Integration of flexible strain sensor using liquid metal into soft micro-actuator

In this study, a flexible strain sensor is devised using corrugated bilayer thin films consisting of poly(3-hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene)-polystyrene(sulfonate) (PEDOT:PSS). In previous studies, the P3HT-based photoactive non-corrugated thin film was shown to generate direct current (DC) under broadband light, and the generated DC voltage varied with applied tensile strain. Yet, the mechanical resiliency and strain sensing range of the P3HT-based thin film strain sensor were limited due to brittle non-corrugated thin film constituents. To address this issue, it is aimed to design a mechanically resilient strain sensor using corrugated thin film constituents. Buckling is induced to form corrugation in the thin films by applying pre-strain to the substrate, where the thin films are deposited, and releasing the pre-strain afterwards. It is known that corrugated thin film constituents exhibit different optical and electronic properties from non-corrugated ones. Therefore, to design the flexible strain sensor, it was studied to understand how the applied pre-strain and thickness of the PEDOT:PSS conductive thin film affects the optical and electrical properties. In addition, strain effect was investigated on the optical and electrical properties of the corrugated thin film constituents. Finally, flexible strain sensors are fabricated by following the design guideline, which is suggested from the studies on the corrugated thin film constituents, and the DC voltage strain sensing capability of the flexible strain sensors was validated. As a result, the flexible strain sensor exhibited a tensile strain sensing range up to 5% at a frequency up to 15 Hz with a maximum gauge factor ~7.

Ultrasensitive and highly stretchable bilayer strain sensor based on bandage-assisted woven fabric with reduced graphene oxide and liquid metal

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

The booming prosperity of strain sensors promotes the regeneration of wearable electronics. However, strain sensors featured with both large workable range and superb sensitivity are still less than satisfaction. Softness and practicality are also of vital importance for wearable electronics. In this work, a flexible wearable strain sensor based on carbonized cotton fabric coated with reduced graphene oxide (rGO) through polydopamine (rGO/PDA/CCF) modification was fabricated, which exhibits fascinating electrical mechanical performance including a large workable range and superb sensitivity. rGO/PDA/CCF was characterized using scanning electron microscopy (SEM), Raman scattering spectroscopy, and X-ray diffraction (XRD) patterns. The electromechanical performance of the rGO/PDA/CCF strain sensor was evaluated. The strain sensors of rGO/PDA/CCF exhibit a large workable strain range (0–120%) and superb sensitivity with strong durability over 800 cycles. The rGO/PDA/CCF strain sensor has a low detection limit down to 0.01% with a good gauge factor (GF) of 132 within the strain range of 0–1%, and also displays fantastic property in terms of sensitivity in other workable range (GF of 26 and 8 within in a strain range of 1–30% and 30–60%, respectively). rGO/PDA/CCF strain sensor was then applied to track the signal of human motions in real time. rGO/PDA/CCF flexible strain sensor can be utilized in detecting almost all human motions with various dimensions. Additionally, the hydrophobicity and the stability at different temperatures endow the rGO/PDA/CCF strain sensor with high efficiency in various usage, which shows its potential for application in health monitor and wearable electronics.

Flexible wearable sensors are emerging as next-generation tools to collect information from the human body and surroundings in a smart, friendly, and real-time manner. A new class of such sensors with various functionality and amenability for the human body is essential for this goal. Unfortunately, the majority of the wearable sensors reported so far in the literature were of a single function (mostly strain sensors) and just a prototype without thinking of continuous mass production. In this paper, we report a series of multifunctional conductive hydrogel/ thermochromic elastomer hybrid fibers with core-shell segmental configuration and their application as flexible wearable strain and temperature sensors to monitor human motion and body/surrounding temperatures. Specifically, a conductive reduced-graphene-oxide-doped poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide (rGO-poly(AMPS-co-AAm)) hydrogel and a thermochromic elastomer containing silicon rubber and thermochromic microcapsules are chosen as strain-sensitive and thermosensitive materials, respectively. A core-shell segmental structure is realized by programming the extrusion of either conductive hydrogel precursor solution or a thermochromic elastomer prepolymer as a core layer via dual-core coaxial wet spinning. Depending on the assembly order and length of the conductive hydrogel and the thermochromic elastomer, the as-prepared hybrid fibers can be used for different purposes, i.e., human-motion monitoring, body or room temperature detection, and color decoration. The strategy described above, i.e., fabrication of core-shell segmental fibers via the wet-spinning method, is especially suitable for mass production in industry and can be further extended to fabricate flexible wearable devices with more components and more functions such as transistors, sensors, displays, and batteries.

