Omni-Purpose Stretchable Strain Sensor Based on a Highly Dense Nanocracking Structure for Whole-Body Motion Monitoring

5G NB-IoT System Integrated with High-Performance Fiber Sensor Inspired by Cirrus and Spider Structures

Real-time telemedicine detection can solve the problem of the shortage of public medical resources caused by the coming aging society. However, the development of such an integrated monitoring system is hampered by the need for high-performance sensors and the strict-requirement of long-distance signal transmission and reproduction. Here, a bionic crack-spring fiber sensor (CSFS) inspired by spider leg and cirrus whiskers for stretchable and weavable electronics is reported. Trans-scale conductive percolation networks of multilayer graphene around the surface of outer spring-like Polyethylene terephthalate (PET) fibers and printing Ag enable a high sensitivity of 28475.6 and broad sensing range over 250%. The electromechanical changes in different stretching stages are simulated by Comsol to explain the response mechanism. The CSFS is incorporated into the fabric and realized the human-machine interactions (HMIs) for robot control. Furthermore, the 5G Narrowband Internet of Things (NB-IoT) system is developed for human healthcare data collection, transmission, and reproduction together with the integration of the CSFS, illustrating the huge potential of the approach in human-machine communication interfaces and intelligent telemedicine rehabilitation and diagnosis monitoring.

Advanced Textile-Based Wearable Biosensors for Healthcare Monitoring

With the innovation of wearable technology and the rapid development of biosensors, wearable biosensors based on flexible textile materials have become a hot topic. Such textile-based wearable biosensors promote the development of health monitoring, motion detection and medical management, and they have become an important support tool for human healthcare monitoring. Textile-based wearable biosensors not only non-invasively monitor various physiological indicators of the human body in real time, but they also provide accurate feedback of individual health information. This review examines the recent research progress of fabric-based wearable biosensors. Moreover, materials, detection principles and fabrication methods for textile-based wearable biosensors are introduced. In addition, the applications of biosensors in monitoring vital signs and detecting body fluids are also presented. Finally, we also discuss several challenges faced by textile-based wearable biosensors and the direction of future development.

High Sensitivity, Wide Linear-Range Strain Sensor Based on MXene/AgNW Composite Film with Hierarchical Microcrack

Stretchable strain sensors suffer the trade-off between sensitivity and linear sensing range. Developing sensors with both high sensitivity and wide linear range remains a formidable challenge. Different from conventional methods that rely on the structure design of sensing nanomaterial or substrate, here a heterogeneous-surface strategy for silver nanowires (AgNWs) and MXene is proposed to construct a hierarchical microcrack (HMC) strain sensor. The heterogeneous surface with distinct differences in cracks and adhesion strengths divides the sensor into two regions. One region contributes to high sensitivity through penetrating microcracks of the AgNW/MXene composite film during stretching. The other region maintains conductive percolation pathways to provide a wide linear sensing range through network microcracks. As a result, the HMC sensor exhibits ultrahigh sensitivity (gauge factor ≈ 244), broad linear range (ɛ = 60%, R2 ≈ 99.25%), and fast response time (<30 ms). These merits are confirmed in the detection of large and subtle human motions and digital joint movement for Morse coding. The manipulation of cracks on the heterogeneous surface provides a new paradigm for designing high-performance stretchable strain sensors.

Carbon-Based Textile Sensors for Physiological-Signal Monitoring

As the focus on physical health increases, the market demand for flexible wearable sensors increases. Textiles combined with sensitive materials and electronic circuits can form flexible, breathable high-performance sensors for physiological-signal monitoring. Carbon-based materials such as graphene, carbon nanotubes (CNTs), and carbon black (CB) have been widely utilized in the development of flexible wearable sensors due to their high electrical conductivity, low toxicity, low mass density, and easy functionalization. This review provides an overview of recent advancements in carbon-based flexible textile sensors, highlighting the development, properties, and applications of graphene, CNTs, and CB for flexible textile sensors. The physiological signals that can be monitored by carbon-based textile sensors include electrocardiogram (ECG), human body movement, pulse and respiration, body temperature, and tactile perception. We categorize and describe carbon-based textile sensors based on the physiological signals they monitor. Finally, we discuss the current challenges associated with carbon-based textile sensors and explore the future direction of textile sensors for monitoring physiological signals.

Design of 3D Controller Using Nanocracking Structure-Based Stretchable Strain Sensor

In this study, we introduce a novel design for a three-dimensional (3D) controller, which incorporates the omni-purpose stretchable strain sensor (OPSS sensor). This sensor exhibits both remarkable sensitivity, with a gauge factor of approximately 30, and an extensive working range, accommodating strain up to 150%, thereby enabling accurate 3D motion sensing. The 3D controller is structured such that its triaxial motion can be discerned independently along the X, Y, and Z axes by quantifying the deformation of the controller through multiple OPSS sensors affixed to its surface. To ensure precise and real-time 3D motion sensing, a machine learning-based data analysis technique was implemented for the effective interpretation of the multiple sensor signals. The outcomes reveal that the resistance-based sensors successfully and accurately track the 3D controller's motion. We believe that this innovative design holds the potential to augment the performance of 3D motion sensing devices across a diverse range of applications, encompassing gaming, virtual reality, and robotics.

