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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15252 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018
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ACS Applied Materials & Interfaces
Highly exfoliated MWNT- rGO ink wrapped Polyurethane foam for Piezoresistive pressure sensor applications Amit Tewaria,b,d, Srinivas Gandlab , Siva Bohmc, Christopher R. McNeilld, Dipti Guptaa, b* a
IITB-Monash Research Academy, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
b
Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai 400076, India.
c
Talga Technologies Ltd, Cambridge Science Park, Unit 15-17 Milton Road, Cambridge CB4 0FQ, United Kingdom.
d
Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia.
*Corresponding Author Email:
[email protected] 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 methyisothiazolinone in appropriate proportion were used to synthesize rGO ink with a hydrophilic nature. Utilizing this hydrophilic rGO ink mixed with multiwall carbon nanotubes (MWNTs), a very simple, low-cost approach is demonstrated for the fabrication of a pressure sensor based on polyurethane foam is 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 resulting in a detectable increase in resistance suitable for detecting small scale motion. At 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.
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Keywords: Reduced graphene oxide (rGO), Multiwall carbon nanotubes (MWNTs), Polyurethane foam (PU), 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 skin1, speech recognition2, sports motion monitoring3, portable healthcare monitoring4, and prosthetic devices5. Until now, various pressure sensing mechanisms have been proposed including piezoelectric sensing6,7, piezoresistive sensing8–10, triboelectric sensing11, transistor sensing12–14, and capacitive sensing15,16. Traditional transistorbased sensors, especially those based on silicon metal-oxide semiconductor field effect transistors, have demonstrated high sensitivity. However, due to their rigidity, they are incompatible with flexible devices. Currently, several flexible pressure sensors have been demonstrated that include capacitive sensors with microstructured rubber dielectric layers10, organic field-effect transistors12 (OTFTs), nanowire active array field-effect transistors17, and reversible interlocking nanofiber piezoresistors18. The fabrication processes for these flexible pressure sensors however typically involve complex processing steps which impacts upon the cost of manufacturing sensor-based products. It is therefore imperative that alternative routes that are simple and cost-effective 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 its several advantages such as their low cost feasibility of processing and convenient signal collection6–8,19. In the past, elastomeric conducting composite materials were conventionally used along with carbon black for fabricating piezoresistive sensors8, but these materials had some limitations such as material instability, lack of sensitivity, and non-reproducibility in the low-pressure regime (
