Metal-Organic Framework Reinforced Highly Stretchable and Durable Conductive Hydrogel-Based Triboelectric Nanogenerator for Biomotion Sensing and Wearable Human-Machine Interfaces

Flexible triboelectric nanogenerators (TENGs) with multifunctional sensing capabilities offer an elegant solution to address the growing energy supply challenges for wearable smart electronics. Herein, a highly stretchable and durable electrode for wearable TENG is developed using ZIF-8 as a reinforcing nanofiller in a hydrogel with LiCl electrolyte. ZIF-8 nanocrystals improve the hydrogel's mechanical properties by forming hydrogen bonds with copolymer chains, resulting in 2.7 times greater stretchability than pure hydrogel. The hydrogel electrode is encapsulated by microstructured silicone layers that act as triboelectric materials and prevent water loss from the hydrogel. Optimized ZIF-8-based hydrogel electrodes enhance the output performance of TENG through the dynamic balance of electric double layers (EDLs) during contact electrification. Thus, the as-fabricated TENG delivers an excellent power density of 3.47 Wm^–2, which is 3.2 times higher than pure hydrogel-based TENG. The developed TENG can scavenge biomechanical energy even at subzero temperatures to power small electronics and serve as excellent self-powered pressure sensors for human-machine interfaces (HMIs). The nanocomposite hydrogel-based TENG can also function as a wearable biomotion sensor, detecting body movements with high sensitivity. This study demonstrates the significant potential of utilizing ZIF-8 reinforced hydrogel as an electrode for wearable TENGs in energy harvesting and sensor technology.     

A Mechanically Robust,Self-Healing,and Adhesive Biomimetic Camouflage Ionic Conductor for Aquatic Environments

Flexible conductive materials capable of simulating transparent ocean organisms have garnered interest in underwater motion monitoring and covert communication applications. However, the creation of underwater flexible conductors that possess mechanical robustness, adhesion, and self-healing properties remains a challenge. Herein, hydrophobic interaction is combined with electrostatic interaction to obtain a solvent-free transparent poly(ionic liquid) elastomer (PILE) fabricated using soft acrylate monomers and acrylate ionic liquids. The synergy of hydrophobic and electrostatic interactions can eliminate the hydration of water molecules underwater, giving the PILE adjustable fracture strength, good elasticity, high stretchability, high toughness, fatigue resistance, underwater self-healing ability, underwater adhesion, and ionic conductivity. As a result, the transparent iontronic sensor generated from the PILE can achieve multifunctional sensing and human motion detection with high sensitivity and stability. In particular, the sensor can also transmit information underwater through stretching, pressing, and non-contact modes, demonstrating its huge potential in underwater flexible iontronic devices.     

Dynamic Photomask‐Assisted Direct Ink Writing Multimaterial for Multilevel Triboelectric Nanogenerator

Triboelectric nanogenerator (TENG) devices are extensively studied as a mechanical energy harvester and self‐powered sensor for wearable electronics and physiological monitoring. However, the conventional TENG fabrication involving assembling steps and using the single property of matrix material suffers from simple devices shape and a single level of mechanical response for sensing and energy harvesting. Here, the printed multimaterial matrix for multilevel mechanical‐responsive TENG with on‐demand reconfiguration of shape is reported. A multimaterial 3D printing approach by using dynamic photomask‐assisted direct ink writing printing together with a two‐stage curing hybrid ink is first developed. Multimaterial structures with location‐specific properties, such as tensile modulus, failure stress, and glass transition temperature for controlled deformation, crack propagation path, and sequential shape memory, are directly printed. The printed multimaterial structure with sequential deformation behavior is used to fabricate a multilevel‐TENG (mTENG) device for multiple level mechanical energy harvesters and sensors. It is demonstrated that the mTENG can be embedded in shoe insoles to achieve both comfortable wearing and motion state monitoring. This work provides a new approach to combine multimaterial 3D printing with TENG devices for functional wearable electronics as energy harvester and sensors.     

