Transparent Strain Sensor Research Articles

Hybrid double-network hydrogel for highly stretchable, excellent sensitive, stabilized, and transparent strain sensors

Nowadays, great effort has been devoted to fabricate flexible wearable sensor with high stretchability, moderate modulus, favorable durability, excellent transparency, and satisfactory sensitivity. In this work, we report the preparation of a hybrid double-network (DN) hydrogel by a simple one-pot method. First, chitosan was added into an AlCl3 solution to form Al3+-chitosan complex (CS-Al3+). Second, the hybrid CS/Al3+-poly(acrylamide) (PAM) DN hydrogels were constructed via in situ polymerization of acrylamide (AM) in present of Al3+-chitosan complex. Thanks to the existence of electrically conductive CS-Al3+ networks, the resulting hybrid DN hydrogel exhibits excellent stretchability, fatigue resistance, transparency, and conductivity. Furthermore, the CS/Al3+-PAM DN hydrogel could be used as strain sensor, and demonstrates many desired virtues, including satisfactory sensitivity (gauge factors of 1.7–12.1), wide detection range (up to 1500%), low limit of discernment (1% strain), high reliability, and excellent durability (1000 cycles). More significantly, the manufactured hydrogel-based strain sensor can be employed as wearable devices to precisely detect various human movements.

Advances in transparent and stretchable strain sensors

Advances in transparent and stretchable strain sensors

Smart Contact Lenses with a Transparent Silver Nanowire Strain Sensor for Continuous Intraocular Pressure Monitoring.

Continuous intraocular pressure (IOP) monitoring can provide a paradigm shift in the management of patients with glaucoma as a facile alternative to conventional diagnostic methods. However, the low sensitivity and functional instability of current IOP sensors have limited their clinical utility in the management of glaucoma. Here, we have developed a smart contact lens integrated with a transparent silver nanowire IOP strain sensor and wireless circuits for noninvasive, continuous IOP monitoring. After confirming the robust stability of the IOP sensor within the smart contact lens in the presence of tears and repeated eyelid blink model cycles, we were able to monitor IOP changes on polydimethylsiloxane model eyes in vitro. In vivo tests demonstrated that our fully integrated wireless smart contact lens could successfully monitor the change in IOP in living rabbit eyes, which was clearly validated by the conventional invasive tonometer IOP test. Taken together, we could confirm the feasibility of our smart contact lens as a noninvasive platform for continuous IOP monitoring of glaucoma patients.

Noninterference Wearable Strain Sensor: Near-Zero Temperature Coefficient of Resistance Nanoparticle Arrays with Thermal Expansion and Transport Engineering.

In this study, non-temperature interference strain gauge sensors, which are only sensitive to strain but not temperature, are developed by engineering the properties and structure from a material perspective. The environmental interference from temperature fluctuations is successfully eliminated by controlling the charge transport in nanoparticles with thermally expandable polymer substrates. Notably, the negative temperature coefficient of resistance (TCR), which originates from the hopping transport in nanoparticle arrays, is compensated by the positive TCR of the effective surface thermal expansion with anchoring effects. This strategy successfully controls the TCR from negative to positive. A near-zero TCR (NZTCR), less than 1.0 × 10-6 K-1, is achieved through precisely controlled expansion. Various characterization methods and finite element and transport simulations are conducted to investigate the correlated electrical, mechanical, and thermal properties of the materials and elucidate the compensated NZTCR mechanism. With this strategy, an all-solution-processed, transparent, highly sensitive, and noninterference strain sensor is fabricated with a gauge factor higher than 5000 at 1% strain, as demonstrated by pulse and motion sensing, as well as the noninterference property under variable-temperature conditions. It is envisaged that the sensor developed herein is applicable to multifunctional wearable sensors or e-skins for artificial skin or robots.

