Capacitive Signal Research Articles

A Highly Sensitive, Conductive, and Flexible Hydrogel Sponge as a Discriminable Multimodal Sensor for Deep‐Learning‐Assisted Gesture Language Recognition

AbstractFlexible multimodal sensors have gained increasing popularity for applications in healthcare and extreme environment operations owing to their all‐around environmental perception and data acquisition capabilities. However, fabricating a magnetism‐mechanics‐humidity multimodal sensor that possesses high sensitivity without signal overlapping while in a facile methodology remains a great challenge. Herein, a highly sensitive, conductive, and flexible hydrogel sponge sensor with discriminable magnetism, mechanics, and humidity sensing capability is proposed, which shows stable pore size (19.30 µm) and satisfactory mechanical properties based on the synergistic hydrogen bonding among sodium alginate, poly(vinyl alcohol) and glycerol. The proposed sensors can not only display favorable humidity sensing ability with rapid response/recovery time (2.5/4 s) but also possess enhanced sensitivities (a gauge factor of 0.46 T−1 for magnetic field, −1.16 kPa−1 for pressure), superior stability and durability (over 8000 cycles). Benefiting from the separated capacitive and resistive response signals, the sensors can precisely distinguish the magnetic, mechanical, and humidity stimuli without cross‐talk. Further, the sensor arrays assisted by the deep learning algorithm are developed to realize gesture language recognition with a high accuracy of 99.17%. It can be believed that this high‐performance sensor will have good prospects in future soft electronics and human‐machine interaction systems.

Balancing stretchability and conductivity: Carbon nanotube layer-enhanced non-ionic conductive hydrogels with a sandwich structure

In non-ionic conductive hydrogels, conductivity often conflicts with mechanical properties, posing a challenge in creating hydrogels that strike a balance between conductivity and mechanical properties. Here, we addressed this issue by integrating carboxylated carbon nanotube (CNT) layers in a sandwich structure onto the surface of the cationic hydrogel (P(AM-co-DMDAAC), PAD), followed by dehydration for densification. This approach simultaneously improved the conductivity of the hydrogel while maintaining its stretchability. The stability of the composite structure was ensured through electrostatic interactions, hydrogen bonding, and physical entanglement between the PAD hydrogel and the CNT layer. Additionally, adjusting the water–solid ratio of the hydrogel allows fine-tuning of its conductivity and stretchability. Remarkably, at a 500 % water–solid ratio, the hydrogel achieved a conductivity of 2.3 S/m. By further reducing the water content, the conductivity could be increased to 25 S/m, significantly higher than similar hydrogels. The PAD-CNT hydrogel found applications in swelling response, tunable resistance wires, and as strain sensors. Notably, the sandwich structure enables the PAD-CNT hydrogel to output capacitive signals, exhibiting a more sensitive response in monitoring human movement. Overall, this study introduces a novel composite structure for conductive hydrogels, holding tremendous potential in flexible conductivity and smart response fields.

A method to calculate the two-phase distribution in a microchannel heat exchanger

Maldistribution seriously impacts the heat transfer performance of the microchannel heat exchanger (MCHX). Fully understanding the two-phase distribution in the heat exchanger is important for advancing academic research and engineering applications. This study introduces a novel method for quantifying two-phase distribution in a microchannel heat exchanger. An experimental setup was developed to measure the local vapor mass fraction in the heat exchanger header. Capacitance signals were measured under inlet vapor mass fractions from 0 to 1, and inlet flow rates of 10, 15, and 25 g s−1 corresponding to a mass flux of 17.47, 26.2, and 43.66 kg m−2s−1, respectively. The local vapor mass fraction in the header was estimated using the capacitance measurements. The mass flow rate in the header, the microchannel tube, and the vapor mass fraction in the tube were calculated using the proposed model. The calculation model was validated against literature data, and the results were analyzed. The analysis reveals the characteristics of vapor mass fraction and mass flow rate distribution in the MCHX and further elaborates on the effects of phase separation, entrainment ratio, and pressure drop balance on the distribution. The proposed method can evaluate distribution in the header and tubes of microchannel heat exchangers, and it is also applicable to other types of two-phase flow devices.

