Multifunctional Iontronic Sensor Based on Liquid Metal-Filled Ho llow Ionogel Fibers in Detecting Pressure, Temperature, and Proximity

Flexible iontronic sensing

The emerging flexible iontronic sensing (FITS) technology has introduced a novel modality for tactile perception, mimicking the topological structure of human skin while providing a viable strategy for seamless integration with biological systems. With research progress, FITS has evolved from focusing on performance optimization and structural enhancement to a new phase of integration and intelligence, positioning it as a promising candidate for next-generation wearable devices. Therefore, a review from the perspective of technological development trends is essential to fully understand the current state and future potential of FITS devices. In this review, we examine the latest advancements in FITS. We begin by examining the sensing mechanisms of FITS, summarizing research progress in material selection, structural design, and the fabrication of active and electrode layers, while also analysing the challenges and bottlenecks faced by different segments in this field. Next, integrated systems based on FITS devices are reviewed, highlighting their applications in human-machine interaction, healthcare, and environmental monitoring. Additionally, the integration of artificial intelligence into FITS is explored, focusing on optimizing front-end device design and improving the processing and utilization of back-end data. Finally, building on existing research, future challenges for FITS devices are identified and potential solutions are proposed.

Highly stretchable, conductive, and self-adhesive starch-based hydrogel for high-performance flexible electronic devices

To achieve the green and sustainable development of environment, biocompatible hydrogels with exceptional ionic conductivity and flexibility are highly desired for intelligent and wearable sensors. However, it remains a great challenge to obtain biopolymer hydrogel-based sensors with high transparency, excellent mechanical properties, and good adhesion ability simultaneously. Herein, starch/polyacrylamide double-network hydrogel is achieved to endow the multifunctionality of traditional hydrogel sensor. Specifically, the resultant hydrogel sensor exhibits wide strain detection range of 2580 %, fast response time of 120 ms, high conductivity of 31.9 mS·m-1, superior sensitivity, remarkable fatigue resistance of 1350 cycles. In addition, multiple hydrogen bonding endows starch/polyacrylamide hydrogel with high mechanical properties and high transparency. Owing to these merits, the hydrogel sensor is capable of discriminating different human motions. Notably, the ionic conducting hydrogels could be employed as single-electrode TENGs for energy harvesting. The multifunctionality and biocompatibility of starch-based hydrogel sensor may offer an inspiration for the future development of next-generation sustainable and wearable electronics.

Capacitive pressure sensors based on bioinspired structured electrode for human-machine interaction applications

Flexible pressure sensor is a crucial component of tactile sensors and plays an integral role in numerous significant fields. Despite the considerable effort put forth, how to further improve sensitivity with ingenious yet easy-to-manufacture structures and apply them to emerging fields such as structure/materials recognition, human motion monitoring, and human-machine interaction remains a challenge. Here, we develop a highly sensitive flexible capacitive pressure sensor featuring a structured electrode layer with embedded microcracks and a dielectric layer with micro-convex structures, which are combined with an iontronic interface. The sophisticated design endows the sensor with superior perceptual performance, showing a relatively linear sensitivity of 1613 kPa-1 in the range of 50 kPa and a detection limit of ∼6.7 Pa. Due to its excellent sensing capabilities, the sensors have been demonstrated for microstructure/material stiffness recognition and human motion monitoring. Furthermore, by integrating a single sensor with an inertial unit, the sensor gains the capability to output multiple sets of instructions. This work provides innovative design inspiration for flexible electronics.

Advanced Morphological and Material Engineering for High-Performance Interfacial Iontronic Pressure Sensors

High-performance flexible pressure sensors are crucial for applications such as wearable electronics, interactive systems, and healthcare technologies. Among these, iontronic pressure sensors have garnered particular attention due to their superior sensitivity, enabled by the giant capacitance variation of the electric double layer (EDL) at the ionic-electronic interface under deformation. Key advancements, such as incorporating microstructures into ionic layers and employing diverse materials, have significantly improved sensor properties like sensitivity, accuracy, stability, and response time. This review highlights advancements in flexible EDL pressure sensors, focusing on structural designs and material engineering. These strategies are tailored to optimize key metrics such as sensitivity, detection limit, linearity, stability, response speed, hysteresis, transparency, wearability, selectivity, and multifunctionality. Key fabrication techniques, including micropatterning and externally assisted methods, are reviewed, along with strategies for sensor comparison and guidelines for selecting appropriate sensors. Emerging applications in healthcare, environmental and aerodynamic sensing, human-machine interaction, robotics, and machine learning-assisted intelligent sensing are explored. Finally, this review discusses the challenges and future directions for advancing EDL-based pressure sensors.

