Mutually Noninterfering Flexible Pressure-Temperature Dual-Modal Sensors Based on Conductive Metal-Organic Framework for Electronic Skin

Flexible Strain Sensor Based on Nickel Microparticles/Carbon Black Microspheres/Polydimethylsiloxane Conductive Composites for Human Motion Detection

Herein, we report a dual-functional flexible sensor (DFFS) using a magnetic conductive polymer composed of nickel (Ni), carbon black (CB), and polydimethylsiloxane (PDMS). The material selection for the DFFS utilizes the excellent elasticity of the PDMS matrix and the synergistic interaction between Ni and CB. The DFFS has a wide strain range of 0-170%, a high sensitivity of 74.13 (140-170%), and a low detection limit of 0.3% strain. The DFFS based on superior performance can accurately detect microstrain/microvibration, oncoming/contacting objects, and bicycle riding speed. Additionally, the DFFS can be used for comprehensive monitoring of human movements. Therefore, the DFFS of this work shows significant value for implementation in intelligent wearable devices and noncontact intelligent control.

Venus Flytrap-Inspired Data-Center-Free Fast-Responsive Soft Robots Enabled by 2D Ni3(HITP)2 MOF and Graphite

The rapid and responsive capabilities of soft robots in perceiving, assessing, and reacting to environmental stimuli are highly valuable. However, many existing soft robots, designed to mimic humans and other higher animals, often rely on data centers for the modulation of mechanoelectrical transduction and electromechanical actuation. This reliance significantly increases system complexity and time delays. Herein, drawing inspiration from Venus flytraps, a soft robot employing a power modulation strategy is presented for active stimulus reaction, eliminating the need for a data center. This robot achieves mechanoelectrical transduction through Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) metal-organic framework (MOF) with an ultralow time delay (256 ns) and electromechanical actuation via graphite. The Joule heating effect in graphite is effectively modulated by Ni3(HITP)2 before and after the presence of pressure, thus enabling the stimulus reaction of soft robots. As demonstrated, three soft robots are created: low-level edge tongue robots, Venus flytrap robots, and high-level nerve-center-controlled dragonfly robots. This power modulation strategy inspires designs of edge soft robots and high-level robots with a human-like effective fusion of conditioned and unconditioned reflexes.

All-Printed Finger-Inspired Tactile Sensor Array for Microscale Texture Detection and 3D Reconstruction

Electronic skins are expected to replicate a human-like tactile sense, which significantly detects surface information, including geometry, material, and temperature. Although most texture features can be sensed in the horizontal direction, the lack of effective approaches for detecting vertical properties limits the development of artificial skin based on tactile sensors. In this study, an all-printed finger-inspired tactile sensor array is developed to realize the 3D detection and reconstruction of microscale structures. A beam structure with a suspended multilayer membrane is proposed, and a tactile sensor array of 12 units arranged in a dual-column layout is developed. This architecture enables the tactile sensor array to obtain comprehensive geometric information of micro-textures, including 3D morphology and clearance characteristics, and optimizes the 3D reconstruction patterns by self-calibration. Moreover, an innovative screen-printing technology incorporating multilayer printing and sacrificial-layer techniques is adopted to print the entire device. In additon, a Braille recognition system utilizing this tactile sensor array is developed to interpret Shakespeare's quotes printed in Grade 2 Braille. The abovementioned demonstrations reveal an attractive future vision for endowing bioinspired robots with the unique capability of touching and feeling the microscale real world and reconstructing it in the cyber world.

