A Highly Sensitive Capacitive-type Strain Sensor Using Wrinkled Ultrathin Gold Films

Stretchable MXene/Carbon Nanotube Bilayer Strain Sensors with Tunable Sensitivity and Working Ranges

Stretchable strain sensors have gained increasing popularity as wearable devices to convert mechanical deformation of the human body into electrical signals. Two-dimensional transition metal carbides (Ti3C2Tx MXene) are promising candidates to achieve excellent sensitivity. However, MXene films have been limited in operating strain ranges due to rapid crack propagation during stretching. In this regard, this study reports MXene/carbon nanotube bilayer films with tunable sensitivity and working ranges. The device is fabricated using a scalable process involving spray deposition of well-dispersed nanomaterial inks. The bilayer sensor's high sensitivity is attributed to the cracks that form in the MXene film, while the compliant carbon nanotube layer extends the working range by maintaining conductive pathways. Moreover, the response of the sensor is easily controlled by tuning the MXene loading, achieving a gauge factor of 9039 within 15% strain at 1.92 mg/cm2 and a gauge factor of 1443 within 108% strain at 0.55 mg/cm2. These tailored properties can precisely match the operation requirements during the wearable application, providing accurate monitoring of various body movements and physiological activities. Additionally, a smart glove with multiple integrated strain sensors is demonstrated as a human-machine interface for the real-time recognition of hand gestures based on a machine-learning algorithm. The design strategy presented here provides a convenient avenue to modulate strain sensors for targeted applications.

Soft mechanical sensors for wearable and implantable applications

Wearable and implantable sensing of biomechanical signals such as pressure, strain, shear, and vibration can enable a multitude of human-integrated applications, including on-skin monitoring of vital signs, motion tracking, monitoring of internal organ condition, restoration of lost/impaired mechanoreception, among many others. The mechanical conformability of such sensors to the human skin and tissue is critical to enhancing their biocompatibility and sensing accuracy. As such, in the recent decade, significant efforts have been made in the development of soft mechanical sensors. To satisfy the requirements of different wearable and implantable applications, such sensors have been imparted with various additional properties to make them better suited for the varied contexts of human-integrated applications. In this review, focusing on the four major types of soft mechanical sensors for pressure, strain, shear, and vibration, we discussed the recent material and device design innovations for achieving several important properties, including flexibility and stretchability, bioresorbability and biodegradability, self-healing properties, breathability, transparency, wireless communication capabilities, and high-density integration. We then went on to discuss the current research state of the use of such novel soft mechanical sensors in wearable and implantable applications, based on which future research needs were further discussed. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

Biomimetic Electronic Skin Based on a Stretchable Ionogel Mechanoreceptor Composed of Crumpled Conductive Rubber Electrodes for Synchronous Strain, Pressure, and Temperature Detection

Electronic skin (e-skin) is showing a huge potential in human-computer interaction, intelligent robots, human health, motion monitoring, etc. However, it is still challenging for e-skin to realize distinguishable detection of stretching strain, vertical pressure, and temperature through a simple noncoupling structure design. Here, a stretchable multimodal biomimetic e-skin was fabricated by integrating layer-by-layer self-assembled crumpled reduced graphene oxide/multiwalled carbon nanotubes film on natural rubber (RGO/MWCNTs@NR) as stretchable conductive electrodes and polyacrylamide/NaCl ionogel as a dielectric layer into an ionotropic capacitive mechanoreceptor. Unlike natural skin receptors, the sandwich-like stretchable ionogel mechanoreceptor possessed a distinct ionotropic capacitive behavior for strain and pressure detection. The results showed that the biomimetic e-skin displayed a negative capacitance change with superior stretchability (0-300%) and a high gauge factor of 0.27 in 180-300% strain, while exhibiting a normal positive piezo-capacitance behavior in vertical pressure range of 0-15 kPa with a maximal sensitivity of 1.759 kPa-1. Based on this feature, the biomimetic e-skin showed an excellent synchronous detection capability of planar strain and vertical pressure in practical wearable applications such as gesture recognition and grasping movement detection without a complicated mathematical or signal decoupling process. In addition, the biomimetic e-skin exhibited a quantifiable linear responsiveness to temperature from 20-90 °C with a temperature coefficient of 0.55%/°C. These intriguing properties gave the biomimetic e-skin the ability to perform a complete function similar to natural skin but beyond its performance for future wearable devices and artificial intelligence devices.

Ultraconformable Capacitive Strain Sensor Utilizing Network Structure of Single-Walled Carbon Nanotubes for Wireless Body Sensing

Capture and real-time recording of precise body movements using strain sensors provide personal information for healthcare monitoring and management. To acquire this information, a sensor that conforms to curved irregular surfaces, including biological tissue, is desired to record complex body movements while acting like a second skin to avoid interference with the movements. In this study, we developed a thin-film-type capacitive strain sensor that is flexible and stretchable on the surface of a living body. We fabricated conductive polymeric ultrathin films ("nanosheets") comprising polystyrene-block-polybutadiene (SB) elastomers and single-walled carbon nanotubes (SWCNTs) (i.e., SWCNT-SB nanosheets) via gravure coating; the SWCNT-SB-coated nanosheets were used as the flexible electrode in a capacitive strain sensor. The dielectric (DE) layer was then prepared using the silicone elastomer Ecoflex 00-30 because its Young's modulus is comparable to that of the epidermis. The normalized capacitance changes (ΔC/C0) in the sensor increased with increasing tensile strain over a range from 0-100%, indicating that the proposed sensor can measure the strain of biological movements, including those of skin and blood vessels. To improve sensor conformability further, the effect of sensor thickness on the gauge factor (GF) was investigated using thinner DE layers by focusing on their flexural rigidity. As a result, the GF increased from 0.64 to 1.13 as the DE layer thickness decreased from 260 to 40 μm. Finally, we evaluated the fabricated sensor's signal stability and mechanical durability, including during wireless sensing when applied to human skin and a vascular model. The ΔC/C0 values varied in response to the bending motion of a finger, dilation of a blood vessel, and the swallowing movement of the throat. These results indicate that our capacitive strain sensor is conformable and functional on biological tissue to enable monitoring of dynamic biological movements (e.g., pulse rate and arterial dilation) without wearer discomfort.

