Enhanced Sensitivity of Iontronic Graphene Tactile Sensors Facilitated by Spreading of Ionic Liquid Pinned on Graphene Grid

Iontronic graphene tactile sensors (i‐GTS) composed of a top floating graphene electrode and an ionic liquid droplet pinned on a bottom graphene grid, which can dramatically enhance the performance of capacitive‐type tactile sensors, are presented. When mechanical stress is applied to the top floating electrode, the i‐GTS operates in one of the following three regimes: air–air, air–electric double layer (EDL) transition, or EDL–EDL. Once the top electrode contacts the ionic liquid in the i‐GTS, the spreading behavior of the ionic liquid causes a capacitance transition (from a few pF to over hundreds of pF). This is because EDLs are formed at the interfaces between the electrodes and the ionic liquid. In this case, the pressure sensitivity increases to ≈31.1 kPa^?1 with a gentle touch. Under prolonged application of pressure, the capacitance increases gradually, mainly due to the contact line expansion of the ionic liquid bridge pinned on the graphene grid. The sensors exhibit outstanding properties (response and relaxation times below 80 ms, and stability over 300 cycles) while demonstrating ultimate signal‐to‐noise ratios in the array tests. The contact‐induced spreading behavior of the ionic liquid is the key for boosting the sensor performance.     

A Highly Stable and Durable Capacitive Strain Sensor Based on Dynamically Super-Tough Hydro/Organo-Gels

Capacitive-type strain sensors based on hydrogel ionic conductors have undergone rapid development benefited from their robust structure, drift-free sensing, higher sensitivity, and precision. However, the unsatisfactory electro-mechanical stability of the conventional hydrogel conductors, which are normally vulnerable to large deformation and severe mechanical impacts, remains a challenge. In addition, there is not enough research regarding the adhesiveness and mechanical properties of the dielectric layer, which is also critical for the mechanical adaptability of the whole device. Here, a dynamically super-tough capacitive-type strain sensor based on energy-dissipative dual-crosslinked hydrogel conductors and an organogel dielectric with high adhesive strength is developed. Combining with the mechanical advantages of the hydro/organo-gels, the capacitive strain sensor exhibits high stretchability and superior linear dependence of sensitivity with a gauge factor of ≈0.8% at 100% strain. Moreover, the sensor displayed ultrastability against various severe mechanical stimuli that can even survive unprecedentedly from extremely catastrophic car run-over by 20 times. With these synergistic mechanical advantages, the capacitive strain sensor is successfully applied as a highly-reliable wearable sensing system to monitor diverse faint physiological signals and large-range human motions.     

Multifunctional Hydrogel Sensor with Curved Macro Cracks: A Strategy for High Sensitivity and Wide Detection Range

Multi-sensing in simple devices, but with a high sensitivity and a large detection range, is desirable for soft machines. Stretchable sensors based on the resistance changes of bulk ionic hydrogels are inherently limited by the single function and low sensitivity at small deformations. Here, a design enabled by a highly cracked hydrogel (HCHG) that is hypersensitive to tensile strain, bending, and tactile force in a wide range is proposed. The mechanism relies on the continuous sharp changes of the cross-sectional area flowing ionic current when pre-cut curved cracks are closed/opened by external load. The high fracture toughness of the hydrogel inhibits the crack propagation, making the sensing robust. By designing the crack patterns, sensitivities of 80 for 0–20% tensile strain and 0.45 kPa⁻¹ for tactile force are achieved. Compared to the sensor made from bulk hydrogel, the sensitivities are enhanced by two and three orders of magnitude, respectively, meanwhile the detectable strain range is maintained (up to 215%). A sandwich design is also developed to distinguish elongation, compression, and bending. Applications of HCHG sensors in manipulating a robotic arm and nondestructive grasping an even softer object by a soft gripper are demonstrated.     

