Self-healing Strain Sensors Research Articles

Fiber-Reinforced Flexible Self-Healing Strain Sensor with Failure-Improving Sensitivity Recovery.

In this work, we address the inherent limitations of porous, flexible, fibrous, and self-healing strain sensors. Specifically, we tackle issues such as the fatigue failure of carbon-fibrous materials and the long-term flow and low mechanical stability of self-healing materials. We achieve this by combining self-healing carbon/PBS blends with fibrous materials, creating a fiber-reinforced self-healing composite. The self-healing carbon/PBS blends provide strain sensitivity and the ability to recover after fatigue and impact failure, while the fibers prevent the long-term flow of material and the scattering of pieces during impact and fatigue failure within the elastic deformation regime, enabling shape recovery. We fabricated composite wearable strain sensors with a viscoelastic functional layer composed of two continuous phases: (i) a self-healing polymer-carbon blend and (ii) long electrospun fibers of commercial polyurethane. This setup also eliminates the other drawbacks of bulk materials, such as nonlinearity of volt-ampere characteristics, irreversibility of deformation, and a low working factor, and allows improvement of the working factor after failure and healing. Most importantly, we discovered that hindered self-healing, like in the case of the MWCNT/PBS system, enables improvement of sensor sensitivity after large strains and failure, which is due to partial failure of the network formed by conductive particles.

A wearable strain sensor based on self-healable MXene/PVA hydrogel for bodily motion detection

Developing flexible, stretchable, and self-healing wearable electronic devices with skin-like capabilities is highly desirable for healthcare and human-machine interaction. Hydrogels as a promising sensing material with crosslinked polymer networks have received widespread attention for decades. However, sensors based on hydrogels suffer from low sensitivity and stability due to their poor electrical conductivity or the movement of nanofillers in hydrogel networks. Herein, a stable, sensitive, and self-healing strain sensor is fabricated by the Ti3C2Tx MXene nanosheets/polyvinyl alcohol (PVA) hydrogel (T-hydrogel). The introduction of MXene increases the number of H-bonds in the PVA hydrogel network and enhances the conductivity, resulting in high sensitivity, stability, and self-healing character. The self-healing T-hydrogel-based strain sensor has a performance close to that of the original sensor. In addition, the device is capable of detecting bodily motions, indicating the potential application in the field of human health monitoring and human-computer interaction.

Ultra-stretchable, high conductive, fatigue resistance, and self-healing strain sensor based on mussel-inspired adhesive hydrogel for human motion monitoring

Conductive hydrogels have been widely applied in various electronics, such as artificial skin, flexible devices, and implantable bioelectronics. However, it is still a challenge to develop high-performance hydrogel with high conductivity without compromising the toughness, stretchability, self-healing, and adhesion properties. Herein, inspired by the mussel-adhesion mechanism, an adhesive hydrogel that simultaneously achieves high conductivity (27.0 S·m−1), ultra stretchability (strain > 3700%), and strong toughness (1930 kJ·m−3) was developed by incorporating polydopamine (PDA)/sodium caseinate (SC) cross-linked network into the poly(acrylamide-co-acrylic acid) (P(AM-co-AA))/Al3+ network. Due to the abundant metal coordination and hydrogen bonds in the network, the obtained hydrogel displayed rapid self-healing capability (healing efficiency, HE > 97%) and excellent fatigue resistance. Moreover, the introduction of PDA-SC cross-linked network endowed the hydrogels with robust adhesion (36.5 kPa for pigskin) and high conductivity. The conductive hydrogel was assembled into a strain sensor, which demonstrated prominent sensing performance with a wide detection range (∼2500%), high sensitivity (gauge factor (GF) of 23.7), fast response time and reliable repeatability. The hydrogel strain sensor can be attached on human body to distinguish and monitor both large and subtle movement signals. The developed conductive hydrogel strain sensor is expected to have promising applications in wearable devices, human–machine interaction, and electrical skin, and to extend for use in other portable and wearable energy related devices with multifunction.

Silicone/Carbon Black-Filled Elastomer-Based Self-Healing Strain Sensor

The recent interest in flexible and stretchable strain sensors reflects their potential applications in various fields, including healthcare monitoring, soft robotics, and electronic skins. In addition to high stretchability and excellent sensing properties, self-healability is also a highly desirable property of stretchable strain sensors as it allows increasing their lifespan and thus reduce electronic waste and cost. In this paper, a self-healing silicone tape is introduced as a self-healable flexible substrate that to fabricate a resistive strain sensor. The sensor has an average gauge factor of 34.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.26 up to 50% stretch. Moreover, the sensor can be cut and healed without the use of any adhesive materials or heating. After cutting, the healed sensor still has an average gauge factor of 2.0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.006 up to 4% stretch, a stable response over 150 stretch-release cyclic tests, and breaks only after 18% of applied tensile strain.

