Large Strain Sensor Research Articles

Flexible Magnetoelectric Fiber for Self-Powered Human-Machine Interactive.

Flexible large strain sensors are an ideal choice for monitoring human motion, but the current use of flexible strain gauges is hindered by the need for external power sources and long-term operation requirements. Fiber-based sensors, due to their high flexibility, excellent breathability, and the ease with which they can be embedded into everyday clothing, have the potential to become a novel type of wearable electronic device. This paper proposes a flexible self-powered strain sensing material based on the electromagnetic induction effect, composed of a uniform mixture of Ecoflex and Nd2Fe14B, which has good skin-friendliness and high stretchability of over 100%. The voltage output of the magnetoelectric composite fiber remains stable over 5000 stretch-release cycles, reaching up to 969 μV. Based on this novel sensing material, a remote smart car control scheme for a human-machine interaction system was designed, enabling real-time gesture interaction.

Biphasic Liquid Metal Composite for Strain Sensor with High Stretchability and Sensitivity

Strain measurement is crucial for the safety and stability of engineering activities. With the increasing demand for sensors and large strain measurement in engineering monitoring, the study on the large strain measurement is deepening. However, the conventional strain sensors based on rigid conductive materials have limited flexibility and low stretchability, and the research of large strain and flexible sensors in engineering still lacks. In this work, a highly stretchable and sensitive flexible strain sensor based on liquid metal‐Nickle microparticles (LMN) composite is developed to detect large strain. The LMN composite, featuring uniformly dispersed Nickle microparticles within the liquid metal matrix, is deposited onto the elastomer Ecoflex base material.The LMN composite based‐sensor exhibits high sensitivity (GF = 16.14644 at strain of 200%), large stretchability (up to 200%) and low hysteresis. The theoretical relationship between the relative resistance changes and strain is derived, and the theoretical derivation is compared with experimental data to verify the accuracy of theoretical derivation. Additionally, the LMN composite based‐sensors can be used to detect pressure and bending signals in engineering. This work highlights the potential of LMN composite in broad applications and can facilitate the application of flexible sensing devices in engineering.

Ternary Liquid Metal Polymer Composite for Stretchable Electronics

AbstractThe small strain characteristics of the traditional flexible conductive composites hinder their realization of skin‐like properties and multi‐scenario applications. The value of liquid metal (LM)‐based elastomers is demonstrated in flexible electronics. Attempts in this area include the development of multifunctional LM‐based elastomers with controllable morphology, superior mechanical performances, and excellent stability. In this work, an efficient strategy is proposed to achieve the fast preparation of the LM patch by uniformly distributing eutectic gallium–indium–tin alloy in deionized water and by constructing the LM‐iron particles‐Ecoflex (LMIE) ternary composite system. In this study, the fabricated LMIE large strain sensor exhibits high sensitivity (the gage factor (GF) = 0.98 when strain ranges from 0% to 60%; GF = 1.91 when strain ranges from 60% to 100%), high linearity (correlation coefficient (R2) = 0.96 when strain ranges from 0% to 60%; R2 = 0.99 when strain ranges from 60% to 100%), a wide strain range of detection (e.g., 1%–100%) and breaking elongation (e.g., 450%), a fast response time (80 ms), excellent repeatability and stability. This work exhibits the excellent potential of multifunctional liquid metal composites in wearable electronics and prediction devices of structural collapse.

An electro-chemo-mechanical theory for hydrogel ionotronics: Application to modeling a capacitive strain sensor and a dynamic large strain actuator

Hydrogel ionotronic devices show substantial potential for use in soft robotics as well as in wearable and implantable actuators and sensors, especially when low weight, high flexibility and optical transparency are important. In this paper we formulate a finite-deformation theory for modeling the coupled electro-chemo-mechanical response of ionic hydrogels, and describe our numerical implementation of the theory in a finite element program. We show that our numerical simulation capability is able to successfully model the response of two ionotronic devices: (i) a capacitance-change based large strain sensor, and (ii) a dynamic large strain actuator which have been previously studied experimentally by Sun et al. (2014) and Keplinger et al. (2013).