Phase fraction measurement of gas-liquid flow based on cyclonic capacitive sensor (CCS) and inverse drift flux method

Abstract As the demand for clean energy sources like natural gas increases, the need for methods to measure the phase fraction of gas-liquid two-phase flows, which have complex and variable flow patterns, becomes more pronounced. This study introduces a phase fraction measurement method based on a cyclonic capacitive sensor. An effective phase fraction prediction model for the gas-liquid flow measurement section is established based on the capacitive signals. Using the drift flux principle, the dimensionless drift velocity of the spiral annular flow is analyzed, and a volume gas fraction (GVF) prediction model based on the ‘inverse drift flux method’ is established. Experimental results show that this method is suitable for a wide range of phase fraction measurements and is highly accurate. When the liquid volume fraction (LVF) is less than 10%, the average relative error of the predicted GVF is 0.3%; for the LVF from 10% to 30%, the average relative error is 1.4%.

Processing of capacitive sensor signal profile for enhancing gestures detection

Processing of capacitive sensor signal profile for enhancing gestures detection

A porous elastomer with a cavity array for three-dimensional plantar force sensing

Human gait is an important indicator for diagnosis of various movement-related disorders. Three-dimensional (3D) plantar force monitoring enables real-time recording of contact forces in both the normal and tangential directions in each step, providing useful clues for identification of irregular gait patterns or gait-related health issues. Most flexible sensors for gait monitoring focus on contact forces in the normal direction, being limited to detect forces in the tangential direction. Herein we have developed a porous elastomer with a cavity array (PECA) to measure 3D contact forces for gait analysis. The sensor consists of two fabric electrodes and a dielectric layer of porous Ecoflex elastomer containing a 5 × 5 cavity array, forming four parallel-plate capacitor units in a soft construction. Therefore, a 3D force in any arbitrary direction generates four unique capacitive electrical signals. Importantly, the PECA includes a well-designed combination of micrometer-scale pores and millimeter-sized cavities, dramatically improving the detection sensitivity and linear elasticity range. We have demonstrated high sensitivities of 0.41 N−1 and 0.23 N−1 in the normal and tangential directions, respectively. The flexible PECA-based sensors have been tested to sensitively distinguish various standing postures and different stride lengths for gait analysis, promising reliable translation into areas such as healthcare monitoring, sports training, and rehabilitation engineering.

Origami single-end capacitive sensing for continuous shape estimation of morphing structures

In this work, we propose a novel single-end morphing capacitive sensing method for shape tracking, FxC, by combining Folding origami structures and Capacitive sensing to detect the morphing structural motions using state-of-the-art sensing circuits and deep learning. It was observed through embedding areas of origami structures with conductive materials as single-end capacitive sensing patches, that the sensor signals change coherently with the motion of the structure. Different from other origami capacitors where the origami structures are used in adjusting the thickness of the dielectric layer of double-plate capacitors, FxC uses only a single conductive plate per channel, and the origami structure directly changes the geometry of the conductive plate. We examined the operation principle of morphing single-end capacitors through 3D geometry simulation combined with physics theoretical deduction, which deduced similar behavior as observed in experimentation. Then a software pipeline was developed to use the sensor signals to reconstruct the dynamic structural geometry through data-driven deep neural network regression of geometric primitives extracted from vision tracking. We created multiple folding patterns to validate our approach, based on folding patterns including Accordion, Chevron, Sunray and V-Fold patterns with different layouts of capacitive sensors using paper-based and textile-based materials. Experimentation results show that the geometry primitives predicted from the capacitive signals have a strong correlation with the visual ground truth with R-squared value of up to 95% and tracking error of 6.5 mm for patches. The simulation and machine learning constitute two-way information exchange between the sensing signals and structural geometry. By embedding part of the origami surface with morphing single-end capacitive sensors, FxC presents a unique solution that leverages both the mechanical properties of origami and sensing properties of capacitive sensing.