A Transparent, and Self-Healable Strain-Sensor E-Skin Based on Polyurethane Membrane with Silver Nanowires

Electronic skin (E-skin) is increasingly utilized in modern society, yet current E-skin technology suffers from issues, such as opacity, hardness, and fragility. To address these challenges, a novel E-skin was developed using polyurethane (PU) as the matrix material and silver nanowires (AgNWs) as the sensing material. By leveraging the small degree of microphase separation and lack of crystallization in the PU, combined with the appropriate length–diameter ratio of the AgNWs, the resulting E-skin exhibited a visible light transmittance of 75%. The E-skin also showed excellent self-healing properties (83.63% efficiency in the third repair) and mechanical properties (with almost no degradation after 60 tensile cycles) due to the reversible dynamic cross-linking network within the PU. The synergistic effect of PU and AgNWs resulted in exceptional sensing performance for the E-skin, with a gauge factor of 46 (when ε = 10%). Moreover, the E-skin demonstrated signal stability during human joint motion monitoring and successfully identified different movement states, highlighting its potential for diverse applications. This research presents a simple yet effective approach for producing transparent, durable, and stable E-skin.

A tough, healable, and recyclable conductive polyurethane/carbon nanotube composite

Recently, conductive composites have been used in flexible electronic devices and have attracted attention. The integration of self-healing, high sensitivity, large tensile strength, environmental stability, and easy recyclability into conductive composites is very desirable yet challenging. Hence, a conductive composite as a flexible strain sensor with a self-healing and recyclability is facilely developed, with a polyurethane (PU) elastomer bearing dynamic boronic ester as the polymer matrix and carbon nanotubes (CNTs) as a conductive filler. Due to the dynamic boronic ester bond and hydrogen bond, the prepared polyurethane conductive composite has good self-healing and mechanical properties. It not only has a high healing efficiency of 78 % but also has a tensile strength of 15.4 MPa and an elongation at break of 420 %. In addition, the prepared conductive composite has high conductivity (0.57 mS/cm) and sensitivity. As a wearable sensor, it can identify human activities in all directions, such as elbow and finger bending, speaking, and facial changes. Consequently, the polyurethane conductive composite prepared in this study exhibited wonderful application potential in wearable electronic devices such as self-healing strain sensors.

A multifunctional MXene-assembled anhydrous gel electronics

Hydrogel devices still cannot overcome the dehydration puzzle. This problem can be solved by using green deep eutectic solvents (DES) instead of water, but the formed gel lacks enough strength and functionality. Herein, we successfully assembled a function-integrated stable anhydrous gel in DES system using active MXene as a function-filler. First, Mxene can catalyze the rapid polymerization of monomers at room temperature. Then, it can also assemble/crosslink the gel through strong interactions with the polymer chains, which endow the gel with great stretchability (∼1750%), self-healing (∼100%), conductivity (1.5 × 10-2 S·m−1), and adhesion (∼7 kPa;pigskin). In addition, the gel also inherits the environmental adaptability of DES, which can achieve nearly 100% weight retention at 40 °C, even 80 °C. Importantly, the application potential of stable functionalized gels in ultra-anti-dryness bioelectrodes, self-healing strain sensors, and electric-generators have been explored and successively demonstrated. The rapid fabrication of functional materials in this new solvent system can be extended to other anhydrous polymer gels for on-demand development.

Self-healing strain sensor based on silicone elastomer for human motion detection

Flexible stretchable sensors based on polymer elastomers and conductive materials for human-computer interaction, health monitoring and soft robots have attracted widespread attention. However, it is difficult for conventional flexible sensors to meet the requirements of wide reflection range, high sensitivity, self-healing and two-dimensional sensing at the same time. Here, we report a novel sensor based on self-healing elastomer and Polypyrrole (PPy). The healed strength and the elongation at break of the elastomer can reach 0.88 MPa and 652.5%, respectively. For sensing functions, the sensor has a wide reflection range of 130% and a high gauge factor (GF) of 1251. Even after cutting-healing, the sensing performance can still be maintained, and the increase in resistance is not obvious. Excellent reliability and stability have also been verified.