Macromolecule conformational shaping for extreme mechanical programming of polymorphic hydrogel fibers

Mechanical properties of hydrogels are crucial to emerging devices and machines for wearables, robotics and energy harvesters. Various polymer network architectures and interactions have been explored for achieving specific mechanical characteristics, however, extreme mechanical property tuning of single-composition hydrogel material and deployment in integrated devices remain challenging. Here, we introduce a macromolecule conformational shaping strategy that enables mechanical programming of polymorphic hydrogel fiber based devices. Conformation of the single-composition polyelectrolyte macromolecule is controlled to evolve from coiling to extending states via a pH-dependent antisolvent phase separation process. The resulting structured hydrogel microfibers reveal extreme mechanical integrity, including modulus spanning four orders of magnitude, brittleness to ultrastretchability, and plasticity to anelasticity and elasticity. Our approach yields hydrogel microfibers of varied macromolecule conformations that can be built-in layered formats, enabling the translation of extraordinary, realistic hydrogel electronic applications, i.e., large strain (1000%) and ultrafast responsive (~30 ms) fiber sensors in a robotic bird, large deformations (6000%) and antifreezing helical electronic conductors, and large strain (700%) capable Janus springs energy harvesters in wearables.

Physical Cross-Linkage Constructed Supramolecular Conductive Hydrogel as Sustainable and Remolded Epidermal Electronics

To balance the comprehensive requirements of toughness, mechanical strength, rapid self-healing, and high conductivity in supramolecular hydrogels toward wearable devices still remain a tough challenge. This paper presents a simple one-step method to construct supramolecular conductive hydrogels (SCHs) through multilevel electrostatic and coordination interactions between amphoteric polymers and metal ions, which are revealed by the density functional theory (DFT) and noncovalent interaction (NCI) analysis. The SCHs integrate stretchability (more than 1500% and 110 kPa), self-healing, remoldability, large strain sensor (0–500%), and long-term monitoring of motion and electrocardiogram (ECG) with decent conductivity (∼2.5 S/m). The self-healing and remolding of SCHs suffer from temperature dependence. Meanwhile, it is worth noting that the remolded SCHs (R-SCHs) retained all of the properties of the original SCHs (O-SCHs), such as conductivity, intelligent sensing, health monitoring, and so on, indicating that the SCHs constructed in physical cross-linking could achieve sustainable recycling in flexible electronics and smart soft materials.

U-shape core-offset fiber sensor with submicrostrain resolution over a 35 millistrain range.

Large strain with submicro resolution is essential for steel structural monitoring; however, the fiber base sensors are limited by the glass extension to be less than 1%. Here, we propose a U-shape core-offset fiber sensor including four fiber segments to realize a large strain sensor. Four fiber segments with slight length differences in between are core-offset fused together to achieve U-shape spring-like microstructure fiber for large transverse bending radius. The reflected high-order modes at three silica/air interfaces interfere to give a broad spectrum due to unequal segment length, which enables continuous strain detection over 35 mɛ. The air and glass hybrid structure of the device enables the large bending, and hence compression and tension measurement can be achieved simultaneously. The strain sensitivity is up to 20.75 pm/µɛ with the strain accuracy of 0.5 µɛ. This novel, to the best of our knowledge, core-offset fiber has high strain sensitivity and large strain range for compression and tension strain measurement. Furthermore, the proposed strain sensor can be fabricated easily for practical applications where large strain with high strain accuracy is needed.

Facile and Large-scale Fabrication of Self-crimping Elastic Fibers for Large Strain Sensors

Stretchable conductive fibers offer unparalleled advantages in the development of wearable strain sensors for smart textiles due to their excellent flexibility and weaveability. However, the practical applications of these fibers in wearable devices are hindered by either contradictory properties of conductive fibers (high stretchability versus high sensing stability), or lack of manufacturing scalability. Herein, we present a facile approach for highly stretchable self-crimping fiber strain sensors based on a polyether-ester (TPEE) elastomer matrix using a side-by-side bicomponent melt-spinning process involving two parallel but attached components with different shrinkage properties. The TPEE component serves as a highly elastic mechanical support layer within the bicomponent fibers, while the conductive component (E-TPEE) of carbon black (CB), multiwalled carbon nanotubes (MCNTs) and TPEE works as a strain-sensitive layer. In addition to the intrinsic elasticity of the matrix, the TPEE/E-TPEE bicomponent fibers present an excellent form of elasticity due to self-crimping. The self-crimping elongation of the fibers can provide a large deformation, and after the crimp disappears, the intrinsic elastic deformation is responsible for monitoring the strain sensing. The reliable strain sensing range of the TPEE/E-TPEE composite fibers was 160%–270% and could be regulated by adjusting the crimp structure. More importantly, the TPEE/E-TPEE fibers had a diameter of 30–40 µm and tenacity of 40–50 MPa, showing the necessary practicality. This work introduces new possibilities for fiber strain sensors produced in standard industrial spinning machines.