<p>Laminated elastomeric bearings are broadly used in large bridges for adjusting the deformation and ensuring the safety of the bridge structure. Interestingly, the bridge bearing’s deformation contains rich information that reflects the complex load conditions of the bridge structure. Nevertheless, the structural health monitoring (SHM) of the bridge bearings has not been carried out effectively due to the special characteristics of bridge bearings’ deformation, including the large range and the existence of varied forms. Along these lines, a novel conductive polymer composite-based flexible strain sensor was fabricated by employing a facile solution-blending method. More specifically, the proposed strain sensor was composed of thermoplastic polyurethane (TPU), multiwall carbon nanotubes (MCNTs), and silane coupling agent KH550. The utilization of the KH550 not only promoted the dispersion of MCNTs within the TPU matrix but also enhanced the interfacial bonding between the MCNTs and the TPU matrix, resulting in the large strain range (~150%) of the silanized MCNT/TPU strain sensor. More specifically, the silanized strain sensor with 3.0 wt% MCNTs demonstrated extraordinary linearity, promising gauge factor (~ 8.12), and great durability under the enforcement of a tensile strain up to the value of 150% through the application of consecutive stretch–release tests. On top of that, the compressive bending sensing tests proved the stable and repeatable bending sensing capability of the proposed silanized strain sensor. Moreover, the silanized strain sensor could effectively monitor the shear and the compressive deformation of the laminated elastomeric bridge bearings. This work provides a new way for fabricating flexible strain sensors with enhanced strain range, which validates the feasibility of the strain sensor for monitoring the bearing’s deformation and provides an experimental basis for developing the SHM of the laminated elastomeric bridge bearings.</p>

Carbon/graphene composite nanofiber yarns for highly sensitive strain sensors

Liquid metals (LM) exhibit high electrical conductivity and excellent ductility, making them highly promising for applications in flexible, stretchable, and wearable sensors. This paper proposes a novel method to overcome the challenge of patterning liquid metals on flexible substrates, which is hindered by their high surface tension. By adding Ni particles to the LM, the surface tension is reduced while maintaining its flowability, enabling easy patterning on flexible substrates. When <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$12 \text{wt}. {\%} \text{Ni}(40 \mu \mathrm{m})$</tex> is added to LM, a gauge factor (GF) of 5.17 can be achieved at 300%. In addition, the mechanism analysis of different GF resulting from varying Ni contents from 0 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$20 \text{wt}. {\%}$</tex> was investigated. The strain sensor with a Ni content of 12 wt.% was subjected to a cyclic performance test of 1000 cycles at 150% strain. The strain sensor proposed in this paper, which adopts a magnet-patterned LM structure, has significant application potential in the field of wearable robots.

A novel flexible strain sensor was developed using a conducting polymer coated on rubber for large strain measurements. A coating of the conducting polymer, polypyrrole, was deposited on a strip of natural rubber through the process of vapour phase polymerisation while the rubber is in a stretched state. This process involves depositing a layer of oxidant on the rubber surface, followed by exposure to pyrrole monomer vapours that polymerize on the oxidantcoated rubber to produce polypyrrole. The change in electrical resistance of the strain sensor was recorded while cyclic strain from 0% to 20% was exerted on it. The gauge factor of the strain sensor was calculated to be 1.86. From repeated electrical resistance-strain measurements, the repeatability of the strain sensor was studied. A hysteresis was observed in a single extensionretraction strain cycle. Further study showed that the observed hysteresis is dependent on the strain rate where lower strain rate resulted in higher hysteresis and vice versa for a higher strain rate. There is also an electrical resistance drift between consecutive extension-retraction cycles. Owing to the flexibility of the rubber, the strain sensor can be used in complex configurations. The strain sensor can also be mounted or attached directly on surfaces to provide low-profile installation where space constraint is an issue. These characteristics offer advantages over traditional strain sensors to be used in applications that were not previously possible.

