A Highly Sensitive and Flexible Capacitive Pressure Sensor Based on Alignment Airgap Dielectric

Abstract

Flexible capacitive pressure sensors with a simple structure and low power consumption are attracting attention, owing to their wide range of applications in wearable electronic devices. However, it is difficult to manufacture pressure sensors with high sensitivity, wide detection range, and low detection limits. We developed a highly sensitive and flexible capacitive pressure sensor based on the porous Ecoflex, which has an aligned airgap structure and can be manufactured by simply using a mold and a micro-needle. The existence of precisely aligned airgap structures significantly improved the sensor sensitivity compared to other dielectric structures without airgaps. The proposed capacitive pressure sensor with an alignment airgap structure supports a wide range of working pressures (20-100 kPa), quick response time (≈100 ms), high operational stability, and low-pressure detection limit (20 Pa). Moreover, we also studied the application of pulse wave monitoring in wearable sensors, exhibiting excellent performance in wearable devices that detect pulse waves before and after exercise. The proposed pressure sensor is applicable in electronic skin and wearable medical assistive devices owing to its excellent functional features.

Keywords: alignment airgap dielectric; capacitive pressure sensor; flexible sensor; high sensitivity.

Figure 1

Fabrication process and structure of the device. (a) Schematic diagram of the micro-needles and mold for the alignment airgap structure. (b) Schematic diagram of the flexible ITO PEN film electrodes (top and bottom) and the alignment airgap dielectric layer (middle). (c) Photograph of the capacitive pressure sensor.

Figure 2

Schematic of the mechanism of the capacitive sensor using an alignment air gap elastomer as a dielectric layer.

Figure 3

Cross-sectional images of the alignment airgap dielectric layer. (a–c) pore size: 300 μm, distance: 1 mm, 1.5 mm, 2 mm. (d–f) pore size: 400 μm, distance: 1 mm, 1.5 mm, 2 mm. (g–i) pore size 500 μm, distance: 1 mm, 1.5 mm, 2 mm.

Figure 4

Pressure−response curves of pressure sensors for various structures (a) with pore size 300−500 μm (increased by 100 μm), distance 1−2 mm (increased by 0.5 mm). (b) Comparison of the pressure sensors’ performance with alignment airgap and with bulk Ecoflex dielectric layer. (c–e) Variation of the relative difference in capacitance with applied pressure ranges from 1−100 kPa.

Figure 5

Characterization of the pressure−sensing performance of the alignment airgap pressure sensor. (a) Detection limit of the sensor. (b) Response and recovery time within 100 ms. (c) Squareshaped pressure with different amplitude. (d) Capacitance in response to a step increase and decrease in pressure. (e) Hysteresis curves of loading/unloading process. (f) Capacitance response of pressure sensor during 10,000 loading/unloading cycles at an applied pressure of 50 kPa.

Figure 6

Practical wrist pulse wave applications using a pressure sensor. (a) Image of wrist pulse wave measurement of the actual pressure sensor. (b) blue indicating the normal pulse and red indicating the pulse after five minutes of exercise.