Elastic, Conductive, and Mechanically Strong Hydrogels from Dual-Cross-Linked Aramid Nanofiber Composites.

Recent research on conductive hydrogels has revealed their potential for building advanced soft bioelectronic devices. Their mechanical flexibility, water content, and porosity approach those of biological tissues, providing a compliant interface between the human body and electronic hardware. Conductive hydrogels could be utilized in many soft tools such as neural electrodes, tactile interfaces, soft actuators, and other electroactive devices. However, most of the available conductive hydrogels exhibit weak mechanical properties, which hinders their application in durable biointegrated systems. Here, we report aramid nanofiber-based hydrogels providing a combination of high elasticity, strength, and electrical conductivity. Highly branched aramid nanofibers (ANFs) provide a robust three-dimensional (3D) framework resembling those in load-bearing soft tissues. When interlaced with poly(vinyl alcohol) (PVA) and cross-linked with both noncovalent and covalent interactions, the nanofiber composites exhibit a high water content of ∼76.4 wt %, strength of ∼7.5 MPa, ductility of ∼407%, and shape recovery of ∼99.5% under cyclic tensile stress of 0.3 MPa. Mobile ions impart a conductivity of ∼2 S/m to the hydrogels, enabling large-strain sensors with stable operation. In addition, the embedded silver nanoparticles afford broad-spectrum antimicrobial activities, which is favorable for medical devices. The versatility of aramid nanofiber-based composites suggests their further possibilities for functionalization and scalable fabrication toward sophisticated bioelectronic systems.

Studying the creep behaviour of strechable capacitive sensor with barium titanate silicone elastomer composite

In this paper, the creep behaviour of stretchable interdigital capacitive (IDC) large strain sensors is studied. A generalized Kelvin-Voigt (GKV) model is used to study the creep behaviour of the sensor’s substrate material, manufactured from silicone elastomer (Ecoflex 00−30) with barium titanate (BTO) filler. Creep experiments are performed on sensors with 10, 20, 30 and 40 wt% BTO nanoparticles with dimensions of 100 nm and 200 nm dispersed in the elastomer. The BTO was used to increase the overall permittivity of the substrate, hence raising the capacitance of the IDC sensor. The effect of BTO on the GKV model parameters was studied in detail through analysis of the creep response. The pristine Ecoflex silicone elastomer is predominately a hyperelastic material, which shows negligible creep, while the addition of BTO particles led to the composite exhibiting creep such that the composite behaves like a visco-hyperelastic material. Hence, this behaviour results in the creep affecting the electrical sensing performance of the capacitive strain sensors during static loading conditions. This information provides insights on the impact of composite composition on creep-resistance and output signal of the sensor (capacitance).

Highly sensitive and large range strain sensor based on synergetic effects with double conductive layer structures

The increasing demand for wearable electronic devices is promoting the development of highly elastic strain sensors, which can monitor various physical parameters and essential factors for realizing next generation electronics. However, it is still challenging to simultaneously achieve wearable strain sensors with sufficient sensitivity within wide stretching sensing ranges under mechanical stimuli in view of their widespread applications, including electronic skin, smart textiles, as well as healthcare monitoring. Herein, we propose a simple, cost-effective strain sensor based on synergetic conductive networks constituted by double-layer microstructures with multi-walled carbon nanotubes and elastomer. The micro-spinous structures of MWCNTs and the peak-valley microstructures have extended stretching sensing ranges with high sensitivities. The double-layer strain sensor exhibits high gauge factors (GF = 12.3 and 29.4) in the two linear stretching strain ranges (0–50 % and 50–200 %) with a long-term reusability, low-hysteresis, and fast response under dynamic loading. This sensor demonstrates excellent mechanical performance in real-time human motion detecting and distinguishing subtle signals, such as vocalization, muscle motions, as well as fingers or wrists.

