Bimodal Tactile Sensor without Signal Fusion for User-Interactive Applications

Visible-to-Near-Infrared Mechanoluminescence in Bi-Activated Spinel Compounds for Multiple Information Anticounterfeiting

Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu3+/Eu2+, Sm3+/Sm2+, and Mn4+/Mn2+. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis-NIR) ML induced by the self-reduction of Bi3+ to Bi2+ in the spinel-type compound (MgGa2O4) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi3+/Bi2+ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (VMg″) is considered as the driving force for the self-reduction of Bi3+ to Bi2+. Furthermore, an oxygen vacancy (VO••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron-hole (VO••/VMg″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis-NIR ML materials toward wider application possibilities.

Multi-Mode Luminescence in Smart Near-Infrared Cr3+/Pr3+ Codoped SrGa12O19 Phosphors Induced by Three Distinct Excitation Mechanisms

Near-infrared (NIR) phosphors have emerged as novel luminescent materials across various fields due to their unique advantages of high penetration and invisibility. However, there is currently a lack of intelligent NIR phosphors that can achieve multimode stimuli responsive for sensing applications. In this study, we employed a high-temperature solid-phase reaction to incorporate Pr3+ into Cr3+-doped gallate magnetite SrGa12O19 phosphor, yielding a multimode luminescent intelligent NIR phosphor. Also, due to the inherent cation vacancies and defects in the matrix, the material not only exhibits brighter photoluminescence but also exhibits distinct NIR mechanoluminescence at a lower load. Notably, Pr3+-doped SrGa12O19:Cr3+ also demonstrates extended persistent luminescence and thermoluminescence effects. Finally, we combined the phosphor with the blue LED chip to develop a new multifunctional NIR pc-LED. Leveraging NIR's unique penetrating ability, it can persist in biological tissues for prolonged periods, enabling optical inspection and offering a novel approach to password protection for anticounterfeiting measures. This intelligent NIR phosphor solution significantly expands the application potential of NIR light in food quality assessment and analysis.

Artificial Multi-Stimulus-Responsive E-Skin Based on an Ionic Film with a Counter-Ion Exchange Reagent

Sensing pressure and temperature are two important functions of human skin that integrate different types of tactile receptors. In this paper, a deformable artificial flexible multi-stimulus-responsive sensor is demonstrated that can distinguish mechanical pressure from temperature by measuring the impedance and the electrical phase at the same frequency without signal interference. The electrical phase, which is used for measuring the temperature, is totally independent of the pressure by controlling the surface micro-shapes and the ion content of the ionic film. By doping the counter-ion exchange reagent into the ionic liquid before pouring, the upper temperature measuring limit increases from 35 to 50 °C, which is higher than the human body temperature and the ambient temperature on Earth. The sensor shows high sensitivity to pressure (up to 0.495 kPa-1) and a wide temperature sensing range (-10 to 50 °C). A multimodal ion-electronic skin (IEM-skin) with an 8 × 8 multi-stimulus-responsive sensor array is fabricated and can successfully sense the distribution of temperature and pressure at the same time. Finally, the sensors are used for monitoring the touching motions of a robot-arm finger controlled by a remote interactive glove and successfully detect the touching states and the temperature changes of different objects.

Contactless User-Interactive Sensing Display for Human-Human and Human-Machine Interactions

Creating a large-scale contactless user-interactive sensing display (CUISD) with optimal features is challenging but crucial for efficient human-human or human-machine interactions. This study reports a CUISD based on dynamic alternating current electroluminescence (ACEL) that responds to humidity. Subsecond humidity-induced luminescence is achieved by integrating a highly responsive hydrogel into the ACEL layer. The patterned silver nanofiber electrode and luminescence layer, produced through electrospinning and microfabrication, result in a stretchable, large-scale, high-resolution, multicolor, and dynamic CUISD. The CUISD is implemented for the real-time control of a remote-controlled car, wherein the luminescence signals induced by touchless finger movements are distinguished and encoded to deliver specific commands. Moreover, the distinctive recognition of breathing facilitates the CUISD to serve as a visual signal transmitter for information interaction, which is particularly beneficial for individuals with disabilities. The paradigm shift depicts in this work is expected to reshape the way authors interact with each other and devices, discovering niche applications in virtual/augmented reality and the metaverse.

