Relative Resistance Change Research Articles

A polypyrrole-dopamine/poly(vinyl alcohol) anisotropic hydrogel for strain sensor and bioelectrodes

Flexible hydrogel-based wearable sensors are of interest for monitoring human movement and electrophysiological signal detection. The high mechanical properties, high conductivity, and sensing sensitivity are the main performance parameters. Inspired by the structure of human muscles, ligaments, and other tissues, a polypyrrole-dopamine/poly(vinyl alcohol) (DA-PPy/PVA) hydrogel with a bionic anisotropic structure is prepared in this study using a directed freeze–thaw strategy. The introduction of DA-PPy nanoparticles can significantly improve the mechanics, adhesion, and conductivity due to the high water dispersibility. Moreover, they can act as physical cross-linking sites to further improve the mechanical properties. The anisotropic structure would influence many properties (mechanical, conductivity, resistance to crack extension, etc.) of DA-PPy/PVA hydrogel. In addition, the prepared DA-PPy/PVA hydrogel exhibits a wide range of strain responses, moreover, its relative resistance change (ΔR/R0) strain curve maintains a line linear within 400% strain. The oriented structure would influence the sensing sensitivity, the gauge factor of anisotropic DA-PPy/PVA in parallel direction is 3.00. In addition, the anisotropic DA-PPy/PVA can effectively motor the physiological electrical signals as a bioelectrode. For example, the acquired electrocardiogram (ECG) and electromyography (EMG) signals not only are clearer than those of the commercial electrodes but show a higher signal-to-noise ratio. It is conceivable that the bionic anisotropic DA-PPy/PVA hydrogels provide a feasible material to construct flexible wearable sensors for monitoring human movement and electrophysiological signals.

MWCNT-reinforced silicone's dual response to human and mechanical motion: Exploring strain sensing behavior and practical applications

This study investigates the human motion sensing capabilities of silicone rubber (SR) composites reinforced with multi-walled carbon nanotubes (MWCNTs). It focuses on assessing the responsiveness of the composites to both compressive and tensile deformations, replicating motions from machines to human movements. The analysis underscores the enhanced sensitivity of the 3 parts per hundred (phr) compressive sample, with gauge factors of 7.5 and 14.44 at distinct linear ranges under 30% strain. During forefinger and thumb pressing, the relative change in resistance was 160 and 350, respectively. Additionally, the study examines the 5 phr sample, revealing a significant 346.67% increase in relative resistance changes during the initial cyclic loading phase under 30% tensile deformation compared to the compression sample. The mechanical properties are also evaluated, showing the 5 phr compressive sample possessing a compressive modulus 180.09% higher than the modulus of the unfilled specimen (1.246 MPa), along with a hysteresis loss of 37.24 kJ. Furthermore, the tensile modulus of the 5 phr specimen exceeds that of the unfilled rubber by 218.84%. The developed composites hold promise for creating sensors capable of tracking athletes' movements in real time, offering immediate performance feedback and post-processing analysis. Moreover, these materials could contribute to the design of rehabilitation devices aimed at assisting individuals with hand injuries in regaining their agility and mobility.

Photoelectric properties of two-dimensional electron gas at the (Ga0.5Al0.5)2O3/SrTiO3 heterointerfaces

We report a two-dimensional electron gas created at the (Ga0.5Al0.5)2O3/SrTiO3 heterointerfaces with different deposition temperatures and thicknesses by a pulsed laser deposition method. It is found that the critical deposition temperature for the metal-to-insulator transition is about 400 °C with a thickness of 43.2 nm. The carrier density and mobility of sample at the deposition temperature of 300 °C are 1012 cm−2 and 67.2 cm2 V−1 s−1 at 50 K, respectively. The thickness of the conductive layer is ∼4.0 nm estimated from the anisotropic magnetoresistance. The bandgaps of films deposited at different temperatures are in the range of 5.6–5.9 eV. Furthermore, the heterointerfaces have a strong response to 360 nm light with a relative resistance change of ∼70%. Our results are related to the charge transfer due to the oxygen defect at the interfaces, which provides a way for the development of third-generation semiconductor optoelectronic devices.

