Crack-based Strain Sensor Research Articles

A mathematical model for predicting the electro-mechanical behavior of biomimetic crack sensors: Effect of crack depth and gap

Various biomimetic microstructures, such as porous, cracks, wrinkles, micro-pyramids, and micro-domes, are applied to improve the sensing performance of mechanical sensors. Among them, the crack-based strain sensors are widely investigated due to high sensitivity and fast response time. To clearly describe the relationship between crack morphology and sensor sensitivity, a mathematical model is developed for investigating the performance of a poly(3,4-ethylenedioxythiophene)-silicon oxide/polydimethylsiloxane (SiOx/PDMS) based crack strain sensor. First, the displacement field of a crack tip is calculated based on the theory of fracture mechanics, and the mathematical relationship between the crack depth, crack gap, and strain is obtained. The predicted crack depth of the SiOx thin film's thicknesses in 7.91 μm (SiOx/PDMS-7.91) is 2.82 μm, with the error of 3.75% compared to the experimental result. Correspondingly, the deviation of SiOx/PDMS-7.91 is 5.74% between the predicted crack gap and the experimental data. Second, above the aforementioned crack tip characteristics, the mathematical model based on crack edges contacts probability (CECP) is used to construct the relationship between the crack tip characteristics, applied stress, and device sensitivity. The maximum predicted sensitivity can reach 3562.68 compared with the experimental data of 3800.44, and the deviation is about 6.26%. Moreover, the CECP model has good universality with the other reported crack-based strain sensors. It can be concluded that crack morphology affects the distribution and quantity of conductive paths. When the strain sensor is subjected to external forces, brittle thin films generate a certain number of bumped-like elements for microcracks. The wider and deeper crack will increase the relative resistance change and the decrease of conductive paths, resulting in a rapid increase in the sensitivity of the strain sensor.

Ultrasensitive, highly stretchable and multifunctional strain sensors based on scorpion-leg-inspired gradient crack arrays

Wearable flexible strain sensors based on conductive films have shown great application potential in many high-tech fields. They have high requirements for synergistic performances of sensitivity, flexibility, stability and durability. However, overcoming the trade-off between high sensitivity and high stretchability is still a great challenge in this research. Here we develop a facile strategy to prepare high-performance flexible strain sensors based on gradient crack arrays inspired by the gradient slits on scorpion legs. The gradient crack arrays with branched or hierarchical features controllably form in a periodic thickness-gradient metal film prepared by one-step masking technique on a flexible substrate. The cracks are progressively open from the thickest film region to the thinnest one under mechanical strain, behaving a unique sensing mechanism. As a result, the gradient crack-based strain sensors possess excellent comprehensive performances including high sensitivity (up to 21000), wide sensing range (∼70 %), low detection limit (0.02 %), quick response speed (below to 80 ms), excellent stability and durability (up to 15,000 cycles at 5 % strain). As demonstration of applications, the sensors are used to monitor various human activities and to express and encrypt information by bending bidirectionally. This work provides a novel way to fabricate highly sensitive, stretchable, robust, multifunctional, crack-based flexible strain sensors, which would greatly expand the applications in flexible electronics and wearable devices.

Enhancement of Sensing Range and Stability of Ultrasensitive Crack-Based Strain Sensor Using Atomized Liquid Metal

Enhancement of Sensing Range and Stability of Ultrasensitive Crack-Based Strain Sensor Using Atomized Liquid Metal

Fatigue crack-based strain sensors achieving flow detection and motion monitoring for reconnaissance robot applications.

Crack-based flexible strain sensors with ultra-high sensitivity under tiny strain are highly desired for environmental perception and motion detection of novel flexible and miniature robots. However, previously reported methods for fabricating crack patterns have often sacrificed the cyclic stability of the sensor, leading to a trade-off relationship between the sensitivity and the cyclic stability. Here, a universal and simple strategy based on fatigue loading with an ultra-large cumulative strain of up to ∼1.2 × 107%, rather than the traditionally quasi-static pre-overloading methods, is proposed to introduce channel cracks in the sensing layer without sacrificing the cyclic stability. The developed flexible strain sensors exhibit high strain-sensitivity (gauge factor = 5798) under tiny strain (< 3%), high cyclic stability (15 000 cycles) and a low strain detecting limit (0.02%). Furthermore, a leaf-like mechanosensor is developed using the fatigue crack-based strain sensor for the realization of multifunctional applications in environment perception and micro-motion detection. Brilliant airflow sensing performance with a wide sensing range (0.93-11.93 m s-1) and a fast response time (0.28 s) for amphibious applications is demonstrated. This work provides a new strategy for overcoming limits of crack-based flexible strain sensors and the developed leaf-like mechanosensor shows great application potential in miniature and flexible reconnaissance robots.

