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Introduction Flexible electronic devices1–4 are expected to open a range of new applications, including flexible displays,5–9 flexible radio frequency (RF) identification devices,10–15 chemical sensors,16–18 flexible large-area physical sensors,19–23 and actuators.24–27 One promising application of flexible electronics that has been attracting attention is sensors for the continuous monitoring of bioinformation, such as pulse28–30 and body temperature.31–35 Such sensor devices would be used in direct contact with human skin and should thus be mechanically flexible such that they are imperceptible; in terms of hygiene, they should also be easily and safely disposable, unlike conventional electronic components. Organic semiconductors are suitable for flexible electronics, yet they are not unique solutions. Another attractive approach is to utilize ultrathin silicon because it exhibits high electronic performance with mechanical flexibility.36–38 Using this approach, Rogers and co-workers demonstrated epidermal electronics36 that can be directly laminated on the surface of human skin to introduce electronic functionalities. However, directly manufacturing sensors or electronic circuits on ultrathin polymeric films with thicknesses of several micrometers or less is a difficult task using conventional semiconductor processes. As discussed in detail later, a reduction

in the total thickness of the devices can improve their mechanical flexibility.39–43 Therefore, new materials, processes, and device structures have to be explored. Organic thin-film devices are expected to yield simultaneously large-area, lowcost, lightweight, and flexible devices because they can easily be fabricated on polymer films by printing processes such as inkjet printing.44–48 In this article, we review recent progress on large-area, ultraflexible, and stretchable organic electronics and related technologies. In particular, ultraflexible organic thin-film devices such as organic thin-film transistors (OTFTs),49–53 organic photovoltaic (OPV) cells,54 and organic light-emitting diodes (OLEDs)55,56 are described. Next, emerging applications using flexible organic devices are presented. Finally, other relevant issues and future prospects are described.

Organic thin-film devices Device and process design to reduce strain Strain occurs whenever any device is bent,39,40 and all semiconductor devices show performance changes in response to strain or deformation. First, strain causes geometrical changes, that is, changes in device dimensions such as the channel length or width of a transistor or the thickness of an insulator film. Such geometrical changes cause modifications in the

Takao Someya, Department of Electric and Electronic Engineering, School of Engineering, The University of Tokyo, Japan; and Exploratory Research for Advanced Technology, Japan Science and Technology Agency, Japan; [email protected] Martin Kaltenbrunner, Soft Matter Physics Department, Johannes Kepler University, Austria; [email protected] Tomoyuki Yokota, Department of Electric an