Small and light strain sensors based on graphene coated human hairs

Sensor-Based Wearable Systems for Monitoring Human Motion and Posture: A Review

In recent years, marked progress has been made in wearable technology for human motion and posture recognition in the areas of assisted training, medical health, VR/AR, etc. This paper systematically reviews the status quo of wearable sensing systems for human motion capture and posture recognition from three aspects, which are monitoring indicators, sensors, and system design. In particular, it summarizes the monitoring indicators closely related to human posture changes, such as trunk, joints, and limbs, and analyzes in detail the types, numbers, locations, installation methods, and advantages and disadvantages of sensors in different monitoring systems. Finally, it is concluded that future research in this area will emphasize monitoring accuracy, data security, wearing comfort, and durability. This review provides a reference for the future development of wearable sensing systems for human motion capture.

Epidermal Inspired Flexible Sensor with Buckypaper/PDMS Interfaces for Multimodal and Human Motion Monitoring Applications

The advancements in the areas of wearable devices and flexible electronic skin have led to the synthesis of scalable, ultrasensitive sensors to detect and differentiate multimodal stimuli and dynamic human movements. Herein, we reveal a novel architecture of an epidermal sensor fabricated by sandwiching the buckypaper between the layers of poly(dimethylsiloxane) (PDMS). This mechanically robust sensor can be conformally adhered on skin and has the perception capability to detect real-time transient human motions and the multimodal mechanical stimuli of stretching, bending, tapping, and twisting. The sensor has feasibility for real-time health monitoring as it can distinguish a wide range of human physiological activities like breathing, gulping, phonation, pulse monitoring, and finger and wrist bending. This multimodal wearable epidermal sensor possesses an ultrahigh gauge factor (GF) of 9178 with a large stretchability of 56%, significant durability for 5000 stretching-releasing cycles, and a fast response/recovery time of 59/88 ms. We anticipate that this novel, simple, and scalable design of a sensor with outstanding features will pave a new way to consummate the requirements of wearable electronics, flexible touch sensors, and electronic skin.

Wearable electronics for heating and sensing based on a multifunctional PET/silver nanowire/PDMS yarn

Stretchable and flexible electronics built from multifunctional fibres are essential for devices in human-machine interactions, human motion monitoring and personal healthcare. However, the combination of stable heating and precision sensing in a single conducting yarn has yet to be achieved. Herein, a yarn comprising poly(ethylene terephthalate) (PET), silver nanowires (AgNWs), and polydimethylsiloxane (PDMS) was designed and prepared. The PET/AgNW/PDMS yarn exhibited high electrical conductivity at ≈3 Ω cm-1 and a large tolerance to tensile strain up to 100% its own length. Only a negligible loss of electromechanical performance was observed after 1700 strain cycles. And an excellent response to applied strain was also achieved across a huge stretching range. The PET/AgNW/PDMS yarn displayed excellent heating performance and outstanding breathability when used in a heating fabric, and excellent sensitivity for monitoring both gross and fine movements in humans when used as a sensor.

3D-Printed Ultra-Robust Surface-Doped Porous Silicone Sensors for Wearable Biomonitoring

Three-dimensional flexible porous conductors have significantly advanced wearable sensors and stretchable devices because of their specific high surface area. Dip coating of porous polymers with graphene is a facile, low cost, and scalable approach to integrate conductive layers with the flexible polymer substrate platforms; however, the products often suffer from nanoparticle delamination and overtime decay. Here, a fabrication scheme based on accessible methods and safe materials is introduced to surface-dope porous silicone sensors with graphene nanoplatelets. The sensors are internally shaped with ordered, interconnected, and tortuous internal geometries (i.e., triply periodic minimal surfaces) using fused deposition modeling (FDM) 3D-printed sacrificial molds. The molds were dip coated to transfer-embed graphene onto the silicone rubber (SR) surface. The presented procedure exhibited a stable coating on the porous silicone samples with long-term electrical resistance durability over ∼12 months period and high resistance against harsh conditions (exposure to organic solvents). Besides, the sensors retained conductivity upon severe compressive deformations (over 75% compressive strain) with high strain-recoverability and behaved robustly in response to cyclic deformations (over 400 cycles), temperature, and humidity. The sensors exhibited a gauge factor as high as 10 within the compressive strain range of 2-10%. Given the tunable sensitivity, the engineered biocompatible and flexible devices captured movements as rigorous as walking and running to the small deformations resulted by human pulse.

