Design and operation of silver nanowire based flexible and stretchable touch sensors

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In recent years wearable devices have attracted significant attention. Flexibility and stretchability are required for comfortable wear of such devices. In this paper, we report flexible and stretchable touch sensors with two different patterns (interdigitated and diamond-shaped capacitors). The touch sensors were made of screen-printed silver nanowire electrodes embedded in polydimethylsiloxane. For each pattern, the simulation-based design was conducted to choose optimal dimensions for the highest touch sensitivity. The sensor performances were characterized as-fabricated and under deformation (e.g., bending and stretching). While the interdigitated touch sensors were easier to fabricate, the diamond-shaped ones showed higher touch sensitivity under as-fabricated, stretching or even bending conditions. For both types of sensors, the touch sensitivity remained nearly constant under stretching up to 15%, but varied under bending. They also showed robust performances under cyclic loading and against oxidation.

I. INTRODUCTION

The demand for touch screen panels has seen dramatic increase in applications such as mobile phones, tablets, and home appliances. It is thus of significant technological importance to develop highly sensitive touch screens. The early touch screen devices mostly rely on resistive sensing, which has several limitations. For instance, it requires physical pressing on the screen, which could cause surface damage/degradation.1 More recently, devices using capacitive,2 acoustic,3 and infrared sensing4 have been developed. Today, the widely used commercial touch screen panels are based on projected capacitive touch technology,5 owing to their durability, optical clarity, and multitouch capability. Indium tin oxide (ITO) is the dominant electrode material used in touch screen panels due to its high conductivity and transparency. However, the brittleness, expensive vacuum-deposition process, and dwindling reserve of ITO limit its further application.6 Nanomaterials have shown promising potential as alternative electrode materials, including carbon nanotubes (CNTs),7–9 graphene,10–13 metal nanowires,14–16 and other metal nanostructures. Wu et al.9 reported that single-walled CNTs show a comparable transparency and conductivity to ITO. Graphene electrodes were reported to achieve ;30 ohms/sq and 98% optical transmittance.10–12 Silver nanowires (AgNWs) are a)

Address all correspondence to this author. e-mail: [email protected] b) These authors contributed equally. DOI: 10.1557/jmr.2014.347 J. Mater. Res., 2014

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emerging candidates. Recently, researchers have fabricated AgNW electrodes with 20 ohms/sq and 80% optical transmittance.14–16 Commercial touch screen panels are mainly based on hard (e.g., glass) and flexible substrates (e.g., PET).1 In recent years, wearable devices have attracted significant attention. In addition to flexibility, stretchability is required to place the wearable devices comfortably on curvilinear and moving surfaces such as joint