Transparent Strain Sensor Research Articles

Flexible mussel‐inspired hydrogel for transparent wearable strain sensors: Investigation of mechanical, physical properties, self‐healing, and electrical conductivity

AbstractNowadays, flexible hydrogels with electrical conductivity have become extremely helpful with broad application scenarios in electronic membranes in human–machine boundaries. However, a design approach to adjust and ameliorate conductivity, mechanical strength, transparency, self‐healing, and good adhesion remains a challenge. One of these strategies is mussels‐mimicking adhesive hydrogels, presenting strong adhesion. Here, we aim to meet the challenge by pursuing a mussel‐inspired design approach, which leads to the formation of a transparent, self‐healing, adhesive, and electrically conductive hydrogel from dopamine (DA), pyrrole (Py), acrylamide (AM), functionalized iron, and copper ions as an advanced electronic sensor. The transparent hydrogels were prepared using a sonication polymerization method. The results have shown that hydrogels containing copper or iron have conductivity (12 × 10−4 or 27 × 10−4 S/cm), mechanical strength (11.2 or 18.4 kPa), elongation at break (EB: 652.231% or 660.198%) and gauge factors (GF: 3.04679, 2.93513). Also, hydrogels can maintain their conductivity if they are twisted or stretched. Self‐healing behavior, swelling, and water‐induced shape memory effect (SME) due to hydrogen bonds and π–π interactions were observed. The behavior of this hydrogel as wearable sensors in integration with different body tissues such as finger, wrist, breathing, and frown was investigated. The obtained hydrogels showed high sensitivity in different areas of the body, fast response time in the strain range of 0%–%500, and high adhesion and transparency.

Sustainable castor oil-derived cross-linked poly(ester-urethane) elastomeric films for stretchable transparent conductive electrodes and heaters

A new class of sustainable castor oil-based cross-linked poly(ester-urethane) elastomer with high transmittance, stretchability, flexibility, thermal and chemical stability was developed for flexible transparent strain sensors and heaters.

Transparent single crystal graphene flexible strain sensor with high sensitivity for wearable human motion monitoring

Human-machine interfaces, smart glasses, touch screens and some electronic skins all require highly transparent and flexible strain sensing components. Flexible strain sensors usually use microstructures or porous active materials to improve sensitivity and response speed, resulting in low transparency of the device. Graphene has attracted extensive attention due to its special physical properties, especially excellent flexibility, and high transmittance. However, previous studies found that it is difficult to obtain high gauge factor in graphene. Thus, graphene cannot be used to monitor subtle deformations. Although previous research has shown that the strain sensors based on the combination of graphene and other materials have high sensitivity, but lack of transmittance. In this paper, we report a single-crystal graphene (SCG) strain sensor fabricated by encapsulating single-crystal hexagonal graphene in a flexible substrate. The strain sensor shows excellent performance with a high gauge factor (16.9) because it is not affected by wrinkles, grain boundaries and additional layers. Further, the strain sensor retains the advantage of high light transmittance of graphene. This sensitive SCG strain sensor has been demonstrated to detect a range of human motions, including finger gestures, tiny vibration of different sounds and heartbeats with high performance. This work further confirms the application of graphene in strain sensors, and provides new ideas for the research of graphene-based transparent strain sensors.

Transparent and Stretchable Piezoresistive Strain Sensors with Buckled Indium Tin Oxide Film

AbstractOwing to excellent performance and a simple mechanism, stretchable piezoresistive strain sensors have been applied to human skin for monitoring physical activities, physiological activities, etc. However, it is still a challenge to simultaneously realize highly sensitive and stretchable piezoresistive strain sensors with high optical transparency. This study reports a transparent and stretchable piezoresistive strain sensor with 2D surface buckling by fabricating ultrathin indium tin oxide (ITO) film on the biaxially pre‐stretched polyacrylate (VHB) elastomer followed by pre‐stretch releasing. To the authors' knowledge, semiconductors are applied for a stretchable piezoresistive strain sensor for the first time. Furthermore, this strain sensor exhibits a high sensitivity of 569, a high transparency of 88% and a high biaxial stretchability of 110% at the same time. This device demonstrates the good long‐term stability over 500 stretching–relaxing cycles. The high sensitivity can be mainly attributed to the piezoresistive effect of the semiconductor where carrier mobility and the resulting resistivity can be significantly changed by the strain. The strain sensors attached to human skin are used to monitor many human motions such as chewing, swallowing, breathing, and walking. ITO‐based strain sensors pave the way toward the development of highly stretchable and sensitive wearable electronics.

