Silicone Reinforced Fabric Sensor with Dual‐Measurement Method: For Human Physiological Signal Detection and Robot Motion Detection

Stretchable strain sensors with high sensitivity or gauge factor (GF), large stretchability, and long-term durability are highly demanded in human motion detection, artificial intelligence, and electronic skins. Nevertheless, to develop high-sensitive sensors without sacrificing the stretchability cannot be realized using simple device configurations. In this work, an acid-interface engineering (AIE) method was proposed to develop a stretchable strain sensor with high GF and large stretchability. The AIE generates a layer of SiO x at the interface between the carbon nanotube (CNT) film and Ecoflex, playing a key role in enhancing the sensor's GF. Compared to devices without AIE (GF = 2.4), the ones with AIE are significantly improved. At an AIE time of 10 min, the GF up to 1665.9 is achieved without sacrificing the stretchability (>100%). The AIE-generated cracks are found to modulate the electrical behaviors and enhance the GFs of sensors with AIE through the crack-induced rapid reduction in the electrical conduction pathway, which is manipulated by the CNTs bridging over the cracks. The device with AIE proves its high mechanical durability through a cycling test (>10 000 cycles) at a high strain up to ∼80%, further paving its practical applications in various human motion detections.

Demand for highly compliant mechanical sensors for use in the fields of robotics and wearable electronics has been constantly rising in recent times. Carbon based materials, and especially, carbon nanotubes, have been widely studied as a candidate piezoresistive sensing medium in these devices due to their favorable structural morphology. In this paper three different carbon based materials, namely carbon black, graphene nano-platelets, and multi-walled carbon nanotubes, were utilized as large stretch sensors capable of measuring stretches over 250%. These stretch sensors can be used in robotic hands/arms to determine the angular position of joints. Analysis was also carried out to understand the effect of the morphologies of the carbon particles on the electromechanical response of the sensors. Sensors with gauge factors ranging from one to 1.75 for strain up to 200% were obtained. Among these sensors, the stretch sensors with carbon black/silicone composite were found to have the highest gauge factor while demonstrating acceptable hysteresis in most robotic hand applications. The highly flexible stretch sensors demonstrated in this work show high levels of compliance and conformance making them ideal candidates as sensors for soft robotics.

The use of complex impedance spectroscopy measurements for improving strain sensor performance

Stretchable sensors hold great potential for monitoring plant physiological parameters and enabling crop identification in smart agriculture. However, achieving long-term, stable, reliable monitoring of plants in dynamic environments, as well as improving crop identification accuracy, remains a substantial challenge, primarily due to the limited biocompatibility of conventional stretchable sensors. Here, we present a highly stretchable and reliable strain sensor based on a graphene/Ecoflex composite. This sensor features a mesh structure that combines graphene’s high electrical conductivity and strain sensitivity with Ecoflex’s excellent stretchability, biocompatibility, and resistance to environmental degradation. By structural optimization, the sensor achieves high sensitivity (gauge factor = 138), a low detection limit (0.1% strain), and high reliability (over 1,500 cycles), along with waterproofing and resistance to both acidic and alkaline conditions. Furthermore, the sensor conforms tightly to various plant leaves and stems without hindering growth, enabling real-time monitoring of plant growth patterns and in situ detection of mechanical damage to predict plant stress. Moreover, assisted by deep learning, it precisely classifies 8 crop types with an accuracy of 95.2%. These demonstrate that stretchable sensors based on mesh graphene/Ecoflex can operate reliably in outdoor agricultural environments even in the face of variable climatic and chemical conditions, providing a practical platform for advancing plant phenomics and smart agricultural robotics.

This paper presents a thorough investigation of the longitudinal gauge factor (GF) at high doping level in columnar polycrystalline-Silicon (poly-Si) nanowire (NW) NEMS devices. It is shown that a high GF (more than 30) can be obtained with concentration about 1020 cm−3. This result is very promising for high volume, low fabrication cost NEMS devices. This high GF is due to the specific piezoresistive behavior of poly-Si when compared to c-Si. We modeled the mechanical properties of the poly-Si and compared them to electrical measurements in order to predict the optimum dopant concentration for high GF. The experimental extraction of the GF has been performed directly on NWs gauge thanks to a new non-destructive method presented in [1].