Synergistic advancements in high-performance flexible capacitive pressure sensors: structural modifications, AI integration, and diverse applications

The development of flexible pressure sensors for monitoring human motion and physiological signals has attracted extensive scientific research. However, achieving low monitoring limits, a wide detection range, large bending stresses, and excellent mechanical stability simultaneously remains a serious challenge. With the aim of developing a high-performance capacitive pressure sensor (CPS), this paper introduces the successful preparation of a single-walled carbon nanotube (SWNT)/polydimethylsiloxane (S-PDMS) composite dielectric with a foam-like structure (high permittivity and low elasticity modulus) and MXene/SWNT (S-MXene) composite film electrodes with a micro-crumpled structure. The above structurally modified CPS (SMCPS) demonstrated an excellent response output during pressure loading, achieving a wide pressure detection range (up to 700 kPa), a low detection limit (16.55 Pa), fast response/recovery characteristics (48/60 ms), enhanced sensitivity across a wide pressure range, long-term stability under repeated heavy loading and unloading (40 kPa, >2000 cycles), and reliable performance under various temperature and humidity conditions. The SMCPS demonstrated a precise and stable capacitive response in monitoring subtle physiological signals and detecting motion, owing to its unique electrode structure. The flexible device was integrated with an Internet of Things module to create a smart glove system that enables real-time tracking of dynamic gestures. This system demonstrates exceptional performance in gesture recognition and prediction with artificial intelligence analysis, highlighting the potential of the SMCPS in human-machine interface applications.

Nanocarbon-based sensors for the structural health monitoring of smart biocomposites

Structural health monitoring (SHM) is a critical aspect of ensuring the safety and durability of smart biocomposite materials used as multifunctional materials. Smart biocomposites are composed of renewable or biodegradable materials and have emerged as eco-friendly alternatives of traditional non-biodegradable glass fiber-based composite materials. Although biocomposites exhibit fascinating properties and many desirable traits, real-time and early stage SHM is the most challenging issue to enable their long-term use. Smart biocomposites are integrated with sensors for in situ identification of the progress of damage and composite failure. The sensitivity of such smart biocomposites is a key functionality, which can be tuned by the introduction of an appropriate filler. In particular, nanocarbons hold promising potential to be incorporated in SHM applications of biocomposites. This review focused on the potential applications of nanocarbons in SHM of biocomposites. The aspects related to fabrication techniques and working mechanism of sensors are comprehensively discussed. Furthermore, their unique mechanical and electrical properties and sustainable nature ensure seamless integration into biocomposites, allowing for real-time monitoring without compromising the material's properties. These sensors offer multi-parameter sensing capabilities, such as strain, pressure, humidity, temperature, and chemical exposure, allowing a comprehensive assessment of biocomposite health. Additionally, their durability and longevity in harsh conditions, along with wireless connectivity options, provide cost-effective and sustainable SHM solutions. As research in this field advances, ongoing efforts seek to enhance the sensitivity and selectivity of these sensors, optimizing their performance for real-world applications. This review highlights the significant advances, ongoing efforts to enhance the sensitivity and selectivity, and performance optimization of nanocarbon-based sensors along with their working mechanism in the field of SHM for smart biocomposites. The key challenges and future research perspectives facing the conversion of nanocarbons to smart biocomposites are also displayed.

Artificial Neuron Devices

Efforts to design devices emulating complex cognitive abilities and response processes of biological systems have long been a coveted goal. Recent advancements in flexible electronics, mirroring human tissue's mechanical properties, hold significant promise. Artificial neuron devices, hinging on flexible artificial synapses, bioinspired sensors, and actuators, are meticulously engineered to mimic the biological systems. However, this field is in its infancy, requiring substantial groundwork to achieve autonomous systems with intelligent feedback, adaptability, and tangible problem-solving capabilities. This review provides a comprehensive overview of recent advancements in artificial neuron devices. It starts with fundamental principles of artificial synaptic devices and explores artificial sensory systems, integrating artificial synapses and bioinspired sensors to replicate all five human senses. A systematic presentation of artificial nervous systems follows, designed to emulate fundamental human nervous system functions. The review also discusses potential applications and outlines existing challenges, offering insights into future prospects. We aim for this review to illuminate the burgeoning field of artificial neuron devices, inspiring further innovation in this captivating area of research.

High-performance flexible strain sensors based on silver film wrinkles modulated by liquid PDMS substrates

Flexible strain sensors based on controllable surface microstructures in film-substrate systems can be extensively applied in high-tech fields such as human-machine interfaces, electronic skins, and soft robots. However, the rigid functional films are susceptible to structural destruction and interfacial failure under large strains or high loading speeds, limiting the stability and durability of the sensors. Here we report on a facile technique to prepare high-performance flexible strain sensors based on controllable wrinkles by depositing silver films on liquid polydimethylsiloxane (PDMS) substrates. The silver atoms can penetrate into the surface of liquid PDMS to form an interlocking layer during deposition, enhancing the interfacial adhesion greatly. After deposition, the liquid PDMS is spontaneously solidified to stabilize the film microstructures. The surface patterns are well modulated by changing film thickness, prepolymer-to-crosslinker ratio of liquid PDMS, and strain value. The flexible strain sensors based on the silver film/liquid PDMS system show high sensitivity (above 4000), wide sensing range (∼80%), quick response speed (∼80 ms), and good stability (above 6000 cycles), and have a broad application prospect in the fields of health monitoring and motion tracking.

The Role of 3D Printing Technologies in Soft Grippers

Soft grippers are essential for precise and gentle handling of delicate, fragile, and easy-to-break objects, such as glassware, electronic components, food items, and biological samples, without causing any damage or deformation. This is especially important in industries such as healthcare, manufacturing, agriculture, food handling, and biomedical, where accuracy, safety, and preservation of the objects being handled are critical. This article reviews the use of 3D printing technologies in soft grippers, including those made of functional materials, nonfunctional materials, and those with sensors. 3D printing processes that can be used to fabricate each class of soft grippers are discussed. Available 3D printing technologies that are often used in soft grippers are primarily extrusion-based printing (fused deposition modeling and direct ink writing), jet-based printing (polymer jet), and immersion printing (stereolithography and digital light processing). The materials selected for fabricating soft grippers include thermoplastic polymers, UV-curable polymers, polymer gels, soft conductive composites, and hydrogels. It is conclude that 3D printing technologies revolutionize the way soft grippers are being fabricated, expanding their application domains and reducing the difficulties in customization, fabrication, and production.