Fabrication of NiCo Bimetallic MOF Films on 3D Foam with Assistance of Atomic Layer Deposition for Non-Invasive Lactic Acid Sensing

Lactic acid (LA) is an important downstream product of glycolysis in living cells and is abundant in our body fluids, which are strongly associated with diseases. The development of enzyme-free LA sensors with high sensitivity and low consumption remains a challenge. 2D metal-organic frameworks (MOFs) are considered to be promising electrochemical sensing materials and have attracted much attention in recent years. Compared to monometallic MOFs, the construction of bimetallic MOFs (BMOFs) can obtain a larger specific surface area, thereby increasing the exposed active site. 3D petal-like NixCoy MOF films on nickel foams (NixCoy BMOF@Ni foams) are successfully prepared by combining atomic layer deposition-assisted technology and hydrothermal strategy. The established NixCoy BMOF@Ni foams demonstrate noticeable LA sensing activity, and the study is carried out on behalf of the Ni1Co5 BMOF@Ni foam, which has a sensitivity of up to 9030 μA mM-1 cm-2 with a linear range of 0.01-2.2 mM and the detection limit is as low as 0.16 μM. Additionally, the composite has excellent stability and repeatability for the detection of LA under a natural air environment with high accuracy and reliability. Density functional theory calculation is applied to study the reaction process between composites and LA, and the result suggests that the active site in the NiCo BMOF film favors the adsorption of LA relative to the active site of monometallic MOF film, resulting in improved performance. The developed composite has a great potential for the application of noninvasive LA biosensors.

Integrating In-Plane Thermoelectricity and Out-Plane Piezoresistivity for Fully Decoupled Temperature-Pressure Sensing

A flexible sensor that simultaneously senses temperature and pressure is crucial in various fields, such as human-machine interaction, artificial intelligence, and biomedical applications. Previous research has mainly focused on single-function flexible sensors for e-skins or smart devices, and integrated bimodal sensing of temperature and pressure without complex crosstalk decoupling algorithms remains challenging. In this work, a flexible bimodal sensor is proposed that utilizes spatial orthogonality between in-plane thermoelectricity and out-plane piezoresistivity, which enables fully decoupled temperature-pressure sensing. The proposed bimodal sensor exhibits a high sensitivity of 281.46 µV K-1 for temperature sensing and 2.181 kPa-1 for pressure sensing. In the bimodal sensing mode, the sensor exhibits negligible mutual interference, providing a measurement error of ± 7% and ± 8% for temperature and pressure, respectively, within a 120 kPa pressure range and a 40 K temperature variation. Additionally, simultaneous spatial mapping of temperature and pressure with a bimodal sensor array enables contact shape identification with enhanced accuracy beyond the limit imposed by the number of sensing units. The proposed integrated bimodal sensing strategy does not require complex crosstalk decoupling algorithms, which represents a significant advancement in flexible sensors for applications that necessitate simultaneous sensing of temperature and pressure.

Bionic Luminescent Skin as Ultrasensitive Temperature-Acoustic Sensor for Underwater Information Perception and Transmission

Bioinspired artificial luminescent skin (L-skin) integrated with multiple sensing functions significantly promotes the development of smart devices. It is considerably challenging to realize underwater sensing technologies. Here, a sharkskin-inspired Eu@HOF-TJ-1@TA L-skin (1) is prepared for both temperature and sound sensing. 1 is an ultrathin and flexible temperature sensor, in 298.15-358.15 K, exhibiting ultrahigh maximum relative sensitivity (97.669% K-1 ) and low minimum uncertainty (0.000 952 K). The temperature response mechanism is analyzed deeply. As a waterproofing acoustic sensor, 1 can monitor sound in both air and water with the greatest sound response frequencies of 400 and 300 Hz in air and water, respectively. The maximum sensitivities of 1 in air and water are 6 593 765.2 and 1 346 124.5 cps Pa-1 , respectively. The response times of 1 in air and water are as fast as 20 and 10 ms. The sound response processes of 1 in air and water are simulated by finite element simulation. Moreover, by using sharkskin-inspired 1, the actual water temperature can be monitored, and a series of water sound information can be recognized by using an artificial neural network. This work proposes a sharkskin-inspired L-skin for temperature and acoustic sensing and promotes the development of underwater sensing technology with high performances.