Stretchable, ultrasensitive strain sensor with high-linearity by constructing crack-based dual conductive network

An increasing attention has been paid to crack-based strain sensors due to their ultrahigh sensitivity. However, two main challenges with such sensors still need to be addressed. Firstly, most crack-based sensors face a trade-off between sensitivity and detection range. Secondly, the problem of low linearity occurs widely among crack-based sensors. This paper presents a simple and efficient method to obtain a multidimensional (1D/2D) crack-based stretchable sensor with dual conductive (electronic/ ionic) network. The process involves creating a hybrid conductive film consisting of Ti3C2Tx MXene nanosheets (MXene) and carbon nanotubes (CNTs) via vacuum-assisted filtration, and developing a flexible substrate with ionic conductivity through convenient UV-initiated curing. The prepared sensor exhibited a broad range of workable strain (0–389 %), high sensitivity (GF = 216.8), rapid response/ recovery (165 ms/ 86 ms) and exceptional durability. Most notably, this strain sensor demonstrates strong linearity (R2 = 0.983) with the synergistic effect of the dual conductive network and cracked hybrid film, an advantage over other crack-based sensors. Therefore, our novel dual-conductive strain sensors, with its superior comprehensive performance, can effectively monitor various large and subtle human movements, including joint bending, pulse, and speech recognition. It is credible that this highly stretchable sensor possesses tremendous potential in human motion monitoring, personal healthcare monitoring, and human–computer interaction.

Highly Conductive and Sensitive Wearable Strain Sensors with Metal/Nanoparticle Double Layer for Noninterference Voice Detection.

Human voice recognition via skin-attachable devices has significant potential for gathering important physiological information from acoustic data without background noise interference. In this study, a highly sensitive and conductive wearable crack-based strain sensor was developed for voice-recognition systems. The sensor was fabricated using a double-layer structure of Ag nanoparticles (NPs) and Ag metal on a biocompatible polydimethylsiloxane substrate. The top metal layer acts as a conducting active layer, whereas the bottom Ag NP layer induces channel cracks in the upper layer, effectively hindering current flow. Subsequently, the double-layer film exhibits a low electrical resistance value (<5 × 10-5 Ω cm), ultrahigh sensitivity (gauge factor = 1870), and a fast response/recovery time (252/168 μs). A sound wave was detected at a high frequency of 15 kHz with a signal-to-noise ratio (SNR) over 40 dB. The sensor exhibited excellent anti-interference characteristics and effectively differentiated between different voice qualities (modal, pressed, and breathy), with a systematic analysis revealing successful detection of the laryngeal state and glottal source. This ultrasensitive wearable sensor has potential applications in various physiological signal measurement methods, personalized healthcare systems, and ubiquitous computing.

From network to channel: Crack-based strain sensors with high sensitivity, stretchability, and linearity via strain engineering

High-performance stretchable strain sensors are highly desirable for various scenarios, such as health monitoring and human-robot interfaces. Here, we propose a universal strain engineering strategy that introduces an inhomogeneous spatial distribution of stress and promotes crack propagation behavior leading to a critical state between network and channel morphologies, achieving stretchable strain sensors with high sensitivity, a wide working range and good linearity. Approaches for introducing soft-rigid interfaces, enlarging elastic modulus mismatches and matching dimensions have been employed to execute the strategy for network-crack strain sensors with collapsed nanocone cluster structures as representatives. The strain sensors can be tuned to realize a gauge factor of 690.95 in a linear working range of 0–40% (R2 = 0.993) or a gauge factor of 113.70 in a larger linear working range of 0–120% (R2 = 0.999). Intraocular pressure monitoring and dynamic facial asymmetry assessment have been demonstrated based on these sensors to show their great application potential.

The Role of Fracture Patterns on Crack-Based Strain Sensors

Abstract Crack-based strain sensors (CBS), which are inspired by a spider's slit organ, can provide highly sensitive measurement with great flexibility. Fracture pattern design holds the key to meeting different sensing needs. In this article, a computational model is developed to understand the role of fracture patterns on sensitivity and sensing range of CBS that consist of a platinum (Pt) conductive layer and a polydimethylsiloxane (PDMS) substrate layer. Through the coupled mechanical–electrical finite element analysis, we find that a single mode I through crack can yield better sensing performance than a nonthrough crack in other orientations or a few discrete nonthrough cracks in the same orientation. Creating multiple mode I through cracks has a negligible effect on sensitivity. However, increasing the number of cracks can lead to a higher sensing range. When the same number of cracks is employed, even crack spacing can yield the highest sensing range. Sensitivity can be effectively improved by increasing the crack depth. Conclusions from the computational analysis can provide useful feedback for design and manufacturing of CBS in different applications.