Highly sensitive, scalable reduced graphene oxide with palladium nano-composite as strain sensor

We report a novel strain sensor based on reduced graphene oxide (rGO) with palladium (Pd) nano-composite. The sensor was fabricated on the SS304 stainless-steel substrate using a screen-printing method. Graphene oxide was synthesized using a modified Hummer's method and reduced using a chemical route. Field emission-scanning electron microscope, x-ray diffraction and Raman spectroscopy were used to characterize the as-synthesized nano-composite. The as-fabricated strain sensor was tested for tensile strain using Micro-universal Test Machine and the change in resistance for different strains was recorded. The sensor response was observed to be stable and linear within the applied strain range of 0-3000 microstrains, and an average gauge factor of 42.69 was obtained in this range.

High-Performance Flexible Ultraviolet Photodetectors Based on AZO/ZnO/PVK/PEDOT:PSS Heterostructures Integrated on Human Hair

Flexible optoelectronics is an emerging research field that has attracted a great deal of interest in recent years due to the special functions and potential applications of these devices in flexible image sensors, optical computing, energy conversion devices, the Internet of Things, and other technologies. Here, we examine the high-performance ultraviolet (UV) photodetectors using AZO/ZnO nanorods/PVK/PEDOT:PSS heterostructures integrated on human hair. Due to the precise interfacial energy-level alignment among all layers and superior mechanical characteristics of human hair, the as-obtained photodetector shows a fast response time, high photoresponsivity, and excellent flexibility. According to integrate 7 heterostructures as 7 display pixels, the flexible UV-image sensor has superior device performance and outstanding flexibility and can produce vivid and accurate images of Arabic numerals from 0 to 9. Different combinations of the two heterostructures can also be used to achieve flexible photon-triggered logic functions, including AND, OR, and NAND gates. Our findings indicate the possibility of using human hair as a fiber-shaped flexible substrate and will allow the use of hair-based hierarchical heterostructures as building blocks to create exciting opportunities for next-generation high-performance, multifunctional, low-cost, and flexible optoelectronic devices.

A highly flexible and multifunctional strain sensor based on a network-structured MXene/polyurethane mat with ultra-high sensitivity and a broad sensing range

Flexible and multifunctional strain sensors with superior properties including high sensitivity, low detection limits, and a wide sensing range are always in high demand for wearable electronics. However, it remains a big challenge to fully satisfy the aforementioned requirements. In particular, there is always a trade-off between high sensitivity and wide sensing range. Here, we developed a multifunctional strain sensor based on a network-structured MXene/polyurethane mat (network-M/P mat) and well balanced the relationship between the sensitivity and sensing range by rationally designing the morphology and microstructures of the sensing device. The network-structured polyurethane mat (network-P mat) was fabricated through a facile and scalable electrospinning technique. The highly conductive MXene sheets were decorated onto the network-P mat through hydrogen bonding or electrostatic interactions. The obtained highly flexible and stretchable network-M/P mat exhibited a superior comprehensive sensing performance that was characterized by high sensitivity (gauge factor up to 228), a low limit of detection (0.1%), a large and tunable sensing range (up to 150%), excellent stability (over 3200 cycles), and multiple functions (lateral strain, vertical pressure, bending and subtle vibration). Based on its superior performance, the network-M/P mat-based strain sensor can detect a full range of body actions and subtle physiological signals (e.g. respirations and pulse waves), demonstrating great potential for applications in artificial electronic skin and wearable health detectors.