Stretchable and Temperature‐Sensitive Polymer Optical Fibers for Wearable Health Monitoring

Stretchable physical sensors that can detect and quantify human physiological signals such as temperature, are essential to the realization of healthcare devices for biomedical monitoring and human–machine interfaces. Despite recent achievements in stretchable electronic sensors using various conductive materials and structures, the design of stretchable sensors in optics remains a considerable challenge. Here, an optical strategy for the design of stretchable temperature sensors, which can maintain stable performance even under a strain deformation up to 80%, is reported. The optical temperature sensor is fabricated by the incorporation of thermal‐sensitive upconversion nanoparticles (UCNPs) in stretchable polymer‐based optical fibers (SPOFs). The SPOFs are made from stretchable elastomers and constructed in a step‐index core/cladding structure for effective light confinements. The UCNPs, incorporated in the SPOFs, provide thermal‐sensitive upconversion emissions at dual wavelengths for ratiometric temperature sensing by near‐infrared excitation, while the SPOFs endow the sensor with skin‐like mechanical compliance and excellent light‐guiding characteristics for laser delivery and emission collection. The broad applications of the proposed sensor in real‐time monitoring of the temperature and thermal activities of the human body, providing optical alternatives for wearable health monitoring, are demonstrated.     

A Puncture-Resistant and Self-Healing Conductive Gel for Multifunctional Electronic Skin

Flexible electronic skins (e-skins) play a very important role in the development of human–machine interaction and wearable devices. To fully mimic the functions of human skin, e-skins should be able to perceive multiple external stimuli (such as temperature, touch, and friction) and be resistant to injury. However, both objectives are highly challenging. The fabrication of multifunctional e-skins is difficult because of the complex lamination scheme and the integration of different sensors. The design of skin-like materials is hindered by the trade-off problem between flexibility, toughness, and self-healing ability. Herein, flexible sodium methallyl sulfonate functionalized poly(thioctic acid) polymer chains are combined with rigid conductive polyaniline rods through ionic bonds to obtain a solvent-free polymer conductive gel. The conductive gel has a modulus similar to that of skin, and shows good flexibility, puncture-resistance, notch-insensitivity, and fast self-healing ability. Moreover, this conductive gel can convert changes in temperature and strain into electrical signal changes, thus leading to multifunctional sensing performance. Based on these superior properties, a flexible e-skin sensor is prepared, demonstrating its great potential in the wearable field and physiological signal detection.     

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

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

Flexible Antiswelling Photothermal-Therapy MXene Hydrogel-Based Epidermal Sensor for Intelligent Human–Machine Interfacing

Conductive hydrogel-based epidermal sensors are regarded with broad prospects in bridging the gap between human and machine for personalized healthcare. However, it is still challenging to simultaneously achieve high sensitivity, wide sensing range, and reliable cycling stability in hydrogel-based epidermal sensors for ultrasensitive human–machine interfacing, along with brilliant antiswelling capability, and near-infrared (NIR) light-triggered dissociation and drug release for further smart on-demand photothermal therapy. Herein, the facile preparation of a flexible multifunctional epidermal sensor from the elaborately fabricated, highly stretchable, and antiswelling MXene hydrogel is presented. It exhibits high sensitivity, wide sensing range (up to 350% strain), and reliable reproducibility for enabling ultrasensitive human-machine interfacing. It displays excellent antiswelling capability for the hydrogel to avoid expanding the wound due to excessive swelling for further reliable wound therapy. Furthermore, it possesses good biocompatibility and robust photothermal performance for the smart photothermal therapy after healthcare monitoring. Meanwhile, the sensor can be triggered to be softened and partly dissociated under the prolonged NIR light irradiation with the transformation of the temperature-sensitive low-melting-point Agar into a sol state and the partial dissociation in the hydrogel to release the loaded drug on demand for synergistically sterilizing bacteria and efficiently promoting wound healing.     

Triboelectric Touch‐Free Screen Sensor for Noncontact Gesture Recognizing

An intelligent human–machine interface (HMI) is a crucial medium for exchanging information between people and electronics. As one of the most important HMI devices, touch screen sensors are widely applied in personal electronics in daily life. However, as the most commonly used touch screen sensor, capacitive sensors can only detect limited kinds of gestures such as touching and sliding. Here, a triboelectric touch‐free screen sensor (TSS) is reported for recognizing diverse gestures in a noncontact operating mode by utilizing the charges naturally carried on the human body. Compared with conventional capacitive sensors, the TSS is capable of detecting various gestures such as the drop and lift of finger with different speeds, making a fist, opening palm, and flipping palm with different directions. Based on the TSS, an intelligent noncontact screen control system is further developed, which is used to unlock the smartphone interface by the noncontact operating mode. This research for the first time proposes the concept that taking the human body itself to participate in triboelectric self‐powered noncontact sensing and provides a touch‐free design concept to develop the next generation of screen sensors. It can alter the usual way that people operating their personal electronics.     

