Fabrication Of Strain Sensors Research Articles

Multimodal E-Textile Enabled by One-Step Maskless Patterning of Femtosecond-Laser-Induced Graphene on Nonwoven, Knit, and Woven Textiles.

Personal wearable devices are considered important in advanced healthcare, military, and sports applications. Among them, e-textiles are the best candidates because of their intrinsic conformability without any additional device installation. However, e-textile manufacturing to date has a high process complexity and low design flexibility. Here, we report the direct laser writing of e-textiles by converting raw Kevlar textiles to electrically conductive laser-induced graphene (LIG) via femtosecond laser pulses in ambient air. The resulting LIG has high electrical conductivity and chemical reliability with a low sheet resistance of 2.86 Ω/□. Wearable multimodal e-textile sensors and supercapacitors are realized on different types of Kevlar textiles, including nonwoven, knit, and woven structures, by considering their structural textile characteristics. The nonwoven textile exhibits high mechanical stability, making it suitable for applications in temperature sensors and micro-supercapacitors. On the other hand, the knit textile possesses inherent spring-like stretchability, enabling its use in the fabrication of strain sensors for human motion detection. Additionally, the woven textile offers special sensitive pressure-sensing networks between the warp and weft parts, making it suitable for the fabrication of bending sensors used in detecting human voices. This direct laser synthesis of arbitrarily patterned LIGs from various textile structures could result in the facile realization of wearable electronic sensors and energy storage.

Ultrasensitive strain sensor based on graphite coated fibrous frameworks for security applications

The increasing demands of low cost, flexible, foldable and disposable electronic devices have directed the attentions of researchers towards applications of paper substrates. Here, we report the fabrication of strain sensor based on graphite-on-paper strategy. Graphite coating is applied on cellulose paper by direct drawing method. SEM and EDS analyses of the graphite electrodes are performed which respectively inform about the morphological features and elemental composition of the samples. Moreover, XRD, FTIR spectroscopy and UV–visible spectroscopy of the graphite electrodes are performed to investigate their structural and optical features. A device is fabricated using the graphite electrode which acts as strain sensor. The sensor responds to applied bending strain by showing changes in its relative resistance. The response of sensor towards applied strain is linear with linearity as 0.997. The sensor responds differently for applied inward and outward bending strains. An exceptional sensitivity of 122,701 is recorded for outward strain and 4,021 for inward bending strain. The response time of the sensor is measured as 0.59 s, while that of recovery time as 0.69 s. The sensor is applied on a door at its axis of motion for which it responds when the door is opened or closed. The results show that the demonstrated sensor could be worthful for advanced security applications.

Surface-modification of silver nanoparticle–based polydimethylsiloxane composite for fabrication of strain sensor

Composites consisting of surface-treated silver nanoparticles (AgNPs) in a polymer matrix were used to fabricate strain sensors. In order to ensure a good interaction of AgNPs with the polymer matrix in the composite, AgNPs were treated with 3-aminopropyltrimethoxysilane (APTMS). Silane-treated AgNPs (AM-AgNPs) contain, on their surface, an efficient coupling agent used in composites and as an adhesion promoter in adhesive formulation. The surface-treated conductive filler exhibits a good interaction with the PDMS matrix and improved composite performance, suggesting a good interface between AgNPs and the polymer matrix in the composite. AM-AgNPs were used as conductive filler in the composite, and an AM-AgNPs–based polydimethylsiloxane composite (AM-AgNPs@PDMS) strain sensor was fabricated via a drop-casting method. The surface morphology of the composite material and the electromechanical properties of the strain sensor were investigated. It was observed that both silane and amine groups were successfully grafted on the surface of the AgNPs, improving the dispersion and interfacial interaction in the PDMS matrix. The fabricated AM-AgNPs@PDMS composite strain sensor exhibited good electrical conductivity and sensing properties, measured as hysteresis behaviour until reaching 20% strain level; good sensitivity (Gauge Factor (GF) = 8.73 at 50% strain level); linearity (R2 = 0.9558); and good stability.