Subject-adaptive Loose-fitting Smart Garment Platform for Human Activity Recognition

The ability to recognize and detect changes in human posture is important in a wide range of applications such as health care and human–computer interaction. Achieving this goal using loose-fit garments instrumented with sensors is particularly challenging, due to the complex interaction between garments and human body. Herein we present a method to detect and recognize human posture with casual loose-fitting smart garments integrated with highly sensitive, stretchable, optical transparent, and low-cost strain sensors. By attaching these sensors to an off-the-shelf casual jacket, we developed a smart loose-fitting sensing garment that enables posture recognition using a deep learning model, domain-adaptive Convolutional Neural Networks–Long Short-Term Memory (CNN-LSTM). This deep learning model overcame the noise and variation due to the complex interaction between loose-fitting garments and human body. Considering that users’ labeled data are usually not available in the training stage, an additional domain discriminator path on the conventional CNN-LSTM model has been introduced to further improve the adaptability. To evaluate the potential of this loose-fitting smart garment, three case studies were conducted under realistic conditions: recognitions of human activities, stationary postures with random hand movements and slouch. Our results demonstrate the potential of the proposed smart garment system for practical applications.

Smart Glove Human-Machine Interface Based on Strain Sensors Array for Control UAV

Smart gloves have attracted much attention as a typical representative of wearable human-machine interface (HMI). Traditional sensors on smart gloves are limited due to high cost, complex manufacture, which can’t meet the requirements of bio-friendliness and sustainability. This paper presents a smart glove human-machine interface based on strain sensor array of conductive hydrogels. A flexible, transparent, stretchable and sensitive hydrogel strain sensor is prepared based on a “one-pot” preparation method. The smart glove was constructed using 3D structural design combined with the sensor array. Combing with the signal processing system, the smart glove can realize the spatial motion control of the Unmanned Aerial Vehicle (UAV). The smart glove based on the hydrogel flexible sensor array showed great application prospects in the field of human-machine interaction of robot and Virtual Reality (VR).

Liquid metal/CNTs hydrogel-based transparent strain sensor for wireless health monitoring of aquatic animals

Strain sensors are intriguing in advanced electronics and are typically composed of active sensing materials. Good elastic recovery, self-healing, good adhesion, high transparency, and great mechano-response are essential but can be rarely met in one material. Here, we report a wireless strain sensor that exhibits tissue-like mechanical properties (softness), is conformable to soft tissues (heart and tail) and has the lowest limit of detection (0.25 % stretch). After introducing multiwalled carbon nanotubes (CNTs) bridged liquid metal (LM) composites (LM/CNTs), the strain sensor simultaneously achieves excellent transparency and conductivity. This LM/CNTs hydrogel overcomes the limitations of the traditional hydrogel and achieves high conductivity (up to 94 S/m), good stretchability (2,200 %), and a high degree of transparency (93 %), in addition to enough adhesion (20 kPa) and rapid self-healing (20 min). The strain sensor device is composed of intrinsically soft LM/CNTs hydrogel as sensing material, a microcontroller, signal-processing circuits, and Bluetooth transceiver. This device is applied to recording high-quality heartbeat signals and clear in-site observation of cardiac bleeding, which has helped a Chinese sturgeon survive after Caesarean. This approach provides an ideal strategy for protecting and saving endangered animals and broadens the applications of strain sensors.

Transparent Electronic Skin from the Integration of Strain Sensors and Supercapacitors

AbstractSome of the major challenges for electronic skin (E‐skin) are to achieve accurate and continuous monitoring of human activities/health condition, eliminating wired external power sources. The ideal E‐skin should be ultra‐sensitive, self‐powered, conformal to human skin, and optically transparent for visual or aesthetic purposes. In this paper, a self‐powered all‐in‐one transparent E‐skin is constructed by integrating a flexible transparent supercapacitor (FTSC) as an energy storage device with a stretchable transparent strain sensor. The assembled FTSC uses oxygen‐deficient molybdenum oxide (MoOx) nanowires and cellulose nanofiber composite as paper electrodes, achieving an excellent combination of high transparency and outstanding energy‐storage capacity (12.1 mF cm−2) to drive the operation of the sensor. Meanwhile, the synergistic effect of 1D nanowires bridging 2D nanosheets in the skeleton network provides the transparent strain sensor with ultra‐high sensitivity (high gauge factor up to 220 under 1% strain), high signal‐to‐noise ratio, fast response time (≈50 ms), and excellent stability. The E‐skin being conformally attached to various parts of the human body enables real‐time detection of subtle physiological signals and joint movements.