Review of Liquid Metal Fiber Based Biosensors and Bioelectronics

Liquid metal, as a novel material, has become ideal for the fabrication of flexible conductive fibers and has shown great potential in the field of biomedical sensing. This paper presents a comprehensive review of the unique properties of liquid metals such as gallium-based alloys, including their excellent electrical conductivity, mobility, and biocompatibility. These properties make liquid metals ideal for the fabrication of flexible and malleable biosensors. The article explores common preparation methods for liquid metal conductive fibers, such as internal liquid metal filling, surface printing with liquid metal, and liquid metal coating techniques, and their applications in health monitoring, neural interfaces, and wearable devices. By summarizing and analyzing the current research, this paper aims to reveal the current status and challenges of liquid metal conductive fibers in the field of biosensors and to look forward to their development in the future, which will provide valuable references and insights for researchers in the field of biomedical engineering.

Hybrid Integration of Wearable Devices for Physiological Monitoring

Wearable devices can provide timely, user-friendly, non- or minimally invasive, and continuous monitoring of human health. Recently, multidisciplinary scientific communities have made significant progress regarding fully integrated wearable devices such as sweat wearable sensors, saliva sensors, and wound sensors. However, the translation of these wearables into markets has been slow due to several reasons associated with the poor system-level performance of integrated wearables. The wearability consideration for wearable devices compromises many properties of the wearables. Besides, the limited power capacity of wearables hinders continuous monitoring for extended duration. Furthermore, peak-power operations for intensive computations can quickly create thermal issues in the compact form factor that interfere with wearability and sensor operations. Moreover, wearable devices are constantly subjected to environmental, mechanical, chemical, and electrical interferences and variables that can invalidate the collected data. This generates the need for sophisticated data analytics to contextually identify, include, and exclude data points per multisensor fusion to enable accurate data interpretation. This review synthesizes the challenges surrounding the wearable device integration from three aspects in terms of hardware, energy, and data, focuses on a discussion about hybrid integration of wearable devices, and seeks to provide comprehensive guidance for designing fully functional and stable wearable devices.

Highly sensitive and wide-range iontronic pressure sensors with a wheat awn-like hierarchical structure

Flexible pressure sensors with high sensitivity and wide sensing range are highly desired in e-skins and wearable electronics. However, there is always a trade-off between high sensitivity and broad sensing range for most flexible pressure sensors. Herein, an artificial wheat awn-like hierarchical structure is designed onto the dielectric layer of the iontronic pressure sensor, realizing both high sensitivity and broad working range. The sensor is constructed by sandwiching a wheat awn-like polyvinyl alcohol/H3PO4 dielectric layer between two transparent electrodes of silver nanowires/thermoplastic polyurethane/ionic liquid. The obtained sensor exhibits a high precision of 1 Pa, a high sensitivity of 47.65 kPa-1 (1-200 Pa), a wide measurement range from 1 Pa to 238 kPa, short response/recovery time of 13 ms/12 ms, outstanding stability over 6000 cycles, as well as good transparency. Considering these excellent properties, the sensor shows promising potential in health monitoring, human-computer interaction, wearable electronics, etc.

Stretchable Iontronic Tactile Sensing Fabric

The iontronic tactile sensing modality has garnered significant attention due to its exceptional sensitivity, immunity to noise, and versatility in materials. Recently, various formats of iontronic tactile sensors have been developed, including droplets, polymer films, paper, ionic gels, and fabrics. However, the stretchability of the current iontronic pressure sensing fabric is inadequate, hindered by the limited stretchiness of the ionic functional fabric. Incorporating a stretchable tactile sensing implement could enhance the wear comfortability by preventing relative movement and ensuring intimate contact between the sensor and the skin. The research focuses on the development of a stretchable iontronic pressure sensing (SIPS) fabric for monitoring diverse aspects of body health and movement in wearable applications. The tactile sensing structure is generated at the iontronic interface between highly stretchable ionic and conductive fabrics. In particular, the ionic fabric is prepared by coating a layer of polyurethane/ionic liquid gel onto a Spandex fabric. To showcase its remarkable sensitivity, stretchability, and ability to detect diverse body information, several application scenarios have been demonstrated including an elastic wristband for precise pulse wave detection, a flexible belt with multitactile sensing channels for respiration and motion tracking purposes, and a stretchable fabric cuff equipped with a high-resolution sensing array comprising 32 × 32 units for accurate gesture recognition.