Dependence of frequency-temperature stability on support tethers in dual-beam piezoresistive sensing MEMS resonators

This paper investigates the dependence of frequency stability over temperature on support tethers in dual-beam piezoresistive length-extensional (LE) mode microelectromechanical systems (MEMS) resonators. The designed dual-beam resonator consists of two identical single-crystal silicon beams, which are mechanically coupled and excited into vibrating in opposite phase to eliminate the inherent capacitive feedthrough signals. Both straight and folded beams are adopted as the support tethers for the reported dual-beam piezoresistive resonators. Quality factor (Q) and temperature distribution across the resonators with various support tethers are investigated by finite element method (FEM) analysis. It is found that folded beam tethers can reduce the support loss and hence improve the Q for the designed dual-beam resonator, while it comes with a tradeoff of high temperature rise on resonator body. The reported dual-beam resonator with straight beam tethers has low temperature rise on the resonator body, which is less sensitive to environmental temperature fluctuations, compared to its counterpart with folded beam tethers. Experimental results show that the fabricated dual-beam piezoresistive resonator with four straight beam tethers achieves a 0.5 ppm frequency shifts in the temperature-control chamber, which is nearly four times better than those with folded beam tethers.

Biosustainable Multiscale Transparent Nanocomposite Films for Sensitive Pressure and Humidity Sensors.

Light weight, thinness, transparency, flexibility, and insulation are the key indicators for flexible electronic device substrates. The common flexible substrates are usually polymer materials, but their recycling is an overwhelming challenge. Meanwhile, paper substrates are limited in practical applications because of their poor mechanical and thermal stability. However, natural biomaterials have excellent mechanical properties and versatility thanks to their organic-inorganic multiscale structures, which inspired us to design an organic-inorganic nanocomposite film. For this purpose, a bio-inspired multiscale film was developed using cellulose nanofibers with abundant hydrophilic functional groups to assist in dispersing hydroxyapatite nanowires. The thickness of the biosustainable film is only 40 μm, and it incorporates distinctive mechanical properties (strength: 52.8 MPa; toughness: 0.88 MJ m-3) and excellent optical properties (transmittance: 80.0%; haze: 71.2%). Consequently, this film is optimal as a substrate employed for flexible sensors, which can transmit capacitance and resistance signals through wireless Bluetooth, showing an ultrasensitive response to pressure and humidity (for example, responding to finger pressing with 5000% signal change and exhaled water vapor with 4000% signal change). Therefore, the comprehensive performance of the biomimetic multiscale organic-inorganic composite film confers a prominent prospect in flexible electronics devices, food packaging, and plastic substitution.

Eliminating Resistance-Capacitance Coupling Shielding for Depicting the Defect Landscape in Perovskite Solar Cells by Capacitance Spectroscopy.

Capacitance spectroscopy techniques have been widely utilized to evaluate the defect properties in perovskites, which contribute to the efficiency and operation stability development for perovskite solar cells (PSCs). Yet the interplay between the charge transporting layer (CTL) and the perovskite on the capacitance spectroscopy results is still unclear. Here, they show that a pseudo-trap-state capacitance signal is generated in thermal admittance spectroscopy (TAS) due to the enhanced resistance capacitance (RC) coupling caused by the carrier freeze-out of the CTL in PSCs, which could be discerned from the actual defect-induced trap state capacitance signal by tuning the series resistance of PSCs. By eliminating the RC coupling shielding effect on the defect-induced capacitance spectroscopy, it is obtain the actual defect density which is 4-folds lower than the pseudo-trap density, and the spatial distribution of defects in PSCs which reveals that the commonly adopted interface passivators can passivate the defects about 60 nm away from the decorated surface. It is further revealed that phenethylammonium ions (PEA+) possess a better passivation capability over octylammonium ions (OA+) due to the deeper passivation depth for PEA+ on the surface defects in perovskite films.