Analysis of the influence of thermal and thermocyclic technology on the properties of maraging steel

The article presents experimental data obtained by studying the influence of changes in the geometry of parts made of maraging steels, depending on the modes of thermal and thermal cycling treatment with heating in the temperature range of α-γ transformation. The experiments were carried out on samples cut from hot-rolled sheets, pre-hardened at a temperature of 1000°C for 1 hour. Using the accelerated testing method, the relative change in resistance to stress corrosion damage was determined depending on the phase composition and aging temperature (strength) of maraging steel.

High-strength fibrous sensors with an enhanced aggregate state for biomechanical monitoring of the Achilles tendon.

Continuous monitoring of biomechanical signals generated from the injured Achilles tendon is essential for the deep understanding of the recovery or rehabilitation process, thus decreasing the risk of secondary injuries. With tissue-like components and adjustable properties, hydrogel-based biomechanical sensors are considered promising materials for human motion detection. However, existing hydrogels are characterized by inferior mechanical properties with strength and modulus typically lower than 1 MPa, as well as poor stability under physiological conditions, which hampers their applications in implantable devices. Moreover, acquiring the stress signal from collected electrical signals remains challenging. Herein, based on the regulation of polymer aggregation, a high-strength fibrous sensor composed of polyvinyl alcohol (PVA) and reduced graphene oxide (rGO) for in vivo monitoring is prepared through a two-step procedure, including freeze-thaw and freeze-soak. Benefiting from the synergy of crystallization, Hofmeister effect and nanocomposite, the hydrogel fibers feature high tensile strength (8.34 ± 0.66 MPa) and elastic modulus (1.15 ± 0.10 MPa). Meanwhile, the removal of salt ions during fabrication improves the water content (69.18 ± 1.47%) and anti-swelling performance of such fibers and minimizes side effects after implantation. It is demonstrated that the fibrous sensor could record the relative resistance changes upon stretching with ideal sensitivity (GF = 1.57) and convert them into bearing stress through formula derivation and calculations. In vitro and in vivo assays further confirm its feasibility for real-time monitoring of joint motion, providing important references for medical diagnosis and treatment.

A stretchable conductive elastomer sensor with self-healing and highly linear strain for human movement detection and pressure response.

Expanding the detection information of wearable smart devices in applications has practical implications for their use in daily life and healthcare. Damage and breakage caused by mechanical injuries and continuous use are unavoidable for polymer matrices so self-healing properties are expected to be conferred on flexible sensors to extend their life and durability. In addition, a good linearity of relative resistance change vs. strain (gauge factor, GF) facilitates the streamlined conversion of electrical signals to 3D information of human motion, whereas existing works on sensors neglect the quantitative analysis of signals. This letter reports a self-healable flexible electronic sensor based on hydrogen bonding and electrostatic interaction between maleic acid-grafted natural rubber (MNR), polyaniline (PANI), and phytic acid (PA). MNR is the flexible matrix and the template for aniline (ANI) polymerization, and PA acts as the dopant and crosslinking agent. The MNR-PANI-PA sensor shows easy self-healing at room temperature, enhanced mechanical behaviour (∼2.5 MPa, 1000% strain), and excellent linearity (GF of 13.8 over 250% strain and GF of 32.0 over 250-100% strain). Due to the highly linear relationship between ΔR/R and bending angle, the electrical signals of human limb movement can output relevant information on bending angle and frequency. By constructing a sensing array, changes in the position and magnitude of applied pressure could also be detected in real-time. Based on these advantages, the MNR-PANI-PA composite sensor is expected to have potential applications in health monitoring, body motion detection, and electronic skins.