A multifunctional flexible sensor based on PI-MXene/SrTiO3 hybrid aerogel for tactile perception

The inadequacy of tactile perception systems in humanoid robotic manipulators limits the breadth of available robotic applications. Here, we designed a multifunctional flexible tactile sensor for robotic fingers that provides capabilities similar to those of human skin sensing modalities. This sensor utilizes a novel PI-MXene/SrTiO3 hybrid aerogel developed as a sensing unit with the additional abilities of electromagnetic transmission and thermal insulation to adapt to certain complex environments. Moreover, polyimide (PI) provides a high-strength skeleton, MXene realizes a pressure-sensing function, and MXene/SrTiO3 achieves both thermoelectric and infrared radiation response behaviors. Furthermore, via the pressure response mechanism and unsteady-state heat transfer, these aerogel-derived flexible sensors realize multimodal sensing and recognition capabilities with minimal cross-coupling. They can differentiate among 13 types of hardness and four types of material from objects with accuracies of 94% and 85%, respectively, using a decision tree algorithm. In addition, based on the infrared radiation-sensing function, a sensory array was assembled, and different shapes of objects were successfully recognized. These findings demonstrate that this PI-MXene/SrTiO3 aerogel provides a new concept for expanding the multifunctionality of flexible sensors such that the manipulator can more closely reach the tactile level of the human hand. This advancement reduces the difficulty of integrating humanoid robots and provides a new breadth of application scenarios for their possibility.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

Asymmetric and Skin-Mimicking Hydrogels with Wide Temperature Tolerance and Superior Elasticity for High-Performance Strain Sensors

Wide temperature tolerance and superior mechanical properties are highly required for composite hydrogels in electronic applications such as electronic skins and soft robotics. In this work, a unique polyacrylamide-based and double-network hydrogel system is designed and fabricated by introducing graphene oxide and glycerol to improve mechanical properties as well as antifreezing and antiheating properties. Maximum stress of the graphene oxide-incorporated hydrogel increases rapidly to 500.0 kPa which is much higher than that of polymetric acrylamide/carboxymethylcellulose sodium hydrogel (281.7 kPa), probably due to the inhibition from graphene oxide in generation and propagation of cracks. With constantly adding glycerol, total elongation and antifreezing and heating properties of the composite hydrogels increase gradually. Especially, sample with 20 vol % of glycerol not only shows stable conductivity and wide temperature tolerance (-50 to 50 °C) but also has ideal strength-toughness match (597.6 kPa and 1263.4%), suggesting that synergistic effect of different layers in the asymmetric structure plays an active role in improvement of mechanical properties.

Visualized Stress-Temperature Sensor with the Zinc Sulfide and Perovskite Glass Ceramics Composite

The visualized dual-modal stress-temperature sensing refers to the ability of a sensor to provide real-time and visible information about both stress and temperature and has indeed attracted significant interest in various fields. However, the development of convenient methods for achieving this capability remains a challenge. In this work, a dual-modal stress-temperature sensor is successfully fabricated using a ZnS/Cu@CsPbBr1.2I1.8 glass ceramics (GCs)/polydimethylsiloxane (PDMS) (ZCP) composite film. The tunable ML color is achieved by modulating the concentration of CsPbBr1.2I1.8 GCs in the ZCP composite films based on the light conversion process from ZnS/Cu to CsPbBr1.2I1.8 GCs. Additionally, the stress and temperature can be visualized simultaneously by integrating the ML intensity and ML color of the ZCP composite film. This feature allows for the real-time monitoring of automotive tire temperature by embedding the ZCP composite film on the tire surface, enabling a strong and stable response to both stress and temperature changes. Overall, this work offers a convenient, efficient, and repeatable approach for achieving visualized dual-modal stress-temperature sensing in the fields of mechanical engineering, structural health monitoring, and intelligent devices.