Bioinspired Pretextured Reduced Graphene Oxide Patterns with Multiscale Topographies for High-Performance Mechanosensors

Highly sensitive mechanical sensing is vital for the emerging field of skin mimicry and wearable healthcare systems. To date, it remains a big challenge to fabricate mechanosensors with both high sensitivity and a wide sensing range. In nature, slit sensilla are crack-shaped sensory organs of arachnids, which are highly sensitive to tiny external mechanical stimuli. Here, inspired by the geometry of slit sensilla, a concept is developed that pretextures reduced graphene oxide (RGO) nanocoating into multiscale topographies with agminated crumples and interlaced cracks (crumpled & cracked RGO) through an efficient and scalable mechanically driven process. Both the sensitivity and the workable range can be facilely tuned by adjusting the crack density. The resulting mechanosensor exhibits a comprehensive superior performance including high sensitivity (a gauge factor of 205 to 3256), a wide and tunable sensing range (from 0-40 to 0-180%), long-term stability (over 5000 cycles), and multiple sensing functions. Based on its excellent performances, the mechanosensor can be used as a wearable electronic to in situ monitor subtle physiological signals and vigorous body actions. The rationally designed crumpled & cracked RGO provides a promising platform for artificial electronic skin and portable healthcare systems.

Highly Sensitive, Ultrastretchable Strain Sensors Prepared by Pumping Hybrid Fillers of Carbon Nanotubes/Cellulose Nanocrystal into Electrospun Polyurethane Membranes

Advanced flexible strain sensors for human motion detection and other potential use have attracted great attention in recent years. However, the preparation of strain sensor with both high sensitivity and large workable strain range remains a challenge. In this work, the carbon nanotube (CNT) suspensions with the assistance of cellulose nanocrystals (CNC) were directly pumped into the porous electrospun thermoplastic polyurethanes (TPU) membranes through a simple filtration process to prepare the flexible strain sensors in one step. The sensitivity and workable strain range of the strain sensors are tunable by changing the mass ratios of CNTs/CNC and the total amount of hybrid fillers. With increase in the total amount of fillers, a change of filler layer from droplet to completely continuous film was observed, resulting in a sharp increase of strain sensitivity. By combining the ultraelasticity of the TPU material and the high sensitivity of hybrid fillers, the strain sensor with large workable strain range (>500%) and high sensitivity (gauge factor = 321) was successfully prepared. Its applications in visual control and full-range human body motion detection were demonstrated, showing its tremendous potential applications in future intelligent electronics.

Stretchable and Highly Sensitive Braided Composite Yarn@Polydopamine@Polypyrrole for Wearable Applications

Flexible wearable devices for various applications have attracted research attention in recent years. To date, it is still a big challenge to fabricate strain sensors with a large workable strain range while maintaining their high sensitivity. Herein, we report the fabrication of highly sensitive wearable strain sensors from braided composite yarns (BYs) by in situ polymerization of polypyrrole (PPy) on the surface of yarns after polydopamine templating (BYs-PDA). The electromechanical performance and strain sensing properties of the fabricated braided composite yarn@polydopamine@polypyrrole (BYs-PDA-PPy) were investigated. Because of the unique braided structure of BYs, the BYs-PDA-PPy strain sensors exhibit fascinating performance, including a large workable strain range (up to 105% strain), high sensitivity (gauge factor of 51.2 in strain of 0%-40% and of 27.6 in strain of 40%-105%), long-term stability and great electrical heating performance. Furthermore, the BYs-PDA-PPy sensors can be used in real-time monitoring subtle and large human motions. The BYs-PDA-PPy strain sensors can also be woven into fabrics for large area electric heating. These results demonstrate the potential of BYs-PDA-PPy in wearable electronics.