Highly Stretchable, Highly Sensitive, and Antibacterial Electrospun Nanofiber Strain Sensors with Low Detection Limit and Stable CNT/MXene/CNT Sandwich Conductive Layers for Human Motion Detection

With the rise of the concept of “Internet of Everything”, the development of wearable sensing devices is growing rapidly. Among them, crack-based strain sensors have received much attention due to their high sensitivity. However, the application of crack-based strain sensors has been limited by the relatively narrow sensing range. Here, a sandwich structure of a CNT/MXene/CNT sensing layer was realized on the flexible thermoplastic polyurethane substrate prepared by electrospinning, followed by layer-by-layer vacuum filtration. The two stable carbon nanotube (CNT) layers broaden the sensing range, and the strain sensor exhibits excellent sensing range (390%) and stability (3000 tensile tests). The strain sensor also shows excellent sensitivity (GF = 2159.5), low detection limit (0.05%), and fast response time (∼50 ms). The sensor is responsive to different movements when being applied to human motion detection, and the smart wearable device based on the strain sensor shows excellent performance in gesture language recognition. In addition, the antibacterial property of the conductive material inherently facilitates the wider application of the strain sensors.

Biodegradable Ecoflex encapsulated bacterial cellulose/polypyrrole strain sensor detects motion with high sensitivity, flexibility and scalability

Microbial production of biopolymers are non-standard materials in wearable resistive-strain sensors, despite established and desirable scale-up, rheological, ecological and economical properties. In this study, we report a biodegradable bacterial cellulose (BC) crack-based strain sensor prepared by in-situ fermentation with polypyrrole (PPy) and encapsulation with Ecoflex (EF) for human-interactive sensing. In-situ microbial fermentation method not only optimizes the distribution state of PPy but also constructs the three-dimensional conductive network, which contributing to the formation of crack-based sensing mechanism. The as-prepared BC/Ppy@EF strain sensor exhibits high sensitivity (gauge factor of 3.21–4.86), large strain ranges (up to 90% strain), ultralow strain detection limit (0.05%) and remarkable long-term stability without any distinct decline in sensitivity after a constant applied stretching of 90% for 1000 cycles. The strain sensor accurately detected a full range of body motions, including subtle vital signs like pulse, respiration and vocalizations, and was successfully integrated into textiles for human–computer interactions. Consequently, this study provides empirical value for the bio-fabrication and all-green construction design of wearable devices, as well as the development of human–computer interaction.

Crack-inducing Strain Sensor Array using Inkjet-Printed Silver Thin Film for Underplate and Off-centered Force Sensing Applications.

The change in resistance upon bending in metal films as thick as 1 mm used for underpanel force touch applications is limited by the low sensitivity, thus requiring high-performance readout circuitry. In this paper, we report inkjet-printed silver thin films having crack-inducing underlayers, which further increases the sensitivity of their resistance changes under deformation. This allows for detecting weak vertical forces even through the plates (force-receiving layer), such as 0.4 or 1.2 mm thick polyethylene terephthalate or 0.4 mm thick glass. The underplate sensors will detect a force level as low as 10 gf, which corresponds to the amount of force required for fingerprint recognition. Furthermore, such highly sensitive strain sensors can potentially solve the inaccuracy issue of wearable devices, which can occur when misplaced sensors detect relatively weak biosignals, such as heart rate and blood pressure. The sensor detects the accurate pulse patterns of the wrist artery even though it is off-centered from the artery by 6 mm or larger. The crack-based strain sensor and its usage as a hidden underplate force sensing device will create various wearable and user-machine interface applications.

Crack-Based Sensor by Using the UV Curable Polyurethane-Acrylate Coated Film with V-Groove Arrays.