Multifunctional Intelligent Wearable Devices Using Logical Circuits of Monolithic Gold Nanowires

Although multifunctional wearable devices have been widely investigated for healthcare systems, augmented/virtual realities, and telemedicines, there are few reports on multiple signal monitoring and logical signal processing by using one single nanomaterial without additional algorithms or rigid application-specific integrated circuit chips. Here, multifunctional intelligent wearable devices are developed using monolithically patterned gold nanowires for both signal monitoring and processing. Gold bulk and hollow nanowires show distinctive electrical properties with high chemical stability and high stretchability. In accordance, the monolithically patterned gold nanowires can be used to fabricate the robust interfaces, programmable sensors, on-demand heating systems, and strain-gated logical circuits. The stretchable sensors show high sensitivity for strain and temperature changes on the skin. Furthermore, the micro-wrinkle structures of gold nanowires exhibit the negative gauge factor, which can be used for strain-gated logical circuits. Taken together, this multifunctional intelligent wearable device would be harnessed as a promising platform for futuristic electronic and biomedical applications.

Flexible Optoelectronic Multimodal Proximity/Pressure/Temperature Sensors with Low Signal Interference

Multimodal tactile sensors are a crucial part of intelligent human-machine interaction and collaboration. Simultaneous detection of proximity, pressure, and temperature on a single sensor can greatly promote the safety, interactivity, and compactness of interaction systems. However, severe signal interference and complex decoupling algorithms hinder the actual applications. Here, this work reports a flexible optoelectronic multimodal sensor capable of detecting and decoupling proximity/pressure/temperature by integrating a light waveguide and an interdigital electrode (IDE) into a compact fibrous sensor. Negligible signal interference is realized by combining heterogeneous sensing mechanisms of optics and electronics, which encodes proximity into capacitance, pressure into light intensity and temperature into resistance. The sensor exhibits a large sensing distance of 225 mm with fast responses for proximity detection, a pressure sensitivity of 0.42 N-1 , and a temperature sensitivity of 7% °C-1 . As a proof of concept, a doll equipped with the sensor can accurately discriminate and detect various stimuli, thus achieving safe and immersive interactions with the user. This work opens up promising paths for self-decoupled multimodal sensors and related human/machine/environment interaction applications.

Stretchable, Adhesive, and Bioinspired Visual Electronic Skin with Strain/Temperature/Pressure Multimodal Non-Interference Sensing

It is highly desirable to construct a single-multimodal sensor that could synchronously perceive multiple stimuli without interference. Here, we propose an adhesive multifunctional chromotropic electronic skin (MCES) that can respond to and distinguish three different stimuli of stain, temperature, and pressure within the two-terminal sensing unit. The mutually discriminating "three-in-one" device converts strain into capacitance and pressure into voltage signals for a tactile stimulus response and produces visual color changes against temperature. In this MCES system, the interdigital capacitor sensor shows high linearity (R² = 0.998), and temperature sensing is realized via reversible multicolor switching bioinspired by the chameleon, showing attractive potential in visualization interaction. Notably, the energy-harvesting triboelectric nanogenerator in MCES can not only detect pressure incentive but also identify objective material species. Looking forward, these findings promise for multimodal sensor technology with reduced complexity and production costs that are highly anticipated in soft robotics, prosthetics, and human-machine interaction applications.

Mechano-luminescence Behavior of Lanthanide-Doped Fluoride Nanocrystals for Three-Dimensional Stress Imaging

Pervasive mechanical force in nature and human activities is closely related to intriguing physics and widespread applications. However, describing stress distribution timely and precisely in three dimensions to avoid "groping in the dark" is still a formidable challenge, especially for nonplanar structures. Herein, we realize three-dimensional (3D) stress imaging for sharp arbitrary targets via advanced 3D printing, owing to the use of fluoride nanocrystal(NC)-based ink. Notably, a fascinating mechano-luminescence (ML) is observed for the homogeneously dispersed NaLuF₄:Tb³⁺ NCs (∼25 nm) with rationally designed deep traps (at 0.88 and 1.02 eV) via incorporating Cs⁺ ions and using X-ray irradiation. Carriers captured in the corresponding traps are steadily released under mechanical stimulations, which enables a ratio metric luminescence intensity based on the applied force. As a result, a significant mechano-optical conversion and superior optical waveguide of the corresponding transparent printed targets demonstrate stress in 3D with a high spatial and temporal resolution based on stereovision. These results highlight the optical function of the 3D-printed fluoride NCs, which cast light into the black boxes of stress described in space, benefiting us in understanding the ubiquitous force relevant to most natural and engineering processes.