Melanin-mimetic multicolor and low-toxicity hair dye

Most commercial permanent hair dyeing technologies are based on the oxidative process of p-phenylenediamine and its derivative materials. However, concerns about their toxicological issues have been raised throughout the years. Herein, we report an innovative surface coloration strategy for fabricating melanin-mimetic multicolor and low-toxicity hair dyes through sodium periodate-induced rapid deposition of eumelanin-like polydopamine (PDA) and pheomelanin-like PDA/cysteine co-deposited coatings on the hair surface. The color and morphology of the resulting hair were characterized in detail by several spectroscopy methods and the possible mechanism for the multi-coloring effects and structural differences of the melanin-mimetic coating was proposed. Our strategy eliminates the use of toxic dye precursors or organic solvents, and the favorable safety of the PDA-based formulations is demonstrated. The fabricated dyes can be applied to hair simply by combing, resulting in uniform multi-coloring effects within a short time. Furthermore, the melanin-mimetic hair dyes have excellent durability and ultraviolet protection performance. This work provides a facile and versatile methodology to develop the next generation of safe, sustainable and multicolor hair dyes and pave new avenues for advancing the field of surface coloration, nanoreactors, nanogenerators, energy storage materials and biomimetic sensing devices.

This study developed novel approaches for melanin-mimetic multicolor and low-toxicity hair dye through rapid deposition of PDA-based coatings on hair.

A High Compressibility Pressure-Sensitive Structure Based on CB@PU Yarn Network

In this work, a piezoresistive sensor structure based on carbon black (CB)@polyurethane (PU) yarn material was developed. Specifically, CB@PU yarn was constructed by the polymer-mediated water-based electrostatic deposition method. The distribution of the yarn was artificially controlled to fabricate conductive networks. The CB conductive layer was efficiently supported by the net-like structure of PU yarn, thus generating collaborative advantage. The as-fabricated pressure sensor not only displayed compressibility of over 97%, but also detected a wide pressure change from 25 Pa to 20 kPa. Furthermore, this sensor exhibited response time of less than 70 ms and reproducibility of over 10,000 cycles. The advantages of the CB@PU network ensured this pressure-sensitive structure enormous potential application in pressure sensitive equipment.

Large-Scale Production of Highly Stretchable CNT/Cotton/Spandex Composite Yarn for Wearable Applications

Incorporation of carbon nanotubes (CNTs) into textiles without sacrificing their intrinsic properties provides a promising platform in exploring wearable technology. However, manufacture of flexible, durable, and stretchable CNT/textile composites on an industrial scale is still a great challenge. We hereby report a facile way of incorporating CNTs into the traditional yarn manufacturing process by dipping and drying CNTs into cotton rovings followed by fabricating CNT/cotton/spandex composite yarn (CCSCY) in sirofil spinning. The existence of CNTs in CCSCY brings electrical conductivity to CCSCY while the mechanical properties and stretchability are preserved. We demonstrate that the CCSCY can be used as wearable strain sensors, exhibiting ultrahigh strain sensing range, excellent stability, and good washing durability. Furthermore, CCSCY can be used to accurately monitor the real-time human motions, such as leg bending, walking, finger bending, wrist activity, clenching fist, bending down, and pronouncing words. We also demonstrate that the CCSCY can be assembled into knitted fabrics as the conductors with electric heating performance. The reported manufacturing technology of CCSCY could lead to an industrial-scale development of e-textiles for wearable applications.

