Dual-Layer Flexible Capacitance Sensor with Wide Range and High Sensitivity

Abstract

Flexible pressure sensors have attracted great attention due to their extensive applications in human–computer interaction and health monitoring. So far, the development of flexible pressure sensors that balance high sensitivity and a wide measurement range remains a challenge. Herein, a double-layer dielectric structure with a surface convex structure is reported for the preparation of flexible capacitive pressure sensors. The double-layer dielectric structure, which is composed of a silicone rubber-based conductive elastomer with a surface micro-convex structure and a PVA-H-based conductive elastomer, balances the advantages and disadvantages of the two conductive elastomer dielectrics. It can form a complete micro-capacitive network under relatively large pressures, enabling the sensor to have high sensitivity at different stages (1.7 kPa⁻¹, 0–104 kPa; 19.14 kPa⁻¹, 104–140 kPa), thus achieving a dual enhancement of sensitivity and sensing range. Additionally, the sensor has been successfully applied to scenarios such as monitoring of human breathing, speaking, and movement, as well as mouse clicks, demonstrating its great potential in the fields of health monitoring and human–computer interaction applications.

1. Introduction

Flexible sensors have received great attention in the past few decades due to their excellent properties such as high stretchability, biocompatibility, and low cost [1,2,3]. Among them, flexible pressure sensors have a wide range of applications in health monitoring devices [4,5,6], robotics [7,8,9], human–machine interfaces [10,11,12,13], and other fields. According to their working mechanism, flexible pressure sensors are mainly divided into resistive [14,15], capacitive [16,17], piezoelectric [18,19], and triboelectric [20,21] types. Among them, flexible capacitive pressure sensors have become a research focus for scholars due to their advantages such as simple structure, high stability and reliability, and low power consumption [22,23,24,25].

Flexible capacitive pressure sensors adopt a “sandwich” layered structure composed of flexible electrode and elastic dielectrics. According to their working principle and structural characteristics, it can be seen that the material and structure of the elastomeric dielectric significantly affect the performance of the pressure sensor [26]. Generally speaking, in order to increase the dielectric constant, elastomeric dielectrics are mostly conductive polymer composite materials, and the matrix materials used are primarily synthetic rubber and hydrogels. Synthetic rubber has excellent elasticity, water resistance, and insulation, and maintains stable physical properties in harsh environments such as acid and alkali, high and low temperatures, hence many studies focus on synthetic rubber, especially polydimethylsiloxane (PDMS) [27,28,29], polyurethane [30,31], and silicone rubber [32] in synthetic rubber. However, rubber has a higher hardness and limited compression deformation capacity, which severely limits the sensitivity of the sensor [33,34]. Therefore, sensors prepared solely with rubber-based dielectrics find it difficult to balance high sensitivity and a wide measurement range, and their application is greatly limited. In the field of health monitoring, such as the detection of physiological signals like respiratory frequency, high-sensitivity sensors are required to accurately capture subtle physiological changes. Due to the insufficient sensitivity of the silicone rubber conductors, they are unable to provide accurate physiological data, making it difficult to meet the high-precision requirements of health monitoring. This restricts their widespread application in this field. Hydrogels are hydrophilic three-dimensional network structures with the characteristics of being sensitive to stimuli and having good flexibility [35,36], and they have been widely used in the field of electronic sensing in recent years. Jing et al. [37] proposed a PVA/CNF hydrogel, based on which the developed capacitive sensor can detect minute pressure changes (such as tiny water droplets) with high sensitivity. Wang Z et al. [38] proposed a PILNM material, and the wearable pressure-sensing textiles prepared based on it have high sensitivity to low pressure and can still reach their low-pressure detection limit under large deformation. However, the measurement range of the above hydrogel-based sensors is very small, and because the dielectric mainly adopts a planar structure, although the sensitivity is improved compared to rubber-based materials, it is still relatively low. Although hydrogels possess high sensitivity and can respond rapidly to minute stimuli, the characteristic of their small measurement range severely restricts their application scope. In the application of sports health monitoring, the large-scale joint movements of the human body also exceed the measurement range of hydrogels, resulting in a significant limitation of their application in this field.

