Ultrasensitive and Highly Stretchable Multifunctional Strain Sensors with Timbre‐Recognition Ability Based on Vertical Graphene

Stretchable/wearable strain sensors are attracting growing interest due to their broad applications in physical and physiological measurements. However, the development of a multifunctional highly stretchable sensor satisfying the requirements of ultrahigh sensitivity (able to distinguish sound frequency) remains a challenge. An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene (VGr) is fabricated using an ultrasonic peeling (UP) method. It can distinguish frequencies of sounds higher than 2500 Hz. Detailed microscopic examinations reveal that their ultrahigh sensitivity stems from the formation of high‐density nanocracks in the graphitic base layer, which is bridged by the top branched VGr nanowalls. These nanocracks cut the VGr film into a large number of nanopieces, which increase the natural frequency of the sensors, enabling the sensors to distinguish the sound frequency. Demonstrations are presented to highlight the sensors' potential as wearable devices for human physiological signal and timbre detections. This is the first multifunctional highly stretchable strain sensor with timbre‐recognition ability.     

Multifunctional ZnO‐Nanowire‐Based Sensor

A simple fabrication of ZnO‐nanowire‐based device and their implementation as a pH sensor, temperature sensor, and photo detector is reported. The presented multifunctional ZnO multiple‐nanowire sensor platform contains a Au finger structure, which is realized by conventional photolithography on a SiO₂ substrate. The nanowires are grown using thermal chemical vapor deposition. In order to detect the physical signals, changes in electrical signals were measured (conductance and current). For temperature sensing, the current behavior from 90 to 380 K under vacuum conditions exhibit a tunneling behavior between spaced nanowires. For photo sensing, the current response between the “on” and “off” states of light was measured when exposed to different wavelengths ranging from UV to visible light. Finally, for pH sensing the conductance was measured between a pH of 5 and 8.5. The ZnO nanowires were protected from chemical attacks by a thin layer of C₄F₈‐plasma‐based coating.     

Stretchable,Transparent, and Self‐Patterned Hydrogel‐Based Pressure Sensor for Human Motions Detection

In this study, a binary networked conductive hydrogel is prepared using acrylamide and polyvinyl alcohol. Based on the obtained hydrogel, an ultrastretchable pressure sensor with biocompatibility and transparency is fabricated cost effectively. The hydrogel exhibits impressive stretchability (>500%) and superior transparency (>90%). Furthermore, the self‐patterned microarchitecture on the hydrogel surface is beneficial to achieve high sensitivity (0.05 kPa^?1 for 0–3.27 kPa). The hydrogel‐based pressure sensor can precisely monitor dynamic pressures (3.33, 5.02, and 6.67 kPa) with frequency‐dependent behavior. It also shows fast response (150 ms), durable stability (500 dynamic cycles), and negligible current variation (6%). Moreover, the sensor can instantly detect both tiny (phonation, airflowing, and saliva swallowing) and robust (finger and limb motions) physiological activities. This work presents insights into preparing multifunctional hydrogels for mechanosensory electronics.     

A Highly Stretchable Fiber‐Based Triboelectric Nanogenerator for Self‐Powered Wearable Electronics

The development of flexible and stretchable electronics has attracted intensive attention for their promising applications in next‐generation wearable functional devices. However, these stretchable devices that are made in a conventional planar format have largely hindered their development, especially in highly stretchable conditions. Herein, a novel type of highly stretchable, fiber‐based triboelectric nanogenerator (fiber‐like TENG) for power generation is developed. Owing to the advanced structural designs, including the fiber‐convolving fiber and the stretchable electrodes on elastic silicone rubber fiber, the fiber‐like TENG can be operated at stretching mode with high strains up to 70% and is demonstrated for a broad range of applications such as powering a commercial capacitor, LCD screen, digital watch/calculator, and self‐powered acceleration sensor. This work verifies the promising potential of a novel fiber‐based structure for both power generation and self‐powered sensing.     