PVA/ChCl Deep Eutectic Polymer Blends for Transparent Strain Sensors with Antifreeze, Flexible, and Recyclable Properties.

Wearable elastic electronic devices have attracted tremendous attention due to their monitoring capabilities for human motion detection. In this work, a hydrogen bond acceptor quaternary ammonium salt, choline chloride (ChCl), has been used to fabricate deep eutectic polymer (DEP) blends with polyvinyl alcohol (PVA). The miscibility, molecular interaction, and physical properties of PVA/ChCl DEP blends were investigated systematically. It is demonstrated that the deep eutectic of PVA/ChCl can be obtained by simple solution blending, and the melting points of both PVA and ChCl are reduced respectively due to the strong hydrogen bond between PVA and ChCl. Due to the elasticity of the PVA/ChCl elastomer and the response of ChCl ions to temperature and humidity, the fabricated sensor showed stable and repeatable resistance changes upon strain, temperature, and humidity variations. We hypothesize that the DEP blend system has potential applications in functional composites and the final PVA/ChCl elastomer composites exhibited high transparent, antifreeze, and recyclable capability, which may be promising for applications in soft/flexible devices.

High-Resolution, Transparent, and Flexible Printing of Polydimethylsiloxane via Electrohydrodynamic Jet Printing for Conductive Electronic Device Applications.

In the field of soft electronics, high-resolution and transparent structures based on various flexible materials constructed via various printing techniques are gaining attention. With the support of electrical stress-induced conductive inks, the electrohydrodynamic (EHD) jet printing technique enables us to build high-resolution structures compared with conventional inkjet printing techniques. Here, EHD jet printing was used to fabricate a high-resolution, transparent, and flexible strain sensor using a polydimethylsiloxane (PDMS)/xylene elastomer, where repetitive and controllable high-resolution printed mesh structures were obtained. The parametric effects of voltage, flow rate, nozzle distance from the substrate, and speed were experimentally investigated to achieve a high-resolution (5 µm) printed mesh structure. Plasma treatment was performed to enhance the adhesion between the AgNWs and the elastomer structure. The plasma-treated functional structure exhibited stable and long strain-sensing cycles during stretching and bending. This simple printing technique resulted in high-resolution, transparent, flexible, and stable strain sensing. The gauge factor of the strain sensor was significantly increased, owing to the high resolution and sensitivity of the printed mesh structures, demonstrating that EHD technology can be applied to high-resolution microchannels, 3D printing, and electronic devices.

Low-hysteresis, pressure-insensitive, and transparent capacitive strain sensor for human activity monitoring

Wearable strain sensors have been widely used for human activity monitoring. Most reported strain sensors have mainly focused on material engineering, high stretchability and large gauge factors. Few works have focused on strain sensor’s robustness and reliability, including low hysteresis, good long-term stability, good electrode material stability, and low coupling effects under multi-input signals, which are the factors that limit practical strain sensor applications. To develop a high-performance strain sensor, we propose a flexible capacitive sensor structure with three-dimensional (3D) interdigital electrodes fabricated by vertically aligned carbon nanotubes. Compared with a traditional resistive strain sensor and a capacitive strain sensor with vertical sandwich electrodes, a strain sensor with horizontal parallel interdigital electrodes can benefit from low cross talk in terms of the normal force and improve substrate transparency. Additionally, embedding 3D electrodes into the substrate improves ultrahigh robustness with a low-pressure coupling effect under normal force. Moreover, compared with other reported works, the electrode variation under strain is small (less than 1.6%), which means that the perturbation of inert properties on device performance is small. Finally, the fabricated strain sensor achieves an ultralow hysteresis (0.35%), excellent pressure-insensitive performance (less than 0.8%), fast response (60 ms), good long-term stability, and good transparency. As an application example, a flexible strain sensor was successfully demonstrated as a wearable device for the precise monitoring of different types of human activities, including bending of the finger, knee, elbow, wrist, and neck with large strain signals and small strain signals generated by a mouth-opening activity. This excellent performance indicates that the flexible strain sensor is a promising candidate for human motion detection, soft robotics, and medical care.