Over the years, several bare metal and crack-based strain sensors have been proposed for various fields of science and technology. However, due to their low gauge factor, metal-based strain sensors have limited practical applications. The crack-based strain sensor, on the other hand, demonstrated excellent sensitivity and a high gauge factor. However, the crack-based strain sensor exhibited non-linear behavior at low strains, severely limiting its real-time applications. Generally, the crack-based strain sensors are fabricated by generating cracks by bending a polymer film on which a metal layer has been deposited with a constant curvature. However, the random formation of cracks produces nonlinear behavior in the crack sensors. To overcome the limitations of the current state of the art, we propose a V-groove-based metal strain sensor for human motion monitoring and Morse code generation. The V-groove crack-based strain sensor is fabricated on polyurethane acrylate (PUA) using the modified photolithography technique. During the procedure, a V-groove pattern formed on the surface of the sensor, and a uniform crack formed over the entire surface by concentrating stress along the groove. To improve the sensitivity and selectivity of the sensor, we generated the cracks in a controlled direction. The proposed strain sensor exhibited high sensitivity and excellent fidelity compared to the other reported metal strain sensors. The gauge factor of the proposed V-groove-induced crack sensor is 10-fold higher than the gauge factor of the reported metal strain sensors. In addition, the fabricated V-groove-based strain sensor exhibited rapid response and recovery times. The practical feasibility of the proposed V-groove-induced crack-based strain sensor is demonstrated through human motion monitoring and the generation of Morse code. The proposed V-groove crack sensor can detect multiple motions in a variety of human activities and is anticipated to be utilized in several applications due to its high durability and reproducibility.

Serpentine-Inspired Strain Sensor with Predictable Cracks for Remote Bio-Mechanical Signal Monitoring.

Flexible strain sensors have attracted intense interest due to their application as intelligent wearable electronic devices. However, it is still a huge challenge to achieve a flexible sensor with simultaneous high sensitivity, excellent durability, and a wide sensing region. In this work, a crack-based strain sensor with a paired-serpentine conductive network is fabricated onto flexible film by screen printing. The innovative conductive network exhibits a controlled crack morphology during stretching, which endows the prepared sensor with outstanding sensing characteristics, including high sensitivity (gauge factor up to 2391.5), wide detection (rang up to 132%), low strain detection limit, a fast response time (about 40ms), as well as excellent durability (more than 2000 stretching/releasing cycles). Benefiting from these excellent performances, full-range human body motions including subtle physiological signals and large motions are accurately detected by the prepared sensor. Furthermore, wearable electronic equipment integrated with a wireless transmitter and the prepared strain sensor shows great potential for remote motion monitoring and intelligent mobile diagnosis for humans. This work provides an effective strategy for the fabrication of novel strain sensors with highly comprehensive performance.

Bioinspired Strain Sensor Using Multiwalled Carbon Nanotube/Polyvinyl Butyral/Nylon Cloth for Wireless Sensing Applications

The mushroom growth of wearable electronics greatly stimulates the demand of flexible strain sensors, and it makes sense to design a manufacturing method that avoids unknown biotoxicity, high-temperature treatment, and redundant technical route. Here, inspired by the geometry of slit receptor on spiders, we prepare a crack-based strain sensor using a facile, cost-effective and large-area-capable method. Conductive inks made of multiwalled carbon nanotubes (MWCNTs) and polyvinyl butyral (PVB) are drop onto nylon cloth, and then pre-stretched to 50% strain to generate long-range ordered and self-organized microcracks. Experimentally, the crack-based strain sensor delivers a superfast response time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 31$ </tex-math></inline-formula> ms), balanced sensitivity and strain range (a gauge factor of 81.45 in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$15-50$ </tex-math></inline-formula> % strain range), and good reliability (>2500 cycles). These excellent sensing properties are attributed to the V-shaped microcrack structure that initiates significant resistance changes during repeated stretching/release. Meanwhile, the nylon substrate provides stable mechanical resilience to meet multiple stretching cycles. Combined with the wireless transmission system, the resistance signal measured by the strain sensor can be transmitted through Bluetooth and displayed in the mobile phone, realizing the real-time monitoring of human health and movements.

Crack-Based Core-Sheath Fiber Strain Sensors with an Ultralow Detection Limit and an Ultrawide Working Range.

With the booming development of flexible wearable sensing devices, flexible stretchable strain sensors with crack structure and high sensitivity have been widely concerned. However, the narrow sensing range has been hindering the development of crack-based strain sensors. In addition, the existence of the crack structure may reduce the interface compatibility between the elastic matrix and the sensing material. Herein, to overcome these problems, integrated core-sheath fibers were prepared by coaxial wet spinning with partially added carbon nanotube sensing materials in thermoplastic polyurethane elastic materials. Due to the superior interface compatibility and the change in the conductive path during stretching, the fiber strain sensor exhibits excellent durability (5000 tensile cycles), high sensitivity (>104), large stretchability (500%), a low detection limit (0.01%), and a fast response time of ∼60 ms. Based on these outstanding strain sensing performances, the fiber sensor is demonstrated to detect subtle strain changes (e.g., pulse wave and swallowing) and large strain changes (e.g., finger joint and wrist movement) in real time. Moreover, the fabric sensor woven with the core-sheath fibers has an excellent performance in wrist bending angle detection, and the smart gloves based on the fabric sensors also show exceptional recognition ability as a wireless sign language translation device. This integrated strategy may provide prospective opportunities to develop highly sensitive strain sensors with durable deformation and a wide detection range.

