A sparse-dense hierarchical structured conductive elastomer composites for highly sensitive and stretchable strain sensor

With the rapid development of soft electronics, flexible and stretchable strain sensors are highly desirable. However, coupling of high sensitivity and stretchability in a single strain sensor remains a challenge. Herein, a kind of conductive elastomer is constructed with poly(dimethylsiloxane) (PDMS) and silylated cellulose nanocrystal (SCNC)/carbon nanotube (CNT) nanohybrids through a facile one-pot solution-casting method. The hydrophobic SCNCs can effectively facilitate the dispersion of CNTs in PDMS and synergistically improve the interfacial compatibility between CNTs and the PDMS matrix, resulting in favorable stress and electron transfer in the polymer network. Due to the outstanding electrical conductivity of CNTs and the excellent dispersity and high mechanical performance of SCNCs, combined with the good compatibility between SCNC-mediated carbon nanotubes (SCNC-CNTs) and PDMS, the resulting composite elastomer (SCNC-CNT/PDMS) shows high electrical conductivity (∼2.77 S m-1), tensile strength (∼5.72 MPa), and fatigue resistance properties. The strain sensor assembled by SCNC-CNT/PDMS demonstrates a high strain range above 100%, appealing strain sensitivity with a gauge factor of 37.11 at 50-100% strain, and long-term stability and durability, which is capable of monitoring both real-time human motions and acoustic vibrations. This work paves a new way for the design and controllable preparation of flexible and stretchable conductive elastomers, demonstrating promising applications in wearable devices and intelligent electronics.

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

Stretchable flexible strain sensors based on conductive elastomers are rapidly emerging as a highly promising candidate for popular wearable flexible electronic and soft-mechanical sensing devices. However, due to the intrinsic limitations of low fidelity and high hysteresis, existing flexible strain sensors are unable to exploit their full application potential. Herein, a design strategy for a successive three-dimensional crack conductive network is proposed to cope with the uncoordinated variation of the output resistance signal arising from the conductive elastomer. The electrical characteristics of the sensor are dominated by the successive crack conductive network through a greater resistance variation and a concise sensing mechanism. As a result, the developed elastomer bionic strain sensors exhibit excellent sensing performance in terms of a smaller overshoot response, a lower hysteresis (∼2.9%), and an ultralow detection limit (0.00179%). What's more, the proposed strategy is universal and applicable to many conductive elastomers with different conductive fillers (including 0-D, 1-D, and 2-D conductive fillers). This approach improves the sensing signal accuracy and reliability of conductive elastomer strain sensors and holds promising potential for various applications in the fields of e-skin and soft robotic systems.

A highly flexible and stretchable strain sensor has been prepared by coating chemical reduction of graphene oxide on electrospun polyurethane nanofiber mats. The sensor exhibits an ohmic behavior regardless of applied strains and the current monotonically increases with the increase of the tensile strain. The morphology and stability of electrospun polyurethane nanocomposite mats were also studied. The flexible and stretchable strain sensor has great potential for practical application such as efficient human-motion detection. This cheap and simple process of graphene layer provides an effective fabrication for graphene stretchable electronic devices and strain sensors due to excellent stability and electrical proper.

Flexible and stretchable strain sensors can be applied for human health monitoring and disease diagnoses via the output of multiple biophysical signals. However, it is still a challenge to fabricate short-peptide-based strain sensors. Here, we prepared a novel polymer-dipeptide double-network hydrogel with excellent stretchability, responsiveness, and stability. The poly(acrylic acid) (PAA) gel, by cross-linking, maintains mechanical and flexible properties, and the fluorenyl methoxycarbonyl-diphenylalanine (Fmoc-FF) network, by non-covalent interactions, is helpful for energy dissipation. With increasing tensile or compression strains, the PAA/Fmoc-FF hydrogel exhibited a high mechanical strength and fast recovery. Moreover, as the presence of KCl improves the electronic conductivity, the hybrid gel exhibited a cyclic strain-stress performance, which is the foundation of a strain sensor. Based on that, its application as a motion sensor was demonstrated by monitoring the movements of human joints, such as the forefinger, wrist, elbow, and knee. Consequently, the hybrid polymer-peptide gel could be an ideal candidate for wearable sensors in the future.