Transparent, highly stretchable, adhesive, and sensitive ionic conductive hydrogel strain sensor for human motion monitoring

Due to the flexibility, ductility and tensile properties, hydrogel is considered as promising candidates for the new generation of wearable flexible devices. However, it is still a challenge to endow multiple functions such as excellent conductivity, high stretchability, self-healing, self-adhesion, and biocompatibility in one hydrogel network. Herein, a two-step method was used to prepare multifunctional transparent tannic acid/sodium alginate/polyacrylic acid/carboxymethyl cellulose/aluminum trichloride (SA/TA/PAA/CMC/Al(III)) hydrogels with high stretchability, toughness, adhesion, and conductivity. This hydrogel can adhere to various substrates both organic and inorganic materials. The highest adhesion strength of the hydrogels to the iron sheet reaches 22.33 kPa with the fracture stress being 0.16 MPa and the toughness 1.01 MJ/m3. Moreover, the hydrogel exhibited repeatable and durable adhesion to human skin and could be peeled off completely without any residual, irritation or allergic reactions. Additionally, the hydrogels also have good repetitive adhesion and strain sensitivity. It was demonstrated that both large-scale and small-scale movements of the human body could be monitored by directly attached to various parts of the human body by the self-adhesion. This work provides a new prospect for the design of the biocompatible hydrogels with transparent, stretchable, self-adhesive, and strain-sensitive properties for potential applications in wearable electronic sensors.

Lithography‐Free Fabrication of Periodic Metal Network for Ultra‐Sensitive Transparent and Unidirectional Strain Sensor in Health‐Monitoring Applications

AbstractTools with the ability of continuous health monitoring have received immense interest in biomedical engineering and the internet of things (IoT). Ultra‐sensitive wearable strain sensors are such tools having the potential to bring cost‐effectiveness in the therapeutic field. The present work describes the development of a transparent strain sensor comprising parallel Au micro‐wires semi‐embedded in polydimethylsiloxane (PDMS). A lithography‐free technique is employed for the cost‐effective fabrication, where a parallel‐crack network, formed by the gravity‐flow of a crackle precursor on an inclined silicon substrate, serves as a stencil for the parallel micro‐wire formations. The ultimate device is realized by transferring the wires into the PDMS substrate. The sensor with ≈85% transmittance shows a resistance change of ≈370% for an applied strain of 2%. The mechanism relies on the formation of nano‐break junctions in the micro‐wires due to applied strain leading to discontinuation in the electron percolation. The unidirectional sensor having such ultra‐high sensitivity and cyclic stability is exploited for the detection of resonance and dissonance oscillations of a coupled cantilever. Besides, the sensor is utilized for speech recognition, gesture detection and radial augmentation index calculation from the blood‐pulse pattern, supporting its feasibility as a potential tool in robotics and biomedical applications.

Directly Printed Embedded Metal Mesh for Flexible Transparent Electrode via Liquid Substrate Electric-Field-Driven Jet.

Flexible transparent electrodes (FTEs) with embedded metal meshes play an indispensable role in many optoelectronic devices due to their excellent mechanical stability and environmental adaptability. However, low‐cost, simple, efficient, and environmental friendly integrated manufacturing of high‐performance embedded metal meshes remains a huge challenge. Here, a facile and novel fabrication method is proposed for FTEs with an embedded metal mesh via liquid substrateelectric‐field‐driven microscale 3D printing process. This direct printing strategy avoids tedious processes and offers low‐cost and high‐volume production, enabling the fabrication of high‐resolution, high‐aspect ratio embedded metal meshes without sacrificing transparency. The final manufactured FTEs with 80 mm × 80 mm embedded metal mesh offers excellent optoelectronic performance with a sheet resistance (R s) of 6 Ω sq−1 and a transmittance (T) of 85.79%. The embedded metal structure still has excellent mechanical stability and good environmental suitability under different harsh working conditions. The practical feasibility of the FTEs is successfully demonstrated with a thermally driven 4D printing structure and a resistive transparent strain sensor. This method can be used to manufacture large areas with facile, high‐efficiency, low‐cost, and high‐performance FTEs.

Highly sensitive, direction-aware, and transparent strain sensor based on oriented electrospun nanofibers for wearable electronic applications

Wearable strain sensors have made great progress in sensing performance, stretchability and durability. However, practical applications of these sensors are still quite challenging because they are incapable of detecting multi-degree-of-freedom strains due to the interference of multidirectional strains. Herein, a high-sensing-performance, direction-aware and transparent strain sensor is reported based on antimony-doped tin oxide oriented nanofiber (ATO-ONF) films prepared by electrospinning. The monolayer ATO-ONF strain sensor shows remarkable anisotropic sensing performance, namely GFs of 250 and 1.2 for the nanofiber orientation and its transverse directions, suggesting the realization of the unidirectional sensing capability of the strain sensor, i.e., only responding to strains along the nanofiber direction. In addition, this strain sensor also exhibits high transparency with a light transmittance of ∼ 80%, and excellent sensing performance including high sensitivity, high linearity, low hysteresis, good repeatability and durability (>2000 cycles). Based on these superior sensing properties, the direction-aware biaxial strain sensor is designed by orthogonally stacking ATO-ONF films, by which the predicted magnitude and direction of the tensile strains agree well with those of the actual strains. Furthermore, the multi-degree-of-freedom applications of direction-aware strain sensors in human motion monitoring and human–machine interaction are demonstrated, showing a great application potential in next-generation wearable electronics.