Nanoscale Enhancement of the Local Optical Conductivity near Cracks in Metallic SrRuO3 Film

Cracking has been recognized as a major obstacle degrading material properties, including structural stability, electrical conductivity, and thermal conductivity. Recently, there have been several reports on the nanosized cracks (nanocracks), particularly in the insulating oxides. In this work, we comprehensively investigate how nanocracks affect the physical properties of metallic SrRuO₃ (SRO) thin films. We grow SRO/SrTiO₃ (STO) bilayers on KTaO₃ (KTO) (001) substrates, which provide +1.7% tensile strain if the SRO layer is grown epitaxially. However, the SRO/STO bilayers suffer from the generation and propagation of nanocracks, and then, the strain becomes inhomogeneously relaxed. As the thickness increases, the nanocracks in the SRO layer become percolated, and its dc conductivity approaches zero. Notably, we observe an enhancement of the local optical conductivity near the nanocrack region using scanning-type near-field optical microscopy. This enhancement is attributed to the strain relaxation near the nanocracks. Our work indicates that nanocracks can be utilized as promising platforms for investigating local emergent phenomena related to strain effects.

Hierarchically Structured Carbon Nanofiber-Enabled Skin-Like Strain Sensors with Full-Range Human Motion Monitoring and Autonomous Self-Healing Capability

Flexible strain sensors that mimic the properties of human skin have recently attracted tremendous attention. However, integrating multiple functions of skin into one strain sensor, e.g., stretchability, full-range motion response, and self-healing capability, is still an enormous challenge. Herein, a skin-like strain sensor was presented by the construction of hierarchically structured carbon nanofibers (CNFs), followed by encapsulation of elastic self-healing polyurethane (PU). The hierarchical sensing structure was composed of diversified CNFs with orientations from highly aligned to randomly oriented, and their different fracture mechanisms enabled the resultant strain sensor to successfully integrate key sensing properties including high sensitivity (gauge factor of 90), wide sensing range (∼80% strain), and fast response (52 ms). These properties, combined with high stretchability (870%) and excellent stability (>2000 cycles), allowed the sensor to precisely detect full-range human motions from large joint motions to subtle physiological signals. Moreover, the strain sensor had spontaneous self-healing capability at room temperature with high healing efficiencies of 97.7%, while the healing process could substantially be accelerated by the natural sunlight (24 h → 0.5 h). The healed sensor possessed comparable stretchability, sensing performance, and accurate monitoring ability of subtle body signals with the original sensor. The biomimetic self-healing functionality along with skin-like sensing properties makes it attractive for next-generation wearable electronics.

High-Performance Strain Sensors Based on Au/Graphene Composite Films with Hierarchical Cracks for Wide Linear-Range Motion Monitoring

Stretchable strain sensors based on nanomaterial thin films have aroused extensive interest for the strain perception of smart skins. However, it still remains challenging to have them achieve high sensitivity over wide linear working ranges. Herein, we propose a facile strategy to fabricate stretchable strain sensors based on Au/graphene composite films (AGCFs) with hierarchical cracks and demonstrate their superior sensing performances. The polydimethylsiloxane substrates were covered with self-assembled graphene films (SAGFs) and sputtered with Au, and then prestretching was applied to introduce hierarchical cracks. The AGCF strain sensors exhibited high sensitivity (gauge factor (GF) ≈ 153) and favorable linearity (R² ≈ 0.9975) in the wide working range (0-20%) with ultralow overshooting (∼1.7% at 20%), fast response (<42.5 ms), and also excellent cycling stability (1500 cycles). Besides, these patternable sensors could further achieve higher GF (∼320) via pattern designing. The dominant effect of the intermediate wrinkled SAGFs in forming hierarchical cracks was studied, and the linear sensing mechanism of the as-formed fractal microstructures was also revealed in detail. Moreover, the AGCF strain sensors were tested for motion monitoring of the human body and electronic bird. Due to the remarkable versatility, scalable fabrication, and integration capability, these sensors demonstrate great potential to construct smart skins.

Double-Layered Conductive Network Design of Flexible Strain Sensors for High Sensitivity and Wide Working Range

For flexible strain sensors, the optimization between sensitivity and working range is a significant challenge due to the fact that high sensitivity and high working range are usually difficult to obtain at the same time. Herein, a breathable flexible strain sensor with a double-layered conductive network structure was designed and developed, which consists of a thermoplastic polyurethane (TPU)/carbon nanotube (CNT) layer (as a substrate layer) and a Ag nanowire (AgNW) layer. The TPU/CNT layer is made of electrospinning TPU with CNTs deposited onto the surface of TPU fibers, and the flexible TPU/CNT mat guarantees the integrity of the conductive path under a large strain. The AgNW layer was prepared by depositing different amounts of AgNWs on the surface of the TPU/CNT layer, and the high-conductivity AgNWs offer a low initial resistance. Benefitting from the synergistic two-layer structure, the as-obtained flexible strain sensor exhibits a very high sensitivity (up to 1477.7) and a very wide working range (up to 150%). Besides, the fabricated sensor exhibits fast response (88 ms), excellent dynamical stability (7000 cycles), and excellent breathability. The working mechanism of the strain sensor was further investigated using various techniques (microscopy, equivalent circuit, and thermal effects of current). Furthermore, the as-fabricated flexible strain sensors accurately detect the omnidirectional human motions, including subtle and large human motions. This work provides an efficient approach to achieve the optimization between high sensitivity and large working range of strain sensors, which may have great potential applications in health monitoring, body motion detection, and human-machine interactions.

Intelligent Nanomaterials for Wearable and Stretchable Strain Sensor Applications: The Science behind Diverse Mechanisms, Fabrication Methods, and Real-Time Healthcare

It has become a scientific obligation to unveil the underlying mechanisms and the fabrication methods behind wearable/stretchable strain sensors based on intelligent nanomaterials in order to explore their possible potential in the field of biomedical and healthcare applications. This report is based on an extensive literature survey of fabrication of stretchable strain sensors (SSS) based on nanomaterials in the fields of healthcare, sports, and entertainment. Although the evolution of wearable strain sensors (WSS) is rapidly progressing, it is still at a prototype phase and various challenges need to be addressed in the future in special regard to their fabrication protocols. The biocalamity of COVID-19 has brought a drastic change in humans’ lifestyles and has negatively affected nations in all capacities. Social distancing has become a mandatory rule to practice in common places where humans interact with each other as a basic need. As social distancing cannot be ruled out as a measure to stop the spread of COVID-19 virus, wearable sensors could play a significant role in technologically impacting people’s consciousness. This review article meticulously describes the role of wearable and strain sensors in achieving such objectives.