Performance and Characteristics of Sprayed Flexible Sensor for Strain Monitoring of Steel Bridges

Monitoring stress and strain at the critical details of steel bridges is essential for ensuring structural integrity. This study introduces a three‐layer flexible strain sensor produced through a spraying process, using flake‐shaped silver‐coated copper powder as the conductive filler and modified acrylic emulsion as the matrix material. The study investigated the impact of size parameters on sensor sensitivity, determining optimal dimensions of 20 mm in length, 2 mm in width, and an initial resistance value ranging from 1.0 Ω to 1.8 Ω. Analysis of the optimized sensor’s performance unveiled high sensitivity and linear response capabilities under low strain conditions with a gauge factor (GF) value of up to 25.6 and a linear correlation coefficient R2 ≥ 0.971 under 300 με. Notably, the sensor exhibits an extremely low strain detection limit of 0.005% and a broad response range spanning from 0.005% to 0.19% strain. It demonstrates swift response and recovery times of 500–800 ms, showcases directional strain response, exhibits good repeatability, and endures durability tests (withstanding 3000 cycles). Furthermore, a fitting formula is proposed to accurately depict the strain and relative resistance change relationship across a wide response range. The study and initial application of this sensor’s sensing characteristics and performance signify its potential for practical engineering applications.

Magnetorheological effect and sensing performance of graphite sheet filled PDMS based magnetorheological elastomer under compression

In order to study the effect of large-sized graphite (Gr) sheet on the magnetorheological (MR) effect and sensing characteristics of MR elastomer (MRE), isotropic and anisotropic Gr-filled MRE samples with different carbonyl iron powder (CIP) contents were fabricated. The effect of Gr sheet on the microstructure, MR effect and sensing characteristics of the MRE samples were experimentally tested, and the mechanisms behind discussed. The results show that in the anisotropic MRE samples, the addition of Gr sheets results in the short particle chains formed between Gr sheets, thus leading to the high MR effect and low resistivity than those of isotropic counterparts. The non-monotonic resistivity responses of the Gr-filled MRE samples during compression were observed owing to the interlayer separation of Gr sheets and the reconstruction of conductive network. A higher piezoresistive response was observed from the isotropic Gr-filled MRE sample filled with the CIP content below the percolation threshold. The resistivities of the Gr-filled MRE samples decline with increasing the applied magnetic field. The isotropic sample filled with lower CIP content shows the higher magnetoresistive effect from the view point of absolute change in resistivity. While for the relative change in resistivity, the anisotropic sample filled with the higher CIP content has the higher magnetoresistive effect.

AuNPs-PDMS Composite Membrane-Based Surface Stress Biosensor for Molecular Detection

Receptor-functionalized membranes provide a new paradigm for developing mechanical biosensors. However, the lower sensitivity of these biosensors is still a challenge. In this research, a sensitive and thin (1.5 [Formula: see text]m) gold nanoparticles-polydimethylsiloxane (AuNPs-PDMS) membrane is fabricated by reducing HAuCl4. Experimental results reveal that surface stress deforms the AuNPs-PDMS membrane, further increasing the resistance due to the tunneling mechanism and conductive percolation. During the functional process, the resistance change is consistent with theoretical prediction. The relative resistance change demonstrates a linear relationship with the glucose concentration. The biosensors exhibit a wider linear range for glucose from 0 to 30[Formula: see text]mM with a lower detection limit of 0.035[Formula: see text]mM. AuNPs-PDMS thin membrane-based surface stress biosensor shows promising application prospects to detect biomolecules.

Flexible Switching Pressure Sensors with Fast Response and Less Bending-Sensitive Performance Applied to Pain-Perception-Mimetic Gloves.

A strategy is proposed herein for preparing a flexible switching piezoresistive pressure sensor, which has a bridge-like structure and inverted micropyramids (IMPs) on its lower conductive substrate. The sensor substrates were prepared by injection compression molding (an industrial manufacturing process) using thermoplastic polyurethane (TPU; an industrial grade polymer). The designed bridge-like structure enables the sensor to obtain a pressure threshold. The flexibility of upper and lower TPU substrates allows them to contact quickly when pressed, and so the sensor exhibits a fast response (as short as 2 ms) and can respond to both static force and dynamic force (up to 50 Hz frequency), which are prominent for the sensor made from TPU. The sensor exhibits less bending-sensitive performance, which is attributed to the conformality of the upper and lower substrates and lower strain on the lower substrate with the IMP under bending. The sensor can amplify signal response at the monitoring limit (the relative resistance change is up to 46%). It can achieve a higher sensitivity in different low-pressure ranges by changing the gap of the bridge-like structure. Moreover, the sensor can obviously and steadily respond to an additional very low pressure under preloading and exhibits good durability performance. As the sensor has a pressure threshold similar to the human pain perception process, a pain-perception-mimetic glove that can identify the external mechanical stimuli but reduces the interference of finger bending is prepared, displaying potential applications of the flexible switching sensor in intelligent wearable protectors.