Oblique Pyramid Microstructure-Patterned Flexible Sensors for Pressure and Visual Temperature Sensing

Flexible tactile sensors have garnered considerable attention in diverse fields. Among them, the sensors integrated with multifunctional tactile sensing features can simultaneously detect various stimuli, such as pressure and temperature, and are thus suitable for practical applications. However, integrating multiple sensor modalities within a solitary pixel invariably encounters various limitations encompassing interplay among disparate sensors, intricate structural design demands, and the complexities and high costs associated with fabrication. Herein, we harness a visual sensing mechanism to synergize with electric sensors, thereby realizing a tactile sensor reliant on thermochromic microstructures for simultaneous pressure and temperature sensing. The thermal distribution could be easily displayed by the color change of the sensor, avoiding inference between the sensing units, which is beneficial for low-cost mass fabrication. A capacitor sensor with dual-scale oblique pyramid microstructures in its dielectric layer is used for the pressure sensing function, resulting in improved sensitivity and an extended measurement range. This innovative tactile sensor design offers insights into tactile sensing mechanisms, paving the way for cost-effective, high-performance, and multimodal sensor fabrication.

Conductive Metal-Organic Framework Nanosheets Constructed Hierarchical Water Transport Biological Channel for High-Performance Interfacial Seawater Evaporation

Solar interfacial water evaporation shows great potential to address the global freshwater scarcity. Water evaporation being inherently energy intensive, Joule-heating assisted solar evaporation for addressing insufficient vapor under natural conditions is an ideal strategy. However, the simultaneous optimization of low evaporation enthalpy, high photothermal conversion, and excellent Joule-heating steam generation within a single material remain a rare achievement. Herein, inspired by the biological channel structures, a large-area film with hierarchical macro/microporous structures is elaborately designed by stacking the nanosheet of a conductive metal-organic framework (MOF), Ni3 (HITP)2 , on a paper substrate. By combining the above three features in one material, the water evaporation enthalpy reduces from 2455 J g-1 to 1676 J g-1 , and the photothermal conversion efficiency increases from 13.75% to 96.25%. Benefiting from the synergistic photothermal and Joule-heating effects, the evaporation rate achieves 2.60 kg m-2 h-1 under one sun plus input electrical power of 4 W, surpassing the thermodynamic limit and marking the highest reported value in MOF-based evaporators. Moreover, Ni3 (HITP)2 -paper exhibits excellent long-term stability in simulated seawater, where no salt crystallization and evaporation rate degradation are observed. This design strategy for nanosheet films with hierarchical macro/microporous channels provides inspiration for electronics, biological devices, and energy applications.

Surface Modification of Super Arborized Silica for Flexible and Wearable Ultrafast-Response Strain Sensors with Low Hysteresis

Conductive hydrogels exhibit high potential in the fields of wearable sensors, healthcare monitoring, and e-skins. However, it remains a huge challenge to integrate high elasticity, low hysteresis, and excellent stretch-ability in physical crosslinking hydrogels. This study reports the synthesis of polyacrylamide (PAM)-3-(trimethoxysilyl) propyl methacrylate-grafted super arborized silica nanoparticle (TSASN)-lithium chloride (LiCl) hydrogel sensors with high elasticity, low hysteresis, and excellent electrical conductivity. The introduction of TSASN enhances the mechanical strength and reversible resilience of the PAM-TSASN-LiCl hydrogels by chain entanglement and interfacial chemical bonding, and provides stress-transfer centers for external-force diffusion. These hydrogels show outstanding mechanical strength (a tensile stress of 80-120 kPa, elongation at break of 900-1400%, and dissipated energy of 0.8-9.6 kJ m-3 ), and can withstand multiple mechanical cycles. LiCl addition enables the PAM-TSASN-LiCl hydrogels to exhibit excellent electrical properties with an outstanding sensing performance (gauge factor = 4.5), with rapid response (210 ms) within a wide strain-sensing range (1-800%). These PAM-TSASN-LiCl hydrogel sensors can detect various human-body movements for prolonged durations of time, and generate stable and reliable output signals. The hydrogels fabricated with high stretch-ability, low hysteresis, and reversible resilience, can be used as flexible wearable sensors.