High electrical self-healing flexible strain sensor based on MWCNT- polydimethylsiloxane elastomer with high gauge factor and wide measurement range

High resolution screen-printing of carbon black/carbon nanotube composite for stretchable and wearable strain sensor with controllable sensitivity

There have been increasing demands and interests in stretchable sensors with the development of flexible or stretchable conductive materials. These sensors can be used for detecting large strain, 3D deformation, and a free-form shape. In this work, a stretchable conductive sensor has been developed using single-walled carbon nanotubes (SWCNTs) and monofunctional acrylate monomers (cyclic trimethylolpropane formal acrylate and acrylate ester). The suggested sensors have been fabricated using a screw-driven microdispensing direct-write (DW) technology. To demonstrate the capabilities of the DW system, effects of dispensing parameters such as the feed rate and material flow rate on created line widths were investigated. Finally, a stretchable conductive sensor was fabricated using proper dispensing parameters, and an experiment for stretchability and resistance change was accomplished. The result showed that the sensor had a large strain range up to 90% with a linear resistance change and gauge factor ∼2.7. Based on the results, it is expected that the suggested DW stretchable sensor can be used in many application areas such as wearable electronics, tactile sensors, 3D structural electronics, etc.

Hetero phase nanocomposite based posture sensor with stretchable connector-sensor interface

Featuring simple device structure, high sensitivity, and excellent reliability, stretchable resistive sensors have developed rapidly due to the high demand for flexible and wearable electronics. Nevertheless, it remains critically challenging to evaluate external stimuli using one simple device for diverse application scenarios. Here, a microstructure is engineered for a stretchable sensor by a facile replication/transferring and a prestretching/releasing process, enabling the device to have discrimination capabilities in the transverse direction (X‐axis) and longitudinal direction (Y‐axis). Consisting of silver nanowires (Ag NWs)/transition metal carbides (MXene)/poly(3,4‐ethylenedioxythiophene):poly (styrene‐sulfonate) (PEDOT:PSS) conducting layer and polydimethylsiloxane (PDMS)/Ecoflex elastomer, the microstructured sensor has a broad strain range of 120% along the X‐axis and a large gauge factor (GF) of 37.44 along the Y‐axis, and shows good stability during 1000 stretching/releasing cycles along two directions, indicating the excellent interfacial connection between the sensing layer and elastomer. As a result, taking advantages of distinct performance along two directions, the proposed stretchable sensor is demonstrated to monitor a variety of human movements and physical stimuli as a wearable and flexible device, revealing its promising potential in diverse application scenarios.

Development of stretchable wearable devices requires essential materials with high level of mechanical and electrical properties as well as scalability. Recently, silicone rubber-based elastic polymers with incorporated conductive fillers (metal particles, carbon nanomaterials, etc.) have been shown to the most promising materials for enabling both high electrical performance and stretchability, but the technology to make materials in scalable fabrication is still lacking. Here, we propose a facile method for fabricating a wearable device by directly coating essential electrical material on fabrics. The optimized material is implemented by the noncovalent association of multiwalled carbon nanotube (MWCNT), carbon black (CB), and silicon rubber (SR). The e-textile sensor has the highest gauge factor (GF) up to 34.38 when subjected to 40% strain for 5,000 cycles, without any degradation. In particular, the fabric sensor is fully operational even after being immersed in water for 10 days or stirred at room temperature for 8 hours. Our study provides a general platform for incorporating other stretchable elastic materials, enabling the future development of the smart clothing manufacturing.