3D Printed Conformal Strain and Humidity Sensors for Human Motion Prediction and Health Monitoring via Machine Learning.

Wearable sensors have garnered considerable attention due to their flexibility and lightweight characteristics in the realm of healthcare applications. However, developing robust wearable sensors with facile fabrication and good conformity remains a challenge. In this study, a conductive graphene nanoplate-carbon nanotube (GC) inkis synthesized for multi jet fusion (MJF) printing. The layer-by-layer fabrication process of MJF not only improves the mechanical and flame-retardant properties of the printed GC sensor but also bolsters its robustness and sensitivity. The direction of sensor bending significantly impacts the relative resistance changes, allowing for precise investigations of joint motions in the human body, such as those of the fingers, wrists, elbows, necks, and knees. Furthermore, the data of resistance changes collected by the GC sensor are utilized to train asupport vector machine with a 95.83% accuracy rate for predicting human motions. Due to its stable humidity sensitivity, the sensor also demonstrates excellent performance in monitoring human breath and predicting breath modes (normal, fast, and deep breath), thereby expanding its potential applications in healthcare. This work opens up new avenues for using MJF-printed wearable sensors for a variety of healthcare applications.

Encapsulated Ag2Se-based flexible thermoelectric generator with remarkable performance

Flexible thermoelectric generators hold great potential for powering wearable electronics. Herein, we employ a novel encapsulation technology to fabricate a Ag2Se-based composite film, which possesses a remarkable room-temperature power factor (PF) of 25.6 μW cm−1 K−2 and a peak PF of ∼27 μW cm−1 K−2 at 380 K. The microscopic origin of the above performance is attributed to the appearance of CuAgSe second phase by our in-situ transmission electron microscopy evidences. Moreover, the encapsulation guarantees the Ag2Se layer in a zero-strain state, contributing to a negligible relative resistance change (<0.2%) even after 2000 bending cycles under a bending radius of 3 mm. The encapsulated flexible thermoelectric generator assembled with six Ag2Se legs demonstrates a maximum power density of 130 W m−2 at a temperature difference of 60 K and is compatible with harsh working environment.

Electrical, mechanical and damage self-sensing properties of basalt fiber reinforced polymer composites modified by electrophoretic deposition

In this work, basalt fiber reinforced polymer (BFRP) with enhanced electrical, mechanical properties and in-situ damage self-sensing capability was prepared using carboxylic carbon nanotubes (COOH-CNTs) modified BF fabrics via electrophoretic deposition (EPD) at different voltages. The BFRP with BF fabrics modified at a deposition voltage of 20 ​V (EPD20-BFRP) showed the lowest electrical resistivity. Compared with unmodified BFRP, the tensile and flexual moduli of EPD20-BFRP increased by 37.5 ​% and 14.9 ​%, respectively. Single-layer EPD20-BFRP exhibited a high gauge factor (GF) of 44.3 during tensile damage self-sensing. The acoustic emission (AE) signals during tensile and flexual damage process agreed well with the relative resistance changes (RRC), which confirmed different damage stages within loading process, such as elastic deformation, damage evolution, crack coalescence, and complete fracture. In addition, the multi-layer BFRP containing a single-layer EPD20-BFRP on the upper or lower surface of laminate exhibited distinct electrical signal responses subjected to the flexural loading.

Template-Free and Stretchable Conductive Fiber with a Built-In Helical Structure for Strain-Insensitive Signal Transmission.