Trench Formation under the Tunable Nanogap: Its Depth Depends on Maximum Strain and Periodicity

Metallic nanogaps have been studied for many years in the context of a significant amount of field enhancements. Nanogaps of macroscopic lengths for long-wave applications have attracted much interest, and recently one dimensional tunable nanogaps have been demonstrated using flexible PET substrates. For nanogaps on flexible substrates with applied tensile strain, large stress is expected in the vicinity of the gap, and it has been confirmed that several hundred nanometer-deep trenches form beneath the position of the nanogap because of this stress singularity. Here, we studied trench formation under nanogap structures using COMSOL Multiphysics 6.1. We constructed a 2D nanogap unit cell, consisting of gold film with a crack on a PDMS substrate containing a trench beneath the crack. Then, we calculated the von Mises stress at the bottom of the trench for various depths and spatial periods. Based on it, we derived the dependence of the trench depth on the strain and periodicity for various yield strengths. It was revealed that as the maximum tensile strain increases, the trench deepens and then diverges. Moreover, longer periods lead to larger depths for the given maximum strain and larger gap widths. These results could be applied to roughly estimate achievable gap widths and trench depths for stretchable zerogap devices.

High-stretchability and low-hysteresis strain sensors using origami-inspired 3D mesostructures

Stretchable strain sensors are essential for various applications such as wearable electronics, prosthetics, and soft robotics. Strain sensors with high strain range, minimal hysteresis, and fast response speed are highly desirable for accurate measurements of large and dynamic deformations of soft bodies. Current stretchable strain sensors mostly rely on deformable conducting materials, which often have difficulties in achieving these properties simultaneously. In this study, we introduce capacitive strain sensor concepts based on origami-inspired three-dimensional mesoscale electrodes formed by a mechanically guided assembly process. These sensors exhibit up to 200% stretchability with 1.2% degree of hysteresis, <22 ms response time, small sensing area (~5 mm2), and directional strain responses. To showcase potential applications, we demonstrate the use of distributed strain sensors for measuring multimodal deformations of a soft continuum arm.

Charge transport transition of PEDOT:PSS thin films for temperature-insensitive wearable strain sensors

In this study, a temperature-insensitive strain sensor that detects only the strain without responding to the temperature was designed. The transport mechanism and associated temperature coefficient of resistance (TCR) of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film were modified through secondary doping with dimethyl sulfoxide (DMSO). Upon DMSO-doping, the carrier transport mechanism of the PEDOT:PSS thin film transitioned from hopping to band-like transport, with a morphological change. At the DMSO doping level, which caused the critical point of the transport transition, the resistance of the thin film was maintained with a change in temperature. Consequently, the TCR of the optimized PEDOT:PSS thin film was less than 9 × 10^-5 K^-1, which is 10² times lower than that of the as-prepared films. The carrier mobility of the PEDOT:PSS thin film was effectively improved with the morphological change due to DMSO doping and was investigated through combinational analysis. Ultimately, the wearable strain sensor prepared using the optimized PEDOT:PSS thin film responded stably to the applied strain with a gauge factor of 2 and exhibited excellent temperature anti-interference.

HACS: Helical Auxetic Yarn Capacitive Strain Sensors with Sensitivity Beyond the Theoretical Limit

The development of flexible strain sensors over the past decade has focused on accessing high strain percentages and high sensitivity (i.e., gauge factors). Strain sensors that employ capacitance as the electrical signal to correlate to strain are typically restricted in sensitivity because of the Poisson effect. By employing auxetic structures, the limits of sensitivity for capacitive sensors have been exceeded, which has improved the competitiveness of this modality of sensing. In this work, the first employment of helical auxetic yarns as capacitive sensors is presented. It is found that the response of the helical auxetic yarn capacitive sensors (termed as HACS) is dependent on the two main fabrication variables-the ratio of diameters and the helical wrapping length. Depending on these variables, sensors that respond to strain with increasing or decreasing capacitance values can be obtained. A greater auxetic character results in larger sensitivities accessible at smaller strains-a characteristic that is not commonly found when accessing high gauge factors. In addition, the highest sensitivity for auxetic capacitive sensors reported thus far is obtained. A mechanism of sensor response that explains both the variable capacitance response and the high gauge factors obtained experimentally is proposed.

Heterogeneous Structure Omnidirectional Strain Sensor Arrays With Cognitively Learned Neural Networks

Mechanically stretchable strain sensors gain tremendous attention for bioinspired skin sensation systems and artificially intelligent tactile sensors. However, high-accuracy detection of both strain intensity and direction with simple device/array structures is still insufficient. To overcome this limitation, an omnidirectional strain perception platform utilizing a stretchable strain sensor array with triangular-sensor-assembly (three sensors tilted by 45°) coupled with machine learning (ML) -based neural network classification algorithm, is proposed. The strain sensor, which is constructed with strain-insensitive electrode regions and strain-sensitive channel region, can minimize the undesirable electrical intrusion from the electrodes by strain, leading to a heterogeneous surface structure for more reliable strain sensing characteristics. The strain sensor exhibits decent sensitivity with gauge factor (GF) of ≈8, a moderate sensing range (≈0-35%), and relatively good reliability (3000 stretching cycles). More importantly, by employing a multiclass-multioutput behavior-learned cognition algorithm, the stretchable sensor array with triangular-sensor-assembly exhibits highly accurate recognition of both direction and intensity of an arbitrary strain by interpretating the correlated signals from the three-unit sensors. The omnidirectional strain perception platform with its neural network algorithm exhibits overall strain intensity and direction accuracy around 98% ± 2% over a strain range of ≈0-30% in various surface stimuli environments.