In Situ Loading of Polypyrrole onto Aramid Nanofiber and Carbon Nanotube Aerogel Fibers as Physiology and Motion Sensors

Nanocomposite conductive fiber has been newly developed as a lightweight material with high flexibility and strong weavability, which can meet the requirements of flexible wearable devices. Herein, lightweight porous aramid nanofibers (ANF) and carbon nanotube (CNT) aerogel fibers coated with polypyrrole (PPy) layers are prepared by a wet spinning method for motion detection and information transmission. The ANF/CNT/PPy aerogel fiber with low density (56.3 mg/cm³), conductivity (6.43 S/m), and tensile strength (2.88 MPa) were used as motion sensors with high sensitivity (0.12) and long life (1000 cycles). At the same time, the differential conductivity of aerogel fibers is utilized to reduce the information transmission time (up to 46%). High- and low-temperature-resistant (-196 to 100 °C) aerogel fibers are also available as a quick heater and ionic solution detector. In summary, the prepared ANF/CNT/PPy aerogel fiber can be used as a multifunctional sensor for human-health detection and motion monitoring.

An integrated microfluidic platform featuring real-time reverse transcription loop-mediated isothermal amplification for detection of COVID-19

An integrated microfluidic platform (IMP) utilizing real-time reverse-transcription loop-mediated isothermal amplification (RT-LAMP) was developed here for detection and quantification of three genes of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; i.e., coronavirus diseases 2019 (COVID-19)): RNA-dependent RNA polymerase, the envelope gene, and the nucleocapsid gene for molecular diagnosis. The IMP comprised a microfluidic chip, a temperature control module, a fluidic control module that collectively carried out viral lysis, RNA extraction, RT-LAMP, and the real-time detection within 90 min in an automatic format. A limit of detection of 5 × 10³ copies/reaction for each gene was determined with three samples including synthesized RNAs, inactive viruses, and RNAs extracted from clinical samples; this compact platform could be a useful tool for COVID-19 diagnostics.

Ultrasensitive, Highly Stable, and Flexible Strain Sensor Inspired by Nature

For advanced flexible strain sensors, it is not difficult to achieve high sensitivity only. However, integrating high sensitivity, high stability, and high durability into one sensor still remains a great challenge. Fortunately, natural creatures with diversified excellent performances have given us a lot of ready-made solutions. Here, scorpion and spiderweb are selected as coupling bionic prototypes, which are famous for their ultrasensitive sensing capacity and excellent structural durability, respectively. Based on that, a bioinspired strain sensor is successfully fabricated. The results demonstrate that the bioinspired strain sensor has a sensitivity of 940.5 in the strain range of 0-1.5% and a sensitivity of 2742.3 between 1.5 and 2.5%. Meantime, this sensor with a spiderweb-like reticular structure has a great improvement in stability and durability. Specifically, the sensor exhibits excellent stability during bending and stretching cycles over 80,000 times. Moreover, the response time and recovery time of the sensor are 169 and 195 ms, respectively. Besides, the sensor also has functions such as vibrating frequency identification due to its low hysteresis. Based on the excellent performance, the sensor can be applied to monitor human body motions serving as wearable electronics.

Flexible Sensory Systems: Structural Approaches

Biology is characterized by smooth, elastic, and nonplanar surfaces; as a consequence, soft electronics that enable interfacing with nonplanar surfaces allow applications that could not be achieved with the rigid and integrated circuits that exist today. Here, we review the latest examples of technologies and methods that can replace elasticity through a structural approach; these approaches can modify mechanical properties, thereby improving performance, while maintaining the existing material integrity. Furthermore, an overview of the recent progress in wave/wrinkle, stretchable interconnect, origami/kirigami, crack, nano/micro, and textile structures is provided. Finally, potential applications and expected developments in soft electronics are discussed.

Development of a Smart Clinical Bluetooth Thermometer Based on an Improved Low-Power Resistive Transducer Circuit

Smart sensors have been used in many engineering monitoring and control applications. This work focuses on the development of a new type of clinical Bluetooth thermometer, based on an improved low-power resistive transducer circuit. Most existing resistive transducers use relatively complicated circuits with higher cost and power consumption. To tackle these problems, especially in real applications, an improved low-power resistive transducer circuit is proposed in this work and is used to develop smart Bluetooth thermometers. The parameters of the resistive transducer circuit are selected by quantitative analysis and optimization to improve the performance of the low-power resistive transducer circuit. The effectiveness of the proposed design technology was verified by tests. The temperature measurement error of the new smart Bluetooth thermometer is less than 0.1 °C, which can not only meet the clinical use requirements but also has lower cost and power consumption.

Stretchable Filler/Solid Rubber Piezoresistive Thread Sensor for Gesture Recognition

Recently, the stretchable piezoresistive composites have become a focus in the fields of the biomechanical sensing and human posture recognition because they can be directly and conformally attached to bodies and clothes. Here, we present a stretchable piezoresistive thread sensor (SPTS) based on Ag plated glass microspheres (Ag@GMs)/solid rubber (SR) composite, which was prepared using new shear dispersion and extrusion vulcanization technology. The SPTS has the high gauge factors (7.8~11.1) over a large stretching range (0-50%) and approximate linear curves about the relative change of resistance versus the applied strain. Meanwhile, the SPTS demonstrates that the hysteresis is as low as 2.6% and has great stability during 1000 stretching/releasing cycles at 50% strain. Considering the excellent mechanical strain-driven characteristic, the SPTS was carried out to monitor posture recognitions and facial movements. Moreover, the novel SPTS can be successfully integrated with software and hardware information modules to realize an intelligent gesture recognition system, which can promptly and accurately reflect the produced electrical signals about digital gestures, and successfully be translated into text and voice. This work demonstrates great progress in stretchable piezoresistive sensors and provides a new strategy for achieving a real-time and effective-communication intelligent gesture recognition system.