Wearable and Implantable Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) are a promising technology to convert mechanical energy to electrical energy based on coupled triboelectrification and electrostatic induction. With the rapid development of functional materials and manufacturing techniques, wearable and implantable TENGs have evolved into playing important roles in clinic and daily life from in vitro to in vivo. These flexible and light membrane‐like devices have the potential to be a new power supply or sensor element, to meet the special requirements for portable electronics, promoting innovation in electronic devices. In this review, the recent advances in wearable and implantable TENGs as sustainable power sources or self‐powered sensors are reviewed. In addition, the remaining challenges and future possible improvements of wearable and implantable TENG‐based self‐powered systems are discussed.     

Stretchable‐Rubber‐Based Triboelectric Nanogenerator and Its Application as Self‐Powered Body Motion Sensors

A stretchable‐rubber‐based (SR‐based) triboelectric nanogenerator (TENG) is developed that can not only harvest energy but also serve as self‐powered multifunctional sensors. It consists of a layer of elastic rubber and a layer of aluminum film that acts as the electrode. By stretching and releasing the rubber, the changes of triboelectric charge distribution/density on the rubber surface relative to the aluminum surface induce alterations to the electrical potential of the aluminum electrode, leading to an alternating charge flow between the aluminum electrode and the ground. The unique working principle of the SR‐based TENG is verified by the coupling of numerical calculations and experimental measurements. A comprehensive study is carried out to investigate the factors that may influence the output performance of the SR‐based TENG. By integrating the devices into a sensor system, it is capable of detecting movements in different directions. Moreover, the SR‐based TENG can be attached to a human body to detect diaphragm breathing and joint motion. This work largely expands the applications of TENG not only as effective power sources but also as active sensors; and opens up a new prospect in future electronics.     

Soft Robotic-Adapted Multimodal Sensors Derived from Entirely Intrinsic Self-Healing and Stretchable Cross-Linked Networks

Flexible sensing technologies that play a pivotal role in endowing robots with detection capabilities and monitoring their motions are impulsively desired for intelligent robotics systems. However, integrating and constructing reliable and sustainable flexible sensors with multifunctionality for robots remains an everlasting challenge. Herein, an entirely intrinsic self-healing, stretchable, and attachable multimodal sensor is developed that can be conformally integrated with soft robots to identify diverse signals. The dynamic bonds cross-linked networks including the insulating polymer and conductive hydrogel with good comprehensive performances are designed to fabricate the sensor with prolonged lifespan and improved reliability. Benefiting from the self-adhesiveness of the hydrogel, strong interfacial bonding can be formed on various surfaces, which promotes the conformable integration of the sensor with robots. Due to the ionic transportation mechanism, the sensor can detect strain and temperature based on piezoresistive and thermoresistive effect, respectively. Moreover, the sensor can work in triboelectric mode to achieve self-powered sensing. Various information can be identified from the electrical signals generated by the sensor, including hand gestures, soft robot crawling motions, a message of code, the temperature of objects, and the type of materials, holding great promise in the fields of environmental detection, wearable devices, human-machine interfacing, and robotics.     

Ionic Tactile Sensors for Emerging Human‐Interactive Technologies: A Review of Recent Progress

Ionic tactile sensors (ITS) represent a new class of deformable sensory platforms that mimic not only the tactile functions and topological structures but also the mechanotransduction mechanism across the biological ion channels in human skin, which can demonstrate a more advanced biological interface for targeting emerging human‐interactive technologies compared to conventional e‐skin devices. Recently, flexible and even stretchable ITS have been developed using novel structural designs and strategies in materials and devices. These skin‐like tactile sensors can effectively sense pressure, strain, shear, torsion, and other external stimuli with high sensitivity, high reliability, and rapid response beyond those of human perception. In this review, the recent developments of the ITS based on the novel concepts, structural designs, and strategies in materials innovation are entirely highlighted. In particular, biomimetic approaches have led to the development of the ITS that extend beyond the tactile sensory capabilities of human skin such as sensitivity, pressure detection range, and multimodality. Furthermore, the recent progress in self‐powered and self‐healable ITS, which should be strongly required to allow human‐interactive artificial sensory platforms is reviewed. The applications of ITS in human‐interactive technologies including artificial skin, wearable medical devices, and user‐interactive interfaces are highlighted. Last, perspectives on the current challenges and the future directions of this field are presented.     