Electronic tongue made of gelatin self-supporting films on printed electrodes to detect lactose

An electronic tongue was developed for the detection of lactose content in commercial foods. This was accomplished by employing optimized detection units comprised of gelatin films and information visualization methods for data analysis. The films incorporating gelatin, tannic acid, and zein, served as the basis for the sensors, whose electrodes were screen printed using carbon black ink. Self-supporting films were produced using various combinations of these materials, some of which had limited solubility in water (from 33% to 36%). They were hydrophobic and yielded reproducible electrical impedance spectra to be used as sensing units. Lactose detection experiments were conducted using various standard concentrations and commercial food samples. Capacitance decreased with lactose concentration at low frequencies, with films lacking a hydrophobic coating showing higher capacitance signals (exceeding 200 nF). Low limits of detection were obtained for the most sensitive films, as low as 2.03 × 10−19 mol/L, comparable to existing biosensors to detect lactose. Combining data from four sensing units in an electronic tongue allowed for the differentiation of lactose concentrations ranging from 1 × 10−20 mol/L to 1 × 10−6 mol/L using the interactive document mapping (IDMAP) projection technique, leading to a silhouette coefficient of 0.716. The discriminatory power of the electronic tongue was validated by distinguishing between lactose-containing and lactose-free food products. These findings highlight the potential of electronic tongues made with sustainable materials for applications in food quality assessment and lactose intolerance management.

Quantification of two-phase flow in a microchannel heat exchanger header based on capacitance measurement

The two-phase distribution has always been one of the research focuses of microchannel heat exchangers, because the maldistribution could seriously affect the heat transfer performance. This paper developed a capacitive method to quantify the two-phase distribution of vapor-liquid flow in the aluminum header of a microchannel heat exchanger. A flexible capacitance sensor was used to quantify the two-phase distribution with R134a. The inlet mass fluxes vary from 17.47 kg m−2 s−1 to 52.42 kg m−2 s−1 and inlet vapor qualities range from 0.2 to 0.6. The normalized time-averaged capacitance signal was processed by a Gaussian function to obtain the mathematical expectation and the standard deviation. The local vapor quality in the header was solved for using a correlation derived from the pre-experiments. The distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. The local vapor quality reached a minimum and remained constant at 0.1 in the middle and lower reaches, unaffected by changes in inlet conditions. As the mass flux increased, both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. Additionally, the increase of mass flux caused the fluctuation of the standard deviation increased, which meant the stability of the flow decreased. The two-phase flow measurement technique based on flexible capacitance sensors proposed in this study realized the extension of the capacitive method from the experimental stage to the industrial application stage.

Design and Scalable Fast Fabrication of Biaxial Fabric Pouch Motors for Soft Robotic Artificial Muscle Applications

Soft pouch motors, engineered to mimic the natural movements of skeletal muscles, play a crucial role in advancing robotics and exoskeleton development. However, the fabrication techniques often involve multistage processes; they lack soft sensing capabilities and are sensitive to cutting and damage. This work introduces a new textile‐based pouch motors with the capacity for biaxial actuation and capacitive sensory functions, achieved through the application of computerized knitting technology using ultrahigh molecular weight polyethylene yarn (Spectra) and conductive silver yarns. This method enables the rapid and scalable mass fabrication of robust pouch motors. The resulting pouch motors exhibit maximum lifting capacity of 10 kg, maximum contraction of 53.3% along the y‐axis, and transverse extension of 41.18% along the x‐axis at 50 kPa pressure. Finite element analysis closely matches the experimental data. The capacitance signals in relation to contraction motion are well suited for detecting air pressure levels and hold promise for applications requiring robotic control. Notably, it effectively elevates an ankle joint simulator at a 20° angle, highlighting its potential for applications such assisting individuals with foot drop. This study presents a practical demonstration of the soft ankle exosuit designed to provide lifting support for individuals facing this mobility challenge.