Three-Axis Tension-Measuring Vitreoretinal Forceps Using Strain Sensor for Corneal Surgery

Precise motion control is important in robotic surgery, especially corneal surgery. This paper develops a new tension-measurement system for forceps used in corneal surgery, wherein contact force is applied only to a specific location for precise control, with precise movements detected by attaching a nano-crack sensor to the corresponding part. The nano-crack sensor used here customizes the working range and sensor sensitivity to match the strain rate of the tip of the forceps. Therefore, the tension in the suture can be sufficiently measured even at suture failure. The printed circuit board attached to the bottom of the system is designed to simultaneously collect data from several sensors, visualizing the direction and magnitude of the tension in order to inform the surgeon of how much tension is being applied. This system was verified by performing pig-corneal suturing.

Flexible and Stable Carbon Nanotube Film Strain Sensors with Self-Derived Integrated Electrodes

The development of flexible and wearable electronic devices has put an increasing demand on electrode systems with seamless connection and high compatibility with the main device, in order to accommodate complex deformation conditions and maintain stable performance. Here, we present a carbon nanotube-integrated electrode (CNTIE) by wet-pulling the ends of a carbon nanotube (CNT) film to form condensed thin fibers that resemble conventional conducting wire electrodes. A flexible strain sensor was constructed consisting of the middle CNT film as the main functional part and the CNTIE as self-derived electrodes, with inherent CNT connection between the two parts. The sensor can be transferred to versatile substrates (e.g., balloon surface) or encapsulated in thermoplastic polymers, exhibiting a large linear response range (up to 1000% in tensile strain), excellent durability and repeatability over 5000 cycles, and the ability to detect small- to large-degree human body motions. In addition, the strain sensor based on the CNTIE hybrid film (MXene/CNT and graphene/CNT) also shows superior linearity and stability at a strain range of 0-800%. Compared with the sensors using traditional silver wire electrodes and separately fabricated CNT fiber electrodes, our CNTIE plays an important role in achieving highly stable performance in the strain cycles. Our self-derived integrated electrodes provide a potential route to solve the incompatibility issues of conventional electrodes and to develop high-performance flexible and wearable systems based on CNTs and other nanomaterials.

Stretchable and Highly Permeable Nanofibrous Sensors for Detecting Complex Human Body Motion

Wearable strain sensors have been attracting special attention in the detection of human posture and activity, as well as for the assessment of physical rehabilitation and kinematics. However, it is a challenge to fabricate stretchable and comfortable-to-wear permeable strain sensors that can provide highly accurate and continuous motion recording while exerting minimal constraints and maintaining low interference with the body. Herein, covalently grafting nanofibrous polyaniline (PANI) onto stretchable elastomer nanomeshes is reported to obtain a freestanding ultrathin (varying from 300 to 10 000 nm) strain sensor that has high gas permeability (10-33 mg h^-1 ). The sensor demonstrates a low weight and can be directly laminated onto the dynamic human skin for long periods of time. The sensor, which produces an intimate connection with solid or living objects, has a stable performance with excellent sustainability, linearity, durability, and low hysteresis. It exibits excellent performance for continuous interrogation of complex movements, mimicking muscle activities, and resembling brain activity. This includes a very precise discrimination of bending and twisting stimuli at different angles (1-180°) and speeds (3-18 rpm) and very low exertion of counter-interference. These results imply the utility of this appraoch for advanced developments of robotic e-skins or e-muscles.

An Ultra-Stretchable Sensitive Hydrogel Sensor for Human Motion and Pulse Monitoring

Ionic hydrogels with intrinsic conductivity and stretchability show great potential in flexible electronics. However, it remains a great challenge to achieve hydrogels with mechanical stretchability, ionic conductivity, optical transparency, and a self-healing ability at the same time. In this paper, we developed a hydroxyethylidene diphosphonic acid (HEDP) assisted poly(vinyl alcohol) (PVA) composite hydrogel to achieve high-performance stretch-sensitive sensor. Through a facile freeze–thaw strategy, the hydrogel could achieve large stretchability (up to 950% strain), good conductivity (10.88 S/m), excellent linear sensitivity (GF = 2.72, within 100% strain), high transparency, and significant self-healing ability. The PVA-HEDP hydrogel-based strain sensor is capable of monitoring various human movements from small scale (e.g., laryngeal vibration while speaking) to large scale (e.g., knee joint movement). Moreover, the multisite sensor array is capable of detecting the subtle differences between the pulse wave features from Cun, Guan and Chi positions, mimicking the three-finger palpation in Traditional Chinese Medicine. This work demonstrates that the composite hydrogel-based flexible sensor provides a promising solution for multifunctional human activities and health monitoring.

Ready-to-wear strain sensing gloves for human motion sensing

Integrating soft sensors with wearable platforms is critical for sensor-based human augmentation, yet the fabrication of wearable sensors integrated into ready-to-wear platforms remains underdeveloped. Disposable gloves are an ideal substrate for wearable sensors that map hand-specific gestures. Here, we use solution-based metallization to prepare resistive sensing arrays directly on off-the-shelf nitrile butadiene rubber (NBR) gloves. The NBR glove acts as the wearable platform while its surface roughness enhances the sensitivity of the overlying sensing array. The NBR sensors have a sheet resistance of 3.1 ± 0.6 Ω/sq and a large linear working range (two linear regions ≤70%). When stretched, the rough NBR substrate facilitates microcrack formation in the overlying metal, enabling high gauge factors (62 up to 40% strain, 246 from 45 - 70% strain) that are unprecedented for metal film sensors. We apply the sensing array to dynamically monitor gestures for gesture differentiation and robotic control.