Ti3C2Tx MXene-Based Multifunctional Tactile Sensors for Precisely Detecting and Distinguishing Temperature and Pressure Stimuli

Although skin-like sensors that can simultaneously detect various physical stimuli are of fair importance in cutting-edge human-machine interaction, robotic, and healthcare applications, they still face challenges in facile, scalable, and cost-effective production using conventional active materials. The emerging two-dimensional transition metal carbide, Ti3C2Tx MXene, integrated with favorable thermoelectric properties, metallic-like conductivity, and a hydrophilic surface, is promising for solving these problems. Herein, skin-like multifunctional sensors are designed to precisely detect and distinguish temperature and pressure stimuli without cross-talk by decorating elastic and porous substrates with MXene sheets. Because the combination of the thermoelectric and conductive MXene with the thermally insulating, elastic, and porous substrate integrates efficient Seebeck and piezoresistive effects, the resultant sensor exhibits not only an ultralow detection limit (0.05 K), high signal-to-noise ratio, and excellent cycling stability for temperature detection but also high sensitivity, fast response time, and outstanding durability for pressure detection. Based on the impressive dual-mode sensing properties and independent temperature and pressure detections, a multimode input terminal and an electronic skin are created, exhibiting great potential in robotic and human-machine interaction applications. This work provides a scalable fabrication of multifunctional tactile sensors for precisely detecting and distinguishing temperature and pressure stimuli.

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.

Skin-Inspired Highly Sensitive Tactile Sensors with Ultrahigh Resolution over a Broad Sensing Range

Flexible tactile sensors with high sensitivity, a broad pressure detection range, and high resolution are highly desired for the applications of health monitoring, robots, and the human-machine interface. However, it is still challenging to realize a tactile sensor with high sensitivity and resolution over a wide detection range. Herein, to solve the abovementioned problem, we demonstrate a universal route to develop a highly sensitive tactile sensor with high resolution and a wide pressure range. The tactile sensor is composed of two layers of microstructured flexible electrodes with high modulus and conductive cotton fabric with low modulus. By optimizing the sensing films, the fabricated tactile sensor shows a high sensitivity of 8.9 × 10⁴ kPa^-1 from 2 Pa to 250 kPa because of the high structural compressibility and stress adaptation of the multilayered composite films. Meanwhile, a fast response speed of 18 ms, an ultrahigh resolution of 100 Pa over 100 kPa, and excellent durability over 20 000 loading/unloading cycles are demonstrated. Moreover, a 6 × 6 tactile sensor array is fabricated and shows promising potential application in electronic skin (e-skin). Therefore, employing multilayered composite films for tactile sensors is a novel strategy to achieve high-performance tactile perception in real-time health monitoring and artificial intelligence.

Skin-interfaced electronics: A promising and intelligent paradigm for personalized healthcare

Skin-interfaced electronics (skintronics) have received considerable attention due to their thinness, skin-like mechanical softness, excellent conformability, and multifunctional integration. Current advancements in skintronics have enabled health monitoring and digital medicine. Particularly, skintronics offer a personalized platform for early-stage disease diagnosis and treatment. In this comprehensive review, we discuss (1) the state-of-the-art skintronic devices, (2) material selections and platform considerations of future skintronics toward intelligent healthcare, (3) device fabrication and system integrations of skintronics, (4) an overview of the skintronic platform for personalized healthcare applications, including biosensing as well as wound healing, sleep monitoring, the assessment of SARS-CoV-2, and the augmented reality-/virtual reality-enhanced human-machine interfaces, and (5) current challenges and future opportunities of skintronics and their potentials in clinical translation and commercialization. The field of skintronics will not only minimize physical and physiological mismatches with the skin but also shift the paradigm in intelligent and personalized healthcare and offer unprecedented promise to revolutionize conventional medical practices.