With the rapid development of intelligent electronic devices, conductive fibers have become very critical to signal transmission devices. However, metal-based rigid conductive wires, such as high-modulus copper and silver wires, are prone to signal failure owing to tensile breakage under large strain conditions. Therefore, strain-insensitive stretchable conductive fibers for signal transmission are critical for next-generation wearable devices. Herein, a stretchable conductive fiber with a built-in helical structure is constructed by a "speed discrepancy" fiber-coating strategy with mass scalable production (60 cm/min). Such a "speed discrepancy" strategy is the key mechanism to template-free fabricate a built-in helical structure of the stretchable conductive fiber. The resultant fiber exhibits high conductivity (873 S/cm), stable insensitive signal transmission with a high quality factor (47.4), and a low relative resistance change (∼6%) under large strain. The built-in helical structure inspired by loofah whiskers endows the fiber with excellent strain insensitivity, and it can withstand large strains. On the proof of concept, our fiber can be seamlessly knitted, woven, and braided into smart textiles as an ideal signal transmission device under large strains, which will undoubtedly promote the development of intelligent electronic textiles and next-generation wearable devices.

Flexible and electrically robust graphene-based nanocomposite paper with hierarchical microstructures for multifunctional wearable devices

Multifunctional and flexible wearable devices play a crucial role in a wide range of applications, such as heath monitoring, intelligent skins, and human-machine interactions. Developing flexible and conductive materials for multifunctional wearable devices with low-cost and high efficiency methods are highly desirable. Here, a conductive graphene/microsphere/bamboo fiber (GMB) nanocomposite paper with hierarchical surface microstructures is successfully fabricated through a simple vacuum-assisted filtration followed by thermo-foaming process. The as-prepared microstructured GMB nanocomposite paper exhibits not only a high volume electrical conductivity of ∼45 ​S/m but also an excellent electrical stability (i.e., relative changes in resistance are less than 3% under stretching, folding, and compressing loadings) due to its unique structure features. With this microstructured nanocomposite paper as active sensing layer, microstructured pressure sensors with a high sensitivity (−4 ​kPa−1), a wide sensing range (0–5 ​kPa), and a rapid response time (about 140 ​ms) are realized. In addition, benefitting from the outstanding electrical stability and mechanical flexibility, the microstructured nanocomposite paper is further demonstrated as a low-voltage Joule heating device. The surface temperature of the microstructured nanocomposite paper rapidly reaches over 80 ​°C when applying a relatively low voltage of 7 ​V, indicating its potential in human thermotherapy and thermal management.

Anisotropic Pressure Sensors Fabricated by 3D Printing‐Aligned Carbon Nanotube Composites

The flexibility and controllability of 3D printing technologies have made them popular for creating integrated wearable sensors. However, producing sensors with 3D anisotropic response is still difficult with current printing materials and techniques. To address this challenge, digital light process (DLP) printing technology with blading function to fabricate 3D anisotropic pressure and strain sensors is used. By applying blading‐induced shearing force, nanoalignment of carbon nanotubes (CNTs) in each layer is achieved, while the continuous liquid interface production of the DLP printing process facilitates good electrical communication between adjacent microlayers. The constructed 3D sensor displays high tensile and pressure responses up to 40 MPa and the ability to detect pressure changes in all directions. Additionally, the resistance anisotropy ratio range is adjustable. Finite‐element method (FEM) simulation shows that the relative change in resistance has an anisotropic distribution that aligns well with experimental results. The work would offer a new way to create 3D anisotropic pressure/strain sensors for advanced flexible and stretchable sensing applications.

In Situ Cancer-Cell-Triggered Visible Changes in Mechanical Properties, Electroconductivity, and Adhesiveness of a MnO2@PD-Based Mineralized Hydrogel.

Herein, a cancer-specific dopamine-conjugated sp2-rich carbonized polymer dot (PD)-encapsulated mesoporous MnO2 (MnO2@PD)-mineralized hydrogel biosensor was developed that offers cancer-induced observable in situ alterations in fluorescence (FL), electrochemical, and mechanophysical properties. Cancer-triggered MnO2 degradation in the hydrogel, prompted by increased levels of glutathione (GSH) and reactive oxygen species (ROS) such as H2O2, leads to PD release and FL restoration, thereby controlling changes in the pore structure and increasing hydrogen bonding, resulting in physiologically visible alterations in mechanical stretchability, viscosity, swelling behavior, and adhesiveness. The pore size of the matrix increased from 21.83 to 36.81 m2/g upon GSH treatment, affecting the viscosity and swellability of the system. The resistance increased from 21.96 ± 1.16 to 30.69 ± 2.01 and 32.21 ± 2.54 kΩ, respectively, confirming the dependence of conductivity changes on H2O2 and GSH treatments. The in vitro treatment with cancer cells (HeLa, PC-3, and B16F10) facilitated a tunable electrochemical sensing performance via redox-mediated MnO2 breakdown by intracellular ROS and GSH, whereas hydrogels treated with normal cells (CHO-K1) showed minimal changes. Cancer-microenvironment-derived water-drop sensing showed three times higher response as compared to the normal cell-treated hydrogel. The sensing capability of the fabricated sensor was validated based on bending-induced relative resistance changes under dry and wet conditions. Moreover, the integration of the developed sensor with a wireless sensor enabled real-time monitoring with a smartphone.