Previous studies have proven that structural design is considered an effective strategy to improve the sensitivity of sensors [39]. For example, Wang Y et al. [40] proposed an ion-conductive hydrogel pressure sensor with a wrinkled surface, which has a sensitivity of 3.19 kPa⁻¹ (0.025–0.5 kPa) and can be used for monitoring joint bending, speaking, and breathing. Xiong Y et al. [41] reported a capacitive pressure sensor with convex microarrays, which has a high sensitivity of 30.2 kPa⁻¹ (0–0.13 kPa) and can monitor physiological signals of the human body and the movement of robotic hands in real time. However, the sensors reported in the above studies cannot balance high sensitivity and a wide measurement range and only exhibit high sensitivity within a small sensing range. This is because the designed microstructures are regular microstructures, which can lead to structural hardening and rapid signal saturation [42].

This paper is dedicated to designing a flexible capacitive pressure sensor with both high sensitivity and a wide measurement range. To achieve this goal, a double-layer capacitive pressure sensor with a surface protrusion structure is proposed. The sensor is constructed from a polydimethylsiloxane (PDMS) encapsulation layer, conductive adhesive tape electrodes, and a double-layer dielectric composed of conductive hydrogel and silicone rubber. Considering the performance evaluation of the sensor, the characteristic structures of the prepared dielectric and the used conductive adhesive tape electrodes are evaluated separately. The sensitivity, measurement range, response/recovery time, response ability under low pressure, and durability of the sensor are analyzed through a performance testing platform. A series of application tests, including human respiration monitoring, speech monitoring, motion monitoring, and mouse click simulation, are carried out on the sensor to explore its feasibility in practical applications.

2. Materials and Methods

2.1. Materials

Conductive elastomeric dielectrics are divided into conductive silicone rubber elastomers and conductive PVA hydrogel (PVA-H) elastomers. The conductive silicone rubber elastomer material uses two-component addition-cure room temperature vulcanizing silicone rubber (RTV-2 silicone) (E620, Shenzhen Hongyejie Technology Co., Ltd., Shenzheng, China) as the matrix, and the conductive PVA-H elastomer material uses PVA (1799, Wuxi Yatai United Chemical Co., Ltd., Wuxi, China) prepared with PVA-H as the matrix; carbonyl nickel powder (CNP) (Particle size 2.5 μm, Xuzhou Jiechuang New Material Technology Co., Ltd., Xuzhou, China) as the soft magnetic particle filler; Dimethyl Silicone Oil (viscosity 100 cs, Shenzhen Hongyejie Technology Co., Ltd., Shenzheng, China) as the plasticizer for the silicone rubber; and Glycerol (Analytical Grade, Sinopharm Group, Beijing, China) as the plasticizer and moisturizer for PVA-H. The sensor electrode material is Polyester Fiber (PET) conductive tape (thickness: 0.11 mm; conductive material: Electroplated Copper–Nickel Alloy). The encapsulation material is PDMS film (DC184, DOWSIL, Auburn, MI, USA). The bonding material is one-component room temperature vulcanized silicone rubber (RTV-1 Silicone) (E43, Wacker Chemie AG, Munich, Germany).