An extremely transparent and multi-responsive healable hydrogel strain sensor

Using natural catechin-assembled nanostructures as electron bridging agents, we developed smart hydrogel strain sensors with excellent performances on transparency, conductivity, self-adhesion, biocompatibility and self-healability.

A facile strategy for fabricating self-healable, adhesive and highly sensitive flexible ionogel-based sensors

An adhesive, self-healable and transparent ionogel-based strain and temperature sensor with high sensitivity (gauge factor up to 6) and a wide sensing range was fabricated based on a green deep eutectic solvent.

Highly transparent, mechanical, and self-adhesive zwitterionic conductive hydrogels with polyurethane as a cross-linker for wireless strain sensors

Zwitterionic hydrogels have attracted a myriad of research interests for their excellent flexibility and biocompatibility as flexible wearable sensors. It is desired to create E-skins that integrate high mechanical strength, sensory sensitivity, and broad adhesion, possessing potential in the fields of intelligent robots and bionic prostheses. In this work, a novel macromolecular cross-linker (MPU) based on waterborne polyurethane (WPU) was designed and applied to synthesize multifunctional conductive hydrogels (PASU-Zn hydrogels). Importantly, in the presence of MPU, the hydrogels exhibited well-balanced mechanical properties (elongation at break 1193%, tensile strength 1.02 MPa, outstanding puncture resistance, and self-recovery abilities). When assembled as wireless strain sensors, PASU-Zn sensors displayed distinguished sensing characteristics to detect mechanotransduction signals of human movements in real-time. Specifically, owing to the dipole-dipole interaction and hydrogen bonding of zwitterions and MPU, the hydrogels have remarkable self-adhesion properties to various surfaces of wood, PDMS, and pigskin, allowing them to stick to skins by themselves without using any adhesive tapes when used. It is deemed that the as-designed zwitterionic hydrogels show great promise for wearable devices and bionic skins.

Cost-Effective Fabrication of Transparent Strain Sensors via Micro-Scale 3D Printing and Imprinting.

The development of strain sensors with high sensitivity and stretchability is essential for health monitoring, electronic skin, wearable devices, and human-computer interactions. However, sensors that combine high sensitivity and ultra-wide detection generally require complex preparation processes. Here, a novel flexible strain sensor with high sensitivity and transparency was proposed by filling a multiwalled carbon nanotube (MWCNT) solution into polydimethylsiloxane (PDMS) channel films fabricated via an electric field-driven (EFD) 3D printing and molding hybrid process. The fabricated flexible strain sensor with embedded MWCNT networks had superior gauge factors of 90, 285, and 1500 at strains of 6.6%, 14%, and 20%, respectively. In addition, the flexible strain sensors with an optical transparency of 84% offered good stability and durability with no significant change in resistance after 8000 stretch-release cycles. Finally, the fabricated flexible strain sensors with embedded MWCNT networks showed good practical performance and could be attached to the skin to monitor various human movements such as wrist flexion, finger flexion, neck flexion, blinking activity, food swallowing, and facial expression recognition. These are good application strategies for wearable devices and health monitoring.

A Surface-Confined Gradient Conductive Network Strategy for Transparent Strain Sensors toward Full-Range Monitoring

The development of transparent and flexible sensors suitable for the full-range monitoring of human activities is highly desirable, yet presents a daunting challenge due to the need for a combination of properties such as high stretchability, high sensitivity, and good linearity. Gradient structures are commonly found in many biological systems and exhibit excellent mechanical properties. Here, we report a novel surface-confined gradient conductive network (SGN) strategy to construct conductive polymer hydrogel-based stain sensors (CHSS). This CHSS showed an ultrahigh stretchability of 4000% strain, transparency above 90% at a wavelength of 600 nm, as well as skin-like Young's modulus of 40 kPa. Impressively, the sensitivity was improved to 3.0 and outstanding linear sensing performance was achieved simultaneously in the ultrawide range of 0% to 4000% strain with a high R-square value of 0.994. With the help of SGN strategy, this CHSS was able to monitor both large-scale and small-scale human motions and activities. This SGN strategy can open a new avenue for the development of novel flexible strain sensors with excellent mechanical, transparent, and sensing performance for full-range monitoring of human activities.