Fibrous MXene Synapse-Based Biomimetic Tactile Nervous System for Multimodal Perception and Memory

Biomimetic tactile nervous system (BTNS) inspired by organisms has motivated extensive attention in wearable fields due to its biological similarity, low power consumption, and perception-memory integration. Though many works about planar-shape BTNS are developed, few researches could be found in the field of fibrous BTNS (FBTNS) which is superior in terms of strong flexibility, weavability, and high-density integration. Herein, a FBTNS with multimodal sensibility and memory is proposed, by fusing the fibrous poly lactic acid (PLA)/Ag/MXene/Pt artificial synapse and MXene/EMIMBF4 ionic conductive elastomer. The proposed FBTNS can successfully perceive external stimuli and generate synaptic responses. It also exhibits a short response time (23 ms) and low set power consumption (17 nW). Additionally, the proposed device demonstrates outstanding synaptic plasticity under both mechanical and electrical stimuli, which can simulate the memory function. Simultaneously, the fibrous devices are embedded into textiles to construct tactile arrays, by which biomimetic tactile perception and temporary memory functions are successfully implemented. This work demonstrates the as-prepared FBTNS can generate biomimetic synaptic signals to serve as artificial feeling signals, it is thought that it could offer a fabric electronic unit integrating with perception and memory for Human-Computer interaction, and has great potential to build lightweight and comfortable Brain-Computer interfaces.

Proximity Sensing Electronic Skin: Principles, Characteristics, and Applications

The research on proximity sensing electronic skin has garnered significant attention. This electronic skin technology enables detection without physical contact and holds vast application prospects in areas such as human-robot collaboration, human-machine interfaces, and remote monitoring. Especially in the context of the spread of infectious diseases like COVID-19, there is a pressing need for non-contact detection to ensure safe and hygienic operations. This article comprehensively reviews the significant advancements in the field of proximity sensing electronic skin technology in recent years. It covers the principles, as well as single-type proximity sensors with characteristics such as a large area, multifunctionality, strain, and self-healing capabilities. Additionally, it delves into the research progress of dual-type proximity sensors. Furthermore, the article places a special emphasis on the widespread applications of flexible proximity sensors in human-robot collaboration, human-machine interfaces, and remote monitoring, highlighting their importance and potential value across various domains. Finally, the paper provides insights into future advancements in flexible proximity sensor technology.

Liquid Metal-Polymer Conductor-Based Conformal Cyborg Devices

Gallium-based liquid metal (LM) exhibits exceptional properties such as high conductivity and biocompatibility, rendering it highly valuable for the development of conformal bioelectronics. When combined with polymers, liquid metal-polymer conductors (MPC) offer a versatile platform for fabricating conformal cyborg devices, enabling functions such as sensing, restoration, and augmentation within the human body. This review focuses on the synthesis, fabrication, and application of MPC-based cyborg devices. The synthesis of functional materials based on LM and the fabrication techniques for MPC-based devices are elucidated. The review provides a comprehensive overview of MPC-based cyborg devices, encompassing their applications in sensing diverse signals, therapeutic interventions, and augmentation. The objective of this review is to serve as a valuable resource that bridges the gap between the fabrication of MPC-based conformal devices and their potential biomedical applications.

Biosignal-integrated robotic systems with emerging trends in visual interfaces: A systematic review

Human-machine interfaces (HMI) are currently a trendy and rapidly expanding area of research. Interestingly, the human user does not readily observe the interface between humans and machines. Instead, interactions between the machine and electrical signals from the user's body are obscured by complex control algorithms. The result is effectively a one-way street, wherein data is only transmitted from human to machine. Thus, a gap remains in the literature: how can information be effectively conveyed to the user to enable mutual understanding between humans and machines? Here, this paper reviews recent advancements in biosignal-integrated wearable robotics, with a particular emphasis on "visualization"-the presentation of relevant data, statistics, and visual feedback to the user. This review article covers various signals of interest, such as electroencephalograms and electromyograms, and explores novel sensor architectures and key materials. Recent developments in wearable robotics are examined from control and mechanical design perspectives. Additionally, we discuss current visualization methods and outline the field's future direction. While much of the HMI field focuses on biomedical and healthcare applications, such as rehabilitation of spinal cord injury and stroke patients, this paper also covers less common applications in manufacturing, defense, and other domains.

Ionogels for flexible conductive substrates and their application in biosensing

Ionogels are highly conductive gels made from ionic liquids dispersed in a matrix made of organic or inorganic materials. Ionogels are known for high ionic conductivity, flexibility, high thermal and electrochemical stability. These characteristics make them suitable for sensing and biosensing applications. This review discusses about the two main constituents, ionic liquids and matrix, used to make ionogels and effect of these materials on the characteristics of ionogels. Here, the material properties like mechanical, electrochemical and stability are discussed for both polymer matrix and ionic liquid. We have briefly described about the fabrication methods like 3D printing, sol-gel, blade coating, spin coating, aerosol jet printing etc., used to make films or coating of these ionogels. The advantages and disadvantages of each method are also briefly summarized. Finally, the last section provides a few examples of application of flexible ionogels in areas like wearables, human-machine interface, electronic skin and detection of biological molecules.