Ultrasensitive,Low‐Voltage Operational,and Asymmetric Ionic Sensing Hydrogel for Multipurpose Applications

Current artificial tactile sensors mostly exploit a variety of electron‐related physical mechanisms to obtain high sensitivity and low detection force. However, these mechanisms are still distinct from the ion‐related biological processes of human's tactile sensation, and are therefore away from the goal of bionic applications. In the past few years, only several types of ionic tactile sensors have been proposed, and they are still subject to low sensitivity. Here, a novel type of ultrasensitive hydrogel tactile sensor is reported based on asymmetric ionic charge injection as the working mechanism, named as asymmetric ionic sensing hydrogel (AISH). With a small external working voltage of only tens of millivolts, these AISH devices show an extremely low detection force of 0.075 Pa, ultrahigh sensitivity of 57–171 kPa^?1, and excellent cycling reliability upon pressing. Applications of these ultrasensitive tactile sensors in fingerprint identification of voice, monitoring of pulse waves, and detection of underwater wave signals are experimentally demonstrated. Combining the merits of simple fabrication process, ionic‐type detection mechanism, and ion injection procedure, such AISH sensors not only reveal a new strategy toward highly sensitive tactile sensors, but also show realistic potential applications in future wearable electronic and bioelectronic devices.     

Ultra-Sensitive and Stretchable Ionic Skins for High-Precision Motion Monitoring

Stretchability and sensitivity are essential properties of wearable electronics for effective motion monitoring. In general, increasing the sensitivity of strain sensors based on ionic conductors trades off elasticity, which results in low sensitivity of the strain sensors at large mechanical deformations. To address this, ion-permeable conducting polymer electrodes with low contact resistance are utilized in ionic gel-based strain sensors. Using a rectangular-shaped ionic gel and ion-permeable electrodes significantly increase the gauge factor of the strain sensor, similar to the theoretical value at a given strain. To further increase the sensitivity of the strain sensor, the ionic gel is patterned with zigzagged tracks that gap apart as the gel stretches, and the gaps close as the gel contracts, leading to a large variation in the relative resistance upon stretching. By combining the zigzagged ionic gel and the ion-permeable electrodes, highly sensitive stretchable sensors are realized with a record-high gauge factor of 173, compared to existing ionic conductor-based stretchable strain sensors. The zigzag-patterned ionic sensor can successfully monitor various motions when attached to the human body. These results are expected to afford promising strategies for developing highly sensitive, stretchable sensing systems for E-skin sensors and soft robotics.     

Stimuli-Responsive Peptide Self-Assembly to Construct Hydrogels with Actuation and Shape Memory Behaviors

Hydrogel actuators, capable of generating reversible deformation in response to external stimulus, are widely considered as new emerging intelligent materials for applications in soft robots, smart sensors, artificial muscles, and so on. Peptide self-assembly is widely applied in the construction of intelligent hydrogel materials due to their excellent stimulus response. However, hydrogel actuators based on peptide self-assembly are rarely reported and explored. In this study, a pH-responsive peptide (MA-FIID) is designed and introduced into a poly(N-isopropyl acrylamide) backbone (PNIPAM) to construct bilayer and heterogeneous hydrogel actuators based on the assembly and disassembly of peptide molecules under different pH conditions. These peptide-containing hydrogel actuators can perform controllable bending, bucking, and complex deformation under pH stimulation. Meanwhile, the Hofmeister effect of PNIPAM hydrogels endows these peptide-containing hydrogels with enhanced mechanical strength, ionic stimulus response (CaCl₂), and excellent shape-memory property. This work broadens the application of supramolecular self-assembly in the construction of intelligent hydrogels, and also provides new inspirations for peptide self-assembly to construct smart materials.     