Flexible strain sensors based on gold nanowire dominoes for human motion detection

Flexible strain sensors have broad application potentials in sports, healthcare and various other fields. However, the large-scale fabrication of strain sensors with both high sensitivity and stretchability simultaneously is still challenging nowadays. Inspired by the game of dominoes, we proposed a strain sensor with a novel mechanism based on gold nanowire (AuNWs) dominoes. Unlike the general piezoresistive strain sensors of micro-crack or percolation mechanisms, AuNWs domino sensors realize the mechanical sensing due to the change of the overlapping area when subjected to external force. The strain sensor performs excellently with a wide detection range(0–250 % strain), an ultrahigh sensitivity (gauge factor of 490 at a strain of 10 %), and high durability and repeatability. The in-situ characterization proved that the overlapped AuNWs dominoes slid and displaced with the increasing strain, and the overlap area decreased before the fracture occurred, thus keeping the effective conducting pathway. Based on its innovative sensing mechanism and superior sensing performance, AuNWs domino sensors effectively suppress conductive path cracks and have the potentials for wearable healthcare devices.

Skin-mimicking strategy to fabricate strong and highly conductive anti-freezing cellulose-based hydrogels as strain sensors

Conductive hydrogels have attracted increasing attention for applications in wearable and flexible strain sensors. However, owing to their relatively weak strength, poor elasticity, and lack of anti-freezing ability, their applications have been limited. Herein, we present a skin-mimicking strategy to fabricate cellulose-enhanced, strong, elastic, highly conductive, and anti-freezing hydrogels. Self-assembly of cellulose to fabricate a cellulose skeleton is essential for realizing a skin-mimicking design. Furthermore, two methods, in situ polymerization and solvent replacement, were compared and investigated to incorporate conductive and anti-freezing components into hydrogels. Consequently, when the same ratio of glycerol and lithium chloride was used, the anti-freezing hydrogels prepared by in situ polymerization showed relatively higher strength (1.0 MPa), while the solvent-replaced hydrogels exhibited higher elastic recovery properties (94.6 %) and conductivity (4.5 S/m). In addition, their potential as strain sensors for monitoring human behavior was analyzed. Both hydrogels produced reliable signals and exhibited high sensitivity. This study provides a new horizon for the fabrication of strain sensors that can be applied in various environments.

Reconfigurable, Stretchable Strain Sensor with the Localized Controlling of Substrate Modulus by Two-Phase Liquid Metal Cells.

Strain modulation based on the heterogeneous design of soft substrates is an effective method to improve the sensitivity of stretchable resistive strain sensors. In this study, a novel design for reconfigurable strain modulation in the soft substrate with two-phase liquid cells is proposed. The modulatory strain distribution induced by the reversible phase transition of the liquid metal provides reconfigurable strain sensing capabilities with multiple combinations of operating range and sensitivity. The effectiveness of our strategy is validated by theoretical simulations and experiments on a hybrid carbonous film-based resistive strain sensor. The strain sensor can be gradually switched between a highly sensitive one and a wide-range one by selectively controlling the phases of liquid metal in the cell array with a external heating source. The relative change of sensitivity and operating range reaches a maximum of 59% and 44%, respectively. This reversible heterogeneous design shows great potential to facilitate the fabrication of strain sensors and might play a promising role in the future applications of stretchable strain sensors.

Flexible and stretchable strain sensors fabricated by inkjet printing of silver nanowire-ecoflex composites

Recently, the fabrication of strain sensors with high sensitivity and high stretchability, which can precisely monitor subtle strains and large mechanical deformations exhibited by the human bodily motions, is critical for healthcare, human–machine interfaces, and biomedical electronics. However, a great challenge still exists i.e. achieving strain sensors with both high sensitivity and high stretchability by a facile, low-cost and scalable fabrication technique. Herein, this work reports Silver nanowires (AgNWs)/Ecoflex based composite strain sensors via inkjet printing technique which precisely deposits functional materials in a rapid, non-contact and maskless approach allowing high volume production. Noteworthily, the fabricated strain sensor display many fascinating features, including high sensitivity (a gauge factor of 13.7), a broad strain sensing range over 30%, excellent stability and reliability (>1000 cycles), and low monitoring limit (<5% strain). These remarkable features allow the strain sensor to effectively monitor various human motions. This work opens up a new path for fabricating nanocomposite thin film-based strain sensors for wearable electronics.

