A Stretchable Array of High-Performance Electrochromic Devices for Displaying Skin-Attached Multi-Sensor Signals

We report on the fabrication of an interactive display system consisting of a stretchable array of electrochromic devices (ECDs) and temperature and strain sensors for visual display of skin temperature and wrist movement. The electrochromic device is prepared by coating poly(3-hexylthiophene) and tungsten trioxide as electrochromic materials on a flexible and transparent electrode of indium-tin-oxide-coated polyethylene terephthalate, and the subsequent assembly of the prepared electrodes using a gel-type electrolyte of Li-mixed 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/poly(methyl methacrylate). The fabricated ECD exhibits three different colors due to the redox reactions of the electrochromic materials at the corresponding potentials: magenta, violet, and blue at-1.5 V, 0.5 V, and 1.5 V, respectively. By using patterned liquid metal Galinstan interconnections, a stretchable array of the ECDs is fabricated based on a strain relieving mechanism with an island-bridge design, resulting in mechanical stability to repeated stretching of up to 30%. A high-performance strain sensor is fabricated by embedding fragmentized graphene foams inside poly(demithylsiloxane) thin film, and a temperature sensor is prepared by embedding multi-walled carbon nanotubes/polyaniline composite in poly(vinyl alcohol) film. With the integrated system, the strain due to wrist bending and the skin temperature measured by the sensors are displayed as changes in the color pattern of the ECD array aided by an Arduino circuit. This work demonstrates the high potential of our system for application to wearable healthcare devices providing real-time visual display of varied bio-signals.