Highly Flexible and Stretchable Nanowire Superlattice Fibers Achieved by Spring‐Like Structure of Sub‐1 nm Nanowires

Conventional inorganic nanowire (NW) fibers are usually not stretchable and elastic, which may limit their practical applications. Inspired by the similarity between inorganic sub‐1 nm NWs and polymer chains in dimension, and helical spring‐like structure of cellulose in cherry bark, highly flexible and stretchable NW superlattice fibers composed of sub‐1 nm GdOOH NWs are fabricated. The NW fibers could be twined, bent, twisted, and tied without any damage. When the strain is less than 10%, the fibers present elastic deformation. The elongation at break of the fibers usually reaches ≈40–50% and the highest elongation could reach ≈86%. Excellent flexibility and stretchability of the NW fibers are attributed to the well‐aligned spring‐like NWs assembled superlattice, which are demonstrated by scanning electron microscopy tests, synchrotron small‐angle X‐ray scattering, and obvious birefringence. Moreover, NW‐nanoparticle (NP) fibers are fabricated, inspired by inorganic nanoparticle–reinforced polymers. The strength is improved compared with the NW fibers. Based on this work, it is possible to fabricate multifunctional, flexible, and stretchable inorganic NW materials composed of different inorganic sub‐1 nm NWs, which may be useful in practical applications.     

An Artificial Peripheral Neural System Based on Highly Stretchable and Integrated Multifunctional Sensors

Prostheses and robots have been affecting all aspects of life. Making them conscious and intelligent like humans is appealing and exciting, while there is a huge contrast between progress and strong demand. An alternative strategy is to develop an artificial peripheral neural system with high-performance bionic receptors. Here, a novel functional composite material that can serve as a key ingredient to simultaneously construct different artificial exteroceptive sensors (AE sensors) and artificial proprioceptive sensors (AP sensors) is demonstrated. Both AP sensors and AE sensors demonstrate outstandingly high stretchability; up to 200% stretching strain and possess the superior performance of fast response and high stability. An artificial peripheral neural system integrated with the highly stretchable AP sensor and AE sensor is constructed, which makes a significant breakthrough in the perception foundation of efficient proprioception and exteroception for intelligent prostheses and soft robots. Accurate feedback on the activities of body parts, music control, game manipulation, and wireless typing manifest the enormous superiority of the spatiotemporal resolution function of the artificial peripheral neural system, all of which powerfully contribute to promoting intelligent prostheses and soft robots into sophistication, and are expected to make lives more fascinating.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

A Microfluidic,Reversibly Stretchable,Large‐Area Wireless Strain Sensor

This article describes the implementation and characterization of a new self‐contained large‐area wireless strain sensor, operating around 1.5 GHz, based on the concept of multi‐layer microfluidic stretchable radiofrequency electronics (μFSRFEs). Compared to existing solutions, the presented integrated strain sensor is capable of remotely detecting repeated high tensile dynamic strains of up to 15% over very large surfaces or movable parts, and gets rid of all hardwiring to external storage or data processing equipment. Unlike conventional electronic devices, the major part of the sensor is a mechanically reconfigurable and reversibly deformable patch antenna, which consists of two layers of liquid metal alloy filled microfluidic channels in a silicone elastomer. A simplified radiofrequency (RF) transmitter composed of miniaturized rigid active integrated circuits (ICs) associated with discrete passive components was assembled on a flexible printed circuit board (FPCB) and then heterogeneously integrated to the antenna. The elastic patch antenna can withstand repeated mechanical stretches while still maintaining its electrical function to some extent, and return to its original state after removal of the stress. Additionally, its electrical characteristics at frequency of operation are highly sensitive to mechanical strains. Consequently, not only is this antenna a radiator for transmitting and receiving RF signals like any other conventional antennas, but also acts as a reversible large‐area strain sensor in the integrated device. Good electrical performance of the standalone antenna and the RF transmitter sub‐module was respectively verified by experiments. Furthermore, a personal computer (PC)‐assisted RF receiver for receiving and processing the measured data was also designed, implemented, and evaluated. In the real‐life demonstration, the integrated strain sensor successfully monitored periodically repeated human body motion, and wirelessly transmitted the measured data to the custom‐designed receiver at a distance of 5m in real‐time.     

