Research Achievements Of Wearable Functional Clothing: Capacitive Pressure Sensor On Fabric Fingers

　　 Abstract This paper introduces a system method of electronic textile pressure sensor based on interdigital capacitor (IDC) printed on fabric. In this study, we proposed a high sensitivity and wide range pressure sensor based on the combination of porous Ecoflex, carbon nanotubes (CNT) and interdigital electrodes. First, in the frequency range of 1 to 300 kHz, the finger capacitor is characterized with silver ink on cotton and polyester fabrics using a precision LCR instrument. Including the influence of fabric on the sensor sensitivity. Secondly, the effects of volume fraction and air gap of carbon nanotubes on the properties of composites are estimated and optimized. The presence of volume fraction carbon nanotubes enhances the bonding strength of composites and improves the deformation ability of sensors. The robustness of the proposed sensor is demonstrated by more than 20000 cycle tests at a high pressure of 400 kPa. Third, the combination of carbon nanotubes and porous dielectrics has realized a wide detection range (400 kPa), with sensitivity ranging from 0.035 (at 400 kPa) to 0.15 Pa_ ? 1KPa ? 1 (at 50 kPa). Finally, the comparison between cotton substrate and polyester substrate shows that the selection of appropriate dielectric substrate will affect the sensor sensitivity and signal output.

Introduction: Nowadays, wearable sensors, especially textile sensors, have become an exciting issue and have aroused great interest of researchers. Among these sensors, the excellent performance of the pressure sensor makes it a promising component in the next generation of flexible electronic products. They are used for commercial purposes and in scientific fields, such as medical monitors, aeronautics, robotics, etc. (Castano&Flatau, 2014; Huang et al., 2019; Seyedin et al., 2019) In addition, they can be attached to the skin or clothes to monitor physiological signals or external pressure under continuous working conditions without disturbing or restricting personal daily activities. Many efforts have been made to develop flexible pressure sensors. There are many methods of measuring pressure using piezoelectricity, piezoelectricity, friction electricity and piezoresistive effect. Among them, capacitive pressure sensor based on parallel plate capacitor is widely used because of its low power consumption, fast response time, simple structure and other advantages. Theoretically, the capacitance of parallel plate capacitor is given by formula (1):

Where is the dielectric constant of the material, the vacuum dielectric constant is, A is the effective area of the upper and lower electrode plates, and d is the thickness or spacing between the two electrodes. By changing, A, d, capacitive sensors can be divided into three types: variable dielectric, variable area (Guo et al., 2019; Wan et al., 2017) and variable spacing (Mahata et al., 2020; Ruth et al., 2020 ε r ε r ε zero ε zero ε r ε r). In this method, the thickness or dielectric layer changes under the action of external force, and at the same time, the capacitance of the sensor changes. Because it depends on the parameters A and d in formula (1), changing the area or thickness will affect the pressure sensitivity ("One Ruee Ultrasensitive Wearable Flexible Low Pressure Sensor | ACS Omega" nd). Therefore, the sensitivity achieved by this method is usually very low (Zang et al., 2015). Most of the methods about flexible sensors focus on improving the sensitivity and flexibility of capacitive pressure sensors. These depend on the deformability of the dielectric layer ("flexible capacitive pressure sensor enhanced by inclined micro column array | ACS Applied Materials&Interfaces" nd; Ruth&Bao, 2020; Wang et al., 2020; Xiong et al., 2020) or the increase of the effective area and thickness ("one rupee super sensitive wearable flexible low-voltage sensor | ACS Omega" nd). However, these methods have the characteristics of slow recovery time, high cost and the complexity of fabricating microstructures. In addition, the high density porosity in the dielectric layer may generate noise and affect the stability and durability of the sensor.

In this work, we propose the design and implementation of a textile pressure sensor based on the calculation of interdigital capacitance. We only focus on the modification of the dielectric layer to improve sensitivity. This method only uses one electrode, so the sensor is not affected by the distance of the dielectric layer, but detects the variable capacitance by changing the relative dielectric constant of the porous polymer layer under compression. The electrode is made of silver paste printed on cotton fabric, which is similar to a comb with a plurality of crossed fingers. In this paper, the efforts to improve the sensitivity are divided into two main studies: the dielectric change of the elastic layer and the generation of particles in the dielectric layer. last,

The rest of this article is divided into the following parts. First, a new pressure technique based on changing the relative permittivity was introduced into the experiment. Secondly, "manufacturing" explains the manufacturing process of the proposed capacitive pressure sensor. Next is "Measurement Results and Discussion", which compares the properties of polyester and cotton substrates, including the effects of sensitivity, cost and durability. Finally, conclusions are drawn in the last section.

Experimental conductive track and transducer principle

Interdigital capacitors use lumped circuit elements called multi finger periodic structures. Unlike parallel plate capacitors, fork capacitors only need one side to detect changes in the material under test (MUT). This design has a higher quality factor than parallel capacitors (Aparicio&Hajimiri, 2002). The working principle of interdigital sensor is the same as that of two parallel plate capacitors. In this structure, the capacitance appears between the narrow gaps of the fingers. When the gap decreases, the capacitance increases accordingly. The shape of the sensor is described by the parameters shown in Figure 1.

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Conclusion:

We propose a new method of manufacturing electronic textile pressure sensors with flexible substrates using interdigital capacitors (IDC). The proposed sensor can achieve high sensitivity and wide range (400 kPa), and the sensitivity range is from 0.035 (at 400 kPa) to 0.15 Pa_ _ ? 1KPa ? 1 (at 50 kPa). Due to the combination of Ecoflex and CNT's high elastic properties, the dielectric layer can achieve excellent durability. In addition, the proposed sensor has fast response and recovery time, and the detection range exceeds 400 kPa. In addition, the influence of the dielectric substrate on the sensitivity performance of the sensor plays a key role in the insensitivity and detection of the signal output. Therefore, proper dielectric substrate should be selected in practical application.

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