Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5185−5195
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Highly Exfoliated MWNT−rGO Ink-Wrapped Polyurethane Foam for Piezoresistive Pressure Sensor Applications Amit Tewari,†,‡,∥ Srinivas Gandla,‡ Siva Bohm,§ Christopher R. McNeill,∥ and Dipti Gupta*,†,‡ †
IITB-Monash Research Academy and ‡Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India § Talga Technologies Ltd., Cambridge Science Park, Unit 15-17 Milton Road, Cambridge CB4 0FQ, United Kingdom ∥ Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia
ACS Appl. Mater. Interfaces 2018.10:5185-5195. Downloaded from pubs.acs.org by TU MUENCHEN on 08/21/18. For personal use only.
S Supporting Information *
ABSTRACT: The fabrication of pressure sensors based on reduced graphene oxide (rGO) as the sensing material is challenging due to the intrinsic hydrophobic behavior of graphene oxide inks as well as the agglomeration of graphene oxide flakes after reduction. Hydrazine (a reducing agent) and a dual-component additive comprising benzisothiazolinone and methylisothiazolinone in appropriate proportion were used to synthesize a rGO ink with a hydrophilic nature. Utilizing this hydrophilic rGO ink mixed with multiwalled carbon nanotubes (MWNTs), a very simple, low-cost approach is demonstrated for the fabrication of a pressure sensor based on polyurethane (PU) foam coated with the MWNT−rGO ink (MWNT− rGO@PU foam). The MWNT−rGO@PU foam-based devices are shown to be versatile pressure sensors with the potential to detect both small-scale and large-scale movements. At low pressure (below 2.7 kPa, 50% strain), the formation of microcracks that scatter electrical charges results in a detectable increase in resistance suitable for detecting small-scale motion. At a higher pressure, the compressive contact of the coated faces of the PU foam results in a sharp decrease in resistance suitable for monitoring of large-scale motion. Moreover, these sensors exhibit good flexibility and reproducibility over 5000 cycles. The versatility of this sensor has been demonstrated in a wide range of applications, such as speech recognition, health monitoring, and body motion detection. The significant advantages of this sensor are that its cost is low, it is easy to fabricate, and it has a versatility that renders it favorable to health-monitoring applications. KEYWORDS: reduced graphene oxide (rGO), multiwalled carbon nanotubes (MWNTs), polyurethane (PU) foam, piezoresistive pressure sensor, sensitivity, microcracks
1. INTRODUCTION Flexible and low-cost sensors will have a high demand in the future because of their applicability in various applications, such as electronic skin, 1 speech recognition, 2 sport motion monitoring,3 portable healthcare monitoring,4 and prosthetic devices.5 Until now, various pressure-sensing mechanisms have been proposed, including piezoelectric sensing,6,7 piezoresistive sensing,8−10 triboelectric sensing,11 transistor sensing,12−14 and capacitive sensing.15,16 Traditional transistor-based sensors, especially those based on silicon metal-oxide semiconductor field-effect transistors, have demonstrated a high sensitivity. However, because of their rigidity, they are incompatible with flexible devices. Currently, several flexible pressure sensors have been demonstrated that include capacitive sensors with microstructured rubber dielectric layers,10 organic field-effect transistors,12 nanowire active array field-effect transistors,17 and reversible interlocking nanofiber piezoresistors.18 The fabrication processes for these flexible pressure sensors, however, typically involve complex processing steps, which impact the cost of manufacturing sensor-based products. It is therefore © 2018 American Chemical Society
imperative that alternative routes that are simple and costeffective be developed for flexible pressure-sensitive materials with good sensitivity across the broad pressure domain. Piezoresistive sensors that convert the applied pressure into an electrical signal are considered to be more demanding due to their several advantages, such as low-cost, feasibility of processing, and convenient signal collection.6−8,19 In the past, elastomeric conducting composite materials were conventionally used along with carbon black for fabricating piezoresistive sensors,8 but these materials had some limitations, such as material instability, lack of sensitivity, and nonreproducibility in the low-pressure regime (
