Highly Sensitive Ultrastable Electrochemical Sensor Enabled by Proton-Coupled Electron Transfer

Flexible piezoelectric materials and strain sensors for wearable electronics and artificial intelligence applications

With the rapid development of artificial intelligence, the applications of flexible piezoelectric sensors in health monitoring and human-machine interaction have attracted increasing attention. Recent advances in flexible materials and fabrication technologies have promoted practical applications of wearable devices, enabling their assembly in various forms such as ultra-thin films, electronic skins and electronic tattoos. These piezoelectric sensors meet the requirements of high integration, miniaturization and low power consumption, while simultaneously maintaining their unique sensing performance advantages. This review provides a comprehensive overview of cutting-edge research studies on enhanced wearable piezoelectric sensors. Promising piezoelectric polymer materials are highlighted, including polyvinylidene fluoride and conductive hydrogels. Material engineering strategies for improving sensitivity, cycle life, biocompatibility, and processability are summarized and discussed focusing on filler doping, fabrication techniques optimization, and microstructure engineering. Additionally, this review presents representative application cases of smart piezoelectric sensors in health monitoring and human-machine interaction. Finally, critical challenges and promising principles concerning advanced manufacture, biological safety and function integration are discussed to shed light on future directions in the field of piezoelectrics.

Reversible Molecule Interactions Enable Ultrastretchable and Recyclable Ionogels for Wearable Piezoionic Sensors

Ionogel-based piezoionic sensors feel motions and strains like human skin relying on reversible ion migrations under external mechanical stimulus and are of great importance to artificial intelligence. However, conventional ion-conductive polymers behave with degraded electrical and mechanical properties after thousands of strain cycles, and the discarded materials and devices become electronic wastes as well. Here, we develop ultrastretchable ionogels with superior electrical properties via the mediation of metal-organic frameworks, whose properties are attributed to reversible molecule interactions inside the material system. Ionogels present excellent mechanical properties with breaking elongation as high as 850%, exceeding most previously reported similar materials, and the high conductivity enables further application in sensor devices. In addition, our ionogels display superior recyclability because of the reversible physical and chemical interactions inside material molecules, which are eco-friendly to the environment. As a result, the ionogel-based piezoionic sensors deliver high sensitivity, flexibility, cyclic stability, and signal reliability, which are of great significance to wearable applications in human-motion detections such as throat vibration, facial expression, joint mobility, and finger movement. Our study paves the way for ultrastretchable and eco-friendly ionogel design for flexible electrochemical devices.

Fully Integrated Multiplexed Wristwatch for Real-Time Monitoring of Electrolyte Ions in Sweat

Considerable progress has already been made in sweat sensors based on electrochemical methods to realize real-time monitoring of biomarkers. However, realizing long-term monitoring of multiple targets at the atomic level remains extremely challenging, in terms of designing stable solid contact (SC) interfaces and fully integrating multiple modules for large-scale applications of sweat sensors. Herein, a fully integrated wristwatch was designed using mass-manufactured sensor arrays based on hierarchical multilayer-pore cross-linked N-doped porous carbon coated by reduced graphene oxide (NPCs@rGO-950) microspheres with high hydrophobicity as core SC, and highly selective monitoring simultaneously for K+, Na+, and Ca2+ ions in human sweat was achieved, exhibiting near-Nernst responses almost without forming an interfacial water layer. Combined with computed tomography, solid-solid interface potential diffusion simulation results reveal extremely low interface diffusion potential and high interface capacitance (598 μF), ensuring the excellent potential stability, reversibility, repeatability, and selectivity of sensor arrays. The developed highly integrated-multiplexed wristwatch with multiple modules, including SC, sensor array, microfluidic chip, signal transduction, signal processing, and data visualization, achieved reliable real-time monitoring for K+, Na+, and Ca2+ ion concentrations in sweat. Ingenious material design, scalable sensor fabrication, and electrical integration of multimodule wearables lay the foundation for developing reliable sweat-sensing systems for health monitoring.

Ionic Polymer-Metal Composites: From Material Engineering to Flexible Applications

ConspectusIonic polymer-metal composites (IPMCs) are one kind of artificial muscles that can realize energy conversions in response to external stimulus with merits of lightweight, scalability, quick response, and flexibility and have been treated as an important platform in artificial intelligence, such as bionic robotics, smart sensors, and micro-electromechanical systems. It is well-known that IPMC devices are mainly composed of one electrolyte layer laminated with symmetric electrode layers and realize energy conversion based on ion migration and redistribution inside the devices. However, several critical issues have greatly impeded the practical applications of IPMC devices, including metal electrode cracks, metal-polymer interface detachment, and water loss in the electrolyte. In the past decade, our group and collaborators have made attempts to address the mentioned critical issues with the purpose of accelerating practical applications of IPMC devices. First, in order to address the metal electrode cracks, we have developed various electrode materials to replace the metal electrode material, such as black phosphorus and graphdiyne. These materials display superior electrical and mechanical properties with enhanced material stability without cracking. Second, to address metal-polymer interface detachment, we have designed robust interfaces for IMPC devices with vertical array structures. The as-prepared interfaces present high ionic conductivity with excellent mechanical stability under bending states. As a result, the IPMC devices deliver high working stability exceeding a million cycles under air conditions. Third, in order to avoid water loss in the electrolyte, especially at ambient conditions, we have developed ionogel electrolytes containing highly stable ionic liquids as active ion sources. The ionogel electrolytes effectively prevent water loss in conventional water-containing electrolytes, just like Nafion electrolytes and greatly improve the working stability of IPMC devices. After addressing the key issues of IPMC devices, we finally obtained many high-performing IPMC devices and explored various intelligent applications of them. For instance, we have demonstrated the smart functions of IPMC devices as sensitive strain sensors, such as sign language recognition, handwriting detection, and human muscle monitoring. In addition, we have developed bionic flying robots with a vibration frequency as high as 30 Hz with the aid of high-performance IPMC actuators. A medical catheter based on IPMC actuators has been put forward by our group and can realize multiple degrees of freedom deformation under a low driving voltage of 2.5 V, which presents great potential for application in medical instruments. Lastly, a perspective on critical challenges and future research directions on IPMC materials and devices is highlighted for accelerating practical application.