Inkjet Printing of Highly Sensitive, Transparent, Flexible Linear Piezoresistive Strain Sensors

Flexible strain sensors are fabricated by using a simple and low-cost inkjet printing technology of graphene-PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) conductive ink. The inkjet-printed thin-film resistors on a polyethylene terephthalate (PET) substrate exhibit an excellent optical transmittance of about 90% over a visible wavelength range from 400 to 800 nm. While an external mechanical strain is applied to thin-film resistors as strain sensors, a gauge factor (GF) of the piezoresistive (PR) strain sensors can be evaluated. To improve the GF value of the PR strain sensors, a high resistive (HR) path caused by the phase segregation of the PEDOT:PSS polymer material is, for the first time, proposed to be perpendicular to the PR strain sensing direction. The increase in the GF with the increase in the HR number of the PR strain sensors without a marked hysteresis is found. The result can be explained by the tunneling effect with varied initial tunneling distances and tunneling barriers due to the increase in the number of HR. Finally, a high GF value of approximately 165 of three HR paths is obtained with a linear output signal at the strain range from 0% to 0.33%, further achieving for the inkjet printing of highly sensitive, transparent, and flexible linear PR strain sensors.

Ultrasonically Patterning Silver Nanowire-Acrylate Composite for Highly Sensitive and Transparent Strain Sensors Based on Parallel Cracks.

It has long been a challenge to develop strain sensors with large gauge factor (GF) and high transparency for a broad strain range, to which field silver nanowires (AgNWs) have recently been applied. A dense nanowire (NW) network benefits achieving large stretchability, while a sparse NW network favors realizing high transparency and sensitive response to small strains. Herein, a patterned AgNW-acrylate composite-based strain sensor is developed to circumvent the above trade-off issue via a novel ultrasonication-based patterning technique, where a water-soluble, UV-curable acrylate composite was blended with AgNWs as both a tackifier and a photoresist for finely patterning dense AgNWs to achieve high transparency, while maintaining good stretchability. Moreover, the UV-cured AgNW-acrylate patterns are brittle and capable of forming parallel cracks which effectively evade the Poisson effect and thus increase the GF by more than 200-fold compared to that of the bulk AgNW film-based strain sensor. As a result, the AgNW-based strain sensor possesses a GF of ∼10,486 at a large strain (8%), a high transparency of 90.3%, and a maximum stretchability of 20% strain. The precise monitoring of human radial pulse and throat movements proves the great potential of this sensor as a measurement module for wearable healthcare systems.

Sensors: Transparent and Stretchable Strain Sensors with Improved Sensitivity and Reliability Based on Ag NWs and PEDOT:PSS Patterned Microstructures (Adv. Electron. Mater. 8/2020)

Transparent and stretchable strain sensors based on Ag nanowires (NWs) and poly(3,4-ethylenedioxythiophene):poly (styrene-sulfonate) (PEDOT:PSS) composite are reported by Xin He and co-workers in article number 1901360. Owing to the unique patterned microstructure of the device and good contact between Ag NWs and PEDOT:PSS, the sensor simultaneously achieves a high sensitivity, large strain range, and excellent stability. The patterned sensor can detect a wide range of human movements and temperature responses in real time.

Transparent and Stretchable Strain Sensors with Improved Sensitivity and Reliability Based on Ag NWs and PEDOT:PSS Patterned Microstructures

AbstractA transparent and stretchable strain sensor based on Ag nanowires (NWs) and poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT:PSS) patterned microstructure is presented. The regular patterns are achieved by a facile replication and transfer process from near‐field electrospun polyacrylonitrile (PAN) grids on the surfaces of polydimethylsiloxane (PDMS) substrates. Owing to the combined advantages for the unique microstructure of sensor, strong adhesion of polymer PEDOT:PSS to underlying layer, and outstanding conductivity of long Ag NWs, the transparent strain sensor possesses the excellent aspects of high sensitivity (gauge factor = 10.2), wide sensing range (0–100%), long‐term reliability for at least 2000 cycles, and distinct temperature responses. Moreover, the patterned strain sensor can be mounted on different body parts to detect a wide range of human movements and temperature responses in real time, providing multifunctional applications in human/machine interaction, electronic skins, and wearable devices.

Ultra-Stretchable, durable and conductive hydrogel with hybrid double network as high performance strain sensor and stretchable triboelectric nanogenerator