A Path Beyond Metal and Silicon:Polymer/Nanomaterial Composites for Stretchable Strain Sensors

Conventional strain sensors based on metals and semiconductors are rigid and cannot measure soft and stretchable objects. Thus, new strain sensors based on polymer/nanomaterial composites are attracting more interest. Although much effort has been dedicated to achieve high values of both sensitivity and stretchability with linearity, this work endeavors to search and establish guidelines for the development of stretchable strain sensors, by critically reviewing conventional sensors and examining recent progress. It starts from introducing key parameters for conventional strain sensors; these parameters are further discussed for their potential impact on new polymer/nanomaterial strain sensors. The work concludes that there are no general benchmarks for conventional strain sensors utilized in industry. From the findings, the authors suggest that stretchable strain sensors should be custom designed and developed to meet particular measurement requirements, in comparison with a generic aim of yielding a sensor with high degrees of stretchability, sensitivity, and linearity. Challenges are discussed, including reliability, calibration to be used as proper gauges, and soft data acquisition systems.     

A Self‐Healable,Highly Stretchable,and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing

Mimicking human skin's functions to develop electronic skins has inspired tremendous efforts in design and synthesis of novel soft materials with simplified fabrication methods. However, it still remains a great challenge to develop electronically conductive materials that are both stretchable and self‐healable. Here it is demonstrated that a ternary polymer composite comprised of polyaniline, polyacrylic acid, and phytic acid can exhibit high stretchability ( ≈ 500%) and excellent self‐healing properties. The polymer composite with optimized composition shows an electrical conductivity of 0.12 S cm^?1. On rupture, both electrical and mechanical properties can be restored with ≈ 99% efficiency in a 24 h period, which is enabled by the dynamic hydrogen bonding and electrostatic interactions. It is further shown that this composite is both strain and pressure sensitive, and therefore can be used for fabricating strain and pressure sensors to detect a variety of mechanical deformations with ultrahigh sensitivity. The sensitivity and sensing range are the highest among all of the reported self‐healable piezoresistive pressure sensors and even surpass most flexible mechanical sensors. Notably, this composite is prepared via a solution casting process, which potentially allows for large‐area, low‐cost fabrication electronic skins.     

Stretchable and Transparent Conductive PEDOT:PSS‐Based Electrodes for Organic Photovoltaics and Strain Sensors Applications

The development of transparent, conducting, and stretchable poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)‐based electrodes using a combination of a polyethylene oxide (PEO) polymer network and the surfactant Zonyl is reported. The latter improves the ductility of PEDOT:PSS and enables its deposition on hydrophobic surfaces such as polydimethylsiloxane (PDMS) elastomers, while the presence of a 3D matrix offers high electrical conductivity, elasticity, and mechanical recoverability. The resulting electrode exhibits attractive properties such as high electrical conductivity of up to 1230 S cm^?1 while maintaining high transparency of 95% at 550 nm. The potential of the electrode technology is demonstrated in indium‐tin‐oxide (ITO)‐free solar cells using the PBDB‐T‐2F:IT‐4F blend with a power conversion efficiency of 12.5%. The impact of repeated stretch‐and‐release cycles on the electrical resistance is also examined in the effort to evaluate the properties of the electrodes. The interpenetrated morphology of the PEDOT:PSS and polyethylene oxide network is found to exhibit beneficial synergetic effects resulting in excellent mechanical stretchability and high electrical conductivity. By carefully tuning the amount of additives, the ability to detect small changes in electrical resistance as a function of mechanical deformation is demonstrated, which enables the demonstration of stretchable and resilient on‐skin strain sensors capable of detecting small motions of the finger.     