2.2. Fabrication Procedures of the Sensor

Figure 1 shows the main fabrication process of the dielectric for the flexible capacitive pressure sensor. The impact of the dielectric on the sensor’s performance cannot be ignored. First, PVA particles were added to deionized water at a mass ratio of 1:10. Subsequently, Glycerol with a mass fraction of 3% was incorporated, and the mixture was stirred at 350 rpm (low speed) for 1 h at 90 °C until the PVA particles were completely dispersed. After the precursor solution of PVA-H is cooled to room temperature, 30 vol% CNP is added and stirred at low speed for half an hour to ensure uniform distribution. The well-mixed precursor solution is then placed in a vacuum drying box with a pressure of −0.1 MPa and left to stand for 5 min before pouring it into a 3D-printed mold. Next, the components A and B of RTV-2 silicone are mixed in a mass fraction of 1:1, and Dimethyl Silicone Oil is added and stirred for half an hour, followed by the addition of 30 vol% CNP, which is stirred at low speed for another half hour to form a fluid-like mixture. The well-mixed silicone rubber/CNP precursor solution is placed in a vacuum drying box with a pressure of −0.1 MPa and left to stand for 10 min. A hollow mold of the same thickness is overlaid on the mold with the precursor solution poured on it, and then the silicone rubber/CNP precursor solution is poured into this mold, allowing it to cover the PVA-H/CNP precursor solution evenly. Finally, the semi-open mold is placed in a uniform vertical magnetic field of 200 mT and magnetized for 2 h to cure, forming the silicone rubber/PVA-H conductive elastomer dielectric. As shown in Figure 2a, the magnetizing device controls the forward and reverse rotation of the motor through a PLC to adjust the distance between the upper and lower permanent magnets, thereby adjusting the magnetic field strength between the magnetic blocks. The semi-open mold filled with the precursor solution is placed between the vertical parallel magnetic fields as shown in Figure 2b. During magnetization, the CNP inside the conductive elastomer forms a chain-like structure under the induction of the magnetic field. Moreover, under a certain magnetic field, a convex structure will spontaneously grow on the surface of the conductive elastomer, as shown in Figure 2c. In addition, a spin coater is also used to spin-coat a suitable thickness of PDMS transparent encapsulation layer onto a glass slide. Two pieces of double-sided adhesive PET conductive tape electrodes cut into the same shape are adhered to the slightly adhesive PDMS film surface, respectively. Finally, the double-layer structured dielectric is placed on the PDMS films with PET conductive tape electrodes, and RTV-1 glue is used to bond the two PDMS films together. Owing to its remarkable initial tackiness, the double-sided adhesive PET conductive tape can be closely affixed to the surface of the marginally adhesive PDMS film and the dried dielectric that has undergone ultrasonic impurity removal treatment. This close attachment ensures effective contact among the electrode, the PDMS film, and the dielectric, thereby maintaining the stability of the electrical conductivity. Moreover, the flexibility of the PET material matches well with that of the PDMS film and dielectric. During cyclic use, even when subjected to bending or stretching, the bonded structure remains intact. For encapsulation, RTV-1 glue is employed to bond the upper and lower PDMS films. Its superior sealing and adhesive properties ensure a secure fixation of the electrodes and dielectric, shielding internal components from moisture, dust, and other contaminants over extended periods. Furthermore, the flexibility of RTV-1 glue allows for seamless adaptation to minor deformations during cyclic use, effectively mitigating internal stress and reducing the risk of joint cracking. A schematic diagram of the flexible capacitive pressure sensor is shown in Figure 1. On one hand, the sensor mimics the multi-layer structure of the skin, including the epidermal layer, dermal layer, and subcutaneous tissue. On the other hand, it introduces microstructures into the PVA-H/silicone rubber-based double-layer structured dielectric, which means that many small convex structures are introduced between the epidermal layer and the dermal layer.

2.3. Characterization and Pressure Sensing Testing

The morphology of the double-layer structured MRE was characterized using a field scanning microscope (Regulus8100, HITACHI, Tokyo, Japan) with an acceleration voltage of 20 KV. The dielectrics were ultrasonically cleaned using an ultrasonic cleaner (Model KQ-300DE, Kun Shan Ultrasonic Instruments Co., Ltd., Kunshan, China) to remove surface impurities. The pressure sensing detection method of the flexible capacitive pressure sensor is as follows: a computer-controlled universal testing machine (HD-B609-S, Haida International Dongguan Haida Instruments Co., Ltd., Dongguan, China) generates pressure to apply an excitation load to the sensor, with the test speed fixed at 0.01 mm/min. Meanwhile, a high-precision LCR meter (LCR-5200, IVYTECH, Suzhou, China) is used to measure the sensor’s capacitance by clamping the wires at both ends of the sample, with the LCR test speed set to slow, the measurement voltage set to 1 V, and the test frequency set to 200 kHz. The sensitivity of the capacitive pressure sensor is defined as δ(ΔC/C₀)/δ P, where ΔC, that is C-C₀, represents the relative change in capacitance value, C is the capacitance value of the sensor after pressure is applied, C₀ is the initial capacitance value of the sensor, and P is the applied pressure.