High performance strain sensor based on buckypaper for full-range detection of human motions

A high-performance strain sensor based on buckypaper has been fabricated and studied. The sensor with an ultrahigh gauge factor of 20 216 can detect a maximum and a minimum strain range of 75% and 0.1%, respectively. During stretching, the strain sensor achieves a high stability and reproducibility of 10 000 cycles, and a fast response time of less than 87 ms. On the other hand, the sensor shows an excellent sensing performance upon pressure. The pressure range, pressure sensitivity and loading-unloading cycles are 0-1.68 MPa, 89.7 kPa-1 and 3000 cycles, respectively. A concept of the optimal value is utilized to evaluate the strain and pressure performances of the sensor. The optimal values of the sensor upon tensile strain and pressure are calculated to be 3.07 × 108 and 1.35 × 107, respectively, which are much higher than those of most strain and pressure sensors reported in the literature. Precise detection of full-range human motions, acoustic vibrations and even pulse waves at a small scale has been successfully demonstrated by the buckypaper-based sensor. Owning to its advantages including ultrahigh sensitivity, wide detection range and good stability, the buckypaper-based sensor suggests a great potential for applications in wearable sensors, electronic skins, micro/nano electromechanical systems, vibration sensing devices and other strain sensing devices.

High-Performance and Multifunctional Skinlike Strain Sensors Based on Graphene/Springlike Mesh Network

The development of skinlike strain sensors that are integrated with multiple sensing functions has attracted tremendous attention in recent years. To mimic human skin, strain sensors should have the abilities to detect various deformations such as pressing, stretching, bending, and even subtle vibrations. Here, we developed a facile, cost-effective, and scalable method for fabrication of high-performance strain sensors based on a graphene-coated springlike mesh network. This composite-based sensor exhibits an incorporation of low detection limit (LOD) for minute deformation (LOD of 1.38 Pa for pressure, 0.1% for tensile strain, and 10 μm for vibration), multiple sensing functions, long-term stability, and wide maximal sensing range (up to 80 kPa for pressure and 110% for tensile strain). On the basis of its superior performance, it can be applied for in situ monitoring of human motions ranging from subtle physiological signals (e.g., pulse, respiration, and phonation) to substantial movements (e.g., finger bending).

A flexible pressure sensor based on rGO/polyaniline wrapped sponge with tunable sensitivity for human motion detection

High-performance stretchable and wearable electronic skins (E-skins) with high sensitivity and a large sensing range are urgently required with the rapid development of the Internet of things and artificial intelligence. Herein, a reduced graphene oxide (rGO)/polyaniline wrapped sponge is prepared via rGO coating and the in situ synthesis of polyaniline nanowires (PANI NWs) on the backbones of sponge for the fabrication of pressure sensors. From the as-prepared flexible sensor, tunable sensitivity (0.042 to 0.152 kPa-1), wide working range (0-27 kPa), fast response (∼96 ms), high current output (∼300 μA at 1 V), frequency-dependent performance reliable repeatability (∼9000 cycle) and stable signal waveform output can be readily obtained. In addition to tiny physiological activities (voice recognition, swallowing, mouth opening, blowing and breath), robust human motions (finger bending, elbow movement and knee squatting-arising) can also be detected in real-time by the flexible sensors based on rGO/polyaniline wrapped sponge. All the results demonstrate that the flexible pressure sensor based on the functional-sponge is a promising candidate for healthcare monitoring and wearable circuitry in artificial intelligence.

Conductive Cotton Fabrics for Motion Sensing and Heating Applications

Conductive cotton fabric was prepared by coating single-wall carbon nanotubes (CNTs) on a knitted cotton fabric surface through a “dip-and-dry” method. The combination of CNTs and cotton fabric was analyzed using scanning electron microscopy (SEM) and Raman scattering spectroscopy. The CNTs coating improved the mechanical properties of the fabric and imparted conductivity to the fabric. The electromechanical performance of the CNT-cotton fabric (CCF) was evaluated. Strain sensors made from the CCF exhibited a large workable strain range (0~100%), fast response and great stability. Furthermore, CCF-based strain sensors was used to monitor the real-time human motions, such as standing, walking, running, squatting and bending of finger and elbow. The CCF also exhibited strong electric heating effect. The flexible strain sensors and electric heaters made from CCF have potential applications in wearable electronic devices and cold weather conditions.

