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Optoelectronics & Communications

Large-area stretchable organic integrated circuits for sensor applications

Devices combining printed elastic conductors and organic active components may enable ‘smart’ surfaces that interact with people in more intuitive ways.
31 August 2009, SPIE Newsroom. DOI: 10.1117/2.1200908.1775

New technologies have been emerging in stretchable electronics1–3 and intensive efforts are being taken to create surfaces with some form of ‘intelligence.’ The biggest challenge has been the development of electrical wiring that is both highly conductive and highly elastic. Towards this end, various types of materials have been developed, such as wavy thin metals,1,2 metal-coated net-films,4 graphene films,5 and single-walled carbon nanotube (SWNT)-fluorinated copolymer composites.3 All exhibit excellent conductivity and mechanical stretchability by exploiting structures such as waves and nets. These materials are made using vacuum evaporation, photolithographic patterning, and mechanical punching or cutting. However, since all of these manufacturing processes are not scalable, it is difficult to apply them to large-area electronics. One solution would be to employ direct printing techniques using inks that have high conductivity and stretchability, but such methods have not been reported. Our group developed an elastic conductor comprising a SWNT-fluorinated copolymer composite.3 However, the material was incompatible with printing due to its low viscosity and tendency to re-aggregate before drying. In addition, improving its elasticity required an additional rubber coating and mechanical processing.

By using an imidazolium-ion-based liquid6 and a jet-milling process, we developed a printable conductor composed of SWNTs uniformly dispersed in a highly elastic fluorinated copolymer rubber (see Figure 1). Using a novel process that uniformly disperses fine bundles of SWNTs in a rubber matrix (without shortening the nanotubes) increases the composite gel viscosity. As a result, this material can be finely patterned using direct printing techniques. A printed elastic conductor requires no additional coating or mechanical processing, can stretch by 118%, and has a high conductivity of 102S/cm. To the best of our knowledge, this is the highest value reported for a stretchable, printable conductor. This material can be used for stretchable wires and contacts in electrical integrated circuits.

Figure 1. (a) Printed elastic conductors on a polydimethylsiloxane sheet patterned by screen printing can be stretched by 100% without damage. (inset) A micrograph of printed elastic conductors with a linewidth of 100μm. (b) A scanning electron micrograph of an elastic conductor surface. SWNT conducting networks are formed in rubber.

In order to demonstrate the feasibility of elastic conductors as electrical wirings, we integrated the conductor with printed organic transistors to fabricate a rubber-like active matrix with an effective area of 20×20cm2: see Figure 2(a). The active matrix sheet can be uniaxially and biaxially stretched by 70–80% without mechanical or electrical damage.3

Figure 2. (a) A rubber-like organic transistor active matrix comprising elastic conductors and printed organic transistors. (inset) A magnified picture of a printed organic transistor with a pentacene channel. (b) A stretchable display can be spread over arbitrary curved surfaces, and is functional even when folded or crumpled (inset).

We also integrated printed elastic conductors with organic transistors7–10 and organic LEDs (OLEDs)11 to realize a rubber-like active-matrix OLED display. We used a 16×16 grid of transistors for driving the display, and an effective active matrix size of 10×10cm2: see Figure 2(b). The display could be stretched by 30–50% and spread over a hemisphere without mechanical or electrical damage. It remained functional even when folded in two or crumpled, indicating excellent durability.12

The materials and integration technology used in this study can also be applied to other types of electronics. This is an important step toward the development of infrastructure for ambient electronics, where a multitude of devices such as sensor networks will be used in daily life to enhance security, safety, and convenience. Integrating a stretchable active matrix with a two-dimensional array of pressure sensors can create a rubber-like artificial skin. In addition, if a matrix is integrated with an array of actuators and mounted on a curved surface, the touch feeling on the surface can be changed electrically. These elastic conductors enable the development of circuits that can be mounted on surfaces which currently lack electrical functionalities. This could lead to the development of intelligent surfaces that can be used as friendly human-electronics interfaces. In the future, such intelligent surfaces will be able to interact with people, objects, and the environment in new ways. Our next steps will be to investigate using these materials to create heat and pressure sensitive devices that connect with people on a more intuitive level, like human skins. Possible applications include steering wheels that analyze perspiration and body temperature to gauge the user's fitness to drive, and mattresses that tilt when they detect body parts under constant pressure.

This study was partially supported by a Grant-in-Aid for Scientific Research (KAKENHI; WAKATE S), and the Special Coordination Funds for Promoting Science and Technology. We thank Hiroyoshi Nakajima, Hiroki Maeda (Dai Nippon Printing Company, Limited), Takanori Fukushima (RIKEN), Takuzo Aida (University of Tokyo), Kenji Hata, and Kinji Asaka (National Institute of Advanced Industrial Science and Technology) for their technical support and valuable discussions.

Tsuyoshi Sekitani
University of Tokyo
Tokyo, Japan

Tsuyoshi Sekitani received his PhD in applied physics from the University of Tokyo in 2003. Since then, he has been a research associate at the Quantum-Phase Electronics Center. In 2009, he joined the Department of Electrical and Electronic Engineering.