3. Results and Discussion

3.1. Characterization

Figure 3a shows a SEM image of the surface protrusion structures self-grown on the silicone rubber conductive elastomer under a vertical curing magnetic field at low magnification. It can be observed from the figure that the surface of the silicone rubber conductive elastomer is uniformly distributed with small protrusions. Figure 3b is a SEM image of a single protrusion structure at high magnification. Figure 3c is a SEM image of the surface of the used PET conductive adhesive cloth electrode. Figure 3d shows the gold-plated material covering the surface of the conductive adhesive cloth fibers at high magnification. It can be seen that the nanoscale metal particles tightly adhere to the surface of the fibers, which ensures the conductivity of the electrode.

3.2. Pressure Sensing Testing

The introduction of the silicone rubber/PVA-H double-layer structured dielectric material has enabled the sensor to exhibit better sensing performance. Figure 4a is a schematic of the test system. The test system consists of three parts: a computer, an LCR meter, and a Universal Testing Machine. Under the condition of room temperature, the Universal Testing Machine is controlled by a computer to apply pressure loads to the sensor. The fixture of the LCR meter is utilized to connect the wires at both ends of the sensor, and the capacitance value of the sensor under the corresponding pressure is then read in real time. In our previous research, it was found that the magnetic field is beneficial to the improvement of the performance of the dielectric. As shown in Figure S1, the dielectric cured under a magnetic field of 200 mT exhibits significantly improved sensitivity. Compared with the dielectric cured without a magnetic field, the maximum improvement factor within 150 kPa can reach 3.71. According to Figure S2, the soft magnetic particles will form chains under the magnetic field, while they are more disorderly distributed without a magnetic field. The relative permittivity of the dielectric with chain-like arranged particles can be greatly changed under the action of the normal force, which enhances the dependence of the capacitance change in the dielectric on the pressure [43]. In addition, a comparative analysis was conducted on the sensitivities of dielectrics with and without surface microstructures when subjected to pressure, under identical curing magnetic field strength conditions. As illustrated in Figure S3, the surface microstructures play a crucial role in augmenting the sensitivity of the dielectric material. Leveraging these findings, the present study opted to utilize dielectrics with surface structures for sensor fabrication. This strategic selection enabled the development of sensors characterized by elevated sensitivity, thereby optimizing their performance metrics for practical applications.

As can be seen from Figure 4b, the sensitivity of the silicone rubber/PVA-H double-layer structured sensor can be divided into two stages: it reaches 1.7 kPa⁻¹ within the pressure range of 104 kPa, and it further reaches 19.14 kPa⁻¹ in the pressure range of 104–140 kPa. The capacitance values acquired beyond a pressure of 140 kPa deviated from stability, manifesting substantial fluctuations. Consequently, only the pressure regime up to 140 kPa is designated as the measurement range of the sensor. To comprehensively elucidate the impact of the double-layer structure on the sensor’s sensitivity and pressure-sensing range, a comparative performance assessment was conducted on sensors assembled with PVA-H dielectrics and silicone rubber dielectrics of identical dimensions. As clearly demonstrated in Figure S4, the PVA-H sensor exhibits a maximum sensitivity 17.7-fold higher than that of the double-layer structured sensor; however, its measurable pressure range is merely 27% of the latter. Conversely, the silicone rubber sensor demonstrates a maximum sensitivity three orders of magnitude lower (0.1%) compared to the double-layer structured sensor, yet offers a pressure sensing range that is 1.2 times broader. As a result, the double-layer structured sensor exhibits a superior ability to reconcile sensitivity and pressure response range compared to its single-layer counterparts, ensuring that the sensor maintains high levels of electrical performance across its operational spectrum.