Rational structural design of vanillin based vitrimers in one-pot to facilely fabricate highly stretchable and sensitive composite strain sensor

The preparation of high performance and multifunctional strain sensors by a facile fabrication process is highly desirable but still challenging. In this study, vanillin based vitrimers were simply synthesized in one-pot by the combination of thiol-epoxy and amine-aldehyde reactions, which was used for strain sensor fabrication. The reaction kinetics were extensively investigated by 1H NMR analysis to demonstrate the feasibility to prepare vitrimers in one-pot. The physicochemical properties and dynamic natures of vanillin based vitrimers were then analyzed by various techniques, such as FTIR, DSC and rheometer. The mechanical properties of vitrimers were tunable via controlling the structure of crosslinkers. Moreover, it exhibited a strong UV-adsorption ability to potentially protect the human skins. Due to the enhanced interfacial interaction between vitrimers and silver nanowire layers, and the dynamic feature of Schiff base, the fabrication of strain sensors with sandwich-structures could be facilely realized. The strain sensors show a wide linear response range (0–100%) and a high gauge factor (15.8), which was comparable to previous sensors prepared by complicated steps. This study has demonstrated the potential to combine dynamic covalent chemistry and sustainability to overcome the general challenges in the fabrication of high-performance strain sensors.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors.

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

A highly sensitive strain sensor with a sandwich structure composed of two silver nanoparticles layers and one silver nanowires layer for human motion detection

The fabrication of strain sensors with high sensitivity, large sensing range and excellent stability is highly desirable because of their promising applications in human motion detection, human-machine interface and electric skin, etc. Herein, by introducing a highly conductive silver nanowire (AgNW) layer between two serried silver nanoparticle (AgNP) layers, forming a sandwich structure, a strain sensor with high sensitivity (a large gauge factor of 2.8 × 105), large sensing range (up to 80% strain) and excellent stability (over 1000 cycles) can be achieved. A combination of experimental and mechanism studies shows that the high performance of the obtained strain sensor is ascribed to the synergy of the highly conductive AgNW layer, astatic AgNP layers and the presence of large cracks in stretching. As a proof-of-concept application, the obtained strain sensor can be used for highly effective human motion detection ranging from large scale motions, i.e. kneel bending and wrist flexion, to subtle scale motions, i.e. pulse and swallowing.

Maskless Formation of Conductive Carbon Layer on Leather for Highly Sensitive Flexible Strain Sensors

AbstractDevelopment of strain sensors on flexible and breathable leather or textile has potential applications in Internet of Things, human–machine interfaces, and motion detection. Nevertheless, the fabrication of strain sensors with high performance on the above substrates in a maskless, scalable, and highly efficient manner is still a challenge. In this study, highly sensitive strain sensors are mask‐freely fabricated on leather by a highly efficient and scalable laser direct writing (LDW) technique in open air without using any additional precursors. The strain sensors are constructed by the carbon flake synthesized by LDW carbonization of collagen fibers in the leather. The LDW carbonized leather‐based device provides maximum sensitivities of ≈2009.5 and ≈50.9 for tension and compression strain detection, respectively. Additionally, the device has a minimum detection strain of 0.005% and a good mechanical stability over 10 000 bending cycles. The high performance of the flexible strain sensors as well as the developed maskless, scalable and highly efficient LDW technique endows the promising applications of the devices in human motion monitoring and external stimuli detection.

Ultrasensitive piezoresistive strain sensors based on CNTs/Ag-NPs coated highly stretchable textile

Highly sensitive and extremely stretchable piezoresistive strain sensors have shown intense applications in wearable electronics. In this context, a comparative study of stretchable strain sensors based on hybrid layers of silver nanoparticles and carbon nanotubes spray coated on stretchable textile is presented in this manuscript. For this persistence, three stretchable and highly flexible electrodes are fabricated: first devoid of pre-strain, second with 25 × 25% multiaxial pre-strain, and third with 50 × 50% multiaxial pre-strain. Pre-strain technique was employed to improve the stretchability and resistance stability of the electrodes. Surface morphology and elemental composition of the as-prepared electrodes were analyzed via scanning electron microscope associated with energy-dispersive detector. The fabricated electrodes were used for the assembly of strain sensors. The sensing measurements revealed high sensitivity, significant stretchability, and fast response of the fabricated strain sensors. Cost-effective and extremely facile fabrication strategy as well as direct fabrication on textile substrates can be significant advancements in fabrication of strain sensors for practical applications in human motion detection and wearable electronics.