Decoupling Transmission and Transduction for Improved Durability of Highly Stretchable, Soft Strain Sensing: Applications in Human Health Monitoring

This work presents a modular approach to the development of strain sensors for large deformations. The proposed method separates the extension and signal transduction mechanisms using a soft, elastomeric transmission and a high-sensitivity microelectromechanical system (MEMS) transducer. By separating the transmission and transduction, they can be optimized independently for application-specific mechanical and electrical performance. This work investigates the potential of this approach for human health monitoring as an implantable cardiac strain sensor for measuring global longitudinal strain (GLS). The durability of the sensor was evaluated by conducting cyclic loading tests over one million cycles, and the results showed negligible drift. To account for hysteresis and frequency-dependent effects, a lumped-parameter model was developed to represent the viscoelastic behavior of the sensor. Multiple model orders were considered and compared using validation and test data sets that mimic physiologically relevant dynamics. Results support the choice of a second-order model, which reduces error by 73% compared to a linear calibration. In addition, we evaluated the suitability of this sensor for the proposed application by demonstrating its ability to operate on compliant, curved surfaces. The effects of friction and boundary conditions are also empirically assessed and discussed.

Highly Sensitive and Reliable Piezoresistive Strain Sensor Based on Cobalt Nanoporous Carbon-Incorporated Laser-Induced Graphene for Smart Healthcare Wearables

The development of highly sensitive, reliable, and cost-effective strain sensors is a big challenge for wearable smart electronics and healthcare applications, such as soft robotics, point-of-care systems, and electronic skins. In this study, we newly fabricated a highly sensitive and reliable piezoresistive strain sensor based on polyhedral cobalt nanoporous carbon (Co-NPC)-incorporated laser-induced graphene (LIG) for wearable smart healthcare applications. The synergistic integration of Co-NPC and LIG enables the performance improvement of the strain sensor by providing an additional conductive pathway and robust mechanical properties with a high surface area of Co-NPC nanoparticles. The proposed porous graphene nanosheets exploited with Co-NPC nanoparticles demonstrated an outstanding sensitivity of 1,177 up to a strain of 18%, which increased to 39,548 beyond 18%. Additionally, the fabricated sensor exhibited an ultralow limit of detection (0.02%) and excellent stability over 20,000 cycles even under high strain conditions (10%). Finally, we successfully demonstrated and evaluated the sensor performance for practical use in healthcare wearables by monitoring wrist pulse, neck pulse, and joint flexion movement. Owing to the outstanding performance of the sensor, the fabricated sensor has great potential in electronic skins, human-machine interactions, and soft robotics applications.

High Multi-Environmental Mechanical Stability and Adhesive Transparent Ionic Conductive Hydrogels Used as Smart Wearable Devices

Ionic conductive hydrogels used as flexible wearable sensor devices have attracted considerable attention because of their easy preparation, biocompatibility, and macro/micro mechanosensitive properties. However, developing an integrated conductive hydrogel that combines high mechanical stability, strong adhesion, and excellent mechanosensitive properties to meet practical requirements remains a great challenge owing to the incompatibility of properties. Herein, we prepare a multifunctional ionic conductive hydrogel by introducing high-modulus bacterial cellulose (BC) to form the skeleton of double networks, which exhibit great mechanical properties in both tensile (83.4 kPa, 1235.9% strain) and compressive (207.2 kPa, 79.9% strain) stress-strain tests. Besides, the fabricated hydrogels containing high-concentration Ca²⁺ show excellent anti-freezing (high ionic conductivities of 1.92 and 0.36 S/m at room temperature and -35 ∘C, respectively) properties. Furthermore, the sensing mechanism based on the conductive units and applied voltage are investigated to the benefit of the practical applications of prepared hydrogels. Therefore, the designed and fabricated hydrogels provide a novel strategy and can serve as candidates in the fields of sensors, ionic skins, and soft robots.

An Elastic and Damage-Tolerant Dry Epidermal Patch with Robust Skin Adhesion for Bioelectronic Interfacing

On-skin patches that record biopotential and biomechanical signals are essential for wearable healthcare monitoring, clinical treatment, and human-machine interaction. To acquire wearing comfort and high-quality signals, patches with tissue-like softness, elastic recovery, damage tolerance, and robust bioelectronic interface are highly desired yet challenging to achieve. Here, we report a dry epidermal patch made from a supramolecular polymer (SESA) and an in situ transferred carbon nanotubes' percolation network. The polymer possesses a hybrid structure of copolymerized permanent scaffold permeated by multiple dynamic interactions, which imparts a desired mechanical response transition from elastic recoil to energy dissipation with increased elongation. Such SESA-based patches are soft (Young's modulus ∼0.1 MPa) and elastic within physiologically relevant strain levels (97% elastic recovery at 50% tensile strain), intrinsically mechanical-electrical damage-resilient (∼90% restoration from damage after 5 min), and interference-immune in dynamic signal acquisition (stretch, underwater, sweat). We demonstrate its versatile physiological sensing applications, including electrocardiogram recording under various disturbances, machine-learning-enabled hand-gesture recognition through electromyogram measurement, subtle radial artery pulse, and drastic knee kinematics sensing. This epidermal patch offers a promising noninvasive, long-duration, and ambulant bioelectronic interfacing with anti-interference robustness.

3D Printing Technology for Smart Clothing: A Topic Review

Clothing is considered to be an important element of human social activities. With the increasing maturity of 3D printing technology, functional 3D printing technology can realize the perfect combination of clothing and electronic devices while helping smart clothing to achieve specific functions. Furthermore, the application of functional 3D printing technology in clothing not only provides people with the most comfortable and convenient wearing experience, but also completely subverts consumers' perception of traditional clothing. This paper introduced the progress of the application of 3D printing from the aspect of traditional clothing and smart clothing through two mature 3D printing technologies normally used in the field of clothing, and summarized the challenges and prospects of 3D printing technology in the field of smart clothing. Finally, according to the analysis of the gap between 3D-printed clothing and traditionally made clothing due to the material limitations, this paper predicted that the rise in intelligent materials will provide a new prospect for the development of 3D-printed clothing. This paper will provide some references for the application research of 3D printing in the field of smart clothing.