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Sensing arrays are prepared on ready-to-wear nitrile butadiene rubber (NBR) gloves

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Solution-based deposition is used to pattern gold sensing arrays on the NBR surface

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The roughness-enhanced sensitivity offers high gauge factors (62-246) to 70% strain

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Motion sensing is demonstrated for gesture differentiation and robotic control

Anisotropic, Wrinkled, and Crack-Bridging Structure for Ultrasensitive, Highly Selective Multidirectional Strain Sensors

Flexible multidirectional strain sensors are crucial to accurately determining the complex strain states involved in emerging sensing applications. Although considerable efforts have been made to construct anisotropic structures for improved selective sensing capabilities, existing anisotropic sensors suffer from a trade-off between high sensitivity and high stretchability with acceptable linearity. Here, an ultrasensitive, highly selective multidirectional sensor is developed by rational design of functionally different anisotropic layers. The bilayer sensor consists of an aligned carbon nanotube (CNT) array assembled on top of a periodically wrinkled and cracked CNT-graphene oxide film. The transversely aligned CNT layer bridge the underlying longitudinal microcracks to effectively discourage their propagation even when highly stretched, leading to superior sensitivity with a gauge factor of 287.6 across a broad linear working range of up to 100% strain. The wrinkles generated through a pre-straining/releasing routine in the direction transverse to CNT alignment is responsible for exceptional selectivity of 6.3, to the benefit of accurate detection of loading directions by the multidirectional sensor. This work proposes a unique approach to leveraging the inherent merits of two cross-influential anisotropic structures to resolve the trade-off among sensitivity, selectivity, and stretchability, demonstrating promising applications in full-range, multi-axis human motion detection for wearable electronics and smart robotics.

Recycled Iontronic from Discarded Chewed Gum for Personalized Healthcare Monitoring and Intelligent Information Encryption

With the explosive development of smart wearable devices, a serious situation that a large amount of energy waste and environmental pollution caused by electronic discarding needs to be solved urgently. Here, as a throwaway waste material, a chewed gum can be reused for the preparation of wearable iontronics simply. A new gum sensor was constructed by regularly stretching a chewed gum in 6 M NaCl aqueous or even a Chinese edible salt solution for increasing the ionic conductivity. This gum sensor can be shaped arbitrarily, and the preparation process is green, pollution-free, with low energy consumption, and repeatable. Herein, this gum sensor can be utilized for real-time human healthcare monitoring effectively (i.e., facial mood changes, finger flexion, long time walking, and continuous ankle movement) and shows a fast response time of 297 ms and a reliable cycling performance in monitoring body motions. Furthermore, the gum sensor (containing edible salt) can act as a signal transmitter for intelligent information encryption and transmission in the light of the international Morse code with excellent repeatability and stability. Hence, this work will greatly possess wide potential application prospects in wearable electronics and information encryption. This gum sensor also provides a ponderable option in the next generation of artificial intelligence devices, which can address the troubles of material selections in sensor preparation.

The Effect of Encapsulation on Crack-Based Wrinkled Thin Film Soft Strain Sensors

Practical wearable applications of soft strain sensors require sensors capable of not only detecting subtle physiological signals, but also of withstanding large scale deformation from body movement. Encapsulation is one technique to protect sensors from both environmental and mechanical stressors. We introduced an encapsulation layer to crack-based wrinkled metallic thin film soft strain sensors as an avenue to improve sensor stretchability, linear response, and robustness. We demonstrate that encapsulated sensors have increased mechanical robustness and stability, displaying a significantly larger linear dynamic range (~50%) and increased stretchability (260% elongation). Furthermore, we discovered that these sensors have post-fracture signal recovery. They maintained conductivity to the 50% strain with stable signal and demonstrated increased sensitivity. We studied the crack formation behind this phenomenon and found encapsulation to lead to higher crack density as the source for greater stretchability. As crack formation plays an important role in subsequent electrical resistance, understanding the crack evolution in our sensors will help us better address the trade-off between high stretchability and high sensitivity.

Strain-Discriminable Pressure/Proximity Sensing of Transparent Stretchable Electronic Skin Based on PEDOT:PSS/SWCNT Electrodes

Pressure/proximity sensing as the essential function of electronic skin (e-skin) has become an emerging technological goal for new-generation electronic devices in a wide variety of application fields, for example, smart electronics, human-machine interaction, and prosthetics. However, the current research lacks pressure/proximity detection of the stretched e-skin, which ignores the key elastic characteristic of skin and hinders the development of e-skin. Here, the pressure/proximity detection of the transparent e-skin in the stretching state is demonstrated based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/single-walled carbon nanotube (SWCNT). The high transparency of the e-skin realizes the visual imperception for wearable electronic systems. The perfect combination of stretchable SWCNT and highly conductive PEDOT:PSS endows the sensors with high stretchability and high discrimination capability toward strain, providing an effective way to overcome the interference of strain to realize accurate pressure/proximity detection of stretched e-skin at different strains.

