Visco-Poroelastic Electrochemiluminescence Skin Devices with Piezo-Ionic Effect

With the development of soft optoelectronics, many researchers are recently interested in electronic skin (e-skin) that can visual feedback of mechanical stimuli for human-machine interaction. In this respect, electrochemiluminescence (ECL) materials have been attracting attention to form alternative light-emitting based e-skin capable of mechanical sensing, because of their high stability, simple fabrication process, and low operating power. In particular, following early research efforts devoted to achieving excellent sensitivity of e-skin, recent design schemes for these devices have focused on strategies for transduction of spatially resolved sensing data into straightforward user-adaptive visual signals. However, many light-emitting based mechanical sensing devices have limitations in terms of low sensitivity to mechanical stimuli and complicated fabrication processes.<br/>Here, we propose the e-skin device exploiting the characteristic mass transfer phenomenon, which is referred to as the piezo-ionic effect, as an alternative strategy capable of transducing mechanical stimuli into visual readout. The material layer comprises a mixture of an ionic transition metal complex luminophore and an ionic liquid within a thermoplastic polyurethane matrix. The proposed material shows visco-poroelastic response to mechanical stress, which induces a change in the distribution of the ionic luminophore in the film, as the piezo-ionic effect. This piezo-ionic effect is exploited to develop a simple device containing the composite layer sandwiched between two electrodes, which is termed ECL skin. Emission from the ECL skin is examined, which increases with the applied normal and tensile stress. Additionally, locally applied stress to the ECL skin is spatially resolved and visualized without the use of spatially distributed arrays of pressure sensors. The simple fabrication and unique operation of the demonstrated ECL skin are expected to provide new insights into the design of materials for human-machine interactive electronic skins.