Self-healing strain-responsive electrochromic display based on a multiple crosslinked network hydrogel

Stretchable electronic devices with self-healing functions that can improve durability are highly recommended as next-generation personal instruments for economic and sustainable society. Here, we report a fabrication of self-healing strain-responsive electrochromic display based on a multiple crosslinked network hydrogel (MCNH) consisting of both hydrophilic and hydrophobic domains. After optimizing the mechanical and self-healing properties of the hydrogel with variation of the chemical crosslinker, N,N'-methylenebisacrylamide, and the ionic crosslinker CaCl2, an extreme mechanical stretchability of up to 2000% strain and shape recovery, and a self-healing efficiency of 83.5% after 8 h at room temperature are obtained. The MCNH-based strain sensor exhibits a fast and linear resistance response with a coefficient of determination of 0.997 over a wide strain range of 100%. The strain sensitivity of the hydrogel remains stable even after 10 repeated self-healing cycles at a single location. As a display application, a novel two-dimensional electrochromic device is fabricated using a hydrogel without depositing an electrochromic material (ECM) on the electrode. ECM-containing gel electrolyte exhibits electrochromic properties through the migration of ions to the electrodes. Coloration/discoloration occurs at a potential bias of 1.7 V with a transmittance change of 76.1% at 547 nm through the chemical oxidation/reduction of ethyl viologen ions in the hydrogel matrix. An integrated system comprising a self-healing strain sensor and an ECD attached to the skin is demonstrated to visually express the applied strain due to finger bending, aided by an external circuit. Such a strain-responsive ECD system preserves a stable performance with the self-healed sensor after a complete bisection. These results suggest the potential application of our newly synthesized hydrogel to various skin-attachable self-healing, and stretchable devices with high durability.

Tough, stretchable and self-healing C-MXenes/PDMS conductive composites as sensitive strain sensors

Due to the good elasticity and stretchability, conductive composites can be widely used as sensitive strain sensors in health monitoring, electronic skin and intelligent robotics. However, the preparation of conductive material that integrates excellent mechanical properties, efficient self-healing properties and sensitive sensing properties remains a challenge. Here, we developed a stable and sensitive self-healing strain sensor based on silicone conductive composites and further evaluated the sensing properties by monitoring the motion of human muscles and joints. Disulfide bonds and multiple hydrogen bonds were introduced into the dynamic silicone supramolecular network through the dehydration reactions of amino groups with carboxyl groups and the addition reactions of amino groups with isocyanate groups. Furthermore, the addition of natural small molecule L-citrulline modified MXenes (C-MXenes) through esterification imparted the material good electrical conductivity. The mechanical strain and stress could reach to 413% and 4.78 MPa, respectively. Due to the dynamic disulfide and multiple hydrogen bonds, the conductive composites could spontaneously achieve self-healing with multiple cycles. Even if the sample was repaired, it could still sense the human motion precisely. Moreover, the material exhibited outstanding sensitive of strain. And the GF (gauge factor) was 3.3821. This excellent performance and simple preparation procedure of silicone conductive composites could be developed as a new generation of wearable sensor devices.

Highly viscoelastic, stretchable, conductive, and self-healing strain sensors based on cellulose nanofiber-reinforced polyacrylic acid hydrogel

Conductive and self-healing hydrogels are among the emerging materials that mimic the human skin and are important due to their probable prospects in soft robots and wearable electronics. However, the mechanical properties of the hydrogel matrix limit their applications. In this study, we developed a physicochemically dual cross-linked chemically modified-cellulose nanofibers-carbon nanotubes/polyacrylic acid (TOCNF-CNTs/PAA) hydrogel. The TOCNFs acted both as a nanofiller and dispersant to increase the mechanical strength of the PAA matrix and break the agglomerates of the CNTs. The final self-healing and conductive TOCNF-CNTs/PAA-0.7 (mass ratio of CNTs to AA) hydrogel with a uniform texture exhibited highly intrinsic stretchability (breaking elongation to ca. 850%), enhanced tensile properties (ca. 59 kPa), ideal conductivity (ca. 2.88 S m− 1) and pressure sensitivity. Besides, the composite hydrogels achieved up to approximately 98.36% and 99.99% self-healing efficiency for mechanical and electrical properties, respectively, without any external stimuli. Therefore, the as-designed multi-functional self-healing hydrogels, combined with stretching, sensitivity, and repeatability, possess the ability to monitor human activity and develop multifunctional, advanced, and commercial products such as wearable strain sensors, health monitors, and smart robots.

