A new strategy for fabricating a stacked flexible capacitive sensor

As an important medium of human-computer interaction, flexible tactile sensors have a vital influence on the intelligence and safety of a system by accurately identifying the contact position and force. For now, most of the existing flexible capacitive pressure sensors adopt an array structure, which still cannot meet the requirements including simple manufacturing, simple wiring, and low cost. This paper innovatively proposes a flexible array-less capacitive tactile sensor, a 30 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times30$ </tex-math></inline-formula> mm sensor prototype, consisting of flexible electrodes and a dielectric layer of a polydimethylsiloxane (PDMS) and multiwalled carbon nanotube (MWCNT) composite. Under the action of a contact force, the contact area is deformed, resulting in a capacitance change in the flexible sensor. First, Poisson’s equation is used to determine the relationship between the electric potential and position. Then, elastic contact mechanics is adopted to describe the relationship between the electric charges and force. Finally, the contact position and force are recognized by measuring the change in capacitance in real time, based on the classical principle of capacitance. Simulation and experimental results verified the effectiveness of the proposed recognition method, showing that the recognition resolution of contact position can reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$500\,\,\mu \text{m}\,\,\times 100\,\,\mu \text{m}$ </tex-math></inline-formula> , and the accuracy of contact force is 95.85%. Theoretically, the proposed flexible array-less sensor in this paper has infinite spatial resolution, and most importantly, it does not rely on complex manufacturing, redundant circuits, or sensor cell arrays. Therefore, it has broad application prospects in related fields such as electronic skin and wearable electronic devices.

The flexible and wearable capacitive sensors have captured tremendous interest due to their enormous potential for healthcare monitoring, soft robotics, and human−computer interface. However, despite recent progress, there are still pressing challenges to develop a fully integrated textile sensor array with good comfort, high sensitivity, multisensing capabilities, and ultra-light detection. Here, we demonstrate a pressure and non-contact bimodal fabric-only capacitive sensor with highly sensitive and ultralight detection. The graphene nanoplatelets-decorated multidimensional honeycomb fabric and nickel-plated woven fabric serve as the dielectric layer and electrode, respectively. Our textile-only capacitive bimodal sensor exhibits an excellent pressure-sensing sensitivity of 0.38 kPa−1, an ultralow detection limit (1.23 Pa), and cycling stability. Moreover, the sensor exhibits superior non-contact detection performance with a detection distance of 15 cm and a maximum relative capacitance change of 10%. The sensor can successfully detect human motion, such as finger bending, saliva swallowing, etc. Furthermore, a 4 × 4 (16 units) textile-only capacitive bimodal sensor array was prepared and has excellent spatial resolution and response performance, showing great potential for the wearable applications.

Because of their importance in obtaining information from people and automatic equipment, flexible pressure sensors (FPS) have been widely used in diverse fields such as electronic skin, soft robots, consumer electronics, health monitoring, and human-computer interaction. Among all kinds of soft pressure sensors, capacitance pressure sensor is characterized by its simple construction, low cost, and stable performance. Although this type of pressure sensor is easily made, it is still a hotspot to increase the sensitivity and extend the efficiency of the system. This paper reviews the related research on flexible capacitive pressure sensors, including working mechanism, capacitor structures, methods to improve the performance of capacitive sensors, and applications. Finally, a comparison is made between the efficient approaches for achieving high-sensitivity, and the developing tendency of flexible capacitance sensor is predicted.The purpose of this thesis is to offer some useful information for the study of highly sensitive and highly sensitive materials for manufacturing flexible capacitive sensors.

Flexible electronic sensors have widespread applications in both traditional and emerging fields, particularly for human-computer interaction (electronic skins, wearable electronic devices) and physical and environmental monitoring for people. Flexible electronic sensors can be bent and folded readily without interfering with their detecting performance. However, researchers still face many difficulties. For instance, traditional flexible materials (such as organic materials) are less sensitive to external signals, and the production progress of flexible sensors is complicated. This review recapitulates applications of nanomaterials in flexible electronic sensors. Firstly, the working principles and applications of three common types of flexible electronic sensors, the piezoresistive sensor, the capacitive sensor, and the piezoelectric sensor, are introduced. Then, the paper summarizes methods for improving sensors performance in health monitoring, disease diagnosis and biological detection by application of different nanomaterials to flexible substrates. Finally, the future development of flexible nanomaterial sensors prospects. It is found that nanocomposites of metal nanomaterials, carbon nanomaterials and other polymers with unique tuned photoelectrical properties show enhanced performance as flexible sensors and further research is needed to improve the material-substrate integration to promote the large-scale application as wearable sensors.