Abstract Hydrogels with integrated attributes of stretchability, conductivity, transparency and robustness have been emerging because of their promising applications in wearable devices, human health monitoring, advanced intelligent systems and energy harvesting. In this paper, we developed stretchable and conductive hydrogels via hybrid double networks approach by combination of rigid physically cross-linked gelatin, tough chemically cross-linked polyacrylamide (PAM) and poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) as conducting component. The double networks can be further interlocked by using physical entanglements and abundant dynamic hydrogen bonds, contributing to the improved mechanical properties and self-recovery ability. A transparent and wearable strain sensor is fabricated by sandwiching the hydrogels with two layers of adhesive polyurethane (PU) tape, exhibiting good sensitivity (gauge factor (GF) = 1.58), ultra-wide sensing range of 0-2850% strain, short response time of 200 ms and superior durability and reproducibility (1200 cycles), which endows the sensitive monitors with effective discernibility for detecting intricate human motions. Importantly, the hydrogel-based device can act as a highly stretchable (300% strain) triboelectric nanogenerator (STENG) for efficient energy harvesting, giving a short circuit current (ISC) of 26.9 μA, open circuit voltage (VOC) of 383.8 V and short-circuit transferred charge (QSC) of 92 nC. The integrated abilities of strain sensing and energy harvesting promise the hydrogels for high performance self-powered wearable devices and stretchable power sources.

Metal‐Free and Stretchable Conductive Hydrogels for High Transparent Conductive Film and Flexible Strain Sensor with High Sensitivity

AbstractConductive hydrogels show promising applications in wearable electronic devices. However, it is still challenging to increase the conductivity as well as the mechanical performance of the conductive hydrogels. In addition, it is more challenging to fabricate ultrathin conductive films with good mechanical strength and high transparency. In this study, a metal‐free flexible conductive hydrogel for flexible wearable strain sensor with high sensitivity is presented. The conductive hydrogel is prepared by polyvinyl alcohol (PVA) templated polymerizing of polypyrrole (PPy) followed by gelating based on the polymerizing and cross‐linking of polyacrylamide (PAAm). The conductive hydrogel is endowed excellent mechanical properties by multiple hydrogen bonds between the interpenetrating network of PVA, PPy, and PAAm. The tensile strength reaches up to 0.2 MPa at 500% and the compression strength reaches up to 1.5 MPa at 90%. It can withstand cyclic loads. The conductivity reaches 0.3 s m−1 and it is sensitive to stretching and compressing. Therefore, strain sensors are prepared based on such hydrogels to make wearable electronic devices, monitoring the subtle and large strains. It is worth noting that the composite material containing PVA has good film‐forming properties. Therefore, ultrathin conductive hydrogel films with high transparency (94.2%), high conductivity (7090 Ω/square) and large‐area are fabricated at low cost.

Transparent, high-strength, stretchable, sensitive and anti-freezing poly(vinyl alcohol) ionic hydrogel strain sensors for human motion monitoring

Transparent, high-strength, stretchable, sensitive and anti-freezing poly(vinyl alcohol) ionic hydrogel sensors were facilely fabricated for human motion monitoring.

A stretchable and transparent strain sensor based on sandwich-like PDMS/CNTs/PDMS composite containing an ultrathin conductive CNT layer

Stretchable and transparent strain sensors based on conductive polymer composites (CPCs) possess potential applications in a variety of fields, including electronic skin, wearable electronics, and human motion detection. However, it is still a challenge to prepare CPC-based strain sensors with both good transparency and broad sensing range by a low-cost, facile, and scalable fabrication method. Herein, we report a simple spray deposition and transfer method to fabricate a stretchable and transparent strain sensor based on the sandwich-like polydimethylsiloxane (PDMS)/carbon nanotubes (CNTs)/PDMS composite containing with an ultrathin and uniform conductive layer. The fabricated strain sensors exhibit superb stretchability, good optical transparency, and excellent sensing performances, which can detect both the subtle and large strains with outstanding stability and repeatability. For example, the strain sensor based on PDMS/CNTs/PDMS composite (containing 0.16 mg/cm2 CNTs on the conductive layer) presents a good optical transmittance of 53.1% at 550 nm, broad sensing range of over 130%, and the ability to monitor both the subtle motions of facial expressions and the large motions of human joints, which has great potential applications in human motion detection, wearable electronics, and electronic skin.

