A wearable and highly sensitive capacitive pressure sensor integrated a dual-layer dielectric layer of PDMS microcylinder array and PVDF electrospun fiber

Highlights

• A wearable pressure sensor developed for mechanical tactile.

• Microcylinder array/electrospun fiber integrated as a dual-layer dielectric structure.

• A novel sensor with sensitivity of 0.6 kPa⁻¹ and response time of 25 ms is realized.

• A 6 × 6 arrayed sensor successfully displays the pressure distribution in a plane.

Abstract

Recently, high-performance flexible pressure sensors have received considerable attention because of their potential application in fitness tracking, human–machine interfaces, and artificial intelligence. Sensitivity is a key parameter that directly affects a sensor's performance; therefore, improving the sensitivity of sensors is a vital research topic. This study developed a dual-layer dielectric structure comprising a layer of electrospun fiber and an array of microcylinders and used it to fabricate a novel high-sensitivity capacitive pressure sensor. A simple, rapid, low-cost, and controllable microstructured method that did not require complex and expensive equipment was adopted. The proposed sensor can efficiently detect capacitance changes by analyzing changes in the fiber and microcylinder structure when compressed. It has high sensitivity of 0.6 kPa⁻¹, rapid response time of 25 ms, ultralow limit of detection of 0.065 Pa, and high durability and high reliability without any signal attenuation up to 10,000 load/unload cycles and up to 5000 bending/unbending cycles. Moreover, it yielded favorable results in real-time tests, such as pulse monitoring, acoustic tests, breathe monitoring, and body motion monitoring. Furthermore, experiments were conducted using a robotic arm, and the obtained results verify that the sensor has different capacitance responses to objects with different shapes, which is crucial for its future applications in smart machinery. Finally, the sensors were arranged as a 6 × 6 matrix, and they successfully displayed the pressure distribution in a plane. Thus, the contributions of the capacitance pressure sensor with a dual-layer dielectric structure in the field of high-performance pressure sensors were verified.

Introduction

Recently, due to their practical applications in human–machine interfaces, intelligent robots, touch panels, electronic skins, and health monitors, flexible pressure sensors have attracted increasing attention [[1], [2], [3], [4], [5], [6]]. The four classic sensing mechanisms of pressure sensors are the piezoresistive effect [[7], [8], [9]], piezoelectricity [[10], [11], [12]], capacitance [[1], [2], [3], [4], [5], [6]], and triboelectric effect [[13], [14], [15], [16], [17]]. Pressure sensors with capacitance mechanisms have structural simplicity, low power consumption, rapid response, and high reliability and stability, and they rapidly and steadily can convert external stimulations into capacitance signals. Therefore, these pressure sensors enable nonliving objects, such as robotic arms, to perceive the same sensations as human skin does during interactions. These sensors can also be integrated into wearable devices to measure human joint movements, breathing, pulse, and languages [[18], [19], [20], [21], [22]].

Classic flexible capacitive pressure sensors are a type of device formed by sandwiching an elastic dielectric layer between flexible electrodes. When pressure is applied on the sensor, the distance between the electrodes changes, which consequently affects the capacitance value. Therefore, manufacturing a three-dimensional structure on the electrode or dielectric layer is presently the main method employed to increase the compressibility and sensitivity of sensors. Studies have verified that the introduction of micropyramid [19,[23], [24], [25]], microwrinkle [26], microconvex [27], tilted micropillar [28], or microcylinder [29,30] structures can effectively improve sensor sensitivity. However, most of these studies have utilized single-sided structures to manufacture sensors; limitations in the morphology of these structures may afford some unimprovable shortcomings, such as limited compressibility, low durability, and increased viscoelastic behaviors. To overcome these shortcomings, double-sided structures were used as dielectric layer. Baek et al. fabricated wrinkles on both sides of polydimethylsiloxane (PDMS) dielectric layers via mechanical stretching [31]. Yoon et al. used sandpaper as a mold to fabricate irregular structures on both sides of the dielectric layer via PDMS remolding [32]. Guo et al. used anodic aluminum oxide (AAO) plates as molds to form double-sided nanocolumn dielectric layers after remolding [29]. These studies have verified that dielectric layers with double-sided structures are more sensitive than those with single-sided structures. Presently, most studies on this topic have only investigated a single material and structure or two types of heterogeneous materials to form a single structure. For example, Ma et al. used BaTiO₃ as an additive to improve the dielectric properties of PDMS and enhanced the sensitivity of the sensor using a micro-wrinkle structure [33]. In other words, no research exists on the integration of two types of heterogeneous materials to form a dual-layer dielectric structure to explore the possibility of capacitive sensors with high flexibility and adjustable sensitivity.

This study developed a dual-layer dielectric structure comprising a polyvinylidene difluoride (PVDF) fiber layer and PDMS microcylinder array. Moreover, its feasibility for application to capacitive pressure sensors was investigated. The high dielectric coefficient of PVDF and the air gap created by electrospinning facilitate the formation of a special bulked layer, and with the incorporation of arrayed microcylinders, a new type of dual-layer dielectric structure is obtained. Compared to dielectric layers with single-layer microcylinder structures, the synergic effects of the two heterogeneous structures can effectively overcome the previously mentioned shortcomings. When pressure is applied on the sensor, the fiber layer releases air in the gaps; even a slight pressure can change the distance between the electrodes, thus increasing the sensor's sensitivity. When the sensor is subject to high pressure, the cylinder deeply embeds into the fiber layer and continues to release the air in the fiber layer. Furthermore, it increases the density of the fiber and cylinder composite layer. This significantly changes the distance between the electrodes, resulting in capacitance changes. When the pressure on the sensor is released, the cylinders pressed into the fibers rapidly widen the distance between the electrodes. The fiber layer compressed because of the pressure rapidly returns to its original bulky state. This type of complementary outcome can increase the sensitivity of the sensor toward changes in the distance between the electrodes and afford low hysteresis, thus increasing the capacitance changes and durability of the sensors. The overall manufacturing process comprises simple and rapid PVDF electrospinning and PDMS pattern transfer methods with large surfaces and controllable microstructures without the need for complicated and expensive equipment. The prepared sensors have a multisectional tunable sensitivity of 0.6 kPa⁻¹ (0–7 kPa), 0.51 kPa⁻¹ (7–15 kPa), and 0.03 kPa⁻¹ (15–50 kPa). Additionally, they afford rapid response time of 25 ms, ultralow limit of detection (LOD) of 0.064 Pa, and high durability and high reliability without any signal attenuation up to 10,000 load/unload cycles and up to 5000 bending/unbending cycles. Furthermore, the sensor yielded favorable results in some practical applications; for example, real-time testing of pulse monitoring, acoustic vibration detection, breathe monitoring, and body motion monitoring. The sensor attached to the gripper of the robotic arm was also evaluated for grasping actions, and the results verify that the sensor has distinguishable capacitance responses to objects with different shapes. Finally, a sensor array of 6 × 6 pixels was fabricated, which successfully projected the pressure distribution onto a flat surface. This study demonstrates the performance improvements of the capacitive sensors using dual-layer microstructured dielectric layers with heterogeneous materials, which will contribute to the development in the field of high-performance pressure sensors.