Dynamically Modulated GaN Whispering Gallery Lasing Mode for Strain Sensor

The continuous development of strain sensors offers significant opportunities for improving human–machine interfaces and health monitoring. The dynamically modulated lasing mode is a novel approach to realize a flexible, noncontact, high color‐resolvability, high‐resolution, and ultrasensitive strain sensor. Here, a flexible strain sensor perceiving stress variations is reported via the dynamical regulation of a GaN whispering gallery lasing mode based on the piezoelectric effect. The refraction index of GaN shows a linear relationship with the applied external tensile strain, resulting in a redshift phenomenon of the lasing mode peak at room temperature due to the predominant function of the piezoelectric polarization in the GaN microwire. Compared with a strain sensor relying on the wavelength shift of a photoluminescence (PL) emission peak, the differences and advantages of a sensor based on the strain‐induced lasing mode variation are also investigated and analyzed systematically. This strain sensor may serve as an essential step toward the color mapping of mechanical signals by optical methods, with potential applications in color‐perceived touching sensing, noncontact stress measurement, laser modulation, and optical communication technologies.     

Self‐Powered Tactile Sensor Array Systems Based on the Triboelectric Effect

With the arrival of intelligent terminals, tactile sensors which are capable of sensing various external physical stimuli are considered among the most vital devices for the next generation of smart electronics. To create a self‐powered tactile sensor system that can function sustainably and continuously without an external power source is of crucial significance. An overview of the development in self‐powered tactile sensor array system based on the triboelectric effect is systematically presented. The combination of multi‐functionalization and high performance of tactile sensors aimed at achieving highly comprehensive performance is presented. For the tactile sensor unit, a development is summarized based on the two primary modes which are vertical contact–separation and single‐electrode. For the pressure mapping array, the resolution is significantly enhanced by the novel cross‐type configuration based on the single‐electrode mode. Integrated with other mechanisms, the performance will be further elevated by broadening of the detect range and realizing of visualization of pressure imaging. Then, two main applications of human–machine interaction (HMI) and trajectory monitoring are comprehensively summarized. Finally, the future perspectives of self‐powered tactile sensor system based on triboelectric effect are discussed.     

Emerging Technologies of Flexible Pressure Sensors: Materials,Modeling, Devices,and Manufacturing

As an important branch of wearable electronics, flexible pressure sensors have attracted extensive research owing to their wide range of applications, such as human–machine interfaces and health monitoring. To fulfill the requirements for different applications, new material design and device fabrication strategies have been developed in order to manipulate the mechanical and electrical properties and enhance device performance. In this paper, the important progresses in flexible pressure sensor development over recent years are selectively reviewed from a material and application perspective. First, an overview of the fundamental working mechanism and the systematic design approach is presented. Particularly, how the theoretical modeling has been used as an auxiliary tool to achieve better sensing performance is discussed. A number of applications, including human–machine interfaces, electronic skin and health monitoring, and certain application‐driven functions, e.g., pressure distribution visualization and direction‐sensitive force detection, are highlighted. Lastly, various advanced manufacturing methods used for realizing large‐scale fabrication are introduced.     

Highly Sensitive,Wearable, Durable Strain Sensors and Stretchable Conductors Using Graphene/Silicon Rubber Composites

Highly sensitive, wearable and durable strain sensors are vital to the development of health monitoring systems, smart robots and human machine interfaces. The recent sensor fabrication progress is respectable, but it is limited by complexity, low sensitivity and unideal service life. Herein a facile, cost‐effective and scalable method is presented for the development of high‐performance strain sensors and stretchable conductors based on a composite film consisting of graphene platelets (GnPs) and silicon rubber. Through calculation by the tunneling theory using experimental data, the composite film has demonstrated ideal linear and reproducible sensitivity to tensile strains, which is contributed by the superior piezoresistivity of GnPs having tunable gauge factors 27.7–164.5. The composite sensors fabricated in different days demonstrate pretty similar performance, enabling applications as a health‐monitoring device to detect various human motions from finger bending to pulse. They can be used as electronic skin, a vibration sensor and a human‐machine interface controller. Stretchable conductors are made by coating and encapsulating GnPs with polydimethyl siloxane to create another composite; this structure allows the conductor to be readily bent and stretched with sufficient mechanical robustness and cyclability.     