M13 Bacteriophage-Assisted Morphological Engineering of Crack-Based Sensors for Highly Sensitive and Wide Linear Range Strain Sensing

Despite their extraordinary mechanosensitivities, most channel-like crack-based strain sensors are limited by their poor levels of stretchability and linearity. This work presents a simple yet efficient way of modulating the cracking structure of thin metal films on elastomers to facilitate the development of high-performance wearable strain sensors. A net-shaped crack structure based on a thin platinum (Pt) film can be produced by coating an elastomer surface with M13 bacteriophages (phages) and consequently engineering the surface strain upon stretching. This process produces a Pt-on-phage (PoP) strain sensor that simultaneously exhibits high levels of stretchability (24%), sensitivity (maximum gauge factor ≈ 845.6 for 20-24%), and linearity (R² ≈ 0.988 up to 20%). In addition, the sensor performance can be further modulated by either changing the phage coating volume or adding a silver nanowire coating to the PoP sensor film. The balanced strain-sensing performance, combined with fast response times and high levels of mechanical flexibility and operational stability, enables the devices to detect a wide range of human motions in real time after being attached to various body parts. Furthermore, PoP-based strain sensors can be usefully extended to detect more complex multidimensional strains through further strain engineering on a cross-patterned PoP film.

Recent progress in controlled nano/micro cracking as an alternative nano-patterning method for functional applications

Generally, cracking occurs for many reasons connected to uncertainties and to the non-uniformity resulting from intrinsic deficiencies in materials or the non-linearity of applied external (thermal, mechanical, etc.) stresses. However, recently, an increased level of effort has gone into analyzing the phenomenon of cracking and also into methods for controlling it. Sophisticated manipulation of cracking has yielded various cutting-edge technologies such as transparent conductors, mechanical sensors, microfluidics, and energy devices. In this paper, we present some of the recent progress that has been made in controlling cracking by giving an overview of the fabrication methods and working mechanisms used for various mediums. In addition, we discuss recent progress in the various applications of methods that use controlled cracking as an alternative to patterning tools.

Advances in Materials for Soft Stretchable Conductors and Their Behavior under Mechanical Deformation

Soft stretchable sensors rely on polymers that not only withstand large deformations while retaining functionality but also allow for ease of application to couple with the body to capture subtle physiological signals. They have been applied towards motion detection and healthcare monitoring and can be integrated into multifunctional sensing platforms for enhanced human machine interface. Most advances in sensor development, however, have been aimed towards active materials where nearly all approaches rely on a silicone-based substrate for mechanical stability and stretchability. While silicone use has been advantageous in academic settings, conventional silicones cannot offer self-healing capability and can suffer from manufacturing limitations. This review aims to cover recent advances made in polymer materials for soft stretchable conductors. New developments in substrate materials that are compliant and stretchable but also contain self-healing properties and self-adhesive capabilities are desirable for the mechanical improvement of stretchable electronics. We focus on materials for stretchable conductors and explore how mechanical deformation impacts their performance, summarizing active and substrate materials, sensor performance criteria, and applications.

A Highly Aligned Nanowire-Based Strain Sensor for Ultrasensitive Monitoring of Subtle Human Motion

Achieving highly accurate responses to external stimuli during human motion is a considerable challenge for wearable devices. The present study leverages the intrinsically high surface-to-volume ratio as well as the mechanical robustness of nanostructures for obtaining highly-sensitive detection of motion. To do so, highly-aligned nanowires covering a large area were prepared by capillarity-based mechanism. The nanowires exhibit a strain sensor with excellent gauge factor (≈35.8), capable of high responses to various subtle external stimuli (≤200 µm deformation). The wearable strain sensor exhibits also a rapid response rate (≈230 ms), mechanical stability (1000 cycles) and reproducibility, low hysteresis (<8.1%), and low power consumption (<35 µW). Moreover, it achieves a gauge factor almost five times that of microwire-based sensors. The nanowire-based strain sensor can be used to monitor and discriminate subtle movements of fingers, wrist, and throat swallowing accurately, enabling such movements to be integrated further into a miniaturized analyzer to create a wearable motion monitoring system for mobile healthcare.

Exploring the limits of sensitivity for strain gauges of graphene and hexagonal boron nitride decorated with metallic nanoislands

The purpose of this work is to clarify the mechanism of piezoresistance in a class of ultra-sensitive strain gauges based on metallic films on 2D substrates ("2D/M" films). The metals used are gold or palladium deposited as ultrathin films (≤16 nm). These films transition from a regime of subcontiguous growth to a percolated morphology with increasing nominal thickness. The 2D substrates are either single-layer graphene or hexagonal boron nitride (hBN). By using either a conductor (graphene) or an insulator (hBN), it is possible to de-couple the relative contributions of the metal and the 2D substrate from the overall piezoresistance of the composite structure. Here, we use a combination of measurements including electron microscopy, automated image analysis, temperature-dependent conductivity, and measurements of gauge factor of the films as they are bent over a 1 μm step edge (0.0001% or 1 ppm). Our observations are enumerated as follows: (1) of the four permutations of metal and 2D substrate, all combinations except hBN/Au are able to resolve 1 ppm strain (considered extraordinary for strain gauges) at some threshold thickness of metal; (2) for non-contiguous (i.e., unpercolated) films of metal on hBN, changes in resistance for these small step strains cannot be detected; (3) for percolated films on hBN, changes in resistance upon strain can be resolved only for palladium and not for gold; (4) graphene does not exhibit detectable changes in resistance when subjected to step strains of either 1 or 10 ppm, but does so upon the deposition of any amount of gold or palladium, even for nominal thicknesses below the threshold for percolation. Our observations reveal unexpected complexity in the properties of these simple composite materials, and ways in which these materials might be combined to exhibit even greater sensitivity.