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.

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.

This article presents the design, fabrication and characterisation of an elastomeric flexible and stretchable strain sensor using a variable resistance fabric. This provides a viable alternative to the microfluid and nanoparticle based flexible strain sensors involving complex fabrication techniques. A nylon-spandex based stretchable fabric serves as the sensing element which is in turn embedded in an elastomeric substrate. The fabricated sensor is experimentally characterised for determining the linearity, hysteresis, stretchability and gauge factor. The Finite Element Method based simulations to predict the operational force range of the sensor corresponding to its strain sensing range has been validated with the experimental results. These sensors have a wide range of potential applications in the constantly developing field of compliant robotics and mechanisms. In future works, these sensors will be utilized in the biomechanical analysis of human movements to track the various joint parameters.

Wrinkled, cracked and bridged carbon networks for highly sensitive and stretchable strain sensors

Silver nanowires (AgNWs), having excellent electrical conductivity, transparency, and flexibility in polymer composites, are reliable options for developing various sensors. As transparent conductive electrodes (TCEs), AgNWs are applied in optoelectronics, organic electronics, energy devices, and flexible electronics. In recent times, research groups across the globe have been concentrating on developing flexible and stretchable strain sensors with a specific focus on material combinations, fabrication methods, and performance characteristics. Such sensors are gaining attention in human motion monitoring, wearable electronics, advanced healthcare, human-machine interfaces, soft robotics, etc. AgNWs, as a conducting network, enhance the sensing characteristics of stretchable strain-sensing polymer composites. This review article presents the recent developments in resistive stretchable strain sensors with AgNWs as a single or additional filler material in substrates such as polydimethylsiloxane (PDMS), thermoplastic polyurethane (TPU), polyurethane (PU), and other substrates. The focus is on the material combinations, fabrication methods, working principles, specific applications, and performance metrics such as sensitivity, stretchability, durability, transparency, hysteresis, linearity, and additional features, including self-healing multifunctional capabilities.

Flexible, stretchable, and wearable strain sensors have attracted significant attention for their potential applications in human movement detection and recognition. Here, we report a highly stretchable and flexible strain sensor based on a single-walled carbon nanotube (SWCNTs)/carbon black (CB) synergistic conductive network. The fabrication, synergistic conductive mechanism, and characterization of the sandwich-structured strain sensor were investigated. The experimental results show that the device exhibits high stretchability (120%), excellent flexibility, fast response (∼60 ms), temperature independence, and superior stability and reproducibility during ∼1100 stretching/releasing cycles. Furthermore, human activities such as the bending of a finger or elbow and gestures were monitored and recognized based on the strain sensor, indicating that the stretchable strain sensor based on the SWCNTs/CB synergistic conductive network could have promising applications in flexible and wearable devices for human motion monitoring.

Simultaneously achieving high piezoresistive sensitivity, stretchability, and good electrical conductivity in conductive elastomer composites (CECs) with carbon nanofillers is crucial for stretchable strain sensor and electrode applications. Here, we report a facile and environmentally friendly strategy to realize these three goals at once by using branched carbon nanotubes, also known as the carbon nanostructure (CNS). Inspired by the brick-wall structure, a robust segregated conductive network of a CNS is formed in the thermoplastic polyurethane (TPU) matrix at a very low filler fraction, which renders the composite very good electrical, mechanical, and piezoresistive properties. An extremely low percolation threshold of 0.06 wt %, currently the lowest for TPU-based CECs, is achieved via this strategy. Meanwhile, the electrical conductivity is up to 1 and 40 S/m for the composites with 0.7 and 4 wt % CNS, respectively. Tunable piezoresistive sensitivity dependent on CNS content is obtained, and the composite with 0.7 wt % filler has a gauge factor up to 6861 at strain ε = 660% (elongation at break is 950%). In addition, this strategy also renders the composites' attractive tensile modulus. The composite with 3 wt % CNS shows 450% improvement in Young's modulus versus neat TPU. This work introduces a facile strategy to fabricate highly stretchable strain sensors by designing CNS network structures, advancing understanding of the effects of polymer-filler interfaces on the mechanical and electrical property enhancements for polymer nanocomposites.