Wearable human-machine interface based on the self-healing strain sensors array for control interface of unmanned aerial vehicle

Flexible strain sensors have been widely adopted for wearable systems in detecting the human activities and health. However, the demonstration of controlling interface based on strain sensor is still limited. Herein, a novel wearable human-machine interface (W-HMI) is proposed based on the signal processing system and self-healing strain sensors array. The strain sensor shows high sensitivity with the Gauge Factor max of 20, wide-detection range of 0.2–3000 % and self-healing ability. The proposed flexible and transparent W-HMI is capable of effectively capturing eyes' motions through four-array strain sensors. Based on the coupling effect of different sensors, W-HMI collects and processes the signals of the four sensors under different movements of the eyes, and then outputs different signals to control the right, left, up and down movements of the UAV. Importantly, the destroyed sensor can still show excellent performance due to its self-healing ability and can be used as a W-HMI to control the movement of UAV, which is an important characteristic to compensate for the vulnerability of W-HMI under continuous tension or extrusion. This innovative, cost-effective, simple-designed W-HMI based on the self-healing strain sensors will open a novel way to its application in the human-machine field.

Inkjet printed self-healable strain sensor based on graphene and magnetic iron oxide nano-composite on engineered polyurethane substrate

In recent years, self-healing property has getting tremendous attention in the future wearable electronic. This paper proposes a novel cut-able and highly stretchable strain sensor utilizing a self-healing function from magnetic force of magnetic iron oxide and graphene nano-composite on an engineered self-healable polyurethane substrate through commercialized inkjet printer DMP-3000. Inducing the magnetic property, magnetic iron oxide is applied to connect between graphene flacks in the nano-composite. To find the best nano-composite, the optimum graphene and magnetic iron oxide blending ratio is 1:1. The proposed sensor shows a high mechanical fracture recovery, sensitivity towards strain, and excellent self-healing property. The proposed devices maintain their performance over 10,000 times bending/relaxing cycles, and 94% of their function are recovered even after cutting them. The device also demonstrates stretchability up to 54.5% and a stretching factor is decreased down to 32.5% after cutting them. The gauge factor of the device is 271.4 at 35%, which means its sensitivity is good. Hence, these results may open a new opportunity towards the design and fabrication of future self-healing wearable strain sensors and their applied electronic devices.

A highly stretchable and intrinsically self-healing strain sensor produced by 3D printing

ABSTRACT One common issue when implementing wearable strain sensors for health-monitoring is the limited service time when they are inevitably subjected to mechanical operation in practical applications. Therefore, integrating multi-functionality with reliable performance is a long-term pursing target. To this end, we have developed a promising strain sensor utilising a facile 3D printing technology of digital light processing (DLP), thereby simultaneously realising super-high stretchability and intrinsic self-healing ability. Owing to the incorporation of carboxyl multi-walled carbon nanotubes (c-CNTs), the over-curing of the N-acryloylmorpholine (ACMO) resin was adequately mitigated, and good electrical conductivity of the nanocomposite was obtained. On the basis of multi-functionality, the strain sensor before and after self-healing can be applied for real-time and accurate detection of human activities. Therefore, it is expected that the highly stretchable and intrinsically self-healing strain sensor will have promising applications in wearable electronics, personal health care, etc.

Three-Dimensional Binary-Conductive-Network Silver Nanowires@Thiolated Graphene Foam-Based Room-Temperature Self-Healable Strain Sensor for Human Motion Detection.

A lot of attention has recently been focused on wearable strain sensors because of their promising applications in the rising areas of human motion detection, health monitoring, and smart human-machine interaction. However, the design and fabrication of self-healable strain sensors with superior overall properties including stretchability, sensitivity, response ability, stability, and durability is still a huge challenge. Herein, we report an innovative self-healable strain sensor with exceptional overall performance constructed with three-dimensional binary-conductive-network silver nanowire-coated thiolated graphene foam (AgNWs@TGF) and room-temperature self-healing functionalized polyurethane (FPU) elastomer. Taking advantage of the good ductility and continuity of the AgNWs@TGF binary structure and the excellent resilience of the FPU, the strain sensor exhibits good stretchability (up to 60% strain), high sensitivity [gauge factor (GF) of 11.8 at 60% strain and detection limit of 0.1% strain], fast response ability (response/recovery time of 40/84 ms), and exceptional durability for 800 cycles of fatigue test. Besides, the highly flexible polydimethylsiloxane chains, strong intermolecular hydrogen bonding, and dynamic exchange reaction of aromatic disulfides ensure the sensor excellent recovery property of electrical conductivity, and the GF of sensor after self-healed only increases slightly. More importantly, the sensor is successfully applied for detecting a variety of human motions including pulse beats, voice recognitions, various joint movements, and handwriting. The method for preparing room-temperature self-healable strain sensor is facile, scalable, and cost-effective. The finds provide a new perspective on fabricating new-generation high-performance and functional strain sensors for health monitoring, wearable electronics, and intelligent robots.