Highly sensitive, wide-pressure and low-frequency characterized pressure sensor based on piezoresistive-piezoelectric coupling effects in porous wood

Lightweight and highly compressible materials have received considerable attention in flexible pressure sensing devices. In this study, a series of porous woods (PWs) are produced by chemical removal of lignin and hemicellulose from natural wood by tuning treatment time from 0 to 15 h and extra oxidation through H₂O₂. The prepared PWs with apparent densities varying from 95.9 to 46.16 mg/cm³ tend to form a wave-shaped interwoven structure with improved compressibility (up to 91.89 % strain under 100 kPa). The sensor assembled from PW with treatment time of 12 h (PW-12) exhibits the optimal piezoresistive-piezoelectric coupling sensing properties. For the piezoresistive properties, it has high stress sensitivity of 15.14 kPa^-1, covering a wide linear working pressure range of 0.06-100 kPa. For its piezoelectric potential, PW-12 shows a sensitivity of 0.443 V·kPa^-1 with ultralow frequency detection as low as 0.0028 Hz, and good cyclability over 60,000 cycles under 0.41 Hz. The nature-derived all-wood pressure sensor shows obvious superiority in the flexibility for power supply requirement. More importantly, it presents fully decoupled signals without cross-talks in the dual-sensing functionality. Sensor like this is capable of monitoring various dynamic human motions, making it an extremely promising candidate for the next generation artificial intelligence products.

Modulus difference-induced embedding strategy to construct iontronic pressure sensor with high sensitivity and wide linear response range

Sensitivity and linearity are two crucial indices to assess the sensing capability of pressure sensors; unfortunately, the two mutually exclusive parameters usually result in limited applications. Although a series of microengineering strategies including micropatterned, multilayered, and porous approach have been provided in detail, the conflict between the two parameters still continues. Here, we present an efficient strategy to resolve this contradiction via modulus difference-induced embedding deformation. Both the microscopic observation and finite element simulation results confirm the embedding deformation behavior ascribed to the elastic modulus difference between soft electrode and rigid microstructures. The iontronic pressure sensor with high sensitivity (35 kPa-1) and wide linear response range (0-250 kPa) is further fabricated and demonstrates the potential applications in monitoring of high-fidelity pulse waveforms and human motion. This work provides an alternative strategy to guide targeted design of all-around and comprehensive pressure sensor.

Dual-Stimuli-Responsive and Anti-Freezing Conductive Ionic Hydrogels for Smart Wearable Devices and Optical Display Devices

Stimuli-responsive hydrogels are a class of important materials for the preparation of flexible sensors, but the development of UV/stress dual-responsive ion-conductive hydrogels with excellent tunability for wearable devices remains a major challenge. In this study, a dual-responsive multifunctional ion-conductive hydrogel (PVA-GEL-GL-Mo₇) with high tensile strength, good stretchability, outstanding flexibility, and stability is successfully fabricated. The prepared hydrogel has an excellent tensile strength of 2.2 MPa, high tenacity of 5.26 MJ/m³, favorable extensibility (522%), and high transparency of 90%. Importantly, the hydrogels have dual responsiveness to UV light and stress, allowing it to be used as a wearable device while responding differently to the UV intensity of different outdoor environments (hydrogels can show different levels of color when exposed to different light intensities of UV light) and can remain flexible at -50 and 85 °C (sensing at both -25 and 85 °C). Therefore, the hydrogels developed in this study have good prospects in different applications, such as flexible wearable devices, duplicate paper, and dual-responsive interactive devices.

Skin-Friendly and Wearable Iontronic Touch Panel for Virtual-Real Handwriting Interaction

Touch panels are deemed as a critical platform for the future of human-computer interaction and metaverse. Recently, stretchable iontronic touch panels have attracted attention due to their superior adhesivity to the human body. However, such adhesion can not be named "real wearable", leading to discomfort for the wearer, such as rashes or itching with long-time wearing. Herein, a skin-friendly and wearable iontronic textile-based touch panel with highly touch-sensing resolution and deformation insensitivity is designed based on an in-suit growing strategy. This textile-based touch panel endows excellent interfacial hydrophilic and biocompatibility with human skin by overcoming the bottlenecks of the hydrogel-based uncomfortable sticky touch interface and low mechanical behavior. The developed touch panel enables handwriting interaction with good mechanical capacity (114 MPa), nearly 4145 times higher than pure hydrogel. More importantly, our touch panel possesses intrinsic insensitivity to wide external loading from the silver fiber (<0.003 g) to even heavy metal block (>10 kg). As proof of concept, the textile-based iontronic touch panel is applied to handwriting interaction, such as a flexible keyboard and wearable sketchpad. This iontronic touch panel with skin-friendly and wearable qualitities is helpful for next-generation wearable interaction electronics.