Dual Mode TENG with Self-Voltage Multiplying Circuit for Blue Energy Harvesting and Water Wave Monitoring

As the most extensive natural energy on earth, ocean wave energy is regarded as a difficult energy to be fully and efficiently utilized because of its low frequency and multi-direction movement. Herein, a versatile blue energy triboelectric nanogenerator (VBE-TENG) fabricated by using dual-mode output terminals with charge excitation strategy is reported, which can harvest varying water-wave energy effectively. Benefiting from the rolling ball on a specific track and the compression rebound characteristics of a spring sheet steel, the carrier can be driven along a specific path through random ocean wave energy, and then the energy is converted into electricity by VBE-TENG. A high peak output power of 34.3 mW is obtained, 2.5 times as much as that of current highest record based on a device unit in blue energy TENG. In addition, the TENG can light 256 LEDs and continuously power commercial electronic devices in wave environments. The average peak voltage of contact-separation TENG is converted into virtual signal via Labview software to provide wave height monitoring as a self-powered sensing system. This work provides a new approach in blue energy TENG toward practical applications.     

All‐in‐One Iontronic Sensing Paper

Paper has been utilized as an ideal platform for chemical, biological, and mechanical sensing for its fibrous structures and properties in addition to its low cost. However, current studies on pressure‐sensitive papers have not fully utilized the unique advantages of papers, such as printability, cuttability, and foldability. Moreover, the existing resistive, capacitive, and triboelectric sensing modalities have long‐standing challenges in sensitivity, noise‐proofing, response time, linearity, etc. Here, a novel flexible iontronic sensing mechanism, referred to as iontronic sensing paper (ISP), is introduced to the classic paper substrates by incorporating both ionic and conductive patterns into an all‐in‐one flexible sensing platform. The ISP can then be structured into 2D or 3D tactile‐sensitive origamis only by the paper‐specific manipulations of printing, cutting, folding, and gluing. Notably, the ISP device possesses a device sensitivity of 10 nF kPa^?1 cm^?2, which is thousands of times higher than that of the commercial counterpart, a resolution of 6.25 Pa, a single‐millisecond response time, and a high linearity (R ² > 0.996). Benefiting from the unique properties of the fibrous paper structures and its remarkable performances, the ISP devices hold enormous potential for the emerging human–machine interfaces, including smart packaging, health wearables, and pressure‐sensitive paper matrix.     

Vacuum-sealed silicon micromachined pressure sensors

Considerable progress in silicon pressure sensors has been made in recent years. This paper discusses three types of vacuum-sealed silicon micromachined pressure sensors that represent the present state of the art in this important area. The devices are a capacitive vacuum sensor, a surface-micromachined microdiaphragm pressure sensor, and a resonant pressure sensor. Vacuum sealing for these devices is accomplished using anodic bonding, films deposited using low-pressure chemical vapor deposition, and thermal out-diffusion of hydrogen, respectively. These sensors emphasize high sensitivity, small size, and excellent stability, respectively. The silicon-diaphragm vacuum sensor uses electrostatic force balancing to achieve a wide pressure measurement range     

MXene Functionalized,Highly Breathable and Sensitive Pressure Sensors with Multi-Layered Porous Structure

Breathable, flexible, and highly sensitive pressure sensors have drawn increasing attention due to their potential in wearable electronics for body-motion monitoring, human-machine interfaces, etc. However, current pressure sensors are usually assembled with polymer substrates or encapsulation layers, thus causing discomfort during wearing (i.e., low air/vapor permeability, mechanical mismatch) and restricting their applications. A breathable and flexible pressure sensor is reported with nonwoven fabrics as both the electrode (printed with MXene interdigitated electrode) and sensing (coated with MXene/silver nanowires) layers via a scalable screen-printing approach. Benefiting from the multi-layered porous structure, the sensor demonstrates good air permeability with high sensitivity (770.86–1434.89 kPa⁻¹), a wide sensing range (0–100 kPa), fast response/recovery time (70/81 ms), and low detection limit (≈1 Pa). Particularly, this sensor can detect full-scale human motion (i.e., small-scale pulse beating and large-scale walking/running) with high sensitivity, excellent cycling stability, and puncture resistance. Additionally, the sensing layer of the pressure sensor also displays superior sensitivity to humidity changes, which is verified by successfully monitoring human breathing and spoken words while wearing a sensor-embedded mask. Given the outstanding features, this breathable sensor shows promise in the wearable electronic field for body health monitoring, sports activity detection, and disease diagnosis.     