Motion Prediction with Artificial Neural Networks Using Wearable Strain Sensors Based on Flexible Thin Graphite Films

A human motion prediction system can be used to estimate human gestures in advance to the actual action for reducing delays in interactive system. We have already reported a method of simple and easy fabrication of strain sensors and wearable devices using pyrolytic graphite sheets (PGSs). The wearable electronics could detect various types of human motion, with high durability and fast response. In this study, we have demonstrated hand motion prediction by neural networks (NNs) using hand motion data obtained from data gloves based on PGSs. In our experiments, we measured hand motions of subjects for learning. We created 4-layered NNs to predict human hand motion in real-time. As a result, the proposed system successfully predicted hand motion in real-time. Therefore, these results suggested that human motion prediction system using NNs is able to forecast various types of human behavior using human motion data obtained from wearable devices based on PGSs.

Monolithic 3D printing of embeddable and highly stretchable strain sensors using conductive ionogels

Medical training simulations that utilize 3D-printed, patient-specific tissue models improve practitioner and patient understanding of individualized procedures and capacitate pre-operative, patient-specific rehearsals. The impact of these novel constructs in medical training and pre-procedure rehearsals has been limited, however, by the lack of effectively embedded sensors that detect the location, direction, and amplitude of strains applied by the practitioner on the simulated structures. The monolithic fabrication of strain sensors embedded into lifelike tissue models with customizable orientation and placement could address this limitation. The demonstration of 3D printing of an ionogel as a stretchable, piezoresistive strain sensor embedded in an elastomer is presented as a proof-of-concept of this integrated fabrication for the first time. The significant hysteresis and drift inherent to solid-phase piezoresistive composites and the dimensional instability of low-hysteresis piezoresistive liquids inspired the adoption of a 3D-printable piezoresistive ionogel composed of reduced graphene oxide and an ionic liquid. The shear-thinning rheology of the ionogel obviates the need to fabricate additional structures that define or contain the geometry of the sensing channel. Sensors are printed on and subsequently encapsulated in polydimethylsiloxane (PDMS), a thermoset elastomer commonly used for analog tissue models, to demonstrate seamless fabrication. Strain sensors demonstrate geometry- and strain-dependent gauge factors of 0.54–2.41, a high dynamic strain range of 350% that surpasses the failure strain of most dermal and viscus tissue, low hysteresis (<3.5% degree of hysteresis up to 300% strain) and baseline drift, a single-value response, and excellent fatigue stability (5000 stretching cycles). In addition, we fabricate sensors with stencil-printed silver/PDMS electrodes in place of wires to highlight the potential of seamless integration with printed electrodes. The compositional tunability of ionic liquid/graphene-based composites and the shear-thinning rheology of this class of conductive gels endows an expansive combination of customized sensor geometry and performance that can be tailored to patient-specific, high-fidelity, monolithically fabricated tissue models.

그래핀/폴리우레탄 나노웹 기반의 스트레인센서 제작 및 호흡측정

그래핀/폴리우레탄 나노웹 기반의 스트레인센서 제작 및 호흡측정

Highly stretchable, anti-corrosive and wearable strain sensors based on the PDMS/CNTs decorated elastomer nanofiber composite

Conductive polymer composite based strain sensors have promising applications in the fields of artificial skin, wearable health-care device, etc. However, fabrication of strain sensors with good stretchability, anti-corrosion, excellent durability and reliability remains challenging. In this work, a superhydrophobic strain sensor based on conductive thermoplastic polyurethane/carbon nanotubes/polydimethylsiloxane (TPU/CNTs/PDMS) was prepared by ultrasonication induced CNTs decoration onto the electrospun TPU nanofiber surface, followed by the PDMS modification. Uniformly dispersed CNTs on the nanofiber surface with a hierarchical structure construct the conductive network. The PDMS layer with a low surface energy endows the nanofiber composite with superhydrophobicity thus anti-corrosion property. The introduction of CNTs/PDMS improves both the Young's modulus, tensile strength and the elongation at break. The superhydrophobicity and conductivity can be maintained after the cyclic stretching-releasing test, displaying excellent durability. When used as a wearable strain sensor, the nanofiber composite is capable of detecting body motion and could work even under harsh conditions (moisture, acid and alkaline environment), showing promising application in wearable electronics.