In intelligent manufacturing and robotic technology, various sensors must be integrated with equipment. In addition to traditional sensors, stretchable sensors are particularly attractive for applications in robotics and wearable devices. In this study, a piezoresistive stretchable strain sensor based on laser-induced graphene (LIG) was proposed and developed. A three-dimensional, porous LIG structure fabricated from polyimide (PI) film using laser scanning was used as the sensing layer of the strain sensor. Two LIG pattern structures (parallel and vertical) were fabricated and integrated within the LIG strain sensors. Scanning electron microscopy, an X-ray energy dispersive spectrometer, and Raman scattering spectroscopy were used to examine the microstructure of the LIG sensing layer. The performance and strain sensing properties of the parallel and vertical stretchable LIG strain sensors were investigated in tensile tests. The relative resistance changes and the gauge factors of the parallel and vertical LIG strain sensors were quantified. The parallel strain sensor achieved a high gauge factor of 15.79 in the applied strain range of 10% to 20%. It also had high sensitivity, excellent repeatability, good durability, and fast response times during the tensile experiments. The developed LIG strain sensor can be used for the real-time monitoring of human motions such like finger bending, wrist bending, and throat swallowing.

Stretchable strain sensors that can detect and monitor human motion have attracted enormous attention in the field. To realize fully the translation of materials for stretchable strain sensors, it is important to make the fabrication way less complex and lessen the production cost. This paper investigated low-cost graphite-VHB strain sensors prepared by simple friction-transfer method. Designed by friction-transfer method, the graphite-VHB strain sensor possesses good flexibility, light weight, high stretchability and tunable range of GF. The gauge factor of the sample was 20 when strain rate was 3% and could be over 1000 when the strain rate was 50%. The addition of protection layer and pre-aging process have contributions to expand the GF of the sample. By attaching our strain sensor to the sample of mechanical arm, the resistance change of the sensor perfectly fitted to the deformation of the arm, which shows its potential in application of soft robotics and mechanical fields. The results showed the friction-transfer method is a simple and low-cost method, featured with environmental advantage that avoids chemical wastes.

Flexible strain sensor has promising features in successful application of health monitoring, electronic skins and smart robotics, etc. Here, we report an ultrasensitive strain sensor with a novel crack-wrinkle structure (CWS) based on graphite nanoplates (GNPs)/thermoplastic urethane (TPU)/polydimethylsiloxane (PDMS) nanocomposite. The CWS is constructed by pressing and dragging GNP layer on TPU substrate, followed by encapsulating with PDMS as a protective layer. On the basis of the area statistics, the ratio of the crack and wrinkle structures accounts for 31.8% and 9.5%, respectively. When the sensor is stretched, the cracks fracture, the wrinkles could reduce the unrecoverable destruction of cracks, resulting in an excellent recoverability and stability. Based on introduction of the designed CWS in the sensor, the hysteresis effect is limited effectively. The CWS sensor possesses a satisfactory sensitivity (GF = 750 under 24% strain), an ultralow detectable limit (strain = 0.1%) and a short respond time of 90 ms. For the sensing service behaviors, the CWS sensor exhibits an ultrahigh durability (high stability > 2×104 stretching-releasing cycles). The excellent practicality of CWS sensor is demonstrated through various human motion tests, including vigorous exercises of various joint bending, and subtle motions of phonation, facial movements and wrist pulse. The present CWS sensor shows great developing potential in the field of cost-effective, portable and high-performance electronic skins.