Development of an Intrinsically Stretchable Electrochromic Display with a Composite of Poly(3,4-ethylenedioxythiophene) and Polyurethane

Abstract

Wearable devices have attracted much attention especially in healthcare for monitoring vital conditions. Stretchability of devices is essentially important to fit the human body and interact with the skin efficiently for biosensing, drug delivery, etc.1,2 Although various components of stretchable wearable devices such as capacitors, batteries, wiring, chemical and physical sensors, etc. have been developed,3 there are few studies on display components of stretchable wearable devices. Recently, electrochromic materials, materials that change color repeatedly by electrical redox reactions, have been studied as one of promising candidates for the display component of sensor devices. From metallic oxides such as WO3 to organic materials such as viologen, various materials have been reported as electrochromic materials. There have been efforts on making an electrochromic device flexible and stretchable. For example, flexible electrochromic displays made of polyaniline deposited on a nonwoven textile were fabricated for the use as a wearable device.4 Stretchable electrochromic displays (ECDs) have also been developed using deformable structures such as coils and waving structures that are fabricated in a top-down approach.5,6 Although these works made a big step toward stretchable ECDs, it still requires complicated fabrication processes that may hinder practical applications. Intrinsically stretchable ECDs, which cause the change of shape by deformation in a molecular level and do not rely on top-down microstructure fabrication, are more desirable for further applications of stretchable ECDs. Here, we developed an intrinsically stretchable ECD with a simple fabrication process. We adopted a composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and polyurethane (PU).7,8 PEDOT is a conductive polymer and it is known as an electrochromic material. It shows near transparent light blue color in an oxidized (conducting) state, and dark blue color in a reduced (insulating) state. Although PEDOT itself has no stretchability, the composite with PU shows enough stretchability with keeping its conductivity.5,6 Therefore we can expect a PEDOT/PU composite shows stretchability and electrochromic behaviors at the same time. To fabricate a PEDOT/PU film, a tetrahydrofuran (THF) solution of an EDOT monomer and PU was spin coated on a glass substrate and baked on a hot plate at 90 °C for 10 minutes for polymerizing EDOT as well as evaporating THF. The fabricated PEDOT/PU film was peeled from the substrate. When voltages of +0.8 V and -0.5 V were applied to the prepared PEDOT/PU film in an electrolyte solution, the film showed reversible color changes between light blue to dark blue, weather the film was stretched or not. Moreover, though we applied potentials to one point of the film, color change occurred at the whole area of the film. This suggests that the PEDOT/PU film works as a freestanding electrochromic material that can be used without additional electrodes. This characteristic property will contribute to the development of the display-integrated stretchable sensor devices. To demonstrate the effectiveness of our stretchable ECD film, a PEDOT/PU film on the glass substrate was cut into two sections to make a cathode and an anode. When voltage was applied between the two electrodes, redox reactions occurred at each electrode. This caused the opposite color changes between the two electrodes, resulting in higher contrast. Furthermore, the PEDOT/PU film was bonded to a double-network (DN) gel as a robust, stretchable support that can serve as a supply source of electrolytes that is required for redox reactions of PEDOT. The fabricated ECD could be wrapped, folded, and twisted, and it fits to a human finger while maintaining its electrochromic properties. It also could be stored at a dried state, and it could swell again before use.