Flexible Pressure Sensors Based on Molybdenum Disulfide/Hydroxyethyl Cellulose/Polyurethane Sponge for Motion Detection and Speech Recognition Using Machine Learning

Flexible pressure sensors with excellent performance have broad application potential in wearable devices, motion monitoring, and human-computer interaction. In this paper, a flexible pressure sensor with a porous structure is proposed by coating molybdenum disulfide (MoS₂) and hydroxyethyl cellulose (HEC) on a polyurethane (PU) sponge skeleton. The obtained sensor has excellent sensitivity (0.746 kPa^-1), a wide detection range (250 kPa), fast response (120 ms), and outstanding repeatability over 2000 cycles. It is proven that the sensor can realize human motion detection and distinguish the touch of varying strength. In addition, a pressure sensing array was fabricated to reflect the pressure distribution and recognize the writing of Arabic numerals. Finally, the sensor performs speech detection through throat muscle movements, and high-accuracy (97.14%) speech recognition for seven words was achieved by a machine learning algorithm based on the support vector machine (SVM). This work provides an opportunity to fabricate simple flexible pressure sensors with potential applications in next-generation electronic skin, health detection, and intelligent robotics.

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.

Increased joint loading induces subchondral bone loss of the temporomandibular joint via the RANTES-CCRs-Akt2 axis

Early-stage temporomandibular joint osteoarthritis (TMJOA) is characterized by excessive subchondral bone loss. Emerging evidence suggests that TMJ disc displacement is involved, but the pathogenic mechanism remains unclear. Here, we established a rat model of TMJOA that simulated disc displacement with a capacitance-based force-sensing system to directly measure articular surface pressure in vivo. Micro-CT, histological staining, immunofluorescence staining, IHC staining, and Western blot were used to assess pathological changes and underlying mechanisms of TMJOA in the rat model in vivo as well as in RAW264.7 cells in vitro. We found that disc displacement led to significantly higher pressure on the articular surface, which caused rapid subchondral bone loss via activation of the RANTES-chemokine receptors-Akt2 (RANTES-CCRs-Akt2) axis. Inhibition of RANTES or Akt2 attenuated subchondral bone loss and resulted in improved subchondral bone microstructure. Cytological studies substantiated that RANTES regulated osteoclast formation by binding to its receptor CCRs and activating the Akt2 pathway. The clinical evidence further supported that RANTES was a potential biomarker for predicting subchondral bone loss in early-stage TMJOA. Taken together, this study demonstrates important functions of the RANTES-CCRs-Akt2 axis in the regulation of subchondral bone remodeling and provides further knowledge of how disc displacement causes TMJOA.

Stretchable and Self-Powered Temperature-Pressure Dual Sensing Ionic Skins Based on Thermogalvanic Hydrogels

Tactile sensors with both temperature- and pressure-responsive capabilities are critical to enabling future smart artificial intelligence. These sensors can mimic haptic functions of human skin and inevitably suffer from tensile deformation during operation. However, almost all actual multifunctional tactile sensors are either nonstretchable or the sensing signals interfere with each other when stretched. Herein, we propose a stretchable and self-powered temperature-pressure dual functional sensor based on thermogalvanic hydrogels. The sensor operates properly under stretching, which relies on the thermogalvanic effect and constant elastic modulus of hydrogels. The thermogalvanic hydrogel elastomer exhibits an equivalent Seebeck coefficient of -1.21 mV K^-1 and a pressure sensitivity of 0.056 kPa^-1. Combined with unit array integration, the multifunctional sensor can be used for accurately recording tactile information on human skin and spatial perception. This work provides a conceptual framework and systematic design for stretchable artificial skin, interactive wearables, and smart robots.