Fully implantable batteryless soft platforms with printed nanomaterial-based arterial stiffness sensors for wireless continuous monitoring of restenosis in real time

Atherosclerosis is a common cause of coronary artery disease and a significant factor in broader cardiovascular diseases, the leading cause of death. While implantation of a stent is a prevalent treatment of coronary artery disease, a frequent complication is restenosis, where the stented artery narrows and stiffens. Although early detection of restenosis can be achieved by continuous monitoring, no available device offers such capability without surgeries. Here, we report a fully implantable soft electronic system without batteries and circuits, which still enables continuous wireless monitoring of restenosis in real-time with a set of nanomembrane strain sensors in an electronic stent. The low-profile system requires minimal invasive implantation to deploy the sensors into a blood vessel through catheterization. The entirely printed, nanomaterial-based set of soft membrane strain sensors utilizes a sliding mechanism to offer enhanced sensitivity and detection of low strain while unobtrusively integrating with an inductive stent for passive wireless sensing. The performance of the soft sensor platform is demonstrated by wireless monitoring of restenosis in an artery model and an ex-vivo study in a coronary artery of ovine hearts. The capacitive sensor-based artery implantation system offers unique advantages in wireless, real-time monitoring of stent treatments and arterial health for cardiovascular disease.

Surface Wrinkling for Flexible and Stretchable Sensors

Recent advances in nanolithography, miniaturization, and material science, along with developments in wearable electronics, are pushing the frontiers of sensor technology into the large-scale fabrication of highly sensitive, flexible, stretchable, and multimodal detection systems. Various strategies, including surface engineering, have been developed to control the electrical and mechanical characteristics of sensors. In particular, surface wrinkling provides an effective alternative for improving both the sensing performance and mechanical deformability of flexible and stretchable sensors by releasing interfacial stress, preventing electrical failure, and enlarging surface areas. In this study, recent developments in the fabrication strategies of wrinkling structures for sensor applications are discussed. The fundamental mechanics, geometry control strategies, and various fabricating methods for wrinkling patterns are summarized. Furthermore, the current state of wrinkling approaches and their impacts on the development of various types of sensors, including strain, pressure, temperature, chemical, photodetectors, and multimodal sensors, are reviewed. Finally, existing wrinkling approaches, designs, and sensing strategies are extrapolated into future applications.

Topological Gradients for Metal Film-Based Strain Sensors

Metal film-based stretchable strain sensors hold great promise for applications in various domains, which require superior sensitivity-stretchability-cyclic stability synergy. However, the sensitivity-stretchability trade-off has been a long-standing dilemma and the metal film-based strain sensors usually suffer from weak cyclic durability, both of which significantly limit their practical applications. Here, we propose an extremely facile, low-cost and spontaneous strategy that incorporates topological gradients in metal film-based strain sensors, composed of intrinsic (grain size and interface) and extrinsic (film thickness and wrinkle) microstructures. The topological gradient strain sensor exhibits an ultrawide stretchability of 100% while simultaneously maintaining a high sensitivity at an optimal topological gradient of 4.5, due to the topological gradients-induced multistage film cracking. Additionally, it possesses a decent cyclic stability for >10 000 cycles between 0 and 40% strain enabled by the gradient-mixed metal/elastomer interfaces. It can monitor the full-range human activities from subtle pulse signals to vigorous joint movements.

Integrated Polypyrrole-Based Smart Clothing with Photothermal Conversion and Thermosensing Functions for Wearable Applications

Integrated smart clothing with photothermal conversion and thermosensing functions is highly desired for next-generation smart wearable applications. Conducting polymer is a promising material that possesses efficient photothermal conversion performance, great sensitivity to temperature change, and excellent processing properties. In this study, we report a new wearable material using the conducting polymer polypyrrole (PPy) as a photothermal and thermosensing layer and nonwoven fabric as flexible textiles to fabricate integrated PPy-based smart clothing (IPSC). The surface temperature of the prepared IPSC can be as high as 68.4 °C with 808 nm near-infrared (NIR) irradiation at a power destiny of 1 kW/m². Meanwhile, a temperature resolution of 1 °C can be achieved for IPSC. These superiorities are in favor of fabricating multifunctional smart wearables to satisfy the needs in future life.

Ultrasonic-Assisted Deposition Method for Creating Conductive Wrinkles on PDMS Surfaces

Harnessing surface wrinkle surfaces in various functional devices has been a hot topic. However, rapidly creating wrinkled surfaces on elastomers of arbitrary shape (especially curved surfaces) is still a great challenge. In this work, an ultrasonic-assisted deposition method has been proposed to achieve nanomodification of the robust layer (e.g., carbon nanotubes (CNTs)) with a labyrinth wrinkle pattern on polydimethylsiloxane (PDMS) fiber, sheet, and porous sponge. It is found that the swelling effect of the dispersion and the ultrasonic treatment play vital roles in the surface wrinkling. As a demonstration, the conductive wrinkled CNTs@PDMS fibers were assembled as stretchable strain sensors. The initial conductivity and the strain-sensing performances could be well tuned by simply adjusting the ultrasonic treatment time. The wrinkled CNTs@PDMS fiber strain sensor exhibited remarkable stretchability (ca. 300%) and good sensitivity, which can be applied in various human motion detection, voice recognition, and air-flow monitoring. It is also expected that the facile ultrasonic-assisted deposition method for surface wrinkling can be extended to fabricate various smart devices with promoted performances.

Multiscale and hierarchical wrinkle enhanced graphene/Ecoflex sensors integrated with human-machine interfaces and cloud-platform

Current state-of-the-art stretchable/flexible sensors have received stringent demands on sensitivity, flexibility, linearity, and wide-range measurement capability. Herein, we report a methodology of strain sensors based on graphene/Ecoflex composites by modulating multiscale/hierarchical wrinkles on flexible substrates. The sensor shows an ultra-high sensitivity with a gauge factor of 1078.1, a stretchability of 650%, a response time of ~140 ms, and a superior cycling durability. It can detect wide-range physiological signals including vigorous body motions, pulse monitoring and speech recognition, and be used for monitoring of human respirations in real-time using a cloud platform, showing a great potential for the healthcare internet of things. Complex gestures/sign languages can be precisely detected. Human-machine interface is demonstrated by using a sensor-integrated glove to remotely control an external manipulator to remotely defuse a bomb. This study provides strategies for real-time/long-range medical diagnosis and remote assistance to perform dangerous tasks in industry and military fields.