The effect of barium titanate ceramic loading on the stress relaxation behavior of barium titanate‐silicone elastomer composites

AbstractThe stress relaxation behavior of barium titanate (BTO)‐elastomer (Ecoflex) composites, as used in large strain sensors, is studied using the generalized Maxwell‐Wiechert model. In this article, we examine the stress relaxation behavior of ceramic polymer composites by conducting stress relaxation tests on samples prepared with varying the particle loading by 0, 10, 20, 30, and 40 wt% of 100 and 200 nm BTO ceramic particles embedded in a Ecoflex silicone‐based hyperelastic elastomer. The influence of BTO on the Maxwell‐Wiechert model parameters was studied through the stress relaxation results. While a pristine Ecoflex silicone elastomer is predominantly a hyperelastic material, the addition of BTO made the composite behave as a visco‐hyperelastic material. However, this behavior was shown to have a negligible effect on the electrical sensing performance of the large strain sensor.

실리콘 고무 기반 탄소나노 복합소재 대변형 스트레인 센서 특성 연구

In this study, a carbon nanotube based flexible strain sensor was experimentally studied to develop a large deformable sensor. The sensor was made of silicone rubber and a multi-wall carbon nanotube as an electrically conductive filler with less than 3 wt% content. Since the nanocomposite sensor has piezoresistivity, its electrical resistivity is changed according to deformation. The sensor was fabricated as a sandwich structure to prevent buckling under large deformation and the electrodes were made with conductive fabrics to be able to connect it on large deformable materials. Flexible strain sensors are less sensitive than commercial strain sensors, but can have sufficient sensing characteristics to be large strain sensors. The output voltage range of the sensor can be enough to cover the large deformation of the sensing object completely due to it lower sensitivity. Further research should develop a high hysteresis compensation algorithm due to viscoelasticity to obtain more accurate rubber parts deformation measure.

Integrated wearable sensors with bending/stretching selectivity and extremely enhanced sensitivity derived from agarose-based ionic conductor and its 3D-shaping

Multi-functional integrated sensing systems have attracted significant attentions for high demands in the areas of flexible and stretchable devices. Herein, a novel selective wearable sensor with bending/stretching force differentiation and superior signal performance is presented. NaCl doped agarose gel (NaCl@AG) is applied as the bio-compatible conductive filler with 3D printed elastomer shaper as the holding matrix for wearable sensors. AG possesses an interesting sol-gel transition property of which NaCl@AG (sol) can be shaped into various stereo-structures through 3D printed micro-channeled elastomer shapers coviniently. 3D printed microporous Copper electrodes were also introduced for enhanced bonding with the NaCl@AG for sensor robustness. Through the integration of both straight and spring channels inside one elastomer shaper, our 3D integrated wearable sensor displayed superior sensing performances as a large-strain sensor with extreme sensitivity (gauge factor of 17 at 500% strain) and capabilities of bending/stretching motion differentiations (selectivity factor of 75 for bending/stretching signals at 97° bend). It can hence simultaneously detect and classify the extent of stretching or bending motions precisely. Sensing mechanisms are found to derive from both the microscopic geometrical deformations of the NaCl@AG associated with the resistance law; and the nanoporous pore size changes of AG associated with ionic diffusions. Such performances cannot be obtained by other conductors under the same circumstances, i.e. NaCl (aq). Overall, this work introduces a new concept for high performing and multifunctional/selective sensors via the combination of bio-compatible materials and novel structural design.

Reusable polymer optical fiber strain sensor with memory capability based on ABS crazing.

A reusable memory capable polymer optical fiber (POF) strain sensor is reported. The fiber consists of an acrylonitrile butadiene styrene (ABS) core and polymethylmethacrylate cladding. The memory capability is derived from stress whitening due to crazing of the ABS core, which can be reversed by heating the fiber close to the ABS glass transition temperature. The probe was characterized under transverse compressive load, macrobending, and tensile loading. Testing shows that the optical properties of the fiber can be reversed to near pristine ABS conditions after thermal recovery and that the POF can be used on the accurate assessment of indentation, flexural and tensile loading, static or cyclical, even after removal of the load. The reusability of the proposed sensor combined with a deeper understanding of the memory mechanisms in POFs are of great interest for the development of new large-strain sensors for modern applications.