Highly stretchable strain sensors with reduced graphene oxide sensing liquids for wearable electronics

Strain sensors with high sensitivity, broad sensing ranges and excellent durable stability are highly desirable due to their promising potential in electronic skins and human-friendly wearable interactive systems. Herein, we report a high-performance strain sensor based on rGO (reduced graphene oxide)/DI (deionized water) sensing elements. The strain sensors were fabricated by using Ecoflex rubber filled with rGO/DI conductive liquids via template methods, making the process simple, low-cost and scalable. The as-assembled strain sensors can be used to reflect both stretching and compressing with high sensitivity (a maximum gauge factor of 31.6 and a pressure sensitivity of 0.122 kPa^-1), an ultralow limit of detection (0.1% strain), and excellent reliability and stability (>15 000 cycles for pressuring and >10 000 cycles for stretching). In particular, the maximum sensing range is up to 400%, much wider than that of the sensor recently reported. More significantly, the strain sensors are able to distinguish between touch/compressive (resistance decrease) and tensile (resistance increase) deformation, which has not been explored before. This interesting property of strain sensors is due to the micro-contact of nanomaterials in a liquid environment. The sensing liquid of the device can be refilled when it fails, and this enables the recycling of the materials and reduces the waste rate. Therefore, it is attractive and promising for practical applications in multifunctional wearable electronics such as the detection of acoustic vibration, human vocalization and other human motions.

A Highly Stretchable Nanofiber-Based Electronic Skin with Pressure-, Strain-, and Flexion-Sensitive Properties for Health and Motion Monitoring

The development of flexible and stretchable electronic skins that can mimic the complex characteristics of natural skin is of great value for applications in human motion detection, healthcare, speech recognition, and robotics. In this work, we propose an efficient and low-cost fabrication strategy to construct a highly sensitive and stretchable electronic skin that enables the detection of dynamic and static pressure, strain, and flexion based on an elastic graphene oxide (GO)-doped polyurethane (PU) nanofiber membrane with an ultrathin conductive poly(3,4-ethylenedioxythiophene) (PEDOT) coating layer. The three-dimensional porous elastic GO-doped PU@PEDOT composite nanofibrous substrate and the continuous self-assembled conductive pathway in the nanofiber-based electronic skin offer more contact sites, a larger deformation space, and a reversible capacity for pressure and strain sensing, which provide multimodal mechanical sensing capabilities with high sensitivity and a wide sensing range. The nanofiber-based electronic skin sensor demonstrates a high pressure sensitivity (up to 20.6 kPa^-1), a broad sensing range (1 Pa to 20 kPa), excellent cycling stability and repeatability (over 10,000 cycles), and a high strain sensitivity over a wide range (up to approximately 550%). We confirmed the applicability of the nanofiber-based electronic skin to pulse monitoring, expression, voice recognition, and the full range of human motion, demonstrating its potential use in wearable human-health monitoring systems.

Continuously Producible Ultrasensitive Wearable Strain Sensor Assembled with Three-Dimensional Interpenetrating Ag Nanowires/Polyolefin Elastomer Nanofibrous Composite Yarn

Fiber-shaped strain sensors with great flexibility and knittability have been tremendously concerned due to the wide applications in health manager devices, especially in human motion detection and physiological signal monitoring. Herein, a novel fiber-shaped strain sensor has been designed and prepared by interpenetrating Ag nanowires (NWs) into polyolefin elastomer nanofibrous yarn. The easy-to-obtain structure and simple roll-to-roll process make the continuous large-scale production of nanofibrous composite yarn possible. The continuous and alternating stretching and releasing reversibly change the contact probability between AgNWs in this interpenetrating network, leading to the variations of electrical resistance of the sensor. The gauge factors of strain sensors are calculated to be as high as 13920 and the minimum detection limit is only 0.065%. In addition, the strain sensor shows excellent durability during 4500 cycles with the strain of 10%. The response times of stretching and releasing strains are 10 and 15 ms, respectively. Furthermore, the strain sensor has been successfully applied in human motion detections both in single yarn and knitted fabrics. The result shows the practicability in applications of monitoring limbs movements, eye motion changes, artificial vocal cords, human pulse, and complex motions, which shows great potential in wearable sensors and electronic skin.