Development of a transparent and stretchable strain sensor based on the dry printing method

Flexible, substrate-based, stretchable sensors have attracted significant attention because they can simulate the complex characteristics of natural skin and have important application value in human motion detection. In this work, we propose a dry printing fabrication method that is performed by mixing carbon nanotubes (CNTs) and silicon dioxide (SiO2) powder, packing it into an SU-8 mold with micro-grid grooves, and then spin coating a small amount of polydimethylsiloxane (PDMS) onto the surface of a PDMS flexible substrate. The coated substrate is then placed on top of the SU-8 mold, so the liquid PDMS can infiltrate the voids in and around the powder. The turning process uses the liquid PDMS’s fluidity and permeability to adhere the CNT-SiO2 mixed powder in the SU-8 mold groove to the flexible substrate, and transfers it to the flexible substrate during the substrate’s removal process from the mold. The result is a microstructure, conductive layer on the PDMS flexible surface. The conductive layer is coated with conductive silver adhesive, which serves as the electrode, and PDMS is used as the sealing layer to complete the sensor preparation. In this experiment, the sensor performance was controlled by adjusting the mass ratio of CNTs to SiO2. The results show that when the CNTs:SiO2 mass ratio reaches 1:1, the sensor exhibits excellent performance, including GF up to 352 and strain range up to 16%. In addition, the electronic skin (E-skin) also has good transparency, with light transmittance reaching upward of 60%.

Investigation of strain sensing mechanisms on ultra-thin carbon nanotube networks with different densities

Transparent, highly stretchable and sensitive strain sensors have been successfully fabricated using carbon nanotube (CNT) networks with various densities as the sensing medium. Owing to the unique two-dimensional percolated structure, the CNT network-based sensors exhibit excellent sensing performance over a wide strain range from 0.5 to 100%. By decreasing the density of CNTs from 152 to 7.2/μm2, their gauge factor is remarkably increased from 0.4 to 50. It is found from the experimental results and theoretical analysis that the sensing mechanism of the CNT networks is transferred from percolation theory to tunneling effect with decreasing the density or increasing the tensile strain of the CNT networks. This study provides a facile technique to prepare high stretchable strain sensors and also fundamental principles for the structural design of the CNT network-based strain sensors.

Graphene nanoparticle strain sensors with modulated sensitivity through tunneling types transition

Highly sensitive strain sensors show great potential for use in wearable health monitoring, autonomous intelligent robots and biomimetic prosthetics. The current resistive strain sensors mainly work through piezoresistors. Here, the robust tunneling mechanism based nanoscale strain sensors with high sensitivity are reported. The strain sensors are fabricated from graphene nanoparticle film. The sensitivity of graphene nanoparticle strain sensors could be tunable through the modulation of tunneling type, suggesting a theoretical support in performance optimization of tunneling strain sensors. The output characterization indicates the direct tunneling (DT) and Fowler–Nordheim tunneling (FNT) are dominant for charge carrier transport in the low voltage and high voltage regions, respectively. It is found that gauge factors are ∼79 at low voltage of 0–4 V, and ∼110 at high voltage of 20–40 V, showing profound dependence on DT and FNT types. The strain sensor bearing 0.3% strain shows a great stability over 100 cycles at bias voltage of 1 V and 40 V, respectively. An integrated strain sensor array with 5 × 5 patterned graphene nanoparticle film on a polyethylene terephthalate substrate is fabricated and demonstrates great spatial strain distribution, guiding the design for flexible and transparent strain sensor e-skins.

Highly Stretchable, Transparent, and Bio‐Friendly Strain Sensor Based on Self‐Recovery Ionic‐Covalent Hydrogels for Human Motion Monitoring

AbstractStretchable, flexible, and strain‐sensitive hydrogels have gained tremendous attention due to their potential application in health monitoring devices and artificial intelligence. Nevertheless, it is still a huge challenge to develop an integrated strain sensor with excellent mechanical properties, broad sensing range, high transparency, biocompatibility, and self‐recovery. Herein, a simple paradigm of stretchable strain sensor based on multifunctional hydrogels is prepared by constructing synergistic effects among polyacrylamide (PAM), biocompatible macromolecule sodium alginate (SA), and Ca ion in covalently and ionically crosslinked networks. Under large deformation, the dynamic SA‐Ca2+ bonds effectively dissipate energy, serving as sacrificial bonds, while the PAM chains bridge the crack and stabilize the network, endowing hydrogels with outstanding mechanical performances, for instance, high stretchability and compressibility, as well as excellent self‐recovery performance. The hydrogel is assembled to be a transparent and wearable strain sensor, which has good sensitivity and very wide sensing range (0–1700%), and can precisely detect dynamic strains, including both low and high strains (20–800% strain). It also exhibits fast response time (800 ms) and long‐time stability (200 cycles). The sensor can monitor and distinguish complicated human motions, opening up a new route for broad potential applications of eco‐friendly flexible strain‐sensing devices.