Coaxial Thermoplastic Elastomer‐Wrapped Carbon Nanotube Fibers for Deformable and Wearable Strain Sensors

Highly conductive and stretchable fibers are crucial components of wearable electronics systems. Excellent electrical conductivity, stretchability, and wearability are required from such fibers. Existing technologies still display limited performances in these design requirements. Here, achieving highly stretchable and sensitive strain sensors by using a coaxial structure, prepared via coaxial wet spinning of thermoplastic elastomer‐wrapped carbon nanotube fibers, is proposed. The sensors attain high sensitivity (with a gauge factor of 425 at 100% strain), high stretchability, and high linearity. They are also reproducible and durable. Their use as safe sensing components on deformable cable, expandable surfaces, and wearable textiles is demonstrated.     

A Single Droplet‐Printed Double‐Side Universal Soft Electronic Platform for Highly Integrated Stretchable Hybrid Electronics

Soft features in electronic devices have provided an opportunity of gleaning a wide spectrum of intimate biosignals. Lack of data processing tools in a soft form, however, proclaims the need of bulky wires or low‐performance near‐field communication externally linked to a “rigid” processor board, thus tarnishing the true meaning of “soft” electronics. Furthermore, although of rising interest in stretchable hybrid electronics, lack of consideration in multilayer, miniaturized design and system‐level data computing limits their practical use. The results presented here form the basis of fully printable, system‐level soft electronics for practical data processing and computing with advanced capabilities of universal circuit design and multilayer device integration into a single platform. Single droplet printing‐based integration of rigid islands and core–shell vertical interconnect access (via) into a common soft matrix with a symmetric arrangement leads to a double‐side universal soft electronic platform that features site‐selective, simultaneous double‐side strain isolation, and vertical interconnection, respectively. Systematic studies of island‐morphology engineering, surface‐strain mapping, and electrical analysis of the platform propose optimized designs. Commensurate with the universal layout, a complete example of double‐side integrated, stretchable 1 MHz binary decoders comprised of 36 logic gates interacting with 9 vias is demonstrated by printing‐based, double‐side electronic functionalization.     

Fingertip‐Skin‐Inspired Highly Sensitive and Multifunctional Sensor with Hierarchically Structured Conductive Graphite/Polydimethylsiloxane Foams

Fingertip skin exhibits high sensitivity in a broad pressure range, and can detect diverse stimuli, including textures, temperature, humidity, etc. Despite adopting diverse microstructures and functional materials, achieving skin sensor devices possessing high pressure sensitivity over a wide linear range and with multifunctional sensing capabilities is still challenging. Herein, inspired by the microstructures of fingertip skin, a highly sensitive skin sensor is demonstrated with a linear response over a broad pressure range and multifunctional sensing capabilities. The porous sensing layer is designed with hierarchical microstructures on the surface. By optimizing the porosity and the graphite concentration, a fabricated skin sensor device exhibits a superior sensitivity of 245 kPa^?1 over a broad linear pressure range from 5 Pa to 120 kPa. For practical application demonstrations, the sensor devices are utilized to monitor subtle wrist pulse and diverse human motions including finger bending, wrist bending, and feet movement. Furthermore, this novel sensor device demonstrates potential applications in recognizing textures and detecting environmental temperatures, thereby marking an important progress for constructing advanced electronic skin.     

A Highly Stretchable Cross‐Linked Polyacrylamide Hydrogel as an Effective Binder for Silicon and Sulfur Electrodes toward Durable Lithium‐Ion Storage

Despite the recent advancement in the in‐practical active materials (e.g., silicon, sulfur) in the rechargeable lithium‐ion energy storage systems, daunting challenges still remain for these high‐capacity electrode material candidates to overcome the severe volume changes associated with the repeated lithiation/delithiation process. Herein, developing a room‐temperature covalently cross‐linked polyacrylamide (c‐PAM) binder with high stretchability and abundant polar groups targeting the construction of high‐performance Si and sulfur electrodes is focused on. The robust 3D c‐PAM binder network enables not only significant enhancement of the strain resistance for working electrodes but also strong affinity to bonding with nano‐Si surface as well as effective capture of the soluble Li₂S_n intermediates, thereby giving rise to remarkably improved cycling performances in both types of electrodes. This rational design of such an effective and multifunctional binder offers a pathway toward advanced energy storage implementations.     