Piezoelectric Nanogenerators Derived Self-Powered Sensors for Multifunctional Applications and Artificial Intelligence

With the arrival of the Internet of Things (IoTs) era, there is a growing requirement for systems with many sensor nodes in a variety of fields of applications. The demands for wireless, sustainable and independent operation are becoming more and more important for large-scale sensor networks and systems. For these purposes, a self-powered sensory system that can utilize the self-harvested energy from its surroundings to drive the sensors and directly sense external stimuli has attracted great attention. The invention and rapid development of piezoelectric generators (PENGs), which take Maxwell's displacement current as the driving force, has been pushing forward research on self-powered active mechanical sensors, electronic skins, and human-robotic interaction. Here, this review starts with a brief introduction of piezoelectric materials, fabrication, and performance improvement. Then, the energy harvesters used for self-power systems based on recent progress are reviewed. After that, PENGs applications toward recent self-powered active sensors are divided into four aspects and highlighted, respectively. Moreover, some challenges and future directions for the self-powered multifunctional sensors are put forward. It is believed that through the continuous investigations into PENG-based self-powered active sensors, they will soon be used in touch screens, electronic skins, health care, environmental monitoring, and intelligence systems.     

Monolithic Dual‐Material 3D Printing of Ionic Skins with Long‐Term Performance Stability

Artificial “ionic skin” is of great interest for mimicking the functionality of human skin, such as subtle pressure sensing. However, the development of ionic skin is hindered by the strict requirements of device integration and the need for devices with satisfactory performance. Here, a dual‐material printing strategy for ionic skin fabrication to eliminate signal drift and performance degradation during long‐term use is proposed, while endowing the ionic skins with high sensitivity by 3D printing of ionic hydrogel electrodes with microstructures. The ionic skins are fabricated by alternative digital light processing 3D printing of two photocurable precursors: hydrogel and water‐dilutable polyurethane acrylate (WPUA), in which the ionically conductive hydrogel layers serve as soft, transparent electrodes and the electrically insulated WPUA as flexible, transparent dielectric layers. This novel dual‐material printing strategy enables strong chemical bonding between the hydrogel and the WPUA, endowing the device with designed characteristics. The resulting device has high sensitivity, minimal hysteresis, a response time in the millisecond range, and excellent repetition durability for pressure sensing. The results demonstrate the potential of the dual‐material 3D printing strategy as a pathway to realize highly stable and high‐performance ionic skin fabrication to monitor human physiological signals and human–machine interactions.     

Cellulose Nanofibrils Enhanced,Strong, Stretchable,Freezing‐Tolerant Ionic Conductive Organohydrogel for Multi‐Functional Sensors

To date, ionic conducting hydrogel attracts tremendous attention as an alternative to the conventional rigid metallic conductors in fabricating flexible devices, owing to their intrinsic characteristics. However, simultaneous realization of high stiffness, toughness, ionic conductivity, and freezing tolerance through a simple approach is still a challenge. Here, a novel highly stretchable (up to 660%), strong (up to 2.1 MPa), tough (5.25 MJ m^?3), and transparent (up to 90%) ionic conductive (3.2 S m^?1) organohydrogel is facilely fabricated, through sol–gel transition of polyvinyl alcohol and cellulose nanofibrils (CNFs) in dimethyl sulfoxide‐water solvent system. The ionic conductive organohydrogel presents superior freezing tolerance, remaining flexible and conductive (1.1 S m^?1) even at ?70 °C, as compared to the other reported anti‐freezing ionic conductive (organo)hydrogel. Notably, this material design demonstrates synergistic effect of CNFs in boosting both mechanical properties and ionic conductivity, tackling a long‐standing dilemma among strength, toughness, and ionic conductivity for the ionic conducting hydrogel. In addition, the organohydrogel displays high sensitivity toward both tensile and compressive deformation and based on which multi‐functional sensors are assembled to detect human body movement with high sensitivity, stability, and durability. This novel organohydrogel is envisioned to function as a versatile platform for multi‐functional sensors in the future.     

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

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

An evaluation of the optical fiber beam as a force sensor

The present study reports the use of an intensity based plastic optical fiber (POF) as a force sensor. Different materials for beam, such as spring steel and mild steel, are used to evaluate the performance of the force sensor during macro-bending. The POF is surface bonded to a beam and subjected to force. The system relies on monitoring the light intensity, as the POF is subjected to transversal loading conditions. Experimentally obtained output of POF which could be measured with negligible hysteresis is compared with finite element analysis in the range between 0.0098 N to 19.613 N. The reproducibility of the sensor is observed in the limit of ±1%. The finding of this study highlights the potential use of POF sensors for various force sensing applications.