Self-healing strain sensor based on silicone elastomer for human motion detection

Our objective was to demonstrate a microminiature magnetoelastic strain gauge that provides both strain and temperature signals without additional sensors. Iron based magnetoelastic materials were embedded within superelastic nickel/titanium (NiTi) tubing. NiTi stress was transferred to the ferrite, causing a permeability change sensed by a tiny coil. The coil/bridge was excited (70 KHz AC), synchronously demodulated, and amplified to produce a voltage output proportional to coil/ferrite impedance. A DC voltage was also applied and separately conditioned to provide an output proportional to coil resistance; this signal was used to provide thermal compensation. Controlled strains were applied and 6 Hz cyclic outputs recorded simultaneously from the magnetoelastic strain gauge and conventional foil strain gauges. The magnetoelastic strain gauge tracked the foil gauge with minimal hysteresis and good linearity over 600 microstrain; repeatability was approximately 1.5 microstrain. The magnetoelastic strain gauge's gauge factor was computed from delta inductance/original inductance under static strain conditions. Temperatures of 25-140 deg C resulted in an uncompensated shift of 15 microstrain/deg C, and compensated shift of 1.0 microstrain/deg C. A sensitive micro-magnetoelastic strain gauge was demonstrated using the same sensor to detect stress and temperature with no moving parts, high gauge factor, and good thermal stability.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

New materials for sputtered strain gauges

Atomic layered molybdenum disulfide (MoS2) is one of the most promising two-dimensional (2D) materials for next-generation microelectromechanical devices, including nanoelectromechanical sensors, flexible tactile sensors, and ultrasensitive miniaturized transducers (1). However, 2D MoS2-based sensors have not yet been put to practical use due to challenges in material synthesis for large areas and mass production of thin film fabrication technology. In this talk, we introduce the synthesis of the MoS2 thin films and the doping effect.Firstly, 2D MoS2 films were prepared using the general low-pressure chemical vapor deposition (LPCVD) method (2). As sources, MoO3 powder and high-purity (99%) sulfur powder were used, and those source temperatures were controlled at 690-710 ºC and 135 ºC under Ar carrier gas in a furnace, respectively. A 2D polycrystalline film was deposited on an oxidized Si substrate. The high-quality continuous films were synthesized at a pressure of 372 Pa. Higher growth pressure leads to higher vapor pressure of MoO3, increasing particle-like MoS2 nucleation. It was found from the observation of the E1 2g vibration mode of the in-plane vibration of Mo and S atoms that the monolayered film was successfully obtained. In order to evaluate the piezoresistivity, the MoS2 film was patterned, and electrodes were formed on the MoS2 resistive elements. A four-point bending method was utilized to apply strain to the MoS2 resistor. As a result, a gauge factor of ~104 was observed for the monolayered film, which is close to the gauge factor of an exfoliated MoS2 monolayer film, indicating that a high-quality monolayer was formed.One of the issues with the mass production of MoS2 films based on LPCVD is the narrow process windows of high-quality MoS2 films. Thus, we developed the simpler method of thin MoS2 film preparation using sputtering. In the first step, thin Mo/Vanadium(V)/Mo films were deposited by magnetron sputtering, and in the second step, sulfurization was performed in the sulfur vapor furnace at 750 ºC, as shown in Fig. 1 (3). As a result, V-doped MoS2 films of a few nanometers thickness with 1T-incorporated 2H structure were synthesized. The doping concentration can be controlled by the sputtering film thickness of the V layer (Fig. 2). The V-doping can reduce the resistivity of the films, which is beneficial to the sensor applications. From the XPS evaluation, S deficiency, which causes the binding energy downshift of the Mo 3d peak, is observed. Piezoresistive elements for evaluation of the piezoresistive effect were prepared as well, as shown in Fig. 3. Observation of piezoresistivity shows the highest gauge factor of ~140 at the V concentration of 15 wt.% (Fig. 4). This two-step film synthesis provides good film uniformity and compatibility with microfabrication, and the doping enhances the piezoresistive performance. References (1) M. Zhu, X. Du, S. Liu, J. Li, Z. Wang, T. Ono, “A review of strain sensors based on two-dimensional molybdenum disulfide, J. Mater. Chem. C 9, 9083, (2021).(2) M. Zhu, K. Sakamoto, J. Li, N. Inomata, M. Toda, T. Ono, " Piezoresistive strain sensor based on monolayer molybdenum disulfide continuous film deposited by chemical vapor deposition", J. Micromech. Microeng. 29, 055002 (2019).(3) M. Zhu, J. Li, N. Inomata, M. Toda, and T. Ono, " Vanadium-doped molybdenum disulfide film-based strain sensors with high gauge factor ", Appl. Phys. Exp. 12, 015003 (2019) Figure 1