Triboelectric Nanogenerator for Harvesting Vibration Energy in Full Space and as Self‐Powered Acceleration Sensor

A spherical three‐dimensional triboelectric nanogenerator (3D‐TENG) with a single electrode is designed, consisting of an outer transparent shell and an inner polyfluoroalkoxy (PFA) ball. Based on the coupling of triboelectric effect and electrostatic effect, the rationally developed 3D‐TENG can effectively scavenge ambient vibration energy in full space by working at a hybridization of both the contact‐separation mode and the sliding mode, resulting in the electron transfer between the Al electrode and the ground. By systematically investigating the output performance of the device vibrating under different frequencies and along different directions, the TENG can deliver a maximal output voltage of 57 V, a maximal output current of 2.3 μA, and a corresponding output power of 128 μW on a load of 100 MΩ, which can be used to directly drive tens of green light‐emitting diodes. Moreover, the TENG is utilized to design the self‐powered acceleration sensor with detection sensitivity of 15.56 V g^‐1. This work opens up many potential applications of single‐electrode based TENGs for ambient vibration energy harvesting techniques in full space and the self‐powered vibration sensor systems.     

Cobalt-Nanoporous Carbon Functionalized Nanocomposite-Based Triboelectric Nanogenerator for Contactless and Sustainable Self-Powered Sensor Systems

Triboelectric nanogenerators (TENGs), which operate in contactless mode and avoid physical contact, are highly attractive for self-powered sensor systems aiming to achieve long-term reliable operation and reduce rubbing friction. Herein, an ultra-flexible and high-performance contactless double-layer TENG (CDL-TENG) is first designed and fabricated using a metal–organic framework-based cobalt nanoporous carbon (Co-NPC)/Ecoflex with MXene/Ecoflex nanocomposite layer for self-powered sensor applications. The porous structure of the Co-NPC provides a high-surface-area of the nanocomposite and the charge storage layer of the MXene/Ecoflex nanocomposite accumulates more negative charge to improve the functionality of the CDL-TENG two and three times, respectively. Compared with Ecoflex film-based TENGs, the fabricated CDL-TENG exhibits an eight-fold slower decay rate owing to charge trapping characteristics, which were confirmed by surface potential measurements. The CDL-TENG shows excellent humidity and acceleration sensitivity of about 0.3 V/% and 2.06 Vs² m⁻¹. The CDL-TENG also offers non-contact position detection performance in the 20 cm range. Furthermore, the CDL-TENG is successfully integrated with mobile-vehicles and an intelligent robot to perform obstacle and human-motion detection. Finally, a contactless door-lock password authentication system was demonstrated. These multifunctional benefits make it useful for numerous applications, including artificial intelligence, human-machine interfaces, and self-powered sensors.     

Skin-Inspired High-Performance Active-Matrix Circuitry for Multimodal User-Interaction

Artificial electronic skin (e-skin), a network of mechanically flexible sensors which can wrap irregular surfaces conformally and quantify various stimuli sensitively, is potentially useful in healthcare monitoring and human-machine interaction (HMI). Although various approaches have mimicked the structures and functions of the human skin, challenges remain with high-density integration, super sensitivity, and multi-functionality. A multimodal and comfortable skin-inspired active-matrix circuitry is reported here with high pixel density (>100 cm^–2) based on all 2D materials, which exhibits excellent performance to detect both mechanical interactions and humidity variations. The ultra-high sensitivity (>400 and ≈ 10⁴ for strain and humidity sensing, respectively), long-term stability (>1000 cycles), and rapid response time for every pixel can fulfill simultaneous multi-stimulus sensing. Accordingly, a respiratory monitor is constructed to realize healthcare monitoring through observing the human breath frequency, intensity, and humidity in real-time. Moreover, the multimodal e-skin breaks through shackles of the contact sensor medium for HMI. 3D strain and humidity spatial mapping can reflect object location information even without contact, avoiding cross-infection of viruses effectively between users during the COVID-19 pandemic. The reported e-skin will broaden applications for future healthcare and human–machine interactive devices.     