Bio-Inspired Mechano-Sensor Based on the Deformation of Slit Wake

Internal mechano-sensors, as an indispensable part of the proprioceptive system of intelligent equipment, have attracted enormous research interest because of their extremely crucial role in monitoring machining processes, real-time diagnosis of equipment faults, adaptive motor control and so on. The mechano-sensory structure with signal-transduction function is an important factor in determining the sensing performance of a mechano-sensor. However, contrary to the wide application of the cantilever beam as the sensory structure of external mechano-sensors in order to guarantee their exteroceptive ability, there is still a lack of an effective and widely used sensory structure to significantly improve the sensing performance of internal mechano-sensors. Here, inspired by the scorpion using the specialized slit as the sensory structure of internal mechano-sensilla, the slit is ingeniously used in the design of the engineered internal mechano-sensor. In order to improve the deformability of the slit wake, the hollowed-out design around the slit tail of biological mechano-sensilla is researched. Meanwhile, to mimic the easily deformed flexible cuticular membrane covering the slit, the ultrathin, flexible, crack-based strain sensor is used as the sensing element to cover the controllable slit wake. Based on the coupling deformation of the slit wake, as well as the flexible strain sensor, the slit-based mechano-sensor shows excellent sensing performance to various mechanical signals such as displacement and vibration signals.

Stress-deconcentrated ultrasensitive strain sensor with hydrogen-bonding-tuned fracture resilience for robust biomechanical monitoring

Recently, rapid advances in flexible strain sensors have broadened their application scenario in monitoring of various mechanophysiological signals. Among various strain sensors, the crack-based strain sensors have drawn increasing attention in monitoring subtle mechanical deformation due to their high sensitivity. However, early generation and rapid propagation of cracks in the conductive sensing layer result in a narrow working range, limiting their application in monitoring large biomechanical signals. Herein, we developed a stress-deconcentrated ultrasensitive strain (SDUS) sensor with ultrahigh sensitivity (gauge factor up to 2.3 × 106) and a wide working range (0%–50%) via incorporating notch-insensitive elastic substrate and micro-crack-tunable conductive layer. Furthermore, the highly elastic amine-based polymer-modified polydimethylsiloxane substrate without obvious hysteresis endows our SDUS sensor with a rapid response time (2.33 ms) to external stimuli. The accurate detection of the radial pulse, joint motion, and vocal cord vibration proves the capability of SDUS sensor for healthcare monitoring and human-machine communications.

Ultrasensitive crack-based strain sensors: mechanism, performance, and biomedical applications

Ultrasensitive crack-based strain sensors: mechanism, performance, and biomedical applications

Flexible strain sensor based on embedded three-dimensional annular cracks with high mechanical robustness and high sensitivity

Crack-based flexible strain sensor have advantages of high sensitivity and high detection resolution, which have shown high promise for applications in human–machine interface, biological health detection, and electronic skin. However, the conductive layer in the commonly used crack-based strain sensors tends to lift off after repeatedly stressing/releasing because of the weak interface bonding property, limiting the robustness of flexible sensors. Moreover, the cracks induced by the flat membrane structure are concentrated on the two-dimensional (2D) surface, which limits the further improvement of the sensor's sensitivity. In this study, a new type of flexible strain sensor was fabricated by embedding the 3D annular cracks in the elastomer. By optimizing the fiber weaving structure, the maximum gage factor of the device could reach 45 within strain range of 0–15%, and the minimum detection accuracy reached 0.1%. The sensor maintains good linearity within 15% and good hysteresis characteristics within 5%. Besides, the interpenetrating structure formed between the elastomer material and the conductive material resulted in the increase in the stability of the interface, endowing the device with highly stable sensing performance and high mechanical robustness (anti-scratch). Experiments show that the device can be cycled 10,000 times under a strain of 10%. Finally, the proposed sensor was able to detect even small vibrations and human motion characteristics, exhibiting great potential for broad range of practical applications.

Enhancing the sensitivity of crack-based strain sensor assembled by functionalized graphene for human motion detection

Enhancing the sensitivity of crack-based strain sensor assembled by functionalized graphene for human motion detection