexible pressure sensor has high sensitivity, fast response capability, and low detection limit. The flexible pressure sensor for electronic skin demonstrates good performance in the application of finger joint movement and word pronunciation recognition, which indicates that it has great potential application in human motion detection and personal health monitoring.     

Programmable and Ultrasensitive Haptic Interfaces Enabling Closed-Loop Human–Machine Interactions

Intuitive, efficient, and unconstrained interactions require human–machine interfaces (HMIs) to accurately recognize users' manipulation intents. Susceptibility to interference and conditional usage mode of HMIs will lead to poor experiences that limit their great interaction potential. Herein, a programmable and ultrasensitive haptic interface enabling closed-loop human–machine interactions is reported. A cross-scale architecture design strategy is proposed to fabricate the haptic interface, which optimizes the hierarchical contact process. The synergistic optimization of the cross-scale architecture between carbon nanotubes and the multiscale sensing structure realizes a haptic interface with ultrahigh sensitivity and a wide detection range of 15.1 kPa⁻¹ and 180 kPa, which are improved by more than 900% over the performance of the common interface. The rapid response time of <5 ms and the limit of detection of 8 Pa of the haptic interface far surpass the somatosensory perception of human skin, which enables the haptic interface to accurately recognize interactive intents. A wireless pressure-data interactive glove (wireless PDI glove) is designed and realizes a round-the-clock operation, noise immunity, and efficient interactive control, which perfectly compensate for the flaws of typical vision and voice recognition modes.     

Bioinspired MXene-Based Piezoresistive Sensor with Two-stage Enhancement for Motion Capture (Adv. Funct. Mater. 18/2023)

Structured piezoresistive membranes are compelling building blocks for wearable bioelectronics. However, the poor structural compressibility of conventional microstructures leads to rapid saturation of detection range and low sensitivity of piezoresistive devices, limiting their commercial applications. Herein, a bioinspired MXene-based piezoresistive device is reported, which can effectively boost the sensitivity while broadening the response range by architecting intermittent villus-like microstructures. Benefitting from the two-stage amplification effect of this intermittent architecture, the developed MXene-based piezoresistive bioelectronics exhibit a high sensitivity of 461 kPa⁻¹ and a broad pressure detection range of up to 311 kPa, which are about 20 and 5 times higher than that of the homogeneous microstructures, respectively. Cooperating with the deep-learning algorithm, the designed bioelectronics can effectively capture complex human movements and precisely identify human motion with a high recognition accuracy of 99%. Evidently, this intermittent architecture of biomimetic strategy may pave a promising avenue to overcome the limitation of rapid saturation and low sensitivity in piezoresistive bioelectronics, and provide a general way to promote its large-scale applications.     

Rational Design of All Resistive Multifunctional Sensors with Stimulus Discriminability

A rational approach is proposed to design soft multifunctional sensors capable of detection and discrimination of different physical stimuli. Herein, a flexible multifunctional sensor concurrently detecting and distinguishing minute temperature and pressure stimuli in real time is developed using electrospun carbon nanofiber (CNF) films as the sole sensing material and electrical resistance as the only output signal. The stimuli sensitivity and discriminability are coordinated by tailoring the atomic- and device-level structures of CNF films to deliver outstanding pressure and temperature sensitivities of ? 0.96 kPa^?1 and ? 2.44%  ° C^?1, respectively, enabling mutually exclusive sensing performance without signal cross-interference. The CNF multifunctional sensor is considered the first of its kind to accomplish the stimulus discriminability using only the electrical resistance as the output signal, which is most convenient to monitor and process for device applications. As such, it has distinct advantages over other reported sensors in its simple, cost-effective fabrication and readout system. It also possesses other invaluable traits, including good bending stability, fast response time, and long-term durability. Importantly, the ability to simultaneously detect and decouple temperature and pressure stimuli is demonstrated through novel applications as a skin-mountable device and a flexible game controller.     

Laser Direct Writing of Ultrahigh Sensitive SiC‐Based Strain Sensor Arrays on Elastomer toward Electronic Skins

Electronic skins (e‐skins) have been widely investigated as important platforms for healthcare monitoring, human/machine interfaces, and soft robots. However, mask‐free formation of patterned active materials on elastomer substrates without involving high‐cost and complicate processes is still a grand challenge in developing e‐skins. Here, SiC‐based strain sensor arrays are fabricated on elastomer for e‐skins by a laser direct writing (LDW) technique, which is mask‐free, highly efficient, and scalable. The direct synthesis of active material on elastomer is ascribed to the LDW‐induced conversion of siloxanes to SiC. The SiC‐based devices own a highest sensitivity of ≈2.47 × 10⁵ achieved at a laser power of 0.8 W and a scanning velocity of 1.25 mm s^?1. Moreover, the LDW‐developed device provides a minimum strain detection limit of 0.05%, a small temperature drift, and a high mechanical durability for over 10 000 cycles. When it is mounted onto human skins, the SiC‐based device is able to monitor external stimuli and human health conditions, with the capability of wireless data transmission. Its potential application in e‐skins is further proved by an LDW‐fabricated device having 3 × 3 SiC sensor array for tactile sensing.     