A Single Electronic Tattoo for Multisensory Integration

Wearable electronics are garnering growing interest in various emerging fields including intelligent sensors, artificial limbs, and human-machine interfaces. A remaining challenge is to develop multisensory devices that can conformally adhere to the skin even during dynamic-moving environments. Here, a single electronic tattoo (E-tattoo) based on a mixed-dimensional matrix network, which integrates two-dimensional  MXene nanosheets and one-dimensional cellulose nanofibers/Ag nanowires, is presented for multisensory integration. The multidimensional configurations endow the E-tattoo with excellent multifunctional sensing capabilities including temperature, humidity, in-plane strain, proximity, and material identification. In addition, benefiting from the satisfactory rheology of hybrid inks, the E-tattoos are able to be fabricated through multiple facile strategies including direct writing, stamping, screen printing, and three-dimensional printing on various hard/soft substrates. Especially, the E-tattoo with excellent triboelectric properties also can serve as a power source for activating small electronic devices. It is believed that these skin-conformal E-tattoo systems can provide a promising platform for next-generation wearable and epidermal electronics.

Quasi-Homogeneous and Hierarchical Electronic Textiles with Porosity-Hydrophilicity Dual-Gradient for Unidirectional Sweat Transport, Electrophysiological Monitoring, and Body-Temperature Visualization

On-skin electronics based on impermeable elastomers and stacking structures often suffer from inferior sweat-repelling capabilities and severe mechanical mismatch between sub-layers employed, which significantly impedes their lengthy wearing comfort and functionality. Herein, inspired by the transpiration system of vascular plants and the water diode phenomenon, a hierarchical nonwoven electronic textile (E-textile) with multi-branching microfibers and robust interlayer adhesion is rationally developed. The layer-by-layer electro-airflow spinning method and selective oxygen plasma treatment are utilized to yield a porosity-hydrophilicity dual-gradient. The resulting E-textile shows unidirectional, nonreversible, and anti-gravity water transporting performance even upon large-scale stretching (250%), excellent mechanical matching between sub-layers, as well as a reversible color-switching ability to visualize body temperature. More importantly, the conducting and skin-conformal E-textile demonstrates accurate and stable detecting capability for biomechanical and bioelectrical signals when applied as an on-skin bioelectrode, including different human activities, electrocardiography, electromyogram, and electrodermal activity signals. Further, the E-textile can be efficiently implemented in human-machine interfaces to build a gesture-controlled dustbin and a smart acousto-optic alarm. Hence, this hierarchically-designed E-textile with integrated functionalities offers a practical and innovative method for designing comfortable and daily applicable on-skin electronics.

Mechanochromic Optical/Electrical Skin for Ultrasensitive Dual-Signal Sensing

Following earlier research efforts dedicated to the realization of multifunctional sensing, recent developments of artificial skins endeavor to go beyond human sensory functions by integrating interactive visualization of strain and pressure stimuli. Inspired by the microcracked structure of spider slit organs and the mechanochromic mechanism of chameleons, this work aims to design a flexible optical/electrical skin (OE-skin) capable of responding to complex stimuli with interactive feedback of human-readable structural colors. The OE-skin consists of an ionic electrode combined with an elastomer dielectric layer, a chromotropic layer containing photonic crystals and a conductive carbon nanotube/MXene layer. The electrode/dielectric layers function as a capacitive pressure sensor. The mechanochromic photonic crystals of ferroferric oxide-carbon magnetic arrays embedded in the gelatin/polyacrylamide stretchable hydrogel film perceive strain and pressure stimuli with bright color switching outputs in the full visible spectrum. The underlying microcracked conductive layer is devoted to ultrasensitive strain sensing with a gauge factor of 191.8. The multilayered OE-skin delivers an ultrafast, accurate response for capacitive pressure sensing with a detection limit of 75 Pa and long-term stability of 5000 cycles, while visualizing complex deformations in the form of high-resolution spatial colors. These findings offer deep insights into the rational design of OE-skins as multifunctional sensing devices.