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.

High Sensitivity and a Wide Sensing Range Flexible Strain Sensor Based on the V-Groove/Wrinkles Hierarchical Array

Flexible strain sensors occupying a large part of human body detection and wearable electronics, which have a wide sensing range and high sensitivity, are crucial in fully monitoring human motion signals. This study proposed a strategy to construct flexible strain sensors based on the V-groove/wrinkles hierarchical array. The V-groove array was prepared on a polydimethylsiloxane (PDMS) substrate through mold transfer printing. The gold film was sputtered on the prestretching PDMS substrate, and the V-groove/wrinkles hierarchical array was formed after strain release. Compared with the sensors based on single-scale wrinkle structures and a V-groove array, the fabricated strain sensor with the hierarchical array showed high sensitivity (maximum gauge factor up to 2,557.71) and a wide sensing range (up to 45%). In addition, the dynamic characteristics of the sensor were investigated in detail, indicating that the sensor had a fast response (less than 130 ms), a low detection limit (0.1% strain), and good stability (almost no performance loss after 10,000 cycles). In practical applications, the sensor was used to detect sizable physical motion and weak physiological signals, demonstrating great potential application value in human motion detection. This study could provide new ideas for preparing high-performance flexible strain sensors.

A 3D-printed neuromorphic humanoid hand for grasping unknown objects

Compared with conventional von Neumann's architecture-based processors, neuromorphic systems provide energy-saving in-memory computing. We present here a 3D neuromorphic humanoid hand designed for providing an artificial unconscious response based on training. The neuromorphic humanoid hand system mimics the reflex arc for a quick response by managing complex spatiotemporal information. A 3D structural humanoid hand is first integrated with 3D-printed pressure sensors and a portable neuromorphic device that was fabricated by the multi-axis robot 3D printing technology. The 3D neuromorphic robot hand provides bioinspired signal perception, including detection, signal transmission, and signal processing, together with the biomimetic reflex arc function, allowing it to hold an unknown object with an automatically increased gripping force without a conventional controlling processor. The proposed system offers a new approach for realizing an unconscious response with an artificially intelligent robot.

Skin bioelectronics towards long-term, continuous health monitoring

Skin bioelectronics are considered as an ideal platform for personalised healthcare because of their unique characteristics, such as thinness, light weight, good biocompatibility, excellent mechanical robustness, and great skin conformability. Recent advances in skin-interfaced bioelectronics have promoted various applications in healthcare and precision medicine. Particularly, skin bioelectronics for long-term, continuous health monitoring offer powerful analysis of a broad spectrum of health statuses, providing a route to early disease diagnosis and treatment. In this review, we discuss (1) representative healthcare sensing devices, (2) material and structure selection, device properties, and wireless technologies of skin bioelectronics towards long-term, continuous health monitoring, (3) healthcare applications: acquisition and analysis of electrophysiological, biophysical, and biochemical signals, and comprehensive monitoring, and (4) rational guidelines for the design of future skin bioelectronics for long-term, continuous health monitoring. Long-term, continuous health monitoring of advanced skin bioelectronics will open unprecedented opportunities for timely disease prevention, screening, diagnosis, and treatment, demonstrating great promise to revolutionise traditional medical practices.

Highly Sensitive and Stretchable MXene/CNTs/TPU Composite Strain Sensor with Bilayer Conductive Structure for Human Motion Detection

The universal application of wearable strain sensors in various situations for human-activity monitoring is considerably limited by the contradiction between high sensitivity and broad working range. There still remains a huge challenge to design sensors featuring simultaneous broad working range and high sensitivity. Herein, a typical bilayer-conductive structure Ti₃C₂T_x MXene/carbon nanotubes (CNTs)/thermoplastic polyurethane (TPU) composite film was developed by a simple and scalable vacuum filtration process utilizing a porous electrospun thermoplastic polyurethane (TPU) mat as a skeleton. The MXene/CNTs/TPU strain sensor is composed of two parts: a brittle densely stacked MXene upper lamella and a flexible MXene/CNT-decorated fibrous network lower layer. Benefiting from the synergetic effect of the two parts along with hydrogen-bonding interactions between the porous TPU fiber mat and MXene sheets, the MXene/CNTs/TPU strain sensor possesses both a broad working range (up to 330%) and high sensitivity (maximum gauge factor of 2911) as well as superb long-term durability (2600 cycles under the strain of 50%). Finally, the sensor can be successfully employed for human movement monitoring, from tiny facial expressions, respiration, and pulse beat to large-scale finger and elbow bending, demonstrating a promising and attractive application for wearable devices and human-machine interaction.

Reversible Mechanochromisms via Manipulating Surface Wrinkling

Mechanochromic structural-colored materials have promising applications in various domains. In this Letter, we report three types of reversible mechanochromisms in simple material systems by harnessing mechano-responsive wrinkling dynamics including (i) brightness mechanochromism (BM), (ii) hue change mechanochromism (HCM), and (iii) viewable angle mechanochromism (VAM). Upon stretching, the BM device exhibits almost a constant hue but reduces light brightness due to the postbuckling mechanics-controlled deformation, while the HCM device can change the hue from blue to red with almost constant intensity because of the linear elastic mechanics-controlled deformation. The VAM device shows a constant hue because of the thin film interference effect. However, the viewable angles decrease with increasing applied strain owing to the light scattering of wrinkles. All of the mechanochromic behaviors exhibit good reversibility and durability. We clearly elucidated the underlying mechanisms for different mechanochromisms and demonstrated their potential applications in smart displays, stretchable strain sensors, and antipeeping/anticounterfeiting devices.