Highly transparent, stretchable, and rapid self-healing polyvinyl alcohol/cellulose nanofibril hydrogel sensors for sensitive pressure sensing and human motion detection

Wearable sensors have emerged as favored novel devices for human healthcare. Current sensors, however, suffer from low sensitivity, non-transparency, and lack of self-healing ability. In this study, we synthesized a polyvinyl alcohol/cellulose nanofibril (PVA/CNF) hydrogel with dual-crosslinked networks for highly transparent, stretchable, and self-healing pressure and strain sensors. The hydrogel contains dynamic borate bonds, metal–carboxylate coordination bonds, and hydrogen bonds, all of which contribute to the hydrogel’s superior dimensional stability, mechanical strength and flexibility, and spontaneous self-healing ability as compared to traditional PVA hydrogels. The developed hydrogel has a moderate modulus of 11.2 kPa, and a high elongation rate of 1900%. It spontaneously self-heals within 15 s upon contact without any external stimuli, has a high transmittance of over 90%, and has excellent compatibility with human fibroblasts. The capacitive sensor developed based on the PVA/CNF hydrogel has high sensitivity to very subtle pressure changes, such as small water droplets. When used as a strain sensor, it was capable of detecting and monitoring various human motions such as finger, knee, elbow, and head movements, breathing, and gentle tapping. The developed hydrogel and sensors not only show great potential in electronic skin, personal healthcare, and wearable devices, but may also inspire the development of transparent, intelligent skin-like sensors.

Ag nanowire-based transparent stretchable tactile sensor recognizing strain directions and pressure

In the last decade, extensive studies have been conducted to realize the functions of human skin based on stretchable electronics. An artificial skin, recognizing complex mechanical stimulation including pressure, strain and shear, and composed of transparent material, is an essential goal but has hardly been achieved. We fabricated a transparent integrated sensor system that can sense the strain direction and normal pressure of applied mechanical stimulation. Each sensor is composed of micropatterned Ag nanowire, forming a composite stretchable conductor with a block copolymer elastomer. The micropatterning and transfer process using thermoplastic elastomer facilitates the transparent conductor to show high transmittance with low sheet resistance at the same time. The designed transparent strain sensor responds linearly to strain, but does not respond to the orthogonal direction, which enables it to have strain-directionality. The applied mechanical signal, comprising normal force and directional strain, can be interpreted through the electrical signal observed from integrated sensors.

Highly Sensitive and Transparent Strain Sensors with an Ordered Array Structure of AgNWs for Wearable Motion and Health Monitoring

Sensitivity and transparency are critical properties for flexible and wearable electronic devices, and how to engineer both these properties simultaneously is dramatically essential. Here, for the first time, we report the assembly of ordered array structures of silver nanowires (AgNWs) via a simple water-bath pulling method to align the AgNWs embedded on polydimethylsiloxane (PDMS). Compared with sensors prepared by direct drop-casting or transfer-printing methods, our developed sensor represents a considerable breakthrough in both sensitivity and transparency. The maximum transmittance was 86.3% at a wavelength of 550 nm, and the maximum gauge factor was as high as 84.6 at a strain of 30%. This remarkably sensitive and transparent flexible sensor has strictly stable and reliable responses to motion capture and human body signals; it is also expected to be able to help monitor disabled physical conditions or assist medical therapy while ensuring privacy protection.

Metal Mesh as a Transparent Omnidirectional Strain Sensor

AbstractThin metal films can be engineered to fabricate strain sensors by conquering their stiffness using wavy, buckle, or wrinkle topography. However, these structures usually operate in uniaxial or biaxial stretching directions, limiting their application in the omnidirectional model. Here a strain sensor is constructed by using metal mesh with honeycomb lattices and multiple domains as the sensitive unit, which simultaneously provides good stretchability and direction independence. Besides, the metal mesh structure makes it possible to obtain a transparent sensor with a sheet resistance of 26 Ω sq−1 at a transmittance of 79%. The sensor exhibits a highly sensitive resistance change with tension strain, demonstrated by quick response when monitoring the finger and muscle motion. The sensor also shows promising potential application in detecting periodic oscillation with selected frequency and even complex mechanical oscillation like glass vibration.