A Novel Microfluidics Droplet-Based Interdigitated Ring-Shaped Electrode Sensor for Lab-on-a-Chip Applications.

This paper presents a comprehensive study focusing on the detection and characterization of droplets with volumes in the nanoliter range. Leveraging the precise control of minute liquid volumes, we introduced a novel spectroscopic on-chip microsensor equipped with integrated microfluidic channels for droplet generation, characterization, and sensing simultaneously. The microsensor, designed with interdigitated ring-shaped electrodes (IRSE) and seamlessly integrated with microfluidic channels, offers enhanced capacitance and impedance signal amplitudes, reproducibility, and reliability in droplet analysis. We were able to make analyses of droplet length in the range of 1.0-6.0 mm, velocity of 0.66-2.51 mm/s, and volume of 1.07 nL-113.46 nL. Experimental results demonstrated that the microsensor's performance is great in terms of droplet size, velocity, and length, with a significant signal amplitude of capacitance and impedance and real-time detection capabilities, thereby highlighting its potential for facilitating microcapsule reactions and enabling on-site real-time detection for chemical and biosensor analyses on-chip. This droplet-based microfluidics platform has great potential to be directly employed to promote advances in biomedical research, pharmaceuticals, drug discovery, food engineering, flow chemistry, and cosmetics.

A novel self-powered sensitive porous ZnO NWs/PDMS sponge capacitive pressure sensor

The demand for flexible sensors has increased due to the explosive expansion of flexible electronic applications, but there are concerns that the development of high-precision measurement is limited. Capacitive pressure sensors are extensively utilized among diverse sensors due to their simple structure, high resolution, and outstanding dynamic response characteristics. Therefore, by sandwiching a porous ZnO NWs PDMS (PZP) sponge dielectric layer between two ITO/PET electrodes, a flexible capacitive pressure sensor with high sensitivity, short response time, and wide working range was prepared. The porous PDMS sponge was prepared utilizing the salt particle as a template, then ZnO NWs in different precise proportions were added to it. The sensitivity of PZP sponge capacitive pressure sensors with different salt particle sizes and ZnO NWs dosages were compared. The research results indicate that when salt particle sizes less than 150 μm, and the amount of ZnO NWs in PZP sponge is increased to 200 mg, the sensor sensitivity is highest in the pressure range of 2.5-7.5 kPa, reaching 0.1279 kPa-1, response time less than 70 ms. When the PZP sponge sensor is used as a self-powered source, it achieves the best output of 1.2 V and 6.8 nA at 5.74 kPa pressure. It provides a new way to tackle the challenge of low sensitivity of flexible sensors to capacitive signal and lays a foundation for expanding the application scenarios of ZnO NWs.

Simultaneous detection of the shuttling motion of liquid metal droplets in channels under alternating pressure and capacitive sensor signals

Implementing a signal-switching mechanism for the selective use of integrated sensors and actuators plays a crucial role in streamlining the functionality of miniaturized devices. Here, a liquid metal droplet (LMD)-based signal-switching mechanism is introduced to achieve such functionality. Pressure modulation with a 100-μm spatial resolution enabled precise control of the position of the LMDs within a channel. After integrating the channel with asymmetrically structured electrodes, the effect of the shuttle-like movement of LMD on the temporal changes in the overall capacitance was investigated. Consequently, analysis of the capacitive peaks revealed the directional movement of the LMDs, enabling estimation of the position of the LMDs without direct observation. In addition, we achieved successful signal extraction from the capacitive sensor that was linked to the activated electrodes, thereby enabling selective data retrieval. The proposed signal-switching mechanism method achieved a detection accuracy of ~0.1 pF. The sensor’s ability to simultaneously detect the LMD position and generate a signal underscores its significant potential for multiplexing in multisensing systems, particularly in concealed environments, such as in vivo settings.