Ultrastretchable and Self-Healing Conductors with Double Dynamic Network for Omni-Healable Capacitive Strain Sensors

Skin-mountable capacitive-type strain sensors with great linearity and low hysteresis provide inspiration for the interactions between human and machine. For practicality, high sensing performance, large stretchability, and self-healing are demanded but limited by stretchable electrode and dielectric and interfacial compatibility. Here, we demonstrate an extremely stretchable and self-healing conductor via both hard and soft tactics that combine conductive nanowire assemblies with double dynamic network based on π-π attractions and Ag-S coordination bonds. The obtained conductor outperforms the reported stretchable conductors by delivering an elongation of 3250%, resistance change of 223% at 2000% strain, high durability, and multiresponsive self-healability. Especially, this conductor accommodates large strain of 1500% at extremely knotted and twisted deformations. By sandwiching hydrogel conductors with a newly developed dielectric, ultrahigh stretchability and omni-healability are simultaneously achieved for the first time for a capacitive strain sensor inspired by metal-thiolate coordination chemistry, showing great potentials in wearable electronics and soft robotics.

Smart Chemical Engineering-Based Lightweight and Miniaturized Attachable Systems for Advanced Drug Delivery and Diagnostics

Smart attachable systems have attracted much attention owing to their capabilities in terms of body performance evaluation, disease diagnostics, and drug delivery. Recent advances in chemical and engineering techniques provide many opportunities to improve device fabrication and applications owing to the advantages of being lightweight and easy to control as well as their battery absence and functional diversity. This review highlights the latest developments in the field of chemical engineering-based lightweight and miniaturized attachable systems, which are mainly inspired by the natural world. Their applications for real-time monitoring, point-of-care sampling, biomarker detection, and controlled release are discussed thoroughly with respect to specific products/prototypes. The perspectives of the field, including persistence guarantee, burden reduction, and personality improvement, are also discussed. It is believed that chemical engineering-based lightweight and miniaturized attachable systems have good potential in both clinical and industrial fields, indicating a large potential to improve human lives in the near future.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors

Conductive hydrogels can be prepared by incorporating various conductive materials into polymeric network hydrogels. In recent years, conductive hydrogels have been developed and applied in the field of strain sensors owing to their unique properties, such as electrical conductivity, mechanical properties, self-healing, and anti-freezing properties. These remarkable properties allow conductive hydrogel-based strain sensors to show excellent performance for identifying external stimuli and detecting human body movement, even at subzero temperatures. This review summarizes the properties of conductive hydrogels and their application in the fabrication of strain sensors working in different modes. Finally, a brief prospectus for the development of conductive hydrogels in the future is provided.

Biomimetic Hierarchically Silver Nanowire Interwoven MXene Mesh for Flexible Transparent Electrodes and Invisible Camouflage Electronics

Flexible transparent electrodes demand high transparency, low sheet resistance, as well as excellent mechanical flexibility simultaneously, however they still remain to be a great challenge due to"trade-off" effect. Herein, inspired by a hollow interconnected leaf vein, we developed robust transparent conductive mesh with biomimetic interwoven structure via hierarchically self-assembles silver nanowires interwoven metal carbide/nitride (MXene) sheets along directional microfibers. Strong interfacial interactions between plant fibers and conductive units facilitate hierarchically interwoven conductive mesh constructed orderly on flexible and lightweight veins while maintaining high transparency, effectively avoiding the trade-off effect between optoelectronic properties. The flexible transparent electrodes exhibit sheet resistance of 0.5 Ω sq^-1 and transparency of 81.6%, with a remarkably high figure of merit of 3523. In addition, invisible camouflage sensors are further successfully developed as a proof of concept that could monitor human body motion signals in an imperceptible state. The flexible transparent conductive mesh holds great potential in high-performance wearable optoelectronics and camouflage electronics.

Electrical Failure Mechanism in Stretchable Thin-Film Conductors

Stretchable thin-film conductors are basic building blocks in advanced flexible and stretchable electronics. Current research mainly focuses on strategies to improve stretchability and widen the range of applications of stretchable conductors. However, stability should not be neglected, and the electrical failure mode is one of the most common stability issues that determines the current range and duration in a circuit. In this work, we report the electrical failure mechanism of stretchable conductors. We find a special failure mode for the stretchable conductors, which can be attributed to the coupling effect between local thermal strains and dynamic resistance changes of the thin film. This creates a vicious circle that significantly differs from traditional conductors. Physical parameters related to this special failure mode are investigated in detail. It is found that this mechanism is applicable to different kinds of stretchable conductors. Based on this finding, we also explore methods to modulate the failure of stretchable conductors. The failure mechanism found here provides a fundamental understanding of the current effect of stretchable circuits and is crucial for designing stable stretchable bioelectrodes and circuits.

Recent Advances in Zwitterionic Hydrogels: Preparation, Property, and Biomedical Application

Nonspecific protein adsorption impedes the sustainability of materials in biologically related applications. Such adsorption activates the immune system by quick identification of allogeneic materials and triggers a rejection, resulting in the rapid failure of implant materials and drugs. Antifouling materials have been rapidly developed in the past 20 years, from natural polysaccharides (such as dextran) to synthetic polymers (such as polyethylene glycol, PEG). However, recent studies have shown that traditional antifouling materials, including PEG, still fail to overcome the challenges of a complex human environment. Zwitterionic materials are a class of materials that contain both cationic and anionic groups, with their overall charge being neutral. Compared with PEG materials, zwitterionic materials have much stronger hydration, which is considered the most important factor for antifouling. Among zwitterionic materials, zwitterionic hydrogels have excellent structural stability and controllable regulation capabilities for various biomedical scenarios. Here, we first describe the mechanism and structure of zwitterionic materials. Following the preparation and property of zwitterionic hydrogels, recent advances in zwitterionic hydrogels in various biomedical applications are reviewed.

Gecko-Inspired Slant Hierarchical Microstructure-Based Ultrasensitive Iontronic Pressure Sensor for Intelligent Interaction

Highly sensitive flexible pressure sensors play an important role to ensure the safety and friendliness during the human-robot interaction process. Microengineering the active layer has been shown to improve performance of pressure sensors. However, the current structural strategy almost relying on axial compression deformation suffers structural stiffening, and together with the limited area growth efficiency of conformal interface, essentially limiting the maximum sensitivity. Here, inspired by the interface contact behavior of gecko's feet, we design a slant hierarchical microstructure to act as an electrode contacting with an ionic gel layer, fundamentally eliminating the pressure resistance and maximizing functional interface expansion to achieving ultrasensitive sensitivity. Such a structuring strategy dramatically improves the relative capacitance change both in the low- and high-pressure region, thereby boosting the sensitivity up to 36000 kPa^-1 and effective measurement range up to 300 kPa. To verify the advantages of high sensitivity, the sensor is integrated with a soft magnetic robot to demonstrate a biomimetic Venus flytrap. The ability to perceive weak stimuli allows the sensor to be used as a sensory and feedback window, realizing the capture of small live insects and the transportation of fragile objects.