Skin-like Ultrasensitive Strain Sensor for Full-Range Detection of Human Health Monitoring

The development of strain sensors with high sensitivity and stretchability, which can accurately detect different human activities such as subtle physiological signals and large-scale joint motions is essential for disease diagnosis and human health monitoring. However, achieving both high sensitivity and stretchability is still an enormous challenge at the moment, particularly for intrinsically stretchable strain sensors. Herein, utilizing large differences in the conductivity and stretchability of micropatterned Au and SWCNTs, we present an ultrasensitive intrinsically stretchable strain sensor by a one-step photolithography process. Its high sensitivity is inspired from spiders' slit organ and the high stretchability is enlightened from spiders' neural pathway. The skin-like sensor exhibits many superior merits, including ultrahigh sensitivity (gauge factors of 7.1 × 10⁴ to 3.4 × 10⁶), wide detection range (up to 100% strain), excellent durability (1000 cycles), ultralow limit of detection (0.1% strain), fast response (1.3 ms), and minimal feature size (≤100 μm). These fascinating merits allow the strain sensor to precisely detect diverse human activities. This work opens up a feasible path to fabricate highly sensitive and stretchable strain sensors, presenting their promising potential in future personalized healthcare, as electronic skins, and being a portable friendly human-machine interaction system.

Bioinspired Electronics for Artificial Sensory Systems

Humans have a myriad of sensory receptors in different sense organs that form the five traditionally recognized senses of sight, hearing, smell, taste, and touch. These receptors detect diverse stimuli originating from the world and turn them into brain-interpretable electrical impulses for sensory cognitive processing, enabling us to communicate and socialize. Developments in biologically inspired electronics have led to the demonstration of a wide range of electronic sensors in all five traditional categories, with the potential to impact a broad spectrum of applications. Here, recent advances in bioinspired electronics that can function as potential artificial sensory systems, including prosthesis and humanoid robots are reviewed. The mechanisms and demonstrations in mimicking biological sensory systems are individually discussed and the remaining future challenges that must be solved for their versatile use are analyzed. Recent progress in bioinspired electronic sensors shows that the five traditional senses are successfully mimicked using novel electronic components and the performance regarding sensitivity, selectivity, and accuracy have improved to levels that outperform human sensory organs. Finally, neural interfacing techniques for connecting artificial sensors to the brain are discussed.

Nano-Cracked Strain Sensor with High Sensitivity and Linearity by Controlling the Crack Arrangement

Studies on wearable sensors that monitor various movements by attaching them to a body have received considerable attention. Crack-based strain sensors are more sensitive than other sensors. Owing to their high sensitivity, these sensors have been investigated for measuring minute deformations occurring on the skin, such as pulse. However, existing studies have limited sensitivity at low strain range and nonlinearity that renders any calibration process complex and difficult. In this study, we propose a pre-strain and sensor-extending process to improve the sensitivity and linearity of the sensor. By using these pre-strain and sensor-extending processes, we were able to control the morphology and alignment of cracks and regulate the sensitivity and linearity of the sensor. Even if the sensor was fabricated in the same manner, the sensor that involved the pre-strain and extending processes had a sensitivity 100 times greater than normal sensors. Thus, our crack-based strain sensor had high sensitivity (gauge factor > 5000, gauge factor (GF = (△R/R₀)/ε), linearity, and low hysteresis at low strain (<1% strain). Given its high sensing performance, the sensor can be used to measure micro-deformation, such as pulse wave and voice.

Wearable, Flexible, Disposable Plasma-Reduced Graphene Oxide Stress Sensors for Monitoring Activities in Austere Environments

In austere environments, for example, in outer space, on surfaces of extra-terrestrial bodies (Moon, Mars, etc.), or under water, technologies that can enable continuous, reliable, and authentic monitoring of movement of human operators and devices can be critical. We report here the production and human body test of wearable, flexible graphene oxide stress sensors suitable for real-time monitoring of body parameters, state and position of humans, and automatic equipment. These sensors have excellent sensitivity and signal strength across a wide strain range, alleviating the need for additional instrumentation for signal processing and amplification. Their low cost makes them virtually disposable, which may benefit such applications as smart clothing. The sensors were fabricated by a concomitant reduction and N-doping of graphene oxide on polydimethylsiloxane in N₂-H₂ plasma. The direct bias and other plasma parameters have a significant effect on the reduction and properties of graphene oxide sensors, as shown by optical emission, Raman and X-ray photoelectron spectroscopies, and X-ray diffraction. Optical emission showed different excitation and ionization processes involving atomic and molecular species in the N₂-H₂ discharge. The photoelectron spectroscopy has confirmed the graphene reduction and introduction of nitrogen doping into the reduced graphene oxide. The bias efficiently controls plasma-induced electric fields, and plasma-related effects determine the N-doping levels. The reduced graphene oxides demonstrate excellent tensile properties, which make them suitable for efficient but cheap stress sensors. This eco-friendly, fast, room-temperature method shows a great potential for fabrication of efficient, flexible sensors.

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line-patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125-145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

One-Step Laser Encapsulation of Nano-Cracking Strain Sensors

Development of flexible strain sensors that can be attached directly onto the skin, such as skin-mountable or wearable electronic devices, has recently attracted attention. However, such flexible sensors are generally exposed to various harsh environments, such as sweat, humidity, or dust, which cause noise and shorten the sensor lifetimes. This study reports the development of a nano-crack-based flexible sensor with mechanically, thermally, and chemically stable electrical characteristics in external environments using a novel one-step laser encapsulation (OLE) method optimized for thin films. The OLE process allows simultaneous patterning, cutting, and encapsulating of a device using laser cutting and thermoplastic polymers. The processes are simplified for economical and rapid production (one sensor in 8 s). Unlike other encapsulation methods, OLE does not degrade the performance of the sensor because the sensing layers remain unaffected. Sensors protected with OLE exhibit mechanical, thermal, and chemical stability under water-, heat-, dust-, and detergent-exposed conditions. Finally, a waterproof, flexible strain sensor is developed to detect motions around the eye, where oil and sweat are generated. OLE-based sensors can be used in several applications that are exposed to a large amount of foreign matter, such as humid or sweaty environments.