Highly Flexible and Sensitive Wearable E-Skin Based on Graphite Nanoplatelet and Polyurethane Nanocomposite Films in Mass Industry Production Available

Graphene and nanomaterials based flexible pressure sensors R&D activities are becoming hot topics due to the huge marketing demand on wearable devices and electronic skin (E-Skin) to monitor the human body's actions for dedicated healthcare. Herein, we report a facile and efficient fabrication strategy to construct a new type of highly flexible and sensitive wearable E-Skin based on graphite nanoplates (GNP) and polyurethane (PU) nanocomposite films. The developed GNP/PU E-Skin sensors are highly flexible with good electrical conductivity due to their unique binary microstructures with synergistic interfacial characteristics, which are sensitive to both static and dynamic pressure variation, and can even accurately and quickly detect the pressure as low as 0.005 N/50 Pa and momentum as low as 1.9 mN·s with a gauge factor of 0.9 at the strain variation of up to 30%. Importantly, our GNP/PU E-Skin is also highly sensitive to finger bending and stretching with a linear correlation between the relative resistance change and the corresponding bending angles or elongation percentage. In addition, our E-Skin shows excellent sensitivity to voice vibration when exposed to a volunteer's voice vibration testing. Notably, the entire E-Skin fabrication process is scalable, low cost, and industrially available. Our complementary experiments with comprehensive results demonstrate that the developed GNP/PU E-Skin is impressively promising for practical healthcare applications in wearable devices, and enables us to monitor the real-world force signals in real-time and in-situ mode from pressing, hitting, bending, stretching, and voice vibration.

A super stretchable and sensitive strain sensor based on a carbon nanocoil network fabricated by a simple peeling-off approach

Despite the tremendous progress in wearable and smart strain sensors, it is still a challenge to develop a highly sensitive, stretchable, and low-cost sensor. Herein, a super stretchable and sensitive strain sensor fabricated by a simple peeling-off approach is reported. The strain sensor is prepared by peeling off a thin as-grown carbon nanocoil (CNC) film from a substrate using a stretchable polydimethylsiloxane (PDMS) film or a flexible adhesive tape. Herein, we took advantage of the spring-like morphology and the original network of the CNCs. The sensor is used to detect pressure, tension, and bend. The strain range and maximum real-time gauge factor reach 260% and 190, respectively, with a rapid response time (less than 12 ms). The contrary resistance responses under tension and bend make it possible to distinguish the direction and type of strain. The sensor is used to monitor a strain over a wide range, from human pulse to the impact of a 0.9 kg weight. The high sensitivity and stretchability, easy and cheap fabrication, and effective interaction with human motions suggest the great potential applications of this sensor in wearable strain sensors and smart systems.

Ultralight Interconnected Graphene-Amorphous Carbon Hierarchical Foam with Mechanical Resiliency for High Sensitivity and Durable Strain Sensors

Ultralight graphene-amorphous carbon (AC) hierarchical foam (G-ACHF) was synthesized by chemical vapor deposition at 1065 °C, close to the melting point of copper. The high temperature leads to the hierarchical structure with an inner layer of graphene and an outer layer of AC. The inner graphene layer with high conductivity and integrity provides high sensitivity. The outer AC layer helps to enhance its durability and mechanical resiliency. The hierarchical structure recovers without damaging the structural integrity after a large strain of 90%. The electrical resistance of G-ACHF remains stable after 200 cycles of compression to a strain level of 50%. The fluctuation of the resistance value remains within ±3%, showing its stability in sensing performance. The pressure sensitivity of G-ACHF reaches as high as ∼11.47 Pa^-1. Finite element simulation reveals that the stress borne by the key position of G-ACHF is 47% lower than that of graphene foam without the AC layer, proving that the AC layer can disperse the stress effectively. With a very low density of 1.17 × 10^-3 g cm^-1, the reversibly compressible G-ACHF strain sensor material exhibits its promising application potential in lightweight and wearable devices.