In addition, the response/recovery time of the silicone rubber/PVA-H double-layer structured sensor is shown in Figure 4c. The response time and recovery time are defined as the time required for the sensor’s relative capacitance change to stabilize when a pressure load is applied and when the pressure load is removed, respectively. It can be observed from the figure, the response time of the PVA-H/silicone rubber-based double-layer structured sensor can reach 120 ms, and the recovery time reaches 180 ms. As can be seen from Figure S5, the response time of the hydrogel sensor is 270 ms, and the recovery time is 120 ms. The design of the double-layer structure has improved the dynamic detection capability of the sensor to a certain extent. Due to the internal soft magnetic particles’ polarization not keeping up with the change in the electric field [53], there is still a certain trend of increase in the sensor’s relative capacitance change after the pressure load is stopped.

To verify the response capability of the silicone rubber/PVA-H double-layer structured sensor at low pressure, a weight ranging from 5 g to 100 g was repeatedly applied to the sensor, and the relative capacitance change under different weight loads was recorded. As can be seen from Figure 4d, when different weights are applied to the sensor, there is a significant difference in the relative capacitance change, which means that the sensor has good resolution at low pressure and can clearly distinguish slight pressure changes. In addition, each weight was repeated five times, and the relative capacitance changes obtained in the five tests were almost identical, indicating that the sensor has a stable resolution when detecting low pressure. This is because the PDMS encapsulation layer has high elasticity, which ensures the sensor’s strong stability.

To verify the durability of the silicone rubber/PVA-H double-layer structured sensor, a pressure of 10 kPa was repeatedly applied to the sensor, and the relative capacitance change was recorded during about 1000 cycles of compression/release testing. As can be seen from Figure 4e, when the pressure is repeatedly applied and released to the sensor, there is a slight downward shift in the lower limit of the relative capacitance change. However, throughout the entire test process, the relative capacitance change remains within the range of −0.04 to 1.25, with no obvious fatigue. This is of great significance for the practical application of the sensor.

Additionally, Figure 4f compares the effective range and sensitivity of the sensor proposed in this work with those of the reported flexible capacitive pressure sensors. It can be intuitively observed that the double-layer structured sensor proposed in this work has an advantage in terms of balancing the effective working range and sensitivity. This further demonstrates that the design of the double-layer structure can enhance the sensing performance of the sensor.

3.3. Pressure Sensing Mechanism

Figure 5 describes the working principle of the silicone rubber/PVA-H-based double-layer structure sensor under different pressures. As shown in the figure, the double-layer dielectric structure undergoes different mechanical deformations under various pressures, which simultaneously affects the contact area between the dielectric and the upper electrode plate, the spacing between the upper and lower plates, and the dielectric properties of the dielectric. Firstly, in the first stage, the double-layer dielectric structure is free of external load, and its effective area is only the contact area between the convex top and the upper electrode. After the internal CNP particles are polarized in the electric field, an equal amount of opposite-sign charges are formed on the surface, and since both silicone rubber and PVA-H matrices used in the dielectric are insulators, the adjacent metal particles, along with the intermediate insulating matrix, form a microcapacitor. The performance of this microcapacitor will change with the mechanical deformation of the dielectric. In the second stage, the applied pressure causes the double-layer dielectric structure to be compressed and mechanically deformed, which means the spacing between the upper and lower plates is reduced. The change in sensitivity at this stage is closely related to the three variables in $C=A\epsilon /d$, but the sensitivity is relatively low. It is particularly noteworthy that the surface convex structure of the upper silicone rubber dielectric increases its contact area as the pressure increases during this stage. Moreover, the micro-capacitors in series in the convex portion gradually transition to parallel capacitors as the mechanical deformation occurs, and the permittivity, ε, changes. However, during this stage, the changes in A and d due to compression contribute relatively little to the improvement in sensitivity, and the formed interface capacitance is small. The microcapacitance network within the double-layer dielectric structure is not dense, and the improvement in dielectric performance is minor. In the third stage, as indicated by $\Delta C/C_0=C/C_0-1$, the sensitivity response of the capacitive pressure sensor is mainly determined by the ratio of the current capacitance value to the initial capacitance. In this stage, the effective contact area of the electrodes no longer changes, and the distance between the upper and lower electrodes changes relatively little. Compared to the second stage, it can be understood that the change in sensor performance at this point is mainly due to the improvement in the dielectric properties of the dielectric. As shown in the figure, with the increase in pressure during this stage, more and more polarized carbonyl nickel powder particles gather at the contact surface between the double-layer dielectrics, forming many parallel capacitors. The emergence of parallel capacitors results in a denser microcapacitance network between the upper and lower dielectrics compared to the initial state. When the pressure reaches a certain level, the microcapacitance network between the dielectrics becomes complete, which is manifested by a rise in sensitivity similar to the second stage. However, if the mechanical deformation is too large, it can lead to excessive aggregation of CNP particles, ultimately causing capacitor breakdown.