With the rapid advancement of information technology, human-computer interaction is undergoing transformative changes of unprecedented scale. Among various interaction technologies, handwriting recognition, being a natural and intuitive input method and consistently holds a significant position. A flexible capacitive pressure sensor, with the sensitivity of 5.734% kPa−1, the response time of 200 ms, and a remarkable cyclic stability (&gt;1000 cycles), is developed by the polydimethylsiloxane (PDMS) with microstructure. Through writing 26 letters ‘A ∼ Z’ on the sensor’s surface, 2600 sets of 200-dimensional capacitive time series signals are generated and collected, which are used to form the customized dataset for handwritten letters. Based on the dataset, a fusion model of Convolutional Neural Network (CNN) and Long Short-Term Memory (LSTM), CNN-LSTM, is constructed to improve the accuracy of handwriting recognition to 98.59%. To verify the efficiency the CNN-LSTM model, Random Forest (RF) and Support Vector Machine (SVM) are constructed to identify different handwritten letters based on the same dataset used for the CNN-LSTM model, and the corresponding recognition accuracies are 94.61% and 95.64%, respectively. All the experimental results demonstrate that the flexible capacitive sensor with great sensation ability can precisely detect and capture different handwritten signals, and the CNN-LSTM model with great feature extraction capability is very suitable to recognize different handwritten signals collected from the sensor.

Intravenous (IV) therapy is extensively used in hospital treatments for infusion of liquid substances directly into a vein. When the medicine has been fully dispensed, an air embolism can be occurred since air in the IV drip bag could enter into the patient’s vein, which is hazardous to the patient’s safety. To prevent this situation, in this paper, design of a wireless detection system using flexible capacitive sensor to detect liquid level of IV drip set is proposed. In the conducted research, a simulation model of capacitive sensor technology was shown by using finite elements analysis (FEA), and its proper geometrical dimension of the capacitive sensor was found out. The studied capacitive sensor was used in this proposed wireless detection system. The system consisted of a wireless capacitance detector for liquid level detection, and a wireless receiver for acquiring the measuring data in another place. The flexible capacitive sensor was fixed on one side of the wireless capacitance detector, and the wireless capacitance detector was placed at the bottom surface of the plastic IV drip bag to measure the changing level of the capacitance. The measured signal from flexible capacitive sensor was converted into digital data by a relaxation oscillator circuit. In addition, a 2.4 GHz wireless communication method was used in this system for wireless data transmission. The proposed flexible capacitive sensor was shown a good performance in the basic experiment of liquid level detection in the plastic IV drip bag, and the wireless detector is able to send the measured data to the receiver in other place by the wireless method. The PC which connected with wireless receiver can give an alarm immediately, when the liquid in the plastic IV drip bag is almost used out. The proposed flexible capacitive sensor is able to attach on the plastic IV drip bag completely, and the experiment data shows that the proposed wireless flexible capacitive sensor detection system is possible to detect the liquid level of the plastic IV drip bag. But in the future, further research is needed.

Porous polytetrafluoroethylene (PTFE) is physically flexible, thermally and chemically stable, relatively inexpensive, and commercially available. It is attractive for various flexible sensors. This paper has studied flexible capacitive humidity sensors fabricated on porous PTFE substrates. Graphene oxide (GO) was used as a sensing material, both hydrophobic and hydrophilic porous PTFE as the substrates, and interdigitated electrodes on the PTFE substrates were screen-printed. SEM and Raman spectrum were utilized to characterize GO and PTFE. An ethanol soak process is developed to increase the yield of the humidity sensors based on hydrophobic porous PTFE substrates. Static and dynamic properties of these sensors are tested and analyzed. It demonstrates that the flexible capacitive humidity sensors fabricated on the ethanol-treated hydrophobic PTFE exhibit high sensitivity, small hysteresis, and fast response/recovery time.

The wind pressure measurement, especially on curved surfaces is imperative in revealing flow characteristics. The flexible sensor with high linear sensitivity over large pressure range is still a significant challenge, especially for commutatively positive and negative pressure measurement. Here, we propose a conformable, range-programmable capacitive sensor that can extremely extend the measuring range but with high linear sensitivity. The key point is to precisely control the reference pressure of the flexible capacitive sensor array through microchannel network. The proposed sensor with reference pressure 0 kPa keeps stable at a highly-linear sensitivity of 0.28 kPa−1 in an initial measurement regime from 0 to 3 kPa, beyond which the linearity changes significantly. Via the tunable reference pressure, the linear ranges can be customized arbitrarily according to different flight conditions and measured positions, but without any deterioration of sensitivity. The theoretical model is built for the flexible capacitive sensor with tunable reference pressure, agreeing well with the experimental and finite element method results. Additionally, the bending effect is discovered when the flexible sensor is conformed on curved surfaces. This surface-mounted sensor skin is tested on a plate and is integrated with acquisition circuits on a standard airfoil NACA0012 in a wind tunnel and compares with the standard destructive method of pressure taps. It shows great potential applications in measuring wind pressure on curved surfaces, such as for “Fly-by-Feel” of unmanned aerial vehicles and wind tunnel test.