Dielectric Modulated Cellulose Paper/PDMS‐Based Triboelectric Nanogenerators for Wireless Transmission and Electropolymerization Applications

Achieving high output performance is the key in the development of triboelectric nanogenerators (TENGs) for future versatile applications. In this study, a novel TENG assembled with porous cellulose paper and polydimethylsiloxane is demonstrated. Through dielectric modulation of the friction materials by the nanoparticles (i.e., BaTiO₃, Ag), the triboelectric outputs increase significantly with the permittivity increase, which is attributed to the enhancement of the charge trapping capability and the surface charge density of the friction materials. The dielectric modulated TENG demonstrates a high output voltage of 88 V and a current of 8.3 µA, corresponding to an output power of 141 µW. Acting as a sensor unit, the TENG can successfully operate in a wireless transmission system, which can remotely monitor the machine operation and deliver the messages associated with finger movements. Moreover, the TENG can also perform as an efficient power source in an electropolymerization system for electropolymerizing polyaniline on a carbon nanotube electrode, holding a great potential to synthesize a high capacitance electrode for supercapacitors. This work provides a simple and efficient way to construct high performance TENGs and promotes their practical applications in the fields of wireless transmission and electropolymerization systems.     

An Artificial Neuromuscular System for Bimodal Human–Machine Interaction

Neuromuscular system enabled muscle functions are critical for body movements, such as rhythmic motions and other complexed movements. Imparting artificial neuromuscular system to advanced robots and interactive systems can potentially improve their sensorimotor coordination and interactivity. Here, an artificial neuromuscular system is reported to mimic the sensing, processing, and manipulation of neuromuscular information, which consists of a triboelectric nano-generator (TENG), SnErO_x neuromorphic transistors (SENTs), and the signal-converting system. The synaptic performance of the SENT is optimized to implement multiple operation modes of muscle upon receiving signals from TENG, including muscle contraction, fast/slow muscle fiber shift, conscious/unconscious muscle movements, and transformation. As a proof-of-concept demonstration, the artificial neuromuscular system is used to develop contact human–machine interaction (HMI) by decoding the surface electromyogram (sEMG) and non-contact HMI based on supercapacitive iontronic effect. Importantly, both HMI demonstrate real-time gesture recognition and robotic manipulation, indicating the potential of developing next-generation smart electronics that desire multiple interaction patterns.     

平面电容传感器液位检测系统设计

为了对高度小于100mm的液位进行非接触式测量,采用液位变化改变平面电容边缘电场参量的方法,对平面电容传感器的工作原理进行了理论分析,研究了平面电容传感器的结构参量对其灵敏度、穿透深度的影响,并对传感器结构参量进行了优化, 基于平面电容传感器, 设计了非接触式低液位检测系统,通过对纯净水、洗洁精溶液和墨汁的实验验证,取得了0mm~100mm范围的液位测量数据。结果表明,该检测系统工作稳定,具有线性输出,重复性误差约为±0.28%,数据修正前的测量误差小于7.8%。这一结果对非接触式较低液位的检测是有帮助的。     

Welcome to the Special Section on Smart Sensors!

The sixteen papers in this special section are devoted to the following topics: CMOS imagers; neuromorphic sensory systems; bio and biomedical services; and other smart sensor topics including capacitive sensors and interface circuits.     

Design of an integrated low-noise read-out system for DNA capacitive sensors

This paper presents an electronic system for a fast DNA label-less detection. The sensitivity of the capacitive sensor in use is improved by depositing an insulating self-assembled monolayer (SAM) over the golden electrodes. The capacitance shift due to the hybridization effect is monitored by means of a charge-sensitive amplifier and digitalized by means of a comparator and a counter. The read-out solution demonstrates the ability to identify a 0.01% variation on the capacitive value of the sensor. Results from measurements with the optimized sensor show the reliability of the electronics. The investigated solution is suitable for monolithic systems or for a micro-fabricated array of sensors. An example of the integrated front-end is described and performances and noise evaluation are reported here.