Structural effects of 3D printing resolution on the gauge factor of microcrack-based strain gauges for health care monitoring

Measurements of physiological parameters such as pulse rate, voice, and motion for precise health care monitoring requires highly sensitive sensors. Flexible strain gauges are useful sensors that can be used in human health care devices. In this study, we propose a crack-based strain gauge fabricated by fused deposition modeling (FDM)-based three-dimensional (3D)-printing. The strain gauge combined a 3D-printed thermoplastic polyurethane layer and a platinum layer as the flexible substrate and conductive layer, respectively. Through a layer-by-layer deposition process, self-aligned crack arrays were easily formed along the groove patterns resulting from stress concentration during stretching motions. Strain gauges with a 200-µm printing thickness exhibited the most sensitive performance (~442% increase in gauge factor compared with that of a flat sensor) and the fastest recovery time (~99% decrease in recovery time compared with that of a flat sensor). In addition, 500 cycling tests were conducted to demonstrate the reliability of the sensor. Finally, various applications of the strain gauge as wearable devices used to monitor human health and motion were demonstrated. These results support the facile fabrication of sensitive strain gauges for the development of smart devices by additive manufacturing.

Wearable Sensors for Remote Health Monitoring: Potential Applications for Early Diagnosis of Covid-19

Wearable sensors are emerging as a new technology to detect physiological and biochemical markers for remote health monitoring. By measuring vital signs such as respiratory rate, body temperature, and blood oxygen level, wearable sensors offer tremendous potential for the noninvasive and early diagnosis of numerous diseases such as Covid-19. Over the past decade, significant progress has been made to develop wearable sensors with high sensitivity, accuracy, flexibility, and stretchability, bringing to reality a new paradigm of remote health monitoring. In this review paper, the latest advances in wearable sensor systems that can measure vital signs at an accuracy level matching those of point-of-care tests are presented. In particular, the focus of this review is placed on wearable sensors for measuring respiratory behavior, body temperature, and blood oxygen level, which are identified as the critical signals for diagnosing and monitoring Covid-19. Various designs based on different materials and working mechanisms are summarized. This review is concluded by identifying the remaining challenges and future opportunities for this emerging field.

Fe3+-coordination Mediated Synergistic Dual-network Conductive Hydrogel as Sensitive and Highly-stretchable Strain Sensor with Adjustable Mechanical Properties

Flexible strain sensors are attracting enormous attention due to their high stretchability and sensitivity that are required for wearable devices and electronic skin. However, diverse application environments require materials whose...

A new strategy for the fabrication of a flexible and highly sensitive capacitive pressure sensor

The development of flexible capacitive pressure sensors has wide application prospects in the fields of electronic skin and intelligent wearable electronic devices, but it is still a great challenge to fabricate capacitive sensors with high sensitivity. Few reports have considered the use of interdigital electrode structures to improve the sensitivity of capacitive pressure sensors. In this work, a new strategy for the fabrication of a high-performance capacitive flexible pressure sensor based on MXene/polyvinylpyrrolidone (PVP) by an interdigital electrode is reported. By increasing the number of interdigital electrodes and selecting the appropriate dielectric layer, the sensitivity of the capacitive sensor can be improved. The capacitive sensor based on MXene/PVP here has a high sensitivity (~1.25 kPa^-1), low detection limit (~0.6 Pa), wide sensing range (up to 294 kPa), fast response and recovery times (~30/15 ms) and mechanical stability of 10000 cycles. The presented sensor here can be used for various pressure detection applications, such as finger pressing, wrist pulse measuring, breathing, swallowing and speech recognition. This work provides a new method of using interdigital electrodes to fabricate a highly sensitive capacitive sensor with very promising application prospects in flexible sensors and wearable electronics.

A multifunctional hollow TPU fiber filled with liquid metal exhibiting fast electrothermal deformation and recovery

Conductive fibers have received considerable interest due to their potential applications in the flexible electronics field. Fabricating a conductive fiber that can realize fast deformation with stretchability for multifunctional applications is still highly appealing. Here, we present a deformable conductive fiber (DCF) fabricated by injecting liquid metal (LM) into a hollow thermoplastic polyurethane (TPU) fiber; the DCF can be shaped into a 2D or 3D shape by an electrothermal method at the thermoplastic transition point of TPU. Combined with the solid-liquid phase transition characteristics of the LM at its melting point, the DCF exhibits a variable shape memory feature at two transition points. We have demonstrated that the double-torsional DCF and the helical DCF can act as a capacitive sensor and an inductive sensor, respectively, and they have both been used for human motion monitoring. In addition, the helical DCF can also act as a stretchable electrode with excellent electrical properties (resistance change <2%) under a maximal mechanical strain of 3300%. Overall, the DCF presents great potential for applications in human motion monitoring, soft robotics and smart electronic textiles.

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.

Tuning Strain Sensor Performance via Programmed Thin-Film Crack Evolution

Stretchable strain sensors with well-controlled sensitivity and stretchability are crucial for applications ranging from large deformation monitoring to subtle vibration detection. Here, based on single-metal material on the elastomer and one-pot evaporation fabrication method, we realize controlled strain sensor performance via a novel programable cracking technology. Specifically, through elastomeric substrate surface chemistry modification, the microcrack generation and morphology evolution of the strain sensing layer is controlled. This process allows for fine tunability of the cracked film morphology, resulting in strain sensing devices with a sensitivity gauge factor of over 10 000 and stretchability up to 100%. Devices with a frequency response up to 5.2 Hz and stability higher than 1000 cycles are reported. The reported strain sensors, tracking both subtle and drastic mechanical deformations, are demonstrated in healthcare devices, human-machine interaction, and smart-home applications.