Recent progress in flexible capacitive sensors: Structures and properties

Flexible capacitive sensors are widely deployed in wearable smart electronics. Substantial studies have been devoted to constructing characteristic material architectures to improve their electromechanical sensing performance by facilitating the change of the electrode layer spacing. However, the air gaps introduced by the designed material architectures are easily squeezed when subjected to high-pressure loads, resulting in a limited increase in sensitivity over a wide range. To overcome this limitation, in this work, we embed the liquid metal (LM) in the internally interconnected porous structure of a flexible composite foam to fabricate a flexible and high-performance capacitive sensor. Different from the conventional conductive elements filled composite, the incompressible feature of the embedded fluidic LM leads to significantly improved mechanical stability of the composite foam to withstand high pressure loadings, resulting in a wider pressure sensing range from 10 Pa to 260 kPa for our capacitive composite sensor. Simultaneously, the metal conductivity and liquid ductility of the embedded LM endow the as-fabricated capacitive sensor with outstanding mechanical flexibility and pressure sensitivity (up to 1.91 kPa-1). Meanwhile, the LM-embedded interconnected-porous thermoplastic polyurethane/MXene composite sensor also shows excellent reliability over 4000 long-period load cycles, and the response times are merely 60 ms and 110 ms for the loading and unloading processes, respectively. To highlight their advantages in various applications, the as-proposed capacitive sensors are demonstrated to detect human movements and monitor biophysical heart-rate signals. It is believed that our finding could extend the material framework of flexible capacitive sensors and offer new possibilities and solutions in the development of the next-generation wearable electronics.

Highly sensitive and stable flexible pressure sensors with micro-structured electrodes

Microstructured flexible capacitive sensor with high sensitivity based on carbon fiber-filled conductive silicon rubber

In recent years, wearable electronic devices have developed rapidly and flexible sensors have been increasingly used in medical, aerospace, automotive, industrial and commercial applications. Flexible pressure sensors are critical in human-robot interaction, physiological signal monitoring and robotic haptics. Among them, capacitive type flexible pressure sensors are widely used due to their unsophisticated structure, good stability, small temperature deviation and environmentally friendly. Currently, most of capacitive sensors adopt the design with an electrical membrane inserted between two electrodes. This paper presents a comprehensive review of the development of flexible capacitive pressure sensor from the perspective of technological and engineering developments. This paper firstly introduces the capacitive pressure sensor by using the working mechanism as an entry point. Secondly, this paper discusses three general methods to increase the sensitivity of pressure sensing, including changing the dielectric microstructure, the dielectric constant, and different dielectric materials. Finally, the advantages and disadvantages of three methods for sensitivity improvement were discussed.

Wearable electronic devices with multifunctions such as flexible, integrated, and self-powered play a crucial role in the fields of health monitoring, motion monitoring, and human-computer interaction. However, their core basic components, flexible pressure sensors, face challenges including poor long-term stability and insufficient real-time sensing accuracy. In order to solve the challenges of long-term, stable, and accurate sensing of the sensor, this paper prepares polydimethylsiloxane (SHPDMS) with intrinsic self-healing property and designs a high-sensitivity self-healing capacitive flexible pressure sensor with dual microstructures (grating microstructured electrodes and microporous dielectric layer) as the substrate based on SHPDMS. Specifically speaking, the self-healing of the sensor under mild conditions was realized by introducing reversible imine bonds with low bonding energy into the polydimethylsiloxane (PDMS) flexible substrate, which solved the problem of the material's long-term service durability. A grating-like microstructure was introduced into the flexible electrode by using a spotted bamboo taro leaf as a template, and a dual microstructure sensor was constructed by combining it with a microporous dielectric layer doped with single-walled carbon nanotubes. This way reduces the elastic modulus of the dielectric layer, improves the dielectric constant of the sensor under loading, and thus significantly improves the sensor's sensitivity and extends the measurement accuracy in a low-stress range. The prepared self-healing flexible sensor achieves a sensitivity of 3.6 kPa-1, a minimum detection limit of 5 Pa, a response recovery time of less than 80 ms, and stability over 5000 cycles, which exceeds most previously reported silicone rubber-based capacitive flexible sensors.

Strip-type flexible capacitive humidity sensor based on composite of polyimide and poly(glycidyl methacrylate): Fabrication, humidity sensitive performance and potential for detecting water content in liquids