Enhanced Sensitivity of Capacitive Pressure and Strain Sensor Based on CaCu3Ti4O12 Wrapped Hybrid Sponge for Wearable Applications

Pressure sensors with highly sensitive and flexible characteristics have extensive applications in wearable electronics, soft robotics, human–machine interface, and more. Herein, an effective strategy is explored to enhance the sensitivity of the capacitive pressure sensor by fabricating a dielectric hybrid sponge consisting of calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO), a giant dielectric permittivity material, in polyurethane (PU). An ultrasoft CCTO@PU hybrid sponge is fabricated via dip‐coating the PU sponge into surface‐modified CCTO nanoparticles using 3‐aminopropyl triethoxysilane. The overall results show that the –NH₂ functionalized CCTO attributes proper adhesion of CCTO with the –OCN group of the PU to enhance interfacial polarization leading to a high dielectric permittivity (167.05) and low loss tangent (0.71) beneficial for flexible pressure sensing applications. Moreover, the as‐prepared CCTO@PU hybrid sponge at 30 wt% CCTO concentration exhibits excellent electromechanical properties with an ultralow compression modulus of 27.83 kPa and a high sensitivity of 0.73 kPa^?1 in a low‐pressure regime (<1.6 kPa). Finally, pressure and strain sensing performance is demonstrated for the detection of human activities by mounting the sensor on various parts of the human body. The work reveals a new opportunity for the facile fabrication of high performance CCTO‐based capacitive sensors with multifunctional properties.     

ANN based CMOS ASIC design for improved temperature-drift compensation of piezoresistive micro-machined high resolution pressure sensor

The paper investigates the temperature-drift compensation of a high resolution piezoresistive pressure sensor using ANN based on conventional neuron model as also the inverse delayed function model of neuron. Using the delayed neuron model, an improvement in temperature-drift compensation has been obtained compared to the conventional neuron model. The CMOS analog ASIC design of a feed forward neural network using the inverse delayed function model of self connectionless neuron for the precise temperature-drift compensation has been presented. The inverse tan-sigmoid function is realized in CMOS implementation by Gilbert multiplier, differential adder and a cubing circuit. The entire design of the circuit has been done using AMS 0.35 μm CMOS model and simulated using Mentor Graphics ELDO simulator. Using the inverse delayed function model of neuron a mean square error of the order of 10⁻⁷ of the neural network has been obtained against a mean square error of the order of 10⁻³ using conventional neuron model for the same architecture of ANN. This brings down the error from 9% for uncompensated sensor to 0.1% only for compensated sensor using the delayed model of neuron in the temperature range of 0-70 °C. Using conventional neuron based ANN compensation, the error is reduced to 1% error.     

Multi‐Layered,Hierarchical Fabric‐Based Tactile Sensors with High Sensitivity and Linearity in Ultrawide Pressure Range

Resistive tactile sensors based on changes in contact area have been extensively explored for a variety of applications due to their outstanding pressure sensitivity compared to conventional tactile sensors. However, the development of tactile sensors with high sensitivity in a wide pressure range still remains a major challenge due to the trade‐off between sensitivity and linear detection range. Here, a tactile sensor comprising stacked carbon nanotubes and Ni‐fabrics is presented. The hierarchical structure of the fabrics facilitates a significant increase in contact area between them under pressure. Additionally, a multi‐layered structure that can provide more contact area and distribute stress to each layer further improves the sensitivity and linearity. Given these advantages, the sensor presents high sensitivity (26.13 kPa^?1) over a wide pressure range (0.2–982 kPa), which is a significant enhancement compared with the results obtained in previous studies. The sensor also exhibits outstanding performances in terms of response time, repeatability, reproducibility, and flexibility. Furthermore, meaningful applications of the sensor, including wrist‐pulse‐signal analysis, flexible keyboards, and tactile interface, are successfully demonstrated. Based on the facile and scalable fabrication technique, the conceptually simple but powerful approach provides a promising strategy to realize next‐generation electronics.     

Air Permeable Vibrotactile Actuators for Wearable Wireless Haptics

Vibrotactile actuators can evoke mechanical stimulations on human skins to induce haptic feedbacks for various human machine interaction applications. However, efforts toward their practical usages encounter several engineering challenges, including wearable comfortability and output abilities. Here, air permeable actuators are developed and embedded in common fabrics for vibrotactile actuation, achieving excellent air permeability of 108 L m⁻² s⁻¹, low preload requirement of 10 mN, high output sensitivity of 0.2 mN/V, and good mechanical durability by surviving 11 million testing cycles. As demonstration examples, a wireless haptic feedback glove is shown to distinguish 32 different English characters and symbols with an overall accuracy of 97.8%, and large size actuators (10 × 10 cm²) are also proved for providing haptic feedback for parts of human body. As such, the proposed system opens a new class of wearable vibrotactile actuators for potential applications in wide fields of metaverse, teleoperation, smart textiles, and robotics.