Flexible electronic devices containing gallium‐based liquid metal (GbLM) play an important role in human–computer interaction and the sensing elements of motion monitoring owing to the intrinsic fluidity and metallic conductivity of the GbLM. High‐aspect‐ratio liquid‐metal patterns have 3D structures, which can increase the heat radiating area of the circuits and the stability of the sensors. However, fabricating high‐aspect‐ratio patterns in high resolution is challenging owing to easy oxidation at the surface layer and the high surface tension of the liquid metal. This study proposes a simple method for the efficient and reproducible fabrication of GbLM patterns. A flexible strain sensor was obtained with high‐aspect‐ratio Galinstan (height/width &gt; 5), linearity (R‐square = 0.9975), stability (ΔR/R0 &lt; 0.8 after 10 000 cycles), and low hysteresis. In addition, the application of the strain sensor in the flexible display and motion measurement of soft hand exoskeletons is demonstrated. The high‐aspect‐ratio liquid‐metal patterns in this work have the potential to extend the application of the presented flexible electronics to the fields of wearable systems and rehabilitation devices.

Hollow-porous fiber-shaped strain sensor with multiple wrinkle-crack microstructure for strain visualization and wind monitoring

Recently, flexible strain sensors with excellent stretchability and sensitivity have attracted increasing interests. Most flexible strain sensors have multifunctional applications in human motion monitoring, transparent and wearable electronic systems, however, they also can be applied in other fields such as structural health monitoring (SHM). In this paper, a flexible strain sensor which was assembled by adhering graphene mesh film on liquid crystal polymer substrate constructed by an efficient, low-cost fabrication strategy for applying in SHM is presented. By means of monitoring the relative resistance change of graphene mesh film of the strain sensor, the strain caused by the minor displacement of the structure could be detected. The experiment results showed that the strain sensor had a high gauge factor (375–473) under different tensile strain (0.1–1.4%) and good stability (1000 test cycles). It can be verified that the sensor has high sensitivity and good stability, which makes it have potential application in SHM.

In the paper the biopolymer substrates were made from nanocellulose (NC), which is extracted from organosolvent reed cellulose using 2,2,6,6-tetramethylpiperidin-1-oxyl reagent (TEMPO). Nickel-based thin films were deposited on the surface of nanocellulose by RF magnetron sputtering. The peculiarities of surface morphology and chemical composition of inorganic material on the surface of biopolymer substrate were studied. Flexible strain sensors were obtained from nickel busbars on the surface of nanocellulose. The influence of overall sizes of nickel busbars on piezoresistive parameters of flexible sensors was established. It was determined that the maximum strain sensitivity is observed for a nickel busbar with a width of 1 mm, a thickness of 250 nm, a length of 15 mm and was 195,8 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⋆</sup> 10-3 %. The magnitude of drift in time was 0,17 %/min. Obtained strain sensors were tested on mechanical strength depending on nickel busbar sizes. It was shown that the presence of nickel thin film increases the strength of strain sensors by 40%: from 13.8 MPa for pure nanocellulose to 19.5-19.6 MPa for nanocellulose with nickel thin films.

Electronic skin (e-skin) with excellent flexibility and comfortable wearability has attracted considerable attention in the past decade. Flexible strain sensors are one of the key elements of e-skins, which are always attached to the joints of humans or robots for motion monitoring. However, most strain sensors previously reported cannot fit different joints to measure the bending angles without calibration. This work aims to propose a flexible strain sensor with an adaptive model for automatically calculating the bending angle of different joints. The proposed sensor is a screen-printed liquid metal strain sensor with a size of 120 mm (L) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times20$ </tex-math></inline-formula> mm (W) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 400\mu $ </tex-math></inline-formula> m (H). The real-time changes in different curvatures as the sensor bend has been investigated. Based on the curvature-strain-bending angle relationship of different joints, we use the Gaussian function to model the bending angle solution for adaptive curvature. Experimental results show that the proposed sensor can be served as the electronic skin on the joint’s surface to measure the bending angle adaptively, which provides a promising path for adaptive motion monitoring in human movement, soft robotics, prosthetic devices, etc.

We report the flexible piezotronic strain sensors fabricated using carbon fiber-ZnO nanowire hybrid structures by a novel and reliable method. The I-V characteristic of the sensor shows high sensitivity to external strain due to the change in Schottky barrier height (SBH), which has a linear relationship with strain. This fabricated strain sensor has a quick, real-time current response under both static and dynamic mechanical loads. The change in SBH resulted from the strain-induced piezoelectric potential is investigated by band gap theory. In this work we develop a new feasible method to fabricate a flexible strain sensor within the fabric adapted to textile structures, able to measure their strain.