Facile and Low-Cost Route for Sensitive Stretchable Sensors by Controlling Kinetic and Thermodynamic Conductive Network Regulating Strategies

Highly sensitive conductive polymer composites (CPCs) are designed employing a facile and low-cost extrusion manufacturing process for both low- and high-strain sensing in the field of, for example, structural health/damage monitoring and human body movement tracking. Focus is on the morphology control for extrusion-processed carbon black (CB)-filled CPCs, utilizing binary and ternary composites based on thermoplastic polyurethane (TPU) and olefin block copolymer (OBC). The relevance of the correct CB amount, kinetic control through a variation of the compounding sequence, and thermodynamic control induced by annealing is highlighted, considering a wide range of experimental (e.g., static and dynamic resistance/scanning electron microscopy/rheological measurements) and theoretical analyses. High CB mass fractions (20 m %) are needed for OBC (or TPU)-CB binary composites but only lead to an intermediate sensitivity as their conductive network is fully packed and therefore difficult to be truly destructed. Annealing is needed to enable a monotonic increase of the relative resistance with respect to strain. With ternary composites, a much higher sensitivity with a clearer monotonic increase results, provided that a low CB mass fraction (10-16 m %) is used and annealing is applied. In particular, with CB first dispersed in OBC and annealing, a less compact, hence, brittle conductive network (10-12 m % CB) is obtained, allowing high-performance sensing.

A Polypyrrole Elastomer Based on Confined Polymerization in a Host Polymer Network for Highly Stretchable Temperature and Strain Sensors

For the purpose of stretchable electronics, broad interests have been paid to elastic conductors by which high tensile strain over 100% can be readily achieved. Here, a scalable-processing, dyeing-like strategy for highly stretchable polypyrrole elastomer (1450% in strain) is conceived without particular topological design. This approach effectively improves the mechanical properties of the classic insoluble polypyrrole by confined polymerization within an elastic polymer network. In terms of the easy processing, it is technically possible to prepare stretchable electronics with arbitrary shape and size for wearable electronics with low cost. The mechanism of interpenetrated networks coexisting with microphase separation is comprehensively illustrated at molecular scale. The as-fabricated polypyrrole elastomers are utilized as temperature or strain sensors for automatic fishing and region-distinct dual signal sensing. Further integration of multiple sensors offers immediate alarm for old people falling at home, which thereby proves its promising potential in practical applications.

Development of a Waterproof Crack-Based Stretchable Strain Sensor Based on PDMS Shielding

This paper details the design of a poly(dimethylsiloxane) (PDMS)-shielded waterproof crack-based stretchable strain sensor, in which the electrical characteristics and sensing performance are not influenced by changes in humidity. This results in a higher number of potential applications for the sensor. A previously developed omni-purpose stretchable strain (OPSS) sensor was used as the basis for this work, which utilizes a metal cracking structure and provides a wide sensing range and high sensitivity. Changes in the conductivity of the OPSS sensor, based on humidity conditions, were investigated along with the potential possibility of using the design as a humidity sensor. However, to prevent conductivity variation, which can decrease the reliability and sensing ability of the OPSS sensor, PDMS was utilized as a shielding layer over the OPSS sensor. The PDMS-shielded OPSS sensor showed approximately the same electrical characteristics as previous designs, including in a high humidity environment, while maintaining its strain sensing capabilities. The developed sensor shows promise for use under high humidity conditions and in underwater applications. Therefore, considering its unique features and reliable sensing performance, the developed PDMS-shielded waterproof OPSS sensor has potential utility in a wide range of applications, such as motion monitoring, medical robotics and wearable healthcare devices.

Highly sensitive metal-grid strain sensors via water-based solution processing

Strain sensor technologies have been spotlighted for their versatility for healthcare, soft robot, and human–robot applications. Expecting large future demands for such technology, extensive studies have investigated flexible and stretchable strain sensors based on various nanomaterials and metal films. However, it is still challenging to simultaneously satisfy parameters such as sensitivity, stretchability, linearity, hysteresis, and mass producibility. In this work, we demonstrate a novel approach for producing highly sensitive metal-grid strain sensors based on an all-solution process, which is suitable for mass production. We investigated the effects of the width of the metal grid and width/spacing ratio on the piezoresistivity of the strain sensors. The metal grid strain sensors exhibited high sensitivity (gauge factor of 4685.9 at 5% strain), rapid response time (∼18.6 ms), and superior strain range (≤5%) compared to other metal-based sensors. We demonstrated that the sensors could successfully convert voice signals and tiny movements of fingers and muscles into electrical signals. In addition, the metal-grid strain sensors were produced using a low-cost procedure without toxic solvent via an all water-based solution process, which is expected to allow the integration of such metal-grid strain sensors into future highly sensitive physical sensing devices.

Highly sensitive strain sensors with metal-grid structures formed by a water-based solution process are presented.

A highly stretchable humidity sensor based on spandex covered yarns and nanostructured polyaniline

Stretchable sensors, as the important components of flexible electronic devices, have achieved progress in a variety of applications for monitoring physical or environmental conditions, such as sound, temperature, vibration, and pressure. However, it still remains a challenge to fabricate high performance stretchable humidity sensors. Herein, we present a novel stretchable humidity sensor, which was fabricated based on an ultrastretchable polyaniline composite fiber. Because of the composite fiber with a “twining spring” configuration (cotton fibers twining spirally around a polyurethane fiber) it maintains a stable electrical conductivity up to a strain of 200%. In addition, the conductivity of the composite fiber remains perfectly stable after 5000 cyclic stretching events of 200% strain. Incorporating the humidity sensitive properties of nanostructured polyaniline, the stretchable humidity sensor based on the composite fiber effectively maintains its humidity sensitivity at different elongations.

In this work, a highly stretchable humidity sensor based on spandex covered yarns and nanostructured polyaniline was fabricated, and maintains its humidity sensitivity well at different elongations.