Binary Synergistic Sensitivity Strengthening of Bioinspired Hierarchical Architectures based on Fragmentized Reduced Graphene Oxide Sponge and Silver Nanoparticles for Strain Sensors and Beyond

Recently, stretchable electronics have been highly desirable in the Internet of Things and electronic skins. Herein, an innovative and cost-efficient strategy is demonstrated to fabricate highly sensitive, stretchable, and conductive strain-sensing platforms inspired by the geometries of a spiders slit organ and a lobsters shell. The electrically conductive composites are fabricated via embedding the 3D percolation networks of fragmentized graphene sponges (FGS) in poly(styrene-block-butadiene-block-styrene) (SBS) matrix, followed by an iterative process of silver precursor absorption and reduction. The slit- and scale-like structures and hybrid conductive blocks of FGS and Ag nanoparticles (NPs) provide the obtained FGS-Ag-NP-embedded composites with superior electrical conductivity of 1521 S cm^-1 , high break elongation of 680%, a wide sensing range of up to 120% strain, high sensitivity of ≈10⁷ at a strain of 120%, fast response time of ≈20 ms, as well as excellent reliability and stability of 2000 cycles. This huge stretchability and sensitivity is attributed to the combination of high stretchability of SBS and the binary synergistic effects of designed FGS architectures and Ag NPs. Moreover, the FGS/SBS/Ag composites can be employed as wearable sensors to detect the modes of finger motions successfully, and patterned conductive interconnects for flexible arrays of light-emitting diodes.

Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate

Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.

High-Performance Strain Sensors with Fish-Scale-Like Graphene-Sensing Layers for Full-Range Detection of Human Motions

Strain sensors with large stretchability, broad sensing range, and high sensitivity are highly desirable because of their potential applications in electronic skins and health monitoring systems. In this paper, we report a high-performance strain sensor with a fish-scale-like graphene-sensing layer. This strain sensor can be fabricated via stretching/releasing the composite films of reduced graphene oxide and elastic tape, making the process simple, cheap, energy-saving, and scalable. It can be used to detect both stretching and bending deformations with a wide sensing range (up to 82% strain), high sensitivity (a gauge factor of 16.2 to 150), ultralow limit of detection (<0.1% strain), and excellent reliability and stability (>5000 cycles). Therefore, it is attractive and promising for practical applications, such as for the full-range detection of human motions.

Sheath-Core Graphite/Silk Fiber Made by Dry-Meyer-Rod-Coating for Wearable Strain Sensors

Recent years have witnessed the explosive development of flexible strain sensors. Nanomaterials have been widely utilized to fabricate flexible strain sensors, because of their high flexibility and electrical conductivity. However, the fabrication processes for nanomaterials and the subsequent strain sensors are generally complicated and are manufactured at high cost. In this work, we developed a facile dry-Meyer-rod-coating process to fabricate sheath-core-structured single-fiber strain sensors using ultrafine graphite flakes as the sheath and silk fibers as the core by virtue of their flexibility, high production, and low cost. The fabricated strain sensor exhibits a high sensitivity with a gauge factor of 14.5 within wide workable strain range up to 15%, and outstanding stability (up to 3000 cycles). The single-fiber-based strain sensors could be attached to a human body to detect joint motions or easily integrated into the multidirectional strain sensor for monitoring multiaxial strain, showing great potential applications as wearable strain sensors.

Decorated CNT based on porous silicon for hydrogen gas sensing at room temperature

A new triple-component sensor for detection of H₂ was developed based on porous silicon and CNTs.