Facile fabrication and performance of robust polymer/carbon nanotube coated spandex fibers for strain sensing

The last decade has witnessed a tremendous growth of research and development in flexible and wearable strain sensors. However, there are still some challenges associated with the fabrication of strain sensors to achieve a high sensitivity and large workable range at low cost. Here, we report on the development of a highly elastic strain sensor based on a commercial spandex fiber coated with a nanocomposite consisting of multi-walled carbon nanotubes (MWCNTs) and thermoplastic polyurethane (TPU) manufactured by a layer-by-layer (LBL) method. The sensor demonstrated outstanding performance with large workable strain, high sensitivity, excellent repeatability and regular signal responses within a wide measuring frequency range of 0.01–1 Hz. Additionally, the effect of ultraviolet irradiation on the sensor performance was also investigated. Application of the sensor in monitoring diverse human motions, such as facial micro expressions and speech recognition, are also demonstrated showing its potential for applications in wearable devices and intelligent robots.

Flexible and Transparent Strain Sensors with Embedded Multiwalled Carbon Nanotubes Meshes.

Strain sensors combining high sensitivity with good transparency and flexibility would be of great usefulness in smart wearable/flexible electronics. However, the fabrication of such strain sensors is still challenging. In this study, new strain sensors with embedded multiwalled carbon nanotubes (MWCNTs) meshes in polydimethylsiloxane (PDMS) films were designed and tested. The strain sensors showed elevated optical transparency of up to 87% and high sensitivity with a gauge factor of 1140 at a small strain of 8.75%. The gauge factors of the sensors were also found relatively stable since they did not obviously change after 2000 stretching/releasing cycles. The sensors were tested to detect motion in the human body, such as wrist bending, eye blinking, mouth phonation, and pulse, and the results were shown to be satisfactory. Furthermore, the fabrication of the strain sensor consisting of mechanically blading MWCNTs aqueous dispersions into microtrenches of prestructured PDMS films was straightforward, was low cost, and resulted in high yield. All these features testify to the great potential of these sensors in future real applications.

MPTMS Treated Au/PDMS Membrane for Flexible and Stretchable Strain Sensors

Au/PDMS membranes are widely used to fabricate strain sensors which can detect input signals. An interfacial adhesion between metal films and polydimethylsiloxane (PDMS) substrates is one of the important factors determining the performance of strain sensors, in terms of robustness, reliability, and sensitivity. Here, we fabricate Au/PDMS membranes with (3-mercaptopropyl) trimethoxysilane (MPTMS) treatment. PDMS membranes were fabricated by spin-coating and the thickness was controlled by varying the spin rates. Au electrodes were deposited on the PDMS membrane by metal sputtering and the thickness was controlled by varying sputtering time. Owing to the MPTMS treatment, the interfacial adhesion between the Au electrode and the PDMS membrane was strengthened and the membrane was highly transparent. The Au electrode, fabricated with a sputtering time of 50 s, had the highest gauge factor at a maximum strain of ~0.7%, and the Au electrode fabricated with a sputtering time of 60 s had the maximum strain range among sputtering times of 50, 60, and 120 s. Our technique of using Au/PDMS with MPTMS treatment could be applied to the fabrication of strain sensors.

Towards inkjet printable conducting polymer artificial muscles

Inkjet printing is a key technology in the field of defined polymer deposition as well as in fabrication of strain sensors. It is also one of the most promising alternatives to prevalent fabrication of conducting polymer actuators. Nevertheless, inkjet printed actuators were not yet realised due to rheological properties of conducting polymer solutions that challenge jetting and the complex solution–membrane interactions, that lead to poor adhesion or uncontrolled infiltration. In order to enable this fabrication method, hybrid hydrophilic-hydrophobic polyvinylidene fluoride (PVDF) ultrafiltration membranes were used. They ensure good adhesion with printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer. Furthermore, PEDOT:PSS compositions that have liquid properties suitable for printing but do not cause uncontrolled infiltration through the membrane were identified. This allowed the realisation of the first inkjet printed PEDOT:PSS based ionic actuators working in air.