Mechanoluminescent-Triboelectric Bimodal Sensors for Self-Powered Sensing and Intelligent Control

Self-powered flexible devices with skin-like multiple sensing ability have attracted great attentions due to their broad applications in the Internet of Things (IoT). Various methods have been proposed to enhance mechano-optic or electric performance of the flexible devices; however, it remains challenging to realize the display and accurate recognition of motion trajectories for intelligent control. Here, we present a fully self-powered mechanoluminescent-triboelectric bimodal sensor based on micro-nanostructured mechanoluminescent elastomer, which can patterned-display the force trajectories. The deformable liquid metals used as stretchable electrode make the stress transfer stable through overall device to achieve outstanding mechanoluminescence (with a gray value of 107 under a stimulus force as low as 0.3 N and more than 2000 cycles reproducibility). Moreover, a microstructured surface is constructed which endows the resulted composite with significantly improved triboelectric performances (voltage increases from 8 to 24 V). Based on the excellent bimodal sensing performances and durability of the obtained composite, a highly reliable intelligent control system by machine learning has been developed for controlling trolley, providing an approach for advanced visual interaction devices and smart wearable electronics in the future IoT era.

Smart Mechanoluminescent Phosphors: A Review of Strontium-Aluminate-Based Materials, Properties, and Their Advanced Application Technologies

Mechanoluminescence, a smart luminescence phenomenon in which light energy is directly produced by a mechanical force, has recently received significant attention because of its important applications in fields such as visible strain sensing and structural health monitoring. Up to present, hundreds of inorganic and organic mechanoluminescent smart materials have been discovered and studied. Among them, strontium-aluminate-based materials are an important class of inorganic mechanoluminescent materials for fundamental research and practical applications attributed to their extremely low force/pressure threshold of mechanoluminescence, efficient photoluminescence, persistent afterglow, and a relatively low synthesis cost. This paper presents a systematic and comprehensive review of strontium-aluminate-based luminescent materials' mechanoluminescence phenomena, mechanisms, material synthesis techniques, and related applications. Besides of summarizing the early and the latest research on this material system, an outlook is provided on its environmental, energy issue and future applications in smart wearable devices, advanced energy-saving lighting and displays.

Multifunctional Flexible Ionic Skin with Dual-Modal Output Based on Fibrous Structure

Flexible wearable electronic devices with multiple sensing functions that simulate human skin in all aspects have become a popular research topic. However, the current expensive and time-consuming means of integration and the complex decoupling process are hampering the further development of multifunctional sensors. Here, an ultraflexible ionic fiber membrane (IFM) prepared by a simple electrospinning technique is reported, which exhibits pressure and humidity sensing properties. With the help of different electrode structures, the IFM-based multifunctional sensor achieved pressure and humidity detection with different sensing mechanisms. Pressure sensing with high sensitivity (49.7 kPa^-1 at 0-30 kPa) and wide detection range (0-220 kPa) was indicated by the capacitive signal. Humidity sensing with high linearity (1.086% per percent relative humidity (RH)) in the range 15%-90% RH was indicated by the resistance signal. In particular, the multimodal output of capacitance/resistance corresponding to pressure/humidity in this study directly addresses the problem of accurately distinguishing the two stimuli. Furthermore, we have demonstrated that the impact between pressure and humidity is negligible when measured simultaneously and independently. Because of the excellent pressure/humidity sensing performance, we have fabricated a smart bracelet and mask for pulse, skin moisture, and breathe monitoring, which indicates the promising future of multifunctional flexible sensors based on IFM in the healthcare field.

Wireless Flexible Magnetic Tactile Sensor with Super-Resolution in Large-Areas

Tactile recognition is among the basic survival skills of human beings, and advances in tactile sensor technology have been adopted in various fields, bringing benefits such as outstanding performance in manipulating objects and general human-robot interactions. However, promoting enhanced perception of the existing tactile sensors is limited by their sensor array arrangement and wire-connected design. Here we present a wireless flexible magnetic tactile sensor (FMTS) consisting of a multidirection magnetized flexible film (perception module) and a contactless Hall sensor (signal receiving module). The flexible magnetic film is composed of NdFeB microparticles and soft silicone elastomer microparticles, and it transfers the unambiguous transduction of external force position and magnitude into magnetic signals. Benefiting from the specific magnetization arrangement and clustering algorithm, only one Hall sensor is needed in FMTS to perceive the magnitude and position of the contact spot simultaneously with super-resolution (2.1 mm average error) on a large area (3600 mm²), and the effective working distance is also greatly extended (∼30 mm), allowing for the full softness and adaptability to diverse conditions. We anticipate that this design will promote the development of soft tactile sensors and their integration into human-robot interaction and humanoid robot perception.