ECT image reconstruction based on sensitive field expansion and optimization

Electrical capacitance tomography (ECT) is an efficient method for addressing the issue of two-phase flow monitoring. Most current methods result in low image reconstruction accuracy due to soft field issues. This paper propose an ECT image reconstruction method based on sensitive field expansion and optimization, which improve reconstruction efficiency and accuracy. Firstly, a sensitivity field optimization method based on flow pattern identification was proposed. The flow pattern recognition determines the flow pattern to which the input signal belongs and selects the sensitive field for the corresponding flow pattern. Secondly, a sensitive field expansion method based on feature extraction is proposed. The ECT image is modeled as a finite rate of innovation signal, sampled using an exponential reproducing kernel. Feature information is extracted from the input signal for data fusion, and the optimized sensitive field is expanded into a new sensitive field distribution matrix by zero-padding and random reorganization. Then, based on optimized and expanded sensitive fields, a sparse image reconstruction method is proposed. Image reconstruction by constructing comprehensive observation equation to obtain the original permittivity distribution vector of the ECT system using the sparsity of the sensitivity matrix and capacitance signal. Finally, the ECT hardware system and the upper computer measurement and imaging integration software are designed. The experimental results show that the method outperforms other existing algorithms in terms of imaging indexes, has better imaging results.

Determination of garlic clove orientation based on capacitive sensing technology

Garlic is a versatile crop of high economic value, with an increase in the growth of the cultivation scale. Planting garlic with bottom-side (root) down and pointy-tip (clove) up is essential in agriculture because the orientation of the clove significantly affects garlic quality. In this study, we propose a clove orientation recognition technique based on capacitance sensing technology, where clove states are determined by utilizing characteristic differences in capacitance variations associated with different orientations of cloves. First, we applied Maxwell simulations to obtain capacitance variation during the falling process. Second, we conducted field experiments using the capacitive sensing device to obtain the capacitance variations during the falling of garlic in an unstructured environment. Third, the continuous capacitance signal collected during the field experiment was segmented into short-term feature signals containing individual garlic fallings. Finally, Long Short-Term Memory (LSTM) and Frequency-Residual-GoogLeNet (F-Res-GoogLeNet) deep models were trained to recognize garlic falling states. Our best model achieved an accuracy of 96.75%, which meets the agricultural requirements for garlic cultivation. This study demonstrates that monitoring capacitance data in field environments can identify garlic cloves’ orientation, eventually enhancing garlic’s final yield, quality, and economic benefits.

A Scanning Microwave Impedance Microscopy Study of α‐In2Se3 Ferroelectric Semiconductor

AbstractVan der Waals ferroelectric semiconductors, which encompass both ferroelectricity and semiconductivity, have garnered intensive research interests for developing novel non‐volatile functional devices. Previous studies focus on ferroelectricity characterization and device demonstration, with little attention paid to the fundamental electronic properties of these materials and their functional structures, which are essential for both device design and optimization. In this study, scanning microwave impedance microscopy (sMIM) is utilized to investigate the ferroelectric semiconductor of α‐phase indium selenide (α‐In2Se3) and its synaptic field effect transistors. α‐In2Se3 nanoflakes of varying thicknesses are visualized through capacitive signal detection, whose responses are consistent with finite element simulations manifesting dependence on both flake thickness and its semiconductor property. sMIM spectroscopy performed on α‐In2Se3‐based metal‐oxide‐semiconductor (MOS) structures reveals typical MOS capacitance‐voltage characteristics, with additional hysteresis arising from the ferroelectric switching of α‐In2Se3. The local conductance state changes of synaptic α‐In2Se3 ferroelectric semiconductor transistors (FeSFET) in response to gate voltage stimuli are effectively detected by in situ sMIM, in good agreement with electrical device transport properties. This work deepens the understanding of ferroelectric semiconductor physics toward their practical device application.

Exploring Key Factors on Capacitive ECG Signal Quality Under Practical Considerations

Exploring Key Factors on Capacitive ECG Signal Quality Under Practical Considerations