Coaxial-helix MXene/PANI-based core-spun yarn towards strain-insensitive conductor and supercapacitor

Stretchable electrical conductors hold great promise in the wearable electronic devices, such as flexible display and human-machine interface. Stable conductivity is vital for wearable devices to maintain their stable performance. However, the electrical resistance of most reported yarns dramatically increases when they are deformed. Herein, we propose a design for a coaxial-helix structure by wrapping the polyaniline/MXene-based elastic core-spun yarn on itself in the form of coils. The fabricated coaxial-helix structured yarns (DCSs) exhibit excellent strain-insensitive properties, including low relative resistance change (25.4%), large strain (200%) and high quality factor (17.95). Moreover, the DCSs can be used as the stretchable electrodes for the supercapacitors, and delivers a linear specific capacitance of 256.7 mF cm−1, which is higher than most stretchable yarns. Therefore, the proposed coaxial-helix structure design creates a new approach for the strain-insensitive stretchable conductor, thereby paving the way for the development of stretchy electronics.

Highly Stable CuS Nanotrough‐Networks Constructed on Arbitrate Substrate for Flexible Supercapacitor

Herein, supercapacitors (SC) are fabricated on arbitrary flexible substrates by constructing copper sulfide nanotrough‐network and electrodeposition of polyaniline (PANI@CuS NN) as electrode. Attributed to its unique hollow structure, the CuS NN current collector demonstrates high conductivity (10.4 Ω □−1) and mechanical flexibility (within 2.3% relative resistance change after more than 1,000 bending times at the radius of 4 mm). The obtained PANI@CuS NN electrode demonstrates more than 218.28 F g−1 specific capacity at the current density of 1 A g−1, remaining 85.08% of the original capacity after 1,000 cycles (5 A g−1). This can be attributed to that CuS NN provides abundant attachment sites for PANI by the large specific surface area. Additionally, a flexible solid‐state supercapacitor is constructed by coating silk fibroin hydrogel as the electrolyte between the PANI@CuS NN and another CuS NN electrode loaded with multiwalled carbon nanotubes (MWCNTs@CuS NN). The capacity is 107.8 F g−1 at the current density of 0.25 A g−1 with excellent cycling stability.

Stretchable PU@CNT/MXene Fiber Fabricated by External Tension for High Performance Strain Sensor and Supercapacitor

AbstractFiber‐based electronic devices can be woven into textiles to provide comfort, durability, and integrated multi‐functionality, and will be widely used in the intelligent wearable field. Herein, by introducing elastic substrates and regulating the external tension during spinning to control the helical structure and optimize mechanical properties, multi‐functional Polyurethane@carbon nanotube (PU@CNT) stretchable fibers with high extensibility and flexibility is prepared for human motion detection and energy storage. A multi‐layered structure consisting of alternating MXene sheets and CNT films is also designed, to promote the relative resistance change of PU@CNT/MXene fibers to 212% (4 times than PU@CNT) when acting as strain sensors, whereas their volume specific capacitance reaches 196 F cm−3 (increases ≈50 times). Additionally, the capacitance of our stretchable fibers supercapacitors maintains at 91.4% and 86.2% when stretched statically (170% strain) or after hundreds of cycles (50% strain), respectively. It is proposed that MXene sheets can fill gaps between CNTs, resulting in more uniform distribution in fibers, which can improve strain sensitivity and long‐cycle capacitance retention. These PU@CNT/MXene fibers with unique multi‐layered hybrid structure and superior performance have potential applications in portable, wearable, and body‐integrated sensors and energy storage devices.