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Highly Sensitive Piezocapacitive Sensor for Detecting Static and Dynamic Pressure Using Ion-Gel Thin Films and Conductive Elastomeric Composites Sun Geun Yoon, Byoung Joon Park, and Suk Tai Chang* School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea S Supporting Information *
ABSTRACT: A new class of simple and highly sensitive piezocapacitive sensors that are capable of detecting static and dynamic pressure changes is reported. The pressure sensor structure is formed by vertically sandw iching a san dpap er-molded carbon nanot ube/p oly(dimethylsiloxane) composite (CPC) dielectric layer between two iongel thin film electrodes. Such a capacitive sensor system enables the distinguishable detection of directional movement of applied pressure as well as static pressure variation by modulating ion distribution in the ion-gel thin films. The resulting capacitive pressure sensors exhibit high sensitivity (9.55 kPa−1), high durability, and low operating voltage (0.1 V). Our proposed pressure sensors are successfully applied as potential platforms for monitoring human physiological signals and finger sliding motions in order to demonstrate their capability for practical usage. The outstanding sensor performance of the pressure sensors can permit applications in wearable electronic devices for human−machine connecting platforms, health care monitoring systems, and artificial skin. KEYWORDS: pressure sensors, piezocapacitive sensors, dynamic pressure detection, ion-gel thin film, conductive elastomeric composite
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hollow and porous structures5,7,8,12,20−22 or arrays of microsized structures, such as domes and pyramids,6,9−11,13,18 have been introduced to the systems. The detection of not only static but also dynamic pressures, including pressure movement,6,21,28 pressure distribution,7,24,28 and discrimination between vicinal touch and physical pressure,17,33 permits the intelligent perception of various user commands to pressure sensors and provides practical applicability as functional touch devices and human−machine connecting platforms. Most studied pressure sensors have used unit-cell array structures as detection platforms for dynamic pressure motions.5,7,24,28 To achieve such array structures, complicated patterning and printing steps are necessary in the fabrication process. Structural designs of capacitive sensors for detecting triaxial forces16 and hydrogel-based touch pads for monitoring human touch and movement34 have been studied as alternative dynamic pressure detectors. Effective and simplestructured transduction platforms for dynamic pressures and various user command modes remain a challenge to achieve improved perception of motion information. Here, we report a simple and novel capacitive pressure sensor that detected both vertical pressure and lateral movement in applied pressure. The pressure sensor comprised an ILcontaining polymer film (ion gel) and carbon nanotube (CNT)/poly(dimethylsiloxane) (PDMS) nanocomposite
INTRODUCTION Pressure sensors that transduce external pressure into electric signals have been extensively investigated in research fields for use in sensitive artificial skin, biomedical prostheses, and electronic devices for human−machine interfaces and health care systems.1−4 It is important for such devices to have properties such as functionality, durability, and low power consumption as circuit components. Various approaches have been used to develop functional pressure sensors; the transduction mechanisms can be subdivided into categories including piezoresistors,5−14 piezocapacitors,15−23 and transistors.24−28 Piezoresistive transduction was studied thoroughly because of its simplicity and sensitivity to diverse strain modes (e.g., tensile strain, compressive strain, and torsion).1,3,29,30 Capacitive transduction has also been applied in pressure sensors because it is highly adaptable in static force measurements and uses a simple capacitor structure (e.g., parallel-plate structures).1,4,15 Transistor-type pressure sensors were proposed based on micropatterning and printing techniques.24,27,28 They utilize unique signal amplification by voltage at the gate terminals, allowing high sensitivities to applied pressures.3 The application of proper materials and structures in these devices is important for optimizing pressure sensor performance and elevating sensitivity to external pressure. Deformable and conductive electrodes have been considered for implementation in pressure sensors, often comprising polymeric substrates incorporated with nanomaterials, 5,17,24 ionic liquids (ILs),23,31,32 and conducting polymers.9,13,28 To achieve high sensitivity to applied stress, easily deformable structures such as © XXXX American Chemical Society
Received: August 6, 2017 Accepted: September 26, 2017
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DOI: 10.1021/acsami.7b11700 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic of I-CPC pressure sensor. Each layer is physically stacked in the order shown in the image. An enlarged image (dashed square) shows the capacitance change mechanism of I-CPC pressure sensor under applied pressure. The same capacitance change mechanism occurs at the bottom interface between the ion gel and sandpaper-molded CPC layers. Schematics of I-CPC pressure sensors (top) and ion distribution in ion gels (bottom) with (b) symmetrically and (c) asymmetrically connected copper electrodes. Pressure is only applied to the sensing area indicated by the dashed squares in panels b and c.
(CPC) layer. IL-embedded polymer matrices can provide electrical conductivity, electrochemical stability, and robust films.32,35−37 We utilized 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) as the conducting IL and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the polymer matrix to prepare the iongel film. The ion-gel film was placed in contact with the CPC layer, forming electric double-layer (EDL) capacitance at the interface. To induce significant changes of EDL capacitance by pressure application, irregular microsized bump structures were introduced to the CPC layer surface via easily accessible sandpaper molds. The ion gel-CPC (I-CPC) pressure sensor was optimized by tuning the surface roughness and concentration of the CNTs and IL. Our pressure sensor showed 9.55 and 4.07 kPa−1 (at 0−0.2 kPa) of pressure sensitivities with no. 120 and no. 220 sandpaper grits, respectively. These sensitivity values are among the highest compared to previous capacitive-type pressure sensors. In addition, high stability under signal detection, low minimum detection limit (