A highly stretchable large strain sensor based on PEDOT–thermoplastic polyurethane hybrid prepared via in situ vapor phase polymerization

Strain sensors based on percolated networks of poly(3,4-ethylenedioxythiophene) (PEDOT) in thermoplastic polyurethane (TPU) matrix fabricated through in situ vapor phase polymerization (VPP) is reported. The hybrid made of elastomer and conductive polymer shows uniform PEDOT distribution on the surface and inner side of the TPU due to the effective penetration, diffusion, and polymerization of EDOT. Performance of sensors using two kinds of oxidants (FeCl3 or iron(III) p-toluenesulfonate(FTS)) are compared and the concentration is varied to obtain the best electromechanical properties. Amount of PEDOT increases with increasing amount of oxidant: thereby improving the electrical conductivity, while the elasticity of the hybrid decreases. The elasticity reduction phenomenon is diminished when FTS is used due to the plasticizer effect of the tosylate ion on the TPU. The performances of PEDOT–TPU hybrids prepared with FTS are excellent in the terms of stretchability (>300%), gauge factor (GF > 10 @ strain 100%), resistance variation reproducibility under various strain modes, small hysteresis, and durability (>1000 cycle). With the electromechanical performance, as well as cheap and scalable production, the PEDOT/TPU sensor holds tremendous prospects on flexible and stretchable devices for human motion monitoring, as well as present strategies on architecture of conductive soft materials.

Embedded large strain sensors with graphene-carbon black-silicone rubber composites

Wearable electronics have generated a strong demand for stretchable, skin-mountable and flexible sensors. This paper provides an effective technique for fabricating conductive networks using graphene-based polymer composites. Graphene-carbon black-silicone rubber (G-CB-SR) composites were cast in a mould, and a film was fabricated to make a composite strain sensor. Here we investigate how the remarkably different geometry affects the effective conductivity and piezoresistive properties of the composites. The electromechanical response of the G-CB-SR composites showed highly reversible and durable behaviour. It was stretchable up to 300%, and the gauge factor (GF) depended on the composition of the composites. The large strain sensors made of high-quality G-CB-SR film was achieved by solution intercalation with low-cost production and easy scalability.

Development of Textile Based Strain Sensor from Polypyrrole

The conducting polymers are polyconjugated, which possess electronic properties of metals, while retaining the mechanical properties and processability of conventional polymers. Conductive polymers can only withstand limited strain before breaking and cannot perform well in evaluating large strains. The aim of this study was to develop a low cost, small to large strain sensor using Polypyrrole and Latex/Polyamide 6 yarn. The stretchable yarn was chosen as the substrate due to its excellent resilience and elasticity. Polypyrrole was coated as thin film onto the substrate by means of vapour deposition technique. The response of resistance of the samples on 2% deformation and relaxation during 40 cycles was analysed. The sensitivity or the change in resistance per unit deformation was used as a tool to figure out the suitability of strain sensor. The high resistive sample gave better sensitivity as well as uniformity as compared to low resistive sample which made it suitable to use as a strain sensor.

P‐136: Resistive Type 2D Mapping Positional Strain Sensor Array for Advanced Tactile Displays

In this research, a new strain sensor array for measuring positional strain distribution have been demonstrated by using silver nanowire coated poly‐urethane fiber and composite blended with carbon black and PDMS, as insensitive and sensitive elements, respectively. Unit strain sensor with both insensitive and sensitive elements can be stretched up to 50 % and has a good sensitivity (Gauge factor; G.F ≈ 6), which is enough to detect human skin's movement. For positional strain distribution under external stimuli, we fabricated large area strain sensor arrays with 10 x 10 pixel and high resolution. Furthermore, to verify the measurement condition and obtained data of sensor array, we performed fine element analysis (FEA) simulation. This simulation result indicated that the fabricated strain sensor array can detect the relative strain values and their strain distribution when a strain was applied to this sensor array.

Large strain detection of SRM composite shell based on fiber Bragg grating sensor

There may be more than 2% strain of carbon fiber composite material on solid rocket motor (SRM) in some extreme cases. A surface-bonded silica fiber Bragg grating (FBG) strain sensor coated by polymer is designed to detect the large strain of composite material. The strain transfer relation of the FBG large strain sensor is deduced, and the strain transfer mechanism is verified by finite element simulation. To calibrate the sensors, the tensile test is done by using the carbon fiber composite plate specimen attached to the designed strain sensor. The results show that the designed sensor can detect the strain more than 3%, the strain sensitivity is 0.0762 pm/με, the resolution is 13.13με, and the fitting degree of the wavelength-strain curve fitting function is 0.9988. The accuracy and linearity of the sensor can meet the engineering requirements.