Omnidirectional Bending and Pressure Sensor Based on Stretchable CNT‐PU Sponge

Bending and pressure sensors are very essential for evaluating external stimuli in human motions; however, most of them are separate devices. Here, two orthogonal carbon nanotube–polyurethane sponge strips (CPSSs) are used, each of which has different resistances when bent or pressed, to fabricate a multi‐functional stretchable sensor capable of detecting omnidirectional bending and pressure independently. Due to the shape of the strip, the resistance of CPSS changes differently when bent along different directions. Based on this feature, two perpendicular CPSSs can reflect information of both bending distance and bending direction. After basic measurement data are obtained, a function set can be formulated to calculate bending distance and bending direction simultaneously. The errors of bending distance and bending angle can be controlled to less than 4%. With the help of the triboelectric effect, which only happens when the device is pressed, the sensor can differentiate bending and pressure effectively, ensuring the device works in complex situations.     

Stretchable Self‐Powered Fiber‐Based Strain Sensor

The rapid development of electrical skin and wearable electronics raises the requirement of stretchable strain sensors. In this study, an active fiber‐based strain sensor (AFSS) is fabricated by coiling a fiber‐based generator around a stretchable silicone fiber. The AFSS shows the sensitive and stable performance and has the ability to detect the strain up to 25%, which is also demonstrated to detect finger motion states. It may play an essential role in future self‐powered sensor system.     

Single‐Thread‐Based Wearable and Highly Stretchable Triboelectric Nanogenerators and Their Applications in Cloth‐Based Self‐Powered Human‐Interactive and Biomedical Sensing

The development of wearable and large‐area fabric energy harvester and sensor has received great attention due to their promising applications in next‐generation autonomous and wearable healthcare technologies. Here, a new type of “single” thread‐based triboelectric nanogenerator (TENG) and its uses in elastically textile‐based energy harvesting and sensing have been demonstrated. The energy‐harvesting thread composed by one silicone‐rubber‐coated stainless‐steel thread can extract energy during contact with skin. With sewing the energy‐harvesting thread into a serpentine shape on an elastic textile, a highly stretchable and scalable TENG textile is realized to scavenge various kinds of human‐motion energy. The collected energy is capable to sustainably power a commercial smart watch. Moreover, the simplified single triboelectric thread can be applied in a wide range of thread‐based self‐powered and active sensing uses, including gesture sensing, human‐interactive interfaces, and human physiological signal monitoring. After integration with microcontrollers, more complicated systems, such as wireless wearable keyboards and smart beds, are demonstrated. These results show that the newly designed single‐thread‐based TENG, with the advantage of interactive, responsive, sewable, and conformal features, can meet application needs of a vast variety of fields, ranging from wearable and stretchable energy harvesters to smart cloth‐based articles.     

A Sensing and Stretchable Polymer-Dispersed Liquid Crystal Device Based on Spiderweb-Inspired Silver Nanowires-Micromesh Transparent Electrode

Polymer-dispersed liquid crystal (PDLC) devices are truly promising optical modulators for information display, smart window as well as intelligent photoelectronic applications due to their fast switching, large optical modulation as well as cost-effectiveness. However, realizing highly soft PDLC devices with sensing function remains a grand challenge because of the intrinsic brittleness of traditional transparent conductive electrodes. Here, inspired by spiderweb configuration, a novel type of silver nanowires (AgNWs) micromesh-based stretchable transparent conductive electrodes (STCEs) is developed to support the realization of soft PDLC device. Benefiting from the embedding design of AgNWs micromesh in polydimethylsiloxane (PDMS), the STCEs can maintain excellent electrical conductivity and transparency even in various extreme conditions such as bending, folding, twisting, stretching as well as multiple chemical corrosion. Further, STCEs with the embedded AgNWs micromesh endow the assembled PDLC device with excellent photoelectrical properties including rapid switching speed (<1 s), large optical modulation (69% at 600 nm), as well as robust mechanical stability (bending over 1000 cycles and stretching to 40%). Moreover, the device displays the pressure sensing function with high sensitivity in response to pressure stimulus. It is conceivable that AgNWs micromesh transparent electrodes will shape the next generation of related soft smart electronics.