3.4. Demonstration of Multifunctional Applications

The silicone rubber/PVA-H double-layer structured sensor is characterized by its ability to balance high sensitivity and a wide measurement range, allowing it to be applied in various scenarios. As shown in Figure 6a, to monitor respiratory rate, the sensor needs to be fixed inside the mask, so that the breath pressure can be tested to monitor respiration. Different frequencies of airflow pressure can represent different breathing rates. The sensor can also detect changes at the throat during human speech. As shown in Figure 6b, the sensor is placed on the throat, so that the pressure changes caused by the movements of the Adam’s apple during speech can be monitored to distinguish different word pronunciations. As can be seen from the figure, there are differences in the relative capacitance changes in the sensor when the volunteer pronounces “Peace and love”. This could potentially be used to apply the sensor in assistive pronunciation teaching, helping people to visually learn new languages. In addition to monitoring human physiological indicators, the sensor can also be applied to monitor the bending of human joints. As shown in Figure 6c,d, when the sensor is fixed to the wrist and elbow, and it transitions from an extended to a bent state, the pressure caused by the joint change leads to an increase in the relative capacitance change in the sensor. The bending motion at the wrist is smaller, so the relative capacitance change is also smaller, whereas the bending motion at the elbow is larger, resulting in a greater relative capacitance change. Repeating the bending motion several times, it can be observed that the sensor can record each bending well, demonstrating good stability. In addition to that, it can also be applied to daily work scenarios. As shown in Figure 6e, when the sensor is fixed to the left button of a mouse, pressing the sensor can simulate a mouse click through changes in pressure. The sensor has a fast response speed, which allows it to effectively simulate single and double clicks of a mouse, suggesting that it can be applied to intelligent touch control devices.

4. Conclusions

In summary, this paper reports on a double-layer dielectric with a surface convex structure, which is used to fabricate a capacitive pressure sensor that balances high sensitivity and a wide measurement range. Thanks to the convex structure formed by the magnetic field and the complete micro-capacitance network formed at higher pressures, the sensor has a wide range of 0–140 kPa, high sensitivity (1.7 kPa⁻¹ from 0 to 104 kPa; 19.14 kPa⁻¹ from 104 to 140 kPa), and good stability with the ability to cycle 1000 times at 10 kPa. Furthermore, the proposed sensor demonstrates broad applicability across various scenarios, including human respiratory rate monitoring, throat phonation detection, joint movement tracking, and mouse click simulation. Its double-layer structure, characterized by high sensitivity and a wide measurement range, offers a practical alternative to the dielectrics of traditional flexible sensors.

In the future, further research will be carried out from the following aspects. On the one hand, a test platform with controllable multi-environmental parameters will be constructed to systematically study the performance variation rules of the sensor under different conditions such as temperature, humidity, and pressure. By combining theoretical calculations and characterization analysis, the influence mechanism of environmental factors on the sensing performance will be deeply revealed. On the other hand, through material adjustment, attempts will be made to compound various conductive materials to further improve the electrical and mechanical properties of the conductive elastomer, to enhance the comprehensive performance of the sensor.