Highly stretchable and self-healing strain sensors for motion detection in wireless human-machine interface

Abstract The highly stretchable, flexible and self-healing strain sensor is considered as a promising wearable device for motion detection in fields such as robotics and human-machine interface. Herein, a strain sensor with high stretchability and sensitivity is designed and fabricated based on the poly(acrylamide) (PAAm) hydrogel. The ionic conductive PAAm hydrogel shows an excellent self-healing property with fast electrical self-healing speed (within 1.8 s) and high self-healing efficiency (99%). The PAAm hydrogel based strain sensor exhibits excellent performance with large stretchability (>900%), high sensitivity (with maximum gauge factor of 6.44), fast response time (~150 ms), and good cycling durability (>3000 cycles). As the wearable device, the strain sensor can detect various human motions and is able to transmit the data to the smart phone combined with a readout and wireless system. Furthermore, a smart glove was fabricated by coupling multiple strain sensors and the corresponding circuit. The smart glove is capable of expressing and recognizing American Sign Language and can be used to control a robotic hand wirelessly through the hand gestures. Thus, the highly stretchable, self-healing hydrogel-based strain sensor shows a potential application for the human-machine interface, personal healthcare and sports training.

Integration of metal-free ATRP and Diels-Alder reaction toward sustainable and recyclable cellulose-based thermoset elastomers

Well-defined sustainable and recyclable thermoset elastomers derived from cellulose, fatty acid and furfural were successfully prepared via photoinduced metal-free ATRP and Diels-Alder (DA) reaction. Firstly, metal free ATRP was applied to prepare a range of thermoplastic cellulose graft copolymers with furfural groups. Then, a modified epoxidized soybean oil bearing 6-maleimidohexanoic group (ESOM) as a crosslinker was employed to perform DA reaction with furfural groups in these cellulose graft copolymers, by which the copolymer formed the dynamic crosslinked network and achieved the thermoset elastomers. The dynamic crosslinked network formed by DA reaction not only could increase the chain entanglement that was associated with the improved flexibility and could contribute to enhancing the mechanical strength up to 166 %, but also endowed these thermoset elastomers with recyclability, excellent shape memory and self-healing property. These thermoset elastomers can be used as self-healing strain sensor and wearable sensing devices after compounding them with carbon nanotubes.

A photoresponsive azopyridine-based supramolecular elastomer for self-healing strain sensors

Optical-stimuli materials have been applied in many fields since it can be activated from external system and localized in time and space. Although, optically healable polymers have been well developed in recent years, to be healed under mild conditions remains a big challenge for these materials. Herein, we report the design and synthesis of a metallo-supramolecular polymer containing azopyridine ligands to address this conundrum. The resulting photo-healable polymer is highly stretchable (up to 735%) with a high toughness (16.33 MJ/m3) and excellent light-healing efficiency of ~90% after three cutting/healing cycles at mild temperature (40 °C) in various harsh conditions (e.g. underwater, subzero temperature). Furthermore, a novel bioinspired microcrack nanostructure design is presented to endow the resulted supramolecular elastomer with outstanding sensitivity to strain, which makes it very suitable for human motion monitoring applications. We believe this work will provide afflatus on future design, fabrication and application of photo-stimuli smart materials.

Self-healing flexible sensor based on metal-ligand coordination

Self-healing is an indispensable property of the next generation wearable electronic devices. However, it is a hugely difficult problem to achieve good self-healing abilities and good elastic properties at different conditions. Herein, we report a self-healing substrate from commercially available methyl vinyl silicone rubber (MVQ) that has been cross-linked through metal-ligand coordinate bonds. The strain sensor was then assembled by coating nanostructured conductive layer (Multiwall carbon nanotubes (CNTs) film and silver film) onto the self-healing elastomeric substrate. The resulting flexible sensors show a desirable healing efficiency under both high temperatures and underwater conditions. Notably, the healed strain sensors can still precisely capture diverse human activities, such as large limb movements and minute physiological activities. The self-healing strain sensors demonstrated in this study will push forward the design and application of stretchable and flexible electronic devices.

Highly Stretchable and Self-Healing Strain Sensor Based on Gellan Gum Hybrid Hydrogel for Human Motion Monitoring

Currently, stretchable and conductive hydrogels have attracted considerable attention because of promising applications in the area of flexible electronic skins. However, integrating stretchability...