Outstanding Synergy of Sensitivity and Linear Range Enabled by Multigradient Architectures

Flexible pressure sensors are devices that mimic the sensory capabilities of natural human skin and enable robots to perceive external stimuli. One of the main challenges is maintaining high sensitivity over a broad linear pressure range due to poor structural compressibility. Here, we report a flexible pressure sensor with an ultrahigh sensitivity of 153.3 kPa-1 and linear response over an unprecedentedly broad pressure range from 0.0005 to 1300 kPa based on interdigital-shaped, multigradient architectures, featuring modulus, conductivity, and microstructure gradients. Such multigradient architectures and interdigital-shaped configurations enable effective stress transfer and conductivity regulation, evading the pressure sensitivity-linear range trade-off dilemma. Together with high pressure resolution, high frequency response, and good reproducibility over the ultrabroad linear range, proof-of-concept applications such as acoustic wave detection, high-resolution pressure measurement, and healthcare monitoring in diverse scenarios are demonstrated.

Highly Sensitive Artificial Skin Perception Enabled by a Bio-inspired Interface

Piezoionic strain sensors have attracted enormous attention in artificial skin perception because of high sensitivity, lightweight, and flexibility. However, their sensing properties are limited by a weak material interface based on physical adhesion, which usually leads to fast performance deterioration under mechanical conditions. In this work, a bio-inspired interface has been reported based on an in situ growth strategy and then utilized for piezoionic sensor assembly. The robust coupling interface provides fast kinetic of ion transfer and prevents interface slippage under external strains. The as-fabricated sensors give high sensing voltage with high sensitivity. It delivers excellent cycling stability with performance retention above 90% over thousands of bending cycles in air. Further, the sensors have been explored as an effective platform for skin perception, and many detections can be realized within our devices, such as skin touch, eye movement, cheek bulging, and finger movement.

Flexible Piezoionic Strain Sensors toward Artificial Intelligence Applications

Flexible piezoionic strain sensors are playing an important role in the field of smart electronic and artificial intelligence. The high sensitivity and superior flexibility make it possible to detect various strain and stress from macro- to microscale precisely. Here, recent progress on flexible piezoionic strain sensors has been summarized into several sections, including sensing mechanism, material engineering, and smart applications. In each section, we present typical scientific works and discuss corresponding critical results. This Account aims to provide broad views for researchers with different academic backgrounds, and then promotes the development of flexible piezoionic strain sensors. Finally, existing challenges and opportunities have been presented to expedite further research works and practical applications of flexible piezoionic strain sensors.

1 Introduction

2 Sensing Mechanism of Flexible Piezoionic Strain Sensors

3 Material Engineering for Flexible Piezoionic Strain Sensors

3.1 Electrolyte Materials for Flexible Piezoionic Strain Sensors

3.2 Electrode Materials for Flexible Piezoionic Strain Sensors

4 Smart Applications of Flexible Piezoionic Strain Sensors toward Artificial Intelligence

5 Conclusion and Perspective

Nanostructure Engineering of Graphitic Carbon Nitride for Electrochemical Applications

Graphitic carbon nitride with ordered two-dimensional structure displays multiple properties, including tunable structure, suitable bandgap, high stability, and facile synthesis. Many achievements on this material have been made in photocatalysis, but the advantages have not yet been fully explored in electrochemical fields. The bulk structure with low conductivity impedes charge-transfer kinetics during electrochemical processes. Excessive nitrogen content leads to insufficient charge transfer, while bulk structures produce tortuous channels for mass transport. Some attempts have been made to address these issues by nanostructure engineering, such as ultrathin structure design, heterogeneous composition, defect engineering, and morphology control. These structure-engineered nanomaterials have been successfully applied in electrochemical fields, including ionic actuators, flexible supercapacitors, lithium-ion batteries, and electrochemical sensors. Herein, a timely review on the latest advances in graphitic carbon nitride through various engineering strategies for electrochemical applications has been summarized. A perspective on critical challenges and future research directions is highlighted for graphitic carbon nitride in electrochemistry on the basis of existing research works and our experimental experience.

Flexible Resistance-Type Strain Sensors toward Monitoring Finger Movements

Flexible strain sensors with superior flexibility and high sensitivity are critical to artificial intelligence, and it is favorable to develop highly sensitive strain sensors by simple and cost-effective methods. We have prepared carbon-nanotubes-enhanced thermal polyurethane nanocomposites with good mechanical and electrical properties for the fabrication of highly sensitive strain sensors. The nanomaterials were prepared through a simple but effective solvent-evaporation method, and cheap polyurethane was used as the main raw material. Only a small quantity of carbon nanotubes (mass content 5%) was doped into a polyurethane matrix with aim of enhancing the mechanical and electrical properties of the nanocomposite. The resulting flexible nanocomposite films show a highly sensitive resistance response under an external strain stimulus. Strain sensors based on these flexible composite films deliver excellent sensitivity and conformality under mechanical conditions, and can detect finger movements precisely at various bending angles.